Accessories for HV and EHV Extruded Cables: Components [1] 3030394670, 9783030394677

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CIGRE Green Books

CIGRE Study Committee B1: Insulated Cables

Accessories for HV and EHV Extruded Cables Volume 1: Components

CIGRE Green Books Series Editor CIGRE International Council on Large Electric Systems (CIGRE) Paris, France

CIGRE presents their expertise in unique reference books on electrical power networks. These books are of a self-contained handbook character covering the entire knowledge of the subject within power engineering. The books are created by CIGRE experts within their study committees and are recognized by the engineering community as the top reference books in their fields. More information about this series at http://www.springer.com/series/15209

Pierre Argaut Editor

Accessories for HV and EHV Extruded Cables Volume 1: Components

With 217 Figures and 111 Tables

Editor Pierre Argaut Héricy, France

ISSN 2367-2625 ISSN 2367-2633 (electronic) ISBN 978-3-030-39465-3 ISBN 978-3-030-39466-0 (eBook) ISBN 978-3-030-39467-7 (print and electronic bundle) https://doi.org/10.1007/978-3-030-39466-0 © Springer Nature Switzerland AG 2021 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Message from the President

CIGRE is the global expert community for electric power systems. It is a nonprofit organization based in Paris. It consists of members from over 100 countries representing 61 national committees. It functions as a virtual organization with members who are experts in their technical field, forming working groups dealing with issues facing the power delivery industry. In 2019, 230 working groups including more than 3000 experts were working together to resolve the identified issues. The output of the working groups is technical brochures. There are over 700 technical brochures, which contain the combined knowledge and practice of engineering experts from all over the world. The brochures are practical in nature enabling the engineer to plan, design, construct, operate, and maintain the power delivery systems as required. CIGRE has over 10,000 reference papers and other documents supporting the brochures and dealing with other relevant technical matters. This Green Book on Accessories for HV and EHV Cable Systems, compiled by Study Committee (SC) B1, “Insulated Cables” provides state of the art in the design and application of Accessories in HV and EHV Cable Systems. The book comprises material from published technical peer-reviewed publications and technical experts in the field. CIGRE is a source of unbiased technical information. Engineers can refer this book without fear of favoring one supplier or country. It is a compilation of the combined expertise of many international experts. Like other CIGRE Green Books this book contains input from several tens of experts; not only one or two. These international experts have provided technical information relevant to readers irrespective of where the readers reside. The views expressed and suggestions made are unbiased objective statements. These can be used as references for engineers to develop standards such as IEC Standards and guidelines within their organizations. This book is a reference book for academia, power transmission engineers, consultants, and users. For each of the chapters of this book, a tutorial has been prepared by the members of the Working Group who published the content of the chapter as a technical brochure or a report in Electra to disseminate this unbiased information. By collecting the knowledge and experience gained over time in the field of cable accessories and systems, this book not only explains the know-how but also the know why. v

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Message from the President

I would like to congratulate those involved from SC B1 who have compiled this book. Many of them have had to work in their spare time for hours to complete this task, for which they worked as volunteers. I specifically want to address on behalf of Cigre all my thanks and gratitude to Pierre Argaut, Past Chair of Study Committee B1 for leading this formidable task. I would recommend this book in forming the basis for Underground Transmission System design activities now and in the future. December 2020

Michel Augonnet Michel Augonnet is a graduate electrical engineer from Centrale Supelec (1973). Forty-two years of his career was related to Alstom Group in the field of power generation, transmission, and distribution, with a focus on electrical systems, control and instrumentation, project management, and sales. Michel is currently the president of SuperGrid Institute (an electrical research and testing laboratory in Lyon, France); a board member of AEG Power Systems, Mastergrid SA; and an alternate board director for ACTOM (PTY) South Africa. Michel is president of CIGRE, and outgoing treasurer and former president of the French NC.

Message from the Chairman of the Technical Council of CIGRE

This Green Book on Accessories for HV and EHV cable systems aims at describing the cumulated experience gained by CIGRE experts in the domain of accessories for transmission underground systems. It is well known that the reliability and performance of a cable circuit is dependent in equal measures on the design of the cable and accessories and on the skill and experience of the person who is assembling the accessory. The cable insulation is manufactured in the factory under controlled process conditions using selected materials of high quality. It is equally important that the same quality measures are employed for the manufacture of the accessories in the factory and for their assembly on site onto the specially prepared cable. It is essential to select the design of accessory to be exactly compatible with the particular cable type and the particular service application. Compatibility should be validated by electrical type approval tests and be supported by prequalification tests, and satisfactory service experience. In particular, the performance of the accessory is dependent on the quality, skill, and training of the jointing personnel and on the use of the specialized tools required for a particular accessory. The chapters of this book form the basis of the information that is needed by the manufacturer and installer of the cable and accessories. For many applications the cable manufacturer also manufactures, supplies, and installs the accessories as part of the complete cable circuit. In the event that the user purchases the accessories separately from the cable, the Green Book will also provide the basis of the information (including tests reports) that should be needed to obtain the appropriate performance of the system, both in the case of a new underground line of “the Power System of the Future” or in the upgrading process of an existing line “to make the best use of the existing power system”. This Green Book has been authored by leading industry, research, and academic professionals, acting as members of Working Groups within Study Committee B1, who published technical brochures and prepared tutorials, providing all stakeholders with high quality and unbiased information.

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Message from the Chairman of the Technical Council of CIGRE

I take the opportunity to acknowledge the editor, the chapters’ authors, and all the numerous contributors in working bodies for this contribution from which the entire global power systems will benefit. Also, I especially acknowledge the leading role of Study Committee B1 – Marco Marelli on the realization of this Green Book. Marcio Szechtman CIGRE Technical Council Chair Marcio Szechtman graduated and received his Ms. Sc. degree in Electrical Engineering from the University of Sao Paulo – Brazil, in 1971 and 1976, respectively. He joined CIGRE in 1981 and became the Study Committee Chair of B4 (DC Systems and Power Electronics) between 2002 and 2008. He received the CIGRE Medal in 2014 and was elected as Technical Council Chair in 2018. Marcio has a long career in R&D Power Systems Centers and since April 2019 was appointed as Eletrobras Chief Transmission Officer.

Message from the Secretary General

In 2014, I had the pleasure to comment on the launching of a new CIGRE publication collection in an introductory message about the first CIGRE Green Book, the one on Overhead Lines, the second one being on Accessories for HV Extruded Cables. The idea to valorise the collective work of the study committees accumulated over many decades, by putting together all the technical brochures of a given field in a single book, was first proposed by Dr. Konstantin Papailiou to the Technical Committee (now Council) in 2011. In 2015, cooperation with Springer allowed CIGRE to publish the Green Book on Overhead Lines again as a “Major Reference Work” distributed through the vast network of this well-known international publisher. In 2016, the collection was enriched with a new category of Green Books, the CIGRE “Compact Series,” to satisfy the needs of the study committees when they want to publish shorter, concise volumes. The first CIGRE compact Book was prepared by Study Committee D2, under the title Utility Communication Networks and Services. The concept of the CIGRE Green Books series has continued to evolve, with the introduction of a third subcategory of the series, the “CIGRE Green Book Technical Brochures” (GBTB). CIGRE has published more than 720 technical brochures since 1969, and it is interesting to note that in the first one, on tele-protection, the first reference was a Springer publication of 1963. A CIGRE Technical Brochure produced by a CIGRE working group, following specific Terms of Reference, is published by the CIGRE Central Office and is available from the CIGRE online library, e-cigre, one of the most comprehensive, accessible databases of relevant technical literature on power engineering. Between 40 and 50 new technical brochures are published yearly, and these brochures are announced in Electra, CIGRE’s bimonthly journal, and are available for downloading from e-cigre. In the future, the Technical Council of CIGRE may decide to publish a technical brochure as a Green Book in addition to the traditional CIGRE Technical Brochure. The motivation of the Technical Council to make such a decision is to disseminate the related information beyond the CIGRE community, through the Springer network.

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Message from the Secretary General

All CIGRE Green Books, are available from e-cigre in electronic format free of charge for the co-authors of the Book. CIGRE plans to co-publish new Green Books edited by the different study committees, and the series will grow progressively at a pace of about one or two volumes per year. This new Green Book, a Major Reference Work prepared by Study Committee B1 on Insulated Cables, is an updated version of the Green Book published in 2014, and is the seventh of this subcategory. This new edition comprises two additional chapters related to the most recent work of SC B1. A second volume of the book will address Accessories in submarine and DC applications. Some chapters will also cover distribution cable systems. It is important to know that each of the chapters of this book is the topic of a dedicated Tutorial prepared by the members of the WG who published the content of the chapter as a Technical Brochure or an Electra Report. I want to congratulate all the authors, contributors, and reviewers of this book, which gives the reader a clear, comprehensive, and unbiased vision of the past, recent, and future developments of Accessories for HV and EHV Cable Systems. Secretary General

Philippe Adam Philippe Adam was appointed secretary general of CIGRE in March 2014. Graduate of the École Centrale de Paris, he began his career in EDF in 1980 as a research engineer in the field of HVDC and was involved in the studies and tests of outstanding projects like the Cross Channel 2000 MW link and the first multiterminal DC link between Sardinia, Corsica, and Italy. After this pioneering period, he managed the team of engineers in charge of HVDC and FACTS studies of the R&D division of EDF. In this period, his CIGRE membership as a working group expert and then as a working group convener in Study Committee 14 was a genuine support to his professional activities. Then, Philippe held several management positions in the EDF Generation and Transmission division in the fields of substation engineering, network planning, transmission asset management, and international consulting until 2000. When RTE, the French TSO, was created in 2000, he was appointed manager of the Financial and Management Control Department, in order to install this corporate function and the necessary tools. In 2004, Philippe contributed to the creation of RTE international activities first as project director and then

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deputy head of the International Relations Department. From 2011 to 2014, he has been the strategy director of Infrastructures and Technologies of the Medgrid industrial initiative. In the meantime, between 2002 and 2012, he served CIGRE as the technical committee secretary and as the secretary and treasurer of the French National Committee from 2009 to 2014. Philippe was appointed secretary general of CIGRE in March 2014.

Preface

Dear Reader, This CIGRE Reference Book on Accessories for HV and EHV Extruded Cables is the first in a series of Reference Books regarding High and Extra High Voltages Cable Systems. This first volume of the Book provides information regarding Recommendations and Guidelines from CIGRE for Design, Installation, and Testing of Accessories for AC extruded cables. Accessories for HVDC extruded cables will be introduced in a second volume together with accessories for submarine applications. The Book compiles the results of the work by several Working Groups and Task Forces of CIGRE Study Committee 21/B1 and Joint Working Groups and Joint Task Forces with other Study Committees. Many experts from Study Committees 21/B1 (HV Cables), 15/D1 (Materials and Emerging Test Techniques) and 33/B3 (Substations) have participated in this work in the last 30 years in order to offer comprehensive, continuous, and consistent outputs. I would like to express my deepest thanks to these WG members who made all this possible. This volume is divided into 11 chapters as follows: ▶ Chapter 1, “Compendium of Accessory Types Used for AC HV Extruded Cables” is the output of WG 21.06, published as TB 177 in 2001 and convened by Z. IWATA (Japan). This chapter is a compendium of accessory types made in 1995. Of course ▶ Chap. 1, “Compendium of Accessory Types Used for AC HV Extruded Cables”, which was written by WG 21.06 around 20 years ago, does not describe some designs of accessories currently offered in the market. Since 2001 new designs of accessories (plug-in connectors, cold shrinkable premoulded accessories) have been introduced in the market. Nevertheless, they are still of the same type as the family of accessories inventoried in 2001. It is thus possible to manage the extension of qualification of the “classical” designs towards the newer innovative designs through the functional analysis described in ▶ Chap. 4, “Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems” and since the basics of interfaces in accessories have been carefully studied and explained in ▶ Chap. 3, “Interfaces in Accessories for Extruded HV and EHV Cables”.

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▶ Chapter 2, “A Guide to the Selection of Accessories” contains further outputs of WG 21.06, (published in TB 177) and proposes a Guide for the Selection of Accessories. The goal was to establish and recommend good practice to obtain the expected performance from accessories. ▶ Chapter 3, “Interfaces in Accessories for Extruded HV and EHV Cables” is the output of TF 21.15, published as TB 210 in 2002 and convened by H. GEENE (Netherlands). This chapter deals with the many interface aspects both electrical and mechanical between cables and joints/sealing ends. ▶ Chapter 4, “Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems” is the output of WG B1.03, published as TB 303 in 2006 and convened by J. BECKER (Belgium). This chapter deals with the Prequalification, Type, Sample, and Routine Testing of Extruded Cables and Joints/Sealing Ends in the range of 170 to 550 kV. ▶ Chapter 5, “Cable Accessory Workmanship on Extruded High Voltage Cables” is the output of WG B1.22, published as TB 476 in 2011 and convened by K. LEEBURN (South Africa). This chapter contains Guidelines with recommended practices in Workmanship. As the continuous increase in voltage levels also resulted in increase of electrical stresses at the interface of cables and accessories, it was necessary to produce Guidelines with recommended practices in Workmanship to ensure reliable installation of joints and sealing ends. ▶ Chapter 6, “Guidelines for Maintaining the Integrity of Extruded Cable Accessories” is the output of WG B1.29, published as TB 560 in 2013 and convened by E. BERGIN (Ireland). This chapter deals with how to maintain the Integrity of Accessories for Extruded Cables and is one of the most important issues, both for new and existing installations. It takes into account the recommendations for a better workmanship as proposed in ▶ Chap. 5, “Cable Accessory Workmanship on Extruded High Voltage Cables” and indicates new ways and procedures to obtain higher reliability from joints and sealing ends. ▶ Chapter 7, “Feasibility of a Common, Dry Type Plug-in Interface for GIS and Power Cables above 52 kV” is the output of JWG B1.B3.33, published as TB 605 in 2014 and convened by P. MIREBEAU (France). This chapter is dedicated to the design and testing of dry type/plug-in GIS terminations. In the conclusion of this report, a new work is recommended to propose a standard design of a GIS termination at voltages up to 145 kV. ▶ Chapter 8, “Test Procedures for HV Transition Joints for Rated Voltages 30 kV up to 500 kV” is the output of WG B1.24, published as TB 415 in 2010 and convened by M. MARELLI (Italy). This chapter gives recommendations for testing AC transition joints. Transitioning from old Cable Technology (LPOF) to New Cable Technology (Extruded) is one of the ways to prepare the Network of the Future while making the Best Use of the Existing Equipment. For DC systems this is also achieved by means of transition joints. A chapter in the second volume of the book will deal with DC Transition joints. ▶ Chapter 9, “Thermal Ratings of HV Cable Accessories” is the output of TF 21.10, published in Electra 212 in 2004, and convened by R. SCHROTH (Germany) and later on by H. GEENE (Netherlands) and is mainly of historical value. It is very

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useful to help the reader to better understand how to match the thermal performance of an accessory with the thermal performance of the cable. This has been taken into account in the existing IEC Standards (IEC 60840 and 62067) and also in Chap. ▶ 4, “Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems”. ▶ Chapter 10, “Test Regimes for HV and EHV Cable Connectors” deals with Cable Connectors. It is the output of TB 758, published in 2019 by WG B1.46 and convened by M. UZELAC (USA) to cover Test regimes for HV and EHV various types of cable conductors, both in Copper or Aluminum. ▶ Chapter 11, “Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power Cables up to 145 kV” is the output of TB 784, published by JWG B1. B3.49 in 2019 to propose a “Standard Design of a Common, Dry type, Plug-in Interface for GIS and Power Cable up to 145 kV” as recommended in ▶ Chap. 7, “Feasibility of a Common, Dry Type Plug-in Interface for GIS and Power Cables above 52 kV”. This JWG was convened by P. MIREBEAU (France) as JWG B1. B3.33. As immediate past Chairman of SC B1, as well as being a Member of several of the Working Groups, I have reviewed these chapters. I can confirm that all of them give unbiased information, which will be useful for those involved in new cable systems’ projects or in upgrading old systems. In this task of reviewing and with the help of the Conveners of the Working Groups, I have been assisted by Mrs. LIU Ying (CN). I would like to express my deepest thanks to all of them. Among all these experts and co-authors, a special mention should be made to two of them who are sadly missed by our community: Jean BECKER (▶ Chap. 4, “Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems”) and Eugene BERGIN (▶ Chap. 6, “Guidelines for Maintaining the Integrity of Extruded Cable Accessories”). This book is dedicated to their memory. We will never forget how good friends and colleagues they were for all of us. Of course, this first volume of the Book will be updated, as soon as new relevant official documents from B1 are available.

Contents

1

Compendium of Accessory Types Used for AC HV Extruded Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zensuke Iwata

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A Guide to the Selection of Accessories Zensuke Iwata

.....................

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3

Interfaces in Accessories for Extruded HV and EHV Cables . . . . . Henk Geene

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4

Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jean Becker

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Cable Accessory Workmanship on Extruded High Voltage Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kieron Leeburn

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Guidelines for Maintaining the Integrity of Extruded Cable Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eugene Bergin

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Feasibility of a Common, Dry Type Plug-in Interface for GIS and Power Cables above 52 kV . . . . . . . . . . . . . . . . . . . . . . . . . . . Pierre Mirebeau

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Test Procedures for HV Transition Joints for Rated Voltages 30 kV up to 500 kV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Marco Marelli

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5

6

7

8

9

Thermal Ratings of HV Cable Accessories . . . . . . . . . . . . . . . . . . . Henk Geene and Reinhard Schroth

401

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Contents

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Test Regimes for HV and EHV Cable Connectors . . . . . . . . . . . . . Milan Uzelac

11

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power Cables up to 145 kV . . . . . . . . . . . . . . . . . . . . Pierre Mirebeau

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About the Editor

Pierre Argaut was graduated as electrical engineer from the “Institut d’Electrotechnique de Grenoble” in 1971. He joined Delle-Alsthom (HV Switchgear Manufacturer) in November 1971 and took several positions before heading the R&D Department on GIS. After being operation manager of South European Pipeline, he joined SILEC in 1988 and retired at the end of 2010. His last position with SILEC was senior vice president. In Study Committee B1 (Insulated Cables), Pierre has held position of working group member (21.09;21.06;21.07;21.17), French SC member, working group convener (B1.19), special reporter (2010), advisory group convener (Tutorial Advisory Group till 2010), and chairman of SC B1 from August 2010 to August 2016. He received the Technical Committee Award in 2000, the Distinguished Member Award in 2002, and the title of Honorary Member of CIGRE in 2016.

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Contributors

Jean Becker Charleroi, Belgium Eugene Bergin Dublin, Ireland Henk Geene Prysmian Group, Product Management HV Accessories, The Hague Area, Netherlands Zensuke Iwata Kamakura, Japan Kieron Leeburn CBI Electric African Cables, Chief Engineer Process and Product in HV, Vereeniging, South Africa Marco Marelli Prysmian Group, System Engineering, Land and Submarine HV and EHV AC/DC Power Cable Systems and Telecom Cable Systems, Milano, Italy Pierre Mirebeau Villebon sur Yvette, France Reinhard Schroth Berlin, Germany Milan Uzelac G&W Electric Co, R&D, HV Cable Accessories, Bolingbrook, USA

Jean Becker: deceased. Eugene Bergin: deceased. Zensuke Iwata has retired. Pierre Mirebeau has retired. Reinhard Schroth has retired. xxi

1

Compendium of Accessory Types Used for AC HV Extruded Cables Zensuke Iwata

Contents 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Types of Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Types of Straight Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Types of Transition Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Types of Y Branch Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Types of Terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Types of Metal Enclosed GIS Terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Types of “Oil Immersed Transformer” Terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Types of Outdoor Terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4 Types of Indoor Terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.5 Types of Temporary Terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix: Glossary of Component Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Glossary of Names for Components Used in Accessories for Extruded Cables . . . . . . . . . . .

1.1

1 2 5 12 15 16 17 21 22 28 33 34 34

Introduction

Chapter 1 categorises the types of accessory designs available for use on HV cables with extruded insulation for ac transmission voltages of 60 kV (75.5 kV) and above. The typical types of extruded cable insulation being low density polyethylene (LDPE), high density polyethylene (HDPE), cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR).

Zensuke Iwata has retired. Z. Iwata (*) Kamakura, Japan e-mail: [email protected] © Springer Nature Switzerland AG 2021 P. Argaut (ed.), Accessories for HV and EHV Extruded Cables, CIGRE Green Books, https://doi.org/10.1007/978-3-030-39466-0_1

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Z. Iwata

The contents were compiled by Cigré Working Group 21-06, as part of a survey into the world-wide usage of accessories up to the year 1992. The purpose of Chapter 1 is twofold; • To organise the designs into logical categories based upon their function and principle of design and • To provide a glossary of the names of their component parts. The collection of accessory designs has been compiled from those known to have been used in service applications or to have been developed in the first thirty year period since the emergence of the extruded type of HV cable. It is important to note that the inclusion of an accessory design does not imply that it has technical merit or is preferred for any particular voltage category, nor does the exclusion of a design imply any censure. Similarly the details of the designs shown are typical and it is acknowledged that design variations exist of equal utility. The Chapter 1 is divided into two parts: Sections 1.2 to 1.3.5: Appendix:

“COMPENDIUM OF ACCESSORY TYPES” “GLOSSARY OF COMPONENT NAMES”

The work has been published as Cigré TB 89 (also included in TB 177) in both English and French versions, these being the dual languages of Cigré. It has been additionally translated into the German language for reference. The dual language versions of the Glossary, cross reference each other, for example, the English version lists each component name together with its French counterpart. Sections 1.2 to 1.3.5 is a compendium of the generic types of accessories. The accessory designs are divided firstly into the main categories of joints and terminations and secondly into sub-categories. For example Type 1.2.1.2, Fig 1.6, is a “straight joint” of the “prefabricated” type, employing “composite” insulation. A diagram is provided for each category of design, together with a description of the accessory and its function. The preferred names of components have been used. Appendix is a glossary in alphabetic order of the preferred names of the component parts. Each preferred name is accompanied by a definition and a list of alternative names. For completeness the list of each type of accessory name have been included under the headings for “straight joint”, “transition joint”, “Y joint”, “metal enclosed GIS termination”, “oil immersed transformer termination”, “outdoor termination”, “indoor termination” and “temporary termination”, together with a cross reference by item number to the Compendium of Accessory Types in Sects. 1.2 to 1.3.5. For example under the heading of “outdoor termination” can be found the “stress cone and insulator” type together with its Part I item number 1.3.3.4.

1.2

Types of Joints

A joint is the insulated and fully protected connection between two or more cables. Also termed “splice”. The following types exist:

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Compendium of Accessory Types Used for AC HV Extruded Cables

• • • •

Straight Joint Transition Joint Screen Interruption Joint Y Branch Joint.

3

Each of the above joint designs is illustrated by a diagram to show the type of insulation. For the purpose of clarity other important design details have been omitted. The design requirements common to each type of joint are: • A high current connection between conductors. • A joint insulation which meets the same performance standard as the cable. • A high current connection to permit the flow of short circuit current between the two cable sheaths or screen wires. • A metallic joint shell or screen wire connection electrically insulated from earth potential to match the insulating integrity of the cable oversheath. • A protection of the joint and cable insulation against the ingress of water. • A protection against corrosion of the joint metal work. In the HV voltage class of greater than 60 kV the majority of extruded cables are of single core construction, thus straight joints of the single core type have been illustrated. Three core joints employ the same types of insulation and are grouped together in one housing as illustrated in Sect. 1.2.2 for transition joints to three core paper insulated cable. Single core joints with screen interruption, Fig. 1.1, have: • A gap in the insulation screen. • An insulated flange in the joint shell. • Bonding leads to permit the adjacent cable sheaths to be connected in the configuration necessary for a specially bonded cable system.

Fig. 1.1 “Taped” straight joint with screen interruption

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The insulated flange can be electrically shorted to make the joint suitable for a solidly bonded cable system. Joints without screen interruption have an electrically continuous insulation screen and continuous joint shell; such joints are used for solidly bonded single core and three core cable systems (Figs. 1.2, 1.3, 1.4, and 1.5 are examples of taped joints without screen interruption). For the purpose of comparison the diagram for each type of joint shows the insulation contained within a metallic joint shell which is plumbed to the cable sheaths, as shown in Fig. 1.1. This provides a complete water barrier and a connection between the cable sheaths. For buried direct installations it is usual to protect and to insulate the metallic shell within a compound filled joint box as shown in Fig. 1.3. For indoor installations, such as tunnels, the joint shell can be insulated by either a) a polymeric sleeve or wrapping, or b) by pedestal insulators as shown in Fig. 1.4. For cables which do not have a metallic sheath an alternative design of joint protection is shown in Fig. 1.5. The cable screen wires are connected across the joint and the joint is protected within either a compound filled waterproof joint box as shown, by a heat shrink sleeve or by a wrapping of elastomeric or adhesive tape.

Fig. 1.2 “Taped” straight joint without screen interruption

Fig. 1.3 Typical protection for a joint with metallic shells for direct burial

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Fig. 1.4 Typical protection for a joint with metallic shells for indoor installations

Fig. 1.5 Typical protection for a joint without metallic shells for direct burial and for indoor installations

1.2.1

Types of Straight Joints

A straight joint connects two cables of the same type. Also termed “straight splice”. • • • • •

“Taped” Joint “Prefabricated” Joint “Field Moulded” Joint “Heat Shrink Sleeve” Joint “Back-to-back” Joint.

1.2.1.1 “Taped” Joints “Self Amalgamating Tape” Type Elastomeric semi-conducting and insulating tapes are wound onto the cable to form the conductor screen, the insulation, the stress control profile screens, the insulation screen and the screen interruption insulation, Figs. 1.1 and 1.2. Stretching the tape during application activates amalgamation between the layers of tape to form a solid mass. “Adhesive Tape” Type Semi-conducting and insulating tapes which have been pre-coated with an adhesive layer are wound onto the cable as described for the “self amalgamating tape” type.

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1.2.1.2 “Prefabricated” Joints Prefabricated straight joints employ insulation that has been preformed and tested in the factory. The usual methods of manufacture are to mould elastomeric insulation and to cast thermoset resin insulation. “Composite” Type Two factory premoulded elastomeric stress cones are inserted into a central insulator of cast thermoset resin, Fig. 1.6. Pressure at the stress cone to central insulator interface and at the cable core interface is maintained by a compression device which is usually comprised of metallic springs. “Premoulded One-Piece” Type A single premoulded elastomeric sleeve forms the insulation as shown in Fig. 1.7, it is complete with insulation, connector screen, stress control profile screens, insulation screens and, where applicable, screen interruption. Interfacial pressure at the sleeve to cable core interface is maintained by the elastic memory of the sleeve.

Fig. 1.6 “Prefabricated composite” joint

Fig. 1.7 “Premoulded one-piece” joint

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Fig. 1.8 “Premoulded two-piece” joint

Fig. 1.9 “Premoulded three-piece” joint

“Premoulded Two-Piece” Type A two-piece joint, Fig. 1.8, is similar to the one-piece type, but the elastomeric insulation is comprised of a large diameter premoulded sleeve which is stretched to fit on top of a smaller diameter elastomeric adaptor moulding. “Premoulded Three-Piece” Type The three-piece joint, Fig. 1.9, has insulation comprised of a large diameter, cylindrical sleeve, elastomeric moulding which is stretched to fit onto two elastomeric adaptor mouldings.

1.2.1.3 “Field Moulded” Joints Field moulded joints employ insulation that is melted, moulded and consolidated to the prepared cable insulation in situ ie. “in the field”. The following types of joints differ in the processes used to form the insulation, these are summarised in Table 1.1. The completed joints all have a similar design as shown in Fig. 1.10.

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Table 1.1 The Typical Methods Used to Form the Insulation of Field Moulded Joints Types of Joint Tape moulded Crosslinked tape moulded

Forming Process of Joint Insulation Taping ! Moulding Taping ! Moulding ! Crosslinking

Extrusion moulded

Extrusion ! Moulding

Crosslinked extrusion moulded

Extrusion ! Moulding ! Crosslinking

Injection moulded

Pellets ! Injection

Crosslinked injection moulded

Pellets ! Injection ! Crosslinking

Block moulded

Premoulded block ! Moulding

Crosslinked block moulded

Premoulded block ! Moulding ! Crosslinking

Diagram

“Tape Moulded” Type The insulation is applied in the form of layers of elastomeric or polymeric tape. A heated mould is fitted around the insulation. The insulation is softened and melted by the heat. Pressure which is generated by the thermal expansion consolidates the melted insulation both to itself and to the cable insulation. The connector screens and insulation screens are usually applied separately to the insulating process.

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Fig. 1.10 “Field moulded” joint

“Crosslinked Tape Moulded” Type As above, but with the addition of a prolonged heating process at elevated temperature, which activates a chemical agent within the tapes to crosslink the insulation. External pressure (hydraulic, pneumatic or mechanical) is applied to prevent the formation of voids. “Extrusion Moulded” Type The prepared cable core is housed within a mould. An extruder containing a rotating screw is used to heat and soften the insulating material before injection into the mould. The insulation is allowed to cool under pressure to consolidate the insulation to the prepared cable core. “Crosslinked Extrusion Moulded” Type As above, but with the addition of a prolonged heating process at elevated temperature to activate the chemical agent within the insulation material after the insulation has been formed around the cable. External pressure is applied to prevent the formation of voids. “Injection Moulded” Type As for the extrusion moulded type, but the insulating material is heated and melted in a cylinder or pot, it is then injected into the mould, either mechanically by a piston, or by the direct application of gas or liquid pressure. “Crosslinked Injection Moulded” Type As above, but with the addition of a prolonged heating process at elevated temperature to activate the chemical agent within the insulation material after the insulation has been formed around the cable. External pressure is applied to prevent the formation of voids. “Block Moulded” Type The insulation is premoulded in the form of two mouldings which are divided longitudinally. These half mouldings are fitted to the prepared cable core and are themselves encased in a heated mould tool to consolidate them to the cable.

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Fig. 1.11 “Heat shrink sleeve” joint

“Crosslinked Block Moulded” Type As above, but with the addition of a prolonged heating process at elevated temperature to activate the chemical agent within the insulation material once the insulation material has been formed around the cable. External pressure is applied to prevent the formation of voids.

1.2.1.4 “Heat Shrink Sleeve” Joint A crosslinked polyolefin sleeve is heated to above its crystalline melting point and is expanded to a large diameter in a factory process. It is then allowed to cool and freeze at the larger diameter. During assembly of the joint the sleeve is positioned over the conductor connector and is again heated to above the crystalline melting point such that the sleeve contracts to form an intimate fit with the cable. The insulation and screens may be supplied as one integral sleeve or as several individual sleeves. Stress control sleeves which are pre-loaded with either a resistive or high permittivity filler may also be applied over the joint to control the stress distribution as shown in Fig. 1.11. 1.2.1.5 “Back-to-Back” Joint These joint designs are derived from certain types of cable terminations and as such are part insulated with either a dielectric liquid or pressurised SF6 gas. Provision is made in the design to withstand the effects of the thermal expansion of these fluid insulants (see item 1.3). “Back-to-Back” Joint with Two Insulators The joint, as shown in Fig. 1.12, is comprised of either a) two metal enclosed GIS terminations which are connected with a short length of SF6 gas insulated busbar trunking, as described in item 1.3.1, or b) two oil immersed terminations which are connected with a short length of liquid insulated busbar trunking, as described in item 1.3.2. These joints can be single phase or three phase. For the latter type, three termination insulators are mounted on a barrier plate. The insulators anchor the conductors and segregate the SF6 gas or insulating liquid from each cable. Some designs employ a mixture of SF6 and N2 gas.

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Fig. 1.12 “Back-to-Back” joint with two insulators

Fig. 1.13 (a) “Back-to-Back” joint with one insulator and fluid insulation. (b) “Back-to-Back” joint with one insulator and solid insulation

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“Back-to-Back” Joint with One Insulator One half of the joint employs an insulator as described in item 1.2.1.5 for the “backto-back” joint with two insulators. In the second half of the joint, Fig. 1.13a, the cable termination is “open”, that is it is terminated as in a “directly immersed” GIS or oil immersed transformer termination. Alternatively, as shown in Fig. 1.13b, the second half of the joint can be insulated by solid insulation formed from tape or an elastomeric sleeve. The single insulator anchors the conductors and centralises the corona shield within the SF6 gas or liquid filled joint shell. It is necessary to seal the “open” cable to prevent ingress of the SF6 gas or liquid into it. “Back-to-Back” Joint Without Insulator Both of the cable terminations are “open” as shown in Fig. 1.14, that is they are terminated as in a “directly immersed” GIS or oil immersed transformer termination. It is necessary to seal both of the “open” cables to prevent ingress of the SF6 gas or liquid into them.

1.2.2

Types of Transition Joints

A transition joint connects two cables of different types, for example a polymeric extruded cable to a self-contained oil filled cable. Transition joints are sometimes employed to connect cables of the same type, but with different conductor sizes. In the latter application they are designed to withstand imbalanced conductor thermomechanical force. • “Polymeric extruded cable to mass impregnated cable” transition joint. • “Polymeric extruded cable to oil filled paper cable” transition joint. • “Polymeric extruded cable to gas pressurised paper cable” transition Joint. ▶ Chapter 8, “Test Procedures for HV Transition Joints for Rated Voltages 30 kV up to 500 kV” of this book is dedicated to tests procedures for HV Transition Joints for Rated Voltages 30 kV to 500 kV.

Fig. 1.14 “Back-to-Back” joint without insulator

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Fig. 1.15 “Polymeric extruded cable to mass impregnated cable” transition joint

Fig. 1.16 “Polymeric extruded cable to oil or gas filled paper cable” transition joint, three core type

1.2.2.1 “Polymeric Extruded Cable to Mass Impregnated Cable” Transition Joint The hydrocarbon compound impregnant in the paper cable is segregated from the polymeric cable by a) a stop ferrule which contains a solid barrier and b) a barrier tape or sleeve which is usually applied over the core of the paper cable to seal onto the ferrule. The joint, as shown in Fig. 1.15, is then insulated with tape or sleeves in the conventional manner. Alternatively the two cables can be segregated using a solid central barrier as described in 1.2.2.2. 1.2.2.2 “Polymeric Extruded Cable to Oil or Gas Filled Paper Cable” Transition Joint, Three Core Type The two cables are segregated by a solid central barrier comprised of three insulated conducting rods in the form of bushings, Fig. 1.16. The barrier is usually premoulded in thermoset resin and is designed to withstand the pressure in the oil or gas filled cable. The joint insulation on the polymeric cable side may be formed of any of the types described for the straight joint (i.e. taped, premoulded elastomeric sleeve, heat

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shrink sleeve etc.). The joint on the paper cable side is usually insulated with either impregnated plain or crepe paper tapes. The solid barrier can also be formed, as shown in Figs. 1.12 and 1.13a, by either a) two sets of three back-to-back GIS terminations, which are connected within a short length of SF6 gas or oil insulated busbar trunking, or b) one set of three back-to-back oil immersed terminations within a short length of oil insulated busbar trunking. For the “oil immersed termination” design of joint, one set of insulators can be omitted and the cable oil can be used to insulate both the busbar trunking and the paper insulated cable, as shown in Fig. 1.13a (but with each paper cable termination of the “open” unscreened type).

1.2.2.3 “Polymeric Extruded Cable to Oil or Gas Filled Paper Cable” Transition Joint, Single Core “Non-Fed” Type The solid barrier contains one bushing, Fig. 1.17, of the type described for the three core cable. With this design of joint it is not possible to feed the oil or gas directly into the central conductor duct of the paper cable. The oil or gas is fed indirectly via the annular gap between the cable core and the metallic sheath. 1.2.2.4 “Polymeric Extruded Cable to Oil or Gas Filled Paper Cable” Transition Joint, Single Core “Fed” Type The joint in Fig. 1.18 employs a central barrier, usually of cast thermoset resin, which closely resembles the stop joint employed to segregate pressure between two single core oil filled cables. The barrier is cylindrical and contains an embedded metallic HV electrode, which is bolted to the conductor connection to form the oil seal. Paper insulation is hand applied onto the paper cable, often with the addition of a stress cone cast in thermoset resin, such that a thin annular channel is formed to permit oil or gas to be fed to the central conductor duct. The insulation on the polymeric cable side can be in the form of the “prefabricated composite” design, as shown in Fig. 1.18, in which an elastomeric stress cone is compressed by springs into the bore of the barrier, or in the form of tape or premoulded elastomeric sleeves, as shown in Figs. 1.7 and 1.17. Alternatively the “back-to-back” type of straight joint, Figs. 1.12, 1.13a, b, can be used to permit gas or oil to be fed directly to the conductor central duct.

Fig. 1.17 “Polymeric extruded cable to oil or gas filled paper cable” transition joint, single core, ‘non-fed’ type

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Fig. 1.18 “Polymeric extruded cable to oil or gas filled paper cable” transition joint, single core, “fed” type

1.2.3

Types of Y Branch Joints

A Y branch joint joins together three cables. In principle any of the types of insulation employed for the straight joint can be considered, however in practice this is a specialised application in which the “prefabricated composite” design, Fig. 1.19, has been the most frequently employed. The three conductor connectors are plugged into an HV electrode. The HV electrode is embedded in a factory cast

Fig. 1.19 “Prefabricated composite” Y branch joint

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thermoset resin barrier from which protrude three half joints of the “composite type”, described in 1.2.1.2.

1.3

Types of Terminations

A termination is the connection between a cable and other electrical equipment. Also termed “pot-head”. • • • • •

Metal or Sealing End enclosed GIS termination Oil immersed transformer termination Outdoor termination Indoor termination Temporary termination.

Each of the above termination designs is illustrated by a diagram to show the type of insulation. For the purpose of clarity other important design details have been omitted. The design requirements common to each type of termination are: • • • •

A high current connection from the cable conductor to an external busbar. Insulation to the same performance standard as the cable. Provision of support to the cable. Ability to withstand cable thermomechanical loads and external forces such as wind, ice and busbar loading. • A high current connection to permit the flow of short circuit current from the cable metallic sheath or shield wires via a bonding lead to the system earth. • A connection to the cable metallic sheath or earth wires which is electrically insulated from earth potential to match the insulating integrity of the cable oversheath. • Protection to the cable insulation and sheath against the ingress of atmospheric water and the ingress of pressurised dielectric liquid or gas from adjacent metalclad busbar trunking. Some termination designs are filled with either a dielectric liquid or pressurised SF6 gas to provide insulation. Provision is required in such designs to withstand the effects of the thermal expansion of these insulants. The incompressible nature of a dielectric liquid requires that an expansion volume be provided. The expansion volume can be formed by an air or gas filled space, usually at the HV end of the insulator, or by either a) an external header tank, b) an external pressurised feed tank, or c) an internal flexible accumulator containing gas usually at the LV end of the termination. In the case of an internal air volume these terminations are only suitable for vertical installation or for max inclined angle or 30 . If they are to be installed inclined, horizontally or inverted then it is usual to fill the termination completely with insulating liquid and to provide either external compensation or an internal flexible accumulator containing gas. An expansion volume is not necessary for gaseous insulation because of its compressible nature, however the termination

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must either be designed to withstand the increased pressure or an external gas cylinder must be connected to limit the pressure to an acceptable level.

1.3.1

Types of Metal Enclosed GIS Terminations

This is a cable termination which connects to a busbar within metal trunking, insulated with gas. The gas is usually pressurised SF6. The busbar is usually connected to switchgear. This type is also termed an “SF6 termination” or “metal enclosed pothead” • • • • •

“Stress cone and insulator” termination “Deflector and insulator” termination “Prefabricated composite dry type” termination “Capacitor cone and insulator” termination “Directly immersed” termination.

1.3.1.1 “Stress Cone and Insulator” Metal Enclosed GIS Termination A hand taped or premoulded one or two-piece moulded elastomeric stress cone or moulded plastic stress cone, is fitted to the prepared cable core, as shown in Fig. 1.20. The cable core can be wrapped with polymeric or paper rolls to a predetermined diameter before the stress cone is fitted. Alternatively the stress cone can be field moulded using one of the methods described for the straight joint (item 1.2.1.3). The prepared cable is housed within an insulator, which is filled with either insulating fluid or SF6 gas. The SF6 gas inside the insulator is usually at a significantly lower pressure than the SF6 gas within the GIS busbar trunking. In some designs either a passageway through the insulator or an external pipe is used to permit the SF6 gas from the GIS to also provide the insulation for the cable termination. It is necessary to seal the cable to prevent ingress of SF6 gas or insulating fluid. In some designs employing a moulded elastomeric stress cone, the stress cone is sealed to the insulator baseplate by a tube to segregate the dielectric fluid or gas from the cable. Some designs employ a mixture of SF6 and N2 gas. The insulator both centralises the conductor within the trunking and anchors it to prevent longitudinal movement due to thermomechanical conductor force. The insulator is usually formed from a cast thermoset resin. Porcelain is also employed. 1.3.1.2 “Deflector and Insulator” Metal Enclosed GIS Termination This is similar to that described in item 1.3.1.1, Fig. 1.20, but has the stress cone replaced by a deflector cone, which is directly insulated by either the dielectric fluid or the SF6 gas contained within the termination. Additional stress control can be employed in the form of a high permittivity layer applied over the cable insulation adjacent to the insulation screen termination. A typical “deflector” cone is shown within an outdoor termination in Fig. 1.29.

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Fig. 1.20 “Stress cone and insulator” metal enclosed GIS termination

1.3.1.3 Prefabricated Composite “Dry” Metal Enclosed GIS Termination A premoulded elastomeric stress cone is inserted into a cast thermoset insulator, Fig. 1.21. Pressure at the stress cone to insulator interface and at the stress cone to cable core interface is maintained by metallic springs or similar compressing device. It is not necessary to fill the insulator with either gas or insulating liquid, thereby dispensing with the need to provide equipment for pressure monitoring, pressure compensation and/or a thermal expansion reservoir. This design is now more and more used. ▶ Chapters 7, “Feasibility of a Common, Dry Type Plug-in Interface for GIS and Power Cables above 52 kV” and ▶ 11, “Standard Design of a Common,

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Fig. 1.21 Prefabricated composite “dry” metal enclosed GIS termination

Dry Type Plug-in Interface for GIS and Power Cables up to 145 kV” of this book which are dedicated to this design.

1.3.1.4 “Capacitor Cone and Insulator” Metal Enclosed GIS Termination This is similar to the “stress cone and insulator” type (item 1.3.1.1) in general layout, but with capacitor stress control instead of geometric stress control.

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Fig. 1.22 “Capacitor cone and insulator” metal enclosed GIS termination

Pre-shaped rolls of polymeric film are wound onto the cable, with sheets of conducting film interleaved in the rolls to form a linear longitudinal voltage distribution, Fig. 1.22. Alternatively a) the capacitor cone can be assembled onto a tubular insulator in the factory and then loosely fitted over the prepared cable core on site, or b) the capacitor cone can be formed from individual toroidal capacitors as shown in Fig. 1.32. The prepared cable is housed in an insulator which is filled with either insulating liquid or SF6 gas.

1.3.1.5 “Directly Immersed” Metal Enclosed GIS Termination The insulator is not employed in this design. The pressurised SF6 gas from the GIS provides the electrical insulation. The stress cone method of stress control

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Fig. 1.23 “Directly Immersed” metal enclosed GIS Termination

is usually employed in this design, Fig. 1.23. It is necessary to seal the conductor and sheath of the cable to prevent ingress of the SF6 gas. It is also necessary for the busbar adaptor to centralise the conductor within the trunking and to prevent or contain longitudinal movement due to conductor thermomechanical force.

1.3.2

Types of “Oil Immersed Transformer” Terminations

This is a termination into oil insulated metalclad busbar trunking, which is usually part of the transformer housing. These closely resemble the types of metal enclosed GIS terminations shown in Figs. 1.20, 1.21, 1.22 and 1.23. A corona shield larger than the one of GIS Termination is usually fitted on HV side of Oil Immersed Transformer Terminations. Also termed “oil immersed transformer potheads”.

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1.3.2.1 “Stress Cone and Insulator” Oil Immersed Transformer Termination See item 1.3.1.1 and Fig. 1.20. 1.3.2.2 “Deflector and Insulator” Oil Immersed Transformer Termination See item 1.3.1.2 and Figs. 1.20 and 1.29. 1.3.2.3 Prefabricated Composite “Dry” Oil Immersed Transformer Termination See item 1.3.1.3 and Fig. 1.21. 1.3.2.4 “Capacitor Cone and Insulator” Oil Immersed Transformer Termination See item 1.3.1.4 and Figs. 1.22 and 1.32. 1.3.2.5 “Directly immersed” Oil Immersed Transformer Termination See item 1.3.1.5 and Fig. 1.23.

1.3.3

Types of Outdoor Terminations

This is a cable termination which interfaces with air insulated equipment and which is subjected to full climatic conditions. Also termed “outdoor pothead” or Outdoor Sealing End. • • • • • • • •

“Prefabricated” elastomeric sheds and stress cone outdoor termination “Heat shrink sleeve” outdoor termination “Elastomeric sleeve” outdoor termination “Stress cone and insulator” outdoor termination “Deflector and insulator” outdoor termination “Prefabricated composite and insulator” outdoor termination “Capacitor cone and insulator” outdoor termination “Prefabricated composite and capacitor cone, and insulator” outdoor termination.

1.3.3.1 “Prefabricated” Elastomeric Sheds and Stress Cone Outdoor Termination A factory premoulded elastomeric moulding, complete with stress cone profile and sheds, is stretched onto the prepared cable insulation, as shown in Fig. 1.24. Alternatively the termination can be formed from a separately moulded stress cone which interlocks with a set of individually moulded sheds. This terminations are generally not self supporting.

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Fig. 1.24 “Prefabricated” elastomeric sheds and stress cone outdoor termination

1.3.3.2 “Heat Shrink Sleeve” Outdoor Termination Heat Shrink “Stress Control Sleeve” Type This termination, Fig. 1.25, is similar to the heat shrink sleeve joint (1.2.1.4). The insulation is formed from a heat shrink sleeve, usually with the assistance of a resistive, or high permittivity method of longitudinal stress control. It is in the form of either a secondary sleeve, a taped winding or a mastic pad. The sheds are usually heat shrunk onto the longitudinal sleeve as the final assembly operation on site. This terminations are generally not self supporting. Heat Shrink “Capacitor Cone Stress Control” Type Stress control is provided by a capacitor cone which is formed from layers of insulating and semiconducting polymeric tapes, usually of the self-amalgamating type. A heat shrink sleeve and sheds are applied overall, as shown in Fig. 1.26. This terminations are generally not self supporting.

1.3.3.3 “Elastomeric Sleeve” Outdoor Termination This type, Fig. 1.27, is similar to the heat shrink sleeve termination shown in Fig. 1.26, except that the insulation is supplied in the form of a moulded elastomeric sleeve, which is stretched onto the prepared cable. Premoulded elastomeric sheds are then stretched onto the sleeve. Sleeves which are pre-loaded with either a resistive,

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Fig. 1.25 Heat shring stress control sleeve outdoor termination

or a high permittivity filler may be applied over the insulation to control the longitudinal stress distribution. This terminations are generally not self supporting.

1.3.3.4 “Stress Cone and Insulator” Outdoor Termination A stress cone is fitted onto the prepared cable core, as shown in Fig. 1.28. The stress cone can be a factory premoulded one or two piece type or alternatively it can be formed in situ by hand taping or field moulding. The cable core can be wrapped with either polymeric or paper rolls to a predetermined diameter before the stress cone is fitted. Alternatively the stress cone can be field moulded using one of the methods described for the straight joint (item 1.2.1.3). The prepared cable is housed within an insulator, which is filled with insulating liquid or gas. The insulator can be formed from either porcelain, thermoset resin or may be of a composite design as shown in Fig. 1.28, inset, with a rigid core of cylindrical or conical shape onto which

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Fig. 1.26 Heat shring capacitor cone stress control sleeve outdoor termination

premoulded elastomeric sheds have been fitted or moulded. In Fig. 1.28 an expansion volume is shown above the liquid level. In some cases the termination is completely filled with liquid and in these terminations external or internal compensation is required for expansion in the form of a pressure tank or a header tank.

1.3.3.5 “Deflector and Insulator” Outdoor Termination A deflector stress control profile made of metal or semiconducting elastomer is fitted onto the prepared cable core, as shown in Fig. 1.29. The prepared cable is housed within an insulator, which is filled with insulating liquid or gas. The insulator can be formed from either porcelain, thermoset resin or may be a composite design with a rigid core of cylindrical or conical shape onto which premoulded elastomeric sheds have been fitted or moulded. Variations of this design exist with and without a high

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Fig. 1.27 “Elastomeric sleeve” outdoor termination

permittivity stress control layer applied adjacent to the cable insulation screen termination.

1.3.3.6 “Prefabricated Composite and Insulator” Outdoor Termination An elastomeric moulded stress cone is inserted into a conical thermoset casting, as shown in Fig. 1.30. Pressure at the stress cone to cable interface is maintained by metallic springs. In the design shown, the stress cone and conical casting also segregate the insulating liquid or gas from the cable. The prepared cable core is housed within an insulator which is filled with either insulating liquid or gas. 1.3.3.7 “Capacitor Cone and Insulator” Outdoor Termination Capacitor Cone and Insulator “Cylindrical Capacitor Cone” Type As shown in Fig. 1.31, pre-shaped cylindrical rolls of polymeric film are wound onto the cable in situ, with sheets of conducting film interleaved to form a linear longitudinal voltage distribution. Alternatively the cylindrical capacitor cone can

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Fig. 1.28 “Stress cone and insulator” outdoor termination

be assembled onto a tubular insulator in the factory and then loosely fitted over the prepared core on site. Capacitor Cone and Insulator “Toroidal Capacitor” Type This capacitor cone, Fig. 1.32, is formed of discrete toroidal shaped capacitors, which are stacked vertically on top of one another and are loosely fitted over the prepared cable core. The prepared cable is housed in an insulator which is filled with either insulating liquid or gas.

1.3.3.8 “Prefabricated Composite and Capacitor Cone, and Insulator” Outdoor Termination As shown in Fig. 1.33, a cylindrical capacitor cone and premoulded elastomeric stress cone is fitted onto the cable to form a linear longitudinal voltage distribution. The premoulded elastomeric stress cone is described in 1.3.3.6, Fig. 1.30, and the cylindrical stress cone is described in 1.3.3.7, Fig. 1.31. The prepared cable is housed in an insulator which is filled with either insulating liquid or gas.

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Fig. 1.29 “Deflector and insulator” outdoor termination

1.3.4

Types of Indoor Terminations

These are essentially outdoor terminations for which the creepage length and sometimes the height of the insulator have been reduced for those situations in which the termination is subjected to neither wet atmospheric conditions nor to air pollution. When sheds are not required, the insulator is of a simple cylindrical or conical shape. The methods of stress control are the same as those described for the outdoor termination, item 1.3.3. Indoor terminations closely resemble the types of outdoor terminations shown in Figs. 1.24, 1.25, 1.26, 1.27, 1.28, 1.29, 1.30, 1.31, 1.32 and 1.33. Also known as “indoor potheads” or Sealing Ends.

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Fig. 1.30 “Prefabricated composite and insulator” outdoor termination

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Fig. 1.31 “Capacitor cone and insulator”, “cylindrical capacitor type” outdoor termination

1.3.4.1 “Prefabricated” Elastomeric Sheds and Stress Cone Indoor Termination See Fig. 1.24. 1.3.4.2 “Heat Shrink Sleeve” Indoor Termination Heat Shrink “Stress Control Sleeve” Type See Fig. 1.25. Heat Shrink “Capacitor Cone Stress Control” Type See Fig. 1.26.

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Fig. 1.32 “Capacitor cone and insulator”, “toroidal capacitor type” outdoor termination

1.3.4.3 “Elastomeric Sleeve” Indoor Termination See Fig. 1.27. 1.3.4.4 “Stress Cone and Insulator” Indoor Termination See Fig. 1.28. 1.3.4.5 “Deflector and Insulator” Indoor Termination See Fig. 1.29. 1.3.4.6 “Prefabricated Composite and Insulator” Indoor Termination See Fig. 1.30.

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Fig. 1.33 “Prefabricated composite and capacitor cone and insulator” outdoor termination

1.3.4.7 “Capacitor Cone and Insulator” Indoor Termination Capacitor Cone and Insulator “Cylindrical Capacitor Cone” Type See Fig. 1.31. Capacitor Cone and Insulator “Toroidal Capacitor” Type See Fig. 1.32.

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1.3.4.8 “Prefabricated Composite and Capacitor Cone, and Insulator” Indoor Termination See Fig. 1.33.

1.3.5

Types of Temporary Terminations

These terminations are usually required to be assembled quickly, to be light in weight and to be small. Their purpose is to enable a cable to be terminated quickly and to be connected in service for a comparatively short duration. It is usually accepted that the temporary termination only has to withstand the system ac voltage and not the full basic insulation level (BIL) of the cable system. The designs and the methods of stress control are the same as those described for the outdoor termination, item 1.3.3. The designs are essentially the same as the outdoor terminations shown in Figs. 1.24, 1.25, 1.26, 1.27, 1.28, 1.29, 1.30, 1.31, 1.32 and 1.33. The types 1.3.5.1, 1.3.5.2 and 1.3.5.3 are light in weight and small. The use of the light weight “composite insulator” Fig. 1.28, inset, can make types 1.3.5.4 to 1.3.5.8 suitable. Also known as “temporary potheads” or temporary Sealing Ends.

1.3.5.1 “Prefabricated Elastomeric Sheds and Stress Cone” Temporary Termination See Fig. 1.24. 1.3.5.2 “Heat Shrink Sleeve” Temporary Termination Heat shrink “Stress Control Sleeve” Type See Fig. 1.25. Heat shrink “Capacitor Cone Stress Control Cone” Type See Fig. 1.26.

1.3.5.3 “Elastomeric Sleeve” Temporary Termination See Fig. 1.27. 1.3.5.4 “Stress Cone and Insulator” Temporary Termination See Fig. 1.28. 1.3.5.5 “Deflector and Insulator” Temporary Termination See Fig. 1.29. 1.3.5.6 “Prefabricated Composite and Insulator” Temporary Termination See Fig. 1.30.

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1.3.5.7 “Capacitor Cone and Insulator” Temporary Termination Capacitor Cone and Insulator “Cylindrical Capacitor Cone” Type See Fig. 1.31. Capacitor Cone and Insulator “Toroidal Capacitor” Type See Fig. 1.32.

1.3.5.8 “Prefabricated Composite and Capacitor Cone and Insulator” See Fig. 1.33.

Appendix: Glossary of Component Names Glossary of Names for Components Used in Accessories for Extruded Cables Adaptor Moulding Pièce moulée d’adaptation A type of “elastomeric moulding” usually of the “stress cone” type employed in prefabricated accessories such as the “premoulded straight joint” of the two and three piece types (1.2.1.2, Figs. 1.8 and 1.9) and some types of stress cones for terminations. These mouldings are designed a) to fit a range of cable sizes, by having different internal bore diameters and b) to have a constant external diameter such that an outer elastomeric cylindricai moulding of constant size can be fitted. The outer moulding is usually designed to be slid back over the cable sheath during jointing. Adhesive Tape Ruban adhésif Tape that is supplied pre-coated with an adherent layer. The tape is usually a polymer with insulating or semi-conducting properties. Anchor Plate Plaque d’ancrage The component which rigidly connects the joint shell of an anchor joint to a concrete or steel structure for the purpose of segregating unequal mechanical loading between two cables. Back-to-Back Straight Joint (1.2.1.5) Jonction “tête-bêche” The joint insulation is part formed from either a “dielectric liquid” or “dielectric gas” under pressure, contained within “trunking”. The design is essentially based upon either two “metai enclosed GIS terminations” or two “oit immersed transformer terminations” connected together within the same trunking. Also termed “back-toback” splice.

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Types of Back-to-Back Straight Joint • Back-to-back joint with two insulators Jonction “tête-bêche” à 2 isolateurs (1.2.5.1, Fig. 1.12) • Back-to-back joint with one insulator Jonction “tête-bêche” à 1 isolateur (1.2.5.2, Fig. 1.13) • Back-to-back joint without insulator Jonction “tête-bêche” sans isolateur (1.2.5.3, Fig. 1.14). Barrier Insulator Isolateur d’arrêt A shaped insulator, usually formed from porcelain or cast resin, which is used a) in a transition joint to prevent the impregnant in the paper cable from entering the polymeric cable, b) in straight joint designs which employ oil of SF6 insulation and c) in some types of straight joint designs to prevent the migration of moisture from a damaged polymeric cable to a sound cable. Barrier Plate Plaque d’arrêt A metal plate to which bushing insulators are clamped within a three core transition joint. Alternatively the thermoset resin plate which is cast integrally with the three bushing insulators. See “bushing”. Base Plate Plaque de base A metal support plate to which the insulator of a cable termination is rigidly bolted. The base plate is usually connected to earth potential a) directly by a local connection or b) by connection to the insulation screen, metallic sheath or shield wires of the cable. See “bonding lead”. Bifurcating Joint Jonction de dérivation See “Y branch joint”. Blind-Head Insulator Isolateur borgne A cylindrical insulator used in a metal enclosed GIS termination which is permanently sealed at its high voltage end in the factory to prevent SF6 gas from entering the termination in service. One method is to have a solid high voltage electrode embedded into an insulator cast in thermoset resin. A plug-in connector is required to transmit current from the conductor to the high voltage electrode. An external adaptor is usually employed to transmit current to the off-going busbar. See “metal enclosed GIS termination”, “busbar adaptor” and “plug-in connector”. Also termed a “closed-top” insulator and a “plug-in” insulator. Bonding Lead C^ a ble de raccordement d’écran

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An ancillary cable comprised of an insulated conductor, which is connected to the cable metallic sheath or screen wires at a) terminations and b) screen interruption joints. The conductor area is normally sized to carry the specified short circuit current of the cable system. The insulation is required to meet the same or a higher voltage performance than the cable oversheath, depending on whether the bonding lead is of the single or concentric conductor type. In all cable systems the bonding leads connect the lower metalwork of the cable terminations to an approved earth system usually via a link box, such that the system short circuit current can be carried along the cable screening wires or metallic sheaths from one earth system to the other. In a “cross bonded” single core cable system one bonding lead conductor is connected to each side of the insulated flange in each screen interruption joint. Concentric bonding leads are recommended, because the reduced surge impedance helps to protect a) the insulated flange and b) the insulated screen gap from flash-over. The concentric bonding leads from the three adjacent joints are usually terminated in a link box, which contains a) links to transpose the cable sheaths or screen wires and b) sheath voltage limiters (SVLs) which are designed to limit the magnitude of transient voltages on the cable oversheath and across the insulated flange. Busbar Barre collectrice The off-going conductor that connects to a) the “conductor stalk” on an outdoor, indoor or oil immersed transformer termination and b) the “busbar adaptor” on a GIS termination. Busbar Adaptor Pièce de connexion The off-going current carrying connection on a “metal enclosed GIS termination” (1.3.1.1–1.3.1.5), which on one face fits to the embedded electrode or to the conductor stalk, and on the second face fits to the particular design of off-going busbar. Bushing Borne traversée A type of “barrier insulator” employed in either a three core transition joint, or a single core transition joint of the non-fed type, to prevent the impregnant in the paper insulated cable from entering the polymeric cable. It is usually formed from cast resin into which a solid conductor rod has been embedded. It has a central flange which is designed to be sealed either to a “barrier plate” or to a “joint shell” The insulation on each side is normally conical or cylindrical in shape, with sufficient length to provide the “creepage” distance. Cable Chamber Cuve d’extrémité

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The metallic “trunking” into which a “metal enclosed GIS termination” is bolted. The trunking at this position is sometimes of different diameter to the trunking that screens the rest of the SF6 gas insulated busbar. Capacitor Cone Cône à répartition capacitive ou cône condensateur Insulation which contains a number of capacitors in series. It is designed to give a linear voltage distribution along either a) the prepared cable core, b) the surface of an insulator, or c) both. Capacitor cones are usually employed in terminations (eg 1.3.1.4, 1.3.2.4, 1.3.3.2, 1.3.3.7 and 1.3.3.8). The cylindrical type of capacitor cone (1.3.3.7, Fig. 1.31) is constructed from metallic or semiconducting foils, which form a series of overlapping cylinders insulated from each other by rolls of polymeric film. This type of cylindrical capacitor cone can be wrapped directly onto the cable core or can be preformed in the factory. Alternatively the capacitor cone can be formed from discrete toroidal shaped capacitors (1.3.3.7, Fig. 1.32), which are slipped over the prepared cable core and stacked one on top of the other. Also termed a “condenser cone”. Cast Barrier Corps isolant en résine thermodurcissable See “resin casting”. Central Insulator Corps isolant en résine thermodurcissable A type of resin casting used in a prefabricated composite type of joint (1.2.1.2, Fig. 1.6). See “resin casting” and “prefabricated straight joint”. Closed Top Insulator Isolateur borgne See “blind-head insulator”. Composite Insulator Isolateur composite An “insulator” used in cable terminations of the “outdoor” (1.3.3), “indoor” (1.3.4) and “temporary” (1.3.5) type. The “composite” insulator has a rigid cylindrical core onto which pre-moulded elastomeric or polymeric “sheds” have been fitted. The “sheds” are usually formed from silicone rubber. The core is usually formed from thermoset resin reinforced with glass fibre. Compression Device Dispositif de compression The device employed to compress the elastomeric “stress cone” in prefabricated designs of joints and terminations of the “composite” type (1.2.1.2, 1.3.1.3, 1.3.3.6 and 1.3.3.8), to achieve an intimate fit with a) the cable core and b) the

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“resin casting”. The device is usually an assembly of components which includes a thrust cone and a set of metallic springs. See also “stress cone”, “resin casting” and “central insulator”. Condenser Cone Cône condensateur See “capacitor cone”. Conductor Rod Barre conductrice A metal rod embedded in a thermoset bushing to enable connection to be made between two cable conductors in a transition joint (1.2.2.2, Fig. 1.16 and 1.2.2.3, Fig. 1.17). See “bushing”. Conductor Screen Ecran sur conducteur The conducting or semiconducting layer at high voltage, which is applied over the conductor and upon which the insulation is applied. See “screen”. Also termed “HV screen”, “HT screen”, “inner screen” and “conductor shield”. Conductor Stalk Tige de sortie A metal connector which terminates the cable conductor to enable a current carrying connection to be made to a busbar. See “busbar”. Connector Connecteur The generic name for the conducting connection between a) cable conductors, b) cable screen wires and c) one cable conductor and a conductor stalk at a termination. The connection can be of the permanent type or of the separable type. See “ferrule”, “MIG weld”, “thermit weld” and “conductor stalk”. Connector Screen Ecran sur connecteur The conducting or semiconducting layer at high voltage, which is applied over the conductor connector and upon which the joint insulation is applied. See “screen”, “connecte” and “ferrule screen”. Also termed “HV screen”, “HT screen” and “connector shield”. Corona Shield Ecran pare-effluves A shaped conducting component positioned around a conductor or busbar connector in a back-to-back straight joint (1.2.1.5, Figs. 1.12, 1.13 and 1.14) and in a termination (e.g. 1.3.3.4–1.3.3.8). Its purpose is to control the electrical stress distribution and thereby prevent the occurrence of corona (partial discharge) in the surrounding liquid or gaseous insulation.

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Creepage Distance Ligne de fuite ou chemin rampant The shortest distance measured along an insulating interface between conducting components at high and Iow voltage. In a joint the creepage distance is that between the end of the “insulation screen” and the exposed “conductor screen”, measured along the interface between the cable core and the applied joint insulation. For a termination the “external creepage” distance is the distance between the upper and Iower metalwork of the insulator, which includes the upper and Iower surfaces of the “sheds”. For an outdoor termination the “protected creepage” is the cumulative surface distance of the underside of each shed. The “external” creepage and “protected” creepage lengths are often specified to meet a particular level of atmospheric pollution. Crosslinked Insulation Isolation réticulée Electrical insulation that has been changed from a thermoplastic to a thermoset material by a chemical reaction which forms links between adjacent long chain molecules. Crosslinked insulation has improved mechanical performance at high operating temperatures because it does not melt. The cross-linked insulation can be formed from a polymer with a semi-crystalline structure such as polyethylene (PE) or from an elastomer with an amorphous structure such as ethylene propylene rubber (EPR). Crosslinking can be achieved by a chemical reaction or by radiation. In the chemical reaction method, an agent termed a curative is contained within the insulation to promote the crosslinking process, usually with the application of heat. Also termed “vulcanised” or “cured” insulation. This process also applies to the crosslinked screens. Cured Insulation Isolation réticulée See “crosslinked insulation”. Deflector Cone Déflecteur A factory formed “stress control profile” of metal or semi-conducting polymer, positioned adjacent to the termination of the cable insulation screen to control stress. it is usually employed in combination with either a liquid or gaseous insulation of high electric strength in accessories such as terminations of the “deflector and insulator” type (1.3.3.5) and straight joints of the “back to back” type. Dielectric Diélectrique An insulating material with high electric strength. Dielectric Fluid Fluide diélectrique

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A “dielectric gas” or “dielectric liquid” with high insulating properties. Dielectric Gas Gaz diélectrique A gas with high insulating properties, such as SF6, N2 or SF6/N2 mixture. These gases are usually pressurised to achieve the required dielectric strength. N2 requires a significantly higher pressurisation than SF6. Dielectric Liquid Liquide diélectrique A liquid with high insulating properties, eg. silicone liquids, synthetic hydrocarbon liquids and minerai oils. Earth Screen Ecran de terre See “insulation screen”. Elastomeric Moulding Corps isolant en élastomère An insulating component that has been premoulded in the factory from an insulating polymer, which has elastic properties (low modulus of elasticity) in the working temperature range of the accessory. The component usually has one or more integral screens formed from a semiconducting elastomer. Also termed a “rubber moulding” or “synthetic rubber” moulding. Embedded Electrode Electrode encastrée See “HV electrode” and “HT electrode”. Extruder Extrudeuse A powered machine used to prepare a) the cable insulation in the factory and b) the joint insulation for some types of field moulded joints (1.2.1.3, Table 1.1). The machine is comprised of a long rotating “screw” with a helical thread-form of “flights” of varying pitch, which is contained within a heated “barrel”. The polymer is fed into one end of the barrel, usually in the form of small pellets. The polymer is melted and compressed into a viscous liquid termed the “melt”, which is homogenous and void free. The melt is extruded from the end of the barrel under the pressure generated by the screw into either a) the cable insulation die tool or b) the joint mould tool). Ferrule Douille de raccordement A cylindrical metal connector between cable conductors or between cable screen wires.

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Types of ferrule: Types de douilles de raccordement • Compression Ferrule Douille à sertir A metal tube into which the conductors to be joined are inserted. The tube is then compressed by a hydraulic press tool. • Flush Ferrule Douille équidiamétrale The ferrule is nominally made flush with the diameter of the conductor, either by a) compressing, b) removing a layer of wire, or c) welding. • Mechanical Bolted Ferrule Douille mécanique à visser The current carrying connection is made by compressing the conductor inside the ferrule by tightening threaded bolts. The bolts are finished flush with the surface of the ferrule. • Soldered Ferrule Douille soudée (à basse température) A metal ferrule is placed around the two conductors. The assembly is then filled with hot molten solder (a lead alloy). • Stop Ferrule Douille borgne The ferrule has a central internal barrier to segregate the conductors. Used a) in a transition joint to prevent the flow of impregnant from a paper cable and b) in a straight joint to prevent the migration of moisture from a damaged cable to a sound cable. • Welded Ferrule Douille soudée (à haute température) The two conductors are fused together by the application of molten metal. See “MIG weld”, “TIG weld” and “thermit weld”. Ferrule Screen Ecran sur douille The conducting or semiconducting layer at high voltage which is applied over the ferrule and upon which the joint insulation is applied. See “screen” and “ferrule”. Also termed “connector screen”, “HV screen”, “HT screen” and “ferrule shield”. Field Moulded Straight Joint (1.2.1.3) Jonction moulée sur site The joint insulation is melted, moulded and consolidated to the prepared insulation of both cables in situ, ie. “in the field”. Also termed “field moulded splice”. Types of Field Moulded Straight Joint (1.2.1.3, Table 1.1) • Tape moulded Rubanée-formée

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• Crosslinked tape moulded Rubanée-formée et réticulée (rubanée cuite) • Extrusion moulded Moulée par extrusion • Crosslinked extrusion moulded Moulée par extrusion et réticulée • Injection moulded Moulée par injection • Crosslinked injection moulded Moulée par injection et réticulée • Block moulded Moulée par blocs • Crosslinked block moulded Moulée par blocs et réticulée. Filling Compound Matière de remplissage An insulating material used to fill some types of accessories. At amblent and at operating temperatures it is substantially solid. During assembly of the accessory it is heated to elevated temperature to reduce the viscosity and permit it to be poured. See “waterproof compound”. Filling Liquid Liquide de remplissage An insulating liquid used to f II some types of accessories, for example straight joints of the “back-to-back type” and terminations which are protected within hollow insulators. See “insulating liquid” and “dielectric liquid”. Filling Oil Huile de remplissage An insulating liquid used to fill some types of accessories. The term “oil” specifically relates to a minerai oil. Flashover Distance Ligne de contournement The shortest distance between high voltage and low voltage metalwork in gaseous or liquid insulation through which an electric discharge (flashover) can occur. GIS Termination Extrémité pour PSEM A termination into Gas Insulated Switchgear. See “metal enclosed GIS termination”. Glass Fibre Box Coquille en fibres de verre.

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A particular type of “joint box”. The box is placed around the joint shell and is filled with compound to provide electrical insulation and corrosion protection (1.2, Fig. 1.3). The box is pre-moulded from a thermoset or thermoplastic resin, which has been reinforced with glass fibres. See “filling compound” and “waterproof compound”. Glass Fibre Reinforcement Renforcement en fibres de verre A bandage of woven glass fibre tape which is impregnated with a thermoset resin. It is applied over a plumb to mechanically reinforce it against a) longitudinal sheath forces and b) internai pressure. Heat Shrink Sleeve Manchon thermorétractable An extruded tube of crosslinked polyolefin is heated in a factory process to above its crystalline melting point and is expanded to a large diameter. It is then allowed to cool and freeze at the larger diameter. During assembly the sleeve is positioned over the accessory and is again heated to above the crystalline melting point, such that the sleeve contracts to form an intimate fit. The insulation and screens may be supplied as one integral sleeve or as several individual sleeves. Stress control sleeves which are pre-loaded with either a resistive or high permittivity filler may also be applied over the accessory to control the stress distribution. Heat shrink sleeves can also be used to form a) the joint protection and b) seals at the conductor, sheath and oversheath. Heat Shrink Sleeve Straight Joint (1.2.1.4) Jonction thermorétractable The joint insulation is formed from one or more “heat shrink sleeves”. Also termed a “heat shrink sleeve splice”. HT Electrode (High Tension) Electrode HT See “HV electrode”. HT Screen (High Tension) Ecran HT See “HV screen” and “conductor screen”. HV Electrode (High Voltage) Electrode HT A shaped conducting component which is embedded within a moulded insulator to electrically screen the conductor connection. Also termed electrode and “embedded” electrode. HV Screen (High Voltage) Ecran HT.

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The conducting or semiconducting layer which is applied over all components at high voltage. See “screen”, “conductor screen” and “ferrule screen”. Also termed “HT screen” and “HV shield”. Indoor Termination (1.3.4) Extrémité intérieure A cable termination which interfaces with air insulated equipment, but which is protected from climatic conditions. These are essentially outdoor terminations, which have had the insulator creepage length and height reduced. Also termed “indoor pothead”. Types of Indoor Terminations: Prefabricated elastomeric sheds and stress cone (1.3.4.1) A jupes en élastomère et à cône déflecteur préfabriqués Heat shrink sleeve type (1.3.4.2) A manchon thermorétractable -stress control sleeve (1.3.4.2, Fig. 1.25) avec manchon de contrôle du champ -capacitor cone stress control (1.3.4.2, Fig. 1.26) avec cône condensateur Elastomeric sleeve (1.3.4.3, Fig. 1.27) A manchon élastomère Stress cone and insulator (1.3.4.4, Fig. 1.28) A cône déflecteur et isolateur Deflector and insulator (1.3.4.5, Fig. 1.29) A déflecteur et isolateur Prefabricated composite and insulator (1.3.4.6, Fig. 1.30) A cône déflecteur composite et isolateur Capacitor cone and insulator (1.3.4.7) A cône condensateur et isolateur -cylindrical capacitor cone (1.3.4.7, Fig. 1.31) avec condensateurs cylindriques -toroidal capacitor (1.3.4.7, Fig. 1.32) avec condensateurs toroïdaux Prefabricated composite and capacitor cone and insulator (1.3.4.8, Fig. 1.33) A cône déflecteur composite, cône condensateur et isolateur.

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Inner Screen Ecran intérieur See “conductor screen”. Insulated Flange Flasque isolant (1.3.4.8) The generic name for the cylindrically shaped insulator which electrically separates a) the two halves of a metallic joint shell or b) the lower metalwork of a termination. The insulated flange can be a) an individuel annular ring of porcelain or thermoset resin (1.2, Fig. 1.1, 1.3.1.5, Fig. 1.23), b) an integral part of a cast resin barrier in a joint (1.2.2.2, 1.2.2.3, 1.2.2.4, Figs. 1.16, 1.17, 1.18, 1.19 and 1.1.3) or c) an integral part of a cast resin insulator in a termination (1.3.1.1– 1.3.1.4). The insulated flange together with the insulated screen gap are essential parts of a specially bonded cable system. See “sectionalising ring” and “screen interruption”. Also termed “insulated ring” and “resin ring”. Insulated Gap Interruption d’écran See “screen interruption”. Insulating Liquid Liquide isolant A liquid with high electrical strength used principally to fill and insulate some types of terminations and back-to-back joints. Examples are minerai oil, synthetic hydrocarbons (such as polyisobutene) and silicones. See aise “dielectric liquid”, “filling liquid” and “filling oil”. Insulating Tape Ruban isolant Tape with high electrical strength can be used to form the insulation or part insulation of an accessory. It can be of the “adhesive” or “self-amalgamating” type or simply of the non-adherent type. Insulation Isolation Material of high electric strength which is applied between the conductor screen and the insulation screen. Insulation Screen Ecran sur isolation The conducting or semiconducting layer at earth potential applied over the insulation. Also termed “earth screen”, “LV screen”, “ground screen”, “outer screen” and “insulation shield”.

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Insulator Isolateur The generic name for pre-formed solid insulation and particularly of the hollow insulators used in cable terminations. In addition to their insulating duty, the “Insulators” employed in cable terminations (1.3.1.1 to 1.3.1.4 and 1.3.3.4 to 1.3.3.8) are designed to a) support the cabie, b) protect the cable insulation, c) contain oil or gas insulation from leakage, d) segregate the cable from external atmospheric conditions e) segregate the cable from the oil or gas insulation used in other manufacturer’s equipment and f) withstand conductor thermomechanical forces. Insulators employed in cable terminations are usually formed from a thermoset casting or from porcelain, with a hollow interior and either a cylindrical or a conical exterior shape. The outer surface of the insulator is often formed in the shape of annuler protrusions termed “sheds”. Insulator designs of the metal enclosed GIS and transformer termination types are sometimes employed in transition joints and straight joints of the “back-toback” type (1.2.1.5). See also “porcelain”, “composite insulator”, “pedestal insulator”, “resin casting”, “shed”, “barrier insulator” and “bushing”. Joint (1.2) Jonction The generic name for the insulated and fully protected connection between two or more cables. It provides an insulated path for the flow of current between the conductors. Also termed a “splice”. Types of Joints See: • “Straight joints” (1.2.1) Jonctions droites • “Transition joints” (1.2.2) Jonctions de transition • “Y branch joints” (1.2.3) Jonctions en Y ou en T – Jonction de dérivation. Joint Box Boıˆte de jonction The generic name for the waterproof housing which is fitted around the joint as part of the “joint protection” (1.2, Figs. 1.3 and 1.5). The box is normally formed from a moulding of thermoplastic polymer or thermoset resin, the latter usually being reinforced with glass fibre (termed a “glass fibre box”). After being assembled around the joint, the box is usually filled with a waterproof insulating compound, such as bitumen, or thermoset resin. See also “joint protection”, “glass fibre box” and “waterproof compound”. Joint Protection Protection de jonction

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The generic name given to the outer coverings of the joint which, depending upon the particular application, a) protect the joint metalwork from corrosion, b) prevent water from entering the joint and c) electrically insulate from earth potential the insulation screens, shield wires and joint shell. Types of protection are “joint shell protection”, “joint” be and “glass fibre box”. Protection can also be formed from tape and heat shrink sleeves. Joint Shell Enveloppe de jonction ou enveloppe métallique The metal tube which contains the joint insulation (1.0, Fig. 1.1), to a) provide electrical continuity between sheaths, b) contain short circuit current within the cable system, c) form an impervious water tight barrier and d) contain the filling of liquid or gas. Joint Shell Protection Protection de l’enveloppe de jonction The covering applied to the joint shell which provides a) anti-corrosion protection and b) electrical insulation from earth potential for insulated sheath systems. Lower Metalwork Pot d’entrée The metal tube or gland connected to the base of a termination insulator which a) seals the cable and b) enables electrical connection to be made to the metal sheath or wire screen. Also termed “bottom” metalwork, “end bell”, “plumbing gland” and “wiping gland”. LV Electrode (Low Voltage) Electrode de terre The shaped conducting component which is embedded within a moulded insulator to provide stress control. Also termed “LV Screen” and “earth electrode”. Metal Enclosed GIS Termination (1.3.1) Extrémité pour station blindée - Extrémité pour PSEM A cable termination which connects to a busbar within metal trunking, insulated with gas. The gas is usually pressurised SF6. The busbar is usually connected to switchgear. Also termed “GIS” termination, “metalclad” termination, “SF6” termination and “metal enclosed GIS pothead”. Types of Metal Enclosed GIS Terminations • Stress cone and insulator (1.3.1.1) A cône déflecteur et isolateur • Deflector and insulator (1.3.1.2) A déflecteur et isolateur

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• Prefabricated composite “dry” type (1.3.1.3) A cône déflecteur composite et isolateur, type sec • Capacitor cone and insulator (1.3.1.4) A cône condensateur et isolateur • Directly immersed (1.3.1.5) Type directement immergé MIG Weld Soudure MIG A weld made by the Metal Inert Gas process in which a consumable wire electrode, usually aluminium, is fed continuously into an electric arc under a shield of inert gas, where it is melted and propelled to fuse into the conductor. Off-Going Connector Raccordement de sortie The device that connects the “conductor staik” in a termination to an off-going busbar. Also termed a “busbar connector”. Oil Immersed Transformer Termination (1.3.2) Extrémité pour transformateur A termination into oil insulated metalclad busbar trunking, this usually being part of the transformer housing. Also termed “oil immersed transformer pothead”. Types of Oil Immersed Transformer Terminations • Stress cone and insulator (1.3.2.1) A cône déflecteur et isolateur • Deflector and insulator (1.3.2.2) A déflecteur et isolateur • Prefabricated composite “dry” type (1.3.2.3) A cône déflecteur composite et isolateur, type sec • Capacitor cone and insulator (1.3.2.4) A cône condensateur et isolateur • Directly immersed (1.3.2.5) Type directement immergé Outdoor Termination (1.3.3) Extrémité extérieure A cable termination which interfaces with air insulated equipment and which is subjected to full climatic conditions. See “termination”. Also termed an outdoor “pothead”. Types of Outdoor Terminations • Prefabricated elastomeric sheds and stress cone (1.3.3.1, Fig. 1.24) A jupes en élastomère et à cône déflecteur préfabriqués • Heat shrink sleeve type (1.3.3.2)

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A manchon thermorétractable -stress control sleeve (1.3.3.2, Fig. 1.25) avec manchon de contrôle du champ -capacitor cone stress control (1.3.3.2, Fig. 1.26) avec cône condensateur Elastomeric sleeve (1.3.3.3, Fig. 1.27) A manchon élastomère Stress cone and insulator (1.3.3.4, Fig. 1.28) A cône déflecteur et isolateur Deflector and insulator (1.3.3.5, Fig. 1.29) A déflecteur et isolateur Prefabricated composite and insulator (1.3.3.6) A cône déflecteur composite et isolateur Capacitor cone and insulator (1.3.3.7) A cône condensateur et isolateur -cylindrical capacitor cone (1.3.3.7) avec condensateurs cylindriques -toroidal capacitor (1.3.3.7) avec condensateurs toroïdaux Prefabricated composite and capacitor cone and insulator (1.3.3.8) A cône déflecteur composite, cône condensateur et isolateur.

Painted Screen Ecran graphité A semiconducting screen normally formed by brushing Iiquid paint onto polymeric insulation. The paint is loaded with a conducting filler, usually dispersed in a resin binder and rendered liquid by a solvent, which subsequently evaporates. See “screen”. Also termed “painted shield”. Pedestal Insulator Isolateur support A short insulator of porcelain or thermoset resin which is used to insulate either a termination baseplate (1.3.3.4–1.3.3.8) or a joint shell (1.2, Fig. 1.4) from earth potential. Also termed “stand-off” insulator and “post” insulator. See “insulator”. Pencil Cône The conical shape into which the insulation of the cable core is formed in a joint (1.2, Fig. 1.1) or termination. Pencils are usually employed in taped joints and in field moulded joints. They are designed to withstand the electrical stresses at the interface between the insulation of the cable and joint. Pencils of shorter Iength are often employed in a cable termination adjacent to the conductor connector to facilitate the application of either a sealing tape or a heat shrink sleeve.

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Plug-In Connector Connecteur embrochable A metal connector which terminates the conductor. It is inserted into a mating electrode usually embedded in a thermoset casting. Current carrying contact is made by sprung connectors or by elasticity of the component. See “blind-head insulator”. Plug-In Insulator Isolateur borgne See “blind head insulator”. Plumb Soudure au plomb lissé A conducting connection and seal which is formed between the metal sheath and a) the joint shell, or b) the termination lower metalwork, by the hand application of a hot lead alloy. Also termed a “wipe”. Porcelain Porcelaine Solid insulation formed from vitrified clay, used primarily for outdoor termination insulators and “pedestal insulators”. The name is sometimes used in the abbreviated foret for “porcelain insulator”. See “insulator”. Pothead Extrémité See “termination”. Prefabricated Straight Joint (1.2.1.2) Jonction préfabriquée Prefabricated straight joints employ insulation that has been preformed and tested in the factory. The usual methods of manufacture are to mould elastomeric insulation and to cast thermoset resin insulation. Also termed “prefabricated splice”. Types of Prefabricated Straight Joint • Composite (1.2.1.2, Fig. 1.6) Composite • Premoulded one-piece (1.2.1.2, Fig. 1.7) Prémoulée en une pièce • Premoulded two-piece (1.2.1.2, Fig. 1.8) Prémoulée en deux pièces • Premoulded three-piece Prémoulée en trois pièces (1.2.1.2, Fig. 1.9). Resin Barrier Corps isolant en résine thermodurcissable See “resin casting”.

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Resin Casting Corps isolant en résine thermodurcissable A rigid insulating component manufactured from a thermoset resin (usually epoxy resin), used to form a) the central insulator in joints of the prefabricated composite type (1.2.1.2), b) the barriers in some types of transition joints (1.3.2 and 1.3.3) and c) the insulator in metal enclosed terminations (1.3.1.1–1.3.1.4). Also termed “resin barrier” and “cast barrier”. See “barrier insulator” and “central barrier”. Rubber Moulding Corps isolant en élastomère See “elastomeric moulding” and “synthetic rubber moulding”. Screen Ecran semi-conducteur A smooth conducting layer in intimate contact with the insulation. It is designed to a) contain the electric field within the insulation and b) minimise the magnitude of the electric stress by the elimination of irregularities in the surface, (termed “stress raisers”). Also termed a “shield”. Screen Interruption Interruption d’écran The “insulated gap” formed in the insulation screen of a joint, which together with the “insulated flange” in the joint sheil, electrically separates the cable screen on one side of the joint from that on the other. Screen interruption joints are necessary for specially bonded cable systems, which together with “bonding leads” and link boxes permit the metallic sheaths or screening wires to be transposed for cross bonding. See “bonding leads”. Also termed “insulated gap”, “sheath interruption” and “sheath segregation”. Screen Termination Arrêt d’écran The name given during jointing to the end position of the cable insulation screen during jointing, after the screen has been removed from the core and the insulation prepared. See “screen”. Sealing End Extrémité See “termination”. Sectionalizing Ring Anneau de sectionnement A type of “insulated flange”. An insulating ring which electrically separates two halves of a) a metal joint shell (1.0, Fig. 1.1) or b) the lower metalwork of a termination (2.1.1, inset and 2.1.5). The ring can be made from a thermoset

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moulding, a reinforced thermoset component or porcelain. Also termed a “resin ring”, when formed from a thermoset moulded or machined component. Self Amalgamating Tape Ruban auto-amalgamant An elastomeric tape which is consolidated to itself by the action of stretching and wrapping. Semiconducting Screen Ecran semi-conducteur A polymeric or elastomeric resin that has been loaded with an electrically conducting filler to give it sufficient conductivity to act as a screen, whilst exhibiting similar thermomechanical properties to the cable and joint insulation. It has a significantly lower conductivity than a metallic screen. The semiconducting screening material can be extruded, moulded or applied in tape form. See “screen”. Also termed “semiconducting shield”. Sheath Closure Etanchéité de gaine The generic name for the seal between the metallic sheath of the cable and either the joint shell or termination lower metalwork. See “plumb”. Sheath Interruption Interruption d’écran See “screen interruption”. Sheath Segregation Interruption d’écran See “screen interruption”. Shed Ailette, Jupe One of the disc shaped protrusions on the outer surface of a termination (1.3.1.1– 1.3.1.5) which increases the effective surface length of the insulator without increasing its height. These are mainly used on outdoor terminations, which are exposed to rain, ice, fog, salt, mist and to atmospheric pollution. Small sheds are sometimes used on indoor, oil immersed transformer and GIS terminations. See “creepage distance”. Also termed a “weather” shed. Shield Écran See “screen”. Splice Jonction See “joint”.

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Straight Joint (1.2.1) Jonction droite A joint which connects two cables of the same type. Types of Straight Joint • Taped Joint (1.2.1.1) Jonction rubanée • Prefabricated Joint (1.2.1.2) Jonction préfabriquée • Field Moulded Joint (1.2.1.3) Jonction moulée sur site • Heat Shrink Sleeve Joint (1.2.1.4) Jonction thermorétractable • Back-to-Back Joint (1.2.1.5) Jonction “tête-bêche” Stress Cone Cône déflecteur A shaped insulating component with a screened “stress control profile”. It is used for stress control at the end of either the low voltage insulation screen or the high voltage electrode (1.3.1.1, 1.2.2.4, 1.3, 1.3.1.3, 1.3.1.5, 1.3.3.1, 1.3.3.4, 1.3.3.6 and 1.3.3.8). It is usually formed from a moulded or machined polymer, elastomer or thermoset resin. Stress Control Electrode Electrode de contrôle de champ The conducting component which controls the electric field in an accessory, it can be placed: a) Adjacent to the end of the insulation screen b) Adjacent to the conductor in a premoulded joint. Stress Control Profile Ecran déflecteur The name for the shape of the conical insulation screen that is positioned adjacent to the termination of the cable insulation screen (1.2, Fig. 1.1). The screen shape can be a) preformed in the factory as part of a “stress cone” moulding or as a “deflector cone” or b) formed during the assembly of the accessory on site by “field moulding” or by the application of insulating tape. Synthetic Rubber Moulding Corps isolant en élastomère See “elastomeric moulding”.

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Taped Straight Joint (1.2.1.1) Jonction rubanée Semiconducting and insulating tapes are wound onto the cable to form the conductor screen, the insulation, the stress control profile screens, the insulation screen and the screen interruption insulation. Also termed “taped splice”. Types of Taped Straight Joint • Self amalgamating tape Ruban auto-amalgamant • Adhesive tape Ruban adhésif Temporary Termination (1.3.5) Extrémités temporaires A cable termination that is designed to be assembled quickly and to be connected in service for a comparatively short time. They are typically smaller and lighter in weight than the permanent types of “outdoor termination” and “indoor termination”. It is usually accepted that they only have to withstand the system ac voltage and not the full basic insulation level (BIL). Also termed “temporary pothead”. Types of Temporary Terminations • Prefabricated elastomeric sheds and stress cone (1.3.5.1 to 1.3.5.8) A jupes en élastomère et à cône déflecteur préfabriqués • Heat shrink sleeve (1.3.5.2) A manchon thermorétractable -stress control sleeve type (Fig. 1.24) avec manchon de contrôle du champ -capacitor cone stress control type (Fig. 1.26) avec cône condensateur • Elastomeric sleeve (1.3.5.3) A manchon élastomère • Stress cone and insulator (1.3.5.4) A cône déflecteur et isolateur • Deflector and insulator (1.3.5.5) A déflecteur et isolateur • Prefabricated composite and insulator (1.3.5.6) A cône déflecteur composite et isolateur • Capacitor cone and insulator (1.3.5.7) A cône condensateur et isolateur -cylindrical capacitor cone type (Fig. 1.31) avec condensateurs cylindriques -toroidal capacitor type (Fig. 1.32) avec condensateurs toroïdaux

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• Prefabricated composite and capacitor cone (1.3.5.8, Fig. 1.33) and insulator A cône déflecteur composite, cône condensateur et isolateur. Termination (1.3) Extrémité The generic name for the connection between a cable and other electrical equipment. Also termed “sealing end” and “pothead”. Types of Terminations • Metal Enclosed GIS Terminations (1.3.1) Extrémités pour PSEM • Oil Immersed Transformer Terminations (1.3.2) Extrémités pour transformateur • Outdoor Terminations (1.3.3) Extrémités extérieures • Indoor Terminations (1.3.4) Extrémités intérieures • Temporary Terminations (1.3.5) Extrémités temporaires Thermit Weld Soudure aluminothermique A weld formed by igniting a mixture of combustible material and powdered metal that melts and fuses to connect two cable conductors. TIG Weld Soudure TIG A weld made by the Tungsten Inert Gas process in which an electric arc is struck between a tungsten electrode and the conductor under a shield of inert gas. A consumable hand held welding rod, usually aluminium, is fed into the arc where it is melted and fused to connect the conductors. Transition Joint (1.2.2) Jonction de transition A joint which connects two cables of different types, for example a polymeric extruded cable to a self-contained oil filled cable. Transition joints are sometimes employed to connect cables of the same type, but with widely differing conductor sizes. Also termed “transition splice”. Types of Transition Joints • Polymeric extruded cable to mass impregnated cable (1.2.2.1) C^able à isolation synthétique – c^able au papier imprégné • Polymeric extruded cable to oil or (1.2.2.2) gas filled paper cable three core

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C^able à isolation synthétique unipolaire c^able tripolaire à huile ou à gaz • Polymeric extruded cable to oil (1.2.2.3) or gas filled paper cable single core, non-fed type C^able à isolation synthétique unipolaire c^able unipolaire à huile ou à gaz, type “non-alimenté” • Polymeric extruded cable to oil or gas (1.2.2.4) filled paper cable single core, fed type C^able à isolation synthétique unipolaire - c^able unipolaire à huile ou gaz, type “alimenté”. Trunking Enveloppe d’extrémité The metallic cylinder that contains a) the SF6 gas in metal enclosed GIS (1.3.1) or b) the oil in a transformer termination. The trunking also forms a) the insulation screen and b) the return conductor for short circuit current. The trunking adjacent to a GIS termination is termed a “cable chamber”. The short length of trunking used to contain the SF6 gas or oil in a joint, of the “back-to-back insulator” type (1.2.1.5) is termed the “joint shell”.

Upper Metalwork Tête d’extrémité The high voltage metal components at the top end of a cable termination, which are normally comprised of the top sealing plate, the clamp ring and the corona shield. Vulcanized Insulation Isolation vulcanisée See “crosslinked insulation”. Waterproof Compound Matière d’étanchéité The generic name for the viscous liquid used to fill a) the joint box and b) the metallic joint shell of some types of joint (1.2, Fig. 1.3). The compound conforms and adheres to the joint components (ie joint shell or screened insulation). It provides electrical insulation and sealing against moisture ingress. Bitumen is one form of compound, this is normally poured hot and cools to a high viscosity liquid. Thermosetting resin is another form of compound, which upon curing forms an adherent solid or elastic mass. See “joint box”, “glass fibre box” and “joint protection” and “filling compound”. Waterproof Seal Etanchéité extérieure

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The seal between the “joint box” and the cable oversheath. It is usually formed from a) thermoset resin putty, b) fibre glass reinforced resin, c) heat shrink sleeve, or d) waterproof tape. The seal is required to contain the compound filling and to prevent the ingress of water. Weather Shed Ailette, jupe See “shed”. Wipe Soudure au plomb lissé See “plumb”. “Y Branch Joint” (1.2.3) Jonction en Y ou T - Jonction de dérivation A joint that connects three cables of nominally the same type. The joint is usually configured in the shape of the letter Y. Also termed “bifurcating joint” and “Y branch splice”.

Zensuke Iwata was born in Tokyo, Japan, on October 5, 1944. He received the B.S. degree in Electrical Engineering from the University of Tokyo, Japan, in 1968. In 1968 he joined the Furukawa Electric Co., Ltd., Japan, where he has been engaged several years in research and development of high-voltage power cables and their accessories before taking other positions in the Furukawa Electric Co. In 2003 he successfully carried out the long-term field test of the real scale H.T. superconducting cable system as the Managing Director and CTO of the Furukawa Electric Co. In 2004 he was appointed as the President of Nuclear Fuel Industries, Japan, and retired in 2014. Zensuke Iwata convened Cigré WG 21.06 which published Technical Brochures 89 and 177. He received the TC Award in 1995. He chaired the ISTC of Jicable in 2003.

2

A Guide to the Selection of Accessories Zensuke Iwata

Contents 2.1 2.2

2.3

2.4

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Compatibility of the Accessory with the Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Number of Cable Cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Cable Constructional Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Conductor Area and Diameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Operating Temperature of the Cable Conductor and Sheath under Continuous, Short Term Overload and Short Circuit Current Loading . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Compatibility of the Accessory with the Type of Cable Insulation and Semiconducting Screens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6 Cable Electrical Design Stresses to be Withstood by the Accessory . . . . . . . . . . . . 2.2.7 Mechanical Forces and Movements Generated by the Cable on the Accessory . . . 2.2.8 Short Circuit Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Compatibility of the Accessory Performance with that of the Cable System . . . . . . . . . . . 2.3.1 Circuit Performance Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Circuit Life Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Metallic Screen Bonding Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4 Earth Fault Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Compatibility of the Accessory with the Cable System Design and Operating Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Type of Cable Installation Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Standard Dimensions for Cable Termination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

61 61 61 61 62 63 63 64 64 64 65 65 65 65 66 66 66 66

Zensuke Iwata has retired. The guide in this Chapter 2 has been prepared by WG 21.06 and published in Cigré TB 177. At the end of the chapter some references are given. They are the original references. Proposed further readings are given in Chapter 4 and following chapters. Z. Iwata (*) Kamakura, Japan e-mail: [email protected] © Springer Nature Switzerland AG 2021 P. Argaut (ed.), Accessories for HV and EHV Extruded Cables, CIGRE Green Books, https://doi.org/10.1007/978-3-030-39466-0_2

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2.4.3 Type of Accessory Installation Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4 Jointing Limitations in Restricted Installation Locations . . . . . . . . . . . . . . . . . . . . . . . . 2.4.5 Mechanical Forces Applied to the Accessory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.6 Climatic Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.7 Type of Accessory Outer Protection Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.8 Situations Requiring Special Accessory Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Verification of Accessory Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Use of the Specific National or International Type Test Specification for the Accessory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2 Use of the Cable Test Specification in the Absence of an Accessory Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.3 Type Test Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.4 Type Tested Accessory in Combination with the Particular Cable . . . . . . . . . . . . 2.5.5 Pre-Qualification Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.6 Satisfactory Service Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.7 Test for Accessories in Specially Bonded Cable Circuits . . . . . . . . . . . . . . . . . . . . . . 2.5.8 Tests for Water Tightness of Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.9 Additional Tests for Cable Terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.10 Pressure Vessel Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Quality Assurance Scheme for Accessory Design and Manufacture . . . . . . . . . . . . . . . . . . . . 2.6.1 The Routine Test Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.2 Quality Assurance Approval for Manufacture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.3 Routine Tests on Prefabricated Moulded Insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.4 Sample Tests on Individual Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Quality Assurance Scheme for Accessory Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.1 Quality Assurance Approval for Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.2 Quality Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.3 Training of Personnel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.4 Assembly Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.5 Special Assembly Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.6 Preparation of the Assembly Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 Compatibility of the Accessory with Specified after Laying Tests . . . . . . . . . . . . . . . . . . . . . . 2.8.1 Voltage Test on Main Insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8.2 Partial Discharge Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8.3 Voltage Withstand Test on the Cable over Sheath and Joint Protection . . . . . . . . . 2.8.4 Current Balance Test on the Cable Sheath and Screening Wires . . . . . . . . . . . . . . . . 2.9 Maintenance Requirements of the Accessory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9.1 Monitoring of Fluid Insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9.2 Voltage Withstand Tests on the over Sheath and Joint Protection . . . . . . . . . . . . . . . 2.9.3 Shelf Life of Accessories for Emergency Spares . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9.4 Availability of Accessory Kits for Emergency Spares . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10 Economics of Accessory Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.1 Cost of the Accessory Complete with all Components . . . . . . . . . . . . . . . . . . . . . . . . 2.10.2 Cost of Guarantee and Insurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.3 Cost of Assembly Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.4 Cost of Preparing the Installation Environment for the Accessory . . . . . . . . . . . . 2.10.5 Cost of Safe Working Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.6 Cost of Special Jointing Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.7 Cost of Training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.8 Comparative Cost of Cable and Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.9 Cost of Verification of Accessory Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Introduction

The reliability and performance of a cable circuit is dependent in equal measures on the designs of the cable and accessory and on the skill and experience of the person who is assembling the accessory. The cable insulation is extruded in the factory under controlled process conditions using selected materials of high quality. It is equally important that the same quality measures are employed for the manufacture of the accessories in the factory and for their assembly on site onto the specially prepared cable. It is essential to select the design of accessory to be exactly compatible with the particular cable type and the particular service application. Compatibility should be validated by electrical type approval tests and be supported by either prequalification tests, or service experience. In particular the performance of the accessory is dependent on the quality, skill and training of the jointing personnel and on the use of the specialised tools required for a particular accessory. The itemised sub-headings below form the basis of the information that is needed by the manufacturer and installer of the cable and accessories. For many applications the cable manufacturer also manufactures, supplies and installs the accessories as part of the complete cable circuit, thus the information is immediately available inhouse. In the event that the user purchases the accessories separately from the cable, then the following items form the basis of the questions that should be asked to the manufacturers of the cable and accessories to ensure that the accessories are suitable.

2.2

Compatibility of the Accessory with the Cable

2.2.1

Number of Cable Cores

The user should determine whether the cable construction is of single, three core or triplex construction (i.e. three single core cables twisted together). The design of the accessory and the method of assembly is dependent upon the number of cable cores; however it is unusual for three core extruded cables to be employed above 60 kV.

2.2.2

Cable Constructional Details

For satisfactory service performance it is most important that the correct size of accessory is selected to suit the particular cable. The outer diameter of the cable insulation, its tolerance and shape are particularly important in the selection of an accessory employing a pre-moulded component, such as an elastomeric stress cone or an elastomeric joint moulding. Such components are designed to fit a specific range of diameters of prepared cable insulation (that is with the insulation screen removed and the insulation smoothed and shaped). The components must not be

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used outside this range. The minimum diameter is determined by the need to achieve sufficient pressure to eliminate voids at the interface with the cable insulation. The maximum diameter is determined by such considerations as a) preventing damage by over stretching during assembly and b) limiting the maximum pressure at the interface such that compression set of the cable insulation and moulded insulation is minimised. The diameter and tolerance ratio of the conductor and of its compaction (the radio of the effective cross sectional area of the metal to the total area occupied) are needed in selecting a connector that will exhibit stable conductivity and high mechanical strength. The diameters and tolerances of the cable metallic barrier and over sheath are needed to ensure that accessory metallic flanges and other components can be passed back over the cable during assembly. The following dimensional and constructional details should be obtained by the user to ensure compatibility of the accessory with the cable: The detailed cable construction should be obtained from the cable manufacturer, which includes the following information as a minimum requirement. Diameters, maximum and minimum tolerances, eccentricity dimensions, construction and material need to be obtained for each of the following cable components: • • • • • • •

Conductor and special features (e.g. water blocking), if any Conductor screen Insulation (ovality and eccentricity dimensions are required) Insulation screen Screen wires, if any Longitudinal water blocking, if any Metallic barrier, if any, for example whether an extruded sheath, a welded sheath, or a laminated foil barrier. Also whether of cylindrical or corrugated form • Over sheath • Armour, if any • Special features (e.g. presence of optical fibre or pilot wires).

2.2.3

Conductor Area and Diameter

The user should ensure that the accessory has been designed and tested for the particular cable conductor size. The electrical performance of an accessory design can become critical on large conductor cables because of the high cable insulation screen stress. The user should ensure that the conductor connections in the complete kit of components are supplied to suit the particular conductor construction. The conductor connection must be capable of carrying the same current as the cable conductor and must be capable of withstanding the cable longitudinal thermomechanical forces, these being proportional to the cross sectional area.

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Operating Temperature of the Cable Conductor and Sheath under Continuous, Short Term Overload and Short Circuit Current Loading

The materials of the accessory must be capable of operating satisfactorily at the operating temperatures specified for the cable. For example, cables with LDPE insulation have a typical continuous maximum operating temperature of 70
C and cables with XLPE insulation have an operating temperature of 90
C, (IEC 840 1988). The short term overload temperatures depend upon the type of cable and application. The temperature of the conductor under short circuit is typically taken as 250
C for XLPE and 160
C for paper insulated cable, (IEC 986 1989). The permitted short circuit temperature of the cable extruded metallic sheath or screen wires is determined by the type of metallic sheath and thermoplastic over sheath, this temperature usually being significantly less than that of the cable insulation.

2.2.5

Compatibility of the Accessory with the Type of Cable Insulation and Semiconducting Screens

2.2.5.1 Physical Compatibility with the Extruded Cable The insulation of the polymeric cable must be identified by the user. There are significant differences between the electrical and mechanical characteristics of extruded insulation. The usual insulants for extruded polymeric cables in the voltage class of 60 kV and above being XLPE (crosslinked polyethylene), LDPE (low density polyethylene), HDPE (high density polyethylene) and EPR (etylene propylene rubber). 2.2.5.2 Chemical Compatibility with the Extruded Cable The type of insulating liquid or lubricant used in joints and terminations should be identified to ensure that these do not affect the properties of the polymeric insulation and semiconducting screens employed in the cable and accessories. For example a) hydrocarbon liquids at elevated temperature can cause swelling of XLPE and EPR insulation and reduction of the conducting properties of screens and b) silicone liquids can have an effect on silicone rubber components. 2.2.5.3 Compatibility with the Paper Insulated Cable In the case of transition joints between polymeric cable and paper insulated cable it is important to establish whether the cable is of the internally or externally pressurised type and whether the fluid dielectric is a gas or a liquid; these details will determine the performance requirements of the barrier plate that segregates the two cables. In the case of mass impregnated non pressurised cables it is important to determine the type of impregnating compound and whether it is of the liquid type or of the non draining type; these details will determine the chemical suitability of the materials employed within the joint to segregate the impregnating fluid from the insulation of the polymeric cable and joint. Penetration of a hydrocarbon impregnating into the polymeric

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cable can result in swelling and modification of the electrical characteristics of the semi-conducting screens and insulation of both the cable and accessory components, thereby reducing their electrical performance. Loss of the impregnating fluid into the polymeric cable can result in eventual electrical failure of the paper cable.

2.2.6

Cable Electrical Design Stresses to be Withstood by the Accessory

The user is advised to obtain the magnitude of the cable stresses at the conductor and insulation screens, or obtain the dimensions of the cable, thereby permitting the stresses to be calculated. The unit of stress is kV/mm calculated at Uo voltage. There are significant differences in the magnitude of the electrical design stress employed in cables, these being dependent upon the type and thickness of insulation, the conductor size, the system voltage and the lightning impulse voltage. It is essential that the accessory has been designed and tested to operate at the particular cable design stress. The stress at the cable insulation screen is of particular significance because this normally determines the maximum design stress in the accessory. The insulation screen stress is usually of higher magnitude in those cables designed for high system voltages and large conductor diameters.

2.2.7

Mechanical Forces and Movements Generated by the Cable on the Accessory

The magnitude of the forces and movements generated by the cable on the accessory depends upon the cable materials, the method of cable manufacture and the type of cable installation design (i.e. rigid or flexible installation). The following mechanical strains are dependent on the cable construction: • Insulation retraction (shrink back) • Insulation radial thermal expansion • Over sheath retraction (shrink back). The following forces are dependent upon the cable construction, current loading, operating temperature, method and type of cable constraint and accessory design: • Conductor thermomechanical thrust and retraction • Sheath thermomechanical thrust and retraction.

2.2.8

Short Circuit Forces

Electromagnetic forces are present during a short circuit between the individual conducting components of the accessory and between the adjacent cables and the accessory. The following information is applicable:

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• Method of constraint of the accessory and cable • Dimensions of the individual accessory components • Method of constraint and the spacing of adjacent cables.

2.3

Compatibility of the Accessory Performance with that of the Cable System

2.3.1

Circuit Performance Parameters

The current rating and optimum circuit economics are dictated by the cable conductor size, cable material costs and the method of installation. To achieve the optimum economical solution it is important that the accessory design is not allowed to limit the performance of the cable. The accessory must therefore match the following cable performance: • Rated voltages (Nominal system voltage U and maximum Um) • Current rating (Current magnitude) • Continuous, cyclic and short time overload. (Current magnitude, time and temperature). • Short circuit rating, “phase to earth” and “phase to phase” (Current magnitude, asymmetry, time and temperature). • Basic impulse level (Withstand voltages for lightning impulse and switching surge). (Flash over voltage for the system insulation co-ordination of outdoor terminations, if specified (IEC 71 1993)).

2.3.2

Circuit Life Required

The accessory should match the design life specified for the particular cable circuit. This is typically requested to be from 20 to 40 years, however some cable circuits are installed as temporary links, for example in an overhead line circuit. Such accessories may be designed to be suitable for quick assembly with a reduction in performance and service life.

2.3.3

Metallic Screen Bonding Requirements

The following information is required on a) the type of bonding leads, (concentric or single conductors) and their conductor size and overall dimensions and b) the type of cable bonding scheme, for example solidly earthed or specially bonded metallic screens.

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• Magnitude of induced sheath or screen wire voltage under normal and short circuit current. • Magnitude of circulating sheath or screen wire current under normal loading. • Magnitude of short circuit current. • Magnitude of specified over sheath lightning withstand voltage and dc withstand voltage, (ELECTRA 128; E.R. C.55/4 1989; ANSI/IEEE 575-1988 1988). It is important that the accessory design incorporates means of connecting the cable screen wires, metallic tapes or sheath and joint shell to the insulation screen.

2.3.4

Earth Fault Requirements

Some Utilities require that short circuit currents be returned within the cable system. The user should ensure that the accessory is also able to contain this current.

2.4

Compatibility of the Accessory with the Cable System Design and Operating Conditions

The user is advised to ensure that accessory design is a) compatible with the particular cable installation design, as this determines the mechanical loading applied, b) capable of being assembled in the site environmental conditions and c) capable of a satisfactory service performance under adverse climatic conditions.

2.4.1

Type of Cable Installation Design

• Rigidly constrained (cable laid direct in the ground or close cleated) • Flexible unconstrained (cable horizontally snaked or vertically waved) • Semi-flexible (cable constrained, but permitted to exhibit a controlled deflection, for example at a bridge crossing or adjacent to gas immersed switch gear) • Unfilled duct.

2.4.2

Standard Dimensions for Cable Termination

The user is advised to ensure the following dimensional compliancies: • Outdoor and indoor termination: Harmonisation with existing equipment of the overall height of the off-going bus bar connector and of the bottom metalwork fixing arrangements to the support structure. • GIS and transformer termination: Harmonisation of the cable termination with both the design of the metal clad switch gear (internal diameter, overall length, off-going bus bar connector, bottom metalwork sealing arrangements and

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pressure) and the design of the support structure (fixing arrangements for the particular cable constraint selected) (IEC 859 1986).

2.4.3 • • • • • • •

Type of Accessory Installation Environment

Laid direct and buried in the ground Jointing chamber (in air) Tunnel Above ground Bridge Tower Shaft.

2.4.4

Jointing Limitations in Restricted Installation Locations

• Space limitations • Time limitations (for example arising from road or rail traffic influences) • Tolerance limitations of assembly personnel (for example arising from extremes of temperature, humidity, vibration, noise and induced voltage).

2.4.5 • • • • • • • • • •

Mechanical Forces Applied to the Accessory

Thermomechanical forces Earthquake Vibration Off-going bus bar at terminations Wind loading on bus bars at terminations Ice loading on bus bars at terminations Short circuit loading on bus bars at terminations GIS pressure Angle of installation of terminations Hydraulic or pneumatic pressure forces at transition joints.

2.4.6

Climatic Conditions

Accessories require to be suitable for the extremes of climatic conditions expected both in service and during assembly. Some types of accessories are required to be assembled under controlled environmental conditions. • Altitude (reduction of electrical strength of air) • Air pollution (reduction of electrical strength of outdoor insulator surface)

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Precipitation (reduction in electrical strength of air and outdoor insulator surface) Salt fog (reduction in electrical strength of outdoor insulator surface) Moisture condensation (reduction in electrical strength of insulator surface) Temperature Atmospheric humidity.

2.4.7

Type of Accessory Outer Protection Required

The accessory protection is required to provide corrosion protection and, for a specially bonded cable circuit, insulation from ground. • • • •

Joint box (laid direct in the ground or in air) Pedestal insulator (in air) Moulded sheet insulation (in air, to protect personnel against electric shock) Metallic fences or screens (in air, to protect personnel against electric shock).

2.4.8

Situations Requiring Special Accessory Protection

• Submerged under water • Termite infestation • Fire risk.

2.5

Verification of Accessory Performance

It is strongly recommended that the performance of the accessories is proven on test with the particular cable type, material, size and manufacture. The following verification items should be checked by the user.

2.5.1

Use of the Specific National or International Type Test Specification for the Accessory

If an applicable type test specification is available for the accessory, then these tests should be undertaken or a type test report provided. A list of world wide test specifications is given in the References.

2.5.2

Use of the Cable Test Specification in the Absence of an Accessory Specification

If a type test specification for the accessory does not exist, then it is recommended to use the type test specification for the cable.

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Type Test Report

The type test report should be obtained, this will give details of the accessory together with the cable size, performance levels and test specification reference. The type test usually includes elevated high voltage testing and load cycle testing of a comparatively short duration (e.g. 20 daily load cycles).

2.5.4

Type Tested Accessory in Combination with the Particular Cable

The cable size recorded in the type test report should be compared to the required cable for the particular application. The radial design stress of the test cable should be equal to or higher than that of the required service cable. The conductor area recorded in the type test report should be equal to or larger than that of the required cable. If the conductor size of the particular cable has not been tested it is permitted to accept tests already performed on a larger conductor (for example on the largest conductor size in the range). Some specifications require the testing of both the largest and smallest conductor sizes in the range, (IEC 840 1988).

2.5.5

Pre-Qualification Tests

The pre-qualification test is an endurance test of extended duration (for example one year). If a recognised pre-qualification test specification does not exist then the user is recommended to ask for confirmation that long term development tests have been undertaken. For pre-qualification tests on accessories for use at system voltages above 150 kV see (ELECTRA 151 1993a, b).

2.5.6

Satisfactory Service Record

Although not essential if the cable and accessories have passed recognized type approval tests and pre-qualification tests it is advisable to check that the accessories have a satisfactory service experience.

2.5.7

Test for Accessories in Specially Bonded Cable Circuits

Type approval reports should be provided to demonstrate the adequacy of the electrical insulation of a) the joint protection and b) the screen interruption. These tests are usually required to be undertaken on the complete accessory together with the cable. The exception is for discrete components, such as pedestal insulators, which for some applications may be tested individually. The tests usually require a combination of lightning impulse, ac and dc voltage withstand tests (ELECTRA 128; E.R. C.66/1 1979).

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Tests for Water Tightness of Joints

Type approval reports should be provided for those joints required to operate partially or fully submerged under water. Typical applications being installation in water logged ground or in jointing chambers or tunnels which are liable to flood. These tests are usually undertaken on the completed joint together with the cable and should require the application of either temperature cycles or current loading cycles whilst under water immersion. For installation in the ground, the test specification may also require the addition of mechanical tests to demonstrate the ability to withstand forces imposed by soil loading and by heavy vehicles, (E.R. C.66/1 1979). For joints in specially bonded cable circuits, installed in the ground, it is usual to provide a type test report which demonstrates performance when subjected to a combination of mechanical loading, water immersion, temperature cycling and elevated voltage withstand tests (item 2.5.7) (E.R. C.66/1 1979).

2.5.9

Additional Tests for Cable Terminations

In addition to the basic electrical type approval test report, the user is advised to seek confirmation of test performance for the following items: • Outdoor and indoor terminations: – Electrical performance of the insulator when subjected to atmospheric pollution in both wet and dry conditions (IEC 815 1986). – Electrical performance of the insulator when subjected to strong sunlight for prolonged periods. This is applicable to polymeric insulators only and not to porcelain insulators. – Mechanical performance of the insulator when subjected to a) cantilever loading to simulate forces from wind, bus bar loading and short circuit electromagnetic loading and b) axial loading to simulate the thermo-mechanical thrust and retraction of the cable conductor. – Electrical withstand performance of the cable termination, complete with cable, under rain spray conditions (IEC 840 1988; IEEE 48-1990 1990; KEMA S10-2). – Measurement on the complete termination of the radio interference level due to partial discharges (corona) in air (IEEE 48-1990 1990). – For application in those countries subject to severe earthquakes, the measurement of the vibration characteristics of the termination complete with its support structure and cable (IEC 1463). • GIS terminations and transformer terminations: – Mechanical performance of the terminations when subjected to a) cantilever loading to simulate the expansion forces of the off-going bus bar and b) axial loading to simulate the thermomechanical thrust and retraction of the cable conductor.

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– Vibration performance of the termination to simulate a) the high frequency vibration generated by the operation of a circuit breaker, b) when applicable, the low frequency vibration generated by an earthquake and c) the vibration of a transformer.

2.5.10 Pressure Vessel Regulations If the accessory contains fluid under pressure or is connected to GIS metal clad trunking it is advisable to check the requirements of National Regulations, if they exist, concerning approval of the design, routine test and type test.

2.6

Quality Assurance Scheme for Accessory Design and Manufacture

Type approval testing is intended to verify that the design has met the specified performance. To ensure consistent manufacturing quality, the user should verify the following items.

2.6.1

The Routine Test Schedule

Compliance with a routine test national specification should be stipulated. Most manufacturers use a more rigorous in-house routine test specification, in which case this should be approved by the user (for example (KEMA S10-2; ENEL spec. DJ 4585)).

2.6.2

Quality Assurance Approval for Manufacture

The user should ensure that the accessory supplier or manufacturer provides evidence of an approved Quality Assurance system for design, manufacture, routine test and traceability complying with an internationally recognised standard, e.g. (BS EN ISO 9001:1994 1994).

2.6.3

Routine Tests on Prefabricated Moulded Insulation

It is strongly recommended that the user specifies that each factory moulded insulating component be subjected, as a minimum requirement, to a routine test comprising an ac voltage withstand test and a partial discharge measurement test. Consideration should also be given to the application of additional routine tests, for example, dielectric loss angle measurement, X-ray examination and ultrasonic examination (KEMA S10-2; ENEL spec. DJ 4585).

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Sample Tests on Individual Components

In addition to the tests on moulded insulation described above, there should be electrical and mechanical tests performed on either all or on representative samples of production. For example (BS EN 50069:1991 1991; EATS 09-10 1976).

2.7

Quality Assurance Scheme for Accessory Installation

Assembly of the accessories onto cable with extruded insulation is the most vulnerable part of a project involving the manufacture and installation of a new cable circuit. Accessories and cables are manufactured and tested under controlled factory conditions, whereas the in-service performance of the accessory is dependent upon the training, skill and reliability of the personnel, who are often required to work under adverse site conditions. For many project applications one company will manufacture the cable and accessories and undertake to complete the installation of the circuit. In other applications the installer may complete the circuit using cable and accessories supplied by different manufacturers. In some applications the installer may only assemble the accessories. For each application the requirements of the QA system are equally rigorous.

2.7.1

Quality Assurance Approval for Installation

The user should ensure that the installer provides evidence of an approved quality assurance system for installation to an internationally recognised standard (BS EN ISO 9001:1994 1994).

2.7.2

Quality Plan

The installer is required to produce a Quality Plan for each project, this includes the project time schedule together with the requirements for suitably qualified personnel, training, on-site storage of components and accessories, tools, testing equipment, constructing materials, assembly instructions, preparation of the jointing environment and records of the assembly work. It is important that the records of assembly are traceable to the location of each accessory in the cable circuit. If purchasing separately, the user is advised to ensure that, for the purposes of traceability, the quality systems of the cable manufacturer, accessory manufacturer and installer are compatible.

2.7.3

Training of Personnel

When selecting the designs of accessories the user should ensure that training courses are available for the jointing and supervisory personnel. It is strongly advised that personnel receive training on the particular designs of accessories and cable.

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Examples of the elements of a training course for assembly personnel are: • General training at specific system voltages with the standard range of accessories required by the user • Repeat training after a defined period for those personnel who have completed general training • Specified training on a new accessory or cable design for those personnel who have completed general training. At the end of the training course the proficiency of the assembly personnel is normally assessed, for example, by a verbal or written examination, by a practical test and preferably by performing on the assembled accessories an electrical partial discharge test and voltage withstand test. Proficiency is recognised at the completion of training by the issue of a certificate, which should be checked by the user as part of the quality plan for a specific project. In many instances a kit of general jointing tools and a set of general assembly instructions is also issued to the personnel following satisfactory completion of training.

2.7.4

Assembly Instructions

The accessory manufacturer is required to supply a complete set of assembly instructions together with drawings of the particular accessory. The instructions should also include lists of the specified assembly tools, the specified consumable materials and the health and safety precautions. Recommendations for the preparation of the assembly environment should also be given. It is important that the user studies the instructions before work begins to ensure that the workplace is correctly prepared and that all the tools and consumable materials are available.

2.7.5

Special Assembly Tools

Most designs of accessories, particularly those operating at higher system voltages, require special tools which are purchased or hired from the accessory manufacturer. The user should ensure that full instructions are provided and that the personnel are trained in their use. These tools may take the form, for example, of a) hydraulic compression presses or welding equipment for connecting the conductors, b) cutting equipment to remove the insulation screen and to shape the cable insulation c) assembly machines which stretch and position pre-moulded elastomeric components, d) taping machines that apply tape and e) heated mould tools and mobile extruders for field moulded joints.

2.7.6

Preparation of the Assembly Environment

It is strongly recommended that the assembly area for both joints and termination to be enclosed within a tent or temporary building, with the objective of providing a

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clean and dry environment. The enclosure should be a) well lit to facilitate accurate preparation of the cable insulation, b) provided with a sound floor and c) lined with sealed materials to facilitate cleanliness. In extremes of climate it is good practice to provide control of temperature and humidity to ensure a) consistent performance of the personnel and b) consistent properties of the polymeric materials.

2.7.6.1 Joint Assembly • An appropriately sized joint bay or chamber. • The provision of a temporary and/or permanent support for the completed joint. 2.7.6.2 Termination Assembly • A permanent support structure. • A temporary weatherproof structure during assembly. • Means of lifting the cable and insulator into position.

2.8

Compatibility of the Accessory with Specified after Laying Tests

When the installation of the cable and accessories has been completed it is standard practice to perform electrical tests to demonstrate that the assembly of the accessories is of satisfactory quality and that mechanical damage to the cable and accessories has not occurred during installation. The following tests can be performed. It is important to ensure that the accessory design is suitable for the particular test:

2.8.1

Voltage Test on Main Insulation

DC tests have been traditionally applied to transmission circuits, however their use on cable with extruded polymeric insulation is not recommended. Experience has shown that the dc voltage test is not always sufficiently sensitive to detect damaged cable insulation or incorrectly assembled accessories and hence prevent them from entering service. In particular the electrical stress distribution under dc voltage in an accessory is usually significantly different from that under ac voltage in normal service. The application of an ac voltage is now under evaluation as an after laying test, either by the application of service voltage from the transmission system or by the application of test voltage from mobile test equipment (ELECTRA 173 1997).

2.8.2

Partial Discharge Detection

Partial discharge detection techniques are at present being developed for some cable and accessory applications to check for the absence of damage to the cable during installation and incorrect assembly of the accessories. Methods are not yet available for this to be done in a simple manner as a routine commissioning test on normal cable circuits (ELECTRA 173 1997).

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Voltage Withstand Test on the Cable over Sheath and Joint Protection

It is usual for specially bonded cable systems, including their accessories, to be subjected to an after laying test comprised of the application of a dc withstand voltage applied to the metallic sheath or screen wires (ELECTRA 128; E.R. C.55/4 1989; IEC 229 1982).

2.8.4

Current Balance Test on the Cable Sheath and Screening Wires

This test is performed on cross bonded cable systems at or adjacent to accessory positions to confirm that a) the bonding connections of the accessory are correct and b) the cable lengths and spacing are symmetrical, such that the magnitude of residual circulating current is of an acceptably low magnitude (E.R. C.55/4 1989; IEC 229 1982).

2.9

Maintenance Requirements of the Accessory

The user should ensure that adequate maintenance tests and checks have been recommended by the cable and accessory suppliers, for example:

2.9.1

Monitoring of Fluid Insulation

Liquid and gas levels: some types of termination, straight joints and transition joints are filled with insulating liquid or gas and may require to be regularly inspected or monitored in service to ensure that neither the liquid or gas have escaped.

2.9.2

Voltage Withstand Tests on the over Sheath and Joint Protection

These tests are similar to the after laying tests, but are usually performed at reduced voltage levels (ELECTRA 128; E.R. C.55/4 1989).

2.9.3

Shelf Life of Accessories for Emergency Spares

The user should ensure that information is provided on the shelf life of the components in an accessory for long term storage as these may vary according to the type of material, the way they are packed and the appropriate temperature and humidity conditions of storage.

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Availability of Accessory Kits for Emergency Spares

The user is recommended to obtain either a sufficient stock of spare accessories or to have an agreement with the manufacturer to supply accessories at short notice. The design of an accessory for emergency use may be different from that installed.

2.10

Economics of Accessory Selection

A comparison of the relative costs of different designs of accessory kits should not be undertaken without giving due consideration to the total costs of installation and assembly. The following are the main items of cost.

2.10.1 Cost of the Accessory Complete with all Components The accessory design should be checked to ensure that it is a complete kit and will be supplied with all the components and assembly instructions for the particular application. Some components that may not necessarily be supplied by all accessory manufacturers are for example a) conductor connections and anti-corrosion protection for joints and b) bus bar take-off connectors and support metalwork for termination.

2.10.2 Cost of Guarantee and Insurance At the higher system voltages it is more usual for the cable and accessories to be supplied, installed and guaranteed as a “turn-key” project. Under such circumstances the guarantee will usually extend to a specified number of years in service. If the user decides to divide the supply and installation of accessories between companies, it is recommended that the cost of financial self insurance be considered, because the responsibility for an accessory failure in service can be difficult to apportion between the accessory manufacturer, the cable manufacturer and the installer.

2.10.3 Cost of Assembly Time The jointing time required to assemble accessories can differ dependent on their design. Similarly the time required to assemble the anti-corrosion protection and the final mechanical support to the accessory can be the over-riding factors in determining the jointing time.

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2.10.4 Cost of Preparing the Installation Environment for the Accessory Accessories require the provision of a weatherproof enclosure together with the environmental conditions necessary for jointing (e.g. good lighting, cleanliness and, when necessary, air conditioning. The supply of electricity and gas may be required). Details are given in item 2.7.6.

2.10.5 Cost of Safe Working Conditions In addition to the cost of constructing the installation environment to comply with the regulations for safe working practices (items 2.10.4 and 2.7.6), the provision may be required for temporary and permanent protection to a) the installer’s personnel from electric shock during assembly and b) the user’s personnel when the accessory is in service.

2.10.6 Cost of Special Jointing Tools There may be significant differences in purchase cost and hiring charges of the tools required for different accessories.

2.10.7 Cost of Training Qualified jointers who are trained to assemble the particular accessory should always be employed. The user should decide whether it will be more cost effective to a) employ qualified and experienced personnel to assemble the accessories, or b) employ qualified and experienced personnel to install the cable and assemble the accessories as part of a turn-key contract, or c) incur the on-going costs of training and regular repeat training for his own personnel.

2.10.8 Comparative Cost of Cable and Accessories The design of the cable can influence the cost of the accessory design. Thus a reduction in the cost of the cable construction may result in an increase in the cost of the accessories. Similarly an increase in the cost of installation by laying longer lengths of cable may achieve a reduction in overall costs by requiring fewer joints.

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2.10.9 Cost of Verification of Accessory Performance If a type test report is not available for the particular cable and accessory in combination then the user is advised to allow for the cost of performing a type approval test. This cost may be born by the supplier, in the case of a turn-key project, but this is less usually so in the case of separately supplied cable and accessories.

References ANSI/IEEE 575-1988.: Guide for the application of sheath-bonding methods for single-conductor cables and the calculation of induced voltages and currents in cable sheaths (1988) BS EN 50069:1991.: Specification for welded composite enclosures of cast and wrought aluminium alloys for gas filled high voltage switch gear and control gear (1991) BS EN ISO 9001:1994.: Quality Systems: Model for Quality Assurance in design, development, production, installation and servicing (1994) E.R. C.55/4.: Insulated sheath power cable systems (1989) E.R. C.66/1.: Type approval testing procedure: protective boxes for use with buried accessories employed on 33kV–400 kV insulated sheath power cable (and for sheath sectionalising insulation embodied in such accessories) (1979) EATS 09-10.: Porcelain insulators for 33, 66, 132, 275 and 400 kV pressure assisted cable outdoor sealing ends (1976) ELECTRA 128.: Guide to the protection of specially bonded cable systems against sheath over voltages (Report of WG 21.07) ELECTRA 151.: Recommendations for electrical tests pre qualification and development on extruded cables and accessories at voltages > 150 (170) kV and  400 kV (420) kV. (December 1993 pp. 15–19: WG 21.03) (1993a) ELECTRA 151.: Recommendations for electrical tests, type, sample and routine on extruded cables and accessories at voltages > 150 (170) kV and  400 kV (420) kV. (December 1993 pp. 21– 29: WG 21.03) (1993b) ELECTRA 173.: After laying tests on high voltage extruded insulation cable systems (Report of WG 21-09) (1997) ENEL spec. DJ 4585.: Prescrizioni per il collaudo di giunti e terminali unipolari cavi isolati con gomma etilenpropilenica IEC 1463.: Bushings – Seismic Qualification IEC 229.: Test on cable over sheaths which have a special protective function and are applied by extrusion (1982) IEC 71.: Insulation Coordination. Part 1 (1993): Definitions, principles and rules. Part 2 (1976): Application guide IEC 815.: Guide for the selection of insulators in respect of polluted conditions (1986) IEC 840.: Tests for power cables with extruded insulation for rated voltages above 30 kV (Um ¼ 36 kV) up to 150 kV (Um ¼ 170 kV)) (1988) IEC 859.: Cable connections for gas-insulated metal-enclosed switch gear for rated voltages of 72.5 kV and above (1986) IEC 986.: Guide to the short-circuit temperature limits of electric cables with a rated voltage from 1.8/3 (3.6)kV to 18/30 (36)kV (1989) IEEE 48-1990.: Test procedures and requirements for high-voltage AC cable terminations (1990) KEMA S10-2.: KEMA specification of requirements to be met by accessories for single-phase power cables with extruded insulation for rated voltages between 50 and 220 kV Section 3.: Summary of world wide usage of accessories for HV extruded cables (Chapter 4.1) (Report of WG 21-06)

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Zensuke Iwata was born in Tokyo, Japan, on October 5, 1944. He received the B.S. degree in Electrical Engineering from the University of Tokyo, Japan, in 1968. In 1968 he joined the Furukawa Electric Co., Ltd., Japan, where he has been engaged several years in research and development of high-voltage power cables and their accessories before taking other positions in the Furukawa Electric Co. In 2003 he successfully carried out the long-term field test of the real scale H.T. superconducting cable system as the Managing Director and CTO of the Furukawa Electric Co. In 2004 he was appointed as the President of Nuclear Fuel Industries, Japan, and retired in 2014. Zensuke Iwata convened Cigré WG 21.06 which published Technical Brochures 89 and 177. He received the TC Award in 1995. He chaired the ISTC of Jicable in 2003.

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Contents 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Terms of Reference of JTF 21/15 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2 Interfaces to be Studied . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3 Materials Involved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Interface Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Smoothness of the Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Contact Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Lubricant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4 Electrical Field Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.5 Temperature and Temperature Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.6 Quality of Accessory Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Long Term Performance of Interfaces in Cable Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Migration of the Lubricant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Movements in the Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Reduction of the Interface Pressure due to Relaxation of Materials . . . . . . . . . . . . . . 3.3.4 Electrical Ageing of Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Recommendations and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.1

81 82 82 83 85 86 87 87 88 90 90 90 91 91 91 92 92 94 94

Introduction

Interfaces in joints and terminations of extruded HV cables have been identified as crucial parts. Some of the mechanisms related to ageing are not well understood. For this reason, a task force has been established to study the behaviour of interfaces in

H. Geene (*) Prysmian Group, Product Management HV Accessories, The Hague Area, Netherlands e-mail: [email protected] © Springer Nature Switzerland AG 2021 P. Argaut (ed.), Accessories for HV and EHV Extruded Cables, CIGRE Green Books, https://doi.org/10.1007/978-3-030-39466-0_3

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accessories for HV and EHV extruded cables. The scope was limited to non-bonded interfaces between solid insulating materials, but included the applied lubricants. The Joint task force 21/15 (JTF21/15) was installed by Study Committee SC21 (Insulated Cables now SC B1) and SC15 (Materials now SC D1) and called “Interfaces in accessories for extruded HV and EHV cables” The members of the task force were cable systems and/or material experts. The report of the work has been published as Cigré TB 210. Within Cigré, interfaces was also subject of study for WG15.10. This working group was concentrating on the material aspects of interfaces and had developed and selected interface models for laboratory testing.

3.1.1

Terms of Reference of JTF 21/15

The joint task force is reviewed the state of the art regarding the interface behaviour in accessories for extruded HV and EHV cables. The targets objectives for JTF 21/15 were: • To evaluate short term behaviour of interfaces (parameters influencing withstand strength) • To evaluate ageing behaviour and relevant parameters • To make practical recommendations for evaluating, testing and installing interfaces in extruded HV and EHV cable systems. The task force did not deal with partial discharges and electrical treeing in interfaces, since these processes are strongly influenced by the type of accessories (materials, design, etc.). The focus was on the parameter settings to prevent partial discharges in interfaces and those phenomena that could lead to partial discharges in service. Partial discharge detection on installed Cable Systems was the subject of Cigré WG 21-16 which published Cigré TB 182 (Cigré WG B1-16 2001).

3.1.2

Interfaces to be Studied

The interfaces to be studied are those in accessories for extruded HV and EHV cables between solid insulating materials. Although the cable is an essential part of the accessory, it will not always be explicitly mentioned (Fig. 3.1). Interfaces in accessories are between: • Rubber insulating body and cable insulation, in a liquid or gas filled termination (a) dry-type termination (b), in a composite joint (c) or joint (d), • Stress-cone and epoxy joint body, in a dry-type termination (b) or composite joint (c) • Adapter sleeve and joint body (e)

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Fig. 3.1 Typical accessory arrangements for HV extruded cables (Peschke and Olshausen 1999)

3.1.3

Materials Involved

Interfaces exist between two solid polymer parts. The materials involved in these interfaces and various related abbreviations are summarized in Table 3.1. The international standards dealing with symbols and abbreviations of polymers respectively rubber and latex latices appear not to be harmonized yet. Polyethylene mainly divides into two big families: LDPE (Low Density Poly Ethylene, density in the range of 0.910-0.925 g/cm3) and HDPE (High Density Poly Ethylene, density in the range 0.941–0.965). In addition to these two main families, it is worth mentioning LLDPE (Linear Low Density Poly Ethylene). In cable manufacturing PE is used both for insulation and for jacketing; the cross-linking of PE (peroxide or silane) leads to XLPE (Cross Linked Poly Ethylene) and is widely used both for MV and HV cables, particularly for allowing high operating temperatures up to 90
C. The general term EPR is used as an abbreviation for Ethylene Propylene Rubber (see Table 3.1). EPR divides into two kinds of polyolefin polymers: EPM and EPDM. EPM represents an Ethylene Propylene copolymer while EPDM denotes a

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Table 3.1 Summary of symbols and abbreviations of plastics Abbreviation in literature SiR, SIR, MVQ, VMQa RTV RTV-2 LR, LSR XLR HTV, HCR

EPR EPM EPM, P

EPDM EPDM, S EPDM, P

EP HDPE, LDPE XLPE

Material (as used in this paper) Silicone rubber Room Temperature Vulcanising, 2-component silicone rubber Liquid Rubber, Liquid Silicone Rubber, Extra Liquid Rubber High Temperature Vulcanising, High Consistency Rubber Ethylene Propylene Rubber Ethylene Propylene Copolymer (‘EPM, P’ stands for peroxide vulcanised EPM) Ethylene Propylene Diene Terpolymer (‘EPDM, S’ stands for sulphur vulcanised; ‘EPDM, P’ see above) Epoxy resin High Density Polyethylene, Low Density Polyethylene Cross-linked polyethylene Grease, silicone oil, paste, lubricant

Abbreviation (as used in this paper) SiR

DIN ISO 1629 Mar 1992 MVQ, VMQ

DIN EN ISO 1043 Part 1 Jan 2000 SI

EPR EPM

EPMb EPM

E/P E/P

EPDM

EPDM

–

Epoxy PE

– –

XLPE Lubricant

– –

EP PE PE-LD PE-HD PE-X –

RTV

LR

HTV

Q indicates rubbers with poly-siloxane-groups in the main chain, e.g. MVQ ¼ Methyl-Vinyl-PolySiloxane b M indicates rubbers with saturated poly-methylene main chain; R indicates rubbers with unsaturated poly-carbon main chain. Note: this deviates from the wide spread use to indicate rubbers in general by R a

terpolymer based on three monomers: Ethylene, Propylene and a non-conjugated Diene. Both grades EPM and EPDM are suitable for peroxide cross-linking while only EPDM allows sulphur cross-linking. Mixing of EPR with other components leads to the final EPR compounds for power cables and accessories applications. SiR consists of a so-called silicone-oxygen (polysiloxane) polymeric main chain, which exhibits high thermal and high UV stability. This main polymeric chain carries methyl-groups as well as other functional groups like vinyl- or hydrogenfunctions. RTV, XLR, LR or HTV (definitions see Table 3.1) rubbers are the most widely used grades in electrical applications. Whereas RTV-2, XLR and LR are

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vulcanised by a so-called addition curing reaction of A- and B-component, HTV rubber normally is peroxide cross-linked. The main differences in RTV-2 versus LR or HTV are viscosity and processing. SiR can be formulated to obtain very low Shore hardness and modulus of elasticity (Strassberger and Winter 1997). Various types of Epoxy and fillers are used for electrical applications. Among them a typical resin system is solid Bisphenol A type [2,2-Bis-(4-hydroxyphenyl)propane)] epoxy resin. Technical production of oligomeric epoxy resins involves reaction of Bisphenol A with Epichlorhydrin to give a reactive intermediate. These reactive epoxy resin intermediates are the basis, which is polymerized (polyaddition) with so called hardeners. Hardeners could be either solid powdery acid anhydrides or aliphatic polyamines resp. polyamidoamines. Cycloaliphatic epoxy resins are cured normally with acid anhydrides. Aminic systems can be hardened at room temperature or below 80
C, acidic systems need temperatures over 80
C for hardening. Results are the various Epoxies, which are classified as duroplastics. Lubricants are widely used as slip-on materials to ease the installation of cable accessories. Silicone greases as well as silicone fluids can be used with either SiR or EPR accessories. Other materials are greases based on fluorinated polymers or polyethylene glycol modified greases. Generally spoken, grease is formulated of a liquid basis polymer and a thickener, mostly silica flour and some additives. In contrast to grease e.g. silicone fluid is a pure silicone polymer. Regarding installation and interface, the degree and speed of migration of the paste into the insulating material are of interest. In the case of a pure fluid the interface may get dry after a certain period of time, while for greases the thickener may remain in the interface (Willems et al. 1995).

3.2

Interface Parameters

Interfaces are characterized by the initial or short-term breakdown strength and longterm ageing properties. In Sect 3.3 the ageing phenomena will be discussed while in this section the focus will be on the initial breakdown strength. The electrical withstand strength of interfaces is influenced by a combination of several parameters. In the design of interfaces the following parameters should be taken into account: • • • • • •

Smoothness of the surfaces Contact pressure on the interface Type of lubricant in the interface Electrical field distribution in the interface Temperature and temperature changes Quality of accessory installation.

Most of the parameters mentioned, interact with each other. In the following paragraphs, the parameters will be discussed separately.

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Smoothness of the Surfaces

According to literature (Kreuger 1989), the discharge in a cavity in insulating materials occurs at approximately the same (or higher) voltage as between equally spaced metal electrodes. This voltage is for a certain gas given by the Paschen curve. According to this curve, the breakdown stress of a gap is depending on the gap distance, type of gas and pressure in the gap. At fixed pressure, the Paschen curve indicates higher breakdown stresses for smaller gaps. In line with the diagram in Fig. 3.2 the smaller the cavities, the higher the stresses at which partial discharges incept. Interfaces without microscopic cavities do not exist. Surface scratches in the order of a few microns are inevitable. In order to avoid partial discharges arising form these scratches, the size of cavities should be limited to a few microns (depending on the electrical stress). Therefore an accurate adaptation of the insulating surfaces in the interface is needed. This can be achieved by smoothening of the insulation surfaces. The surface smoothness of moulded rubber or epoxy insulators can be achieved by equivalent surface smoothness of the moulds. Smoothness in the order of a few micrometers can be achieved without difficulties. The surface smoothness of the cable insulation, however, is depending on the applied peeling and smoothing techniques, performed on site in a less well-controlled environment. This part of the interface depends on the jointer skills. Therefore it is desirable, particularly for high stress accessory designs, to prescribe the required smoothing technique for the

Fig. 3.2 Breakdown strength of air gaps, derived from the Paschen Curve (Kreuger 1989)

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preparation of the cable insulation. Although interface pressure is discussed in a separate paragraph, it is mentioned here, that the pressure in the interface and the adaptability of the insulating materials, determine the sensitivity to surface irregularities. Typical example for calculating the surface smoothness from the Paschen curves (Kreuger 1989). Assume a typical radial stress in the cable insulation, adjacent to the interface, of 3.6 kV/mm at Uo (radial stress at the insulation screen of the cable) and assume that the accessory is required to be free of partial discharges at 2 Uo, then the inception stress in a possible gap has to be higher than: Egap ¼ er,PE =er,cav  Emin  2 ¼ 2:3=1  3:6  2 ¼ 16:7 kV=mm Reading from Fig. 3.2, the cavity size in such case has to be smaller than about 20 μm. Sanding with grade 400 leads to a roughness Rz ¼ 10 μm (Rz ¼ RmaxRmin). Assuming that the mould irregularities are significant smaller (order of 1 μm) the achieved cable insulation smoothness with paper grade 400 is sufficient.

3.2.2

Contact Pressure

As described in paragraph 3.2.1, the sensitivity of the interface to irregularities depends on the interface pressure (Fig. 3.3). High interface pressures minimize the size of micro cavities in the interface. In practice, two different methods to achieve the required interface pressure have proven to be suitable (Fig. 3.3): • Application of external mechanical forces, using springs. This method is often used in the so-called inner cone model of composite joints or dry-type terminations, pushing a rubber part against an epoxy body • Use of elasticity of the rubber body expanded on the cable insulation. This method is often used in slip-on joints and stress-cones. Expansion percentages of 5–50% are common practice. The interface pressure obtained by this method depends on: – E-modules of the applied rubbers – Strain of the rubbers – Wall thickness of the rubber.

3.2.3

Lubricant

Lubricants are basically used to relieve the friction between rubber parts and the other insulating materials during installation. Silicone fluids and greases are commonly used for this application.

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y

→

p

x

z

material strength

XLPE

electrical interface strength

E

SIR

→

Rz=const. electrical strength of air transition zone (nearly) only cavities

(nearly) no cavities

Interface pressure, surface smoothness Fig. 3.3 Electrical interface strength vs. interface pressure and surface smoothness (Kunze 2000a)

Lubricants also tend to increase the initial breakdown strength (Fig. 3.4). However it is not recommended to use lubricants for filling cavities. On the long-term, lubricants probably migrate (at least partly), resulting in a more or less dry interface, possibly leaving air gaps behind. The migration rate of the lubricant depends on: • • • •

Type of lubricant Type of insulating materials Contact pressure Temperature.

Another issue regarding lubricants is the presence of air bubbles trapped in the interface during installation. In particular greases with a high viscosity are more likely to enclose air bubbles. The design of the accessory or the applied installation method should prevent the formation of air bubbles (Fig. 3.4).

3.2.4

Electrical Field Distribution

During operation of the cable system, the interfaces are subjected to electrical stresses (Fig. 3.5). The following stress characteristics can be distinguished: • Direction • Amplitude • Distribution.

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Fig. 3.4 Effect of lubricant oil on the breakdown strength of a typical XLPE-SIR interface (Kunze 2000a) Fig. 3.5 Typical electrical field distribution in the interface of a pre-moulded joint

90% 70% 50% 30% 10%

Enorm

E tan

x

The component along the surface of the insulators is called the parallel, longitudinal or tangential electrical stress and is generally regarded as the most important one. Also the amplitude (e.g. for inception of partial discharges) and the distribution (e.g. for electrical treeing) will affect the interface behaviour. It is preferred that areas with highest electrical stress coincide with highest interface pressure. In most of the accessory designs the shape of the stress-cones and embedded electrodes control the electrical field distribution (Fig. 3.5).

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3.2.5

Temperature and Temperature Changes

It is known that operating temperatures influence the withstand stresses of insulating materials. Although this might contribute to the weakening of the interface, thermomechanical effects usually have a much larger influence at the interface, as they could lead to movements along the interface. Reasons are e.g. differences in thermal expansion coefficients or external mechanical forces. In this respect temperature changes i.e temperature cycling and/or temperature gradient (are of more) are of more importance than temperature itself. High temperatures in cable and accessories can lead to deformations of the cable insulation. During the cooling down period of cable and accessories thermal shrinkage of materials will occur. This may result in pressure changes at the interface and should be properly taken into account during design of accessory design.

3.2.6

Quality of Accessory Installation

The installation of accessories is considered as the most critical step in realizing a cable system. In particular the interfaces are influenced by the installation, since in most cases the cable insulation is prepared on site. The insulation surface must be prepared most carefully. Installation instructions must clearly indicate the cable preparation, including the smoothing technique and/or the required smoothness. During installation, also the positioning of components is of utmost importance. The installation procedure and instructions, including the drawings, must be clear and unambiguous. During the installation the following aspects should be taken into account, • • • • • •

Final cable diameter and roundness Straightness of the cable Smooth and regular insulation surface Smooth and regular transition from insulation screen to cable insulation Correct positioning of accessory parts Dryness and cleanliness.

It is essential that the jointers shall be well trained, to provide the necessary skills. See ▶ Chaps. 2, ▶ 5 and ▶ 6 of this book for further information.

3.3

Long Term Performance of Interfaces in Cable Accessories

The ageing of interfaces can be considered as a change in one or more parameters mentioned above, leading to a decrease of the electrical withstand strength of the interface. Electrical ageing of interfaces, as a result of intrinsic electrical ageing of the applied materials, is not likely to occur. The electrical stresses in the interface are low compared to the withstand stresses used for cables or accessories (Kunze

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2000a). More likely, mechanical and thermo-mechanical effects change the interface parameters. Thermo-mechanical effects can cause formation of cavities and in extreme situations even gaps between the insulating materials. It is obvious that gaps or large voids cause partial discharging, followed by electrical treeing or tracking in the interface. The formation of cavities and gaps in interfaces can be the result of a combination of effects, such as: • • • • •

Migration of the lubricant Movements in the interface Reduction of the interface pressure due to relaxation of materials Electrical ageing of interfaces Contamination of the interface.

3.3.1

Migration of the Lubricant

It is general practice to use lubricants in accessories to relieve the friction between cable and accessory parts during installation. Already during the installation most of the lubricant is pushed out of the interface. The remaining film of lubricant will disappear in time, due to migration into the insulation materials. Depending on the type of lubricant and the materials applied, the migration time can vary between hours and years. Greases composed of fluid and solid filler can dry out (migration into the insulating materials), leaving filler behind. Sufficient interface pressure in combination with the smoothness of the surfaces will prevent the formation of cavities.

3.3.2

Movements in the Interface

Interfaces have been shaped carefully in order to obtain a secure fit between the insulating surfaces. If the insulating surfaces can move in respect to one another, due to thermal expansion or external forces, the interface pressure can decline locally causing a weak spot and in some extreme cases even gaps in the interface. The insulating surfaces shall be shaped in such a way that if movements can occur, this will not lead to the formation of gaps.

3.3.3

Reduction of the Interface Pressure due to Relaxation of Materials

Interface pressure can decrease due to deformation of the cable insulation or due to relaxation of the rubber. Deformation of the cable insulation can occur at high temperatures. The accessory design has to prevent unacceptable deformation of the cable insulation.

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Relaxation of the rubber can occur in those designs where the interface pressure is achieved by expanding the rubber sleeve onto the cable insulation. To prevent critical low interfaces pressures, the setting of the rubber should stabilize at a safe value. Important parameters describing the setting of rubber are the so-called compression set and tension set. In the case of interface pressure applied by external means (i.e. springs), their mechanical design has to secure a sufficient pressure level during the lifetime.

3.3.4

Electrical Ageing of Interfaces

Due to the fact that partial discharge is a symptom of an insulation defect, the inception and occurrence of partial discharges may accompany the electrical ageing of interfaces as well. Because of the different accessory designs on the market, it is impossible to deduce a single relationship between the magnitude of partial discharges and the remaining lifetime. However, in individual cases, partial discharge characteristics and its development may give indications for the incipient failure (Smit et al. 1997; Smit 1999).

3.4

Testing

There are several ways interfaces in HV accessories can be tested. Roughly speaking we can distinguish between laboratory testing and on site testing. Laboratory tests can be performed on many levels: • Material test • Model tests on material samples • System tests on cable and accessories. As none of these tests can solely represent the characteristics of the interface completely, the optimum has to found in a combination of these tests. Regarding an interface, the breakdown strength of the insulation materials itself is of minor importance, due to the lower electrical stresses in the interface section. More important for the interface breakdown strength are the mechanical properties of the materials, i.e.: • • • •

Modulus of elasticity Hardness Compression set or tension set Surface roughness.

The right combination of these mechanical properties has to ensure a tight fit between the insulation surfaces, thus leading to the required electrical interface performance.

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Tests on interface models offer the possibility of a statistical result evaluation of different interfaces at relatively low cost. Cigré WG15.10 has dealt with the subject of models intensively (Nagao et al). An important conclusion that can be drawn from their work is that different model types should be used to investigate different aspects of electrically and/or mechanically stressed interfaces. A model for real accessory design purposes should be a realistic simulation of the practical interface situation (Kunze 2000b; Nagao et al) or e.g by using a lower voltage class accessory of the same design (Smit et al. 1997). A good interface design has to prevent inception of partial discharges. Therefore the models studying electrical treeing in interfaces can be of importance for basic material investigation. For the evaluation of interface design parameters, partial discharge free arrangements are more suitable. The results of model tests can directly be used for comparison of different parameter settings. The absolute values can be transferred to real accessory designs, using special algorithms, but should be done with great care. Once a design is completed, prototypes subjected to system tests are indispensable for the qualification of interfaces. Development tests on systems should represent electrical, thermal and thermomechanical service conditions. For this reason it is recommended to include the following conditions in the development test program: • • • • •

Elevated electrical stresses to accelerate ageing Mechanical stress due to simulate installation and service conditions Thermo-mechanical stress during load cycles Transient voltages as impulse voltage Partial discharge monitoring (continuously or periodically).

In particular the thermo-mechanical conditions are most complicated to predict. The thermo-mechanical forces highly depend on the design and the way of installation e.g.: flexible and rigid installations, load and environmental conditions. During the development and qualification of HV cable systems, the accessories should be installed in practical worst-case condition (maximum mechanical forces). Testes should incorporate heating cycles and voltage simultaneously. During the cycle period, partial discharge measurements should be made at different conductor temperatures. After thermo-mechanical ageing it is recommended to perform the impulse voltage test, in order to detect possible weakening of the interface. Outdoor terminations sometimes have to operate at low temperatures. This circumstance can represent a more critical condition for interfaces than elevated temperatures. Testing of these terminations in cold conditions and varying temperatures should be taken into account during the development. To verify the performance of the interface and the complete accessory, the prequalification test as recommended by Cigré (WG 21.03 1993) and standardized by IEC (62067) is a necessary and reliable method for testing. The final step in commissioning a cable system is the after laying test. The AC voltage test is an important one to verify the correct preparation and installation of

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the accessories on site (Nagao et al). Testing with DC voltage is not recommended. The electrical field distribution in the interface for DC voltage can differ completely from AC voltage (IEC60840 1999). If there is a need for monitoring accessories in service (e.g. higher failure rate than normal), the most appropriate test method is on-line partial discharge monitoring. The partial discharge tests are preferably executed under different environmental and/or load conditions, in order to determine the thermo-mechanical impact on the interface. The recommended frequency of testing will highly depend on the nature of the discharge pattern and accessory type. In high stress accessories (e.g. slip-on joints for EHV) partial discharges in interfaces in the order of a few pico-coulombs can lead to breakdown within hours, while for some low stress accessories (e.g. outdoor termination) partial discharges in interfaces can be withstood sometimes for several years.

3.5

Recommendations and Conclusions

Interfaces in HV and EHV cable accessories should be designed in such a way that under operating conditions always a tight fit between cable and accessory or between other insulating bodies is secured. Once the insulating surfaces do not adapt carefully, cavities will be formed leading to inception of partial discharges. A proper interface design does not allow partial discharges. Below the inception level of partial discharge, no detectable ageing will take place. Once discharges have been ignited, accelerated ageing will start, most probably leading to electrical treeing in the interface and finally failure of the accessory. The reliability of interfaces in HV and EHV extruded cable accessories is strongly dependent on the mechanical and thermo-mechanical design of the accessories and the interaction with its environment, i.e. way of installation and service conditions. During the development of cable systems, these circumstances have to be taken into account. For this reason the long term or prequalification tests of the entire system (cable and accessories) is of eminent importance. The quality of the interface depends on the cable surface preparation. This has to be ensured by clear procedures, adequate quality management systems (e.g. ISO 9001 (1994)) and skilled jointers. Although there is no general relation between the partial discharges and the remaining lifetime, trend analyses by means of online partial discharge monitoring can give indication if risk of failure is involved for the type of PD patterns observed.

References Cigré WG 21.03.: Recommendations for electrical tests prequalification and development on extruded cables and accessories at voltages > 150 (170) and  400 (420) kV, Elektra No 151 (December 1993) Cigré WG B1-16.: Partial Discharge Detection in Installed HV Cable Systems. Cigré Technical Report 182 (April 2001)

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Densley, J., Nadolny, Z.: PD characteristics of model interfaces for extruded cable systems – influence of contaminants, Cigré WG15-10 Fournier, D., Lamarre, L.: Effect of pressure and length on interfacial breakdown between two dielectric surfaces. In: IEEE International Symposium on Electrical Insulation, Baltimore, June 7–10, 1992, pp. 270–272 Geene, H.T.F., van der Wijk, G.P., Pultrum, E.: Development and qualification of a new 400kV XLPE cable system with integrated sensors for diagnostics, Cigré 1998, paper 21-103 Gockenbach, E., Kunze, D.: Makroskopische, innere Grenzflächen in Hochspannungskabelgarnituren, VDE Fachtagung ‘Einfluss von Grenzflächen auf die Lebens-dauer elektrischer Isolierungen’ (Bad Nauheim, 21–22 Sept 1999) IEC 62067.: Power cables with extruded insulation and their accessories for rated voltages above 150kV (Um¼170 kV) up to 500kV (Um¼550 kV) – Test methods and requirements IEC 60840.: Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um¼36 kV) up to 150 kV (Um¼170 kV)-Tests methods and requirements (1999) Imai, N., Andoh, K.: Development of pre-fabricated joints for 275 kV XLPE cables, Jicable (1991), paper A.5.4 ISO 9001.: Quality Systems-Model for quality assurance in design, development, production, installation and servicing (1994) Kärner, H., Kodoll, W., Seifert, J., Tanaka, T., Nagao, M.: Interfacial phenomena affecting electrical insulating properties in composites, Cigré WG15-10 Kreuger, F.H.: Partial Discharge Detection in High-Voltage Equipment. Butterworth & Co, London (1989) Kunze, D.: Eine neue Muffengeneration für VPE-isolierte Höchstspannungskabel, Elektrizitätswirtschaft, Jg. 96 (1997), Heft 26 Kunze, D.: Untersuchungen an Grenzflächen zwischen Polymerwerkstoffen unter elektrischer Hochfeldbeans-pruchung in der Garniturentechnik VPE-isolierter Hochspannungskabel. Dissertation Uni Hannover, Shaker Verlag Aachen (2000a). ISBN 3-8265-7721-3 Kunze, D.: Macroscopic internal interfaces in high voltage cable accessories, Cigré session (2000b), paper 15-203 Nadolny, Z., Braun, J.M., Densley, R.J.: Effect of mechanical pressure and silicone grease on partial discharge characteristics for model XLPE transmission cable joint. In: Proceedings of ISH’99 (London, August 1999) Nagao, M., Ka, S., Murramotto, Y., Tanaka, T.: Model specimens for testing interfacial properties in EHV extruded cable splices and preliminary results, Cigré WG15-10, 15-10-Nagao-01-98 Nagao, M., Ka, S., Murramotto, Y., Tanaka, T.: Model specimens for testing interfacial properties in EHV extruded cable splices and preliminary results, Cigré WG15-10, 15-10-Nagao-01-99 Peschke, E., Olshausen, R.V.: Cable Systems for High and Extra-High Voltage. PIRELLI, Publicis MCD Verlag, Erlangen (1999). ISBN 3-89578-118-5 Report on Internal interfaces of modern electrical insulation systems, Cigré WG 15-10 Meeting Palais des Congres Paris, France, September 1, 1998 Ross, R.: Dealing with interface problems in polymer cable terminations. IEEE Electr. Insul. Mag. 15(4), 5–9 (1999) Ross, R.: Investigating and monitoring the reliability of interfaces in polymer cable terminations. In: Proceedings at IEEJ Kansai Meeting on Insulation Diagnosis Ross, R., Megens, M.G.M.: Aging of interfaces by discharging. In: Proceedings of ICPADM (2000) Ross, R., Megens, M.G.M.: An interface testing cell for multi-stress ageing. In: Proceedings ISEIM 98 Smit, J.J.: Life management of electrical infrastructure. In: Cigré SC15-Symposium “Service Ageing of Materials in HV Equipment”, Sydney (1999), key-note paper Smit, J.J., Gulski, E., Pultrum, E.: Partial discharge fault analysis of SIR-based cable terminations. In: Proceedings of International Symposium on High Voltage Engineering, Montreal, vol. 4, p. 513 (1997) Strassberger, W.: Silicone Elastomers for Transmission and Distribution Strassberger W., Winter H.-J.: Silikonelastomere in der Mittel- und Hochspannungstechnik. ETG Fachbericht Nr. 68, pp. 7–14. VDE-Verlag GmbH, Berlin, Offenbach (1997)

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Strassberger, W., Winter, H.-J.: Silikonelastomere . . ., ETG-Fachbericht Tanaka, T.: Polymer interfaces, associated with electric insulation systems. Cigré Colloquium on Advanced Materials (Boston, 18 Aug 1997) Willems, H.M.J., Geene, H.T.F., Vermeulen, M.R.: A new generation of HV and EHV extruded cable systems, Jicable (1995), paper A.1.6

Henk Geene has a Master’s degree in Electrical Engineering from the Technical University in Eindhoven, the Netherlands. Shortly after graduation, he joined the Dutch cable manufacturer NKF (nowadays part of the Prysmian Group) where he started as an engineer to develop High and Extra High Voltage cable accessories. Currently, he is responsible for product management and sales of the Prysmian high voltage accessories. He is past Dutch Member of Cigré Study Committee D1, Convener TF15/21 “interfaces in HV cable accessories,” Convener TF21.10 “thermal ratings of HV cable accessories,” participated as a Member in several Cigré Working Groups, and is currently Chairman of the IEEE Insulated Conductors Committee (ICC). He is author of several papers and publications on a wide range of subjects in the field of high voltage cable accessories and their interaction with other components in the cable systems.

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Contents 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Scope and Terms of Reference of WG B1.06 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3 Experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Long Duration Test on EHV Cable Systems (170 < Um < 550 kV) . . . . . . . . . . . . . . . . . . . 4.2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Revision of the Present Prequalification Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Changes in a Prequalified Cable System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4 Recommendations to IEC 62067 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Long Duration Test on HV Cable Systems (36 < Um
170 kV) . . . . . . . . . . . . . . . . . . . . . . 4.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Prequalification Test for HV Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Exchanges and Modifications in a Prequalified HV Cable System . . . . . . . . . . . . . 4.3.4 Recommendations to IEC 60840 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Annexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1 Terms of Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Sensitivity of Partial Discharges in XLPE Cable Insulation to Change of Electrical Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3 Functional Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.4 Tests From Functional Analysis not in IEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

102 102 103 103 107 107 110 112 121 122 122 124 127 132 133 134 134 135 146 183 187

Jean Becker: deceased. J. Becker (*) Charleroi, Belgium © Springer Nature Switzerland AG 2021 P. Argaut (ed.), Accessories for HV and EHV Extruded Cables, CIGRE Green Books, https://doi.org/10.1007/978-3-030-39466-0_4

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Summary IEC test requirements have evolved over the years from the component-based approach in IEC 840 to the system based approach. Accessories are considered together with the cable, in IEC 62067 Ed.1 and in the most recent edition of IEC 60840 Ed.3. In its meeting in Madrid of 2001, Study Committee B1 decided to install a Task Force TF 21.11 to get SC B1 prepared to issue future recommendations for evolutions of IEC 62067 taking into account the expected innovations in cable technology, the need to reduce the time-to-market and the overall cost to introduce new evolutions as well as service experience collected by the Cable Industry. TF 21.11 issued in 2002 a proposal of Terms of Reference and Scope of Work for a new Working Group which was launched as WG B1.06 in Paris Study Committee meeting in 2002. In July 2005 WG B1.06 circulated its final draft of report for approval by SC B1. Comments were received from France, Japan, The Netherlands and Italy. As time schedule for issuing the new Edition of IEC 62067 Ed.1 was critical, SC B1 agreed to go in more detailed recommendations and decided in its meeting of Rosenön (SE), September 2005 to launch a Task Force to finalise the report and write clear and practical recommendations for appropriate changes in IEC 62067 Ed.1 and IEC 60 840 Ed.3. This chapter published as Cigré TB 303 is the result of the Work of WG B1.06 and of the Task Force. Section 4.1 of this chapter is an Introduction, which recalls and details the Scope of Work and the Terms of Reference and gives an overview of the service experience of HV and EHV cable systems so far as well as a survey of experience obtained by testing EHV cable systems. At voltages up to and including 150 kV extruded insulation has largely superseded paper-insulated cables for new installations. Much of the service experience with HV XLPE cable systems is based on cables with moderate design stresses. A new generation of “slim-design” HV cables is being developed, with similar technology and design stresses to those seen in EHV XLPE systems. Hence historical service experience with HV cable systems is not necessarily a good guide to the likely future service experience of these novel systems. XLPE has only recently become the insulation of choice for many utilities for EHV transmission circuits. The introduction of XLPE for longer transmission circuits has been facilitated by the use of a one-year heat cycle voltage test called Prequalification (PQ) test, which was recommended by Cigré in 1993 and afterwards specified in IEC 62067 Ed.1 in 2001. Following the successful completion of a number of PQ programmes, some large 400 and 500 kV cable circuits have been installed and commissioned. There is still limited experience with EHV XLPE cable systems. The designs, manufacturing methods and materials employed in joints and terminations differ significantly amongst manufacturers. Thus the service experience from any particular system cannot necessarily be taken as a guide to the likely service experience of other systems. The long term behaviour of an EHV system has to be demonstrated by a well specified PQ test.

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Section 4.2 covers long duration tests on EHV cable systems and the different features, which are examined: • • • • •

Design concept Electrical performance of cable and accessories Performance of a cable system under prolonged heat cycling Aspects of installation design and practice Ability of the Installer to joint in realistic conditions rather than laboratory conditions.

From the existing service experience in both long duration tests and in operation as mentioned in Sect. 4.1, it is confirmed that a Prequalification test (PQ test) is still necessary to demonstrate the long-term reliability. Improvements of this test are proposed such as measurement of partial discharges as a mean to provide early warning and offer possibility of repair before failure. A procedure to be adopted in case of failure of a component during the PQ test is introduced and a modification to the final impulse test is proposed. Then changes in an already prequalified cable system are evaluated. A procedure of extension of qualification is recommended and a table is given to indicate in main cases of changes the test sequence to adopt, instead of repeating the complete PQ test. A new test called Extension of Qualification Test (EQ test) is proposed mainly in case of changes of or in accessories. This test shall be performed in a laboratory on one or more samples of complete cable of the already prequalified cable system. At least two accessories of each type that need the extension of qualification shall be tested. A total of 80 heating cycles shall be carried out of which the last 20 cycles shall be under a voltage of 2 U0. As a summary and conclusion from its reflections WG B1.06 makes the following recommendations to IEC for further consideration in future editions of IEC 62067: • To maintain a Prequalification (PQ) test for the basic qualification of a new cable system. • To allow in case of a failure of an accessory the continuation and completion of the PQ test for the undisturbed components of the loop. • To introduce in case of less significant changes/modifications at prequalified components a simplified long-term test (80 cycles) called “Extension of Prequalification (EQ) test”. • To perform the lightning impulse test at the end of the PQ test at the complete test loop or, in case of practical problems with test equipment, in any other test arrangements, which include the accessories. • To include sample tests at accessories in IEC 62067 Ed.1 as in IEC 60840 Ed.3. These tests are intended to check not only the intrinsic quality of the accessory, but also the quality of the installation, which is critical at the EHV level. Section 4.3 similarly covers long duration tests on HV cable systems. Due to experience on EHV cable systems it becomes more common nowadays to produce cables with reduced insulation thickness at the high voltage level. This leads

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to higher dielectric stresses nearly as high as in the EHV field not only at main insulation but also at the interfaces between cables and accessories. In the meantime, new types of accessories are appearing on the market, of course with no earlier experience. These accessories should be able to fit to the older types of cables with thicker insulation and the newer types of cables with reduced insulation (changes of an existing HV link with a new cable type or repair of an older link). Taking into account that service experience collected so far on HV cable systems working at usual stresses was rather good, the Working Group recommend that cable systems should be considered rather than cables or accessories alone when higher stresses are adopted. After giving detailed examples of calculated stresses (AC and impulse) in different types of accessories, the WG recommends to adopt a prequalification procedure when electrical stresses are above given limits. A Prequalification (PQ) test shall be performed only on cable systems where the calculated nominal electrical stresses at the conductor screen will be higher than 8 kV/mm and/or at the insulation screen higher than 4 kV/mm. This Prequalification test can be omitted in some special cases listed in Sect. 4.3. Contrary to the Prequalification test for EHV systems, in this case the test is simplified because it can be performed in a laboratory and 180 cycles are required. The proposed layout of cable system is described as well as the test sequence. Then changes in a prequalified cable system are addressed. The Extension of Prequalification test (EQ) is proposed to be the same as for EHV systems. As a summary and conclusion from its reflections WG B1.06 makes the following recommendations to IEC for further consideration in future editions of IEC 60840: • To introduce a Prequalification (PQ) test for those HV cable systems where the calculated nominal electrical stress at the conductor screen will be higher than 8 kV/mm and/or at the insulation screen higher than 4 kV/mm. This test needs not to be performed if – Cable systems with the same constructions and accessories of the same family have been prequalified for higher rated voltages – Equivalent long term tests have been already successfully carried out – Good service experience at cable systems with equal or higher stresses can be demonstrated • To allow in case of a failure of an accessory the continuation and the completion of the PQ test for the undisturbed components of the test loop. • To introduce in case of less significant changes/modifications at prequalified components a simplified long-term test (80 cycles) called “Extension of Prequalification (EQ) test”. • To perform the lightning impulse test at the end of the PQ test at the complete test loop or, in case of practical problems with test equipment, in any other test arrangements, which include the accessories.

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Section 4.4 lists the conclusions of the Working Group. Main ones are: • EHV Cable systems: there is not sufficient service experience on EHV cable systems collected so far to introduce major changes to the existing initial Prequalification test. This PQ test has to be repeated in case of extension of the range of approval. Within the range of approval, a new test called Extension of Qualification test is proposed to control changes in already prequalified cable systems instead of repeating the complete PQ test. This new test can be carried out on a laboratory loop and will comprise 80 heating cycles combined with voltage application at 2 U0 for the last 20 cycles. • HV Cable systems: a Prequalification test is recommended for design stresses above 8 kV/mm on the conductor or 4 kV/mm over insulation. This test can be carried out on a laboratory loop and will comprise 180 heating cycles combined with voltage application at 1.7 U0. This PQ test has to be repeated in case of extension of the range of approval. Section 4.5 contains all the Annexes introduced in previous sections Annex 4.5.1: Terms of Reference Annex 4.5.2: Sensitivity to PD In this Annex the sensitivity of partial discharges in XLPE cable insulations to change of electrical stresses is investigated. The conclusion is that dimensional changes, especially reduction of insulation wall thickness, can result in considerable higher stresses at defects such as voids or fissures, thus increasing the risk of inception of partial discharges. Annex 4.5.3: Functional analysis • A “Functional Analysis Method” is recommended as means for a systematic assessment of the significance of changes/modifications at components of a cable system and thus for the choice of the appropriate test (e.g. PQ or EQ). • Based on the application of the “Functional Analysis Method” to the most important components of actual cable systems, guides to test procedures are given in case of – Exchange of a cable and/or accessory in a prequalified cable system – Modification of a cable in a prequalified cable system – Modification of an accessory within the same family in a prequalified cable system Annex 4.5.4: Tests missing in IEC As a result of the functional analysis exercise (see Annex 4.5.3), a number of tests that are not included in IEC 60840 Ed.3and IEC 62067 Ed.1 have been identified. These tests are generally performed as development tests and are summarized in Annex 4.5.4 for future consideration by IEC. Annex 4.5.5: References.

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4.1

Introduction

4.1.1

General

Extra high voltage (EHV) cable systems are designed and built to transmit bulk electrical power. For this reason, it is imperative that they attain the highest possible reliability. At the same time, the transmission capacity of the high voltage (HV) cable links is continuously increasing. This is why reliability considerations of cable installations in this category are also becoming very important. Accessories are an integral part of a cable system. Their performance together with that of the cable determines the overall reliability of the circuit. Accessories are installed by hand, and therefore their reliability is determined by a combination of good design, clear instructions, quality assurance (QA) and the skill of the fitters. The long-term Prequalification (PQ) test was developed to build confidence in the operation of XLPE cable at EHV levels. A PQ test demonstrates the quality of the overall design of the system together with the quality of assembly. IEC test requirements have evolved over the years from the component based approach in IEC 840 to the system based approach, where accessories are considered together with the cable, in IEC 62067 Ed.1 and the most recent edition of IEC 60840 Ed.3. The IEC has published series of test specifications for HV and EHV cables, accessories and cable systems: • In 1988, the first specification was published. IEC 840 (renamed later as IEC 60840) is for cables up to 150 kV (Um ¼ 170 kV) [1]. In this specification, type tests, routine and sample tests were prescribed for cables only. • In 1999 IEC revised this specification and IEC 60840 Ed.2 was published, in which accessories were included in type testing [2]. • In 2004 IEC published a third edition, IEC 60840 Ed.3, in which type tests on cable system and routine and sample tests on prefabricated accessories were introduced [2]. • In December 1993 Cigré Working Group 21.03 published in Electra recommendations for PQ tests, type tests, sample and routine tests for extruded cables and their accessories for voltages above 150 kV (Um ¼ 170 kV) up to 400 kV (Um ¼ 420 kV) [3, 4]. In 1997, the voltage range was extended to 500 kV (Um ¼ 525 kV) [5, 6]. In these recommendations, type tests, routine and sample test procedures were based on those from IEC 840. Long term AC stressing together with heat cycles up to the maximum operating temperature followed by impulse tests were prescribed to demonstrate the long-term electrical and thermo-mechanical performance of the system (PQ test). It was considered to be imperative that the test set-up reflected real installation conditions. Test specifications recommended by WG 21.03 were then implemented in IEC 62067 Ed.1 [7] published in October 2001. The PQ tests recommended by Cigré and incorporated into IEC 62067 Ed.1 have been accepted worldwide. As a result, only those manufacturers whose products have passed long-term PQ tests have been allowed to participate in subsequent EHV cable projects.

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4.1.2

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Scope and Terms of Reference of WG B1.06

It is inevitable that over a period of time an approved cable system undergoes some changes, such as modifications to the cable construction, higher stress, new type of accessories, new manufacturing processes, etc. However, there is little incentive to the manufacturer to make incremental improvements to the product, since these might invalidate the previous “approval” and require the long and expensive PQ test to be repeated. Reducing the amount of testing needed would encourage manufacturers to introduce design improvements or measures to reduce cost. For this reason, Cigré Study Committee B1 launched Working Group WG B1.06 with the task of revising the qualification procedures for underground high voltage cable systems. The WG was asked to examine how it might be possible to qualify a modification to a cable system without making the full set of tests which are presently recommended or specified in standards. All tests, PQ and type tests were to be reviewed, although the PQ test has received greatest attention, as it is the most costly and the longest. The full scope and terms of reference of the WG are given in Annex 4.5.1.

4.1.3

Experience

4.1.3.1 Ageing of Extruded Polymeric Insulation The ageing behaviour of extruded insulation of cables under dielectric and/or thermal stress has been studied extensively [8–16]. No significant ageing could be detected, even with the most sophisticated test methods presently available [15]. Locally there may be weak points in the insulation: impurities or voids in the insulation or protrusions at the interface with the semi-conductive screens. These defects may initiate ageing or accelerate degradation of the cable insulation. In modern cable factories great care is taken to avoid these defects: extrusion of extremely clean insulation materials, extrusion of very smooth semi-conductive materials, careful handling of insulation and semi-conductive materials, etc. Before leaving the factory, the finished cable is submitted to a routine electrical withstand test together with measurement of partial discharges. The effectiveness of these routine tests on HV cables is good as there are very few cable breakdowns on installed systems. Other ageing factors (i.e. factors that may affect the capability of components to fulfil their roles) are described in the 1992 report from WG 21.09 published in ELECTRA 140, “Considerations of ageing factors in extruded insulation and accessories” [17]. The main “ageing” factors considered in this report are the locked-in mechanical stresses and shrinkage of cable insulation or outer sheath, maximum operating or overload temperatures affecting radial expansion and possible permanent deformation and reducing AC and impulse break down strength, deformation under temperature and externally applied mechanical stress, etc. The conclusion of this report is: “the extruded polymeric insulants used in high voltage cables do not appear to exhibit property changes that can be measured easily or that can be said to be significant in terms of cable life reduction when

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contaminants from external sources, e.g. water, oil and sulphur are avoided. For accessories the same may be stated.” In order to evaluate, to some extent, the electrical ageing aspects of a cable system, a type test is prescribed in IEC 60840 Ed.3. In IEC 62067 Ed.1, the PQ test is supposed to evaluate the long-term electrical, thermal and mechanical behaviour of the cable system in an environment near to the conditions in the field.

4.1.3.2 Experience with HV Extruded Cable Systems up to and Including 150 kV The evolution of XLPE MV and HV systems commenced in the 1960s. In the 1970s, the first commercial 90-132-154 kV XLPE systems were installed in Europe and in Japan. The results of a survey made by Cigré WG 21.09 on cable, associated accessories and service stresses and lengths installed in different countries were published in ELECTRA 139 in 1991 [18]. Cigré WG 21.10 has published in Electra 137 [19] a survey on the service performance of HV AC cable systems. The failure rate of extruded cable systems was very low (0.1 failures per 100 circuit km per year on cables and accessories external failures were not included). A new Working Group (WG B1.10) was set up in 2004 to update the service experience data. Results are published in Cigré TB 579, April 2009. There are a number of designs of joints and terminations currently in use and an industry standard has not yet evolved. WG 21.06 has described and illustrated the different types of accessories in use [20], ▶ Chap. 1, “Compendium of Accessory Types Used for AC HV Extruded Cables” of this book. At voltage up to and including 150 kV extruded insulation has largely superseded paper-insulated cables for new installations. Much of the “good” experience with HV XLPE cable systems is based on older cable with moderate design stresses. A new generation of “slim-design” HV cables is being developed, with similar technology and design stresses to those seen in EHV XLPE systems. Hence historical service experience with HV cable systems is not necessarily a good guide to the likely future service experience of these novel systems. 4.1.3.3 Experience with EHV Extruded Cable Systems at Voltages above 150 kV Whilst cable with extruded insulation is in general use for electricity distribution and at the lower transmission voltages, XLPE has only recently become the insulation of choice for many utilities for EHV transmission circuits. The introduction of XLPE for longer transmission circuits has been facilitated by the use of the PQ test. 4.1.3.3.1 Prequalification Test Experience As cable makers started to develop EHV XLPE cable systems, they needed testing programmes both to monitor their own progress and to give customers confidence in the products being developed. Initially, these testing programmes were agreed on a local or national basis. For example, France used a 250-cycles test for 6000 hours at

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pffiffiffi 3 U0, while Belgium adopted a 100-cycles test at 2 U0. Japan used a half-year test at relatively low electrical stress based on the degradation factor of the insulation system. Plans to install major 400 kV cable systems led Cigré to set up a working Group to consider an international test specification. The tests were developed to give confidence that cable systems passing the tests would have a fault rate in service lower than 0.2 faults/100 km/year. In 1993 Cigré WG 21.03 published a test program for cable systems above 150 (170) kV [3, 4] and IEC published a specification based on these documents IEC 62067 Ed.1 in 2001 [7]. In IEC 62067 Ed.1 the definition of the PQ test is as follows: “a test made before supplying on a general commercial basis a type of cable system covered by this standard, in order to demonstrate satisfactory long term performance of the complete cable system. The PQ test need only be carried out once unless there is a substantial change in the cable system with respect to material, manufacturing process, design and design levels”. It is useful to examine some of the early French experience of prequalification testing. The French (EDF) specification required a long-term test of duration 6000 hours, p although many of the tests were continued beyond this. The specified ffiffiffi voltage was 3 U0 for 250 heat cycles (167 cycles at maximum service temperature and 83 cycles at emergency temperature). Figure 4.1 summarizes long-term test results on 220 kV cable at the EDF laboratories. Design stresses for the cables (at U0) were 8.5 kV/mm at the conductor screen and 4.2 kV/mm at the insulation screen.

12

10

Number of failures

8

5

4

2

0

1200 1450 2500 3000 4000 5000 6000 7000 11000 12000 14000 16000 30000 30000 Time (hours)

Fig. 4.1 Results of long-term tests on 220 kV cable

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The defects causing breakdown in less than 10 hours were mounting errors in accessories (5 in joints and 2 in terminations). Eight of the test lengths that failed prematurely contained artificial defects in their terminations. These tests were to simulate a defect found in the field. They showed that this type of PQ test is effective in distinguishing between defective and well-made accessories. The tests are all from the early stages of development of 220 kV cable systems (pre 1980). Only 2 breakdowns occurred on cables themselves (150 h and 36000 h). All the other breakdowns were in the accessories. This indicates the important role of accessories in determining the overall reliability of the cable system and the importance of carrying out tests on the cable and accessories as a system. Some of the tests carried out by connecting the cable between phase and earth of the 400 kV network highlighted the problems associated with this approach. Four of breakdowns occurring after about 6000 hours happened during a thunderstorm with lightning strikes falling on the adjacent overhead line. The defects responsible for the cable failures could not be determined because the cables had experienced the full short circuit current of the 400 kV network and suffered significant local damage at the failure site. The use of a dedicated test transformer provides far better control of the test voltage (avoiding system disturbances) and limiting the short circuit current allows better forensic examination of the failure site. The occurrences of failures over a wide range of times (up to 16000 hours) suggests that it is not advisable to reduce the duration of the 8760 hours PQ test. In 2001 Parpal [22] summarized the early experience from a number of PQ tests [23–30]. Subsequently, most of the major EHV cable makers have successfully completed PQ tests on 400 or 500 kV XLPE cables often with large conductors (2000 and 2500 mm2).

4.1.3.3.2 Service Experience The evolution of EHV systems followed with the first EHV XLPE systems being installed in the voltage range 220–275 kV in the late 1970s. Widespread commercial use of XLPE cables up to 230 kV was not seen until the 1980s. The first 275 kV XLPE systems with joints were installed in Japan in 1989 [21]. These were qualified using Japanese utility specifications. In France cables insulated with low-density polyethylene (LDPE) preceded XLPE and were first installed at 225 kV in 1969. Since then more than 1000 km of LDPE cable has been installed with field-moulded joints and around 600 km of high-density polyethylene (HDPE) cables with good service experience (Fig. 4.2). The first 400 kV LDPE cables were installed in France in 1985 and XLPE in 1999. In total, 40 km of cable and 21 back-to-back joints have been installed. The world’s first 500 kV XLPE system was commissioned in Japan in 1988 and two subsequent circuits were commissioned in 1988 and 1991. These were relatively short circuits without joints. Following the successful completion of a number of PQ programmes, some large 400 and 500 kV cable circuits have been installed and commissioned. These are summarized in Table 4.1.

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Fig. 4.2 345 kV Cables installed in a long Tunnel in Korea

Although the service experience is limited, all the EHV installed systems subjected to the IEC 62067 Ed.1 PQ procedures have, to date, demonstrated satisfactory behaviour. A more precise picture about the service experience of these cable systems became available when WG B1.10 “Update of service experience on underground and submarine cables” concluded his task in 2009. See Cigré TB 379 [19 bis].

4.2

Long Duration Test on EHV Cable Systems (170 < Um < 550 kV)

4.2.1

General

The prequalification (PQ) test was introduced to compensate for the lack of service experience with XLPE insulated cables above 150 kV (Um ¼ 170 kV). The PQ test checks the performance of the cable/accessory system under realistic conditions. Features examined include: • Design concept (for example, some early PQ tests at CESI Laboratory showed that a number of taped joints designs did not perform well in long-term tests). • Electrical long-term performance of accessories and cable. • Performance of the cable system under prolonged heat cycling (e.g. thermalmechanical aspects, shrinkage). The 20 thermal cycles specified in the type test are not sufficient to test for the effect of cable insulation shrink-back within the assembled accessories. • Aspects of installation design and practice. For example, PQ tests have shown that insufficient attention was paid by some installers to arrangements for clamping the cable adjacent to the joint.

Country Denmark (Copenhagen: Southern cable route) [54] Denmark (Copenhagen: Northern cable route Germany (Berlin/ BEWAG MitteFriedrichshain) Germany (Berlin/ BEWAG FriedrichshainMarzahn) Japan (Tokyo)(3) [55]

Type of joints(1) CPFJ

PMJ

CPFJ+ PMJ

CPFJ+ PMJ

EMJ

Rated (ϕ–ϕ) voltage (kV) 400

400

400

400

500

264

30

48

42

Number of joints 72

0/12

0/12 (double systems)

0/12 (double systems)

3/3

Number of outdoor/SF6 terminations 3/3

T

T

T

DB

Type of installation(2) DB

39.8

5.5

6.3

12

Route length (km) 22

2

2

2

1

Number of circuits 1

2000

2000

2500 Cu/2400 (4)

1998

1999

Commissioning year 1997

1600 Cu/1100

1600 Cu/1100

1600 Cu/800

Conductor crosssection/ Transmission capacity in Winter Mm2/(MVA) 1600 Cu/975

Table 4.1 Major XLPE cable systems at 400 kV and above. (Data supplied by Cigré WG B1.07, March 2006-TB 338)

108 J. Becker

CPFJ +PMJ PMJ CPFJ

PMJ

PMJ

PMJ

400

400

380

380

400 400

PMJ

400

66

30

3

96 60

96

12

12/0

6/6

6/0

36/0 0/6

12/0

12/12

DB&T&M

DB&D

DB&D T

T

D&M

8.4

5.2

2.25

14.5 20

12.8

1.3(5)

CPFJ ¼ Composite prefabricated joint, PMJ ¼ premoulded joint and EMJ ¼ extruded moulded joint (2) T ¼ tunnel, DB ¼ directly buried, D ¼ ducts, D&M ¼ ducts and manhole (3) Cable system prequalified following Japanese Specifications [48] (4) 1200 MVA/circuit with forced cooling in the future. 900 MVA/circuit now (5) 15 core kms/4 circuits X3 phases ¼ 1.3 km

Denmark (Jutland) United kingdom (London) The Netherlands (Rotterdam) Austria (Wienstrom) Italy (Milan)

United Arab Emirates (Abu Dhabi) Spain (Madrid)

1200 Cu/1400 2000 Cu/2100

2

1600 CU/1000

1200 Al/1200 2500 Cu/1600

2500 Cu/1720

800 Cu/not available

2

1

2 1

2

4

2006

2005

2005

2004 2005

2004

2000

4 Qualification Procedures for HV and EHV AC Extruded Underground Cable. . . 109

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J. Becker

• The ability of the installer to mount accessories under realistic conditions rather than in the test laboratory. In some cases, this has highlighted the need for improvements in jointer training. Although not every aspect of the work is tested, the PQ process gives a good indication of the overall competence of the supplier of the cable system. Although some manufacturers have learnt rapidly from problems in the early PQ tests, this knowledge has not been widely shared. Accessories (and sometimes cables) are still experiencing problems during PQ and type testing and sometimes also in service. The WG has the opinion that the state of the art of XLPE cable technology is not sufficiently advanced that the competence of every manufacturer can be assumed without evidence. Until such time that a significant body of service experience has built up, the WG feels that a PQ test is still necessary.

4.2.2

Revision of the Present Prequalification Test Procedure

The prequalification test procedure detailed in IEC 62067 Ed.1 has been reviewed by the WG. The main items addressed in order to look for a possible simplification/optimization were: • The range of Type and PQ approval in relation with the calculated nominal electrical stresses • The duration of the heating cycle voltage test • The procedure in case of a component failure during the test • The voltage control at the end of the test. Regarding a possible widening of the present range of type and PQ approval WG looked very carefully to the sensitivity to change of electrical stress in relation with the generation of partial discharges. It is recommended to keep the relevant sub-clauses in IEC 62067 Ed.1 and in IEC 60840 Ed.3 as they are, see Annex 4.5.2.

4.2.2.1 Duration of the Heating Cycle Voltage Test There is no clear evidence for recommending that the number of 180 heat cycles, presently specified, should be reduced. It is well known that, if the heating cycles were not correct in terms of temperature drop, the thermo-mechanical behaviour of the cable system would not be checked adequately. There is, also, no possibility to reduce the overall length of the test to 180 days, because a daily heat cycle is not possible in many practical conditions. In fact, for cables installed in air or for buried smaller conductor cables with one day cycle it will be possible to reach, at the end of the cooling period, a conductor temperature near to ambient temperature. When testing a large conductor cable buried in the ground it is not always possible to achieve a correct cooling temperature in 16 hours, due to the long thermal time constant of the cable and/or its

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surrounding. Thus the actual duration of the test could vary from 6 months for a cable system installed in a tunnel to one year for large conductor cables installed in the ground. In order to avoid a significant difference in duration of the prequalification test as a function of the cable construction and installation conditions, WG B1.06 suggests for the heating cycle voltage test of the prequalification test the duration of one year. In addition, the practical experience detailed at Sect. 4.1.3.3 has shown that the duration of the voltage application is an important factor, because failures in accessories during PQ testing have occurred throughout the one-year test period [22, 23, 28, 29, 49, 50, 52]. This is another reason why the one-year heat cycle test with AC voltage on the system is needed to check the long-term electrical behaviour of the system under test. Because of these two reasons, it is recommended to maintain the present one-year duration (8760 hours). Where partial discharge (PD) tests have been performed at regular intervals during the long-term test, it was noted that PD activity could initiate at any moment during the test. For this reason it is recommended to perform partial discharge measurements on the test assembly to provide an early warning of possible degradation and to enable the possibility of a repair before failure. The WG considers that it is not easy to specify a PD test on the whole loop as compulsory, because it can be difficult to achieve an adequate sensitivity when carrying out a PD test at an unscreened location. This makes it currently difficult to define a level of background noise that can be achieved in practice.

4.2.2.2 Procedure in Case of a System Component (Cable and/or Accessory) Failure during the Test The present IEC 62067 Ed.1 standard does not allow any failure during the heating cycle voltage test. However, taking into account that this test is very onerous, the WG considers that replacement/repair of an accessory failed during the test should be allowed and the test continued because the replaced/repaired accessory cannot influence the behavior of the other ones. On the contrary it is not allowed to repair a cable failure, unless it is caused by an accidental external damage or the same cable has already been prequalified. At the end of the 180 loading cycles/one year test and impulse test only the successfully tested accessories will be prequalified, while the accessory/ies subjected to repair or replacement will not be prequalified. However, it is in the option of the cable/accessory manufacturer to continue the test on the replaced/repaired accessory/ies until it/they complete the 180 loading cycles/one year test and impulse test. In this case also these accessories are prequalified. In the event that during this test another component, fully prequalified (including the final impulse test), fails, it is possible to repair it and continue the test until the second run is completed on the replaced accessory/ies.

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The failure of the component already prequalified (which in this case acts as a laboratory test component) does not violate its prequalification and should not be mentioned in the test report.

4.2.2.3 Final Control Test A lightning impulse withstand test is an effective way of demonstrating that interfacial pressure within accessories has not relaxed during the PQ test. Also it enables to check that no irregularity in the semi-conducting screen of the cable has been generated due to thermo-mechanical forces at for instance bends. As such, the impulse test on the complete test loop should form an important part of the PQ test sequence. At present, IEC 62067 Ed.1 requires a hot lightning impulse test on one or several pieces of cable cut from the PQ test loop. As an alternative to this, a test on the whole loop is permissible. The WG has the opinion that after long-term testing, a hot lightning impulse test on the whole test loop is desirable as a check on the insulation properties at the interfaces, in the accessories and in the cable. In fact a check on the insulation properties of the cable only is not sufficient. The WG recommends that IEC 62067 should require a test on the whole loop. Only in case suitable impulse test equipment is not available at the test site, the impulse test can be carried out in any other test arrangement on cable sections, which include the accessories from the test loop, again as a check on the insulation properties at the interfaces, in the accessories and in the cable (1).

4.2.3

Changes in a Prequalified Cable System

4.2.3.1 Evaluation of Changes in a Prequalified System According to IEC 62067 Ed. 1 the PQ test need only be repeated if there is “a substantial change in the cable system with respect to material, manufacturing process, design and design levels”. A substantial change is defined as one, which might adversely affect the performance of the cable system and puts the responsibility on the supplier to make the detailed case that a change is not substantial. There is however no internationally agreed method by which the supplier and purchaser can evaluate the evidence and come to a decision as to whether the PQ test or other tests need to be repeated or not. In order to cover this matter and to evaluate what changes must be considered as “substantial”, the WG has adopted the Functional Analysis Method described in Annex 4.5.3. This method correlates the function performed by a certain item to the tests required to check that function. As a result a number of changes to the components of a cable system that require the repetition of the long-term test were identified. 1()

Note: If the energy of the impulse generator is not sufficient to test the whole cable length, the test loop could be cut, without moving the accessories, into appropriate sections, which then are available to be tested with auxiliary terminations.

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However it was realized that some of these changes do not require the repetition of a full Prequalification test, but a simplified long-term test called “Extension of Prequalification (EQ) test” could be adopted (see Sects 4.2.3.2 and 4.2.3.3). The Working Group considers that allowing a shorter laboratory based long-term test rather than requiring the cable/accessory maker to repeat the full PQ test, will encourage incremental improvements in technology, whilst reducing the risk to the customer. The main situations of changes that can be found in practice are: • The exchange of components (cable or accessories) already prequalified in systems from different manufacturers and/or from different plants of the same manufacturer. • The modification to cable components in a prequalified system • The modification to accessory components in a prequalified system. As far as the accessories are concerned, it is important to observe that accessories of different manufacturers may significantly differ in design, material and construction. For instance a premoulded joint and a taped joint are substantially different and require a specific Prequalification test (if not already prequalified). For this reason the concept of accessory families for each type of accessory, i.e. joints, metal enclosed terminations and outdoor/indoor terminations has been introduced. The names of the accessory families are defined in ▶ Chap. 1, “Compendium of Accessory Types Used for AC HV Extruded Cables” (Cigré TB 89 [20]) and are summarized in Table 4.2. The three main types of changes that can occur in a prequalified system and the relevant type of qualification test required in each case are discussed below. 4.2.3.1.1 Exchange of Cable and/or Accessory in a Prequalified Cable System In Table 4.3 a guide to the selection of test procedures is given for the extension of the prequalification of a prequalified cable system in case of an exchange of cable and/or an accessory by another cable and/or accessory (from the same family or from another family), see Table 4.2. The selected procedure depends on the calculated electrical stresses at the insulation screen of the other cable and/or accessory with respect to the calculated electrical stresses at the insulation screen of the originally prequalified cable system and on the prequalification of the cable system containing that other cable and/or accessory. 4.2.3.1.2 Modification to the Cable in a Prequalified Cable System In Table 4.4 a guide to the selection of test procedures is presented for the extension of the prequalification tests or of the type tests in case of a modification to the cable in a prequalified cable system. The selected test procedure depends on the type of modification of the relevant cable component. As far as type tests are concerned, in some cases only the relevant clauses of IEC 62067 Ed.1 type test procedure covering the function of the specific item changed are considered. The origin of the type of modification may be a change in material, manufacturing process, design or design level, as indicated for information in Table 4.4.

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Table 4.2 Accessory family definitions Accessory families 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 2 2.1 2.2 2.3 2.4 2.5 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7

Joints Taped joints Pre-moulded joints Composite joints Field moulded joints Heat shrink sleeve joints Back to back joints Transition joints Branch joints Metal enclosed terminations families (SF6 and oil immersed) Stress cone and insulator type Deflector and insulator type Prefabricated composite dry type Capacitor cone and insulator type Directly immersed termination type Indoor and outdoor terminations Prefabricated elastomeric sheds and stress cone type Heat shrink sleeve type Elastomeric sleeve type Stress cone and insulator type Deflector and insulator type Capacitor cone and insulator type Prefabricated composite and capacitor cone and insulator type

Table 4.3 Test procedures in case of an exchange of a cable and/or accessory in a prequalified cable system Cable and/or accessory cable Joint Metal enclosed Termination Outdoor Termination

Already qualified on another cable system within the same or higher insulation screen stress 12.5 + XX2) (non electrical TT + EQ1) XX2) (EQ) XX2)

Already qualified on another cable system with a lower insulation screen stress or not qualified 12 + 13.2 (electrical and non electrical TT + PQ) 12 + 13.2 (TT + PQ) 12 + 13.2

(EQ) XX2) (EQ)

(TT + PQ) 12 + 13.2 (TT + PQ)

The numbers given refer to the respective clauses in IEC 62067 Ed.1 EQ consists of the bending test, 60 heat cycles without voltage and the electrical type tests 2) (XX) Clause to be added in the standard 1)

Cable Insulation

Cable semi-conductive inner and/or outer screen

Component Cable Conductor

Type of modification Larger cross-section Copper to Aluminium Insulated wires (enamelled or oxidized...) Stranded to solid conductor Water tightness Change of origin (supplier or production plant) Transfer extrusion line (see cable insulation) Different quality of semicon Change of base resin Change in cross linking package (peroxide/ antioxidant) Nature of polymer (XLPE, LDPE, HDPE, EPR) Higher conductor stress, no increase of insulation screen stress Increase of insulation screen stress

Modification

✓

✓ ✓ ✓ ✓ ✓ ✓

✓ ✓

M

✓

P ✓ ✓ ✓ ✓ ✓ ✓ ✓

✓

✓

✓

D ✓

Table 4.4 Guide to the selection of tests because of modifications to a cable in a prequalified EHV cable system

12 12 12

✓ ✓

DL ✓

–

– –

– – –

EQtest – – (xx) (xx) –

(continued)

13.2

13.2 –

IEC 62067 Ed.1 Clause number PQT-test test 12 13.21) 12 – 12.5 – 12.5 – 12.5.14 – 123) 2) 2) 123) – 123) – 12 –

4 Qualification Procedures for HV and EHV AC Extruded Underground Cable. . . 115

Different types of metal screen Different type of materials Different processes Change in bonding material and/or process to metal screen

Type of modification New extrusion line or transfer of extrusion line with earlier experience in-house New extrusion line, or transfer of extrusion line without earlier experience Change of laying, material, thickness

Modification

v ✓ ✓ ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓

P ✓

M ✓

✓

✓

D

DL

12.5.4 12.5.14 (if required) 12.4.4 + 12.4.10 12.54) 12.54) 12.54)

12

– – – –

–

13.2

IEC 62067 Ed.1 Clause number PQT-test test 12 –

(XX) Clause to be added in the standard Remark: in type test, only the relevant clauses are applicable  M: change in material; P: change in manufacturing process; D: change in design (construction); DL: change in electrical design stress level 1) If higher calculated dielectric stresses at the insulation screen, clause 13.1 2) Same as for insulation 3) If outside Range of Type Approval, clause 12.2 4) As appropriate to outer sheath materials

Cable Metallic screen Cable Outer sheath

Cable Bedding (layer over extruded semicon screen)

Component

Table 4.4 (continued)

– – – –

–

EQtest

116 J. Becker

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4.2.3.1.3

Modification to an Accessory within the Same Family in a Prequalified Cable System In Table 4.5 a guide to the selection of test procedures is presented for the extension of the Prequalification tests or of the Type tests in case of a modification to an accessory (joint and/or termination), within the same accessory family (see Table 4.2) in a prequalified cable system. Table 4.5 Guide to the selection of tests because of modifications to an accessory within the same family in a prequalified EHV cable system IEC 62067 Ed.1 Clause number PQT-test Test – –

EQtest (xx)

–

–

(xx)

–

13.2

–

–

–

(xx)

✓

–

–

(xx)

✓

Annex D Annex D

–

–

–

–

–

–

(xx)1)

–

–

(xx)1)

–

–

(xx)1)

Modification Component Joints

Terminations: - outdoor - indoor - metal enclosed + SF6 + oil-immersed

Type of modification Higher calculated electrical stress design and construction Compound of main insulation body (same base resin) Changing nature of polymer, (EPR, Silicone....) Material of semi-con electrodes Fixation of cable ends on either side of the joint Screen interruption Outer screen and Protection design, (Filling/water tightness), Outlet of bonding leads Higher electrical stress design of stress cone (or smaller metal clad for GIS or transformer terminations) Change in nature of Filling medium (e.g. oil to gas....) Change in the formulation of the stress cone

M

P

✓

✓

✓

✓

D

DL ✓

✓

✓

✓

✓ ✓

✓

(continued)

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J. Becker

Table 4.5 (continued)

Modification Component

Type of modification compound but with the same base polymer Change of the base polymer (EPR, Silicone, . . .) of the stress cone compound Change of insulator material for indoor or outdoor terminations. Change of insulator design or manufacturer of GIS/Transformer insulator

M

P

D

✓ 2)

✓ 2)

✓

✓ 2)

✓ 2) ✓

DL

IEC 62067 Ed.1 Clause number PQT-test Test

EQtest

–

–

(xx)

12

–

–

12

–

–

(xx) Clause to be added in the standard M: change in material; P: change in manufacturing process; D: change in design (construction); DL: change in electrical design stress level 1) When can be demonstrated that the thermo mechanical aspects have no significant influence on the performances of the termination a Type Test may be sufficient 2) In case of elastomeric insulators (“silicone” or “EPR”) climatic and pollution test according to IEC 61109 Annex C should be considered 

The selected test procedure depends on the type of modification of the relevant accessory. The origin of the type of modification may be a change in material, manufacturing process, design or design level, as indicated for information in Table 4.5.

4.2.3.2 Basic Principles of the Extension of Prequalification (EQ) Test The Extension of Prequalification consists of a period of 60 days of thermal pre-conditioning without applied voltage, carried out in laboratory conditions, followed by the electrical part of the type test. The 60 daily heat cycles without voltage plus the 20 heat cycles of the type test applied to the test loop are intended to allow relaxation of most of the mechanical stresses trapped in the cable insulation during manufacture. This relaxation results in retraction of the XLPE insulation wherever the cable is cut. The retraction of the cable insulation within an accessory, if not provided with a specific anti-retraction device, can initiate partial discharge activity, leading to failure

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Qualification Procedures for HV and EHV AC Extruded Underground Cable. . . 35.0

Retraction of oversheath

30.0

Retraction of insulation

25.0 Retraction (mm)

119

20.0 15.0 10.0 Evelution of the retraction

5.0 0.0 0

10

20

30

40

50

60

70

80

-5.0 Number of heat cycles

Fig. 4.3 Retraction test on a 5-meter long 1000 mm2 500 kV cable with XLPE insulation and PE sheath

of the accessory. In fact the considerations made at paragraph Sect. 4.1.3.1 and the TB “Interfaces in accessories for extruded HV and EHV cables” [53], indicate that thermal cycles stressing is the main failure mechanism in accessories, which typically include due to their intrinsic construction a number of interfaces. As shown by Fig. 4.3 the retraction of the XLPE insulation is practically completed after about 60–80 heat cycles. In addition the heat cycles are producing a radial expansion and retraction of the cable insulation and of the accessory components that can also influence the interfacial pressure in accessories. In order to be able to perform the EQ test in a laboratory, limiting the engagement of the HV test equipment, it has been decided to perform these pre-conditioning cycles without applied voltage. Being the EQ test performed in a laboratory, i.e. with well-defined thermal conditions, it is possible to carry out the heat cycles in approximately one day also with very large cross-section cables. To prove that the heat cycling has not affected the integrity of the test loop a complete electrical type test (as defined in IEC62067 Ed.1) is used. As the installation design conditions can significantly affect the thermo-mechanical behaviour of the accessories, the test arrangement shall take this into account. For instance a rigid installation can be simulated by suitably cleating the cable at each side of a joint. In order to comply with this requirement a minimum length of 10 m of free cable between accessories is specified for the EQ test. To maintain a similar degree of risk for the shorter duration of the EQ test compared to the duration of Prequalification test, at least two accessories of the same type, instead of one, shall be included in the test loop submitted to the extension of prequalification.

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Fig. 4.4 Extension of Prequalification test loop for a joint intended for flexible and rigid installations

Rigid installation

Flexible installation

>10 m

4.2.3.3 Procedure of the Extension of Prequalification Test The sequence of tests for the extension of prequalification is summarized below. The numbers in brackets refer to clause numbers of present tests in IEC 62067 Ed.1. • Check of the insulation thickness of cable for electrical type test to determine the test voltage values (12.4.1) • Bending test without final PD test (12.4.4) • The test assembly may be installed in a laboratory and shall consist of at least 2 accessories of the same type that is to be prequalified. There shall be at least 10 m of cable between accessories. The minimum length of the test loop shall be at least 30 m. If a joint submitted to EQ has to be used in both flexible and rigid installations, one joint shall be installed in a flexible configuration, the other rigid. Where a joint is designed for use only in rigid installations, then both joints shall be rigidly fixed. Similarly, for a joint intended only for flexible installations, both joints shall be installed in a flexible test configuration. An example of the test loop is shown in Fig. 4.4. • The loop shall have a U bend with a diameter specified in 12.4.4 – The partial discharge test defined in 12.4.5 shall be carried out here to check the quality of the assembled accessories. • The thermal preconditioning test consists of 60 heat cycles with no voltage applied. The heat cycles shall be as given in 12.4.7, i.e. a minimum of 8 hours of heating followed by at least 16 hours of natural cooling. The steady state conductor temperature shall be between 5  C and 10  C above the maximum cable operating temperature for at least two hours. At the end of the cooling period the conductor temperature shall be within 15  C of ambient temperature, with a maximum of 45  C. • Continue with the partial discharge test (12.4.5) followed by the full sequence of electrical type test. No failure shall occur. Note: In case of modification of the cable, in order to cover all the requirements of the type test it is necessary to perform also the non-electrical test on the cable as specified in 12.5.

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121

Recommendations to IEC 62067

As a summary and conclusion from its reflections WG B1.06 makes the following recommendations to IEC for further consideration in future editions of IEC 62067: • To maintain unchanged the present one-year Prequalification (PQ) test for the basic prequalification of a new cable system (as a check on e.g. the long-term electrical and thermal-mechanical behaviour). • To allow in case of a failure of an accessory the continuation and completion of the PQ test for the undisturbed components (cable and other accessories) of the loop. • To perform partial discharge measurements on the prequalification test assembly during the PQ test to provide an early warning of possible degradation and to enable the possibility of repair before failure. • To perform the lightning impulse test at the end of the PQ test on the complete test loop or, in case of practical problems with test equipment, on cable samples including each type of accessory. The intention is to check the insulation properties at the interfaces, in the accessories and in the cable. • To maintain unchanged the present range of Type and PQ approval, see Annex 4.5.2. • To introduce a simplified long-term test (80 cycles) called “Extension of Prequalification (EQ) test” (see Sect. 4.2.3.3) in case of exchange of prequalified components (cable and/or accessories) with other components that are already prequalified in other cable systems with the same or higher calculated electrical stress at the insulation screen of the subjected system or in case of modification of a cable or an accessory within the same family in a prequalified cable system (see Sect. 4.2.3). • For engineering purposes a “Functional Analysis Method” (see Annex 4.5.3) is recommended as a mean for a systematic assessment of the significance of changes/modifications at components (cables and accessories) of a cable system and thus for the selection of the appropriate test (PQ or EQ test). • To introduce guides to the selection of appropriate test procedures, based on the application of that “Functional Analysis Method”, to the most important components of actual cable systems, in case of – Exchange of a cable and/or accessory in a prequalified cable system (Table 4.3) – Modification of a cable in a prequalified cable system (Table 4.4) – Modification of an accessory within the same family in a prequalified cable system (Table 4.5) • To include sample tests on accessories, which are presently “under consideration” within IEC 62067 Ed.1, following the wording of IEC 60840 Ed.3. For accessories, where the main insulation cannot be routine tested, the partial discharge and high voltage test should be introduced, but only on one accessory of each type per contract. These tests are intended to check not only the intrinsic quality of the accessory (design and materials), but also the quality of the installation (equipment and jointers skill), factors that are very important at the EHV level. • To align the definition of type tests (sub-clause 3.2.3 of IEC 62067 Ed.1) to the definition of the prequalification test to ease the potential use of the Tables 4.3,

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4.4, and 4.5 as a guide for the selection of test procedures in case of changes on prequalified cable systems. • It is worth noting that, as a result of the application of the “Functional Analysis Method” (see Annex 4.5.3), a number of tests have been identified that are not included in IEC 62067 Ed.1. These tests are generally performed as development tests and are summarized in Annex 4.5.4 for future consideration by IEC.

4.3

Long Duration Test on HV Cable Systems (36 < Um ≤ 170 kV)

4.3.1

General

Traditionally EHV cables are working at a significantly higher stress than HV cables (see Fig. 4.5). Due to increased competition and good experience with (very) high AC stresses (12–15 kV/mm and even more) on EHV cable systems it becomes more common nowadays to produce cables with reduced insulation thickness [31–33] at the HV level. This leads to higher dielectric stresses nearly as high as in the EHV field not only at main insulation, but also at the interfaces between cables and accessories. The development of reduced insulation thickness cables at the HV level was successful due to the experience gained by the major cable makers at the production, testing, installation and good service of EHV cable systems. Also new types of accessories are appearing on the market, of course with no earlier experience [34–40]. These accessories should be able to fit to the older types of cables with thicker insulation and the newer types of cables with reduced insulation (changes of an existing HV link with a new cable type or repair of an older link). The following remarks are of major importance:

Working stress (kV/mm)

100 Impulse stresses

10 AC stresses

1 10

100 Voltage level (kV)

Fig. 4.5 AC and impulse conductor stresses of XLPE cables

1000

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• The new highly stressed cables with reduced insulation thickness are generally produced by experienced cable makers that are using their know-how from EHV technology: development and production teams, production equipment and process, material handling and control as well as their well trained teams for installation in the field. • Failures, if any, during type or after installation tests, tend to occur at the interfaces between accessories and cables when new systems are tested [41, 42, 45–47]. To assess the reliability of a new HV cable system whose working stresses are similar to those of the EHV systems, the experienced cable makers rely on the PQ and type tests they have performed on EHV cable systems. • For highly stressed cable systems, type tests are not necessarily sufficient [52]. In several countries long-term tests have been performed on new HV cable systems [32, 33, 43, 44, 48]. Working Group members recommend that cable systems should be considered rather than cables or accessories alone, see also [46]. There exist arguments for and against long-term tests on HV cable systems. For: • The electrical service stresses and mainly lightning impulse stresses at the core (interface with prefabricated accessories) of HV cables are becoming almost as high as the stresses of EHV cables. • Practical shrinkage tests on cable lengths of 5 meters or more have shown that cable insulation and over-sheath is only stabilized after around 60 to 80 heat cycles (see Fig. 4.3). This is confirmed by experts and by the outcome of PQ tests [23, 28, 49, 50] • New accessory designs are coming to the market • The countries that have performed long-term tests on new cable systems before installing them in the network have reported very good service experience • Test laboratories that conduct type tests for many suppliers have published the results of their experience: the failure rate of MV and HV type tests is growing [45]. The reasons for this could be the lack of experience of some manufacturers with higher dielectric stresses in cables, the lack of experience with new type of accessories or all these combined • Breakdowns in service are recorded on recently commissioned systems. Against: • The good and sometimes long experience of some cable makers with HV cables and cable systems (cables with “reasonable” stresses) • In some countries where long-term tests are not performed on new cable systems before installing them in the network, service experience is good, as know-how has been gained during many years in the HV field • The reluctance of some utilities or other end-users to test entire systems if they wish to buy their HV cables and accessories separately.

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Prequalification Test for HV Systems

4.3.2.1 Range of Prequalification Test As accessories typically include due to their intrinsic construction a number of interfaces, the thermal cycles stressing is their most common failure mechanism (apart from jointing errors). That’s why the critical dielectric stresses in service for which long term testing should be recommended for HV cable systems depend very much on the interface stresses with accessories. For prefabricated accessories (joints or stress-cones), mainly impulse stresses at the cable insulation screen are most critical. For instance a 400 kV cable has a ratio BIL/U0 lower than HV cables. That means that HV cables with an AC stress of 4 kV/mm at insulation surface reach an impulse level of 36 kV/mm, as high as a 400 kV system, which has an AC interface stress of 5.5 kV/mm, etc. (see Table 4.6). A practical example: A 1600 mm2 400 kV cable with an insulation thickness of 26 mm has a service stress at the insulation surface of 6.5 kV/mm and an impulse stress at the insulation surface of 40 kV/mm. A 1600 mm2 150 kV cable with an insulation thickness of 15 mm has a service stress of 4.7 kV/mm at the insulation screen, i.e. 30% lower than the 400 kV cable, but an impulse stress of 40.3 kV/mm at the insulation surface, the same as the 400 kV cable. Based on the above considerations, a PQ test is recommended for cable systems with insulation screen stresses above 4 kV/mm. As shown in Fig. 4.5 HV cables have operated at conductor stresses below 8 kV/mm. In addition, for joints that are taped or field-molded also the AC and impulse stresses at the conductor screen are critical. So, if the conductor stresses are higher than 8 kV/mm, it is recommended to perform the long-term test on the system. These conclusions are also supported by the evaluation of the sensitivity of electric stress to changes in dimensions reported in Annex 4.5.2. In fact these calculations show that for a reduction of the insulation thickness of 5%, the thickness of a defect between the cable insulation and the premoulded accessory must be reduced of about 21%, in order to avoid partial discharges. If the operating stresses at the insulation screen and at the conductor screen are below 4 kV/mm and 8 kV/mm respectively, they compare with those found in cable Table 4.6 AC and BIL stresses at the insulation screen for different voltage levels Um (kV) U0 (kV) BIL (kV) Ratio BIL/Uo Routine Test (kV) AC-stress for a BIL-stress of 36 kV/mm at insulation surface (kV/mm) AC-stress for a BIL-stress of 40 kV/mm at insulation surface (kV/mm)

52 26 250 9.6 65 3.74

72.5 36 325 9.0 90 3.99

123 64 550 8.6 160 4.19

145 76 650 8.5 190 4.21

170 87 750 8.6 218 4.18

420 220 1425 6.5 440 5.56

4.16

4.43

4.65

4.68

4.64

6.17

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systems already installed for a long time. Even if most of these cables and accessories have only been type tested, service experience is generally considered good. So the prequalification test shall be performed only on cable systems where the calculated nominal electrical stresses at the conductor screen will be higher than 8 kV/mm and/or at the insulation screen higher than 4 kV/mm. The prequalification test shall be performed except: • If cable systems including a cable of similar construction and accessories of the same family have been prequalified for higher rated voltages or • If an alternative long term test has been carried out on cable systems with the same construction and accessories of the same family and the manufacturer can demonstrate good service experience (2) with cable systems with equal or higher calculated nominal electrical stresses on the conductor and insulator screens, in the main insulation part(s) and in boundaries of the accessories.

4.3.2.2 Prequalification Test Procedure As HV cables are less rigid and have generally smaller conductor cross sections than EHV cables, the thermo-mechanical aspects are less critical than for EHV cable systems, so the WG has considered that laboratory conditions, instead of an actual outdoor installation, could be adopted, in this case, for the PQ test. Being the test performed in a laboratory, i.e. with well defined thermal conditions, it is possible to carry out correctly the heat cycles in approximately one day also with very large conductor cross section cables, so 180 daily cycles instead of one year duration is permissible. This has the advantage of halving the time to market of these new HV cable systems, combined with a reduction of the overall cost of the PQ test. The disadvantage is that the long-term behavior of the electrical insulation of the cable system is checked in only half a year. For these kinds of cable systems, it was judged as acceptable. The proposed layout of cable system is: • Length of cable: one length with a minimum of 20 m without accessories and at least 10 m for the other lengths between two accessories. The total length depending on the number of accessories in the system under test • Number of accessories: at least one of each type • Test could be performed in a laboratory and not necessarily in a situation simulating the real installation conditions. Where thermo-mechanical aspects have to be considered, special test arrangements could be considered. Figures 4.6 and 4.7 show examples of methods to simulate the thermo-mechanical forces on joints, when a cable is installed in a duct or rigidly fixed. • The test is to be performed on the cable system (cables and accessories).

2

() Unfortunately it is difficult to give a specific rule about the evaluation of a good service experience, but the matter has to be dealt with, case by case, between the supplier and the purchaser.

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Fig. 4.6 Possible test layout for cable installed in a duct

J1 J3

J2

Fig. 4.7 Possible test layout for cable anchored on the ground

The test procedure is: • Test voltage: 1.7 U0 • Number of heating cycles: 180 cycles, cycle duration not less than 24 hours. The heating by conductor current shall be applied for at least 8 h. The cable conductor temperature remote from the accessories shall be maintained within the temperature limits of 0  C to 5  C above the maximum conductor temperature in normal operation for at least 2 h of each heating period. This shall be followed by at least 16 h of natural cooling to a conductor temperature within 10  C of the test ambient temperature, with a maximum of 45  C. No breakdown shall occur.

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Note: partial discharge measurements are recommended to provide an early warning of possible degradation. • Tests after long term testing: hot lightning impulse test on the complete loop • Examination of the test loop after completion of these tests.

Note: The replacement/repair of an accessory failed during the heating cycle voltage test is allowed and the test can be continued, because the replaced/ repaired accessory cannot influence the behavior of the other ones. On the contrary it is not allowed to repair a cable failure, unless it is caused by an accidental external damage or the same cable has already been prequalified. At the end of the 180 loading cycle voltage test and the impulse test, only the successfully tested accessories will be prequalified, while the accessory/ies subjected to repair or replacement will not be prequalified. However, it is in the option of the cable manufacturer to continue the test on the replaced/repaired accessory/ies until it/they complete the 180 loading cycles voltage test and the impulse test. In this case also these accessories are prequalified. In the event that during this continued test another component, fully prequalified (including the final impulse test), fails, it is possible to repair it and continue the test until the second run is completed on the replaced accessory/ies. The failure of the component already prequalified (which in this case acts as a laboratory test component) should not be mentioned in the test report.

4.3.3

Exchanges and Modifications in a Prequalified HV Cable System

4.3.3.1 Evaluation of Changes and Modifications in a Prequalified System As far as the guide to the selection of test procedures in case of an exchange of a cable and/or an accessory with another one or of a modification of a cable and/or an accessory of a prequalified cable system is concerned, the same considerations made at paragraph Sect. 4.2.3 apply. Also the extension of prequalification test (EQ) is the same as for EHV systems (see Sect. 4.2.3.1), where Tables 4.3, 4.4, and 4.5 (for IEC 62067 Ed.1) have to be replaced by Tables 4.7, 4.8, and 4.9 (for IEC 60840 Ed.3). 4.3.3.2 Procedure of the Extension of Prequalification (EQ) Test for HV Cable Systems The basic principles of Sect. 4.2.3 apply.

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Table 4.7 Test procedures in case of an exchange of a cable and/or accessory in a prequalified HV cable system Cable and/or accessory cable Joint Metal enclosed Termination Outdoor Termination

Already qualified on another cable system within the same or higher insulation screen stress 12.4 + YY2 (non electrical TT + EQ1) YY2) (EQ) YY2)

Already qualified on another cable system with a lower insulation screen stress or not qualified 12 + ZZ3 (electrical and non electrical TT + PQ) 12 + ZZ3) (TT + PQ) 12 + ZZ3)

(EQ) YY2) (EQ)

(TT + PQ) 12 + ZZ3) (TT + PQ)

The numbers indicate the respective clauses in IEC 60840 Ed.3 EQ consists of the bending test, 60 heat cycles without voltage and the electrical type tests 2 (YY) Clause to be added in the standard 3 (ZZ) Clause to be added in the standard 1

Table 4.8 Guide to the selection of tests because of modifications to a cable in a prequalified HV cable system

Component Cable Conductor

Cable semiconductive inner and/or outer screen

Modification Type of modification Larger crosssection Copper to Aluminium Insulated wires (enamelled or oxidized....) Stranded to solid conductor Water tightness Change of origin (supplier or production plant) Transfer extrusion line (see cable insulation)

M

P ✓

✓

✓

✓

✓

D ✓

✓

✓ ✓ ✓

✓ ✓

✓

✓

✓

DL ✓

IEC 60840 Ed.3 Clause number PQT-test test 12 (ZZ)1

EQtest –

12

–

–

12.4

–

(YY)

12.4

–

(YY)

12.5.14 123

–

–

2

2

(continued)

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Table 4.8 (continued)

Component

Cable Insulation

Cable Bedding (layer over extruded semicon screen) Cable Metallic screen

Modification Type of modification Different quality of semicon Change of base resin Change in cross linking package (peroxide/ antioxidant Nature of polymer (XLPE, LDPE, HDPE, EPR) Higher conductor stress, no increase of insulation screen stress Increase of insulation screen stress New extrusion line or transfer of extrusion line with earlier experience in-house New extrusion line, or transfer of extrusion line without earlier experience Change of laying, material, thickness

Different types of metal screen

M ✓

P

D

DL

IEC 60840 Ed.3 Clause number PQT-test test – 123

EQtest –

✓

123

–

–

✓

12

–

–

12

(ZZ)

–

✓

12

–

–

✓

12

(ZZ)

–

✓

✓

✓

✓

✓

12

–

✓

✓

12

(ZZ)

✓

✓

✓

12.4.4 12.4.18 (if required)

✓

✓

✓

12.3.3 + 12.3.8

–

–

(continued)

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Table 4.8 (continued)

Component Cable Outer sheath

Modification Type of modification Different type of materials Different processes Change in bonding material and/or process to metal screen

M ✓

✓

P ✓

D

DL

IEC 60840 Ed.3 Clause number PQT-test test – 12.44

EQtest –

✓

12.44

–

–

✓

12.44

–

–

Note: (YY) and (ZZ), Clauses to be added in the standard Remark: only the relevant clauses are applicable for type tests  M: change in material; P: change in manufacturing process; D: change in design (construction); DL: change in electrical design stress level 1 If higher calculated nominal dielectric stresses at the insulation screen, clause 16.1 2 Same as for insulation 3 If outside Range of Type Approval, clause 12.1 4 As appropriate to outer sheath materials

The sequence of tests for the Extension of Prequalification (EQ) test is summarized below. The numbers in brackets refer to clause numbers of IEC 60840 Ed.3. • Check on the insulation thickness of cable for electrical type test to determine the test voltage values (12.3.1) • Bending test without final PD test (12.3.3) • The test assembly may be installed in a laboratory and shall consist of at least 2 accessories of the same type that is to be prequalified. There shall be at least 10 m of cable between accessories. The minimum length of the test loop shall be at least 30 m. If a joint submitted to EQ has to be used in both flexible and rigid installations, one joint shall be installed in a flexible configuration, the other rigid. Where a joint is designed for use only in rigid installations, then both joints shall be rigidly fixed. Similarly, for a joint intended only for flexible installations, both joints shall be installed in a flexible test configuration. An example of the test loop is shown in Fig. 4.4. • The loop shall have a U bend with a diameter specified in 12.3.3 – The partial discharge test defined in 12.3.4 shall be carried out here to check the quality of the assembled accessories. • The thermal preconditioning test consists of 60 heat cycles with no voltage applied. The heat cycles shall be as given in 12.3.6, i.e. a minimum of 8 hours of heating by conductor current followed by at least 16 hours of natural cooling. The steady state conductor temperature shall be between 5  C and 10  C above the maximum cable operating temperature for at least two hours. At the end of the cooling period the conductor temperature shall be within 15  C of ambient temperature, with a maximum of 45  C.

Change of insulator design or manufacturer of GIS/Transformer insulator

Change of insulator material for indoor or outdoor terminations.

✓ 2 v 2

✓ 2 ✓ 2

✓

✓

✓ ✓

✓

✓

Outer screen and Protection design, (Filling/water tightness), Outlet of bonding leads Higher electrical stress design of stress cone (or smaller metal clad for GIS or transformer terminations) Change in nature of Filling medium (e.g. oil to gas....) Change in the formulation of the stress cone compound but with the same base polymer Change of the base polymer (EPR, Silicone, . . .) of the stress cone

✓ ✓

✓ ✓ ✓

D

✓ ✓

P

M

Type of modification Higher calculated electrical stress design and construction Compound of main insulation body (same base resin) Changing nature of polymer, (EPR, Silicone....) Material of semi-con electrodes Fixation of cable ends on either side of the joint Screen interruption

✓

DL ✓

12

12

–

– –

–

–

– –

(ZZ)

(ZZ)1 (ZZ)1

(ZZ)1

–

EQtest (ZZ) (ZZ) – (ZZ) (ZZ) –

–

– –

IEC 60840 Clause number PQT-test Test – – – – – (YY) – – – – Annex – D Annex – D – –

NOTE: (YY) and (ZZ) Clauses to be added in the standard  M: change in material; P: change in manufacturing process; D: change in design (construction); DL: change in electrical design stress level 1 When can be demonstrated that the thermo mechanical aspects have no significant influence on the performances of the termination a Type Test may be sufficient 2 In case of elastomeric insulators (“silicone” or “EPR”) climatic and pollution test according to IEC 61109 Annex C should be considered.

Terminations: - outdoor - indoor - metal enclosed + SF6 + oil-immersed

Component Joints

Modification

Table 4.9 Guide to the selection of tests because of modifications to an accessory within the same family in a prequalified HV cable system

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• Continue with the partial discharge test (12.3.4) followed by the full sequence of the electrical type test (see 12.3.2 item b to h). No failure shall occur.

Note: In case of modification of the cable, in order to cover all the requirements of the type test it is necessary to perform also the non-electrical test on the cable as specified in 12.4.

4.3.4

Recommendations to IEC 60840

As a summary and conclusion from its reflections WG B1.06 makes the following recommendations to IEC for further consideration in future editions of IEC 60840: • To introduce a prequalification (PQ) test for those HV cable systems where the calculated nominal electrical stress at the conductor screen will be higher than 8 kV/mm and/or at the insulation screen higher than 4 kV/mm (see Sect. 4.3.2). This test need not to be performed if – Cable systems with the same constructions and accessories of the same family have been prequalified for higher rated voltages – If equivalent long term tests have been already successfully carried out on cable systems with the same construction and accessories of the same family and a good service experience at cable systems with equal or higher stresses can be demonstrated • To allow in case of a failure of an accessory during the test the continuation and the completion of the PQ test for the undisturbed components (cable and other accessories) of the test loop. • To perform partial discharge measurements on the prequalication test assembly during the PQ test to provide an early warning of possible degradation and to enable the possibility of repair before failure. • To perform the lightning impulse test at the end of the PQ test on the complete test loop or, in case of practical problems with test equipment, on cable samples including each type of accessory. The intention is to check the insulation properties at the interfaces, in the accessories and in the cable. • To maintain unchanged the present range of Type and PQ approval, see Annex 4.5.2 • To introduce a simplified long-term test (80 cycles) called “Extension of prequalification (EQ) test” (see Sect. 4.3.3.2) in case of exchange of prequalified components (cable and/or accessories) with other components that are already prequalified in other cable systems with the same or higher calculated electrical stress at the insulation screen of the subjected system or in case of modification of a cable or an accessory within the same family in a prequalified cable system (see Sect. 4.3.3). • For engineering purposes a “Functional Analysis Method”, see Annex 4.5.3, is recommended as means for a systematic assessment of the significance of changes/modifications at components of a HV cable system and thus for the choice of the appropriate tests (PQ or EQ) or Type Tests (TT).

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• Based on the application of the “Functional Analysis Method” to the most important components of actual HV cable systems (see Annex 4.5.3), guides to the selection of test procedures are given in case of – Exchange of a cable and/or accessory in a prequalified cable system (Table 4.7) – Modification of a cable in a prequalified cable system (Table 4.8) – Modification of an accessory within the same family in a prequalified cable system (Table 4.9) • To align the definition of type tests to the definition of the prequalification test (see sub-clause 3.2.3 of IEC 62067) to ease the potential use of the tables (Tables 4.7, 4.8 and 4.9) as a guide for the selection of test procedures in case of changes on prequalified HV cable systems. • To include an electrical sample test as a check on the properties of the insulation of HV cables, see Table 4.17, item 25 • It is worth noting that, as a result of the functional analysis exercise (see Annex 4.5.3), a number of tests that are not included in IEC 60840 Ed.3 have been identified. These tests are generally performed as development tests and are summarized in Annex 4.5.4 for future consideration by IEC.

4.4

Conclusions

In this chapter the testing procedures of HV and EHV extruded cables and related accessories have been reconsidered extensively. Improvements in both effectiveness (can it be done better) and efficiency (can it be done faster) have been proposed. In the final proposals the desirable improvement and its practical feasibility are balanced carefully, resulting in a series of well thought practical propositions to improve extruded cable testing. The main results are summarized below: • EHV Cable systems: there is no sufficient service experience on EHV cable systems collected so far to change the existing initial Prequalification test. This PQ test has to be repeated in case of extension of the range of approval. Within the range of approval, a new test called Extension of Qualification (EQ) test is proposed to control changes in already prequalified cable systems instead of repeating the complete PQ test. This new test can be carried out on a laboratory loop and will comprise 80 heating cycles combined with voltage application at 2 U0 of the electrical type test. • HV Cable systems: a prequalification test is recommended for design stresses above 8 kV/mm on the conductor or 4 kV/mm over insulation. This test can be carried out on a laboratory loop and will comprise 180 heating cycles combined with voltage application at 1.7 U0. This PQ test has to be repeated in case of extension of the range of approval. Within the range of approval, a new test called Extension of Qualification (EQ) test is proposed to control changes in already prequalified HV cable systems. This new test can be carried out on a laboratory loop and will comprise 60 heating cycles without voltage and followed by the full

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•

• • • •

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sequence of the electrical type tests, with 20 heat cycles combined with voltage application at 2 U0. For both EHV and HV cable systems the PQ test will be completed with an impulse test on the full loop to check that no degradation in the system, especially at the interface with accessories, has occurred. (Only in case suitable impulse test equipment is not available at the test site, the impulse test can be carried out in any other test arrangement including each type of accessory taken from the loop). For both PQ and EQ tests, PD tests are recommended to provide an early warning of possible degradation and to enable the possibility of a repair before failure. Several tables are provided as guides to determine the appropriate test sequence in case of changes or modifications. Functional Analysis is a good tool to help engineers to manage a wide range of potential changes. Examples are given in Annex 4.5.3. In Annex 4.5.4 tests from Functional Analysis, not yet published in IEC standards, are summarized. This list of tests will be handed to IEC TC 20 for further consideration.

4.5

Annexes

4.5.1

Terms of Reference

4.5.1.1 Title Revision of Qualification Procedures for High Voltage and Extra High Voltage AC Extruded Underground Cable Systems. 4.5.1.2 Scope After the official qualification of a cable system, there are possible changes (new cable construction, higher stress, new extrusion line, new process, new type of accessories. . .) especially if this cable system is manufactured over several years, and the question raised is to examine how it is possible to qualify this new system without making the full set of tests which are presently recommended or specified in standards. A WG has been launched on this item limited to AC extruded cable systems. All tests, prequalification and type tests will be reviewed, even if the prequalification test should be examined first, as it is the most costly and the longest. 4.5.1.3 Terms of reference For the range of AC extruded underground cable systems for voltages above 30 kV up to 500 kV, review and complete the qualification procedures for the different HV voltage ranges with the goal to come quickly and economically to the market with innovative solutions but without jeopardizing the reliability of the installed system: • Propose tests where there are lacks e.g. short circuit tests, climatic tests on terminations. . . • Evaluate whether in high voltage systems up to 150 kV a long term test has to be recommended above given dielectric service stresses or where the innovation is not built on earlier experience

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• Define what “earlier experience” means • In case of major innovation in EHV cable systems, evaluate whether long term test can be replaced by shorter ones, which should be defined by the WG. In order to build up a guide of qualification procedures depending on earlier qualification(s) at the same and/or different voltage levels and on field experience.

4.5.2

Sensitivity of Partial Discharges in XLPE Cable Insulation to Change of Electrical Stress

Contents 1. Introduction 2. Cable standards and electric stress 3. Sensitivity of electric stress to change of cable dimensions 4. Determination of risk of discharge caused by change of dimensions 5. Effect of change of cable dimensions on discharge free operation 6. Conclusions

4.5.2.1 Introduction In Cigré WG B1.06, a question was raised whether the range of type approval and of PQ approval could be widened. This Annex provides details of the analysis made for this purpose. Dielectric stress level variations specified in relevant sub-clauses of the current IEC standards were used as a starting point in this work. As a first step of the process, we can determine the rate of change of electric field strength inside a radial cable with respect to: • The change of the insulation inner radius, without changing the insulation thickness • The change of the insulation thickness, without a change of the inner radius. The obtained relationships are the so-called sensitivities of field strength to a dimensional change. They can indicate, for example, what effect a 1% change of the insulation thickness has on the field strength at the insulation outer surface. In the next step, the sensitivity values together with Paschen’s curve can be used to calculate the change of the discharge inception voltage in insulation defects. The Paschen curve describes the relationship between breakdown voltage and size of defect together with gas pressure within the defect. In this study we are concerned with discharge free operation of the cable. Therefore, it is more useful that instead of the discharge inception voltage the maximum defect size is determined from the above relationship at a given voltage across the defect and at a given pressure. Two types of defects are considered in this study: a spherical void located on the conductor screen interface and a fissure located on the outer insulation surface. The former can be seen as a worst-case scenario of a manufacturing fault while the latter can represent a defect at the interface to accessories. The model calculations presented in this study are first order estimations of the risk of electrical discharge in voids from a change in electric stress at insulation

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interfaces caused by dimensional changes of insulation. In the practical situation, stresses at the interfaces may be different because of the actual geometry. However the model calculations describe the worst-case situation.

4.5.2.2 Cable Standards and Insulation Stress Sub-clauses 12.3.1 of IEC 60840 Ed. 3 and 12.4.1 of IEC 62067 Ed. 1 state that the test voltage values have to be adjusted if insulation thickness of the tested cable system exceeds 105% of the nominal insulation thickness with the upper limit at which the adjustment is allowed of 115%. The range of type approval sub-clause 12.1 of IEC 60840 and sub-clause 12.2 of IEC 62067 state that the approval is valid for systems in which: • Calculated nominal conductor stress at the conductor screen does not exceed 110% of the tested system • Calculated nominal insulation stress at the insulation screen does not exceed the insulation screen stress of the tested system. In this context, we are considering whether it is possible to widen the range of approval without jeopardizing performance of the cable system.

4.5.2.3 Sensitivity of Insulation Stress to Change of Cable Dimensions Electric field strength at a radial location x in a concentric cable is expressed as Ex ¼

U0 1 x ln Rr

ð4:1Þ

where r is the inner radius of the insulation, R is the outer radius of the insulation w is the insulation thickness We are interested in the field strength at two locations within insulation, namely, on the inner surface and on the outer surface. Using R ¼ r + w, we get U0 1 r ln rþw r

ð4:2Þ

U0 1 r þ w ln rþw r

ð4:3Þ

Er ¼ ER ¼

which is also shown in Fig. 4.8. Electrical stress sensitivity is defined as the rate of change of field strength with respect to a given dimension. In this work, we consider the rate of change of field with respect to insulation inner radius, r, and with respect to insulation thickness, w.

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Fig. 4.8 Electric field strength in a concentric cable

4.5.2.3.1 Sensitivity to Change of Inner Radius Sensitivity to change of inner radius only is obtained by differentiating Equations (4.2) and (4.3) with respect to r, which gives   dEr U w kV  1 ¼ 2 0rþw mm dr r ln r ðr þ wÞ ln rþw r   dEr U0 w kV ¼  1 rþw mm dr ðr þ wÞ2 ln rþw r ln r r

1 mm

ð4:4Þ

1 mm

ð4:5Þ

if U0 is in kV and all dimensions are in mm. 4.5.2.3.2 Sensitivity to Change of Insulation Width Differentiating (4.2) and (4.3) with respect to w gives the following expressions of sensitivity to change of insulation width only dE r U0 kV 1 ¼ dw r ðr þ wÞ ln rþw2 mm mm r  rþw  U 0 ln r þ 1 kV 1 dER ¼ 2 dw ðr þ wÞ ln rþw mm mm

ð4:6Þ

ð4:7Þ

r

4.5.2.3.3 Sensitivity Per Unit The absolute value of sensitivity, as described by Eqs. (4.4), (4.5), (4.6), and (4.7), is useful only for a specific cable if its operating voltage and relevant dimensions are known. To make sensitivity expressions more general, we can represent them in per unit terms. The per unit base for the field change dEr and dER will be the respective nominal field intensities Er and ER. The per unit base for the dimension change dr and dw will be the respective dimensions r and w. These lead to the following results.

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dE =E dE r dE r w 1 ¼ r r¼ r ¼ 1 dr PU dr Er r þ w log rþw dr=r r   dER =ER dE R r dE R r w 1 ¼ ¼  1 ¼ dr PU dr ER r þ w r log rþw dr=r r dE =E dEr dE w w 1 ¼ r r¼ r ¼ dw PU dr Er r þ w log rþw dw=w r   dE R =ER dER w dE R w 1 ¼ ¼ þ1 ¼ r þ w log rþw dw PU dr ER dw=w r

ð4:8Þ ð4:9Þ ð4:10Þ ð4:11Þ

The obtained expressions can now be used to estimate sensitivity values for a range of cable designs. The operating voltage will be reflected in the value of the insulation width, w, while the conductor size will be reflected in the value of the inner radius r. 4.5.2.3.4 Numerical Example To provide a more specific numerical example of sensitivity calculations we consider two cables whose dimensions are tabulated in Table 4.10. These data were applied to sensitivity Eqs. (4.8), (4.9), (4.10), and (4.11) and the obtained results are shown in Table 4.11. Table 4.11 results show that a decrease of insulation thickness by 1% leads to an increase of the field strength at the insulation screen by approximately 1.2% in both types of cables. At the same time, an increase of the inner radius by 1% leads to an increase of field intensity at the insulation screen by approximately 0.2%. Similar calculation results are shown graphically in Fig. 4.9a, b for a range of values of r and w. The obtained plots show that variations of the field strength Table 4.10 Dimensions of two example cables

Rated voltage Uo/U (kV) Conductor cross-section Conductor radius Insulation inner radius (r) Insulation thickness (w) Insulation outer radius (R)

127/220 kV 2000mm2 28.95 mm 30.95 mm 20.80 mm 51.75 mm

220/380 kV 1600mm2 24.4 mm 27.0 mm 26.6 mm 53.6 mm

Table 4.11 Sensitivity to dimensional change in two example cables Change of r only Change of w only Change of r only Change of w only

Rated voltage 127/220 kV 127/220 kV 220/380 kV 220/380 kV

dEr/Er 0.22 dr/r 0.78 dw/w 0.28 dr/r 0.72 dw/w

dER/ER + 0.18 dr/r 1.18 dw/w + 0.22 dr/r 1.22 dw/w

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Qualification Procedures for HV and EHV AC Extruded Underground Cable. . .

a

Field sensitivity to change of insulation inner radius

0.4

dEr/dw 220 kV dER/dw 220 kV dEr/dw 380 kV dER/dw 380 kV

0.3 0.2 Field sensitivity (PU)

139

0.1 0 -0.1 -0.2 -0.3 -0.4

20

22

24

26

28

30

32

34

36

38

40

Inner radius (mm) Field sensitivity to change of insulation width b

0

dEr/dw 220 kV dER/dw 220 kV dEr/dw 380 kV dER/dw 380 kV

Field sensitivity (PU)

-0.2 -0.4 -0.6 -0,8 -1 -1.2

15

20

25

30

35

Insulation width (mm)

Fig. 4.9 Field strength sensitivity to change of dimension for a range of values of the insulation inner radius and width

sensitivity to dimensional change are relatively small within a large range of dimension values. The sensitivity values are also very similar for the two types of cables considered. This is an important observation, which allows the definition of sensitivity to be generalized for the entire range of practical cable designs.

140

J. Becker

Now, we will consider how these changes of electric field strength affect the risk of discharge in the interface regions in the presence of voids.

4.5.2.4 Determination of Risk of Discharge Caused by Change of Dimensions 4.5.2.4.1 Size of Discharge-free Defects We consider two types of defects in which discharges can occur • Spherical void located on the conductor screen interface • Fissure located on the insulation screen interface. Spherical defects near the conductor screen usually originate from manufacturing faults and they can be detected by factory quality control tests. Fissure defects at the interface with accessories pose a greater risk because they can be introduced during installation and they are not as easily detected. Both types of defects are found in most practical cable systems in operation. At a given field strength a discharge free operation is possible if size of defects is sufficiently small. A discharge free operation may be still possible even under increased field strength if the defect size is further reduced. The maximum size of discharge free defects at operating voltage Uo can be estimated from sensitivity of defect size to a relative increase of the field strength in the insulation, dEins/Eins, at the defect location. This is done with the help of Paschen curve shown in Fig. 4.10. In the context of this analysis, the Paschen curve can represent relationship between breakdown voltage, Ubd, within a defect of given size at given gas pressure inside the defect (Bar-mm). Instead of breakdown voltage, the breakdown field strength can be obtained from the curve, as shown in Fig. 4.11. Ubreakdown [kV] 1000

100

10

1

0.0 0.001

0.01

0.1

1 Bar mm

Fig. 4.10 Paschen’s curve for air at 20  C

10

100

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Ubd (kV) Ebd (kV/mm) 40 35 30 25

Ubd (kV) from Paschen

20

Ebd (kV/Barmm) and (kV/mm, 1B)

15 10 5 0 0.01

0.0

1

Defect dimension (Barmm)

Fig. 4.11 Breakdown voltage and breakdown field strength as a function of defect dimension in mm from Paschen curve at p ¼ 1 Bar

U

U εi

εe Ei E0=U/d

d

E0=U/d

εe

E d

Ee

εi Ei=E013εe/(εi+2εe)

Ei=E0(εe/εi) Ei=2.26E0

(a)

Ei=1.23E0

(b)

Fig. 4.12 Electric field calculations inside a fissure (a) and spherical defect (b)

The field intensity within the defect can be determined from the field strength in the insulation at the defect location considering also permittivity difference between the defect (gas, ε ¼ 1) and surrounding insulation (XLPE, ε ¼ 2.26). In a radial field, such as in the case of fissure at the interface to an accessory, a simple ratio of dielectric permittivity of insulation material and of gas inside the defect gives the field increase factor inside the fissure. This is illustrated in Fig. 4.12a. For a gas filled sphere the field increase factor is equal to 1.23, as shown in Fig. 4.12b. Taking into account the field increase factors from Fig. 4.12 and the breakdown field strength of the defect from Fig. 4.11 we can obtain the relationship between the size of a breakdown free defect and the field strength in the surrounding dielectric. It is quite clear that nearly twice stronger field is needed to initiate

142

J. Becker

Fig. 4.13 Maximum field intensity in the surrounding dielectric at p ¼ 1 Bar, with no partial breakdown in the defect

30. 25.

Ei(kV/mm)

20. 15. sphere

10. 5. 0. 0.0

0.

1

-mm 1B

discharges in a spherical void in comparison with fissure. The related curves are depicted in Fig. 4.13. Using Eqs. (4.2) and (4.3) we can calculate partial discharge inception voltage from field intensity for a defect located on the conductor screen interface and the insulation screen interface respectively. Typically, we consider a fissure on the insulation screen and a spherical void at the conductor screen interface. 1 R U sphere ¼ pffiffiffi Er r ln i r 2

kVrms

ð4:12Þ

1 R ¼ pffiffiffi ER R ln U fissure i r 2

kVrms

ð4:13Þ

A range of values of inception voltage as a function of defect size can be obtained for the two cables listed in Table 4.10 by substituting to Eqs. (4.12) and (4.13) appropriate field intensity values from Fig. 4.12 together with respective cable dimensions. The obtained results together with the cable nominal voltage are shown in Figs. 4.14 and 4.15. The comparison of the discharge inception voltage Ui with the nominal voltage U0 on these plots gives the maximum size of a discharge free defect of approximately 30 μm (at p ¼ 1 Bar) in the 127/220 kV cable and approximately 20 μm (at p ¼ 1 Bar) in the 220/380 kV cable.

4.5.2.4.2

Size Sensitivity of Discharge Free Defects to Change of Field Strength The Paschen curve in the form of field strength vs. defect size, Fig. 4.11, shows that the higher the field intensity the smaller defect size must be for a discharge free operation of the cable. We are interested in determining the relative rate of change of the defect size with respect to the relative change of field intensity in the dielectric at a given location.

Qualification Procedures for HV and EHV AC Extruded Underground Cable. . . Inception voltage (kV-rms) for a 127/220 (245) kV XLPE cable, 2000 mm2, at conductor screen, sphere 350.00 300.00 250.00 200.00 150.00 100.00 50.00 0.00 0.01

0.1

1

143

Inception voltage (kV-rms) for a 127/220 (245) kV XLPE cable, 2000 mm2, at insulation screen 350.00 Inception voltage (kg-rms)

Inception voltage (kg-rms)

4

300.00 250.00 200.00 150.00 100.00 50.00 0.00 0.01

10

0.1

1

10

Bar.mm

Bar.mm

Ui (kV-rms) spleet U0 (kV-rms)

Ui (kV-rms) bol U0 (kV-rms)

Fig. 4.14 Partial discharge inception voltage, as a function of the size at p ¼ 1 Bar, in a spherical void at the conductor screen interface and in a fissure at the insulation screen interface in a 127/220 kV, 2000 mm2 cable

400.00 350.00 300.00 250.00 200.00 150.00 100.00 50.00 0.00 0.01

Inception voltage (kV-rms) for a 220/380 (420) kV XLPE cable, 1600 mm2, at insulation screen, Inception voltage (kg-rms)

Inception voltage (kg-rms)

Inception voltage (kV-rms) for a 220/380 (420) kV XLPE cable, 1600 mm2, at conductor screen, sphere

0.1

1

10

450.00 400.00 350.00 300.00 250.00 200.00 150.00 100.00 50.00 0.00 0.01

Bar.mm

0.1

1

10

Bar.mm

Ui (kV-rms) bol U0 (kV-rms)

Ui (kV-rms) spleet U0 (kV-rms)

Fig. 4.15 Partial discharge inception voltage, as a function of the size at p ¼ 1 Bar, in a spherical void at the conductor screen interface and in a fissure at the insulation screen interface in a 220/380 kV, 1600 mm2 cable

  dðBar  mmÞ dEi ¼f Bar  mm Ei

ð4:14Þ

These parts of the Paschen curve where Barmm > 0.01 (> 0.01 mm at p ¼ 1 Bar) can be modelled in sections as an exponential function of the following form Ebd ¼ bðBar  mmÞa

ð4:15Þ

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J. Becker

Differentiating both sides of (15) with respect to Barmm gives dE bd =dðBar  mmÞ ¼ baðBar  mmÞa1 ¼ a Ebd =ðBar  mmÞ

ð4:16Þ

At the same time Ebd ¼ εi Ei

ð4:17Þ

dE bd ¼ εi dEi

Substituting (4.17) to (4.16) and rearranging gives the desired form of sensitivity dðBar  mmÞ 1 dE i ¼ Bar  mm a Ei

ð4:18Þ

in which a is the exponent coefficient of the exponential function modelling the Paschen curve. Equation (4.18) allows us to obtain sensitivity values from estimating the value of exponent a from the Paschen curve. Taking the logarithm of both sides of (4.15) gives log Ebd ¼ a log ðBar  mmÞ þ log ðbÞ

ð4:19Þ

Now, the values of a and b can be found from the curve Ebd ¼ f(Barmm) in Fig. 4.11. The obtained results are shown in Tables 4.12 and 4.13. Values of 1/a shown in Table 4.13 indicate that within the pd. area of 0.03– 0.1 mm at p ¼ 1 Bar a 1% increase of the electric field strength will require a 2% reduction of the defect size for a discharge free operation of the cable. Within the pd. area of 0.1–0.3 mm at p ¼ 1 Bar the corresponding reduction of the defect size is Table 4.12 Data points from Paschen’s curve in Fig. 4.10 Barmm 0.01 0.03 0.1 0.3 1 3

log (Bar-mm) 2 1.5228 1 0.5229 0 0.477

Table 4.13 Estimated parameters

Barmm 0.01–0.03 0.03–0.1 0.1–0.3 0.3–1 1–3

Ubd (kV) 0.35 0.55 1.00 2.10 5.00 11.0

a 0.59 0.50 0.26 0.30 0.28

Ebd (kV/mm) 35 18.3 10 6.67 5 3.67

Log (b) 0.34 0.50 0.74 0.70 0.70

log Ebd 1.544 1.262 1.000 0.824 0.699 0.565

1/a 1.7 2.0 3.8 3.3 3.5

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3.8%. For the range of 0.1–3 mm, the reduction of the defect size is stated to be 3.5% in case of a 1% increase of the electric field strength.

4.5.2.5 Effect of Change of Cable Dimensions on Discharge Free Operation A change of insulation dimensions will result with a change of electric field intensity. This was considered in Sect. 4.5.2.3 and defined as field strength sensitivity to change of inner radius and to change of width of insulation. We will combine these findings with results of Sect. 4.5.2.4 to determine conditions of discharge free operation if dimensions of cable insulation change. There are two main cases that need to be considered • Decreased insulation thickness (“slim” design) • Increased conductor size. At the same time, we concentrate on the case of possible discharges occurring in a fissure located at the insulation screen interface as more important from the practical point of view. 4.5.2.5.1 Cable Systems with “Slim” Design By recalling results from Table 4.11 in Sect. 4.5.2.3 we know that a 1% reduction of insulation width will result with a 1.2% increase of the field strength at the insulation screen interface. At the same time, the defect size sensitivity to change of field for a discharge free operation is stated to 3.5, see Table 4.13. Therefore, the combined effect will be 1.2%  3.5 ¼ 4.2% of reduction of permissible size of defects for a discharge free operation. From this, it is straightforward to see that a 5% decrease of insulation width leads to a necessary reduction of the size of contributing defects by 21%. 4.5.2.5.2 Cable Systems with Increased Conductor Size Results presented in Table 4.11 show that a 1% increase of the conductor radius contributes to a 0.2% increase of the field strength on the insulation screen interface. This gives the total effect of 0.2%  3.5 ¼ 0.7% decrease of permissible size of contributing defects. An increase of the conductor cross-section from 1600 mm2 to 2000 mm2 results with an approximate increase of the insulation inner radius by 12%. This corresponds to a necessary decrease of permissible size of contributing defects by approximately 8.4%.

4.5.2.6 Conclusions The effect of change of insulation dimensions on the size of discharge free defects in high voltage cables was analyzed in this Annex. The following results were obtained for the two types of design considered.

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J. Becker

4.5.2.6.1 “Slim” Design For each 1% reduction of the insulation width the reduction of fissure radial dimension by approximately 4.2% is necessary for a discharge free operation. Considering reduction of insulation width of 5%, as specified in the current range of type approval, will require a reduction of the fissure size by 21%. 4.5.2.6.2 Increased Conductor Size An increase of the conductor cross-section size from 1600 mm2 to 2000 mm2 without changing the insulation width will result with a necessary decrease of permissible size of contributing defects located on the insulation screen interface by approximately 8.4%. 4.5.2.6.3 Conclusion The above results show that widening the current range of type approval described by IEC 60840 and IEC 62067 will lead to an unacceptable increase of risk of partial discharge in operating cable systems. Therefore, it can be concluded from this study that the range of type approval in relevant IEC standards ought not to be changed.

4.5.3

Functional Analysis

Use of Functional Analysis in case of Changes in Cable and Accessory Components of HV and EHV Systems.

4.5.3.1 Introduction International Standards provide plant manufacturers with a consistent set of tests that allows them to demonstrate that their equipment meets certain minimum criteria. For the purchaser, testing to International Standards provides a degree of assurance that the plant or equipment can be operated safely and reliably. Although testing to International Standards is often time consuming and expensive, once a manufacturer and purchaser have agreed that the test requirements have been met, the product is “approved” and further purchases of the same product are relatively simple. One advantage of testing to International Standards is that many purchasers worldwide will accept the test evidence, without the tests having to be repeated. A significant disadvantage of International Standards is that once a product is “approved” there is little incentive to the manufacturer to make incremental improvements to the product, since these would invalidate the “approval” and require the type approval tests to be repeated. This Annex sets out a process whereby the significance of any change can be evaluated and the need for further testing agreed. 4.5.3.2 Functional Analysis Method In principle, it should be possible to classify any change to a cable as a change that might change the performance characteristics (“major” change), requiring a repeat of

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147

type testing, or as a “substantial change” requiring a repeat of prequalification testing, or a “minor” change requiring little or no repeating of testing. The tests in international cable standards have evolved either to simulate the service conditions experienced by the cable (system) (often in an accelerated manner) or to test for the presence of specific defects and deficiencies. The tests have evolved to ensure that a cable (system) is able to survive the various functions that it has to perform during its lifetime and their resulting stresses (electrical, mechanical and chemical). For example, a cable has to be bent during production, transportation and installation, so the IEC Standards include a bending test. The Working Group has extended this principle to examine how a change to the cable design or manufacture might affect its performance and hence what further testing might be required. The methodology adopted by the WG was first to perform a functional analysis to consider the functions performed by each part of a cable’s and accessories construction and how each function is tested presently in IEC 60840 Ed.3 and IEC 62067 Ed.1. The WG then considered a number of possible changes a manufacturer might make to the cable system. The functional analysis was then used to determine what further testing might be required. The result of this exercise is summarized in Table 4.14 as far as the constituents of the cable itself are concerned and in Tables 4.15 and 4.16 for the components of joints and terminations respectively. It must be mentioned that this analysis is based on the present knowledge and could be updated and used as guidance for future work. For each constituent part, the table identifies; • The functions performed by that component • The feature which identifies if the function is being fulfilled or the threat posed if it is not • Comments that give further details of the threat or alternative test procedures. The tests in IEC 60840 Ed.3 or IEC 62067 Ed.1 which check that the function is being performed satisfactorily • Some tests described in the Functional Analysis are missing in IEC60840 Ed.3 and 62067 Ed.1 (e.g. short-circuit, side-wall pressure, etc.). The full list of missing tests is given in Annex 4.5.4. The main changes to cables and accessories in a prequalified cable system, which require the repetition of the type test (or part of it) or of the prequalification tests or of the extension of qualification, are summarized in Tables 4.4 and 4.5 in part 4.2 of this chapter. The cable supplier and purchaser may use the functional analysis to discuss if any change to a constituent part might lead to a change in the performance characteristics or reliability of the cable system. This is particularly useful where the change is outside the scope of Tables 4.4 and 4.5 of part 4.2.

Cable’s component A Conductor

-a. Prevent longitudinal water penetration -a. No corrosion when using Al conductor

3 Water blocking 4 Chemical properties

-b. No degradation due to layers over conductor (tape or semi-con)

-a. Bending before type tests (S: §12.3.3/§12.4.4)

-a. Satisfy the minimum bending radius without harmful mechanical deformation -b. Support pulling during installation

2 Mechanical properties

-a. Water penetration test (T: §12.4.18/§12.5.14) -a. Examination after type test (IEC60840/ IEC 62067) or long term PQ test (IEC 62067) - b. Compatibility test (T: §12.4.4/§124)

-b. No test

-b. Calculation of thermal short circuit temperaturre

Specification/Threat C -a. No overheating with nominal current

-b. Limit temperature with thermal short circuit current

Function or Property B 1 Electrical conductivity

Test to check the functionality (Relevant paragraphs of IEC 60840 Ed.3/62067 Ed. 1 in brackets –IEC 62067 Ed. 1 in Italics) D -a. Resistance measurement (S: §10.5/§ 10.5)

b. Limitation of pulling force to be given by the manufacturer a. Test if water blocking properties required a. If Al conductor avoid: water ingress +chemical species (e.g. contaminations, solvents)

Comments E a. Measurement to check design Verification of AC resistance of large conductors with insulated wires or other special properties: See WG B1-03 – Large cross-sections and composite screen designs: For large cross-sections, AC resistance test should be performed as type test (see TB TB 272 of Cigré WG B1-03 from 2005) b. If temperature rise is considered dangerous, a short circuit test is proposed as a development test. Check that there is no degradation of tapes and extruded semi-con after short circuit test

Table 4.14 Functional Analysis of a High Voltage Cable and cable components (Abbreviations: Routine: R, Sample test: S, Type test: T, Development: D, Prequalification: PQ)

148 J. Becker

Tape over conductor (optional)

-a. Resistance sufficiently low to avoid PD and to avoid voltage over semicon during fast transients

-a. Good mechanical properties

-a. No degradation of conductor or semi-con -b. Stability of electrical resistivity after heat cycles

-a. Avoid penetration of semi-con into conductor

2 Mechanical properties

3 Chemical properties

4 Interface with other components

-a. Low interface resistivity with connectors -b. Thermal-mechanical expansion/ deformation (Influence of shape of wires on interface of extruded semi-con with insulation) -c. Avoid water penetration

1 Electrical resistance

5 Interface with other components or accessories b. PQ not in IEC 60840 Ed.3

-b. Thermal cycles: 20 (T: §12.3.6/§12.4.7) or 180 (PQ: §13.2.3 on cable system) -c. Water penetration test (T: §12.4.18/§12.5)

(continued)

c. Water penetration may lead to chemical degradation of the insulation mainly if the conductor metal is aluminum -a. PD measurement (R,T; §9.2/§9.2- a. Covered by type tests §12.3.4/§12.4.5) +Lightning impulse test (T; 12.3.7/ §12.4.9) + examination (T: §12.3.8/ §12.4.10) -a. Bending test (T: §12.3.3/§12.4.4) Examination after completion of the type test (T: §12.3.8/§12.4.10) -a. Compatibility test on whole cable (T:§12.4.4/§12.5.4) -b. Resistance measurement after b. Can be confirmed by test §12.3.9/ heat cycles §12.4.11 on semicon indirectly and its data should be shown by manufacturer if required -a. Visual examination after manufacturing in hot oil + impulse test (S; 12.3.7/§12.4.9)

a. Taken into account in accessories part

-a. See accessories

4 Qualification Procedures for HV and EHV AC Extruded Underground Cable. . . 149

Cable’s component A Inner extruded semiconducting screen

-a. Resistivity measurement before and after ageing (T: §12.3.9/§12.4.11) -b. Resistivity measurement before and after ageing (T: §12.3.9/§12.4.11)

-a. Stability of electrical properties with ageing

3 Chemical properties

-b. Stability of electrical resistivity after heat cycles

-a. Bending + 20 thermal cycles (T: §12.3/§12.4) -b. Bending (T: §12.3.3/§12.4.4) + 20 thermal cycles (T§12.3.6/§12.4.7) -c. Heat cycles (T§12.3.6/§ 12.4.7) and shrinkage test (T §12.4.13)

-a. Resistance to mechanical bending during manufacturing and installation -b. Stability of form with thermal constrains -c. Shrinkage of semicon

Specification/Threat C -a. Smoothening of electrical field at the insulation interface (avoid protrusions, voids, contaminants). -b. Avoid PD at inner surface of insulation -c. Resistivity sufficiently low, to avoid PD, in the whole range of service temperatures and to avoid voltage over semicon during fast transients

Test to check the functionality (Relevant paragraphs of IEC 60840 Ed.3/62067 Ed. 1 in brackets –IEC 62067 Ed. 1 in Italics) D -a. AC (R, T, PQ §9.3/§9.3, §12.3.6/ §12.4.7, §13.2.3 and impulse test (T, PQ: §12.3.7/§12.4.9, §13.2.4 -b. PD tests (R, T: §9.2/§9.2§12.3.4/§12.4.5) -c. Resistivity measurement (T: §12.3.9/§12.4)

2 Thermalmechanical properties

Function or Property B 1 Electrical properties

Table 4.14 (continued)

b. 20 cycles are considered sufficient by the WG to evaluate this functionality c. For type tests, the shrinkage test acts as a check on the properties of the material. For the long term test, the shrinkage test is not necessary when these long term tests are performed successfully on the cable system

Comments E a. Some standards and customer’s specifications prescribe to check on a sample that there are no protrusions, voids or contaminants larger than their permissible level. It is a good tool to check the quality of interfacesprotrusions and contamination. However electrical routine tests are considered necessary to have a view of the quality on the full length of the cable and are still mandatory.

150 J. Becker

4 Interface with conductor and insulation

-c. Compatibility test on whole cable (T:§12.4.4/§12.5.4) +Resistivity measurement of semicon before and after compatibility test (T: §12.3.9/§12.4.11)

-a. Compatibility test on whole cable (T:§12.4.4/ §12.5.4) -b. Type test (T: §12.3/§12.4) or long term test (PQ: §13.2) -c. Bending + heat cycling (min 80 cycles -D) + PD: ((T: §12.3/§12.4

-d. Examination of surface in hot oil

-c. Compatibility with layers or conductor below and with insulation

-a. Compatibility with conductor or layers below and with insulation (See above and underneath) -b. No degradation of insulation by migration of low molecular species -c. Good bonding of extruded semiconducting screen with insulation

-d. No deformation of semicon surface due to penetration of semicon between the conductor’s wires

(continued)

b. No PQ long term test in IEC 60840 Ed.3 c. Good bonding of extruded semiconducting screen with insulation is generally not a problem but should be checked at least once with a new material. Tests could be performed on model cables. 20 cycles are not considered enough to check whether the bonding with insulation remains correct. When 180 PQ cycles are performed, this property is indirectly checked d. Quality control

c. PIXE (Particle induced X-ray Emission) measurements have shown that there is no ionic migration from the semi-con into the insulation during heat cycling tests or in service. However, there is some diffusion of the antioxidant and low molecular weight species during the curing process and afterwards Some customers ask to check the moisture content in the semicon (150 (170) kV and
500 (525) kV” [6] Cigré SC21 Website paper Doc 97.07, 1997, Working Group 21.03 “Recommendations for electrical tests prequalification and development on extruded cables and accessories at voltages >150(170) kV and
500 (525) kV” [7] IEC 62067 Ed.1, October 2001,” Power Cables with extruded insulation and their accessories for rated voltages above 150 kV (Um ¼ 170 kV) up to 500 kV (Um ¼ 550 kV) – Test methods and requirements” [8] “Evolution of ac breakdown strength of XLPE HV cable after long term test, and correlation with physical properties” 4th International Conference on Conduction and Breakdown in Solid Dielectrics, Sestri Levante, Italia (1992). J. Bezille, H. Janah, J. Becker and H. Schädlich [9] “Evolution of ac and impulse breakdown strength of HV cable after long term test. Correlation with physical properties” C.E.I.D.P., Arlington, Texas, USA 94, Annual Report, pp. 582–87 (1994). J. Bezille, H. Janah, J. Chan and M. D. Hartley, See also Jicable 95, Versailles, France, paper A.8.3 (pp. 212–14)

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[10] “Les limites des études de vieillissement sur le polyéthylène”, Jicable 95, Versailles, France, paper B.9.1 (pp. 476–80) F. Duchateau et al. [11] “Optimisation des procédures physico-chimiques d’analyses destinées à l’étude du vieillissement du polyéthylène, isolant pour c^ables d’énergie” Jicable 95, Versailles, France, paper B.9.2 (pp. 481–87) F. Duchateau et Co-auteur [12] “Investigation of XLPE insulations after high stress ageing”, Jicable 95, Versailles, France, paper B.9.4 (pp. 494–99) H. Schädlich and J. Klass [13] “AC field ageing of power cables”, Jicable 99, Versailles, France, paper B 3.3 (p405) J. L. Parpal, P. Mirebeau, D. Coelho, H. Janah, F. Gahungu, J. Cardinaels et D. Meurer [14] “Assessment under high dielectric field of the long term behaviour of power polyethylene insulant”, Jicable 99, Versailles, France, paper B 4.2 (pp 424–429) R. Clavreul, M. H. Luton, J. Berdala, H. Janah and P. Laurenson [15] “Evaluation of modelling of thermo-electric ageing of XLPE insulated power cables: the ARTEMIS outcome”, Jicable 03, Versailles, France, paper B.7.5 (pp 525–530) C. Laurent and A. Campus [16] “Electrical Degradation and Breakdown in Polymers”, IEE Materials and Devices Series 9, Peter Peregrinus Ltd, 1992, ISBN 0-86341-196-7 Dissado L. A and Fothergill J.C [17] ELECTRA 140, February 1992, Working Group 21.09 “Considerations of ageing factors in extruded insulation cables and accessories” [18] ELECTRA 139, 1991, Working Group 21-09 “Working Gradient of HV and EHV Cables with Extruded Insulation and its Effects” [19] ELECTRA 137, 1991, Working Group 21-10 “Survey of the service performance on HV AC cable systems” [19 bis] Cigre TB 379, April 2009, Working Group B1,10 “Update at Service Experience of HV Underground and Submarine Cable Systems” [20] Cigré TB 89, 1994, Working Group 21.06, “Accessories for HV extruded cables, types of accessories and terminology” [21] “Development and Installation of Long-Distance 275-kV XLPE Cable Lines in Japan”, Cigré paper 21-102, Paris 1990. K. Kaminaga, T. Asakura, Y. Ohashi, Y. Mukaiyama [22] “Prequalification testing of EHV XLPE cable system”; Understanding and Managing Underground Transmission and Distribution Cables, CEA Workshop, June 10–13, 2001. Jean-Luc Parpal. [23] “New 400kV XLPE Long Distance Cable Systems, Their First Application of the Power supply of Berlin” Cigré paper 21-109, Paris 1998 Henningsen C. H., Muller K. B., Polster K. and Schroth R. G [24] “Development of High Stress HV and EHV XLPE Cable Systems”, Cigré paper 21– 108, Paris 1998 Attwood J. A., Gregory B., Dickenson M., Hampton R.H. and Svoma R [25] “Development of a 420 kV XLPE Cable System for the Metropolitan Power Project in Copenhagen”, Cigré paper 21-201, Paris 1996 P. Andersen, M. Dam-Andersen, L. Lorensen, O. Kjaer Nielsen, S.H. Poulsen, B.S.Hansen, T. Tanabe, S. Suzuki [26] “State of the Art in EHV XLPE Cable Systems”, Jicable 99, Versailles, France, paper A 2.2 (pp 44–49) A. Bolza, D. Kunze, S. Norman, S. Pöhler

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[27] “Long term test of 500kV XLPE cables and accessories” Cigré, Paper 21-202, Paris 1996 Kaminaga K., et al. [28] “Prequalification Testing of 345kV Extruded Insulation Cable System” Cigré paper 21-101, Paris 1998 Parpal J.L. et al. [29] “Prequalification testing of 290/500(525) kV extruded cable system at IREQ”, Jicable99, Versailles, France, paper A.2.3 (pp 50–55) Parpal J. L. et al. [30] “Development and Qualification of a new 400 kV Cable System with Integrated Sensors for Diagnostics”, Cigré paper 21-103, Paris 1998 G.P. Van der Wijk, E. Pultrum, H.T.F. Geene [31] “Qualification of a highly electrically and mechanically stressed AC cable system”; Jicable 03, Versailles, France, paper A.2.1 (pp 38–44) Erisson et al. [32] “Design of a new 150 kV cable system for the Belgian electrical network”, Jicable 99, Versailles, France, paper A.1.5 (pp 25–30) Couneson/Argaut et al. [33] “150 kV underground links in Belgium: A new technical stage for XLPE insulated cables”; Cigré paper 21–101, Paris 2000 Couneson/Becker et al. [34] “Development of factory expanded cold shrinkable joint for HV XLPE cables”; Jicable 03, Versailles, France, paper A.5.1 (pp 148–153) Kobayashi et al. [35] “Development of cold shrinkable joints for 110-230 kV XLPE cables”; Jicable 03, Versailles, France, paper A.5.3 (pp 164–169) Nakamura et al. [36] “Super compact rubber block joint with high dielectric constant layer”; Jicable 03, Versailles, France, paper A.6.1 (pp 175–180). Ninobe et al. [37] “Micro varistor based field grading elements for HV terminations”; Jicable 03, Versailles, France, paper A.6.3. (pp 186–190) Gramespaker et al. [38] “Anti explosion protection for HV porcelain & composite terminations”; Jicable 03, Versailles, France, paper A.6.2 (pp 86–190) Gahungu et al. [39] “New dry outdoor termination for HV extruded cables”; Jicable 03, Versailles, France, paper A.6.4 (pp 191–196) Dejean et al. [40] “Plug-in type connection technique using HV-connex on encapsulated components in high voltage equipment up to Um ¼ 245 kV”; Jicable 03, Versailles, France, paper A.6.5 (pp 197–198) Deister et al. [41] “Type testing of cables and accessories”; Jicable 99, Versailles, France, paper C.10.1 (pp 880–883) Berlijn et al [42] “Type testing of cables and their accessories, some statistics”; CIRED May 2003 Pultrum et al. [43] French specification for HV cables: NF C 32 352 “Insulated or protected cables for power systems. Single core cables with polymeric insulation of rated voltages above 30 kV (Um ¼ 36 kV) up to 150 kV (Um ¼ 170 kV)” [44] French specification for EHV cables: C 33 253 “Insulated cables for power systems. Single core cables with polymeric insulation of rated voltages above 150 kV (Um ¼ 170 kV) up to 500 kV (Um ¼ 525 kV)” [45] “Testing of extruded cables: experiences in type testing, PQ testing and test after installation. What do we learn from it?”; Cigré paper B1-104, Paris 2004 E. Pultrum, S.A.M. Verhoeven [46] Contribution to the discussion of the Cigré SC B1 session, Paris 2004 E. Dorison

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[47] ELECTRA 205, December 2002, TF21-05 “Experiences with AC Tests after installation on the main insulation of polymeric (E) HV cable systems” [48] JEC-3408-1997, Standard of The Japanese Electrotechnical Committee, “High Voltage Tests on Cross-Linked Polyethylene Insulated Cables and their Accessories for Rated Voltage From 11kV up to 275kV”; The institute of Electrical Engineers of Japan [49] “Prequalification testing of EHV XLPE cable systems”; CEA Workshop, June 10–13, 2001 J.L Parpal. [50] “Prequalification testing of 290/500 kV Extruded cable system at IREQ”; Jicable 99, Versailles, France, paper A 2.3 (pp 50–55) J.L Parpal at al. [51] “EHV XLPE cable systems up to 400 kV – more than 10 years filed experience”; Cigré paper B1-102, Paris 2004 W. G. Weissenberg, U. Rengel, R. Scherer [52] “Prequalification test of 400 kV XLPE cable system “; Jicable 95, Versailles, France, paper A.1.3 (pp 11–15) Helling K., Henningsen C.G., Polster K., Bosotti O., Mosca W., Tellarini [53] Cigré TB 210, 2002, Joint Task Force 21/15 “Interfaces in accessories for extruded HV and EHV cables” [54] “Development of a 420kV XLPE Cable System for the Metropolitan Power Project in Copenhagen”; Cigré paper 21-201, Paris 1996 [55] “Construction of the world’s first long-distance 500kV XLPE cable line”; Cigré paper 21-106, Paris 2000

Jean Becker was born on 30 August 1938. He got a diploma as Electrical engineer AIM AILG in Electronics and Electrotechnique from the University of LIEGE in Belgium as: “Master of Science in Engineering.” From 1964 to 2003 he has been in the Electrical Cable Business. Involved in the development and testing of all kinds of cables (low, medium, and high voltage, communication cables, special cables), in the manufacturing of low, MV, HV, and EHV cables, in the development and testing of HV and EHV accessories in the design of HV and EHV links, and the installation of HV and EHV cables. He was the Competence Center Manager of the Extra High Voltage Cables for the Nexans Group during the last 10 years of his career in this business career. Since 1978, he was a Member of IEC TC20-WG16, dealing with the international specifications of low, medium high, and extra high voltage cables, accessories, and cable systems. Since 1985 he was in Cigré as an expert in HV and EHV cable systems. He has been the Convener of two Cigré Working Groups and contributed to several other Cigré WGs as an expert. Since his retirement in 2003, he was a consultant. As such he continued to work with Cigré and has been involved as an expert in breakdown problems of HV and EHV cable systems. Jean was serving as Secretary of the ISTC of Jicable 2015 when he suddenly passed away in April 2015, leaving the Cable Community in great sorrow.

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Cable Accessory Workmanship on Extruded High Voltage Cables Kieron Leeburn

Contents 5.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Inclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Exclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Related Literature and Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1 Related Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2 Additional Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 General Risks and Skills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Technical Risks and Required Specific Skills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.1 Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.2 Insulation Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.3 Metallic Sheath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.4 Oversheath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.5 Installation of Joint Electric Field Control Components . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.6 Installation of Termination Electric Field Control Components . . . . . . . . . . . . . . . . . 5.6.7 Outer Protection of Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.8 Filling of Terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.9 Handling of Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7 Skills Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.1 Aspects to be Tested . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.2 Methods of Qualification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.3 Certification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.4 Duration of Certification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.5 Upskilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.6 New Accessory Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

192 192 194 194 194 194 194 195 195 197 197 202 209 217 217 227 230 233 233 237 238 238 239 239 240 240

Published as Cigré TB 476 in October 2011 K. Leeburn (*) CBI Electric African Cables, Chief Engineer Process and Product in HV, Vereeniging, South Africa e-mail: [email protected] © Springer Nature Switzerland AG 2021 P. Argaut (ed.), Accessories for HV and EHV Extruded Cables, CIGRE Green Books, https://doi.org/10.1007/978-3-030-39466-0_5

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5.8 Set Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.1 Organisation of Jointing Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.2 Positioning of Joint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.3 Environmental Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.4 Cable End Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.5 Verification of Each Step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.6 Measuring of Diameters, Ovality, Concentricity, Position . . . . . . . . . . . . . . . . . . . . . . . 5.8.7 Safety and Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.8 Environmental Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.9 Quality Insurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A: Model Certificate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix B: QA Document . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.1

240 240 241 241 241 241 242 242 242 242 243 249 256

Summary

This Chapter 5 (Published as Cigré TB 476) covers workmanship associated with the jointing and terminating of AC land cables incorporating extruded dielectrics for the voltage range above 30 kV (Um ¼ 36 kV) up to 500 kV (Um ¼ 550 kV). Cigré TB 476 is a complement of Cigré TB 177 (See ▶ Chaps. 1, “Compendium of Accessory Types Used for AC HV Extruded Cables” and ▶ 2, “A Guide to the Selection of Accessories”), the recommendations of which are not questioned in this chapter. A short section covers general risks and skills, but the bulk of the chapter focuses on the specific Technical Risks and the associated skills needed to mitigate these risks. This is done for each installation phase. This Chapter is not an Instruction Manual, but rather gives guidance to the reader on which aspects needs to be carefully considered in evaluating the execution of the work at hand. The supplier’s Instruction Manual is considered the primary source of technical information. A section on skills assessment helps the qualification of jointers. Finally, attached appendices give samples of a certificate and QA documentation. This chapter is intended for a broad range of readers. It is risk mitigation focussed so the reader can develop his personal use for the document.

5.2

Introduction

High Voltage cable accessories are manufactured using high quality materials and very sophisticated production equipment. Recent technical and technological developments in the field in their design, manufacturing and testing have made it possible to have pre moulded joints and stress cones for terminations up to 500 kV as well as cold shrink joints for up to 400 kV. One conclusion of Cigré TB 379 – Update of service experience of Underground and Submarine cables – is that internal failure rates of accessories, particularly on XLPE cable are higher than other components and are of great concern. Focus on quality control during jointing operations must be maintained. Many utilities have adopted the “system approach” by purchasing the cables as well as the major accessories from same supplier. Some of these utilities would also

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request that the link should be installed by the supplier or by a contractor under the supplier’s supervision in a “turn Key” fashion. The main advantage of this approach is that the entire responsibility for the materials and workmanship is clearly the supplier’s. Some customers have adopted the component approach by purchasing the cables and the accessories from different suppliers and to entrust the installation to a third party. In all cases, it is imperative that, the installation be carried out by qualified jointers who follow the jointing instructions provided by the supplier. International standards such as IEC and IEEE provide the necessary guidelines concerning the interface between cables and accessories. However, it is highly recommended that the responsible engineer should satisfactorily verify the compatibility of the different components of the link. It is of vital importance to manage the interface between the cables and the accessories in order to reduce the potential technical risk. One of the trends that have been developing in the international cable technology is the reduction of the cable insulation thickness and the corresponding increase of electrical stresses. This tendency is based on a better knowledge and an improved quality of the insulating material and the extrusion process. The cables and accessories are made under well-defined factory conditions. Their quality and reliability are assured by adherence to well defined specifications. The accessories, however, are mounted on site, and notwithstanding that this job is done by skilled and trained jointers, it is often performed in more delicate and undefined conditions than in the factory. It is noted that most of the new HV links will be built using XLPE insulated cables. With the imminent retirement of experienced jointers, a major shortage in this field has been identified. There are few well structured training programmes and accreditation processes in place in order to meet demand. Jointer skills are vital in ensuring the reliability of the new links. This chapter captures the state of the art of Jointing. It is considered the Best Practice by the members of the SC B1 Study Committee as of 2009. It is acknowledged that other practices which are not explicitly covered in this brochure are not necessarily bad practices. Great care should be exercised and the approach agreed where a departure from this chapter is envisaged. Where alternative techniques are detailed, no preference is intended nor implied unless specifically mentioned. Diagrams are provided to illustrate the concept described and should not be interpreted literally. Working under induced voltages or currents is not considered in this Chapter. As mentioned in Sect. 5.8.7, in this case precautions have to be taken to eliminate or minimize the risk further work is in progress by WG B1.44. Note: For the range above 36 kV, the risks associated with jointing are considered significant due to the risk of a Medium 7Voltage (MV) jointing (continued)

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philosophy being applied to High Voltage (HV) cables. Cigré TB 303 (▶ Chap. 4, “Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems” of this book) indicates the qualification procedures for HV and EHV AC extruded underground cable systems.

5.3

Scope

5.3.1

Inclusions

The Scope is limited to all accessories of: • • • • •

Extruded dielectric cables; AC cables; Land cables; HV Cables covered by IEC 60840; EHV Cables covered by IEC 62067; Note: Asymmetric joints (eg different conductor material; conductor size; insulation thickness; . . .etc. . .) are not specifically covered as the permutations are too numerous. Where these are encountered, each of the components should be evaluated in terms of the Technical Risks and the required General and Specific Skills needed.

5.3.2

Exclusions

The scope specifically excludes: • • • • • • • • •

After installation tests; Cable pulling and Laying; Direct Current Cables; Fault finding; Fluid Filled Cables; Maintenance; Submarine Cables; Superconducting Cables; Transition joints between Fluid Filled and Polymeric cables.

5.4

Related Literature and Terminology

5.4.1

Related Literature

• IEC 60050 Chapter 461: electric cables.

5

•

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The defined vocabulary can be assumed valid throughout this brochure except where specific note to the contrary is made. Cigré TB 177 –Accessories for HV cables with extruded insulation. This brochure is still a valid guide to the selection of accessories (▶ Chaps. 1, “Compendium of Accessory Types Used for AC HV Extruded Cables” and ▶ 2, “A Guide to the Selection of Accessories”). Annex in ▶ Chap. 1, “Compendium of Accessory Types Used for AC HV Extruded Cables” concerns the terminology (update of TB 89). It is adopted in its entirety except where specific note to the contrary is made. Cigré TB 194 – Construction, Laying and installation techniques for extruded and self contained fluid filled cable systems. Cigré TB 210 JTF 21/15 – Interfaces in high voltage accessories (▶ Chap. 3, “Interfaces in Accessories for Extruded HV and EHV Cables”). AEIC CG4-97 Guide for installation of extruded dielectric insulated power cable system rated 69 kV through 138 kV (2nd ed.) TB 379 Update of service experience of HV Underground and Submarine Cable Systems (2004–2008)

5.4.2

Additional Terminology

• Jointing – A process referring generically to all types of assembly/mounting of both joints and terminations. The term splicing is used in North America. • Jointer – A person skilled in the art of Jointing. The term splicer is used in North America. • Due Care – This refers to familiarity with the specific activity, tool or material being handled. It is intended to stress the importance of understanding and precisely executing the work to be carried out. • Technical Risk – An aspect, which, if not mitigated, could lead to the premature failure of the cable and/or accessory. • Good practices – Recommendation, based on practical experience, which can mitigate Technical Risks. • Work phases – Installation steps during cable workmanship of cable accessories. • General skills – Skills normally acquired by jointers through training/exposure to common HV cable accessories. • Specific skills – Skills not commonly acquired. Requires specific training. In this brochure tables conclude the general and specific skills and technical risks related to a work phase.

5.5

General Risks and Skills

The quality and performance of any new link or replaced joints and terminations are highly dependent on the skills and competence of the jointers who need to ensure the proper installation of these accessories under less than ideal field conditions (Table 5.1).

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Table 5.1 General risks and required skills Work Phase Preparing the jointing area

Risks Accidents leading to cable or accessories damage Electric shocks Traffic accidents Collapse of joint bay or trench

Cable preparation Cable straightening

Underside cable is blind Over heating of insulation Mechanically damaging the cable Incomplete removal of graphite or semi-conductive coating Cutting cable too short

Cable outer sheath cleaning Cutting the cable Preparing cable insulation Plumbing

Installing terminations All phases Access to site with installation under voltage

Rough insulation surface leading to bad interface between cable and accessory Local fire Burns and loss of life or materials

Falling, injury Personal injuries Personal injuries

General skills Sense of organisation and selection of proper tools and equipment Knowledge of electricity (voltage induction, absence of voltage, phasing etc.) Proper grounding connections Familiarity with safety and security measures Proper bracing Due care Use of electric heaters Use of hydraulic equipment Due care Proper measurements Use of an electric saw Proper use of sanders Meticulous sanding Mastering the use of an open flame (torch) Mastering the use of fire extinguisher Working on scaffolding and at heights First aid help and reanimation Training to have the authorisation to work on site with installation under voltage

Systematic and compulsory training is required by all High Voltage jointers. However, other basic and general skills are also important. These include: • • • • • • • •

Sense of observation and organisation; Environmental and safety awareness; Problem solving, sometimes called “common sense”; Ability to read and interpret drawings and instructions in the relevant language; Good knowledge of materials and their physical and mechanical properties; Good familiarity and handling of different electric and hydraulic tools; Good understanding of electricity; Precision in taking physical measurements. Other essential basic attributes include:

• Patience; • Dexterity; • Discipline;

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• • • •

Sense of engagement; Responsibility; Physical fitness; Mental fitness.

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Technical Risks and Required Specific Skills

The primary source of technical information is the instruction manual supplied by the accessory manufacturer. The required skills listed here are generally ordered from the conductor outwards. It should be emphasised that this is not the order of assembly of the accessory (some components need to be “pre-parked” before the conductor is joined as these components cannot be added later). It is essential that the jointer be well trained in the necessary skills. Each element below first describes the procedure and then the associated known risks as well as the essential skill set needed. In addition it is emphasised that adherence to the instruction manual is essential.

5.6.1

Conductors

5.6.1.1 Conductor Preparation The preparation phase includes: • Cutting conductors according to the relevant instruction manual; • Removing insulation using an approved tool; • Protecting the cable from damage and metallic particles, while cutting the conductor; • Cleaning the insulation surface with an approved solvent, if it has been contaminated; • Removing tapes and powders; • Cleaning conductor wires of fillers or coating compounds before jointing.

5.6.1.2 Compression Deep indentation, hexagonal and other techniques of crimping are considered here. These techniques include: • Cleaning any enamel coating e.g. by applying heat or abrasion if applicable, otherwise MIG/TIG welding must be adopted (Figs. 5.1 and 5.2); • Deforming the ferrule and conductor by deep indentation or compression. It is suitable for both copper and aluminium conductors; • Using an hydraulic press; • Carefully choosing the correct dies or punch and ensuring their compatibility with the press jaws; • Checking the number, position and sequence of compression as it may vary depending on the conductor size and the compression tool capability. Aluminium

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Fig. 5.1 Cable end prepared for top connector application

Fig. 5.2 Copper ferrule suitable for compression

conductors usually require longer ferrules and more compression positions than copper conductors; • Removing, any sharp edges or marks from the ferrule, after compression, unless it is screened by a metallic shield (Figs. 5.3, 5.4, and 5.5). Good practice includes: • Performing a trial compression on a spare sample of the actual conductor using a spare ferrule and the actual tools and dies available.

5.6.1.3 MIG/TIG Welding This technique includes: • Using arc welding in gas with a feed wire of copper or aluminium; • Ensuring that the wire is appropriate to the welding machine/method;

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Fig. 5.3 Examples of presses for compression Fig. 5.4 Hexagonal compression

• Cutting the conductor ends diagonally to form a V shape when placed in the welding jig; • Avoiding overheating of insulation during welding. Heat sinks or forced coolers are generally applied on both sides of the exposed conductor and temperature monitored with thermocouples; • Removing enamel where enamelled copper wires are MIG/TIG welded, prior to welding; • Removing any sharp edges or marks from the connector, after welding. Good practice includes: • Performing trial MIG/TIG Welds on a spare sample of the actual conductor using the actual jigs and welding equipment.

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Fig. 5.5 Deep indentation

5.6.1.4 Thermit Weld Sometimes called exothermic welding or Cadweld. It uses chemical reagents in a reusable crucible, placed above a mould specifically designed for the conductors being welded. This technique includes: • Placing the correct quantity of reactants in a crucible; • Taking care to avoid porosity and cavities in the welding mass due to any presence of moisture or filler in the conductor; • Carefully setting the gap between the conductors; • Setting the crucible and mould assembly; • Firing the reactants so they drop into the mould, melting the ends of the conductors together (Fig. 5.6); • Dressing the weld. During this process, the presence of any porosity should be noticeable. Note: A safety and health risk is the high explosive reaction and formation of gases that prevent this technique being used in confined areas. Good practice includes: • Preheating the mould to remove moisture; • Performing a trial thermit weld on a spare sample of the actual conductor (or connector) and the actual weld metals and moulds available (Fig. 5.7).

5.6.1.5 Mechanical Connection This technique uses bolts to apply pressure to the underlying conductor. It can be used on both copper and aluminium conductors. Either bolts are tightened until they shear ensuring the correct connection force or they are tightened by a torque wrench

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Fig. 5.6 MIG welding

Fig. 5.7 Thermit Weld

to a specified torque. One connector may cover multiple conductor sizes. These connectors do not require special tools. This technique includes: • Tightening the bolts in sequence as prescribed by the instruction manual; • Removing any sharp edges or marks from the connector after breaking the head of bolts by torque; • Filling any holes if applicable (Figs. 5.8 and 5.9). Good practice includes: • Restraining the connector while applying tightening torque for small conductor sizes (Table 5.2).

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Fig. 5.8 Mechanical connectors with shear bolts

Fig. 5.9 Mechanical connectors tightened with torque wrench

Table 5.2 Technical risks and specific skills for conductor connection techniques Work phase Conductor preparation Compression and indentation MIG/TIG welding Thermit weld

Mechanical connection Finishing of connectors

5.6.2

Technical Risks Contamination of insulation Wrong dies or punch Wrong press Overheating of the cable Moisture Porosity Overheating of the cable insulation Incorrect gap Bolt too deep

Specific Skills Cleaning of conductor and filler removal

Sharp edges Depressions

Due care

Due care Cleaning of conductor and filler removal Specific MIG/TIG WeldingTechniques Cleaning of conductor and filler removal Thermit Welding

Due care

Insulation Preparation

The preparation of the cable insulation is considered to be the most critical step in the installation of accessories on extruded cables.

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5.6.2.1 Straightening In general, accessories require straightness of the cable during preparation. This can be achieved by either cold straightening or hot straightening techniques. Cable has the tendency to bend again as a result of the elastic memory of the insulation. 5.6.2.1.1 Cold Straightening This technique includes: • Straightening cable by bending if it has a solid Aluminium conductor. After mechanical straightening, the cable will remain in its corrected position; • Straightening of cables with stranded conductors by bending the cable beyond its straight position and letting it return to a straight neutral position. 5.6.2.1.2 Hot Straightening This technique includes: • Heating the insulation to the specified temperature for the specified duration; • Cooling it down while the cable is fixed in a straight position. The temperature and duration can vary depending on conductor size as well as, insulation material and thickness (Table 5.3).

5.6.2.2 Stripping of Insulation Screen During this step, it is essential to follow the instruction manual requirements especially with regard to: • prepared core diameter; • required roundness of the cable insulation. Methods used for removing the screen are peeling, scraping and hot stripping or a combination of these. 5.6.2.2.1 Peeling This common technique includes: • Carefully setting the tool to minimise the loss of insulation; Table 5.3 Technical risks and specific skills for cable straightening techniques Work phase Cold straightening Hot straightening

Technical risks Excessive bending

Specific skills Due care

Overheating due to inadequate temperature control Hot core is more prone to mechanical damage

Operation of heating equipment

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• Moving the peeling tool which contains a specially shaped knife in a circular direction to remove the screen. It is inevitable that during peeling some core insulation will be removed too (Figs. 5.10 and 5.11). Good practice includes: • Performing a peeling trial on an off-cut of the actual cable to be jointed to check the setting of the tool. 5.6.2.2.2 Scraping In most cases this technique uses glass and includes: • Moving a fragmented, sharp piece of glass at a shallow angle over the insulation screen, thereby removing the semiconducting layer until the cable insulation becomes visible;

Fig. 5.10 Hot straightening of cable while fixed in straight position

Fig. 5.11 Examples of peeling tools

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• Repeating around the circumference ensuring even removal while avoiding flat spots, cuts and dents. The scraping method results in a minimum loss of cable insulation, but requires great skill. Scraping can be combined with peeling in order to reduce the installation time (Fig. 5.12). 5.6.2.2.3 Hot Stripping This can only be performed on cables with strippable screens. This technique includes: • Heating the insulation screen with a torch or hot air gun; • Cutting the screen longitudinally; • Stripping the pieces like a banana. Hot stripping is a common method for cables with EPR insulation. The amount of heat applied should be carefully controlled (Table 5.4).

5.6.2.3 Preparing the End of the Insulation Screen It is essential that the transition from the insulation screen to the cable insulation surface is: • Correctly tapered without depression particularly in the insulation; • Smoothly prepared without any step; Fig. 5.12 Scraping by glass

Table 5.4 Technical risks and specific skills for removing the insulation screen techniques Work phase Peeling Scraping Hot stripping

Technical risks Uneven travel of the peeling tool Blunt tool Removing too much insulation if the cable is not completely round Flat spots on the insulation surface Cuts/dents in the insulation surface Burning of cable surface

Specific skills Appropriate to insulation type Glassing Handling the torch on semiconducting layer

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• Within the specified dimensional tolerances. Irregularities in this area can lead to a mismatch between field control body and the cable insulation causing field enhancement and reduction of the interface pressure. The end of the insulation screen can be chamfered by means of peeling or scraping. Peeling tools used for this purpose contain a specially angled knife. The chamfer can also be achieved by carefully scraping with glass. Sometimes semi-conducting paint is used to achieve a fine tapered transition. Good practice includes: • Checking the peeling tool settings by performing a trial on a spare piece of cable.

5.6.2.4 Smoothening the Insulation Surface The quality of the interface between the cable insulation and field control body significantly affects the reliability of the joint (▶ Chap. 3, “Interfaces in Accessories for Extruded HV and EHV Cables”: ref. Cigré TB 210, Cigré JTF 21/15 Interfaces in high voltage accessories). Installation instructions should clearly indicate the cable preparation details, including (Fig. 5.13): • The smoothing technique • The required degree of smoothness • Dimensional tolerances. The methods of smoothing the insulation surface include polishing, melting and a combination of polishing and melting. 5.6.2.4.1 Polishing This common technique involves circumferential sanding of the insulation to remove grooves remaining from the peeling or scraping process. Generally emery cloth is used with grain sizes ranging from 150 to 400 grit. For EHV accessories, a grain size finer than 400 grit may be needed to achieve sufficient smoothness. Fig. 5.13 Chamfering of the end of the insulation screen

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5.6.2.4.2 Melting This phase achieves a smooth insulation surface by melting and deforming the cable insulation surface (Figs. 5.14; Table 5.5). Melting techniques include: • Applying a heat shrinkable tube made from fluorine or silicone rubber over the cable insulation and shrinks it to fit on the insulation surface. The temperature of the insulation surface is then controlled to above the melting point of the cable insulation. The smoothness of the inner surface of the tube is transferred to the insulation surface during the application of heat; • Applying heat directly to the surface of the insulation. This is usually done with a hot air gun rather than a flame which could scorch the surface of the insulation (Table 5.6). Flat spots in the cable surface should be avoided, as these could result in areas of low interfacial pressure.

5.6.2.5 Cleaning of Insulation The cable insulation surface has to be thoroughly cleaned in order to remove any residue left during the insulation preparation.

Fig. 5.14 Polishing of the insulation

Table 5.5 Technical risks and specific skills for preparing the end of the insulation screen Work phase Peeling

Scraping Painting

Technical risks Uneven travel of the peeling tool Blunt tool Removing too much insulation if the cable is not completely round Cuts/dents in the insulation surface Thick edge (untapered)

Specific skills Handling peeling tools Glassing Due care

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Table 5.6 Technical risks and specific skills for smoothening the insulation surface Work phase Polishing (sanding) Melting

Technical risks Surface roughness Incorrect diameter Excessive ovality Eccentric core preparation Flat spots on the insulation surface Overheating, burning and deformation of cable surface Damage during removal of heat shrink tube

Specific skills Due care Heating technique Due care

Table 5.7 Technical risks and specific skills for cleaning the cable insulation surface Work phase Cleaning with solvent

Technical risk Wrong solvent Too long exposure Dissolving of semiconducting paint Cross contamination

Specific skills Due care

This is best achieved by using a lint free cloth or tissue wetted with an appropriate cleaning fluid. Only solvents supplied with the jointing kit, or specifically defined (full chemical name), should be used due to the risk of incompatibility. Where semi-conducting paint has been used, be aware that the solvent can remove the paint. Water based cleaning fluids are strongly discouraged as they might leave moisture or residues like soap on the surface. Good practice includes • Cleaning from the conductor end towards the semiconducting screen cut and disposing of the cloth thereby preventing contamination (Table 5.7).

5.6.2.6 Shrinkage Some insulation has stretch memory introduced into its molecular structure during the extrusion process. When heated (during load) the insulation may revert to its relaxed state. This shrinkage can cause a mismatch of the field control components. Three known methods of mitigating this risk are: • Locking the insulation e.g. by applying a clamp over the joint connector which grips into specially peeled grooves in the insulation; • Pre-shrinking the insulation to ensure all potential shrinkage in the joint has already taken place; • Tolerating the anticipated shrinkage in the design of the joint.

5.6.2.7 Lubrication Lubricants are used to relieve the friction between different surfaces (cable and accessories) during installation. Lubricants can fill possible gaps and increase the initial breakdown strength. It is recommended that jointers do not take advantage of

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Table 5.8 Technical and specific skills for limitation of the cable insulation shrinkage Work phase Insulation groove Pre-shrinking

Technical risk Wrong dimensions Overheat the insulation

Specific skills Due care Handling of equipment

Table 5.9 Technical risks and specific skills for lubrication phase Work phase Lubrication

Technical risk Polluting of lubricated surface

Technical skill Due care

this feature, as lubricants will eventually be absorbed by the insulating materials, resulting in reduced breakdown strength. Lubricants used are commonly based on silicone oil or silicone grease. The lubricant should be supplied in the jointing kit, or specified by the accessory manufacturer to ensure compatibility with cable and accessory components (Tables 5.8 and 5.9). Care should be taken to avoid contamination by pollutants sticking to the lubricant.

5.6.3

Metallic Sheath

The metallic sheath on cables is usually applied as a moisture barrier and mechanical protection and/or to conduct sheath currents (inductive and capacitive) and fault currents. The connection between metallic sheath and accessory casing (joint shell or wiping bell of the termination) should maintain these characteristics.

5.6.3.1 Welded Aluminium Sheath (WAS) 5.6.3.1.1 Preparation of Cable Sheath This preparation phase includes: • Cutting the welded aluminium sheath and the outer sheath perpendicular to the cable axis; • Taking great care to avoid damaging the underlying cable core; • Using specifically designed tools for these operations; • Making longitudinal cuts to remove the sheath and facilitate the mechanical and electrical connections to other metallic components and allowing more room for subsequent steps. 5.6.3.1.2 Metallic Sheath Continuity Two methods can be used to maintain the earth screen continuity:

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Fig. 5.15 Connection on top of the aluminium sheath

Welded aluminium sheath Outer sheath

Accessory casing Plumbing

CABLE CORE

Connection on Outside of the Aluminium Sheath

This connection phase includes (Fig. 5.15): • Peeling of the non-metallic outer sheath. This is usually done by applying heat to the outer sheath; • Cleaning of the aluminium surface to eliminate, amongst other things, the factory applied glue and the aluminium oxide; • Tinning of the accessory casing which is generally in copper or aluminium; Connecting the WAS to the accessory casing by using the plumbing technique.

Connection under the Aluminium Sheat

This connection phase includes: • • • •

Making longitudinal cuts of the aluminium sheath and the outer sheath; Opening out the aluminium sheath bonded to the outer sheath; Inserting a connecting piece under the aluminium sheath; Ensure an electrical and mechanical contact between the connecting piece and the aluminium; • Tinning the accessory casing which is generally in copper or aluminium; Joining the connecting piece to the accessory casing by using the plumbing technique or by mechanical assembly.

Additional Copper Wire Insulation Screen

Where a copper wire screen is applied in combination with the WAS, the connection can include the following: • Plumbing the wires into the tin wipe; • Connecting the copper wires with the accessory casing using mechanical means (eg lug); • Connecting the copper wire screen of both cable ends (for joints) using mechanical means (eg ferrule);

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• Connecting the copper wires directly with the bonding cables. (This can only be applied if the copper wire screen is rated to handle the sheath currents). Reinforcement

Glass fibre reinforced epoxy resin may be required over the tin wipes to improve their mechanical strength (Fig. 5.16; Table 5.10).

5.6.3.2 Corrugated Sheaths: Aluminium (CAS); Copper (CCS); Stainless Steel (CSS) The techniques described apply to all three types unless specifically indicated otherwise. The corrugation can be helical or discrete ring shape. 5.6.3.2.1 Preparation of Cable Sheath The preparation phase includes: • • • •

Removing of the outer sheath perpendicularly to the cable axis; Cleaning of bitumen and other coatings; Removing of the oxide layer (CAS) by brushing vigorously; Applying tin coating to a section of the corrugated sheath using the appropriate flux; • Removing of the corrugated sheath at the tinned section and rounding the remaining edge. Note: On CSS, longitudinal cutting is not recommended due to the hardness of the stainless steel. One method of preparation is to drill a hole on a crest of the corrugation and insert a special cutter in the hole. The sheath is cut along the helical crest. After completion of one revolution, the trough is cut by special scissors.

Accessory casing Welded aluminium sheath

Plumbing or mechanical assembly

Tightening device

Liaising piece

CABLE CORE

Fig. 5.16 Connection under the aluminium sheath

Outer sheath

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Table 5.10 Technical risks and specific skills for WAS connection Work phase Preparation of cable sheath Metallic screen continuity Connection on top of WAS

Metallic screen continuity Connection under WAS Plumbing

Copper wire screen

Reinforcement

Technical Risks • Cutting into the underlying layers • Overheating underlying layers • Weak metallic screen continuity due to an incomplete cleaning or preparation of the aluminium sheath or/and the metallic connection before plumbing • Faulty earth screen continuity due to an improper mechanical connection • Damage to underlying layers • Overheating the cable core • Melting of the aluminium sheath • Enclosing voids in the tin wipe, which could lead to a weak connection • Poor plumbing connection of the wire screen • Poor mechanical connection due to wrong dies or press • Uncured resin • Faulty glass-fibre tape application

Specific Skills • Handling of specific tools for this work • Removing the glued outer sheath • Removing glue • Tinning and Plumbing Techniques • Controlling the heat of the torch • Due care

• Plumbing Techniques • Controlling the heat of the torch

• Plumbing technique • Crimping techniques • Resin mixing and glass-fibre application techniques

Good practice includes: • Cutting the metallic sheath on the crest of the corrugations. 5.6.3.2.2 Metallic Screen Continuity Two common methods used to make electrical connection onto corrugated sheaths are Plumbing and Soldering (Fig. 5.17). Plumbing

This technique includes: • • • •

Applying a special tin alloy under heat to the surface of CAS; Applying plumbing grease as a soldering flux, while heat is applied with a torch; Deforming, compacting and smoothening the tin alloy by means of wiping; Building up the valleys to provide a flat surface (platform) for making earthing connections easier; • Joining the connecting piece using similar plumbing techniques. These connection pieces can be solid or flexible.

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Outer sheath

Apply tin coating

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Cut on crest and round remaining

Fig. 5.17 Preparation of cable sheath

Good practice includes: • Leaving as much metallic sheath on the cable as possible to act as a heat sink during the plumbing process as well as protecting the underlying layers from splatter during the wiping process; • Moving the torch circumferentially to ensure even heat distribution; • Applying generous amount of tallow will keep the area of the wipe cool. Note: Making a good, solid and smooth tin wipe, requires specific skills from the jointers. Jointers that have been used to work with fluid filled accessories should have the required specific skills. Soldering

Where the connecting piece is braided tinned copper, this technique includes: • • • •

Tinning the surface of the sheath using the appropriate flux; Fastening the braid to the tinned sheath using tinned copper binding wire; Soldering the braid and binding wire to the tinned aluminium (copper) sheath; Repeating the above on the joint or termination casing as appropriate.

5.6.3.2.3 Additional Copper Wire Insulation Screen Where a copper wire screen is applied (specifically for CSS), the connection can include the following: • Plumbing the wires into the tin wipe; • Connecting the copper wires with the accessory casing using mechanical means (eg lug); • Connecting the copper wire screen of both cable ends (for joints) using mechanical means (eg ferrule); • Connecting the copper wires directly with the bonding cables. (This can only be applied if the copper wire screen is rated to handle the sheath currents).

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Good practice includes: • Leaving as much metallic sheath on the cable as possible to act as a heat sink during the plumbing process as well as protecting the underlying layers from splatter during the wiping process; • Moving the torch circumferentially to ensure even heat distribution; • Applying generous amount of tallow will keep the area of the wipe cool; • Cutting the metallic sheath on the crest of the corrugations. 5.6.3.2.4 Reinforcement Glass fibre reinforced epoxy resin may be required over the tin wipes to improve their mechanical strength.

5.6.3.3 Lead Sheath 5.6.3.3.1 Preparation of Cable Sheath This preparation phase includes: • Cutting the lead sheath taking great care to avoid damaging the underlying cable core. A safe method is to make one shallow (partial) circumferential cut and two shallow longitudinal cuts toward the end. A tool specifically designed for this operation should be used; • Tearing the lead strip between the longitudinal cuts; • Tearing the sheath along the circumferential cut (Table 5.11). 5.6.3.3.2 Metallic Screen Continuity Plumbing is the most common method for connecting the lead sheath with the accessory casing. 5.6.3.3.3 Additional Copper Wire Insulation Screen Where a copper wire screen is applied in combination with the lead, the connection can include the following: • Plumbing the wires in the tin wipe; • Connecting the copper wires with the accessory casing using mechanical means (e.g. lug); • Connecting the copper wire screen of both cable ends (for joints) using mechanical means (e.g. ferrule); • Connecting the copper wires directly with the bonding cables. (This can only be applied if the copper wire screen is rated to handle the sheath currents). 5.6.3.3.4 Reinforcement Glass fibre reinforced epoxy resin may be required over the tin wipes to improve their mechanical strength.

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Table 5.11 Technical risks and specific skills for Corrugated Sheath connections Work phase General

Technical Risks Connecting cable screens where not intended

Cable Preparation

Overheating the cable core during oversheath removal Cutting into underlying layers

Tinning

Use of wrong metal Lack of brushing into valleys Uneven circumferential heat (dry joint) Excessive heat damaging underlying layers Accidental oxidization of surface by touching or other material contamination Use of wrong gas Use of wrong metal Excessive heat damaging underlying layers Wipe exceeds tinned area Use of wrong gas Poor plumbing connection of the wire screen. Poor mechanical connection due to wrong dies or press Weak metallic screen continuity due to a bad cleaning or preparation of the aluminium sheath or/and the metallic connection before plumbing Cutting into underlying layers

Platform Wipe (CAS)

Copper wire screen (CSS) Connection

Cutting of metallic sheath Braided tinned copper soldering Reinforcement

Wrong flux Overheating underlying layers Insufficient braided copper pieces Uncured resin Faulty glass-fibre tape application

Specific skills Know risks and function of screen connections Controlling the heat of the torch Handling of specific tools for this work Tinning Techniques Controlling the heat of the torch

Due care Plumbing Techniques

Plumbing techniques Crimping techniques Tinning and Plumbing Techniques Handling of specific tools for this work Soldering Techniques Due care Resin mixing and glass-fibre application techniques

5.6.3.4 Laminated Sheaths: Aluminium Polyethylene Laminate (APL); Copper Polyethylene Laminate (CPL) 5.6.3.4.1 Preparation of Cable Sheath This preparation phase includes: • Carefully applying heat; • Scraping until a clean surface is achieved. 5.6.3.4.2 Metallic Screen Continuity This connection phase can include: • Connecting the APL/CPL layer directly to the accessories with a roll spring, or

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• Folding the aluminium (APL) or copper (CPL) back over the spring and applying an additional external connection. Sometimes an additional copper fabric tape is used under the contact spring on either side. Folding back requires multiple longitudinal cuts before bending outwards to ensure there is a clean aluminium surface facing outwards. Where a semiconducting coating is bonded to the inside of the APL this will need to be removed as well to ensure a good contact. CPL also allows for: • Plumbing, which makes it possible to establish a metal enclosed, moisture proof connection between the cable sheath and the accessory casing. Great care has to be taken when applying the heat for plumbing (Table 5.12; Fig. 5.18); • Soldering with a soldering iron which is usually preferred. However this can be achieved only if an additional tinned copper foil is used to cross the gap between the cable sheath and the accessory casing.

5.6.3.4.3 Additional Copper Wire Insulation Screen If a copper wire screen is applied in combination with APL/CPL, it is essential to establish an electrical contact between the copper wire screen of the cable and the laminated aluminium/copper sheath. This can be achieved by: • Bending the copper wires back and clamping using a roll spring, or • Collecting the wires together and crimping in a lug or ferrule.

Table 5.12 Technical risks and specific skills for lead sheath connection Work phase Cable Preparation

Technical Risks Overheating the cable core during oversheath removal

Cutting of lead sheath Plumbing

Cutting into the cable core

Copper wire screen

Reinforcement

Overheating the cable core Melting of the lead sheath Enclosing voids in the tin wipe, which could lead to a weak connection Poor plumbing connection of the wire screen. Poor mechanical connection due to wrong dies or press Uncured resin Faulty glass-fibre tape application

Specific skills Controlling the heat of the torch Handling of specific tools for this work Handling of specific tools for this work Plumbing Techniques Controlling the heat of the torch Plumbing techniques Crimping techniques

Resin mixing and glass-fibre application techniques

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Fig. 5.18 Arranging metallic sheath continuity with a contact spring

5.6.4

Oversheath

Prior to work, jointer should be aware about the cable design e.g. extruded semiconductive layer or graphite applied on the PVC or Polyethylene oversheath.

5.6.4.1 Case of Graphite Coating This phase includes: • Cleaning the graphite for a specified distance from its end; • Removing all traces of graphite using a clean cloth moistened with a suitable solvent; • Abrading the previously washed area using aluminium oxide tape or coarse glass paper to ensures that the embossed lettering is completely removed. It is essential that the serving is abraded and any graphite that may be embedded in the extruded sheath is removed; • Performing an appropriate resistance measurement to confirm effective removal of any conductive layers (Table 5.13).

5.6.4.2 Case of Extruded and Bonded Semi-Conducting Layer Removal by shaving with a spoke shave or glass slides. • Cleaning the semi-conducting layer for a specified distance from its end; • Performing an appropriate resistance measurement to confirm effective removal of any conducting layer.

5.6.4.3 Low Smoke, Zero Halogen, Enhanced Flame Performance Sheaths In the case of “special sheath materials”, advice should be sought from the cable manufacturer.

5.6.5

Installation of Joint Electric Field Control Components

Check that the body is in good condition and that all surfaces (inside and outside) are completely clean and free from defects. On joints, the accessory body is temporarily parked on one cable core before connecting the conductor. Special tools are

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Table 5.13 Technical risks and specific skills for laminated sheath connections Work phase Cutting the cable sheath Removing the PE cable sheath by heating

Opening the Aluminium/ Copper foil Plumbing/soldering

Technical risks Damaging the cable core (cutting) Overheating the cable core Poor contact due to bad cleaning Damage to thin Aluminium/ Copper layer Damaging the cable core (cutting) Excessive heat damaging underlying layers

Specific skills Handling of specific tools for this work Controlling the heat of the torch

Handling of specific tools for this work Plumbing and soldering techniques

Table 5.14 Technical risks and specific skills for oversheath preparation Work phase Cleaning or removing of the conductive or semiconducting layer Removing of the embossed lettering

Technical Risks Local overheating and fire risk due to incomplete cleaning which can lead to surface currents Graphite concentration

Specific Skills Due care Due care

generally required to guide the movement of the accessory body into the final position and subsequently to align it correctly. The tools may include: • • • • • •

Movable supports; Chain hoists; Special clamps; Special seals; Dry nitrogen; Lubricating grease which reduces the friction between cable core and the accessory body. Only use the lubrication specified in the instruction manual (Tables 5.14 and 5.15).

5.6.5.1 Slip on Prefabricated Joint The slip on technique represents the most common way of installing field control components. These field control components are usually made from silicon rubber, (e.g. RTV, LSR and HTV) or EPDM and have an integrated conductive deflector. This deflector takes over the field control at the end of the semi-conductive insulation screen of the prepared cable core. The joint body should be checked to ensure that it is in good condition and that all surfaces (inside and outside) are completely clean prior to installation.

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Table 5.15 Common Technical risks and specific skills for joint field control components Work phase Core preparation Joint parking

Technical risk Dimension/tolerance

Specific skill Due care

Cutting or damaging the joint body Damage to conductive varnishes Selection of wrong tools Use of wrong tools Use of wrong grease

Handling of sensitive components

Fig. 5.19 Lubrication of the joint

The installation phase includes: • Lubricating the inner surface of the joint body and the cable core with grease or other liquids specified by the manufacturer; • Slipping the joint body on the cable core into a parking position in order to prepare the conductor connection. It is advisable to temporarily cover the conductor during the positioning phase (Figs. 5.19, 5.20, 5.21, and 5.22); • Checking for smoothness and cleanliness before the joint body is slipped on to the prepared cable core; • Slipping the joint body into the final position after making the conductor connection; • Using chain hoists or other auxiliary tools to help move the joint body. • Good practice includes: • Special movable supports are used in order to guide the movement of the accessory body; • Chain hoists with suitably auxiliary slip-on rings might be used in order to pull the accessory body; • The use of reference marks on the cable core to ensure correct positioning of the joint body; • Check the correct position of the accessory body which should be in accordance with the instruction manuals. During positioning the accessory body, any slight bulging of the body caused by the pulling process may in some cases be smoothed out by slightly turning the body on the cable;

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Fig. 5.20 Lubrication of the cable

Fig. 5.21 Joint body slipping on the cable core

Fig. 5.22 Final position of the joint body

• Temporary cover on cable conductor to ensure that the joint body is not damaged during parking; • Ensuring that instructions are followed where a specific installation tool is to be used for locating the accessory body.

5.6.5.2 Expansion Joints Cold-shrink pre-moulded bodies are expanded either in the factory or on site. In the case of factory expanded joints, no expansion is required on site. All that has to be checked is that the joint has not exceeded its expiry date.

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Site expanded joints have to be expanded onto a carrier tube in the field, just before fitting them on the cable. Particular skills and tools are required for making a field expanded joint or termination. The tools vary with each manufacturer and the jointer must be trained in their use. The jointer should ensure that the expanded body is positioned correctly as specified by the manufacturer (Table 5.16). Field expansion steps: • Environmental aspects include: – Checking to ensure that accessory components and expansion tools are in good conditions, perfectly clean and free of defects; – Carrying out expansion in a protected (dust-free environment in order to avoid having impurities trapped between the carrier tube and the pre-moulded body); – Ensuring that ambient temperature and humidity are in accordance with manufacturer instructions (Fig. 5.23). • Lubrication: – Applying the specified quantity and type of lubricating oil/grease on the carrier tube and/or inside the body before starting the field expansion operation; – Applying lubrication, if specified, to the surface of cable insulation where the body will be positioned. For both factory and field expanded joints:

Table 5.16 Technical risks and specific skills for the “slip-on” prefabricated joint installation Work phase Accessory pulling Preparation

Parking

Slipping on

Technical risk Any damage or cutting into the body Wrong joint body Mis-alignment of the cable Contamination of components prior to slide on Incorrect application of lubrication Wrong preparation dimensions Damage while positioning in parking position due to sharp edges of the conductor Incorrect positioning of joint body Damage by use of chain hoists Damage of joint body and/or cable core due to use of wrong tools Damage due to improper fixation of the accessory body Damage to semi-conductive paint

Specific skills Due care Due care Cleaning with specified solvents and the correct use of grease

Due care

Due care Awareness of the behaviour of accessory bodies with different sizes during movement on the cable core Application of formulas or graphs for calculating the premoulded body position respect to reference marks made on the cable Handling of specific tools

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Fig. 5.23 Joint body expansion onto a smooth carrier tube

• Positioning on the cable. The expanded body must be initially parked over the cable during the conductor jointing operations. The body can remain expanded only for the time specified by the manufacturer according to the material characteristics and the expansion rate. Then it will be moved and positioned to the final position and the carrier tube removed. • Shrinking on the cable. According to the different design of carrier tubes (i.e. helical tube or smooth onethe tube can be removed by hand or by a specific tool) (Figs. 5.24, 5.25, 5.26, and 5.27; Table 5.17).

5.6.5.3 Field Taped Joints These cable accessories are formed by applying an insulating tape on suitably prepared cable ends in the field (installation site). Fusing is only effective if the tapes are stretched by the correct amount. The taping can be done manually or with the help of a machine. Taping by the machine achieves a higher quality and/or performance of the joint. Field taped joints include: • • • • • •

Self-fusing (or self-amalgamating) rubber tape, which is usually made of EPR; Semi-conductive self-fusing rubber tape; Lead tape or copper braid; PVC tape; Waterproofing rubber tape; High permittivity field grading tape (if required), etc... Some tapes are impregnated with silicon oil to fill overlap gaps. The taped profile and geometry forms the electrical stress control of the joint. The taping phase includes:

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Fig. 5.24 Example of lubrication and fitting of a smooth carrier tube for a stress cone and a joint body

Fig. 5.25 Removal of helical carrier tube from the joint body

Fig. 5.26 Removal of smooth carrier tube from joint

Fig. 5.27 Cutting of tube from cable core

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Table 5.17 Technical risks and specific skills for field expansion of premoulded body Work phase General Expansion of the body

Carrier tube removal

Technical risks The risks listed under Table 6.5.1 apply here Presence of defects on the carrier tube (e.g. sharp edges or cracks) Presence of defects on the expansion tooling (e.g. damaged nose) Breaking of the carrier tube during removal Damage to the cable insulation

Specific skills The skills listed under Table 6.5.1 apply here Due care

Ability to correctly remove the carrier tube. Knowledge about handling the special tools required for the removal of the smooth carrier tube Handling of special tools and equipment

Table 5.18 Technical risks and specific skills for field taped joints Work phase Preparation Taping

Technical risk Contamination Mis-alignment of the cable Wrong tension during taping Wrong stress control profile (diameter and length) Wrong settings of taping machine

Specific skills Due care High skill in hand taping Knowledge of setting and operating taping machines Reading technical drawings and measuring

• Applying the tapes at the right pace, stretch, tension (Table 5.18); • Ensuring that the correct profile is achieved and that air gaps and voids are managed. Frequent measurement is required; • Ensuring that the transition point between the taped insulation and the cable core semiconducting layer is correctly applied; • When a tapping machine is used, setting the parameters as per operating and jointing instruction manuals. These settings include tension, pitch and return limits; • Continuing the semiconducting layer over the joint with a semiconducting tape; • Depending on the instruction, lead tape may be applied over the semi-conducting tape to ensure continuity (Fig. 5.28).

5.6.5.4 Field Moulded Joints (Extruded or Taped) Tape Moulded and Extrusion Moulded joints are highly specialised. These proprietary joints are usually installed by the manufacturer and are thus not covered here. 5.6.5.5 Heatshrink Sleeve Joint Heatshrink insulation is commonly used in Medium Voltage cable joints and has recently been available for some High Voltage applications. Heatshrink preparation phase includes:

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y

B

Stress control profile

Rj

dy (x, y) A

dx

Ri Cable insulation r x

Conductor

Fig. 5.28 Taped profile and geometry

Table 5.19 Technical risks and specific skills for heatshrink joint Work phase Shrinking

Technical risk Wrong order of tube insertion Change in positioning during shrinking Uneven shrinking Folds in tube

Specific skills Shrinking of thick wall tubes

• Careful positioning of the insulation in the heat shrinkable tube; • Taking into account dimensional changes of the tube with the application of heat; • Uniformly applying the heat and controlling the temperature in order to ensure thetubes are shrunk uniformly.

5.6.5.6 Prefabricated Composite Type Joint This joint consists of an epoxy insulation unit in which an electrode for shielding the electric field of the connector is embedded and the premoulded stress cones are made of rubber. Pressure is applied at the interfaces by a compression device which is usually comprised of metallic springs. These joints can also be used to connect cables having different conductor cross sections and/or different insulation thicknesses (Table 5.19). Assembly phase includes: • Ensuring the straightness of the cable is within the prescribed limits given in the instruction manual; • Parking the joint shells, epoxy and rubber insulators; • Lubricating the appropriate surfaces with the specified lubricant; • Connecting the conductors with compression type connector.; • Fixing the Epoxy insulation unit and Connector completely by fitting the HV electrode embedded in the Epoxy insulation unit to connector;

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Epoxy insulation unit

Compression device

Connector

Premoulded insulator

Joint shell

HV electrode

Fig. 5.29 Prefabricated composite joint Table 5.20 Technical risks and specific skills for prefabricated composite joint Work phase Insert parts Setting the epoxy unit and Slide of premoulded stress cone

Assemble the spring unit

Technical risk Wrong order of inserting parts Setting in wrong place Wrong centering of the cable to epoxy insulation unit Damage to cable insulation Lack of the designed pressure

Specific skills Due care Handling of specific tools for this work Due care Due care

• Fitting and compressing with the springs the premoulded stress cones against the epoxy insulation unit (Fig. 5.29; Table 5.20) • Wiping the joint shells to the cable sheaths if applicable.

5.6.5.7 Plug-in Joint Plug-in type joints are based on a premolded joint body, with integrated metal ring for locking the cable ends in the joint. The cable end preparation, i.e. installing the plugs, peeling and smoothening of the cable insulation, require equivalent skills and tools as other premolded or prefabricated joints. This applies also to the installation of the joint covering. These joints can also be used to connect cables having different conductor cross sections and/or different insulation thicknesses. Specific tools and skills are needed to plug in the prepared cable ends. The tool can be based on hand driven chain hoists or a hydraulically driven plug-in frame (Fig. 5.30). 5.6.5.8 Pre-moulded Three Piece Joint This joint consists of three pre-moulded parts. Two cable adapters containing the stress control profiles and the main joint sleeve. Assembly phase includes: • Ensuring the straightness of the cable is within the prescribed limits; • Parking the joint main sleeve and other outer components;

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Fig. 5.30 Cable plug-in tool

Table 5.21 Technical risks and specific skills for plug-in type joints Work phase Inspection of cable end Inserting cable ends Locking of the plugs

• • • •

Technical risk Damaged plugs Mis-alignment of the cable in case of hand driven chain hoists Not locking

Specific skills Check plug and cable ends Due care Pull back check of cable

Lubricating the appropriate surfaces with the specified lubricant Pushing on the cable adapters; Connecting the conductors including installation of corona shield; Pushing over the joint main sleeve (Table 5.21; Fig. 5.31).

5.6.6

Installation of Termination Electric Field Control Components

In order to successfully install terminations, the jointer must possess certain skills and abilities. These depend on the following aspects: the technology of the terminations, the voltage levels and the manufacturer of the cable and accessories. The procedures and skills as detailed in Sect. 0 above apply. Installation of HV cable terminations present an additional set of challenges. As most terminations are installed in a vertical position at a few meters from the ground, special preparation of the work area is needed. Usually, a scaffolding system is built around each cable (or the three cables) to facilitate access to cable and reduce strain on the jointers. Some environmental protection against dust, wind, rain and snow is also advisable.

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HV electrode

Cable adapter

Stress profile

Screen connection

Conductor connection

Shrinkable sleeves with moisture barrier

Fig. 5.31 Three piece joint

Table 5.22 Technical risks and specific skills for pre-moulded three piece joint Work phase Parking Core preparation Pushing on of elastomeric parts

Technical risk Wrong placing Wrong measures Bad surface Wrong position

Specific skills Due care Handling of specific tools for this work Due care

See also Table 5.16

Particular situations such as installation on poles, high voltage pylons and in underground power generating station transformer vaults may require special work area arrangement to ensure safety and ease of access.

5.6.6.1 Slip-on Prefabricated Field Control Components The risks and skills as detailed in Sect. 0 above apply (Table 5.22). 5.6.6.2 Plug-in Terminations Plug-in type terminations consist of a field control component usually made from silicon rubber (e.g RTV, LSR and HTV) or EPDM, and an insulator made from epoxy resin. This insulator represents the interface to switchgears, transformers or bushings. Two designs are commonly available: • Inner Epoxy Cone – based on a rubber stress cone pushed into the epoxy insulator, achieving the required interface pressure by means of metal springs; • Outer Epoxy Cone – based on a rubber mould, pushed onto a conical bushing of the epoxy insulator, achieving the interface pressure by stretching the rubber mould.

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Table 5.23 Technical risks and specific skills for cone plug-in terminations Work phase Installing the epoxy insulator Inserting cable ends (with or without stress cone) Locking of the plugs (if applicable)

Technical risk Damaging the insulator Unequal tightening of the bolts Mis-alignment of the cable Damaging the cable and or stress cone Not locking

Specific skills Due care Check torques Due care

Check locking

Fig. 5.32 Plug-in terminations based on inner cone and outer cone model

The cable related part is based on slip-on or plug-in technology. In this chapter only the plug-in technology is considered. Installation skills and risks for field control components and insulators can be taken from Sect. 5.6.5.1. In addition, the handling of the insulator has to be considered representing an additional interface to be cleaned and prepared according to given instructions specified by the manufacturer (Table 5.23; Fig. 5.32).

5.6.6.3 Taped Terminations This kind of cable accessory is formed by taping (paper) a field control element (taped cone) in the field (installation site). This technique includes: • Applying impregnated papers (conductive and insulating) specified by the manufacturer, according to the given measurements and instructions. Alternating layers of conductive and non-conductive tapes establish a field control element

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to be embedded finally in an insulator with a fluid insulation (insulating oil e.g. silicone oil or polyisobutylene). The papers (width, length and thickness), number of layers and profile of the taped cone are specified by the manufacturer; Checking the profile several times during taping. At each time, the jointer should estimate the final diameter and length of slope based on the diameter of remaining tape. Usually the profile angle is smaller at the start of the slope and increases further up; Applying vacuum treatment to ensure that voids included in the paper wrapped cone will disappear; Heating and degassing the filling compound using special equipment in order to remove any water and gas content. During this procedure the termination has to be evacuated according to the given instructions. This procedure can last several hours. During this time all values have to be controlled and monitored; Avoiding contamination of the vessels and other equipment with water. The environmental conditions (temperature, humidity, dust) should be considered here.

5.6.6.4 Heatshrink Sleeve Insulated Terminations The risks and skills, as detailed in Sect. 0, Heat-shrink sleeve joint apply here. 5.6.6.5 Prefabricated Composite Dry Terminations The risks and skills, as detailed in Sect. 5.6.5.6, Prefabricated composite type joint apply here.

5.6.7

Outer Protection of Joints

5.6.7.1 Polymeric Outer Protection by Taping and/or Heatshrink Tubes Heat shrink tubes allow the installation of a watertight joint without the application of a compound filled outer protection. The heat shrink tubes are often equipped with an internal hot-melt glue layer (Fig. 5.33). The application of tubes includes: • Parking heat shrink tubes on one or both cable ends, prior to joint installation; • Moving heat shrink tubes into place after inner joint components have been applied; • Shrinking the tubes in position; • Avoiding excessive heat as it will lead to melting or even burning of the material; • Ensuring sufficient heat is applied to enable the shrinking process and melting of the glue; • Installing an under lying tape layer if the heat shrink is not adequate to withstand the sheath voltage requirements. A self fusing polymeric tape is often used for this purpose (Tables 5.24 and 5.25).

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Fig. 5.33 Dry type termination

Table 5.24 Technical risks and specific skills for taped terminations Work phase Taped Accessories (terminations)

Vacuum treatment

Technical risk Wrong order of material Wrong positioning and diameter of the specified layers as specified by the manufacturer Wrong positioning of conductive layers Lack of cleanliness and dryness of all components during taping Inclusion of particles or voids during taping must be avoided Improper degassing

Specific skills High skill in taping. Operation of taping equipment Dealing with measuring tools such as vernier-callipers Read technical drawings well

Handling of the equipment and basic knowledge about the behavior of fluids and vacuum treatment

Table 5.25 Technical risks and specific skills for polymeric outer protection (taping and/or heatshrink tubes) Work phase Shrinking

Technical risks Burning the polymeric material in case of too much heat Insufficient melting of the hot-melt glue in case of too little heat Folds in the tubes Gaps between overlapping layers, resulting in leaks Wrong amount of tension during taping

Specific skills Heat control Taping

Alternatively, a fully taped outer sheath is possible. A special tape is needed to establish a water tight barrier, an insulating sheath and mechanical protection at the same time.

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5.6.7.2 Outer Protection Assembly Outer protection housings (including metal protectors, coffin boxes, etc) represent a cover for joints to be filled with compounds. They can be made from e.g. PVC, PE, GRP (glass reinforced polyester) or ABS (acrylonitrile butadiene styrene). They may consist of a tube (to be positioned in the correct parking position) or two half pipes (installed after positioning of joint body). The function of the outer protection housing is to act as a container for the filling compounds referred to in 0 below. This assembly phase includes: • Parking of the outer protection if applicable; • Positioning the components according to the instruction manual; • Sealing all interfaces and openings to the environment by means of recommended methods and materials like silicone, putty or self amalgamating silicone tapes; • Filling as soon as possible to ensure no build up of moisture in the outer protector due to humidity. In the case where a joint is to be fixed to a support structure this should be done before the outer protector is filled with compound.

5.6.7.3 Filling Compounds for Joint Protections (Joint Boxes) The function of the filling compound is to establish a corrosion protection of the joint body (joint shell) and cable, to improve the thermal conductivity, to avoid the penetration of water into the joint (waterproof compound) and to ensure that the screen interruption design is maintained. Products used for the filling include: • • • •

Cold pouring RTV (Room Temperature Vulcanization); Mixed (Two component resin); Heated (bitumen); Insulating fluid or gas. The filling phase includes:

• Preparing the work taking account of the environmental conditions such as temperature, humidity, dust and dirt; • Heating of the compound (if applicable); • Mixing of the filling compound. Taking care to ensure the correct amounts of additives are added, the mixing creates a chemical reaction and can be easily influenced. Too little catalyst/hardener can result in undercuring. Too high temperature can cause precuring in the mixing pot (Table 5.26); • Filling by using a pump or pouring the compound. Good practice includes (when applicable): • Taking a sample from each batch or joint in order to ensure that it has cured properly; • Considering the physical position of the joint in order to avoid air pockets;

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Table 5.26 Technical risks and specific skills for outer protection assembly Work phase Outer protection assembly

Technical risks Poor sealing of interfaces leading to loss of compound, poor filling, etc. and leading to water penetration Over tightening of bolts leading to cracking of outer protection Incorrect horizontal positioning of outer protector leading to poor filling

Specific skills Due care

• Not moving the joint before the compound is vulcanized or stable; • Preventing contamination to ground and waterways.

5.6.8

Filling of Terminations

There are types of terminations that are to be filled with insulating compound, typically they are metal enclosed GIS terminations and outdoor terminations. They can be filled with insulating liquid or gas. For taped terminations the process is covered in Sect. 5.6.6.3. In the case where the termination is to be filled with compound the manufacturers filling instruction is to be followed; filling compound may include such items as polybutene, silicon oil or other dielectric fluid, gas or mixed two component resins. The different filling compounds involves different steps, the main steps in preparation phase includes: • Preparing the work taking account of the environmental conditions such as temperature, humidity, dust and dirt; • Heating of filling compound to the correct temperature in order to facilitate filling (if applicable); • Evacuating the chamber (if specified); • Mixing of components (if applicable); • Filling by using a pump (compound or gas) or pouring the compound. Good practice includes (when applicable): • Taking a sample from each batch or termination in order to ensure that it has cured properly (when applicable); • Considering the physical position of the termination and be aware about the risk of including air bubbles inside the chamber; • Preventing contamination to ground and waterways.

5.6.9

Handling of Accessories

5.6.9.1 Supporting of Accessory The support structure design for cable and accessories should be part of the civil engineering. This should be done prior to cable installation and should not be improvised on site.

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Jointers need to ensure that the supporting structure is installed according to prepared drawings and/or instructions (Table 5.27; Fig. 5.34). This assembly phase includes: • Applying the right amount of torque to the closing screws. Too much torque can result in deformation of the polymeric jacket, while too little torque will reduce the friction between cable (and/or joint) with the support structure and give inadequate support and constraint. Flame treating the extruded polythene sheath and applying a resin impregnated tape system, in order to mechanically reinforce the cable/accessory interface where thermomechanical forces and movement might be experienced (Tables 5.28 and 5.29). Table 5.27 Technical risks and specific skills for filling compounds Work phase Preparation

Technical risks Contamination

Heating filling compound Filling of compounds (joints)

Overheating of compound Underheating of compound Insufficient hardeners or accelerators Premature curing due to heat Enclosed air pockets

Check of outer protection condition

Wrong order of materials May be impossible to visually check the vulcanization status afterwards

Fig. 5.34 Wet type termination

Specific skills Due care in cleanliness of equipment and components General skills and knowledge in handling the equipment The use of special equipment (e.g. mixer) under clean conditions Able to check and verify the correct position Able to check the status of the vulcanization. General skills and knowledge in handling the equipment

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Table 5.28 Technical risks and specific skills for filling of terminations Work phase Preparation

Technical risk Contamination

Heating filling compound Evacuating chamber Mixing of components Filling

Overheating of oil Underheating of oil Not evacuated enough Incorrect curing of compounds with A and B mixing components Water or humidity presence during filling Underfilling or overfilling Air bubbles if the compound is filled or mixed improperly

Specific skills Due care in cleanliness of equipment and components Handling the equipment Handling of equipment Mixing and handling compounds and tools Handling the specific tool or method required

Table 5.29 Technical risks and specific skills for installation of supporting of accessory Work phase Closing the clamps on the cable (and joint) Erecting steelwork

Mechanical reinforcement of accessories

Technical risks Applying the wrong torque on the closing screws Steelwork modification or the adjustment on site may weaken the support or cause steelwork corrosion at a later date Burning of the PE oversheath Wrong resin and/or deficient application

Specific skills General skills and knowledge in handling with equipment General skills

Heating techniques

5.6.9.2 Lifting of Accessories It is sometimes necessary to lift accessories. It is necessary to lift all or part of the terminations when fitting to the structure. The lifting phase includes: • Setting up the lifting equipment • Adopting appropriate safety techniques and adherence to sling load ratings • Securing the lifting device to the accessory taking care not to damage any components. Supplier guidance should be adopted • Lifting the accessory ensuring that the cable is not restrained as this may dislodge some internal components (Fig. 5.35; Table 5.30).

5.6.9.3 Special Bonding Configurations and Link Box Installation The jointer must have the skill to install the bonding leads (single and concentric), the link boxes, SVLs, etc. associated with the particular bonding scheme adopted for the cable circuit. Particular attention is drawn to the removal of the conducting layer, if supplied, on the bonding leads in order to ensure integrity of the bonding design. 5.6.9.4 Sensor Connections Many types of sensors can be installed on the cable accessories. These include:

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Fig. 5.35 Example of lifting of cable terminations by fixing the lifting device in a) the cable respectively b) the upper metalwork of the termination

Table 5.30 Technical risks and specific skills for lifting/moving accessory Work phase Lifting/ moving

• • • • •

Technical risks Cable damage and displacing the internal parts due to bending or twisting of the cable Broken insulator Cracked Porcelain Scratched polymeric sheds

Specific skills General skills and knowledge in handling with equipment Operator to be trained in relevant aspect of lifting/moving for accessory Rigging skills

Temperature; Distributed temperate sensing; Pressure (leak); Partial discharge; G as density, etc.

The kind of sensors used depends on the accessories and the requirements of the manufacturer or the user. It should not be assumed that the HV cable jointer automatically has the required sensor connection skills. The specific sensor supplier should provide input on the suitable skill set needed. Special care should be taken where insulated sheath systems are employed as the sensors must not compromise the earth isolation.

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Table 5.31 Technical risks and specific skills for special bonding configurations Work phase Connecting bonding lead

Filling link box with compound e.g. bitumen

Technical risks Sheath bonding connection error Disturbed seal on bonding lead may lead to water ingress Similar risks as in 0

Specific skills Knowledge of bonding design Due care General skills and as 0

Table 5.32 Technical risks and specific skills for sensor connections Work phase Connecting sensors

Technical risks Sheath bonding connection error Disturbed seal on bonding lead may lead to water ingress

Specific skills Due Care

Note, these activities can be done by a third party (Tables 5.31 and 5.32).

5.6.9.5 Fibre Optics In some specific installations, cables may be installed with optical fibres, mainly used for temperature sensing. Fibre optic cables can be: • Integrated in the cable; • Attached to the cable sheath from outside; • Blown/pulled into a separate tube. Usually the fibre optics have to be connected in splice boxes. Additional and very different skills are necessary in order to make fibre optic connections. The general handling of fibre optics has to be done very carefully. For fibre optics integrated in the cable, the splice box is usually attached to the cable close to the joint. It is common practice to integrate the splice box into the coffin box or protection housing of the joint. In order to connect the fibre optics to the splice box, one side of the cable must be cut with an over length, taking the position of the splice box beyond the joint into account (Table 5.33). When sheath interruption joints are installed (eg cross bonding), it is common to use two splice boxes (one on each side of the joint) with an additional intermediate fibre optic splice box made from non conductive material without any outer metallic protection.

5.7

Skills Assessment

Since education and training differs for each country, it is not appropriate to dictate the method of assessment and certification. It is recommended that the certifying authority, normally the accessory manufacturer, keeps an up to date record of the jointing competencies tested and certified. The methodology of assessment should also be stated. Where no formally structured assessment and certification is

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Table 5.33 Technical risks and specific skills for installation of fibre optics Work phase Handling the optical fibre

Technical risks Breaking the optical fibre

Removing the stainless steel fibre protection tube

Damaging the cable core with the steel tube Short circuiting of the sheath interruption of the joint with the steel tube

Specific skills Fibre splicing Knowledge in handling optical fibre Due care

available, the methodology described in Sects. 5.7.1, 5.7.2, 5.7.3, 5.7.4, 5.7.5, and 5.7.6 should be used.

5.7.1

Aspects to be Tested

Modern high voltage accessories often seem fairly simple in design e.g. pre-molded joints. This may lead to the incorrect assumption that a jointer with a skill-set suitable for assembling low voltage accessories can be easily up skilled to high voltage accessories. However, very careful assembly is needed for high voltage accessories, as these accessories will operate at very high voltage stresses and, as a result, the margin for error in assembly is very low. Further, the impact of a system outage is very high. It is essential that the jointer has the skill-set appropriate to the accessory being assembled or, if a team is assembling the accessory, then the team should have the full skill-set between them. Of course in the latter case each jointer should be limited to working only in his area of competency. In order to determine the jointer’s skill-set he should be tested for relevant competencies, as outlined in Sect. 5.6 above. There may be three levels of competence: • Apprentice (not allowed to do jointing on their own); • Jointer (allowed to do jointing on their own); • Supervisor (highly experienced and could train others).

5.7.2

Methods of Qualification

It is advisable that qualification involve three elements: • Theoretical understanding of WHY a particular aspect is important; • Observation that a jointer has understood and executed the instructions correctly; • Electrical and mechanical testing of the final accessory assembly.

5.7.2.1 Theoretical This test should demonstrate the jointer’s basic understanding of the theoretical aspects of the assembly processes e.g. importance of smooth surfaces, cleanliness, avoiding nicks, etc.

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5.7.2.2 Training on the Job and Observation In this case the jointer has to demonstrate his skill-set under the supervision of a jointing supervisor, experienced in the type of accessory being assembled and in the skill-set required. Quality assurance checklists provide a useful tool for on the job training verification. The full assembly of the accessory should be developed into a check list, starting at materials checking, jointing tent conditions, tools checking, etc. – a typical overview check list is contained in “5.Appendix A” The jointer must complete the checklist, as he/she assembles the accessory and the jointing supervisor certifies it is done correctly. It is recommended that each jointer keeps a logbook of all the accessories assembled (type and voltage class). 5.7.2.3 Testing: Electrical & Mechanical Extensive testing is not always possible, because of practical reasons or is not financially feasible. Higher voltage systems generally have higher stresses. They also have higher consequence of failures. As the stress increases, the risk of failure also increases. It is thus prudent to conduct as many of the following tests as is financially or technically practical. Following the tests the accessory should be disassembled to see if there are any problems and, if noted, the jointer should be further trained in this area until his competency is established. The tests proposed must considered the particular skills that jointers will use. Some tests which can be done are HV tests, PD tests or impulse tests at specified IEC levels.

5.7.3

Certification

A Jointing Supervisor or an Installation Engineer, who has suitable experience and authority, should certify the jointer, following their completion of Sect. 5.7.2 above. The Certificate should indicate: • Voltage class applicable; • Accessory cover; • List of skill-sets covered. Mechanism used for Certification (Sects. 5.7.2.1, 5.7.2.2, and 5.7.2.3) – if Sect. 5.7.2.3, give test details.

5.7.4

Duration of Certification

While it is preferable that a jointer keeps his skills up to date by having a continuous programme of work, it is recognised that this rarely happens. Very often there can be

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long breaks between periods of jointing activity and there may be a possibility that there would be a reduction in skills. It is for this reason that the concept of a duration attached to certification is introduced. If the jointer is regularly using his skill-set then there is no need for re-certification. In the event that there is a significant gap in the jointer’s work programme, then the jointer may need to be re-certified. The re-certification should take place in accordance with the relevant parts of Sects. 5.7.2 and 5.7.3 above. The log book described in Sect. 5.7.2.2 above will help in the evaluation of the need for re-certification.

5.7.5

Upskilling

A case may arise where a jointer has a fairly good skill-set, but needs to gain some more skills for a new accessory to be installed that is not too different from accessories he has previously installed. In this case it may be sufficient for the jointer to be tested and certified for the additional skills required. We would advise to err on the side of caution.

5.7.6

New Accessory Type

If the jointer is required to install an accessory, with which he is not familiar, then he should be fully trained in all of the necessary skills outlined in Sect. 5.6 above, tested as per Sect. 5.7.2 and certified as per Sect. 5.7.3, as appropriate, before he commences installation.

5.8

Set Up

While, not part of the accessory, set up is complimentary to the accessory, and is thus covered here. It is recommended the steps outlined below are followed:

5.8.1

Organisation of Jointing Location

Joints are installed in different locations such as, joint bays, manholes, vaults, tunnels, etc. The installation crew should ensure that: • The jointing kits are verified for contents and expiry dates where appropriate; • The layout of the jointing space is compatible with the required dimensions to carry out the jointing activities; • There is adequate space available for tools and equipment as well as the joint components; • There is adequate electric power supply, lighting, ventilation and other necessary services; • Safety of the personnel is assured through careful planning.

5

Cable Accessory Workmanship on Extruded High Voltage Cables

5.8.2

241

Positioning of Joint

The jointer should ensure that: • The joint is positioned and supported in accordance with designs; • The design instruction is followed to ensure the joint does not overheat during operation due to incorrect depth or backfill; • The joint should be positioned in the joint bay in such a manner that in the event of a failure that replacement is possible without serious disruption. Allowance should also be made for those designs that incorporate rigid joint designs.

5.8.3

Environmental Conditions

The jointer should ensure that the conditions are suitable with for jointing with respect to: • • • • •

Temperature; Humidity; Dust; Pollution; Salt.

Depending on the jointing instruction, the joint bay environmental control may vary from a simple single skinned jointing tent with no temperature or humidity control to a double skinned tent or jointing container with careful temperature and humidity control. In addition the possibility of the jointer perspiring too much must be considered.

5.8.4

Cable End Inspection

At the commencement of jointing, great care should be exercised to inspect the cable pulling head and tail. They should be very carefully removed and inspected for moisture penetration, as should the remaining cable – this can be done visually, but it is best done by immersing a small sample of conductor in hot oil; if there is moisture present then the oil will crackle. Cable should not be jointed if there is moisture in the conductor. This should be the subject of discussions between the Installation Company and the Client.

5.8.5

Verification of Each Step

This should be covered in the Instruction Manual Quality Assurance checklists.

242

5.8.6

K. Leeburn

Measuring of Diameters, Ovality, Concentricity, Position

This should be covered in the Instruction Manual Quality Assurance checklists.

5.8.7

Safety and Health

In any jointing operation it is vital that considerable attention is paid to the safety and health of the jointing operatives and their assistants. Amongst the items that should be considered and precautions taken to eliminate or minimise the risk are: • • • • • • • • • • • • • • • • • • • •

Tripping; Sunburn/sunstroke; Falling from ladder or into joint bay or other; Ground Subsidence; Electrocution/Earthing; Induced voltages from parallel circuits; Water, drowning; Gas; Traffic; Attack by animals; Attack by people; Lifting/handling; Noise; Handling hazardous substances; EMF exposure; Explosion/Fire; Failure of parallel power circuits; Equipment failure; Inadequately trained staff and supervision; Compliance with local safety laws and Regulations. All of the above should be the subject of a detailed documented risk assessment.

5.8.8

Environmental Aspects

In completing installation operations it is necessary to comply with all relevant local environmental laws and regulations. In any case, the environmental impact shall be kept as low, as is reasonably possible.

5.8.9

Quality Insurance

The quality insurance is treated in TB 177 (see ▶ Chap. 2, “A Guide to the Selection of Accessories”).

Lubrication

Shrinkage

Cleaning of insulation

Basic Has theoretical knowledge-

CERTIFICATE

-capable to make joint

Operational 5-10 years experience

Supervisory

Cable Accessory Workmanship on Extruded High Voltage Cables

Smoothing the insulation surface

Preparing the end of the semi conducting insulation screen

Stripping of semi conductive insulation screen

Straightening

Insulation Preparation

Mechanical Connection

Thermit Weld

Deep indentation

MIG/TIG Welding

Round and Hexagonal Compression

Conductor preparation

Construction and Procedure description

Conductors

Organisation of jointing location

Check that end bell, etc are passed down the cable, before jointing commences, so that they are available in the right position for use later in the jointing installation

Check cable serving, sheath, semi-conducting layer and insulation removed in truction drawing

Check cable placed in position with correct bending radius

Safety and Health

Cable End Inspection

Environmental Conditions

Positioning of Joint

Organisation of jointing location

Set Up

List below to be reorganised so that it complements with respect to the sequence in which each operation carried out

Jointing Instruction number/date/revision no:

Accessory; Type. voltage class/

Appendix A: Model Certificate

5 243

ents Field expansion Taped Accessories (Joints) Field moulding Extrusion or taped Heatshrink tube insulation. Coldshrink tube insulation. Prefabricated composite type joint Prefabricated composite type termination Polymeric outer protection by taping and/or heatshrink tubes

Oversheath Preparation of oversheaths Extruded PVC Extruded Polyethylene Low Smoke, Zero Halogen, Enhanced Flame Performance Sheaths Mechanical Reinforcement of Accessories

Metallic sheath Welded Aluminium Sheath Construction and procedure description Preparation of cable sheath Metallic screen continuity Copper wire screen Corrugated Seamless Aluminium (CSA), Copper (CCS), Stainless Steel (CSS) Plumbing Tig Welding Lead Sheath {highlight risks and care for cutting} Aluminium Polyethylene Laminate (APL) Copper Polyethylene Laminate (CPL) Corrugated Cu Stainless steel

Basic Has theoretical knowl-

CERTIFICATE Supervisory with 5-10 years experience

Operational jointer -capable to make joint

244 K. Leeburn

d spring loading, if

Expiry Date

Certifying Authority

Signed By

Environmental Aspects

Safety

Fibre optics

Special features Sensor connections on system

Common parts about accessory installation (joints and terminations) Supporting of accessory Lifting of accessories tion

Taped Accessories (Terminations) Heatshrink tube insulation Fitting OD porcelain/cast resin insulator and top metal erminations

necessary, to ensure pressure is maintained Installation of plug-in types

Outer Protection of Joints Filling compounds (joints)

Basic Has theoretical knowl-

CERTIFICATE Supervisory with 5-10 years experience

Operational -capable to make joint

5 Cable Accessory Workmanship on Extruded High Voltage Cables 245

YYYYY

re q u i re d re q u i re d re q u i re d re q u i re d

R e q u i re d R e q u i re d R e q u i re d R e q u i re d R e q u i re d R e q u i re d

Environmental Conditions

Cable End Inspection

Safety and Health

Check cable placed in position with correct bending radius

Check cable serving, sheath, semi-conducting layer and insulation removed in t r u c t i o n d r aw i n g

re q u i re d

re q u i re d re q u i re d re q u i re d

R e q u i re d R e q u i re d R e q u i re d

Organisation of jointing location

Construction and Procedure description

Conductor preparation

Round and Hexagonal Compression

required required required required

required required required required required

Preparing the end of the semi conducting insulation screen

Smoothing the insulation surface

Cleaning of insulation

Shrinkage

Lubrication

required

required required

required required

Stripping of semi conductive insulation screen

re q u i re d

re q u i re d

re q u i re d

re q u i re d

re q u i re d

re q u i re d

re q u i re d

re q u i re d

re q u i re d

re q u i re d

re q u i re d

re q u i re d

5-10 years experience

Super visor y

Straightening

Insulation Preparation

Mechanical Connection

Thermit Weld

Deep indentation

MIG Welding

Conductors

re q u i re d

R e q u i re d R e q u i re d

Check that end bell, etc are passed down the cable, before jointing commences, so that they are available in the right position for use later in the jointing installation

re q u i re d

re q u i re d

re q u i re d

R e q u i re d

-capable to make joint

O p e rat i o n a l

Organisation of jointing location

Has theoretical knowledge-

Basic

Positioning of Joint

Set Up

List below to be reorganised so that it complements with respect to the sequence in which each operation carried out

Jointing Instruction number/date/revision no:

D sealing end (with internal stress cone) with cu condr, lead sheath and PE serving with PD test facility

CERTIFICATE FILLED IN SAMPLE

246 K. Leeburn

nts

Filling compounds (joints)

Outer Protection of Joints

Polymeric outer protection by taping and/or heatshrink tubes

Prefabricated composite type termination

Prefabricated composite type joint

Coldshrink tube insulation.

Heatshrink tube insulation.

Field moulding Extrusion or taped

Taped Accessories (Joints)

Field expansion

Mechanical Reinforcement of Accessories

Low Smoke, Zero Halogen, Enhanced Flame Performance Sheaths

Extruded Polyethylene

Extruded PVC

Preparation of oversheaths

Oversheath

Stainless steel

Corrugated Cu

Copper Polyethylene Laminate (CPL)

Aluminium Polyethylene Laminate (APL)

Lead Sheath {highlight risks and care for cutting}

Tig Welding

Plumbing

Corrugated Seamless Aluminium (CSA)

Copper wire screen

required

required

re q u i re d

required

required

required

required

re q u i re d

required

required

required

Metallic screen continuity

required

required

5-10 years experience

Super visor y

required

-capable to make joint

O p e rat i o n a l

Preparation of cable sheath

R e q u i re d

Has theoretical knowledge-

Basic

Construction and procedure description

Welded Aluminium Sheath

Metallic sheath

CERTIFICATE FILLED IN SAMPLE

5 Cable Accessory Workmanship on Extruded High Voltage Cables 247

d spring loading, if necessary,

R e q u i re d

Environmental Aspects

Expiry Date

Certifying Authority

Signed By

R e q u i re d

on system

Safety

Fibre optics

Sensor connections

Special features

re q u i re d

re q u i re d

required

required to know about PD

required

required tion

required

required

-capable to make joint

O p e rat i o n a l

Supporting of accessory

Has theoretical knowledge-

Basic

required

Lifting of accessories

Common parts about accessory installation (joints and terminations)

erminations

Fitting OD porcelain/cast resin insulator and top metal

Heatshrink tube insulation

Taped Accessories (Terminations)

CERTIFICATE FILLED IN SAMPLE

Installation of plug types

to ensure pressure is maintained

re q u i re d

re q u i re d

required

required to know about PD

required

required

required

required

5-10 years experience

Super visor y

required

248 K. Leeburn

d and managed

Preparing the end of the semi conducting insulation screen

Stripping of semi conductive insulation screen

sets required from list

QA Requirement value/description

Value

QA checked by jointer and ok

Signed by

Date

Any comments

Cable Accessory Workmanship on Extruded High Voltage Cables

Straightening

Insulation Preparation

Mechanical Connection

Thermit Weld

Deep indentation

MIG Welding

Round and Hexagonal Compression

Conductor preparation

Construction and Procedure description

Conductors

Check that end bell, etc are passed down the cable, before jointing commences, so that they are available in the right position for use later in the jointing installation

Check cable serving, sheath, semi-conducting layer and insulation ointing instruction drawing

Check cable placed in position with correct bending radius

Check fully detailed jointing instruction supplied covering all items listed above and below

Cable End Inspection for no damage /water

Environmental Conditions

Positioning of Joint

Checking that all jointing tools and other required equipment are on site

Checking that all jointing materials and consumables are on site

Organisation of jointing location

Set Up

Joint/Accessory Drawing number:

Jointing Instruction number/date/revision no:

Accessory Type/voltage class/number:

QA DOCUMENT

Appendix B: QA Document

5 249

preparation

Prefabricated composite type joint

Coldshrink tube insulation.

Heatshrink tube insulation.

Field moulding Extrusion or taped

High Voltage Heat-resistant tape

High Voltage tape

Taped Accessories (Joints)

Field expansion

Mechanical Reinforcement of Accessories

nts

Low Smoke, Zero Halogen, Enhanced Flame Performance Sheaths

Extruded Polyethylene

Extruded PVC

Preparation of oversheaths

Oversheath

Stainless steel

Corrugated Cu

Copper Polyethylene Laminate (CPL)

Aluminium Polyethylene Laminate (APL)

Lead Sheath

Tig Welding

Plumbing

Corrugated Seamless Aluminium (CSA)

Copper wire screen

Metallic screen continuity

Preparation of cable sheath

Construction and procedure description

Welded Aluminium Sheath

Metallic sheath

Lubrication

Shrinkage

Cleaning of insulation

Smoothing the insulation surface

QA DOCUMENT sets required from list

QA Requirement value/description

Value

QA checked by jointer and ok Signed by

Date

Any comments

250 K. Leeburn

d spring loading,

on system

tion

sets required from list

QA Requirement value/description Value

QA checked by jointer and ok

Signed by

Date

Any comments

Cable Accessory Workmanship on Extruded High Voltage Cables

Signature of Supervisory Jointer

Signature of Jointer

Environmental Aspects

Safety

Fibre optics

Sensor connections

Special features

Lifting of accessories

Supporting of accessory

Common parts about accessory installation (joints and terminations)

rminations

Fitting OD porcelain/cast resin insulator and top metal

Heatshrink tube insulation

Taped Accessories (Terminations)

Installation of plug-in types

if necessary, to ensure pressure is maintained

Filling compounds (joints)

Polymeric outer protection by taping and/or heatshrink tubes

Prefabricated composite type termination

assemble the spring unit

setting the epoxy unit and slide of premoulded insulator

sleeve compression

insert parts

QA DOCUMENT

5 251

required required

Check cable placed in position with correct bending radius

Check cable serving, sheath, semi-conducting layer and insulain jointing instruction drawing

required required

Round and Hexagonal Compression

Thermit Weld

Deep indentation

MIG Welding

required

Construction and Procedure description

Conductor preparation

Conductors

required Check that end bell, etc are passed down the cable, before jointing commences, so that they are available in the right position for use later in the jointing installation

required

required

Check fully detailed jointing instruction supplied covering all items listed above and below

managed

Cable End Inspection for no damage /water

required

per J.I

per J.I

per J.I

description

values

value

description

list

description

dwg

required

Environmental Conditions d and

list

required

Checking that all jointing tools and other required equipment are on site

Positioning of Joint

description list

required required

dimensional value to be achieved

to be achieved and dimensions checked

to be achieved

to be achieved

to be achieved

to be achieved

to be achieved

to be achieved

to be achieved

to be achieved

to be achieved

to be achieved

to be achieved

QA Requirement Value value/ description

Checking that all jointing materials and consumables are on site

required from list

Organisation of jointing location

Set Up

Joint/Accessory Drawing number : YYYYYY

Jointing Instruction number/date/revision no : XXXXXX

OD sealing end (with internal stress cone) with cu cond, lead sheath and PE serving with PD test facility

QA DOCUMENT FILLED IN SAMPLE

yes yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

Signed by

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

QA c h e c k e d by jointer and ok

xxxxx

xxxxx

xxxxx

xxxx

xxxx

xxxx

xxxx

xxxx

xxxx

xxxx

xxxx

xxxx

xxxx

Date

Any comments

252 K. Leeburn

required

required required

Lubrication

Construction and procedure description

Preparation of cable sheath

required

required

required

per JI

per JI

per JI

per JI

per JI

per JI

per JI

per JI

per JI

per JI

per JI

per JI

per JI

description of process and value to be achieved

description of process and value to be achieved

description of process and value to be achieved

description

description of process and value to be achieved

description

description

description

description of process and value to be achieved

description of process and value to be achieved

description of process and value to be achieved

description of process and value to be achieved

description of process and value to be achieved

QA Requirement Value value/ description

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes yes

yes

yes

yes

Signed by

yes

yes

yes

QA c h e c k e d by jointer and ok

xxxxx

xxxxx

xxxxx

xxxxx

xxxxx

xxxxx

xxxxx

xxxxx

xxxxx

xxxxx

xxxxx

xxxxx

xxxxx

Date

Any comments

Cable Accessory Workmanship on Extruded High Voltage Cables

Low Smoke, Zero Halogen, Enhanced Flame Performance Sheaths

Extruded Polyethylene

Extruded PVC

Preparation of oversheaths

Oversheath

Stainless steel

Corrugated Cu

Copper Polyethylene Laminate (CPL)

Aluminium Polyethylene Laminate (APL)

Lead Sheath

Tig Welding

Plumbing

Corrugated Seamless Aluminium (CSA)

Copper wire screen

Metallic screen continuity

Welded Aluminium Sheath

required

reqiuired

Shrinkage

Metallic sheath

required required

required

Preparing the end of the semi conducting insulation screen

Smoothing the insulation surface

required

Cleaning of insulation

required

Stripping of semi conductive insulation screen

required from list

Straightening

Insulation Preparation

Mechanical Connection

QA DOCUMENT FILLED IN SAMPLE

5 253

ents

per JI per JI

required required required

tion

per JI

required

per JI

per JI

per JI

per JI

description

description

description

description

description of process and value to be achieved

description of process and value to be achieved

description of process and values to be achieved

QA Requirement Value value/ description

Lifting of accessories

required

required

required

required from list

Supporting of accessory

Common parts about accessory installation (joints and terminations)

rminations

Fitting OD porcelain/cast resin insulator and top metal

Heatshrink tube insulation

Taped Accessories (Terminations)

Installation of plug types

nd spring loading, if necessary, to ensure pressure is maintained

Filling compounds (joints)

Polymeric outer protection by taping and/or heatshrink tubes

Prefabricated composite type termination

assemble the spring unit

setting the epoxy unit and slide of premoulded insulator

sleeve compression

insert parts

preparation

Prefabricated composite type joint

Coldshrink tube insulation.

Heatshrink tube insulation.

Field moulding Extrusion or taped

High Voltage Heat-resistant tape

High Voltage tape

Taped Accessories (Joints)

Field expansion

Mechanical Reinforcement of Accessories

QA DOCUMENT FILLED IN SAMPLE

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

Signed by

yes

yes

QA c h e c k e d by jointer and ok

xxxxx

xxxxx

xxxxx

xxxxx

xxxxx

xxxxx

xxxxx

Date

Any comments

254 K. Leeburn

Signature of Supervisoy Jointer

Signature of Jointer

re q u i re d re q u i re d

required

Safety

on system

required for PD detection

required from list

Environmental Aspects

Fibre optics

Sensor connections

Special features

QA DOCUMENT FILLED IN SAMPLE

g e n e ra l

g e n e ra l

per JI

per JI

g e n e ra l

g e n e ra l

description of process and value to be achieved

description

QA Requirement Value value/ description

ye s ye s

ye s

yes

yes

ye s

yes

Signed by

yes

QA c h e c k e d by jointer and ok

xxxxx

xxxxx

xxxxx

xxxxx

Date

Any comments

5 Cable Accessory Workmanship on Extruded High Voltage Cables 255

256

K. Leeburn

References International Electrotechnical Commission IEC 60050 Chapter 461: electric cables Cigré TB 89 – Accessories for HV Extruded Cables (Chapter 1) Cigré TB 177 – Accessories for HV cables with extruded insulation (Chapters 1 and 2) Cigré TB 194 – Construction, Laying and installation techniques for extruded and self contained fluid filled cable systems Cigré TB 210 JTF 21/15 – Interfaces in high voltage accessories (Chapter 3) Association of Edison Illuminating Companies AEIC CG4-97 Guide for installation of extruded dielectric insulated power cable system rated 69 kV through 138 kV (2nd ed.) Cigré TB 272 – Large Cross Sections and Composite Screen Design Cigré TB 379 – Update of service experience of HV Underground and Submarine Cable Systems Cigré TB 446 – Advanced Design of Metal Laminated Coverings: Recommendations for Tests. Guide for Use Operational Feed-Back

Kieron Leeburn has a B.Sc. Electrical Engineering from the University of the Witwatersrand in South Africa. He is employed by CBI-Electric: African cables as their Chief Engineer covering product and process design and innovation. He has participated in a number of working groups in study committee B1 (insulated cables), and convened B1–22 one on Accessory Workmanship. He has represented Africa on CAG B1 (customer advisory group on insulated cables) since its inception. He received the Cigré Technical Committee Award in 2011 for outstanding contribution to the work of SCB1. He is a Member of IEC TC 20 WG16 (high voltage cables). He is a Member of the South African Institute of Electrical Engineers.

6

Guidelines for Maintaining the Integrity of Extruded Cable Accessories Eugene Bergin

Contents 6.1 Review of Recent Experience with Failures of Outdoor and Filled Terminations and Non-buried Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1 Review of Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.2 Review the Consequences of Termination Failures for Cables within Substations and Outside . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3 Survey by B1–29 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 The Role of Improved Materials, Design, Assembly and Quality Control in Mitigating the Effects of Termination and Non-buried Joint Failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Survey Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Design and Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.4 Quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 The Role of Testing and Condition Monitoring in Minimising the Incidence or Severity of Termination and Non-buried Joint Failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Condition Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Existing Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.2 New Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix 1: Terms of Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix 2: Bibliography/References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IEC Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CIGRE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jicable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

261 261 267 267 272 272 277 287 287 288 288 294 296 297 297 298 299 299 299 300 301

Eugene Bergin: deceased. Published as Cigré TB 560 in December 2013 E. Bergin (*) Dublin, Ireland © Springer Nature Switzerland AG 2021 P. Argaut (ed.), Accessories for HV and EHV Extruded Cables, CIGRE Green Books, https://doi.org/10.1007/978-3-030-39466-0_6

257

258

E. Bergin

Appendix 3: Reminder Chapter 5/TB 476 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix 4: Short Circuit Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Low Energy External Fault (Through-fault i.e. Breakdown Outside the Accessory) . . . . . High Energy Internal Fault (Internal Fault i.e. Breakdown Inside the Accessory) . . . . . . . . Simulation of the Fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix 5: Condition Monitoring Techniques for Terminations and Non-buried Joints . . . .

302 305 305 307 307 308

Executive Summary This work was motivated by the occurrence of disruptive failures of cable terminations and the consequential risks. The original scope of the Working Group (WG) was limited to land XLPE cable systems 110 kV and above. Although priority was given to outdoor and oil-immersed terminations, joints that are not directly buried were also included. The Terms of Reference are attached as 6.Appendix 1. Following discussions within the Working Group on the terms of reference, it was agreed that: • Bonding and earthing, including SVL failures, were, in the main, not to be included. • Any relevant learning points from PE cable accessories were to be included, although polyethylene (PE) cables are no longer installed. • There should be no time restriction on assets covered by the survey, as the relative newness of XLPE cable technology would naturally limit the scope. • The scope was extended to cover voltage ranges from 60 kV and above, as relevant failures at these voltage levels have also occurred and designs are similar to those being used at higher voltages. • Priority was given to outdoor, oil-immersed and GIS terminations, but joints that are not directly buried were also to be considered. Those items that needed to be considered and complied with to minimise the failure rate for terminations and non-buried joints are listed below, following detailed analysis by WG B1–29. Development, Prequalification and Type Tests The nature and scope of tests to be carried out when developing (new) cables and/or accessories have not been formally standardised and it has been left up to the individual producers/manufacturers to use their knowledge and philosophy to design such tests. However, in the early 1990’s the Cigré Task Force 21.03 published comprehensive recommendations for development tests on extra high-voltage cables with extruded dielectric, including the associated accessories. It was recommended that development tests for accessories focus on the following aspects: • Analysis of chemical, electrical and mechanical behaviour of materials • Long-term voltage test under thermal load cycles • Impulse and/or AC step voltage tests, where appropriate, with maximum conductor temperature. • Short circuit/disruptive discharge tests.

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Type tests in IEC62067 and IEC 60840 focus mainly on the withstand levels of cables and accessories with respect to a.c. or impulse stresses. They do not supply much information on the long-term behaviour of components, as the longest voltage test in these standards is limited to 20 days or 20 cycles of heating and cooling. The issue of long term tests (typically 1 year) is dealt with in Prequalification Tests in IEC 60840 and is to be carried out if the electrical stresses at the design voltage Uo exceed 8.0 kV/mm at the conductor screen and 4.0 kV/mm at the insulation screen. Fluid leakage is a significant cause of termination breakdown and this concern has to be addressed e.g. through final examination, as in IEC 62067 and 60840 standards, which states: “Examination of the cable system with cable and accessories with unaided vision shall reveal no signs of deterioration (e.g. electrical degradation, moisture ingress, leakage, corrosion or harmful shrinkage) which could affect the system in service operation.”

Factory Quality Control (QC) It is essential that full quality control is exercised in the manufacture and supply of terminations and joints. This applies to all the sub-components of each accessory e.g. stress cones, jointing material, compounds, etc. A full set of suitable tests e.g. dimensional checks, electrical tests, as appropriate, should be established and implemented. The different components of an accessory should be packaged in such a way as to avoid damage and moisture ingress during transport. Delicate components, such as stress cones, should be shipped in sealed plastic containers. A detailed list of these components should be included in each box together with a complete set of assembly instructions. Recommended handling, storage conditions and expiry dates for any components should also be provided. On Site Quality Control It is essential that full quality control is exercised on site with respect to the jointing area set-up, including the control of dust, humidity and temperature and the use of the correct jointing tools in good condition. In addition it is essential that suitable jointing instructions and drawings are supplied and that checks are carried out to ensure that the proper jointing material is supplied to site, in good condition and not past it’s expiry date. Finally a proper check-off list (inspection/test plan) should be used to make sure the jointing is done properly and in accordance with instructions. Jointer Certification As the quality of cable preparation and accessory installation plays a significant part in the reliability of XLPE accessories, it is critical that cable jointers have sufficient knowledge and training to carry out the task. It is therefore important that jointers are continually assessed to ensure competence and to maintain a high standard of workmanship. These training records and an up-to-date CV of previous works can be requested for review. Jointers should have valid up-to-date certification, as contained in Cigré TB 476, for the accessory they intend to assemble.

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Tools The minimum required tools are: • Those found in a standard tool box, such as knives, screwdrivers, wrenches, spanners, etc. • Specific tools for conductor jointing, insulation and semi-conducting screen preparation, installing pre-molded stress cones, metallic sheath, screen and armour connecting, inner and oversheath finishing. Specific tools and consumables shall be specified by the cable and accessory supplier/s. Jointing Instructions and Drawings Jointing instructions and drawings should be part of the quality assurance system. This is particularly crucial where accessories and cables are supplied by different providers. It is essential that the correct and suitable jointing instructions and drawings are used and that they are delivered with the accessory. Site Testing It is strongly recommended that an AC voltage test should be carried out on the insulation of the cable system in accordance with IEC Standards. Maintenance and Condition Monitoring In order to reduce the likelihood of failure of a termination or a non-buried joint, an inspection and test regime is recommended to monitor the condition of accessories. Many techniques are available to assess the condition of XLPE cable accessories. However, these techniques vary significantly with regards to practicality, availability of test equipment and the level of expertise required. The condition monitoring techniques employed should generally be assessed on a case by case basis and assessed against the requirements and cost of monitoring compared to the consequence of a failure. A list of the currently available techniques is contained in 6. Appendix 5. In the event of oil or compound leakage or other incipient failure mechanism, a risk assessment should be carried out and corrective action taken if necessary. Risk Assessment The continued use of any accessory should be based on: • • • • • • •

Public and employee safety The criticality of the circuit The history of the circuit and its accessories The potential repair time The potential cost of an outage to complete the repair The potential cost of an outage, if a failure occurs Potential damage from the failure

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• • • • • •

Potential cost of the damage Effect on reputation, licence compliance and potential for prosecution Effectiveness of any monitoring system adopted Availability of monitoring tools and trained personnel The cost of monitoring Potential for damage of the accessory due to external factors.

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In case of a failure in service the first step is to verify if the cable systems (cable and accessories) has been subjected to the tests (development, prequalification, type, sample, routine), as requested by the relevant IEC standards or Cigré recommendations. Following that one should investigate manufacture, delivery, installation and operation to determine the source of the fault. In the case of new cable systems, utilities should try to adopt designs that either do not experience disruptive discharge and/or have been tested to ensure the impact is kept to a minimum.

6.1

Review of Recent Experience with Failures of Outdoor and Filled Terminations and Non-buried Joints

The Working Group carried out a review of published literature on the subject and also carried out a survey of the experience of the Working Group members’ and Study Committee B1 members’.

6.1.1

Review of Literature

The first step taken was to review existing literature and determine what was relevant to the study of accessory failures. It was agreed reviews should be short and take the following format: • Cause of defect • Consequence of the defect • Corrective steps taken.

6.1.1.1 Cigré, Jicable and Other Technical Literature Nothing of particular relevance was found in the published Cigré literature. A recent paper for Jicable 2011 (A.5.4) described a failure in an XLPE cable termination installed in a 400 kV GIS substation and the remedial actions taken. Another Jicable 2011 paper (A.3.7) summarised the experiences of three European TSO’s. It showed that only a small part of the total cable circuit outage time is due to the actual repair time. More time was spent on other aspects, such as approvals to enter the premises, arranging the proper permissions to start repair works, cleaning the area and getting the necessary parts to site. The relevant literature is listed in 6. Appendix 2.

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6.1.1.2 Statistics Cigré TB 379 “Update of Service Experience of HV Underground and Submarine Cable Systems” supplied the statistics in Table 6.1 below regarding XLPE terminations. There is no information in Cigré TB 379 for non-buried joints. The table below gives an overview of the number of terminations installed on XLPE cables (including PE and EPR) in the period 2001–2005. Later statistics are not available in a Cigré TB, but the WG addressed this in Sect. 6.1.3 below by gathering up-to-date experience from those 14 countries that responded to the WG survey enquiry. The table below (Table 6.2) indicates the failure rates over the same time period (2000 to 2005): In Table 6.1, for the period 2001–2005, we can see that for the HV cable systems (60 to 219 kV) the use of outdoor composite insulators is already a commonly used technology. For EHV (above 219 kV) this technology is only starting. The same findings are made with regard to the use of dry type GIS terminations. From Table 6.2 we can see that the failure rate on terminations for EHV cable systems (above 219 kV) is around 5 times higher than that for the HV cable systems (60–219 kV). Table 6.3 gives indicates the failure rate per type of termination and is grouped for the voltage levels 60–219 and 220–600 kV. For a relatively high number of failures on terminations, the type of the terminations was not specified. As a result, the reader must be careful when comparing the different types of terminations. The information as shown in Tables 6.1, 6.2 and 6.3 is based upon replies received by WG B1–10 to their questionnaire. For further information regarding these statistics we refer to Cigré TB 379. 6.1.1.3 Workmanship Cigré TB 476 “Cable Accessory Workmanship on Extruded High Voltage Cables” was published in October 2011 and is published in this Book as ▶ Chap. 5, “Cable Accessory Workmanship on Extruded High Voltage Cables”. This Sect. 6.1.1.3 is substantially reproduced from that Cigré TB and ▶ Chap. 5, “Cable Accessory Workmanship on Extruded High Voltage Cables”. Cigré TB 476 covers workmanship associated with the jointing and terminating of AC land cables, incorporating extruded dielectrics for the voltage range above 30 kV (Um ¼ 36 kV) and up to 500 kV (Um ¼ 550 kV). This brochure is a complement of Cigré TB 177 (see ▶ Chaps. 1, “Compendium of Accessory Types Used for AC HV Extruded Cables” and ▶ 2, “A Guide to the Selection of Accessories” of this book). A short chapter covers general risks and skills, but the bulk of the document focusses on the specific technical risks and the associated skills needed to mitigate these risks. This is done for each phase of the installation. This Cigré TB is not an Instruction Manual, but rather gives guidance to the reader on which aspects need to be carefully considered in evaluating the execution of the work at hand. High voltage cable accessories are manufactured using high quality materials and very sophisticated production equipment. Recent technical and technological developments in the field of their design, manufacturing and testing have made it possible to have pre-molded joints and stress cones for terminations up to 500 kV, as well as cold shrink joints up to 400 kV.

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Table 6.1 Terminations installed on XLPE cables (including PE and EPR) in the period 2001– 2005 AC ACCESSORIES VOLTAGE RANGE kV

YEAR OF INSTALLATION

60 to 109

110 to 219

220 to 314

315 to 500

>500

2001 2002 2003 2004 2005 2001 2002 2003 2004 2005 2001 2002 2003 2004 2005 2001 2002 2003 2004 2005 2001 2002 2003 2004 2005

EXTRUDED CABLES (EPR, PE or XLPE) O u td o o r Termination

Outdoor Termination

- Porcelain

- composite insulator 27 15 21 24 21 131 128 163 190 285 0 0 6 9 3 0 0 0 0 12 0 0 0 0 0

531 753 513 483 600 267 282 546 226 187 135 63 102 66 60 12 0 0 0 28 0 0 0 0 0

Outdoor Termination - Dry Porcelain 12 27 15 24 51 159 216 51 63 162 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Outdoor Termination - Dry Composite Insulator 75 69 96 186 138 32 35 83 32 41 0 0 0 0 12 0 0 0 36 0 0 0 0 0 0

GIS or Transformer Termination

GIS or Transformer Termination - Dry

0 6 5 2 3 116 77 130 98 106 54 30 0 3 3 0 0 0 0 12 0 0 0 0 0

311 296 225 190 225 394 565 447 366 389 135 12 42 27 42 0 0 12 0 0 0 0 0 0 0

Table 6.2 Failure rates of terminations over the period 2000 to 2005 Failure rates based on all replies

Termination

Xlpe cables (AC) 60– 220– 219 kV 500 kV 0,006 0,032

ALL VOLTAGES 0,007

Termination

60– 219 kV 0,005

220– 500 kV 0,018

ALL VOLTAGES 0,006

Termination

60– 219 kV 0,011

220– 500 kV 0,050

ALL VOLTAGES 0,013

A. Failure Rate – Internal Origin Failures Failure rate [fail./yr. 100 comp.] B. Failure Rate – External Origin Failures Failure rate [fail./yr. 100 comp.] C. Failure Rate – All Failures Failure rate [fail./yr. 100 comp.]

Voltage range kV 60 to 219

Cable type Extruded (XLPE, PE or EPR)

Accessory type Outdoor Termination - Fluid filled - Porcelain Outdoor Termination - Fluid filled - Composite insulator Outdoor Termination - Dry Porcelain Outdoor Termination - Dry Composite insulator Outdoor Termination -Type not specified Outdoor Termination -Total GIS or Transformer Termination - Fluid filled GIS or Transformer Termination - Dry 2 2 1 17 37 0 19

1954 1353 0 52152 4222 20771

Total numbers of faults 15

2619

Total number of accessories in 2005 46226

Table 6.3 Failure rates by type of termination over the period 2000 to 2005

0,019

0,015 0,000

0,020

0,024

0,019

Total failure rate 0,007

0,015

0,007 0,000

0,000

0,024

0,019

0,002

0,006 0,000

0,020

0,000

0,000

Failure rates Cause of failure Internal External 0,003 0,003

0,002

0,002 0,000

0,000

0,000

0,000

Unknown 0,001

264 E. Bergin

220 to 500

Extruded (XLPE, PE or EPR)

Outdoor Termination - Fluid filled - Porcelain Outdoor Termination - Fluid filled - Composite insulator Outdoor Termination - Dry Porcelain Outdoor Termination - Dry Composite insulator Outdoor Termination -Type not specified Outdoor Termination -Total GIS or Transformer Termination - Fluid filled GIS or Transformer Termination - Dry

5 0 0 0 18 23 2 2

1493 61 0 53 0 1607 2447 637

0,071

0,330 0,016

0,000

0,000

0,000

0,075

0,071

0,215 0,016

0,000

0,000

0,000

0,030

0,000

0,086 0,000

0,000

0,000

0,000

0,045

0,000

0,029 0,000

0,000

0,000

0,000

0,000

6 Guidelines for Maintaining the Integrity of Extruded Cable Accessories 265

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One of the conclusions of Cigré TB 476 is that internal failure rates of accessories, particularly on XLPE cable, are higher than other components and are of great concern due the larger impact of a failure. Therefore the focus on quality control during jointing operations must be maintained. Many utilities have adopted the “system approach” by purchasing the cables as well as the major accessories from the same supplier. Some utilities also request that the link should be installed by the supplier or by a contractor under the supplier’s supervision in a “turnkey” fashion. The main advantage of this approach is that the entire responsibility for the materials and workmanship is clearly the supplier’s. Some customers have adopted the component approach by purchasing cables and accessories from different suppliers and entrusting the installation to a third party. In all cases, it is imperative that the installation be carried out by qualified jointers, who follow the jointing instructions provided by the accessory supplier. International standards such as IEC and IEEE provide the necessary guidelines concerning the interface between cables and accessories. However, it is strongly recommended that the responsible engineer should verify the compatibility of the different components of the link. It is of vital importance to manage the interface between the cables and the accessories in order to reduce the potential technical risk, e.g. cables and pre-molded accessories having non-compatible diameters or other non-compatible dimensions or characteristics. One of the international trends in cable technology has been the reduction of the cable insulation thickness and the corresponding increase in electrical stress. This tendency is based on better knowledge, increased quality of the insulating material and improvements in the extrusion process. Cables and accessory components are made under well-defined factory conditions and their quality and reliability are assured by adherence to well defined specifications. However, the accessories are assembled on site and, notwithstanding that this job is carried out by skilled and trained jointers, it is often performed in more delicate and less controlled conditions than in the factory. This means that correct assembly is even more important, because, with the increased stress level due to the reduced insulation thickness, bad workmanship will, sooner or later, lead to a breakdown of the accessory. It is noted that the majority of the new HV cable links being considered will use XLPE insulated cables. Cigré TB 476 captured the state of the art of jointing and is considered the best practice internationally. It is acknowledged that other practices, which are not explicitly covered in this brochure, are not necessarily bad practices. Great care should be exercised and the approach agreed when departing from practices recommended in Cigré TB 476. While Cigré TB 476 does not directly refer to failures or the consequences of failures, it is a comprehensive document on the assembly of cable accessories. If used properly it can provide vital advice on the avoidance of failures due to bad workmanship.

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6.1.2

267

Review the Consequences of Termination Failures for Cables within Substations and Outside

6.1.2.1 Cigré, Jicable and other Technical Literature In the case of Cigré the only consequences are the repair times that are covered in Sect. 6.1.2.2 below. 6.1.2.2 Statistics From Cigré TB 379, average repair times in days for XLPE systems are set out in the Table 6.4 below. This average repair time was calculated for all the reported failures on extruded cables for the corresponding voltage levels. No separate values were calculated for specific types of accessory. The definition of repair time as used in the questionnaire by B1–10 is the following: Repair time is the cumulative period of time required to mobilize resources, locate and repair the failure. The repair time associated with a failure is of fundamental importance since the summation of repair times is required to obtain a measure of non-availability, which from a reliability viewpoint is of greater significance than fault rate. 6.1.2.3 Workmanship Cigré TB 476 does not specifically refer to the consequences of failures, except to indicate the potential damage in the area, the very serious transmission system consequences with potential safety implications, loss of load, loss of customers, poor public relations and potential loss of revenue and additional costs.

6.1.3

Survey by B1–29

The Working Group compiled a survey to be completed by all members of the WG and SC B1 members, whose country were not represented on the Working Group. The survey was split into the voltage ranges recommended by Cigré below (Fig. 6.1): • • • •

50–109 kV 110–219 kV 220–314 kV 315–500 kV.

Table 6.4 Average repair time for cables in days

60 to 219 kV 220 to 500 kV

15 days 25 days

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Fig. 6.1 Failure due to poor workmanship (surface scratch due to bad workmanship)

Replies were received from 14 countries. Terminations and non-buried joints were dealt with separately. The survey results may be summarised as follows:

6.1.3.1 Survey on Terminations • A total of 61 failures were reported • Most of the installations were inside substations with only 6 being in a public area • The voltage range was from 51 to 400 kV, with the main installations being in the 50–150 kV range • The installation year varied from 1972 to 2010 • The year of failure varied from 1988 to 2010 Most installations had commissioning tests and, in most cases, voltage tests were carried out as part of commissioning • Most installations were outdoor (37) • The outdoor housings were generally filled with silicon oil or polybutene and the GIS (Gas Insulated Substation) housings were mainly unfilled • Most AIS (Air Insulated Substations) installations had composite or polymeric outer housings – 18 had porcelain housings. However it should be noted that failures in porcelain housings are likely to be more serious in view of the shards that are created during the fault • The terminations were mainly installed by a manufacturer, with only 15 being installed by a utility or contractor • The conductor sizes varied from 100 to 2500 sq. mm and were both copper and aluminium • The metallic shield varied from lead to aluminium foil to copper wires • In nearly all cases the cable and termination were from the same manufacturer • In most cases prequalification test had not been completed • Nearly all termination designs had undergone type tests

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• In only a few cases were maintenance test carried out – varying from a serving test, DC test and thermovision tests • The pollution design ranged varied from normal to serious • The causes of failure were listed as: – Termination Design • Moisture ingress due to inadequate sealing. • Pre-molded component breakdown. • Breakdown of insulating material. – Manufacture • Poor adherence of pre-molded components within stress cone • Rough surface of metallic parts leading to Partial Discharge • In one case manufacture was identified, but a reason was not given. • Poor fluid quality leading to internal discharges. – Workmanship • Damage to primary insulation during jointing. • Poor fluid treatment prior to filling. • Poor XLPE surface preparation. • Poor preparation of the outer semi-conducting layer. • Copper particles between cable and stress cone. • XLPE shavings left in position between cable and stress cone. • Incorrect application of stress cone. • Cable not sufficiently straightened prior to jointing. – Overload • No cases reported in the returned survey results. – Overvoltage • Four cases due to switching/lightning surge. – Animals • No cases reported in the returned survey results. – Weather Effects • No cases reported in the returned survey results. – Cable Insulation Inadequacies • Two cases, no details supplied. – Bonding Problems • Thermal runaway due to a metal sheath being solidly bonded during installation. This was not in accordance with the specified bonding design, which was based on single point bonding. • Poor earth connection due to mechanical movement causing flash-over. – Fluid/Gas Problems • Partial discharge caused by solidifying silicon oil. • Multiple failures due to leaks of insulating oil. – External Damage /Sabotage • No cases reported in the returned survey results. – Others • Failure of pressure relief system, leading to loss of insulating fluid. • Consequences of Failure – fire, outage time, collateral damage, reputation

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Fig. 6.2 50 kV porcelain outdoor cable termination, leaking high viscous insulating oil at bottom flange

– Most cases resulted in a disruptive failure and some collateral damage that required a lengthy repair outage (Fig. 6.2). • Actions Taken – New Design • Method for earthing of sheath improved • Change in specifications for pre-molded parts – New Tests • No new tests were specified in the returned surveys. – New Installation Specification • Improved termination fluid filling and treatment processes • Changes made to compounds used during jointing and methods for handling compounds • Suitable hold and witness points introduced during jointing • New XLPE shaping techniques implemented • Improvements made to Jointing Instructions – Risk Management • On-Line PD tests introduced. • Exclusion zones set up around termination, including screening walls. – Repair/Corrective Action • Changed whole joint/ termination. • Changed stress cone only. All faults required some form of repair or corrective action to be taken. – Preventative Action • In many cases sealing ends that were leaking insulating fluid were replaced or repaired before an electrical failure occurred.

6.1.3.2 Survey on Non-buried Joints • 27 failures were reported: 12 of the failures in premolded joints and 11 in taped joints. The remaining four failures being EMJ (extruded molded) or transition joints.

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• The location of the joints was generally not stated. • The voltage range was 50 to 314 kV, but the taped joints were in the lower voltage range. • Core sizes varied from 400 to 2000 mm2 with both copper and aluminium conductors. • Most joint casings were unfilled. • The installations were mainly carried out by the manufacturer. • It was not clear if the joints and cables were from the same manufacturer • In general the joints were type tested. • Most joints were commissioned with DC voltage tests (both insulation and serving). • There was no maintenance testing before failure. • Many joints failed within 1–2 years of commissioning. • The causes of failure were attributed as follows: – Joint Design • Incorrect stress cone internal diameter. • Incorrectly shaped embedded electrode. • Poor tape design. – Manufacture • Defective manufacture of stress cone that contained voids. • Poor quality stress cone material. • Water penetration via a crack, due to a manufacturing defect within the metallic casing. – Installation • Damaged insulation during jointing. • Poor shaping of XLPE. • Voids created, due to poor shaping of insulating tapes. • Incorrect positioning of stress cones. • Cable inadequately plugged into joint body. • Metallic particle contamination. • Loss of earthing connection to screen wires, due to poor soldering. • Racking or tray system that permitted joint movement. – Overload • No cases of failure were attributed to overload. – Overvoltage • One reported case was attributed to a possible lightning strike. – Animals • There were no failures attributed to animals. – Weather Effects • In only two cases failures were attributed to weather effects, namely water penetration. The water penetration in joints may be a design/material/workmanship issue – Unknown • One case was listed as unknown. • Consequences of Failure – No consequences were provided in the survey replies. • Actions Taken

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– New Design • In most cases where joint design was identified as the cause of failure, the joint was redesigned. – New Tests • Post-installation PD testing of joints was introduced in many cases. – New Installation Specification • Hold and witness points were introduced including photographic records. • New guidance on joint protection and waterproofing was introduced. • Clean room conditions introduced to joint bays. • Improvements were made to jointing instructions. – Risk Management • Joints identified as potential failure candidates were replaced with either joints of a different design from the same manufacturer or joints from a different manufacturer. • Inspection, partial discharge testing and X-Raying of all joints installed from the same manufacturer were carried out. – Repair/Corrective Action • In most cases the affected joints were removed, which required the insertion of a new piece of cable and 2 joints and the joint bay was extended to fit the new joints – Other • A new reinforced racking design was introduced

6.2

The Role of Improved Materials, Design, Assembly and Quality Control in Mitigating the Effects of Termination and Non-buried Joint Failures

This section examines how matters may be improved with respect to materials, design, assembly and quality control in preventing termination and non-buried joint failures and mitigating their effects. As part of this process, the results of the survey are reviewed to identify the causes of faults and steps identified that could be taken to ensure these faults did not occur. It should be noted that some of the measures identified in the Survey Results Sect. 6.2.1 below may be repeated to some extent in the Sections 6.2.2, 6.2.3, and 6.2.4 dealing with Materials, Design, etc. This was done to ensure the Cigré TB is as complete as possible.

6.2.1

Survey Results

It is of considerable importance that the results of the survey in Sect. 6.1.3 are taken into account and that, where causes were identified, these are acknowledged and steps are taken to avoid these causes in the future. The causes and recommended mitigations are listed below:

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6.2.1.1 Terminations 6.2.1.1.1

Design

Cause Unsuitable top O ring seal used leading to moisture ingress Powder separation of chemical mixture. Earthing conductors slipping off metal sheath in termination by sliding over PE sheath. Circulating current flowing through insulator screen causing overheating and damage. Pre-molded insulation degradation at extremely low temperatures Damage due to thermal cycling.

Interface design. Degradation of components in stress cone. GIS copper corona shield with thin layer having whiskers, leading to PD and breakdown. Stress cone interface contaminants

Mitigation Use appropriate O ring and fit properly Ensure correct compounds are used and installed correctly Ensure correct installation. Use checklist for installation. Ensure the correct bonding design is installed Ensure design suitable for operating temperatures high and low Design and test for heat conditions. (Snaking cable before terminating to minimise conductor expansion into the termination) Change components or design Use appropriate materials and enhance the interface design Consider extended Prequalification Tests. Design corona shield materials for use in GIS cable termination box. Inspect all components prior to fitting. Jointer trained on fitting accessory, as recommended in 6.Appendix 3 Ensure clean conditions when jointing

6.2.1.1.2 Manufacture One case was identified but no details were supplied – no additional mitigation proposed. 6.2.1.1.3

Workmanship

Cause Jointer damaged insulation Poor XLPE surface shaping – copper contaminants between cable and stress conecontaminants invasion of oil Shavings of copper contamination during the insertion of pre-molded insulation

Mitigation Follow 6.Appendix 3 Consider use of inspection test plans (ITP’s) Follow 6.Appendix 3 Consider use of inspection test plans (ITP’s) Follow 6.Appendix 3 Consider use of inspection test plans (ITP’s)

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E. Bergin

Poor surface of outer semi conducting layerdefective position of compression device Void generation between epoxy and stress cone Plastic wrap is used for protection during construction. Void generation at cable/ stress cone interface by overbending of cable and shaving cable insulation too much. Generation of crack in epoxy insulator by stressing it more than it was designed. Overbending of cable. Void generation at cable/ stress cone interface by conductor centering error, when conductor sleeves were compressed Wrong insert position 6.2.1.1.4

Mitigation Ensure appropriate design and installation of lightning protection, when required.

Weather Effects

Cause Lightning Water entry Connection broken, due to mechanical overload Jointing with high relative humidity 6.2.1.1.6

Follow 6.Appendix 3 Consider use of inspection test plans (ITP’s)

Overvoltage

Cause One case due to switching/ lightning surge 6.2.1.1.5

Follow 6.Appendix 3 Consider use of inspection test plans (ITP’s) Follow 6.Appendix 3 Consider use of inspection test plans (ITP’s) Follow 6.Appendix 3 Consider use of inspection test plans (ITP’s)

Mitigation Ensure lightning protection used, when needed Follow 6.Appendix 3 and use proper O ring and fit it properly (it could be a design/material problem) Ensure that not overbend Use of an enclosed air conditioned work environment Follow 6.Appendix 3

Bonding Problems

Cause Metal sheath incorrectly bonded on a single core cable, resulting in a sheath circulating current that overheated and damaged the termination Bad connections; poor design of wiping gland leading to mechanical movement, sparking and failure

Mitigation Ensure bonding design is followed Carry out checks during commissioning Ensure design suitable for operating temperatures high and low and installed properly.

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6.2.1.1.7

Fluid/Gas Problems

Cause Partial discharge in fluid Leaking fluid or gas

6.2.1.1.8

275

Mitigation Ensure correct fluid is used and that fluid is properly treated and tested and that it is at the right level. Check where fluid or gas is leaking from, repair if necessary, and top up. Replace termination or component causing the leak.

Others

Cause Unknown – breakdown just above stress cone Contaminants noticed at the cable stress cone interface Moving cables after installation

Mitigation Ensure design is suitable for high and low operating temperatures Remove Follow 6.Appendix 3 Ensure cables do not exceed their thermomechanical design limits, are properly clamped and are not physically disturbed

6.2.1.2 Non-buried Joints 6.2.1.2.1

Design

Cause Stress cone with incorrect inner diameter Shape of embedded electrode not right Poor tape design

6.2.1.2.2

Mitigation Ensure joint is suitable for use on specified cable after cable is prepared Ensure design is compatible Ensure adequate Prequalification and Type Tests are carried out Ensure material used has the right properties and installation instructions. Consider Prequalification Testing

Manufacture

Cause Defective manufacture of stress cone (voids) Poor material quality

Mitigation Ensure manufacturer’s QC system is adequate Consider Prequalification testing Ensure manufacturer’s QC system for materials is adequate Consider Prequalification testing

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Water penetration from a crack, because of manufacture problem with metallic sheath

6.2.1.2.3

Workmanship

Cause Jointer mistakes causing damage to insulation and poor insulation shield shaping. Water penetration, metallic contaminants, wrong inset position. Poor adhesion of stress cone Metallic contaminants in the insulation tape. Void generation with poor tape shaping. Contaminants. External damage by jointing tool, when connection box was assembled. Fibrous contaminant in extruded insulation. Clamping of screen wires caused damage of outer semi-conducting layer Loose flakes of applied semiconducting coatings in joint assembly.

6.2.1.2.4

Ensure manufacturer’s QC system is adequate

Mitigation Follow 6.Appendix 3 Consider use of inspection test plans (ITP’s)

Follow 6.Appendix 3 Consider use of inspection test plans (ITP’s) Follow 6.Appendix 3 Consider use of inspection test plans (ITP’s)

Follow 6.Appendix 3 Consider use of inspection test plans (ITP’s)

Follow 6.Appendix 3 Ensure proper procedures followed, adequate drying time and care in positioning of the joint body.

Overvoltage

Cause In only one case was joint damage attributed to possible lightning strike

6.2.1.2.5

Mitigation Ensure appropriate lightning protection is used.

Weather Effects

Cause In only two cases was failure attributed to weather effects, namely water penetration.

Mitigation Follow 6.Appendix 3 Consider use of inspection test plans (ITP’s). Adequately designed casing (coffin) filled with waterproof compound.

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277

Design and Materials

In considering the design of terminations and joints it is necessary to consider the materials to be used, the pressures in different parts of the accessory assembly, the different electrical characteristics, etc.

6.2.2.1 Air Insulated Terminations Air Insulated Terminations are generally used outdoor to terminate cables in air insulated substations. They may have porcelain or composite insulators and may be filled or unfilled. The design adopted may depend on the local environment with respect to the required basic impulse level voltage (BIL), maintenance requirements, pollution (industrial and ocean), reliability and altitude. Surface creepage distances may need to be increased in areas of high pollution, excessive sea spray or at high altitudes. 6.2.2.1.1 Porcelain Insulators Glazed electrical grade porcelain is the most common and widely installed insulator. It has high reliability in terms of electrical and mechanical performance. It requires periodic maintenance (cleaning) to remove pollution deposits from the insulator surface (sheds). It has high resistance to surface tracking. Porcelain production is a mature technology and can be provided for MV to EHV cable terminations and for both AC and DC application. However, porcelain can be susceptible to external mechanical damage and to electrical failure (internal or external). It can shatter on termination failure with pieces of glazed porcelain and other debris projected over the surrounding area by the force of the failure. The potential for injury or damage to adjacent equipment in the surrounding area is high. 6.2.2.1.2 Composite or Polymeric Insulators There are many types of composite insulators available on the market. The most common design consists of a fibreglass tube covered by elastomeric sheds (silicone). This solution is much lighter than a porcelain insulator and is normally much easier to handle during installation. However, the bond between silicon rubber and the epoxy glass fibre pipe must be certified as this can be a weak point (Fig. 6.3). Composite insulators are available up to EHV applications, even though at this stage there is no long term operational experience at EHV levels. Composite insulators have many advantages. In particular they have proven to be reliable even under exceptional events such as earthquakes, system faults and vandalism. They also provide good insulation performance due to their silicone housing and the intrinsic hydrophobic characteristic of this material. Well designed composite insulators have limited ageing. They give satisfactory performance in heavily polluted areas, where no cleaning or special maintenance is necessary and this can provide important economic savings.

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Fig. 6.3 Composite insulator filled with synthetic oil

Their technical and economic advantages are of particular significance in the EHV and UHV range of accessories. This is because of their design flexibility (single pieces of 10 m or more may be manufactured), relative low weight (10–30% of a corresponding porcelain insulator), ease of handling for manufacturing and installation and their ability to withstand stresses, such as seismic events and high levels of pollution. From the point of view of end-users, a very important feature of composite insulators is safety. They reduce the potential for manual handling injury during delivery and installation. Since they are not brittle, the risk following an internal fault, with the associated projection of material, is greatly reduced compared with porcelain. The satisfactory long term performance of composite insulators is directly related to electrical and mechanical design, good selection of the material, good manufacturing processes and quality control. Environmental constraints of the installation site such as the required BIL, temperature, barometric pressure (for high altitude), presence of aggressive gases, pollution, and humidity should be taken into account in the design. Qualification procedures can help to qualify the technology and the materials and assure the performance during the required life time of the insulator and these are dealt with in detail in Cigré TB 455 “Aspects for the Application of Composite Insulators to High Voltage (72 kV) Apparatus”. A range of biological growths have been reported on composite insulators leading to a reduction of the hydrophobicity. However, the overall performance of the composite insulator design generally remains satisfactory. Bird attacks have also been reported, but this appears to be a problem related to insulators in some countries and usually only happens when de-energised or before the insulators are put into service. Another consideration is whether vapour could permeate directly through the sheds and walls of the housing (polymeric materials are generally slightly permeable

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for vapour) or through the bonding area between flanges and fibre-reinforced plastic (FRP) tube. Investigations and service experience indicate that the amount of moisture ingress due to these mechanisms is below the quantities which can pass through a good sealing system. Quantities can easily be controlled by internal desiccants as is usual practice for much of the HV apparatus in the electric power system. In the case of terminations/ sealing ends this is often accomplished by using filling compounds. Nevertheless research continues in an attempt to better understand these mechanisms and to derive minimum design requirements on composite hollow core insulators used for HV apparatus applications. Most damage in composite insulators can be attribute to errors during transport, un-packing, re-packing, manipulation and storage of the insulators. These aspects are dealt in detail in Cigré TB 455 “Aspects for the application of Composite Insulators to High Voltage (72 kV) Apparatus”, Chap. 9, “Handling and Maintenance”. In this chapter, procedures and rules are given for: unpacking, repacking, storage, handling and cleaning. A composite termination has the advantages of a simple structure. Its anti-pollution capacity depends mainly on the number of sheds and their size and orientation. The terminal must be installed upright, it cannot be installed inclined or curved. Porcelain and composite terminations are compared in the Table 6.5 below. It can be seen that each outer housing material has its advantages and disadvantages. The selection of the appropriate termination body depends on the particular installation conditions. The satisfactory performance of composite terminations is dependent on the inner electrodes and the electric field distribution within and along the termination. This, in turn, depends on the top electrodes, the insulator material, the inner electrodes, non-linear coatings, cable make-up; etc. All of these components must be designed, manufactured and installed to control the operating electrical stresses. 6.2.2.1.3 Latest Developments The latest developments on the market provide two alternative solutions:• Self Supporting Terminations – A termination filled with silicon based leak-proof gel that replaces the traditional liquid fluids. This solution has been tested up to EHV, but service experience is available only up to 132 kV. The filling procedure has to be strictly controlled to ensure proper filling (Table 6.5; Figs. 6.4 and 6.5). – A fully dry termination, where no liquid or filling is used • Supported or Flexible Type A Prefabricated Outdoor Termination • This type of termination has elastomeric sheds and an external stress cone. The stress cone and the sheds form one single factory-tested premolded component and they are widely used in the voltage class up to 150 kV. With this termination type a completely “dry” design is obtained. Note this termination is not self supporting and must be connected to an overhead conductor or to another component e.g. a surge arrester, able to support the termination. • Disruptive–proof Outdoor Terminations i.e. terminations that are designed to limit the consequence of an internal power arc, etc.

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Table 6.5 Comparison of Porcelain and Composite Insulators Element Environmental

Chemical

Mechanical

Rating Performance Other Properties

Porcelain Insulators Can shatter Periodic cleaning required Poor pollution performance It’s earthquake performance is not so good Impermeable to animal attack even when unenergised Not hydrophobic Compatibility with SF6 by-products and oil Can shatter under fault conditions

Composite Insulators Safe/Inert Limited cleaning required

High performance in polluted areas Good earthquake performance Possible attack by animals during storage and while unenergised Hydrophobic Compatibility of filling material to be checked Will not shatter but may split Low weight High weight Vulnerable to Less susceptible to vandalism vandalism No moisture ingress Possible moisture ingress through through the insulator from outside.1 the insulator from outside.1 1 Note for both types of insulators there may still be some moisture ingress through the top and bottom metal components or gaskets No practical temperature limit Temperature limits of 55 to (temperature limits exceed those of +110  C other components) Lot of experience, but relatively long Limited service experience manufacturing time Because of its weight it’s not so easy Because of its weight its relatively to handle and install. Heavy manual easy to handle and install handling or mechanical assistance required Can be damaged (cracked or chipped) Not so likely to be damaged by handling and installation. Small damage can be repaired in-situ.

One must also bear in mind the effect of insulation retraction on the termination. Retraction is a result of the mechanical stress formed in the insulation during the manufacturing process. When the cable is cut, in order to install the accessory, the insulation may retract on the accessory and lead to a failure. This must be taken into account in the accessory design (Fig. 6.6).

6.2.2.2 GIS and Oil Immersed Terminations EHV and HV cables may also be directly terminated in SF6 insulated switchgear (GIS) and transformers to eliminate air-insulated interfaces. This solution has the significant advantage of markedly reducing substation area requirements and costs in urban, suburban and industrial plant locations. It also eliminates insulation contamination from pollutant deposits and reduces exposure to lightning and vandalism.

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Fig. 6.4 Example of a 170 kV composite cable termination

Fig. 6.5 Example of a Self Supporting Fluidless Cable Termination

GIS and oil immersed terminations have similar construction, except for the use of a larger top corona shield on the termination in order to reduce the top-end stress. The electrical stress control for GIS and oil immersed terminations follows the same approach usually employed for outdoor terminations i.e. it uses a premolded stress relief cone, which is fitted over the cable insulation. The cable is then

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Fig. 6.6 Example of a Dry Type Supported Termination

accommodated inside a cast epoxy resin bushing which separates the cable from the pressurised SF6 or the oil in the termination end box. The space inside the epoxy bushing can be filled with insulating fluid or SF6 gas. In order to eliminate any risk of leakage of this fluid or gas from inside the epoxy bushing, a new generation of dry type SF6 and oil immersed terminations have been developed. In these dry terminations there is no insulating fluid or gas between the epoxy insulator and the stress cone, because the latter is in intimate contact with the inner surface of the bushing; the pressure of the stress-cone at the cable core interface as well as at the inner epoxy insulator surface can be obtained by means of compression devices such as springs or by special design of the polymeric part. It should be noted that currently there is a Joint Working Group B1/B3.33 examining the “Feasibility of a common, dry type interface for GIS and Power cables of 52 kV and above” (2009–2012) and a Cigré TB is to be issued shortly by this WG by the end of 2013, ▶ Chaps. 7, “Feasibility of a Common, Dry Type Plugin Interface for GIS and Power Cables above 52 kV” and ▶ 11, “Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power Cables up to 145 kV”.

6.2.2.3 Insulation Medium Terminations are generally filled with a dielectric fluid, usually a synthetic (polybutene or silicone based) insulating liquid, at or slightly above atmospheric pressure. The type and quantity of the fluid depends on the specific design of the termination. Poor quality of the liquid or contamination, due to external factors (humidity, water ingress, metallic or other polluting particles, etc), can reduce the electrical performance of the fluid and result in termination failure. One of the most common issues with the use of fluid is the risk of leakage through the sealing point areas, typically the weld/plumbing between the cable metallic screen and the bottom part of the termination or the mechanical seal onto the stress cone. A well-made seal depends mostly on the skill of the jointers.

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There are also designs that use SF6 gas as the insulation medium, but this solution has to bear in mind the environmental concerns of using SF6 gas.

6.2.2.4 Connectors The connector electrically and mechanically joins the conductors of two cables or the cable and the top connector of a termination. Thus the connector must exhibit good electrical conductivity to avoid temperatures higher than that of the conductor in any operating condition and also present sufficiently high mechanical pull-out (tensile) strength to withstand thermo mechanical stresses during operation. It should be noted that WG B1.46 is currently working on Conductor Connectors (Mechanical and Electrical Testing). The final report of WG B1.46 is reproduced in ▶ Chap. 10, “Test Regimes for HV And EHV Cable Connectors” of this Book. The following types of connectors are used for extruded cable connections:6.2.2.4.1 Compression Connector This connector includes a tube of the same material as the cable conductor into which the conductors to be joined are inserted. The tube is then compressed by a hydraulic press. The compression connector is the most commonly used type, because it is easy to install and does not require heat (Fig. 6.7). The cross section of the connector is at least equal to the cross section of the conductors to be joined. When the connector is exposed to an electric field, as in taped joints, it is necessary to provide suitable chamfers at both ends to minimize the effects of longitudinal electrical stresses. A special bimetallic connector is used when it is necessary to join a copper conductor to an aluminium conductor. These connectors are half copper and half aluminium. The two connector halves are joined in the factory by friction welding. Some companies use a copper alloy connector for both copper and aluminium conductors. 6.2.2.4.2 Cad Welding Another way is to make a connection of copper and aluminium conductors by Cad-welding on site, though Cad welding is not used that often for aluminium. This is an exothermic welding process in which metal and metal oxide powders are placed in a special crucible mold around the parts to be welded. This mixture is Fig. 6.7 Compression connector

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ignited resulting in a short high temperature reaction, causing the flow of molten metals to form a localised solid connection. 6.2.2.4.3 Soldered or Brazed Connector Soldered connectors are used with small conductor cross sections (below 630mm2) and with cables having a short circuit current temperature below 160  C, because the solder can become soft during the cable system operation. Brazed connectors do not present this problem, but are more difficult to make. 6.2.2.4.4 MIG or TIG Welded Connection The two conductors are fused together by the application of molten metal. A Metal Inert Gas (MIG) or Tungsten Inert Gas (TIG) welding process is applied in this case. Due to the high temperature developed during the process, air or water cooling clamps are required on both sides of the weld, in order not to damage the cable insulation The welding process is used for large aluminium conductors and for insulated wire copper conductors; in the latter the burning of the wire insulation, if necessary, ensures a good contact between strands. This technology requires an operator with a very high skill level and is time consuming (Figs. 6.8 and 6.9). This weld provides a connection with an electrical conductivity, which is equivalent to that of the conductor itself. The connection is not subject to instability due to decrease of contact pressure as a result of load cycling. However the tensile strength of the welded connector is significantly (50 to 60%) lower than the ultimate tensile strength of the conductor, due to the annealing of the conductor near the weld. If necessary, for submarine cables, the tensile strength can be improved by round compressing the conductor and the weld (hardening process) (Fig. 6.10). 6.2.2.4.5 Plug-in Connector Two metal connectors, that terminate the conductor, are jointed through elastic or multi contact spring loaded contacts that are able to carry the current. Locking pins

Fig. 6.8 Examples of Cad Welding

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Fig. 6.9 Example of a MIG Weld

Fig. 6.10 Welding of an aluminium conductor

can be used to anchor the two parts together. Plug-in connectors can easily join conductors of different materials and cross section. One of the advantages of a plug-in connection is the shorter length of the joint.

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Fig. 6.11 Plug-in connector (male contact) on prepared cable end

6.2.2.4.6 Mechanical Bolted Connector (Shear Bolts) With these connectors compression of the conductors inside a ferrule is made by tightening threaded bolts. The bolts shear off at a predetermined torque and are then finished flush with the surface of the connector. These connectors are extensively used in MV accessories, and may also be used in HV joints or terminations, subject to checking their short circuit current and current loading capacity. The compatibility of these connectors with the termination or joint design must be checked. These connectors have a diameter larger than the compressed connectors and care must be taken to ensure there are no bits of bolt protruding above the connector surface. Before using shear connectors consideration must be given to tensile strength during load cycling and pull out (Fig. 6.11). 6.2.2.4.7 Mechanical Bolted Connector With these connectors compression of the conductors inside a ferrule is made by tightening threaded bolts. These connectors are extensively used in MV accessories, and may also be used in HV joints or terminations, subject to checking their short circuit current and current loading capacity. The compatibility of these connectors with the termination or joint design must be checked. These connectors have a diameter larger than the compressed connectors and care must be taken to ensure there are no bits of bolt protruding above the connector surface.

6.2.2.5 Non-buried Joints Non-buried joints locations may be in tunnels, on bridges, in underground chambers or similar enclosures. Non-buried joints for XLPE cables usually have premolded joint bodies with additional covering for protection against moisture and mechanical damage. The additional covering could be heat shrink tubes or metal housings with additional insulating housings/coffins (Fig. 6.12). Transition joints for XLPE to oil filled cable are often installed as non-buried joints in underground chambers. They use metal-tubes combined with epoxy insulators as a barrier between the different insulating materials – XLPE and fluid impregnated paper. In the case of transition joints full quality control must take

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Fig. 6.12 Example of a bolted connector

into account electrical and mechanical stresses for both sides of the joint and any interface locations. Water can seep into a non buried joint, if any earth or bonding wire connections to the joint are not sealed properly.

6.2.3

Assembly

Cigré TB 476 is a comprehensive document on assembly and quality control of XLPE accessories and the contents pages are attached as 6.Appendix 3. It gives guidance on aspects of cable accessory workmanship that need to be carefully considered in evaluating the execution of the work, including the specific technical risks and the associated skills needed to mitigate them (Fig. 6.13). Where a termination is to be filled with compound, the manufacturers filling instruction should be followed. Filling compounds may be such items as polybutene, silicon oil or other dielectric fluid or gas.

6.2.4

Quality Control

Joints and terminations are delivered to site as kits, which in turn are made up of many components It is vital to have quality control on all components. The main insulation is either the premolded joint body or premolded stress-cone, and the testing requirements for these are as defined in IEC60840 and IEC62067. The manufacturer shall demonstrate or guarantee that the components forming the accessory are the same as those tested to IEC standards. Each component has a specific function, whether it is secondary insulation, oil, gas or air tightness, mechanical protection, conductor or sheath connection, etc. It is

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Fig. 6.13 Example of non-buried joints: 145 kV single core cable joints installed in a cable jointing chamber/manhole

essential that the manufacturer has in place quality control plans that define the tests to be carried out and their frequency and these should be related to the function of the component. The inspection or testing may include visual, dimensional, mechanical, dielectric, pressure, whether as an incoming control from sub-suppliers or as final control as semi-finished products (insulators for example). Components must be inspected according to drawings and specifications with given tolerances, and there must be no deviations outside the given tolerances. Final checking must be done on delivery to site to ensure the right quantity and quality of materials has been delivered. Of course the QC aspects with respect to jointing, as set out in Cigré TB 476, must also be followed. This applies in particular to the certification/ approval for the jointers and the site conditions.

6.3

The Role of Testing and Condition Monitoring in Minimising the Incidence or Severity of Termination and Non-buried Joint Failures

6.3.1

Testing

6.3.1.1 General In order to prove that a cable system meets the expectations of the customer the role of testing at all stages of design, supply and in-service is clearly important for both the supplier and end-user. In addition, once a cable system is in service, it may be beneficial to carry out in-service testing to assess the condition of the system and its components. This section will examine the types of testing and condition monitoring

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that may be carried out, when assessing a cable system. This is not intended to be exhaustive, but to provide guidance on the areas that should be considered. The level of testing required for a cable system should be decided on by the customer, based on risk and performance requirements. International standards for underground cable systems generally provide design rules and testing procedures to assess a cable system and to ensure it meets the requirements for reliable operation during its design life. These generally focus on prevention of failure, rather than actions that can be taken to mitigate the consequences of a fault. Some National Standards or individual utility specifications have introduced fault simulation testing and specify requirements for the performance of the system under these conditions e.g. an internal arc test is carried out by some utilities to evaluate the consequence of an internal fault – there is a requirement for this within IEC 62271 requirements for switchgear testing. It should be noted that a cable system incorporates the cable, terminations, joints, internal terminations and joint components, filling media, connectors, screen connections, bonding etc., and great care must be exercised in testing to ensure that all of the components are properly represented and identified in testing regimes.

6.3.1.2 Development Testing Development testing is carried out by the cable accessory supplier during the design of a new accessory. The results of these tests may indicate to the manufacturer and, where required, the customer, any changes and improvements that can be made to a cable accessory. An example of development tests are the environmental tests including salt/ fog, rain and pollution tests, carried out on composite insulators, which are not covered by cable international standards. These tests are carried out by manufacturers to demonstrate the long term performance of their products and are carried out to in-house test specifications. IEC61462 ed. 1.0 covers the test procedures for Composite Insulators for AC Overhead Line with Nominal Voltage greater than 1000 volts. Results of development testing are generally not specified by customers, but may help to inform a decision on the suitability of a cable termination or joint for use for a particular application or in a particular location, for example the suitability of terminations for use in areas of high pollution. Development tests are performed by the manufacturer during the development of a new accessory and are intended to ensure the accessories long term performance and to assess safety margins. The tests include: • • • • •

Analysis of electrical, mechanical and material compatibility Electrical tests up to breakdown and mechanical and thermal tests on prototypes Wet and pollution test on outdoor terminations Electrical and thermal tests of connectors Mechanical tests on premolded components (on the insulators and connectors)

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• Fire and disruptive failure performance, including Internal Power Arc test on terminations in accordance with 6.Appendix 4. 6.3.1.2.1 Insulators IEC 61462 “Composite hollow insulators –pressurised and unpressurised insulators for use in electrical equipment with rated voltage greater than 1000 V” specifies both design and type test requirements for self supporting composite insulators. The tests in this IEC standard are designed to provide information on material selection, manufacturing processes, material thickness and adhesion and end fitting material selection an attachment. To complete the project of developing a new accessory, construction drawings shall be prepared of all components and a full size prototype shall be manufactured and subjected to tests. If the prototype includes specific components such as premolded parts, composite and epoxy resin insulators, it is necessary to develop the technology to produce these components. The tests should show the limit in the performance of the accessory and guarantee a proper safety margin with respect to test values stated in the relevant IEC standard. Tests carried out must ensure that the entire family of accessories is able to withstand the stresses, which they may be subjected to in their operational life (Fig. 6.14). The termination may be exposed to a saline solution of a different concentration depending on the level of pollution it will experience. In this condition it is then subjected to an AC voltage test. For composite insulators with a polymeric coating, which are subject to aging of the surface, the pollution test is performed after an aging of 1000 hours in saline fog or an electrical cycle-environmental of 5000 hours (see IEC 62 217).

Fig. 6.14 Salt-fog test on insulator

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6.3.1.2.2 Connectors Development testing may also be done for connectors. Thermal cycles are performed on connectors and contacts used in the accessories following the standards of IEC 61238–1, currently restricted to medium voltage. During the test, measurements of temperature and electric resistance as a function of time are taken. Short circuit current tests are also performed on the connectors, Testing of connectors is the topic of TB 758 which is reproduced in ▶ Chap. 10 “Test Regimes for HV and EHV Cable Connectors” of this book. 6.3.1.2.3 Filling Fluids Before using any type of oil or fluid within a specific housing material, equipment manufacturers should have verified its full compatibility with materials and assembly processes, including health and safety. This is especially of interest where new types of fluids or other fillers are considered. Some manufacturers have developed their own qualification procedures, specifying test conditions in terms of temperature, duration, safety and final acceptance criteria. This forms part of the development tests.

6.3.1.3 Prequalification Test Prequalification testing, as in IEC 62067 & 60840, is only specified for cable systems above 150 kV or where the conductor screen stress is designed to be greater than 8 kV/mm or the insulation screen stress is designed to be greater than 4 kV/mm, Prequalification tests are long term tests that are carried to assess the performance of a cable system and attempt to replicate in-service duty. The test arrangement should be representative of installed conditions, e.g. fixed and flexible sections and contain both joints and terminations to give a true replication of the cable system. These tests are intended to verify the thermo-mechanical and electrical behaviour of the cable and accessories. In some local standards it is also a requirement to monitor and record the pressure of any insulating mediums used in order to assess the robustness of any sealing arrangements. After testing, all accessories are to be examined to check for any changes or deterioration that might affect the performance (Fig. 6.15).

Fig. 6.15 Tests on connectors

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6.3.1.4 Type Test Type tests are carried out on the complete cable system and are required for all voltages and design stresses. These tests provide a minimum requirement to show specific cables and accessories are fit for a specific purpose. Type tests, as specified in IEC 60840 & IEC 62067, focus mainly on the cable system shortterm voltage withstand performance. They include AC, over-voltage and lightning transients combined with material aging effects. Following completion of these tests, the cable system must be shown to be partial discharge free or to have a level of discharge below a certain requirement. If any partial discharge is present, even below the level specified, it may be prudent to identify the cause of this discharge. Once tests are completed it is important to disassemble all accessories and closely inspect them for any signs of electrical activity or physical changes, which may not have caused an electrical discharge, but may cause mechanical or operational problems. The interpretation shall be based on the previous experience with development, prequalification and other type tests (Fig. 6.16).

Fig. 6.16 Type Test loop of 400 kV system

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6.3.1.5 Short Circuit Tests The WG identified that short-circuit behaviour was not addressed by any IEC standard relating to HV cable systems. Several utilities have independently taken the step of specifying an additional type test to check the behaviour of terminations (especially those containing insulating fluid) when they are subjected to short circuits. Two cases need to be considered • A low energy external fault. In this case the fault current passes though the conductor. The fault is external to the accessory. • A high energy internal fault. In this case the fault is the result of component failure or arcing inside the accessory. Consideration, depending on the design and installation, should be given to whether it is necessary to do one or both of the above tests to cover the worst case condition. These tests are detailed in 6.Appendix 4.

6.3.1.6 Sample Tests Sample test requirements are outlined in IEC 60840 and 62067. These tests are to be carried out on a specified number of components and complete accessories during a production run. For accessories, where the main insulation cannot be routine tested, IEC 60840 states that a partial discharge and an AC voltage test shall be carried out on a fully assembled accessory. For individual components the characteristics of each component shall be verified in accordance with the specifications of the accessories’ manufacturer, either through test reports from the supplier of a given component or through internal tests. Also the components shall be inspected against their drawings and there shall be no deviation outside the declared tolerances.

6.3.1.7 Routine Tests Routine tests are carried out on some accessory components to be supplied. These tests should form part of a robust quality control regime and provide confidence in accessories’ quality. As part of these tests, the main insulation of prefabricated accessory designs is required to undergo AC voltage and partial discharge tests. Finally each component should be visually inspected for defects. Insulators filled with oil, gas, or other material should also undergo a pressure test before delivery.

6.3.1.8 Tests on Filling Materials Filling materials, like polybutene or synthetic oil, are selected based on the material parameters and characteristics and they are approved during the development, prequalification and type tests. –specification IEC 60836 covers silicon oil.

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It is recommended that a “finger print” of the filling material be determined after delivery, as this “finger print” might be useful during condition assessment programs or failure analysis. Well established material “finger print” techniques are • • • •

AC electrical strength Dielectric dissipation factor Fourier transform infrared spectroscopy (FTIR) Thermal gravimetric analysis (TGA).

6.3.1.9 Commissioning Tests Commissioning tests are carried out on the assembled cables, joints terminations, bonding and earthing once the installation is completed. They are the final tests performed on the cable system prior to energising and provide the final check that the system has been correctly designed and installed. The requirements for commissioning tests will vary depending on the type of circuit installed and the consequences of failure. There are very few tests that can be carried out that will prove the long term life of cable, joints and terminations. However, it is recommended that an AC insulation test is carried out with partial discharge monitoring, if possible, of all joints and terminations. Ideally this is carried out using a resonant test voltage generator. This allows the cable system to be energised off-line and at low energy and so there is a minimised risk of a disruptive accessory failure during the test. The tests may give an early warning of potential failure points, before a later breakdown of the complete cable system in service leads to bigger problems. The commissioning tests should be performed according to the relevant IEC standard. It is possible to carry out an AC test by energising the termination with system voltage (soak test) and using on-line partial discharge monitoring. This is not ideal, as noise from the system can mask discharge activity occurring within the accessory. In addition, if a breakdown does occur this will lead to a disruptive failure of the joint or termination (as the full system short circuit current is available to flow through the failed accessory) and may lead to an outage and power disruption. Such a failure presents both a safety risk on site and introduces a significant delay to commissioning of the circuit while the affected components are replaced. A DC oversheath test should also be carried out to ensure the cable system and its accessories are insulated from earth.

6.3.2

Condition Monitoring

As indicated in Cigré TB 420 Generic Guidelines for Life Time Condition Assessment of HV Assets and Related Knowledge Rules, it is recommended that a good database of information is established for each piece of equipment as it ages. Useful information on the aging process during the full service life includes loading, maintenance test results, fault history, general ambient and environmental conditions and details of any site incidents.

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295

Fig. 6.17 On site Commissioning Test (in this set up three mobile tests sets needed simultaneously, because of cable length)

Fig. 6.18 Discharge tracks on cable PE outer serving due to a defect. The discharge tracks are a consequence of fault localisation pulses

To effectively manage the aging of HV cable accessories, a structured methodology to analyse and prevent in-service failures is recommended. A suggestion for such methodology is given in Cigré TB 420, clause 4.2. The final step in this

296

E. Bergin

methodology is to gather the outputs from this process into a management strategy which can be used for: • Preventative maintenance, • Decisions on equipment change-out • Improvement in the specification, design or manufacture of new equipment. Regarding preventative maintenance, there are many possible approaches to monitoring the condition of terminations and non-buried joints. These vary from visual inspection to on-line monitoring or regular testing while out of service, etc. The monitoring to be carried out depends on: • • • • • • • • • •

The importance of the circuit The history of the circuit and its accessories The potential repair time The potential cost of the outage Potential cost of the damage Effect on reputation Potential damage from the failure Effectiveness of the monitoring system adopted Availability of monitoring tools and trained personnel Cost of monitoring.

A list of current Condition Monitoring Tools is detailed in 6.Appendix 5. To assist in the selection of a monitoring tool, each tool is described under a number of headings including:• Experience – the level of working experience of each condition monitoring tool is categorized as either well established (“W”) or under development (“D”). • Effectiveness – one diagnostic monitoring tool may be considered (based on costs, time and results) as more effective than another in finding damages or degradations that will lead eventually to system failure; categorized here as useful (“U”) and less useful (“L”). • Level of expertise required – whether high or low level expertise is required i.e. a technician/engineer trained in the particular tool being used or is a general operative sufficient to operate the tool. • Cost.

6.4

Recommendations

The aim of the WG has been to produce a Cigré TB that could be used by designers, manufacturers, contractors and utilities to increase the integrity of terminations and non-buried joints. Many approaches to this subject are possible, depending on the factors outlined in Sect. 6.3.2 above. Two cases need to be considered:

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297

Fig. 6.19 Example of condition monitoring technique: The X-ray photo of cable outdoor termination used to check any internal displacement of the top-connector

• where the accessories are on an existing cable circuit • where the accessories are to be installed on a new cable circuit.

6.4.1

Existing Circuits

For existing circuits the following considerations apply:• • • • • • • • • •

The importance of the circuit The history of the circuit and its accessories The potential repair time The potential cost of the outage Potential damage from the failure Potential cost of the damage Effect on reputation Effectiveness of the monitoring system adopted Availability of monitoring tools and trained personnel Cost of monitoring.

6.4.2

New Circuits

If a new circuit is being installed then it seems appropriate to use proven composite terminations (unfilled, if possible) and proven joints. The designs should comply with IEC 60840 and 62067 as far as PQ and Type testing, Routine and Site Test are concerned. There should be a full QC system in the factory for both cables and accessories. Of course both joints and terminations should be

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installed fully in accordance with the manufacturer’s instructions, and in accordance with Cigré TB 476. When new accessories are being installed a decision will have to be made on what condition monitoring, if any, is necessary. Refer to recommendations of Sect. 6.3.2.

6.5

Conclusions

The following conclusions resulted from the work carried out by this working group: • The survey completed by this WG has shown that disruptive discharge has been experienced in terminations and non-buried joints. • Utilities are concerned about these discharges. • In the case of installing new cable systems, utilities should try to adopt designs that either do not experience disruptive discharge and/or that have been tested to ensure the impact is kept to a minimum. • Full quality control procedures should be employed during the manufacture, delivery, storage and the installation process. • Jointers should be fully certified, have experience of the accessory to be installed and their work should be checked/monitored/ inspected. • All materials and jointing tools used should be appropriate for the work, be in good condition, have been correctly stored and be within their expiry dates. • The site conditions should be suitable with respect to space, safety, dust, pollution, humidity and temperature. • On-site testing at an elevated voltage level, as prescribed in the IEC standards, is strongly recommended during commissioning. • A risk analysis should be done to determine the corrective actions required for existing accessories, which have experienced disruptive discharge or it is suspected they may do so in the future. This can vary from leaving the accessory in service to partial or full replacement. Whether it is decided to go for full or partial replacement, steps 3 to 8 above should be followed. • If it is decided to do condition monitoring on existing or new circuits, then the following items need to be considered – The importance of the circuit – The history of the circuit and its accessories – The potential repair time – The potential cost of the outage – Potential cost of the damage – Effect on reputation – Potential damage from the failure – Effectiveness of the monitoring system adopted – Availability of monitoring tools and trained personnel – Cost of monitoring

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Appendix 1: Terms of Reference

Study Committee No: B1 WORKING BODY FORM Group No: WG B1.29 Name of Convener: Eugene Bergin (Irl) TITLE of the Working Group: Guidelines for maintaining the integrity of XLPE transmission cable accessories Background: The work is motivated by the occurrence of disruptive failures of cable end terminations, with consequent risks for personal and material loss and damage. Terms of Reference: The scope shall be limited to land XLPE cable systems at 110 kV and above. Priority shall be given to outdoor and oil-immersed terminations, but also joints (that are not directly buried) shall be considered. The work shall concentrate on recent incidents, but near misses shall also be included in the analysis. The WG shall: Review recent experience with failures of outdoor a Review the consequences of termination failures for cables within substations and outside. Examine the role of design, assembly and quality co res Examine the role of testing (development, type, routine & after-laying) and condition monitoring in minimising the incidence or severity of termination failures At the SC B1 meeting in 2010, the WG shall provide recommendations on possible extensions of work into joints (not directly view at the B1 annual meeting in 2011. Deliverables: An Executive Summary article for Electra A full report to be published as a TB A Tutorial Created: 2008

Duration: 3 years

Convener e-mail: [email protected] WG members from: AU, BE, BR, CA, FR, DE, IN, IT, JP, KR, NL, NO, ES, CH, UK, US

Other stakeholding SC’s: B2, B3, C3 Approval by TC Chairman:

Date:

2008

Appendix 2: Bibliography/References IEC Standards IEC 60840 Ed 3 Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um ¼ 36 kV) up to 150 kV (Um ¼ 170 kV) –Test methods and requirements

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IEC 62067 Ed 2 Power cables with extruded insulation and their accessories for rated voltages above 150 kV (Um ¼ 170 kV) up to 500 kV (Um ¼ 550 kV) – Test methods and requirements IEC 62217 Ed. 1: Polymeric insulators for indoor and outdoor use with a nominal voltage greater than 1 000 V – General definitions, test methods and acceptance criteria. IEC 61462 Ed. 1.0: Composite insulators – Hollow pressurized and unpressurized insulators for use in electrical equipment with rated voltage greater than 1000 V – Definitions, test methods, acceptance criteria and design recommendations IEC 62271:High voltage switchgear and control gear – Part 209: Cable connections for gas-insulated metal-enclosed switchgear for rated voltages above 52 kV – Fluidfilled and extruded insulation cables – Fluid-filled and dry-type cable-terminations IEC 61039: General Classification of insulating liquids IEC 60815–1 TS Ed. 1.0: Selection and dimensioning of high-voltage insulators for polluted conditions – Part 1: Definitions, information and general principles IEC 60836 Ed 2.0 b 2005 Specification for unused silicon insulating liquids for electrotechnical purposes. IEC 61109 Ed 2 Insulators for overhead lines – Composite suspension and tension insulators for AC. systems with a nominal voltage greater than 1 000 V – Definitions, test methods and acceptance criteria

CIGRE Title of Electra Paper Electra No. 243 Update of Service experience of HV underground and submarine cable systems 235 Statistics on AC underground cables in power networks 210 Current cable practises in Power Utilities (A report on the recent AORC Panel Regional Workshop in Malaysia) 204 General overview on experience feedback methods 141.1 Service experience of cables with laminated protective covering. 137 Survey of the service performance on HV AC cables. 212 Thermal ratings of HV cable accessories 203 Interfaces between HV extruded cables and accessories TB Title of TB 220 502 High Voltage On Site Testing with Partial Discharge Measurement 476 Cable Accessory Workmanship on Extruded High Voltage Cables 455 Aspects for the Application of Composite Insulators 420 Generic Guidelines for Life Time Condition Assessment of HVAssets and Related Knowledge Rules

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379 338 303 279 211 210 177 89

301

Update of Service experience of HV underground and submarine cable systems Statistics on AC underground cables in power networks Revision of Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems Maintenance of HV Cables and Accessories Preparation of guidelines for collection and handling of reliability data Interfaces between HV extruded cables and accessories Accessories for HV cables with extruded insulation Accessories for HV extruded cable. Types of accessories and terminology

Title of Session Paper Session Paper No. 21–01 Studies of Impurities and Voids in Cross-linked Polyethylene Insulated Cables. Pre-fabricated Terminations. 21–02 Plastic insulated cable with voltage dependent core screen.

Jicable Jicable 2011 paper A.3.7 “Return of Experience of 380 kV Power Cable Failures” from Sander MEIJER (TenneT TSO), Johan SMIT, Xiaolin CHEN (Delft University of Technology), Wilfried FISCHER (50 Hertz Transmission GmbH), Luigi COLLA (Terna S.p.A.) Jicable 2011 paper A.5.4 “Remedial action and further quality assuring measures after a failure in a 400 kV GIS cable termination” from Frank JAKOB, Frank KOWALOWSKI, Claus KUHN, Wilfried FISCHER (50 Hertz Transmission GmbH), Sigurdur A. HANSEN (Südkabel GmbH) Jicable 2011 paper A.5.3 “Dry terminations for high voltage cable systems” from Pascal STREIT (NEXANS) Jicable 2003 paper A.6.2 “Anti-explosion protection for HV porcelain and composite terminations” from Gahungu, Cardinaels, Streit, Rollier (Nexans) Jicable 2003 paper A.6.4 “New dry outdoor terminations for HV extruded cables” from DEJEAN (PIRELLI France), QUAGGIA, PARMIGIANI (PIRELLI Italy), GOEHLICH (Technical University of Berlin);. Jicable 1999 paper A.5.4 “Development of synthetic and composite terminations for HV and EHV extruded cables” (LE PURIANS from EDF R&D and JUNG from EDF CNIR – RTE Jicable 1995 paper A.3.2 “Composite EHV terminations for extruded cables” (ARGAUT, LUTON from SILEC and JOULIE, PARRAUD from SEDIVER.

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Appendix 3: Reminder Chapter 5/TB 476 TB 476 Cable Accessory Workmanship on Extruded High Voltage Cables Oct 2011 Table of Contents 1 Summary 4 2 Introduction 4 3 Scope 6 3.1 Inclusions 6 3.2 Exclusions 6 4 Related Literature and Terminology 6 5 General risks and skills 8 6 Technical risks and required specific skills 10 6.1 Conductors 10 6.1.1 Conductor preparation 10 6.1.2 Compression 11 6.1.3 MIG/TIG Welding 12 6.1.4 Thermit Weld 12 6.1.5 Mechanical Connection 13 6.2 Insulation Preparation 15 6.2.1 Straightening 15 6.2.2 Stripping of insulation screen 16 6.2.3 Preparing the end of the insulation screen 18 6.2.4 Smoothening the insulation surface 19 6.2.5 Cleaning of insulation 20 6.2.6 Shrinkage 21 6.2.7 Lubrication 21 6.3 Metallic sheath 22 6.3.1 Welded Aluminium Sheath (WAS) 22 6.3.2 Corrugated Sheaths: Aluminium (CAS); Copper (CCS); Stainless Steel 25 (CSS) 6.3.3 Lead Sheath 28 6.3.4 Laminated sheaths: Aluminium Polyethylene Laminate (APL); Copper 30 Polyethylene Laminate (CPL) 6.4 Oversheath 32 6.4.1 Case of graphite coating 32 6.4.2 Case of extruded and bonded semiconducting layer 32 6.4.3 Low Smoke, Zero Halogen, Enhanced Flame Performance Sheaths 32 6.5 Installation of joint electric field control components 33 6.5.1 Slip on prefabricated joint 34 6.5.2 Expansion joints 37 6.5.3 Field Taped Joints 40 6.5.4 Field Molded Joints (Extruded or taped) 41 6.5.5 Heatshrink sleeve joint 41 6.5.6 Prefabricated composite type joint 42 6.5.7 Plug-in joint 43 6.5.8 Pre-molded three piece joint 44 6.6 Installation of termination electric field control components 45 6.6.1 Slip-on prefabricated field control components 45 6.6.2 Plug-in terminations 45 6.6.3 Taped Terminations 47

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Guidelines for Maintaining the Integrity of Extruded Cable Accessories

6.6.4 Heatshrink sleeve insulated terminations 6.6.5 Prefabricated composite dry terminations 6.7 Outer Protection of Joints 6.7.1 Polymeric outer protection by taping and/or heatshrink tubes 6.7.2 Outer Protection Assembly 6.7.3 Filling compounds for joint protections (joint boxes) 6.8 Filling of Terminations 6.9 Handling of Accessories 6.9.1 Supporting of accessory 6.9.2 Lifting of accessories 6.9.3 Special bonding configurations and link box installation 6.9.4 Sensor connections 6.9.5 Fibre optics 7 Skills Assessment 7.1 Aspects to be tested 7.2 Methods of qualification 7.2.1 Theoretical 7.2.2 Training on the job and observation 7.2.3 Testing – Electrical & Mechanical 7.3 Certification 7.4 Duration of certification 7.5 Upskilling 7.6 New Accessory type 8 Set Up 8.1 Organisation of jointing location 8.2 Positioning of Joint 8.3 Environmental Conditions 8.4 Cable End Inspection 8.5 Verification of Each Step 8.6 Measuring of Diameters, Ovality, Concentricity, Position 8.7 Safety and Health 8.8 Environmental Aspects 8.9 Quality Insurance 9 Bibliography

303

48 48 49 49 50 51 52 53 53 54 56 56 57 58 58 58 58 58 59 59 60 60 60 61 61 61 61 61 62 62 62 62 62 63

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Appendix 4: Short Circuit Tests The possibility of two types of fault has to be considered – a fault external to the accessory and a fault inside the accessory. Both faults will have very different impacts on the accessory. The external fault may cause rapid heating of the conductor and result in a build up of pressure, if there is fluid or gas present in the accessory. The internal fault results in fault current flowing through the insulation of the accessory with high energy being dissipated in the insulation and filling medium and this may cause large thermo-mechanical and pressure changes inside the accessory.

Low Energy External Fault (Through-fault i.e. Breakdown Outside the Accessory) In the case of a system fault in another part of the electrical system external to the accessory, the fault current passes through the conductor of the termination or joint. Testing for such a case is carried out on terminations, joints (buried and non-buried) installed as in service. The test installation shall be in accordance the requirements of the specification and rules of each System Operator. This test should be performed on terminations and joints connected by the specified cables, which have either already gone through a type test or have gone through at least ten thermal cycles. The aim is to study the effects of a simulated external fault on the accessories, including a check that pressure relief devices in terminations do not break during an external short-circuit.

Simulation of the Fault The accessory shall be installed in a suitable circuit to permit the fault current to flow through the accessory. Position of the Fault The fault shall be external to the accessory being tested External Fault Withstand Test The test is performed with AC. In order to prevent fade-out of the electrical arcing, the test will be performed with a symmetrical start-up on a voltage crest. The current is injected from the cable to the accessory. The test voltage shall be at least 20 kV. Examples are given in the Table 1 and each country will have its own set of values depending on system configurations and fault conditions (Table 6.6). Due to safety regulations, testing terminations that contain SF6 gas is no longer allowed, as some gas by-products that may be generated by internal arcing are harmful. Replacing SF6 with air (or nitrogen) has to be carefully considered, since there are a lot of differences between arcs in SF6 and air. WG A3.20 is currently carrying out studies on this question.

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Table 6.6 Short Circuit Levels at Different Operating Voltages France Voltage kV U(Um)

Short-Circuit Parameters Three-phase Short-circuit Intensity and Single-Phase Short-circuit duration Intensity, duration, 63 (72,5) a) 20 kA – 1 s 8 kA – 1,7 s – 0,2 s b) 31,5 kA – 0,5 s 90 (100) a) 20 kA – 1 s 10,3 kA – 1,7 s – 0,2 s b) 31,5 kA – 0,5 s 225 (245) 31,5 kA – 0,5 s 31,5 kA – 0,5 s – 0,16 s 400 (420) a) 63 kA – 0,5 s a) 63 kA – 0,5 s – 0,07 s b) 40 kA – 0,5 s b) 40 kA – 0,5 s – 0,06 s NOTE – Cases a) and b) depend on the grid characteristics and short-circuit power of the grid. Ireland Voltage Short-Circuit Parameters kV U(Um) Three-phase Short-circuit Intensity and Single-Phase Short-circuit duration Intensity, duration, 110 (123) a) 31.5 kA – 1.0 s a) 31.5 kA – 1.0 s b) 40.0 kA – 1.0 s b) 40.0 kA – 1.0 s 220 (245) 40 kA – 1.0 s 40 kA – 1.0 s 400 (420) 50kA – 1.0 s 50kA – 1.0 s NOTE – a) outside Dublin b) in Dublin Cases Netherlands Voltage Short circuit parameters kV Three-phase Short –circuit Intensity and Single-phase Short-Circuit U (Um) duration Intensity, duration 50 (72.5) 9 kA – 0.5 sec 9 kA – 0.5 sec 15 kA – 1.0 sec 12.5 kA – 1.0 sec 25 kA – 1.0 sec 15 kA – 1.0 sec 110 (123) 30 kA – 0.5 sec 25 kA – 0.5 sec 40 kA – 1.0 sec 25 kA – 1.0 sec 150 (170) 30 kA – 0.5 sec 15 kA – 0.5 sec 40 kA – 1.0 sec 30 kA – 1.0 sec 50 kA – 1.0 sec 40 kA – 1. 0 sec 220 (245) 40 kA – 1.0 sec 27 kA – 1.0 sec 380 (420) 50 kA – 0.5 sec 50 kA – 0.5 sec 50 kA – 1.0 sec 50 kA – 1.0 sec 63 kA – 0.5 sec 63 kA – 1.0 sec 63 kA – 1.0 sec The short-circuit levels are depending on the protection settings, imposed by the grid owner, and the position of the cable system in the grid: close to a power plant or more remote

Requirements On completion of the test, the pressure relief shall be observed to have operated correctly. The whole test shall be recorded and filmed with a high-speed camera

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(at least 1000 images per second) in order to witness and analyse the behaviour and reaction of the termination and fixing installation devices.

High Energy Internal Fault (Internal Fault i.e. Breakdown Inside the Accessory) The test is carried out on a termination or joint installed as in service. The installation shall be in accordance with the requirements of the specification and rules of each System Operator. The aim is to study the external effects generated by the accessory during the simulation of an internal arc fault. This test is intended to check that the accessory does not disruptively eject components that might cause external damage.

Simulation of the Fault An internal fault is initiated by drilling a hole in the main insulation of the cable within the termination or joint. A 1.5 mm2 copper wire shall connect the conductor to the metallic screen/sheath or to a metallic piece itself connected to the screen/sheath.

Position of the Fault In the case of a termination or joint having a stress cone, the fault is initiated by drilling a hole at the top of the stress cone to the conductor in order to connect the 1.5mm2 copper wire. Internal Fault Withstand Test The test is performed with AC. In order to prevent from the fade-out of the electrical arcing, the test will be performed with a symmetrical start-up on a voltage crest. The rms. value and duration of the phase-to-earth short-circuit are given in the table above. The current is injected from the cable to the termination or joint. The test voltage shall at least 20 kV. Due to safety regulations, testing accessories which contain SF6 is not allowed any more, as some by-products that may be generated in case of arcing are considered harmful. Replacing SF6 by air (or nitrogen) has to be considered carefully, since there are a lot of differences between arcs in SF6 and air. Requirements On completion of the test, no solid debris shall be observed at a distance of more than 3 metres from the termination or joint. The whole test shall be recorded and filmed with a high-speed camera (at least 1000 images per second) in order to witness and analyse the behaviour and reaction of the termination or joint and fixing installation devices.

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Appendix 5: Condition Monitoring Techniques for Terminations and Non-buried Joints Condition monitoring techniques for terminations and associated auxiliary components are summarized in the tables in this Appendix under the following headings: Condition Details the specific diagnostic tool for monitoring the monitoring tool termination/auxiliary components Component Identifies the component to which the monitoring tool can be used on Event/Cause Application of the ‘condition monitoring tool’ reduce the detected probability of the here mentioned event that cause the cable system failure, damage or degradation On/off-line Condition monitoring techniques are categorized as capable of being done either on-line (cable system in service) or off-line (cable system must be switched out) Experience The level of working experience of each condition monitoring tool is categorized as either well established (‘W’) or under development (‘D’). Effectiveness One diagnostic monitoring tool is considered as more effective in finding damages or degradations that will lead eventually to system failure than other tools, considering costs and time versus result, categorized here as useful (‘U’) and less useful (‘L’). Frequency Suggested interval of application of the monitoring tool versus the cable life time cycle. Please note that most monitoring tools will be selected based on the service experience of the termination type and hence the frequency. Primary/secondary Primary tests are considered as the minimal test one shall test perform on a cable system after it has been put into service. Secondary tests will be selected to monitor or discriminate terminations with (suspected) specific defects, based on (service) experience. Cost Indication of cost per test. Range: minor costs < +, ++, ++ + > considerable costs. These costs are for the test only cost and do not include cost of preparatory work, outages and other associated expenses. Expertise Indication of required skills to perform the test. Range: less skilled personal < +, ++, +++ > skilled personal Reference Reference source of the monitoring technique

Dielectric loss Terminations, angle testa non buried joints

PD testing Terminations, (various non buried methods, such joints as: acoustic, UHF, Radio interference, voltage test)

2

3

Terminations, non buried joints, cable

Serving test (DC test)

Component

1

Condition Monitoring No. Tool

Detecting assembly errors, low contact pressure at interface, shrink back of cable insulation, contamination of internal insulation fluid and/or gas due to aging or

Ingress of water in insulation area

Pollution on support insulators or screen separations D

W

Off- W line/ On- D line

Offline

Offline

U

L

U

Depending on service experience

Depending on service experience

Annual

On/ Event or Cause OffDetected Line Experience Effectiveness Frequency

Short Circuit Levels at Different Operating Voltages

P

S

P

Primary/ Secondary Test

++ (on-line) +++ (off-line)

+++

+

Cost

+++

+++

+

Guidelines for Maintaining the Integrity of Extruded Cable Accessories (continued)

CIGRÉ TB 279, Table 6.4, item 5

CIGRÉ TB 279, Table 6.4, item 3

CIGRÉ TB 279, Table 6.4, item 2

Expertise Reference.

6 309

Visual Terminations, inspection non buried with visible or joints UV light

6

Terminations, non buried joints

X-ray

5

Terminations, fluid filled non buried joints

Chemical and Physical analysis of insulating fluid, such as: DGA, Tan delta, Water content, Particles etc.

Component

4

Condition Monitoring No. Tool

Offline

Surface pollution, mechanical damage, uncontrolled

Online

Movement of Offcable due to line thermal cycling or poor clamping

Contamination of internal insulating fluid.

leaking, insulator tracking,

W

W

W

U

L

U

Annual

Depending on service experience

Annual

On/ Event or Cause OffDetected Line Experience Effectiveness Frequency

Short Circuit Levels at Different Operating Voltages (continued)

P

S

P

Primary/ Secondary Test

+

++

++

++

CIGRÉ TB 279, Table 6.4, item 8

CIGRÉ TB 279, Table 6.4, item 7

CIGRÉ TB 279, Table 6.4, item 6

Expertise Reference.

+ +++ (N.B. does not include the costs of taking the sample)

Cost

310 E. Bergin

Visual inspection with IR on current carrying components

Leakage current measurement

7

8

Terminations

Terminations, non buried joints

Insulator surface pollution, surface tracking or damage

As with Item 6 above plus detecting possible hotspots on top-connector and earthing circuit

movement of cable, cable clamping, tracking marks on outdoor insulators, ferrule retraction, leakages, corrosion, animal attack, vandalism.

Online

Online

w

w

L

U

Depending on service experience

5 yearly

s

s

+++

+

++

++

Guidelines for Maintaining the Integrity of Extruded Cable Accessories (continued)

CIGRÉ TB 279, Table ▶ 5.4, item 25

CIGRÉ TB 279, Table 6.4, item 8

6 311

Surface wetting characteristics (STRI method)b

Continuous measurement of fluid or gas pressures and/or low pressure alarms

10

11

Auxiliary

Terminations

Test at Terminations, elevated non buried voltage: AC, joints VLF, DC with or without DLA (Item 2) and/or PD (Item 3).

Component

9

Condition Monitoring No. Tool

Indication of falling fluid / gas pressure

Extrinsic surface pollution on outdoor polymeric insulators

Main insulation / stress cone/ interface defects

Online

Offline

Offline

W

W

w

U

L

U

p

Primary/ Secondary Test

Continuous S

Depending S on location

Depending on service experience

On/ Event or Cause OffDetected Line Experience Effectiveness Frequency

Short Circuit Levels at Different Operating Voltages (continued)

+

+

+++

Cost

+

++

+++

CIGRÉ TB 279, Table ▶ 5.4, item 23

CIGRÉ TB 279, Table ▶ 5.4, item 26

IEEÉ St 48, clause 8.6 (DC), IEC 60840, IEC62067

Expertise Reference.

312 E. Bergin

Visual inspection

Voltage test on Earthing and Failure of SVL OffSVL cross bonding to operate at line boxes. rated voltage

15

16

Testing alarm settings and signals for fluid/gas pressure monitoring

Earthing and Water ingress cross bonding in link box, boxes. Condition of any insulating compounds, Link arrangement

Auxiliary

Testing of fluid/gas monitoring equipment

Offline

Offline

Online

14

Leakage of internal insulating fluid/gas from termination

SF6 sniffers or Auxiliary cameras

Online

13

Leakage of internal insulating fluid/gas from termination

Regular gauge Auxiliary maintenance and calibration.

12

W

W

W

W

W

L

L

L

L

L

Depending on service experience (usually

Annual

Annual

Annual

Annual

S

p

p

s

s

+

+

+

++

+

+

+

++

+

+

Guidelines for Maintaining the Integrity of Extruded Cable Accessories (continued)

CIGRÉ TB 279, Table 6.4, item 10

CIGRÉ TB 279, Table ▶ 5.4, item 32

CIGRÉ TB 279, Table ▶ 5.4, item 31

CIGRÉ TB 279, Table ▶ 5.4, item 23

CIGRÉ TB 279, Table ▶ 5.4, item 23

6 313

Excessive temperature rise

Off or on line

Integrity of Offearthing circuit line

W

W

U

L

L

P

Continuous s

As per 16

Continuous S

Primary/ Secondary Test

++

+

+++

Cost

Distinction between cable and termination might be a problem STRI hydrophobicity classification guide provides a coarse value of the wetting status, reference is made here to IEC TS 62073

Cable and joint

Optical fibre

19

b

a

Auxiliary

Measurement of earthing system electrical resistance

18

D

done on same outage as serving test)

On/ Event or Cause OffDetected Line Experience Effectiveness Frequency

Earthing and Failure of SVL Oncross bonding to operate at line boxes. rated voltage

Continuous SVL monitoring

Component

17

Condition Monitoring No. Tool

Short Circuit Levels at Different Operating Voltages (continued)

+++

+

+++

TB 247





Expertise Reference.

314 E. Bergin

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315

Eugene Bergin received a Bachelor of Engineering degree from the University College Dublin. He had over 30 years experience in the area of HV cables and has worked in many countries – China, Japan, Korea, Bahrain, Dubai, Saudi Arabia, Turkey, France, Germany, Sweden, etc. He has been a Member of Cigré for over 35 years and served as a Member, Secretary, and Convener of many Working Groups. He received the Cigré Technical Committee Award in 2000 and the Cigré Distinguished Member Award in 2004. In his most recent commitments, he was Convener of both the Customer Advisory Group and the Trenchless Technology Working Group B1.48. Eugene was in the final stage of publication of the report of this Working Group on Trenchless Technologies (TB 770) when he passed away peacefully in October 2018. He is missed by all his colleagues as a great professional and a warm-hearted friend.

7

Feasibility of a Common, Dry Type Plug-in Interface for GIS and Power Cables above 52 kV Pierre Mirebeau

Contents 7.1 Introduction and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 General Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Definitions and Terms (According to IEC 62271-209) . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.3 Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 GIS Cable Terminations Installation Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2 Experience of Dry Type Insulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Design of Dry Type GIS Terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Differences in Design of Barrier Insulator, Inner Cone Type . . . . . . . . . . . . . . . . . . . . 7.4.2 Differences in Design of Barrier Insulator, Outer Cone Type . . . . . . . . . . . . . . . . . . . 7.4.3 Requirements for Standardization of a Common Interface . . . . . . . . . . . . . . . . . . . . . . 7.5 Where the Plug-in Concept Could Be Applicable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.1 Geometrical Installation Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.2 Safety Practices and Constraints during Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.3 Testing Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.4 Conclusion Regarding Testing Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Qualification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6.1 State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6.2 Where the Plug-in Common Interface Could be Applicable . . . . . . . . . . . . . . . . . . . . 7.6.3 Qualification of new Insulator or Stress Cone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7 Feasibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.1 Definition Feasibility (Cost Involved) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.2 Qualification Feasibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8 Market Acceptance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8.1 Current Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

319 320 321 321 321 325 325 325 331 334 335 335 336 339 339 350 351 353 354 354 362 362 362 362 363 364 364

Pierre Mirebeau has retired. P. Mirebeau (*) Villebon sur Yvette, France e-mail: [email protected] © Springer Nature Switzerland AG 2021 P. Argaut (ed.), Accessories for HV and EHV Extruded Cables, CIGRE Green Books, https://doi.org/10.1007/978-3-030-39466-0_7

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7.8.2 Future Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8.3 Where the Plug-in Common Interface Could be Recommended . . . . . . . . . . . . . . . . 7.9 Conclusion and Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

364 366 366 367

Abstract

Since 1986 the connection between GIS substations and cables has been managed by a dimensional standard establishing electrical and mechanical interchangeability between cable terminations and gas insulated metal enclosed switchgear. Within this framework termination suppliers design their own components: insulator, stress cone (for the two available options inner cone and outer cone) and the connection inside the termination. The responsibility limit between the switchgear manufacturer and the cable termination manufacturer is at the interface SF6/insulator. Considering the large number of substations and planning difficulties due to the fact that the cable system is not usually defined at the time of switchgear manufacture, a joint working group has been set up by Cigré within committees B1 and B3. The group has to investigate the possibility of a standardised common interface insulator for the dry type and plug-in cable termination, which could be supplied independently from the remaining termination components. Starting from review of GIS cable termination designs and actual installation practices for all voltage levels, the joint working group has studied: • Operational experience in common interface design, for medium voltage and for a specific utility • Constraints in terms of civil works, space, weight of cables and terminations • Compliance with standards • Implications of the common interface insulator for the market • Qualification requirements • Applicable range (voltage and size) • Estimated cost for testing and qualitative advantages of the common interface (financial benefit versus development and qualification costs was not evaluated) • New limits of responsibility (insulator/stress cone and insulator/SF6) • Market trend. Taking into account all the above, as per the TOR of the group, WG B1-B3.33 advises Study Committees B1 and B3 to set up a new working group with the following Terms of Reference. The Working group should recommend a functional design of an insulator with a common interface with the following scope of work: • Voltage is
145 kV AC • Current is
1000A, short circuit
40 kA 1 sec • Cross sections are
1000 mm2 Cu or 1600 mm2 Al

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319

• Technology has to be defined (inner or outer cone), with a detailed evaluation of technical advantages/disadvantages of the two technologies. • The number of sizes has to be defined; the short circuit current can be altered for the smallest sizes. • Dimensions of insulator components have to be defined (current connection, electrical design and properties, mechanical design and properties). • The type and dimension of the main current connection has to be defined • Consider the consequence of a termination failure. • Consider upgrading of the cable link for higher current loads. • Consider installation constraints, with a special focus on the basement dimensions. • The design has to meet the requirements of IEC 62271-209 and IEC 60840 • The initial and cross qualification processes. The stress cone design and material, the lubricant and the design of the compression device should be left to the discretion of the accessory manufacturer within the limits of the standardised cable terminations properties. Cigré TB 303 and the work of WG B1.44 and WG B1.46 should be taken into account.

7.1

Introduction and Scope

The interface between High Voltage cable and switchgear is defined by IEC 62271-209. In particular IEC 62271-209 defines two types of dry-type cable connections for gas insulated switchgear above 52 kV. The limit of supply of the cable termination manufacturer is the insulator. Type A connection incorporates elastomeric electrical stress control component inside the insulating barrier. For type B, the elastomeric electrical stress control component is located externally. IEC 62271-209 covers specifically the connection assembly with a separating insulating barrier between the cable insulation and the gas of the switchgear, which is the case of dry terminations. It does not address specifically the plug in issues. Regarding Medium Voltage, EN 50181 Standard was published in 1997 describing “Plug-in type bushings above 1 kV up to 36 kV for equipment other than liquid filled transformers” This document gave full details of bushings which were fitted to power equipment (such as switchgear) to make a cable connection to the equipment. The insulator could be customized to suit the design of the equipment on that side, but was required to have standardized dimensions on the cable side, such that a “separable connector” (plug in cable/stress cone assembly as per the definitions of chapter 2.1) could be plugged in on the cable side. The separable connector could then be supplied by one of several possible suppliers. The current version of EN 50181 was published in 2010. In this version the upper limit of the applicable voltage range was raised to 52 kV.

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With the above background interest has been raised to extend the principle of a common insulator interface to higher voltages with the potential benefits that cable connections from different manufacturers would be interchangeable in a single insulator. Investigation of this proposal was undertaken within Cigré, by forming a joint working group (JWG) between Study Committees B1 (cables) and B3 (switchgear). The working group JWG B1-B3.33 was formed and has produced the current document.

7.1.1

Scope

The scope of the work of JWG B1-B3.33 is to consider the feasibility of a common dry-type interface for GIS connections for AC extruded cable systems for voltages above 52 kV, considering the following aspects: • Examine the conditions around the switchgear and installation issues, including the supporting system (also called site issues) • Consider the impact of large cross sections • Consider safety prectices during works • Consider the testing procedures for GIS/Terminations and cables at the factory and on site (overlapping or missing items). • Propose measures to reduce the potential consequences of the GIS insulation failure. • Propose measures to reduce the potential consequences of the cable termination insulation failure1 • Review the existing standards addressing the qualifications and extension of qualification procedures applicable to GIS terminations. • Define the relevant qualification procedures needed if any. • Identify the limit of suppliers’ responsibility. • Estimate the overall technical and practical feasibility of the common design definition and qualification, insulator manufacturers' qualification and the cable manufacturers' qualification and the cost involved. • Once the feasibility window has been determined, survey the market (manufacturers and end users) • Recommend or not to go to a second step with the launching of a new WG B1-B3. XX to go in detail into the design of the standard components (shape, dimensions, properties, . . .).

A WG “B1.29 Guidelines for maintaining the integrity of XLPE cable accessories” was decided in 2009. The group considered that this question is in B1-29 scope and is not to be addressed here. Report from B1.29 is TB 560 published in December 2013 and published as ▶ Chap. 6, “Guidelines for Maintaining the Integrity of Extruded Cable Accessories” of this Book.

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7.2

Definitions

7.2.1

General Layout

321

The general layout of the dry type GIS cable terminations as described in IEC 62271209 is shown Figs. 7.1 and 7.2.

7.2.2

Definitions and Terms (According to IEC 62271-209)

The definitions and terms of the different components of the dry type GIS cable terminations as described in IEC 62271-209 are shown Figs. 7.2, 7.3, and 7.4.

7.2.2.1 Cable-Termination (IEC 62271-209) Equipment fitted to the end of a cable to ensure electrical connection with other parts of the system and to maintain the insulation up to the point of connection. Two types are described in this standard.

7.2.2.1.1 Fluid-Filled Cable-Termination (IEC 62271-209) Cable-termination which comprises a separating insulating barrier between the cable insulation and the gas insulation of switchgear. The cable-termination includes an insulating fluid as part of the cable connection assembly.

Fig. 7.1 General layout of dry GIS termination (in ref to IEC 62271-209)

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Fig. 7.2 Dry-type cable connection assembly – Typical arrangement (IEC 62271-209)

GIS main circuit end terminal Connection interface Plug-in connector of insulator Insulator assembly Insulator Cable connection enclosure Flange (if needed)

Plug-in connector of cable

Cable/stress cone assembly

Stress cone

Cable gland Cable

Fig. 7.3 Identification of the different parts of GIS termination, inner cone type design

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Connection interface Insulator assembly

Cable connection enclosure Insulator Plug-in connector of insulator

Stress cone

Plug-in connector of cable

Cable/stress cone assembly

Cable gland

Cable

Fig. 7.4 Identification of the different parts of GIS termination, outer cone type design

7.2.2.1.2 Dry-Type Cable-Termination (IEC 62271-209) Cable-termination which comprises an elastomeric electrical stress control component in intimate contact with a separating insulating barrier (insulator) between the cable insulation and the gas insulation of the switchgear. The cable-termination does not require any insulating fluid.

7.2.2.2 Plug-in Cable Termination Cable termination where cable/stress cone assembly can be engaged into the insulator assembly that is already installed into sealed GIS enclosure. 7.2.2.2.1 Locked Plug-in Type Cable Termination Plug-in cable termination where conductor of the cable is interlocked with the insulator assembly and cannot be removed without disassembling insulator assembly from the GIS enclosure.

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7.2.2.2.2 Plug-in, Plug-out Type Cable Termination Plug-in cable termination where the plug-in assembly may be removed from the barrier insulator assembly without disassembling the insulator assembly from the GIS enclosure. 7.2.2.2.3 Locking Plug-in, Plug-out Type Cable Termination Plug-in cable termination where conductor of the cable is interlocked with the insulator assembly and can be removed without disassembling the insulator assembly from the GIS enclosure.

7.2.2.3 Insulator Assembly Assembly of insulator, plug-in connector of insulator and flange if needed. 7.2.2.4 Insulator Separates insulating fluid (SF6) of GIS enclosure from the cable/stress cone assembly. 7.2.2.5 Plug-in Connector of Insulator Provides connection to GIS main circuit end terminal and to plug-in connector of cable. 7.2.2.6 Plug-in Connector of Cable Provides connection between cable conductor and plug-in connector of insulator. 7.2.2.7 Main-Circuit End Terminal (IEC 62271-209 and Compliant with IEEE 1300) Part of the main circuit of a gas-insulated metal enclosed switchgear forming part of the connection interface. 7.2.2.8 Cable Connection Enclosure (IEC 62271-209 and Compliant with IEEE 1300) Part of the gas-insulated metal-enclosed switchgear which houses the cabletermination and the main-circuit end terminal. 7.2.2.9 Cable Connection Assembly (IEC 62271-209 and Compliant with IEEE 1300) Combination of a cable-termination, a cable connection enclosure and a main-circuit end terminal, which mechanically and electrically connects the cable to the gas-insulated metal enclosed switchgear. 7.2.2.10 Cable System (IEC 62271-209) Cable with installed accessories. Cable System (IEC 62271-209) Cable with installed accessories.

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7.2.3

325

Units

7.2.3.1 Pressure All pressure values in this document are given in bar as relative pressure. 7.2.3.2 Rated Voltages (IEC 60840 and 62271) The symbols U0, U and Um are used to designate the rated voltages of cables and accessories where these symbols have the meanings given in IEC 60183: U0 ¼ the rated r.m.s. power-frequency voltage between each conductor and screen or sheath for which cables and accessories are designed U ¼ the rated r.m.s. power-frequency voltage between any two conductors for which cables and accessories are designed Um ¼ the maximum r.m.s. power-frequency voltage between any two conductors for which cables and accessories are designed. It is the highest voltage that can be sustained under normal operating conditions at any time and at any point in a system. It excludes temporary voltage variations due to fault conditions and the sudden disconnection of large loads. Unless specified differently all voltages mentioned in this brochure are considering Um values.

7.3

Experience

7.3.1

GIS Cable Terminations Installation Examples

The following pictures show some examples of installation. They include all kinds of GIS terminations.

7.3.1.1 Um 362 ~ 550 kV 7.3.1.1.1 Vertical Installations Vertical installation with cables entering from below is the most widely used. The presence of a basement is of importance (Figs. 7.5 and 7.6). • In case of no basement, the support structure must be high enough to comply with the bending radius. • When there is a proper sized basement, there is more freedom regarding the cable termination implementation. 7.3.1.1.2 Horizontal Installations Horizontal installation is selected when there is no basement and limited height. Horizontal snaking is performed to lower the conductor thrust on the insulator (Figs. 7.7 and 7.8).

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Fig. 7.5 GIS vertical arrangement examples

7.3.1.1.3 Inclined Installations Inclined installation can be chosen as a trade-off between height and bending radius, when there is no basement (Fig. 7.9).

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Feasibility of a Common, Dry Type Plug-in Interface for GIS and Power. . .

Fig. 7.6 Installation works, placing GIS enclosure over the 1600 mm2 Cu 345 kV cable termination

Fig. 7.7 GIS horizontal arrangement examples

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Fig. 7.8 GIS horizontal arrangement study example (before clamping implementation design)

5000

328

44590

33090

EL. 1523.50

installation under 30° angle, 400kV XLPE cable, 250

4750

4750

30°

EL. 1522.10 FEEDER 1 REACTOR 1

FEEDER 2 REACTOR 2

Fig. 7.9 GIS inclined arrangement examples

7.3.1.2 Um 245 ~ 300 kV Vertical installation with cables entering from below is the most widely used. The presence of a basement is of importance. 7.3.1.2.1 Vertical Installation Vertical installation is the most widespread; generally cables are installed from below (Fig. 7.10).

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Fig. 7.10 Libya 245 kV vertical outdoor installation 800 mm2, no basement (not plug-in)

Sometimes cables are installed from above (Fig. 7.11).

7.3.1.3 Um 123 ~ 170 kV 7.3.1.3.1 Vertical Installation Almost all 123–170 kV installations are in vertical arrangement (Figs. 7.12 and 7.13). Note the restricted space to fit the termination, and once installed, no room to position the Surge Voltage Limiters. 7.3.1.3.2 Horizontal Installation Figures 7.14 and 7.15.

7.3.1.4 Um 72.5 ~ 100 kV 7.3.1.4.1 Vertical Installation Note on the left picture the restricted room for installation works that is performed through the basement concrete floor (Fig. 7.16).

330

Fig. 7.11 245 kV vertical indoor installation, 1400 mm2 (not plug-in)

Fig. 7.12 145 kV vertical outdoor installation 800 mm2

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Fig. 7.13 Plug in type 123 ~ 170 kV vertical indoor installations

Fig. 7.14 145 kV horizontal installation 500 mm2 plug-in (compact GIS)

7.3.2

Experience of Dry Type Insulator

7.3.2.1 History of Dry Plug-in Termination Dry type insulators were introduced first in Germany at 170 kV level according to Fig. 7.17.

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Fig. 7.15 Installation works Plug-in of cable into dry type GIS termination (outer cone model)

Fig. 7.16 72.5 kV vertical installations

7.3.2.2 German Experience of Plug-in Plug-out Interchangeable GIS Termination A large German utility has experienced a common interface with two epoxy suppliers and two cable makers for 110 kV cables. Projects were based on standard cables: • 630 mm2 and 800 mm2 in one city (dimensions fixed as per KG4023 specification), • 640 mm2 to 1000 mm2 in another city.

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Cross section/mm2 2500

2010

2008

2000

2004 2002

2000

1000 800

1996 1998

145

170

300

550

Fig. 7.17 Year of introduction of dry type GIS termination as a function of voltage and cable cross section in Germany

The electric stress was (and still remains) conservative: 17 mm insulation wall thickness gives stresses lower than currently used similar cables. Development History The goal was to ease the planning of the interface components between the GIS and cable suppliers though the specification of a standard connection. First Step 1997, Planning Design Cable manufacturer (A) was giving the insulator to the GIS manufacturer. Then the cable/stress cone assembly was plugged in on site As there were only standard cable types (including standard dimensions) the utility went further to the full standardized termination (insulator and stress cone). Second Step 1998, Product Qualification One single cable manufacturer (A) had a termination ready. Third Step 1999, Market Opening The utility qualified a second cable termination manufacturer (B) for the same insulator dimensions. Then a long term test was performed with: • 4 GIS manufacturers • 2 cable termination manufacturers (A and B)

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• 2 insulator manufacturers (A and C) • There were cross qualifications among suppliers (but not all combinations). Currently, to qualify a new supplier the utility gives a list of components to be used including insulators, and the cable supplier has to perform a type test. There are 90 terminations in in the area of the utility. On the network, termination plug-out occurred only a few times (for fault location purposes). There has not been any termination breakdown up to now, and the utility did not face a responsibility issue in case of breakdown. The cable and termination dimensions and design have not changed up to now (no electric stress increase).

7.3.2.3 USA Experience In USA a large utility from the east coast has a very positive experience with plugin type 115 kV vertically installed terminations. These XLPE to GIS, 3500 kcmil, 36 single phase termination units installed in 2008 have provided the company with a trouble and maintenance free service. The terminations are part of two 115 kV interconnection circuits that separately connect a 115 kV air insulated substation and a generating facility to a major 345 kV GIS substation. Two transformers have been installed in between these UG lines to step up the voltages from 115 kV to 345 kV. All of these terminations have been installed outdoors, with provision of suitable additional ground clearances on their bases. This extra space at the bottom provided the utility with flexibility of fitting the cable terminations inside their GIS cable enclosure. The installation was much easier and quicker in comparison with fixed type conventional GIS terminations. The insulator and cable/stress cone assembly came from a single manufacturer. The termination is of locked plug-in type. The USA has no experience of common interface design or use.

7.4

Design of Dry Type GIS Terminations

Cable and GIS manufacturers had met together in the early 80’s to write a technical specification which establishes electrical and mechanical interchangeability between cable-terminations and the gas-insulated metal-enclosed switchgear and determines the limits of supply. This resulted in 1986 in the first edition of the Technical Specification IEC 859. Now updated and transformed in international standard IEC 62271-209. In the frame of this technical specification, each manufacturer developed, with its own technology and solution, products that comply with the interchangeability requirements, for the specified inner and outer cone types.

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7.4.1

335

Differences in Design of Barrier Insulator, Inner Cone Type

All barrier insulators comply with the dimensions and requirements of IEC 62271209, however there are differences in the connection part, the plug in connector, the high voltage screen, the insulator material, geometry, finish and the shield break design which are represented in Fig. 7.18.

7.4.2

Differences in Design of Barrier Insulator, Outer Cone Type

Similar to the inner cone design, all barriers insulators comply with the dimensions and requirements of IEC 62271-209, however there are differences in the connection part, the plug in connector, the high voltage screen, the insulator material, geometry, finish and the shield break design which are represented in Fig. 7.19.

Fig. 7.18 Design of barrier insulator – inner cone type

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Fig. 7.19 Design of barrier insulator – outer cone type

7.4.3

Requirements for Standardization of a Common Interface

This paragraph deals with the requirements that are to be set in addition to current standards; they are not intended to replace them. A common interface implies a common inner surface of the insulator. If a full interchangeability has to be provided, the connector shielding has to be common as well because the radius at the bottom of the electrode and the distance to the interface affects the field distribution at the rubber body interface. As a consequence the minimum epoxy thickness is defined. To comply with IEC 62271-209, each voltage class requires a specific insulator (voltage classes are 72.5 to 100 kV, 123 to 170 kV, 245 to 300 kV, and 362 to 550 kV). For a given voltage class, depending on the cable termination design several insulator sizes might be needed to cover all cross sections.

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7.4.3.1 Insulator For the standardisation of each insulator, the following requirements should be defined. 7.4.3.1.1 Dimensions and Tolerances • Compliance with 62271-209 Fig. 5 • Inner cone or outer cone model • Plug-in connector of insulator • Lock -in system (if any) • High Voltage screen (connector shielding) • Insulator and shape of cone, smoothness • Insulation shield break ring (if integrated in the insulator). • Ground screen electrode (if any) • Fixing of cable/stress cone assembly. 7.4.3.1.2 Dielectric Parameters • Dielectric properties of insulator (permittivity, tan δ) • Dielectric performance requirement (electrical withstand level, PD level) • Insulation shield break ring (if integrated in the insulator). The voltage requirements of the insulation shield break are the same as the ones of the sheath sectionalising insulation of joints in IEC 60840 and IEC 62067 appendix G. Paragraph G4.3. G.4.3.1 gives the DC voltage level and G.4.3.2 gives the lightning impulse voltage level in table G.1 column “each part to earth/bonding leads
3 m”. • Other standards and specifications may specify higher requirements. • Resistance to SF6 by-products, if applicable. 7.4.3.1.3 Mechanical Parameters • Fixing for the pressure device (springs) and cable gland attachment (geometry and strength) ! tensile tests to agree on • Resistance to internal pressure. • Resistance to internal fault (refer to brochure WG B1-29) • Design maximum SF6 outside pressure • The design maximum SF6 outside pressure is defined by IEC 62271-209 paragraph 6.1: 8.5 bar abs. and the type test level is according to paragraph 6.104 of IEC 62271-203 (pressure test on partitions) • Resistance to cantilever force according to paragraph 6.2 of IEC 62271-209 • Mechanical properties of epoxy : hardness, elongation, tensile strength, modulus, maximum permissible temperature, type of test and test level to be agreed • Smoothness of the epoxy/stress cone interface (test to be defined) • Test at limit temperatures according to paragraph 6.106.2 of IEC 62271-203 (insulator thermal performance) • Quality of connection interface according to paragraph 5.4 of IEC 62271-209, • Quality of connector interface, to be defined. • Tightness test according to paragraph 6.106.3 of IEC 62271-203.

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7.4.3.1.4 Routine Test • Voltage test and PD measurement test according to paragraph 7.1.101 & 7.1.102 of IEC 62271-203 the pressure being the minimum functional pressure for insulation as per IEC 62271_209 Fig. 1, with the deviation range of paragraph 8.2.1. • Tightness test according to paragraph 7.4 of IEC 62271-203 • Design and visual checks according to Fig. 5 of IEC 62271-209 • Pressure test according to paragraph 7.104 of IEC 62271-203. 7.4.3.1.5 Type Test and Prequalification Test • Type Test of insulator according to GIS standard (see above) and type test of the entire termination according to accessory standard (IEC 60840 and 62067). • Type test of connector interface, test to be defined. • Pre-Qualification test of entire termination, for voltages covered by IEC 62067.

7.4.3.2 Stress Cone The cable accessory manufacturer will have to comply with the dimensions and tolerances which will be defined by the insulator standard paragraph Dimension and Tolerances. 7.4.3.2.1 Design Considerations • The lubricant should be compatible with the epoxy (this is presently the responsibility of the cable manufacturer) ! compatibility test to be defined • The arrangement has to apply a pressure that is lower than the maximum internal pressure of the insulator. • The electrical stress induced in the insulator must be within the acceptable range of insulator (electric field calculation to be provided). 7.4.3.2.2 Routine Test • Routine test values of the stress cone. (e.g. Voltage test and PD measurement test) according to IEC 60840 and IEC 62067. 7.4.3.2.3 Type Test and Prequalification Test • Type Test of the entire termination according to accessory standard (IEC 60840 and 62067). • Pre-Qualification test of entire termination, for voltages covered by IEC 62067 or for voltages covered by IEC 60840 in the case where the stress at the cable insulation screen is higher than 4 kV/mm.

7.4.3.3 Plug in Connector and Other Parts of the Termination 7.4.3.3.1 Type Test • Plug in connector must be Type tested of according to the recommendations of the works of Cigré WG B1-46 “Conductor Connectors: Mechanical and Electrical Test”.

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Connection and other parts of the termination should be in the supply scope of the stress cone manufacturer, because they depend strongly on the cable conductor and screen design.

7.5

Where the Plug-in Concept Could Be Applicable

7.5.1

Geometrical Installation Constraints

7.5.1.1 GIS Termination Installation Procedures We distinguish 2 cases of metal enclosure installation, 1. Metal enclosure installed after cable termination installation 2. Metal enclosure already on place before termination installation and two types of cable termination installation (a) Stress cone plugged in the preinstalled insulator in the cable termination enclosure (b) Insulator fitted on the stress cone, then assembly installed in the GIS metal enclosure. These 4 cases are sketched (Fig. 7.20) Advantages and Constraints of the Different Types of Installation Solution 1a • GIS metal enclosure can be factory tested • Requires a temporary protection around the cable/stress cone assembly (mechanical protection, moisture protection, etc., always required unless metal enclosure is installed immediately after termination assembly) • Requires free space above metal enclosure (could require a higher ceiling) • For safety reasons, reduced pressure inside GIS adjacent gas compartment during cable termination installation work. • Needs both installers for final assembly (both of them have major work to do) • Cable termination enclosure must be disassembled on site. • Gas operation must be performed prior and after assembly.

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Fig. 7.20 Types of installation procedure

Solution 1b • GIS metal enclosure cannot be factory or site tested with the insulator • Could require a temporary protection around the cable termination (mechanical protection, moisture protection, . . ., depending of environmental conditions, stand-by duration and risk of impact during handling) • Requires free space above metal enclosure (could require a higher ceiling) • For safety reasons, reduced pressure inside GIS adjacent gas compartment during cable termination installation work.

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• Needs both installers for final assembly (cable termination installer has minor work, which would be to remove eventual protection, adjust cable termination height, install SVLs) • Gas operation must be performed after assembly.

Solution 2a • GIS connection parts could be installed prior cable termination. • GIS metal enclosure can be factory tested (high voltage + gas tightness test) including insulator assembly. • Could require more space under metal enclosure (cable bending for termination installation) • For safety reasons, reduced pressure inside metal enclosure during termination plug-in. • Does not need both installers at the same time (but GIS installer or customer would have to come after termination installation to fill the metal enclosure at rated pressure).

Solution 2b • GIS metal enclosure cannot be factory or site tested with the insulator • Could require more space under metal enclosure (cable bending for termination installation) • For safety reasons, reduced pressure inside GIS adjacent gas compartment during cable termination installation work. • Needs both installers for final assembly

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• Gas work must be performed after assembly.

All these solutions are presented as vertical installation, but they are also applicable for horizontal installation. In such case only civil work types will be different, but space constraints will remain the same. Solution 2a is the subject of this brochure. However: Solution 1a, is not fully satisfactory for the GIS manufacturer as the GIS has to be opened during installation Solutions 1b and 2b are not in the scope of the present brochure as they don’t consider a common interface. Solutions 1.a and 1.b give much less civil work constraints as the cable termination can be realised in final position and in such case no room is necessary to move the cable. As a disadvantage if no free space is available there is no possibility to have some cable overlength for eventual repair or rerouting of the link in the switchgear.

7.5.1.2 Civil Work Constraints From paragraph 7.5.1.2 we present what are the distance requirements for solution 2. a., knowing that for such solution it is not possible to avoid a large free space for cable snaking in order to plug the termination in. All calculated values are based on minimum bending radius. On a practical basis, these values have to be increased to ensure a realistic proper installation (for instance free spaces or distances may need to be increased up to 20%). In addition to this description a free length has to be considered in the case of a flexible installation as it is needed during line operation to release thermo-mechanical stresses on the cable terminations. 7.5.1.2.1

Height Between the Bottom of Metal Enclosure/Epoxy Insulator and Lower Floor The height between the bottom of metal enclosure/epoxy insulator and the lower floor must be at least the termination length (which is more than epoxy insulator

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height) + the straight length of cable under the termination (at least 1 m where cable clamps will be installed) + the minimum cable bending radius (20 D) see note2. Often, it will be worth considering the largest cable size for the minimum permanent bending radius, as a utility could need to upgrade its substation or install additional GIS modules with larger cables. Here is an example (Fig. 7.21): • Diameter of cable to be used for the current project ¼ 100 mm • Diameter of largest cable which could be used (for future eventual upgrading) ¼ 120 mm. So the height between floor and bottom of GIS enclosure has to be increased by 420 mm to accommodate potentially future larger cables (Fig. 7.22).

straight cable legth

termination height

H min (project)

D

20

2

0 D= 12

420

D=

10

0

H min (optimal)

Fig. 7.21 Height vs. cable diameter

Installation cable radius based on cable manufacturers recommendation (Nexans catalogue, ERA technology report) for aluminium or lead sheath

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Fig. 7.22 Typical view of a basement installation

Fig. 7.23 Horizontal length for cable snaking

7.5.1.2.2 Free Space for Cable Snaking Necessary for Plug-in Operation Horizontal Cable Snaking – Vertical Plug-in This drawing shows that in order to allow termination insertion, it is necessary to have a minimum length available for cable bending (usually horizontal or vertical) (Fig. 7.23). Vertical Cable Snaking – Vertical Plug-in When the cable is laid in ducts the plug-in operation can be applied when the distance between the base plate and the duct is large enough to make a vertical snaking (Fig. 7.24).

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Fig. 7.24 Vertical length for cable snaking

7.5.1.2.3

Free Space for Cable Snaking Necessary in Case of an Intermediate Floor In this paragraph we describe the snaking length for plug-in operation. In case the termination has to be built on the lower floor, the available space between lower and intermediate floors is very critical. Figure 7.25 shows a theoretical cable arrangement in a basement to plug in a terminationwith: • D: cable diameter • H bas.: minimum basement height ¼ 20D + D (practically 2 m minimum) • L bas.: minimum free cable length in basement according to H bas. allowing a maximum vertical snaking • Ls: available length due to cable snaking. The summary Table 7.1 (rounded values), is based on typical cables with an aluminium foil screen bonded to the outer sheath:

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Fig. 7.25 Cable arrangement in a basement for termination plug in operation

Table 7.1 Snaking vs. free space

Cable type / Uo D (mm) Weight (kg/m) H bas. (m) L bas. (m) Ls (m) Weight of cable to move (kg)

Cable 1 630 mm2 Al 63 kV 65 4.5 1.36 5.9 0.93 33

Cable 2 630 mm2 Cu 110 kV 80 9.6 1.68 7.3 1.14 86

Cable 3 1000 mm2 Cu 220 kV 100 17 2.10 9.1 1.43 190

Cable 4 1600 mm2 Cu 220 kV 120 23 2.52 10.9 1.71 310

Cable 5 2500 mm2 Cu 500 kV 150 40 3.15 13.6 2.14 680

If we consider that minimum basement height (H bas.) is 2 m, the available cable length will increase for smaller cables, as long as free cable length (L bas.) can be made slightly longer. For example, considering Cable 1 from Table 7.1, and the arrangement of Fig. 7.26 the above table values are modified as per Table 7.2. From this example we can see that with basement length (L bas.) only 55 cm longer, available cable length (Ls) increases from 0.93 to 1.77 m. The necessary length in basement also depends where and how the cable termination is prepared (horizontal preparation on lower floor, vertical above intermediate floor . . .) 7.5.1.2.4 Floor Hole Size when Cable Is Crossing an Intermediate Floor There are different parameters that need to be taken into consideration. Distance Between Bottom of Metal Enclosure/Epoxy Insulator and Intermediate Floor In some case the bottom of the metal enclosure is located within the floor hole. Under such circumstances the diameter of the floor hole has to facilitate the cable termination installation works and shall not just cover the dimensional requirements of the GIS cable enclosure (Fig. 7.27).

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Fig. 7.26 2 m high basement for small diameter cable snaking

Table 7.2 Cable type D (mm) Weight (kg/m) H bas. (m) L bas. (m) Ls (m) Weight of cable to move (kg)

Cable 1 630 mm2 Al 63 kV 65 4.5 2.00 6.45 1.77 43

Closed ferrous magnetic loops should not be introduced around the single phase power cables by items such as external steelwork, and concrete reinforcement. Size of Floor Hole (in Line with the Termination) When the cable termination can be prepared on the intermediate floor, there is no additional requirement due to the plug-in concept. When the cable termination has to be prepared on the lower floor, a larger hole is needed, depending of the type of termination (3-phase – single phase), and the SVL position, For a single phase termination, the hole has to be at least equal to external diameter of the base of the epoxy insulator + extra space depending on the SVL position, size and rated voltage (voltage clearance). Figures 7.28 and 7.29 show an example of floor hole size study for a 72/100 kV 3-phase GIS metal enclosure, with a distance of 150 mm between bottom of metal enclosure and intermediate floor. The use of 15 kV SVL requires specific arrangement (SVL are in green). • Distance between SVL and earth or metallic parts: min. 130 mm • Distance between 2 SVL: min. 160 mm.

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Fig. 7.27 Tri-phase GIS termination installation

Fig. 7.28 Profile view of 15 kV SVL installation

The two dimensions vertical distance of the bottom of the metal enclosure and diameter of the floor hole are somehow linked. Whenever the vertical distance gets narrow (e.g. below 500 mm) the size of the floor hole should be increased in order to facilitate the mechanical installation and construction during insertion of the cable/ stress cone assembly from below. This leads to additional constraints on the metal enclosure metallic support structure design.

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Fig. 7.29 Bottom view of SVL installation

Cable stiffness increases when temperature decreases. At low temperature, plug in operations are not feasible unless the cable has been heated. If cable temperature is lower than +5 C, it has to be heated over this temperature. The structure to support the cable in the basement must also allow for the installation operations, which need large forces, especially for large cross sections of cables and/or cold temperatures. It is worth remembering that fixing of lifting tools etc. to the metal GIS enclosure or its related components is not permitted.

7.5.1.3 Conclusions Regarding Geometrical Installation Constraints Conclusion from geometrical constraint on solution 2a with the table space-weightcondition Advantages and disadvantages of the different installation conditions are summarised in Table 7.3. The installation survey shows that the case 2a is the only one where the plug-in concept can be considered. In this case, the evaluation of geometrical constraint for termination preparation and plug-in shows that the main controlling parameters are the cable diameter, weight, and the available space. Regardless of the civil works, the plug-in concept looks easy to implement for smaller and lighter cables. i.e. less than 100 mm in diameter and 15 kg/m. For large size and heavy cables, the constraints rapidly become more severe. Refer to Table 7.4, where the practical cases are written with green letters, difficult ones with orange letters, and almost impossible ones with red letters. However, the plug-in is always possible if it has been taken into account at the design stage of the civil works as it may require additional installation procedures and efforts, adapted handling means and extended free space.

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Table 7.3 Type of installation a Strong coordination between cable termination and GIS installers. Necessity to disassemble the Cable termination enclosure. Increased probability to damage the stress cone. Reduced civil works No advantage for further standardization of the insulator

Case 1

Type of installation b Always possible. Reduced civil works Need for coordination between cable termination and GIS installers. Not plug-in procedure.

No advantage for further standardization of the insulator Possible with restrictions: Available space for moving the cable Necessary strength for lifting the cable. Not plug-in procedure. Same civil works as 2a No advantage for further standardization of the insulator

Possible with restrictions: Align stress cone and insulator axis during insertion Available space for moving the cable Necessary strength for lifting the cable

Case 2

Further standardization of the insulator can be evaluated

Table 7.4

Cable type / Uo D (mm) Weight (kg/m) H bas. (m) (at least 2 m) L bas. (m) Ls (m) Weight of cable to move (kg) Comments

Cable 1

Cable 2

Cable 3

Cable 4

Cable 5

63kV 65 4.5 2.00 6.45 1.77 43

110kV 80 9.6 2.00 7.60 1.53 100

220 kV 100 17 2.10 9.1 1.43 190

220 kV 120 23 2.52 10.9 1.71 310

500 kV 150 40 3.15 13.6 2.14 680

*depending on site

*depending on site

with - D: cable diameter - H bas.: minimum basement height = 20D + D (practically 2 m minimum) - L bas.: minimum free cable length in basement according to H bas. allowing a maximum vertical snaking - Ls: available length due to cable snaking

nge letters, almost impossible red letters.

7.5.2

Safety Practices and Constraints during Installation

These recommendations are specific to GIS plug in cable terminations and come in addition to normal practices in the electric civil works environment

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7.5.2.1 Voltage • DISCONNECT THE VOLTAGE! Two open gaps are needed, for instance circuit breaker and line disconnector • Earth the part between the circuit breaker and the outgoing disconnector switch with the earthing switch. 7.5.2.2 Gas Pressure during Installation During GIS manufacturing, installation and delivery: All insulators are pretested, tested in modules or complete in factory, and on site after installation. This requires adjusting the compartment pressures several times during the different phases of the GIS delivery. • • • •

The design pressure is 7.5 bars relative (reference to IEC 62271-209). During transportation, the pressure is decreased to 0,5 bar During works, the pressure in adjacent compartments is decreased to 0.5 bar. The customer or an authorized third party can perform the pressure decrease and refill. There may be legal regulation regarding authorized persons. • Supervision performed by experienced people or GIS manufacturer is recommended. • SF6 maintenance equipment is available at the customer premises. • Decrease of pressure is specified in the operating/maintenance manual. During the plug-in of the cable termination: Uncontrolled forces or mistakes during the plug-in operation are more dangerous with high gas pressure. Similar to the work practice on the GIS compartments, the Cigré WG recommends decreasing the pressure of the cable box to 0.5 bars relative for the above safety reason during the termination installation. Note that during the manufacturing of insulator and termination stress cone, it is common practice to first perform the pressure test of the insulator according to IEC 62271-203 and the maximum pressure of IEC 62271-209, then to perform the dielectric test of the stress cone. During the installation of the stress cone, the pressure in the cable box must be reduced to 0,5 bar.

7.5.3

Testing Constraints

All insulators and stress cones have to be tested.

7.5.3.1 Tests on Insulator Before Supply At insulator manufacturer (or at the GIS manufacturer facility by agreement between GIS and insulator manufacturers): Dimensions and tolerances according to paragraph Dimensions and Tolerances of Sect. 7.4.3.1.1

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Compliance with IEC 62271-209 Fig. 5, Plug-in connector of insulator, Lock-in system (if any), Insulator and shape of cone, smoothness, Insulation shield break ring (if integrated in the insulator).

Routine tests according to paragraph Routine Test of Sect. 7.4.3.1.4. • Voltage test and PD measurement test according to paragraph 7.1.101 & 7.1.102 of IEC 62271-203, the pressure being the minimum functional pressure for insulation as per IEC 62271_209 Fig. 1, with the deviation range of paragraph 8.2.1, • Tightness test according to paragraph 7.4 of IEC 62271-203, • Design and visual checks according to Fig. 5 of IEC 62271-209, • Pressure test according to paragraph 7.104 of IEC 62271-203.

7.5.3.2 Tests of the Stress Cone on a Cable Termination Assembly with a Host Insulator By accessory manufacturer: as per paragraph 9 of IEC 60840 or IEC 62067. 7.5.3.3 Tests After Installation GIS Without installed cable, test is according to IEC 62271-203 or ANSI C 37.122-2010. The common interface insulator causes no special issue except the test of the insulator, which is already prescribed in IEC 62271-209 paragraph 8.1 when it is pre-installed during GIS manufacturing. Cable The cable system shall be tested after installation according to paragraph 8.3 of IEC 62271-209. When the termination is not plug-in type, it is fitted inside the GIS enclosure. Different testing arrangements can be implemented: • A SF6 to air bushing has to be temporarily installed on the GIS after a disconnected area. See Fig. 7.30. • A termination is available at the other end (outdoor termination or SF6/air bushing). This is used to test the cable without emptying the cable box. If the GIS busbar is not disconnected, there may be impact on the GIS enclosure in case of termination failure. When the termination can be plugged in Different testing arrangements can be implemented: • The GIS has a SF6/air bushing. It can be used for cable after installation test (for cases above 245 kV – below 245 kV there is usually no bushing as part of the

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Fig. 7.30 Example of after installation test equipment

GIS) without emptying the cable box. If the GIS busbar is not disconnected, there may be impact on the GIS enclosure in case of termination failure. • A termination is available at the other end (outdoor termination or SF6/air bushing). This is used to test the cable without emptying the cable box. If the GIS busbar is not disconnected, there may be impact on the GIS enclosure in case of termination failure. • GIS to GIS link or no termination available. The cable box must be open, or use of a “dummy” accessory before plug in. The final arrangement and the epoxy busbar cone inside the GIS cable termination are not tested. • GIS to GIS; as an alternative to the bullet point above, install a voltage lead outside of the insulator. Depending on the GIS design, it can be a temporary GIS termination, which is later on removed. The test of the cable system via GIS should be made in agreement with the GIS manufacturer.

7.5.4

Conclusion Regarding Testing Constraints

Due to the weight of the complete cable and the handling issues in the case where there is no available termination for performing after installation test without moving the cable, the cross section should be less than 1000 mm2 Cu or 1600 mm2 Al.

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7.6

Qualification

7.6.1

State of the Art

7.6.1.1 Medium Voltage Standards There are 2 current standards EN 50180 and mainly EN 50181 (European standards from CENELEC) which standardize the interface profile. Connectors in Medium Voltage are similar to stress cones in our brochure. EN 50180:2010: Bushings above 1 kV up to 52 kV and from 250 A to 3.15 kA for liquid filled transformers. Introduction The object of this European Standard is to specify the requirements to ensure interchangeability of bushings having highest voltages above 1 kV up to 52 kV and rated currents from 250 A up to 3150 A for insulating liquid filled transformers. Scope (Chapter 1) This European Standard is applicable to ceramic and resin insulated bushings having highest voltages above 1 kV up to 52 kV, rated currents from 250 A up to 3150 A and frequencies from 15 Hz up to 60 Hz for insulating liquid filled transformers. This standard establishes essential dimensions, to ensure interchangeability of bushings and to ensure adequate mounting and interchangeability of mating plug-in separable connectors of equivalent ratings. EN 50181:2010: Plug-in type bushings above 1 kV up to 52 kV and from 250 A to 2.50 kA for equipment other than liquid filled transformers. Introduction The object of this European Standard is to specify the requirements to ensure interchangeability of bushings for maximum voltages above 1 kV up to 52 kV and rated currents from 250 A to 2500 A for equipment other than insulating liquid filled transformers. Scope (Chapter 1) This European Standard is applicable to insulated bushings for maximum voltages above 1 kV up to 52 kV, rated currents from 250 A up to 2500 A and frequencies from 15 Hz up to 60 Hz for equipment other than liquid filled transformers. This European Standard establishes essential dimensions, to ensure adequate mounting and interchangeability of mating plug-in separable connectors of equivalent ratings. Definitions in EN 50181:2010 are not in line with ones of this brochure. Some examples are given hereunder.

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Plug in Type Bushing (Chapter 3.1) Bushing one end of which is immersed in an insulating medium which has customized dimensions according to insulation requirements for the specific application and the other end designed to receive a separable insulated cable connector without which the bushing cannot function Separable Connector (Chapter 3.2) Fully insulated termination permitting the connection and disconnection of the cable to and from the mating plug-in type bushing Interface Type (Chapter 3.3) Bushing dimensions that insure mechanical and electrical interchangeability of bushing and separable connector of similar rating and type. NOTE: Each interface type is designated by a letter or a number. Bail Holder (Chapter 3.4) Fixture which facilitates anchoring of an externally mounted device (called the bail) designed to prevent undesirable separation of a separable connector and a bushing.

7.6.1.2 Medium Voltage Qualification Experience Qualifications were carried according to market needs. Most connector suppliers are also bushing suppliers, not necessarily to be installed together. No crossed qualifications are required. Bushing material is not mentioned in EN 50181 (and mentioned as porcelain or resin in EN 50180), but an elastomeric bushing should not be considered as a possible solution according to these interchangeability standards. As long as the interface is in accordance with the standard, the most critical point concerns the lubricant to be used at the interface. Some utilities have been facing problems of disconnection with silicone connectors which absorb the lubricant and stick to the bushing. Consequently a disconnection often leads to damage of such connectors. Hence suppliers have to provide the associated lubricant which is compatible with their connector (EPDM or Silicon Rubber) and considered neutral regarding compatibility with the bushing resin. In case of fault, an examination is usually performed in order to identify the fault source. However in MV the material cost remains very low compared to examination or other investigations costs, so it is not worth engaging further investigations to determine responsibilities. Usually the connector manufacturer supplies the replacement parts. Unplugging could be necessary for temporary link installation (main purpose), or in case of fault (fault location, repair . . .), but it is almost never used. Lessons Learned from Qualification and Installation Experience There are important points to validate:

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• Compatibility between connector and lubricant • Conductor connection • Insulator resin choice, which determines its electrical performance, but also on its molding process or machining ability, mainly to ensure a well-controlled interface roughness.

7.6.1.3 High Voltage Standards Interface Standards Table 7.5. Cable System Standards The Table 7.6 shows the most common applicable standards depending on the different countries. Table 7.5 GIS TERMINATIONS FOR EXTRUDED CABLES COLLATION OF RELEVANT STANDARDS Country Standard Title List of Type Tests (TT)

Country

Standard

Title

International and EN

IEC 622712092007

USA

IEEE 13002011

High-voltage switchgear and controlgear -Part 209: Cable connections for gas-insulated metal-enclosed switchgear for rated voltages above 52 kV. Fluid filled and extruded insulation cables Fluidfilled and dry-type cableterminations Guide for Cable Connections for Gas Insulated Substations

Terminations alone

List List of PQ of Extension PQ Tests Tests As part of cable systems only

As part of cable systems This standard is an interface standards gives recommended arrangements for dielectric tests on GIS terminations. The dielectric tests for type and PQ tests are specified in relevant IEC standards for particular type of cable. It makes reference for insulators to routine tests specified in IEC 62271-203.

This guide is carbon copy of IEC 62271-2092007 in regard to GIS connections for extruded cables. In addition it specifies dimensional requirements for GIS connections for laminated cables.

Standard IEC 60840-2011

IEC 62067-2011

Country International

International

Power cables with extruded insulation and their accessories for rated voltages above 150 kV (Um ¼ 170 kV) up to 500 kV (Um ¼ 550 kV) – Test methods and requirements

Title Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um ¼ 36 kV) up to 150 kV (Um ¼ 170 kV) – Test methods and requirements

GIS TERMINATIONS FOR EXTRUDED CABLES COLLATION OF RELEVANT STANDARDS

Table 7.6

Terminations are type tested as part of cable system only.

List of Type Tests (TT) Terminations alone • PD amb • 20 cycles at 2U0 (8 h heating, min 2 h @95– 100 C for EPR and XLPE) • PD amb • PD hot • Hot BIL followed by 2.5U0 15 min • Visual inspection
4 kV/mm at insulation screen.

• PD amb • 20 cycles at 2U0 (8 h heating, min 2 h @ 95– 100 C for EPR and XLPE) PD amb • PD hot • SIV hot (for Um  300 kV) • Hot BIL followed by

As part of cable systems • PD amb • 20 cycles at 2U0 (8 h heating, min 2 h @ 95– 100 C for EPR and XLPE) PD amb • PD hot • Hot BIL followed by 2.5U0 15 min • Visual inspection >4 kV/mm at insulation screen.

Feasibility of a Common, Dry Type Plug-in Interface for GIS and Power. . . (continued)

List of PQ Extension List of PQ Tests Tests As part of cable systems only • PD amb • 180 cycles at 1.7U0 • Hot BIL (if done on • entire test rig) 60 cycles, • Visual inspection no voltage • 20 cycles at 2U0 • PD amb • PD hot • Hot BIL followed by 2.5U0 15 min • Visual inspection 4 kV/mm at insulation screen. min 180 cycles at 1.7U0 • PD amb (1 year test) • • Hot BIL (if done on 60 cycles, entire test rig) no voltage • Visual inspections • 20 cycles at 2U0 • PD amb • PD hot • SIV hot

7 357

Standard

Cigré TB 303

Country

International

Changes in a qualified cable system

Title

GIS TERMINATIONS FOR EXTRUDED CABLES COLLATION OF RELEVANT STANDARDS

Table 7.6 (continued)

Change of insulator material for indoor or outdoor terminations. – > new TT Change of insulator design or manufacturer of GIS/Transformer insulator -> newTT Change in the formulation of the stress cone compound but with the same base polymer – > EQ Change of the base polymer (EPR, Silicone,...) of the stress cone –> EQ

List of Type Tests (TT) Terminations alone As part of cable systems 2.5U0 15 min • Visual inspection

(for Um  300 kV) • Hot BIL followed by 2.5U0 15 min • Visual inspection

List of PQ Extension List of PQ Tests Tests As part of cable systems only

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IEEE 48-2009

AEIC-CS9-06 Association of Edison Illuminating Companies representting Utilities

USA

USA

Specification for Extruded Insulation Power Cables and their Accessories Rated Above 46 kV Through 345 kV AC

IEEE Standard for Test Procedures and Requirements for Alternating- Current Cable Terminations Used on Shielded Cables Having Laminated Insulation Rated 2.5 kV through 765 kV oi Extruded Insulation Rated 2.5 kV through 500 kV

• PD amb • 3.5-3.9U0AC, 1 min • DC, 15 min • Cold BIL • Hot BIL • PD amb • 30 cycles at 2U0 (each cycle min 6 h at emergency temp +0/ 5 C (105 C for XLPE and 130 C for EPR, cooling process specific) • PD amb • 2.5U0 AC, 6 h • Cold BIL • Cold SIL (for 345 kV and above) • PD amb • Visual inspection Per IEEE 48 Not considered

• For class
170 kV: per IEC60840 (if required by purchaser spec) • For class >170 kV: per 62067 • Additional 90 cycles at emergency temp +0/ 5 C (105 C for XLPE and 130 C for EPR) are required • For class
170 kV: per IEC60840 (if required by purchaser spec) • For class >170 kV: per 62067 The load cycling test for both classes should be done at emergency temp +0/5 C (105 C for XLPE and 130 C for EPR)

Feasibility of a Common, Dry Type Plug-in Interface for GIS and Power. . . (continued)

Not considered

Not considered

Not considered

7 359

Standard JEC-3408-1997

RTE Spec No. 214 ind.3 full current ref.: NT-IMRCNERDL-ML2010-00214 (+amendments of 14 Oct 2011 and 4 Apr 2012)

Country Japan

France

RTE specification Single phase cables and accessories for underground links from 63 kV to 400 kV AC

Title High voltage tests on XLPE insulated cables and their accessories for rated voltage from 11 kV up to 275 kV

GIS TERMINATIONS FOR EXTRUDED CABLES COLLATION OF RELEVANT STANDARDS

Table 7.6 (continued)

GIS cable terminations have to be in accordance with IEC 62271-209 This specification is based on IEC 60840 & 62067. + additional tests on base insulating ring:  lightning impulse test (for 63, 90 & 225 kV ¼ 50 kV; for 400 kV ¼ 62.5 kV) – AC test under rain condition (all voltages ¼ 20 kV for 15 min)

List of Type Tests (TT) Terminations alone As part of cable systems • 30 days daily cycling at 90 C or 105 C, at 1.48 times max. phase-to-ground cable voltage (U0) or • 1 h hot AC at 2.53 E0 or 1 h cold AC at 3.04 E0 • Hot or cold lightning Impulse, 3+, 3 shots

List of PQ Extension List of PQ Tests Tests As part of cable systems only • 1/2 year daily cycling Not at 90 C or 105 C, at considered 1.32 U0 • Hot or cold lightning impulse, 3+, 3 shots • 10 min hot or cold AC PQ test 180 cycles, 6000h for voltages
150 kV, 1 year for 245 kV and 420 kV.

360 P. Mirebeau

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361

Table 7.7 Guide to the selection of tests because of modifications to an accessory within the same family in a prequalified EHV cable system Modification Component Terminations:  Outdoor  Indoor  Metal enclosed + SF6 + Oil-immersed

Type of modification Higher electrical stress design of stress cone (or smaller metal clad for GIS or transformer terminations) Change in nature of Filling medium (e.g. oil to gas. . .) Change in the formulation of the stress cone compound but with the same base polymer Change of the base polymer (EPR, Silicone, ...) of the stress cone Change of insulator material for indoor or outdoor terminations. Change of insulator design or manufacturer of GIS/Transformer insulator

Ma

Pa

Da

V V

V

V2)

V2)

V2)

V2)

V

V

DLa V

IEC 62067 Ed.1 Clause number TPQ- EQtest test test – – (xx)1)

–

–

(xx)1)

–

–

(xx)1)

–

–

(xx)

12

–

–

12

–

–

When can be demonstrated that the thermo mechanical aspects have no significant influence on the performances of the termination a Type Test may be sufficient. In case of elastomeric insulators (“silicone” or “EPR”) climatic and pollution test according to IEC 61109 Annex C should be considered (xx) Clause to be added in the standard a M, change in material; P, change in manufacturing process; D, change in design (construction); DL, change in electrical design stress level

In every case voltages >170 kV are worth a full system consideration. Voltages
170 kV are either considered as a commodity (example: China, Middle East, Thailand), or considered as a system (example: France, Italy, some end users in USA). Cigre Brochure: Revision of Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems Cigré TB 303 deals with extension of qualification. Interchangeability leads to a type test. Table 7.7 is an extract of Table 2.4 of TB 303: GIS Partition and Insulator Standards The insulator specification is part of the IEC standard 62271-203 “Gas-insulated metal-enclosed switchgear for rated voltages above 52 kV” where most important

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parts of the outdated EN 50089 “Cast resin partitions for metal-enclosed gas-filled high-voltage switchgear and controlgear” (1994) have been included. • Type test (Chapter 6.1 and 6.104) – Tightness test – Voltage test including PD measurement – Burst test (with a burst pressure result >3  design pressure) – Thermal performance • Routine tests (chapter 7.1, 7.4 and 7.104) – Visual inspection – AC voltage test including PD measurement – Pressure test (2  design pressure/1 min.) – Tightness test.

7.6.2

Where the Plug-in Common Interface Could be Applicable

Due to the qualification issues and specially the need for prequalification, the common interface should be restricted to voltages up to and including 170 kV. The detailed technology and the number of sizes will be defined by the next Cigré working group.

7.6.3

Qualification of new Insulator or Stress Cone

We consider here terminations for voltage
170 kV. When insulator, stress cone and termination assembly have been qualified according to paragraph 4.3, the qualification of a new insulator with the same stress cone or a different stress cone with the same insulator needs a Type Test of the termination assembly. The details of the Type Test arrangement as well as the range of approval need to be defined by the next Cigre working group (see Sect. 7.9).

7.7

Feasibility

In this section, the conclusions from sections 7.6: “the common interface should be restricted to voltages up to and including 170kV” and 7.5 “Due to the weight of the complete cable and the handling issues, the cross section should be less than 1000 mm2 Cu or 1600 mm2 Al” are taken into account. The cost of different development phases are addressed below.

7.7.1

Definition Feasibility (Cost Involved)

Definition and production of the plug in system Due to variations in the manufacturing processes as a function of different suppliers, the design and manufacturing costs cannot be addressed by the working group.

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Feasibility of a Common, Dry Type Plug-in Interface for GIS and Power. . .

363

Definition and production of insulator: • Design of the geometry: Cigré WG to come (see Terms Of Reference at the end of the brochure) • Engineering • Mould design • Eachinery • Prototypes. All this work needs several man-years Definition and production of stress cones: • The work needed to design, engineer, prototype stress cones. The cost is between two to five times more than for the insulator, depending on the stress cone sizes that are needed per voltage level (according to the core diameters).

7.7.2

Qualification Feasibility

The qualification has to be performed according to the standardisation requirements (paragraph 4.3): The market acceptance study (paragraph 8) shows that the customer will accept only combinations stress cone/insulator that have been tested as a system. The cost of the necessary tests is approximately: Initial component and system qualification: Insulator • Development tests are about 100 k€ • The Type Test of paragraph 7.4.3.2 cost about 200 k€ (test of connector not included) • A preliminary Type Test is about 200 k€ Stress Cone • Development tests are about 100 k€ at the supplier premises • The Type Test of paragraph 7.4.3.1 is about 200 k€ including laboratory costs, cable and accessory, installation works. System • A type test is needed (no prequalification because of the voltage level) i.e. 200 k€. • If the cable stresses at the insulation screen is larger than 4 kV/mm (ref. IEC 60840) a prequalification test is required. This cost about 400 k€. Cross Qualification (paragraph 6.2) • Due to market acceptance, a type test is needed for all combinations. It costs about 200 k€ per combination.

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• When a stress cone has been prequalified and the insulator is from a different manufacturer, a new prequalification test is not necessary as the stresses on the insulator are not influenced by the insulation screen stress of the cable.

7.8

Market Acceptance

7.8.1

Current Status

For voltages above 170 kV full qualification of the cable system is required. Each link is bought as a system (refer to IEC 62067). For voltages up to 138 kV, There are two opposite trends: some customers move to the system approach, others go to commodity approach: cable and accessories are bought separately. 170 kV is a special case where the cable system stresses can be similar to the 245 kV level and the cable cross section is larger. For this voltage, the system approach prevails. When the cable termination supplier is not chosen at the time the GIS has to be delivered (around 70% of cases), the GIS is not pre equipped with the insulator and the last compartment is not tested. It has to be open at the cable installation time. There are gas works and risk of pollution.

7.8.2

Future Status

For voltages lower than 170 kV, if stress cones and insulators are provided by different manufacturers, there is one more limit of responsibility barrier as compared to the cable system case. It is the stress cone – insulator interface. In case of failure, the responsibility for the failure is less easy to address than at the insulator/SF6 interface. End users are concerned by this change as there is no clear limit of responsibility. All combinations of stress cone/insulator must be tested. If the cable accessory is installed by a third party contracted by the end user, there is one more layer of responsibility. Improper cable clamping, contamination, or pressure spring compression may lead to defect and further complicate allocation of responsibility in case of a fault. The end user can limit the complication of responsibility by giving the contract to a limited number of parties. The main benefits are that more flexibility is given to the end user and overall logistics costs are reduced. Note: The financial balance of common interface benefits versus the design development and qualification costs was not investigated. Market acceptance drivers are given in the hereunder table (Table 7.8).

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365

Table 7.8 In favour of interchangeability Qualification

Routine test of insulator and rubber Routine test of termination assembly Routine test of GIS manufacturing

Planning

–

Installation on GIS

Cable enclosure fully tested. (presently fully tested only when the termination supplier is selected together with GIS) Engineering of cable and GIS independent. Easier logistics for GIS manufacturer. Higher flexibility for the end user

Less storage – less logistics – easier planning No opening of GIS on site and pre-tested (true for any pre-installation of insulator, whatever the interface)

Installation of cable termination

Spare parts

Comment

Final termination assembly not routine tested.

Costs Cost manufacturing (insulator and rubber)

Cost GIS manufacturing

Detrimental to interchangeability Cost of a cross qualifications of insulators and stress cones –

More difficult and costly logistics for the cable accessory manufacturer.

In practice the cables are ordered later than GIS.

Cable system design: less optimised due to variability of components. (ref. Vattenfall experience 3.2.2). Cable cost can be affected.

New investments and qualification to perform due to new design of insulator and stress cone (paragraph 7).

The termination can only be installed when the GIS is on site

More space needed to plug in (see 5.1) as compared to 5.1.1 case 1 situation. This could impact civil works cost.

Installation should avoid any torsion on the cable.

Easier logistics (continued)

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Table 7.8 (continued)

Utility & industrial users impact Spare parts Flexibility for choice of cable supplier in case of upgrading Responsibility allocation After installation test of the cable system with GIS termination.

7.8.3

In favour of interchangeability Easier Higher

Detrimental to interchangeability

More difficult with more parties. Responsibility allocation in case of problem.

Comment To be in the application range of insulator

End user should minimize the number of parties. The test must be performed through the high voltage bus bar of the GIS or from the remote end outdoor termination.

Where the Plug-in Common Interface Could be Recommended

As coming from the market acceptance drivers the common interface should be limited to the commodity market: • Voltage 145 kV and less. • Current 1000 A and less • Short circuit 40 kA during 1 s and less.

7.9

Conclusion and Recommendations

Taking into account the above considerations and specially the market trend in some countries towards a commoditisation of the High Voltage cables lower or equal to 145 kV, the working group thinks that there is room in these voltage levels for a standard design in parallel with the present designs. As per the TOR of the group, B1.B3-33 recommends Study Committees B1 and B3 to set up a new working group with the following Terms of Reference. The Working group should recommend a functional design of an insulator with a common interface with the following scope of work:

7

Feasibility of a Common, Dry Type Plug-in Interface for GIS and Power. . .

• • • •

Voltage is
145 kV AC (Um) Current is
1000 A, short circuit
40 kA 1 s. Cross sections are
1000 mm2 Cu or 1600 mm2 Al Technology has to be defined (inner or outer cone), with a detailed evaluation of technical advantages/disadvantages of the two technologies. The number of sizes has to be defined, the short circuit current can be altered for the smallest sizes. Dimensions of insulator components have to be defined (current connection, electric design and properties, mechanical design and properties). The type and dimension of the main current connection has to be defined Consider the consequence of a termination failure. Consider the upgrading of the cable link for higher current loads. Consider the installation constraints, with a special focus on the basement dimensions. The design has to meet the requirements of IEC 62271-209 and IEC 60840 Define the initial and cross qualification processes.

• • • • • • • •

367

The stress cone design and material, the lubricant and the design of the compression device should be left to the discretion of the accessory manufacturer within the limits of the standardised insulator properties. Cigré TB 303 and the work of WG B1.44 and WG B1.46 should be taken into account. Acknowledgments The Working Group wishes to thank T. Klein (DE), D. Kunze (DE) and M. Obst (DE) for their active support.

References All standards that are in the documents: Cigré TB 303 (Chapter 4) Cigré WG B1.29: Guidelines for maintaining the integrity of XLPE cable (Chapter 6) Cigré WG B1.46: Conductor Connectors: Mechanical and Electrical Test (Chapter 10)

Pierre Mirebeau, who graduated from the “École Supérieure de Physique et Chimie Industrielles” (Paris), has headed high-voltage R&D for Nexans over the past 25 years. As a Member of Cigré since 2005, he contributed to a variety of subjects, including testing of DC extruded cables, life management of buried AC lines, advanced designs of laminated metallic coverings, dry type interfaces for Gas-Insulated Switchgear and power cables, and the environmental impact of cable links. In recognition to this work, he was granted the Technical Committee Award for 2011. He is an Active Member of the International Electrotechnical Commission (IEC) standardization body, and the Institute of Electrical and Electronics Engineers (IEEE), where his presentations on development techniques for

368

P. Mirebeau HVDC Links with synthetic insulation in 2001 and his collaborative (IEEE + IEC) presentation on cable terminations for gas insulated switchgears in 2006 were awarded “best presentation.” He also holds several important patents relating to lead-alloy composition, cable designs, and polymer material composition. He is the Liaison Member between IEC TC 20 and Cigré B1 (both regarding insulated cables), and between CIBRE B1 and Cigré B3 (substations and electrical installations).

8

Test Procedures for HV Transition Joints for Rated Voltages 30 kV up to 500 kV Marco Marelli

Contents 8.1

8.2 8.3

8.4 8.5

8.6

8.7 8.8

8.9

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.3 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.4 Condition Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normative References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Definition of Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Development Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2 Routine Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.3 Sample Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.4 Type Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.5 Prequalification Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.6 Electrical Test after Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Test Cables and Transition Joint Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Development Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.1 Electrical Development Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.2 Non-Electrical Development Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Routine Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6.1 Extruded Cable Side of the Transition Joint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6.2 Paper Cable Side of the Transition Joint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Type Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8.2 Range of Type Test Approval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8.3 Type Test Arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8.4 Type Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prequalification Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.9.1 General and Range of Prequalification Test Approval . . . . . . . . . . . . . . . . . . . . . . . . . 8.9.2 Prequalification Test Arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

370 370 370 371 372 372 373 373 373 373 373 373 374 374 374 374 374 375 375 375 375 376 376 376 377 379 382 382 383

M. Marelli (*) Prysmian Group, System Engineering, Land and Submarine HV and EHV AC/DC Power Cable Systems and Telecom Cable Systems, Milano, Italy e-mail: [email protected] © Springer Nature Switzerland AG 2021 P. Argaut (ed.), Accessories for HV and EHV Extruded Cables, CIGRE Green Books, https://doi.org/10.1007/978-3-030-39466-0_8

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M. Marelli

8.9.3 Prequalification Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical Test after Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.10.1 DC Voltage Test of the Oversheath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.10.2 AC Voltage Test of the Insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A Considerations for Transition Joints for Other Types of Paper Cable . . . . . . . . . . . A.1 Cables to IEC 60141-2: – Internal Gas-Pressure Cables and their Accessories for Alternating Voltages up to 275 kV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.2 IEC 60141-3: – External Gas- Pressure (Gas Compression) Cables and their Accessories for Alternating Voltages up to 275 kV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.3 IEC 60141-4: – Oil-Impregnated Paper-Insulated High Pressure Oil- Filled Pipe-Type Cables and their Accessories for Alternating Voltages up to and Including 400 kV . . . . . . Appendix B Design Features, Performance and Necessity for Performing Type Tests for Transition Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.1 Back-to-Back Transition Joint with Two Insulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.2 Back-to-Back Transition Joint with One Insulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.3 Composite Type Transition Joint (Three-Core, Single Core) . . . . . . . . . . . . . . . . . . . . . . . . . B.4 Single-Core or Three-Core Type with Bushing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.5 Methodology for Assessing Test Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix C List of Type and Prequalification Tests of Cable Systems . . . . . . . . . . . . . . . . . . . . . . . Appendix D Transition Joint Experience Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix E Terms of Reference for WG B1-24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.10

8.1

Introduction

8.1.1

General

384 385 386 386 387 387 388 389 390 391 392 393 394 395 395 395 399 400

This chapter is the editorial and graphical revision of the Cigré TB 415, prepared by the WG B1.24 and published in June 2010.

8.1.2

Background

The use of extruded cables is increasing for transmission and distribution circuits in preference to cables with paper insulation (either kraft paper or polypropylene paper laminate). The number of manufacturers of paper cable is also decreasing, therefore the availability of such cables for repair works or re-routing will be very limited in the near future. Consequently it is becoming more common for a length of extruded cable to be introduced into a paper cable circuit requiring transition joints for the interconnection of the two cable types. Cigré set up WG B1.24 to review this subject and issue a report including: • A review of existing designs of transition joints. • A review of the existing international standards and the extent to which they cover the testing of transition joints.

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Test Procedures for HV Transition Joints for Rated Voltages 30 kV up to. . .

371

• Recommendations about aligning voltage levels to those specified in IEC Standards. • Definition of test regimes for transition joints for routine, sample, type, prequalification and after installation tests. This Chap. 8 is the report of WG B1.24 published as Cigré TB 415.

8.1.3

Scope

The purpose of these new recommendations is to give general guidance for tests on high voltage transition joints. These recommendations are valid for transition joints between paper-insulated low pressure oil filled cables and extruded insulation cables with rated voltage from 30 kV up to 500 kV. Transition joints for single core cables as well as 3-core cables are covered. The use of these recommendations in relation to other paper cable types, e.g. high pressure oil filled or gas pressure types is addressed in Appendix A.

Note: The 30 kV voltage level is included in order to cover the full range of cables covered by IEC 60141

Different types of transition joints are used to connect oil-filled to extruded insulation high voltage cables, such as: • Back-to-back transition joint • Composite transition joint. Tests on joints between cables with similar type of insulation are not considered in this document, even if they are used between cables with different conductors or different screens. Although the application of high voltage transition joints for interconnection of different cable systems is likely to increase, the quantity of transition joints compared to the quantity of standard accessories required will be low. There will also be a large variety of cable constructions which have to be connected using transition joints. The number of type tests may be limited due to the availability of suitable paper insulated cables, thus guidance is given about the range of approval. Comments on the need for a long term prequalification test are also made. Wherever well known and type tested components are used, for instance symmetric back-to-back transition joint designs (e.g. comprising two SF6 terminations in a common chamber), a type test and prequalification test of the combination may be omitted.

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8.1.4

M. Marelli

Condition Assessment

In many cases the existing oil-filled cable on which a transition joint will be installed will have been in service for many years and hence diagnostic tests may be advisable to assess the cable condition. A full review of such procedures is given in reference (Cigré Electra 1998). In the event that the cable is found to be in as-new condition then it would generally be considered unnecessary to carry out any special diagnostic tests after installation of a transition joint. Practices for maintenance of HV cable circuits are described in reference (Cigré TB 279) and are not considered further in this report.

8.2

Normative References

The following documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. • IEC 60141 Test on oil-filled and gas-pressure cables and their accessories. – Part 1: Oil-filled, paper-insulated, metal-sheathed cables and their accessories for alternating voltages up to and including 400 kV – Part 2:Internal gas-pressure cables and their accessories for alternating voltages up to 275 kV – Part 3: External gas-pressure (gas compression) cables and their accessories for alternating voltages up to 275 kV – Part 4: Oil-impregnated paper-insulated high pressure oil-filled pipe-type cables and their accessories for alternating voltages up to and including 400 kV • IEC 60229 Electric cables – Tests on extruded oversheaths with a special protective function • IEC 60230 Impulse tests on cables and their accessories • IEC 60840 Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um ¼ 36 kV) up to 150 kV (Um ¼ 170 kV) – Test methods and requirements. • IEC 60885-3 Electrical test methods for electric cables. Part 3: Test methods for partial discharge measurements on lengths of extruded power cables • IEC 62067 Power cables with extruded insulation and their accessories for rated voltages above 150 kV (Um ¼ 170 kV) up to 500 kV (Um ¼ 550 kV) – Test methods and requirements. • EN 50299 Oil-immersed cable connection assemblies for transformers and reactors having highest voltage for equipment Um from 72,5 kV to 550 kV.

8

Test Procedures for HV Transition Joints for Rated Voltages 30 kV up to. . .

8.3

Definition of Tests

8.3.1

Development Tests

373

Internal tests made by the manufacturer before a new transition joint is type tested and taken into service. Details of such development tests are proprietary and shall be determined by the manufacturer.

8.3.2

Routine Test

Tests made by the manufacturer on each manufactured component to check that the component meets the specified requirements.

8.3.3

Sample Test

Tests made by the manufacturer on samples of components taken from a complete accessory, at a specified frequency, so as to verify that the finished product meets the specified requirements.

8.3.4

Type Test

Tests made before supplying on a general commercial basis a type of accessory covered by this recommendation, in order to demonstrate satisfactory performance characteristics to meet the intended application. Note: Once successfully completed, these tests need not be repeated, unless changes are made in the accessory with respect to materials, manufacturing process, design or design electrical stress levels, which might adversely change the performance characteristics.

8.3.5

Prequalification Test

Tests made before supplying on a general commercial basis a type of accessory covered by this recommendation, in order to demonstrate satisfactory long term performance of the accessory. Note 1: The prequalification test need only be carried out once unless there is a substantial change in the accessory with respect to material, manufacturing process, design or design electrical stress levels. (continued)

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Note 2: A substantial change is defined as that which might adversely affect the performance of the accessory. The supplier should provide a detailed case, including test evidence, if modifications are introduced, which are claimed not to constitute a substantial change.

8.3.6

Electrical Test after Installation

Tests made to demonstrate the integrity of the cable system as installed.

8.4

Test Cables and Transition Joint Characteristics

For the purpose of carrying out tests described in this document and recording the results, the cables and accessory shall be identified. The relevant characteristics as given in IEC 60141-1, IEC 60840 and IEC 62067 shall be known or declared.

8.5

Development Tests

Development tests are carried out to prove the main electrical and non-electrical characteristics of the transition joint. Details of such development tests shall be at the discretion of the manufacturer, examples of possible tests are given in the following clauses.

8.5.1

Electrical Development Tests

Electrical development tests can be adopted from the type test recommendations of this document, but may have increased test voltage levels. The duration of withstand tests as well as the number of impulses during impulse voltage test may be increased, too. Examples for electrical development tests: • AC voltage test • Partial discharge test • Lightning impulse voltage test. If new types of conductor connections are used as part of the transition joint design, the necessity for development tests of such connections should also be considered.

8.5.2

Non-Electrical Development Tests

Non-electrical development tests are considered to demonstrate the sufficient tightness of the external transition joint housing as well as the pressure maintaining barrier insulators between the different insulating fluids.

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During operation joint housings and barrier insulators are subjected to hydraulic pressure. Barrier insulators and related sealing systems may also be subjected to vacuum during the installation process. (Vacuum processing is often used during assembly work on paper-insulated cable accessories). Examples for non-electrical development tests: • Pressure test • Vacuum leak test • Thermo-mechanical test.

8.6

Routine Test

8.6.1

Extruded Cable Side of the Transition Joint

The prefabricated stress control component of a transition joint for U >30 kV (Um >36 kV) shall undergo partial discharge and voltage tests according to IEC 60840 or IEC 62067 using a test arrangement which may be chosen from the following: a) On a transition joint installed on a cable. b) By using a host accessory into which a component of a transition joint is substituted for test. c) By using a simulated accessory rig (in place of a cable) in which the electrical stress environment of a main insulation component is reproduced. In cases b) and c) the test voltage shall be selected to obtain electrical stresses at least the same as those on the component in a complete transition joint when subjected to the test voltages specified. Note: The prefabricated stress control component of a transition joint consists of the components that come in direct contact with the cable insulation and are necessary to control the electric stress distribution in the accessory.

8.6.2

Paper Cable Side of the Transition Joint

The hydraulic tests specified in IEC 60141 shall be made on each accessory to which the relevant clauses apply.

8.7

Sample Test

Due to the small numbers of transition joints which are expected to be supplied under single orders, sample tests will not normally be appropriate.

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Note: In special cases sample tests may be agreed between manufacturer and customer.

8.8

Type Test

8.8.1

General

The tests specified in this clause are intended to demonstrate the satisfactory performance of transition joints. The type test may be omitted: • As defined in the Range of Type Test Approval (see 8.8.2) or • If the transition joint is a combination of existing type tested accessories. • An example is a back-to-back transition joint. In the case of three-core cables or three-core transition joints then if the cables and cores within the joint are fully screened then it is permitted to carry out the electrical type tests on one core only, or on a single core joint of similar electrical design. Reference to Appendix B may be made to assist in determining the need for type tests. A summary of type tests on transition joints is given in Appendix C. Note 1: If suitable paper-insulated cable is unavailable, type testing will not be possible, thus approval of a transition joint design is dependant on agreement between manufacturer and customer, subject to consideration of any relevant test data. Note 2: In the event that breakdown occurs in the paper-insulated cable or within the joint and the primary cause is attributable to the quality of the paper-insulated cable then approval of a transition joint design is dependant on agreement between manufacturer and customer, taking into account the extent of tests passed and any other relevant test data.

8.8.2

Range of Type Test Approval

When a type test has been successfully performed on a transition joint for connecting cables of specific conductor cross-sections, and of the same rated voltage and construction, the type approval shall be considered as valid for a transition joint within the scope of these test recommendations with other conductor cross-sections, rated voltages and with other cables provided that all the conditions of a) to c) are met: (a) The voltage group is not higher than that of the tested transition joint.

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Note: In this context, transition joints of the same rated voltage group are those of rated voltages having a common value of Um, highest voltage for equipment, and the same test voltage levels as given in Table 8.1.

(b) The transition joint has the same or similar construction to that of the tested transition joint. Note: Transition joints of similar construction are those of the same type and manufacturing process of stress control elements and major insulation components. Repetition of the electrical type tests is not necessary on account of the differences of the cable insulation material, of the conductor or connector type or material, or of the protective outer covering, unless these are likely to have a significant effect on the results of the test.

(c) The calculated nominal electrical stresses within the main insulation parts of the transition joint and at the cable and accessory interfaces do not exceed those of the tested transition joint, or equal or higher electrical stresses at the relevant locations are well proven in other accessories. Note: Larger conductor cross-sections than tested are allowed within this limitation.

A type test certificate signed by the representative of a competent witnessing body, or a report by the manufacturer giving the test results and signed by the appropriate qualified officer, or a type test certificate issued by an independent test laboratory, shall be acceptable as evidence of type testing.

8.8.3

Type Test Arrangement

The transition joint shall comply with the tests specified in 8.8.4.1 and 8.8.4.2. The minimum length of free cable between accessories shall be 5 m. One sample of each transition joint type shall be tested. The accessory shall be assembled on the cables in the manner specified by the manufacturer’s instructions, with the grade and quantity of materials supplied, including lubricants and insulating fluids if any. There should be provision for measuring internal pressure of insulating fluid in the lapped cable compartment during the test. In units that are intended to operate with internal oil pressure, whether such pressure is from the cable system or a separate source, the maximum pressure during the test must not exceed the minimum design operating pressure +25%. If the

U Kv 30–33 45–47 60–69 110– 115 132– 138 150– 161 220– 230 275– 287 330– 345 380– 400 500

1 Rated voltage

Uo kV 18 26 36 64

76

87

127

160

190

220

290

145

170

245

300

362

420

550

3 Value of Uo for determination of test voltages

2 Highest voltage for equipment Um kV 36 52 72.5 123

Table 8.1 Test Voltages

435

330

285

240

190

131

114

4 Partial discharge measurement of 8.8.4.3 1,5Uo kV 27 39 54 96

580

440

380

320

254

174

152

5 Heating cycle voltage test of 8.8.4.4 2Uo kV 36 52 72 128

493

374

323

272

216

148

129

6 Heating cycle voltage test of 8.9.3.3 1,7Uo Kv 30 44 61 109

750 1050

– –

1175

1050

950

1550

1425

1175

1050

650

–

850

kV 170 250 325 550

8 Lightning impulse voltage test of 8.8.4.5 and 8.9.3.4

kV – – – –

7 Switching impulse voltage test of 8.8.4.5

580

440

380

320

254

218

190

2Uo kV 36 65 90 160

9 AC Voltage test after impulse voltage test of 8.8.4.5 and 8.9.3.4

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accessory includes an SF6 gas filled compartment then the gas pressure must be set so that at 20  C the pressure is no greater than the minimal functional pressure for insulation specified for the accessory +0.02 MPa. Neither the cables nor the accessories shall be subjected to any form of conditioning not specified in the manufacturer’s instructions which might modify the electrical, thermal or mechanical performance. During tests a) to f) of 8.8.4.2, it is advisable to test joints with their outer protection fitted. If it can be shown that the outer protection does not influence the performance of the joint insulation, e.g. there are no thermo-mechanical or compatibility effects, the protection need not be fitted.

8.8.4

Type Test Procedure

8.8.4.1 Test Voltage Values Test voltages shall be in accordance with the values given in the appropriate column of Table 8.1. Prior to the type tests of the transition joint, the insulation thickness of the extruded cable used shall be measured and the test voltage values adjusted, if necessary, as stated in IEC 60840 or IEC 62067. In case of difficulty in achieving impulse and ac test voltages for paper cable the test values may be agreed upon between manufacturer and customer. Note: If suitable oil filled cable of the required insulation thickness is not available then it is allowed to use a cable with a greater insulation thickness and to reduce the insulation thickness in the region where the joint is to be installed to the required level. As an alternative it is also allowed to adjust the test voltages in order to achieve the required electrical stress values.

8.8.4.2 Tests and Sequence of Tests Transition joints shall be subjected to the following sequence: (a) Partial discharge measurement at ambient temperature (see 8.8.4.3). (b) Heating cycle voltage test (see 8.8.4.4). (c) Partial discharge measurements (see 8.8.4.3). • At ambient temperature and • At high temperature The measurements shall be carried out after the final cycle of item b) above or, alternatively, after the lightning impulse voltage test in item d) below. (d) Switching impulse voltage test (required for Um 300 kV, see 8.8.4.5). (e) Lightning impulse voltage test followed by a power frequency voltage test (see 8.8.4.5). (f) Partial discharge measurements, if not previously carried out in item c) above.

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(g) Tests of outer protection for buried joints (see 8.8.4.6). (h) Pressure leak test: this test can be carried out on a separate sample of the transition joint. All components encapsulating the paper-insulated cable need to be assembled and a pressure and leak test performed (see 8.8.4.7). (i) Examination of the transition joint after completion of the above tests (see 8.8.4.8).

8.8.4.3 Partial Discharge Measurements The measurements shall be performed in accordance with IEC 60885-3, the sensitivity being 5 pC or better. Measured values are for information purposes only. The test voltage shall be raised gradually to and held at 1,75Uo for 10 s and then slowly reduced to 1,5Uo (see Table 8.1). When performed at high temperature, the test shall be carried out on the assembly which shall be heated until the cable conductors reach a steady temperature 0 K to 10 K above the maximum conductor temperature(s) in normal operation. The conductor temperature shall be maintained within the stated temperature limits for at least 2 h.

8.8.4.4 Heating Cycle Voltage Test Even though the extruded and oil filled cables may be of the same cross-section and voltage it is likely that the thermal characteristics of the cables are very different. Thus it is unlikely to be possible to heat the test assembly so that both cables achieve their required temperature, using conductor current alone. It is thus acceptable to use conductor current with the addition of heater tapes, thermal insulation or current heating of the sheath of one or both of the cables in order to ensure that the required temperatures are reached for both cables. See IEC 60840 or IEC 62067 for determination of actual cable conductor temperatures. The assembly shall be heated until the cable conductor in each case reaches a steady temperature 0 K to 10 K above the maximum conductor temperature in normal operation as specified in the relevant cable standard. The heating shall be applied for at least 8 h. The conductor temperatures shall be maintained within the stated temperature limits for at least 2 h of each heating period. This shall be followed by at least 16 h of natural cooling. The extruded cable shall cool to within the temperature defined for the type test heating cycle voltage test in IEC 60840 or IEC 62067. The conductor current during the last 2 h of each heating period shall be recorded. The cycle of heating and cooling shall be carried out 20 times. During the whole of the test period a voltage of 2Uo shall be applied to the assembly. Interruption of the test is allowed provided 20 complete heating cycles in total under voltage are completed.

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Note: Heating cycles with a conductor temperature higher than 10 K above the maximum conductor temperature in normal operation are considered valid.

8.8.4.5 Impulse Voltage Tests 8.8.4.5.1 Switching Impulse Voltage Test A switching impulse voltage test shall be carried out on transition joints of voltage Um 300 kV. The assembly shall be heated as stated in Sect. 8.8.4.4, until the cable conductors reach a steady temperature 0 K to 10 K above the maximum conductor temperatures in normal operation. These temperatures shall be maintained within the stated temperature limits for at least 2 h. Note: If, for practical reasons, the test temperature cannot be reached, additional thermal insulation may be applied. The impulse voltage shall be applied according to the procedure given in IEC 60230 with standard switching impulse withstand voltage levels according to Table 8.1. The transition joint shall withstand without failure 10 positive and 10 negative voltage impulses.

8.8.4.5.2

Lightning Impulse Voltage Test Followed by a Power Frequency Voltage Test The assembly shall be heated as stated in Sect. 8.8.4.4, until the cable conductors reach a steady temperature 0 K to 10 K above the maximum conductor temperatures in normal operation. These temperatures shall be maintained within the stated temperature limits for at least 2 h. The impulse voltage shall be applied according to the procedure given in IEC 60230 with standard lightning impulse withstand voltage levels according to Table 8.1. The transition joint shall withstand without failure 10 positive and 10 negative voltage impulses. After the lightning impulse voltage test, the assembly shall be subjected to a power frequency voltage test at 2Uo for 15 min (see Table 8.1). At the discretion of the manufacturer, this power frequency voltage test may be carried out either during the cooling period or at ambient temperature. No breakdown of the transition joint shall occur.

8.8.4.6 Tests of Outer Protection for Buried Joints These tests shall be performed according to IEC 60840 or IEC 62067 as appropriate, unless already covered by the range of approval for these tests as specified in the relevant standard.

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8.8.4.7 Pressure Leak Test 8.8.4.7.1 Leak Test The vacuum leak test is to be performed as per manufacturer’s instructions when applicable. 8.8.4.7.2 Pressure Test Apply 2 times rated internal pressure for 1 h. Leakage shall be detected at the end of this period by visual examination of the test specimen and by pressure drop. This test may be performed at the end of the type tests. No leak or rupture shall occur.

8.8.4.8 Examination Examination of the transition joint, whenever possible, by dismantling, with normal or corrected vision without magnification, shall reveal no signs of deterioration which could affect the system in service operation (e.g. electrical degradation, corrosion, harmful shrinkage or leakage, in particular across any seal separating the extruded and oil filled cables).

8.9

Prequalification Test

8.9.1

General and Range of Prequalification Test Approval

The tests specified in this clause are intended to demonstrate the satisfactory long term performance of transition joints. The prequalification test may be omitted: • If the transition joint is a combination of existing type tested paper-insulated and prequalification tested extruded insulation accessories. • If a transition joint of the same design has been prequalified for higher rated voltages. • For those accessories suitable for cables with insulation screen stress less than or equal to 4,0 kV/mm covered by IEC 60840. • If the manufacturer can demonstrate good service experience with transition joints of the same family with equal or higher calculated electrical stresses on the insulation screen of the extruded cable and in the main insulation. • If the manufacturer has fulfilled the requirements of an equivalent long term test following a national or customer specification on similar transition joints. It is recommended that prequalification of a new design of transition joint can be achieved by carrying out tests based on IEC 62067 prequalification test (▶ Chap. 4, “Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems” of this book) but may be installed in a laboratory as per the recommendations made in Cigré TB 303 for extension of a prequalification test. The details of this test are described in Sects. 8.9.2 and 8.9.3.

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A prequalification test certificate signed by the representative of a competent witnessing body, of a report by the manufacturer giving the test results and signed by the appropriate qualified officer, or a prequalification test certificate issued by an independent test laboratory shall be acceptable as evidence of prequalification testing. Note 1: If the manufacturer so wishes then a transition joint can be included in a full prequalification test to IEC 60840 or IEC 62067, in which case no additional prequalification testing will be required. In this test the transition joint must be placed in a rigid installation condition as this is generally the most onerous. Note 2: If suitable paper-insulated cable is unavailable, type testing will not be possible, thus approval of a transition joint design is dependant on agreement between manufacturer and customer, subject to consideration of any relevant test data. Note 3: In the event that breakdown occurs in the paper-insulated cable or within the joint and the primary cause is attributable to the quality of the paper-insulated cable then approval of a transition joint design is dependant on agreement between manufacturer and customer, taking into account the extent of tests passed and any other relevant test data.

8.9.2

Prequalification Test Arrangement

The minimum length of free cable between accessories shall be 5 m. One sample of each transition joint type shall be tested. The accessory shall be assembled on the cables in the manner specified by the manufacturer’s instructions, with the grade and quantity of materials supplied, including lubricants and insulating fluids if any. There should be provision for measuring internal pressure of insulating fluid in the lapped cable compartment during the test. In units that are intended to operate with internal oil pressure, whether such pressure is from the cable system or a separate source, the maximum pressure during the test must not exceed the minimum design operating pressure +25%. If the accessory includes a gas filled compartment then the gas pressure must be set so that at 20  C the pressure is no greater than the minimal functional pressure for insulation specified for the accessory +0,02 MPa. If the prequalification of the transition joint is to qualify the joint for use both in flexible and in rigid installations, the joint shall be installed in a rigid configuration. Otherwise the joint shall be installed in a flexible configuration. If the joint is installed for test in a rigid configuration, the manufacturer of the joint shall consider the aspects of the design which might affect operation in a flexible installation and subject to agreement between manufacturer and customer the prequalification shall apply to both rigid and flexible installations.

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Neither the cables nor the accessories shall be subjected to any form of conditioning not specified in the manufacturer’s instructions which might modify the electrical, thermal or mechanical performance.

8.9.3

Prequalification Test Procedure

8.9.3.1 Test Voltage Values Test voltages shall be in accordance with the values given in the appropriate column of Table 8.1. Prior to the prequalification tests of the transition joint, the insulation thickness of the extruded cable used shall be measured and the test voltage values adjusted, if necessary, as stated in IEC 60840 or IEC 62067. In case of difficulty in achieving impulse and ac test voltages for paper cable the test values may be agreed upon between manufacturer and customer. Note: If suitable oil filled cable of the required insulation thickness is not available then it is allowed to use a cable with a greater insulation thickness and to reduce the insulation thickness in the region where the joint is to be installed to the required level.

8.9.3.2 Tests and Sequence of Tests The normal sequence of the prequalification tests shall be as follows: • Installation of the transition joint which is subject to the prequalification on the relevant cables. • Heating cycle voltage test (see 8.9.3.3). • Lightning impulse voltage test (see 8.9.3.4). • Examination of the cable system with cable and accessories shall be carried out after completion of the tests above (see 8.9.3.5).

8.9.3.3 Heating Cycle Voltage Test Even though the extruded and oil filled cables may be of the same cross-section and voltage it is likely that the thermal characteristics of the cables are very different. Thus it is unlikely to be possible to heat the test assembly so that both cables achieve their required temperature, using conductor current alone. It is thus acceptable to use conductor current with the addition of heater tapes, thermal insulation or current heating of the sheath of one or both of the cables in order to ensure that the required temperatures are reached for both cables. See IEC 60840 or IEC 62067 for determination of actual cable conductor temperatures. The assembly shall be heated until the cable conductors reach a steady temperature 0 K to 10 K above the maximum conductor temperature(s) in normal operation. Note: If the conductor temperature exceeds the upper limit the test is still valid.

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The heating shall be applied for at least 8 h. The conductor temperatures shall be maintained within the stated temperature limits for at least 2 h of each heating period. This shall be followed by at least 16 h of natural cooling. The conductor current during the last 2 h of each heating period shall be recorded. The cycle of heating and cooling shall be carried out 180 times. A voltage of 1.7Uo (see Table 8.1) shall be applied to the assembly during the whole of the test period. Interruption of the test is allowed provided 180 complete heating cycles in total under voltage are completed. Note 1: The test period is determined by the time required to complete thermal cycles and will be a minimum of 180 days. Note 2: Heating cycles with a conductor temperature higher than 10 K above the maximum conductor temperature in normal operation are considered valid. Note 3: Partial discharge measurements are recommended to provide an early warning of possible degradation and to enable the possibility of a repair before failure.

8.9.3.4 Lightning Impulse Voltage Test The assembly shall be heated as stated in the preceding section, until the cable conductor reaches a steady temperature 0 K to 10 K above the maximum conductor temperature in normal operation. The conductor temperature shall be maintained within the stated temperature limits for at least 2 h. The lightning impulse voltage shall be applied according to the procedure given in IEC 60230. The assembly shall withstand without failure or flashover 10 positive and 10 negative voltage impulses of the appropriate value given in Table 8.1. No breakdown of the insulation or flashover shall occur. 8.9.3.5 Examination Examination of the transition joint, whenever possible, by dismantling, with normal or corrected vision without magnification, shall reveal no signs of deterioration which could affect the system in service operation (e.g. electrical degradation, corrosion, harmful shrinkage or leakage, in particular across any seal separating the extruded and oil filled cables).

8.10

Electrical Test after Installation

Tests on newly installed transition joints are carried out when the installation of the cable and its accessories has been completed.

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If required the new cable section with extruded insulation may be tested separately according to its relevant standard before the transition joint is installed. The test voltages recommended for general use are given in Table 8.2, however test regimes should be evaluated on an individual basis to take into account the condition of an existing cable system. A d.c. oversheath test according to clause 8.10.1 and an a.c. insulation test according to clause 8.10.2 are recommended.

8.10.1 DC Voltage Test of the Oversheath 8.10.1.1 New Cable Section with Extruded Insulation The voltage level and duration specified in clause 5 of IEC 60229 shall be applied between each metal sheath or concentric wires or tapes and the ground. 8.10.1.2 Existing Cable Section (Paper-Insulated) The voltage level and duration of the test should follow the local practice.

Note: Where it is required to test the oversheath of the two cable sections separately it is recommended to install a sheath sectionalised transition joint.

8.10.2 AC Voltage Test of the Insulation The a.c. test voltage to be applied shall be subject to agreement between the purchaser and contractor. The waveform shall be substantially sinusoidal and the frequency shall normally be between 20 Hz and 300 Hz. However if the capacitance of the cable is such that this cannot be achieved, then subject to agreement between purchaser and contractor, the minimum frequency may be reduced to 10 Hz. A voltage according to Table 8.2 shall be applied for 1 h.

Note: For installations, which have been in use, lower voltages and/or shorter durations may be used. Values should be determined, taking into account the age, environment, history of breakdowns and the purpose of carrying out the tests.

Alternatively, a voltage of Uo may be applied for 24 h. In addition to the a.c. voltage test, partial discharge measurements may be carried out, especially on the extruded cable part of the transition joint. The result should be recorded for information and future reference.

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Table 8.2 AC Test Voltages after Installation Rated voltage (kV) 30–33 45–47 60–69 110–115 132–138 150–161 220–230 275–287 330–345 380–400 500

Value of U0 (kV) 18 26 36 64 76 87 127 160 190 220 290

Existing cable 5 years (kV) 36 (2Uo) 52 (2Uo) 72 (2Uo) 128 (2Uo) 132 (1,73Uo) 150 (1,73Uo) 180 (1,4Uo) 210 (1,3Uo) 250 (1,3Uo) 260 (1,2Uo) 320 (1,1Uo)

Existing cable >5 years (see note) (kV) 29 (1,6Uo) 42 (1,6Uo) 58 (1,6Uo) 103 (1,6Uo) 106 (1,4Uo) 122 (1,4Uo) 152 (1,2Uo) 192 (1,2Uo) 228 (1,2Uo) 260 (1,2Uo) 320 (1,1Uo)

Note: the threshold of 5 years is indicative only; test regimes should be evaluated on an individual basis to take into account the condition of an existing cable system and local practices where these exist

Appendix A Considerations for Transition Joints for Other Types of Paper Cable The main body of this document specifically addresses transition joints connecting low pressure oil filled cables and extruded cables. The extruded cable types are those covered by IEC 60840 and IEC 62067. However three other main types of paper cable exist and are covered by IEC 60141 parts 2, 3 and 4. Specific differences which should be taken into account when testing transition joints for use on these types of cables are given in this Appendix.

A.1 Cables to IEC 60141-2: – Internal Gas-Pressure Cables and their Accessories for Alternating Voltages up to 275 kV Routine Test: – A hydraulic test as specified in the main body of this report should be carried out. In addition a gas leak test is required for the casing on the paper cable side of the joint at maximum operating pressure for 24 h. There shall be no leakage. AC Test Voltages for Heating Cycle Voltage Tests: – Test voltages with heating cycles are not specified in IEC 60141. The test voltages given in the main body of this report may be used. However the manufacturer should consider the values to be applied in relation to the known performance of the particular cable and adjust the AC test voltages if appropriate. Lightning Impulse Test and AC Voltage Test After Impulse Voltage Test: – As specified in IEC 60141-2 the lightning impulse test voltage is calculated according to the formula:

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• Test voltage ¼ 6Uo + 40 kV and the AC test voltage according to: • Test voltage ¼ 1,7Uo + 10 kV This results in the following values reported in Table 8.3. Note: These test voltages are lower than those specified in the main body of this document.

After installation test: the test may be carried out at Uo for 24 h. If a one hour AC test is proposed then the condition of the cable should be taken into account in determining the voltage. Nevertheless the test voltage should not exceed the value specified in Table 8.2 of the main document.

A.2 IEC 60141-3: – External Gas- Pressure (Gas Compression) Cables and their Accessories for Alternating Voltages up to 275 kV Routine Test: – A hydraulic test as specified in the main body of this report should be carried out. In addition the following tests to IEC 60141-3 are required where applicable: • A gas leak test is required for the casing on the paper cable side of the joint, if it is exposed to gas pressure in service, at maximum operating pressure for 24 h. There shall be no leakage. Table 8.3 Test Voltages, case of internal gas-pressure cables 1 Rated voltage U kV 30–33 45–47 60–69 110–115 132–138 150–161 220 to 230 275 to 287

2 Highest voltage for equipment Um kV 36 52 72,5 123 145 170 245 300

3 Value of Uo for determination of test voltages Uo kV 18 26 36 64 76 87 127 160

8 Impulse voltage test of 8.8.4.5 and 8.9.4.4 6Uo + 40 kV 148 196 256 424 496 562 802 1000

9 AC Voltage test after impulse voltage test of 8.8.4.5 and 8.9.4.4 1,73Uo +10 kV 41 55 72 121 141 161 230 287

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• If the accessory is subjected in normal service to small transient differences between oil pressure and gas pressure then the interface between the oil and gas regions shall be subjected to a gas pressure difference of 3 bar for 1 h. There shall be no leakage. AC Test Voltages For Heating Cycle Voltage Tests: – Test voltages with heating cycles are not specified in IEC 60141. The test voltages given in the main body of this report may be used. However the manufacturer should consider the values to be applied in relation to the known performance of the particular cable and adjust the AC test voltages if appropriate. Lightning Impulse Test And AC Voltage Test After Impulse Voltage Test: – As specified in IEC 60141-3 the lightning impulse test voltage is calculated according to the formula: • Test voltage ¼ 6Uo + 40 kV and the AC test voltage according to: • Test voltage ¼ 1,7Uo + 10 kV This results in the following values reported in Table 8.4. Note: These test voltages are lower than those specified in the main body of this document.

After installation test: the test may be carried out at Uo for 24 h. If a one hour AC test is proposed then the condition of the cable should be taken into account in determining the voltage. Nevertheless the test voltage should not exceed the value specified in Table 8.2 of the main document.

A.3 IEC 60141-4: – Oil-Impregnated Paper-Insulated High Pressure Oil- Filled Pipe-Type Cables and their Accessories for Alternating Voltages up to and Including 400 kV Routine Test: – It is recommended that a hydraulic test in accordance with IEC 60141-1 is carried out on the fluid filled side of the transition joint. There shall be no leakage. AC Test Voltages For Heating Cycle Voltage Tests: – Test voltages with heating cycles are not specified in IEC 60141. The test voltages given in the main body of this report may be used. However the manufacturer should consider the values to be applied in relation to the known performance of the particular cable and adjust the AC test voltages if appropriate.

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Table 8.4 Test Voltages, case of external gas-pressure (gas compression) cables 1 Rated voltage U kV 30–33 45–47 60–69 110– 115 132– 138 150– 161 220 to 230 275 to 287

2 Highest voltage for equipment Um kV 36 52 72,5 123

3 Value of Uo for determination of test voltages Uo kV 18 26 36 64

8 Impulse voltage test of 8.8.4.5 and 8.9.4.4 6Uo + 40 kV 148 196 256 424

9 AC Voltage test after impulse voltage test of 8.8.4.5 and 8.9.4.4 1,73Uo +10 kV 41 55 72 121

145

76

496

141

170

87

562

161

245

127

802

230

300

160

1000

287

Lightning Impulse Test And AC Voltage Test After Impulse Voltage Test: – As specified in IEC 60141-4 the lightning impulse test voltage is defined by the manufacturer of the cable. In practice the lightning impulse voltages given in Table 8.2 of this report are those normally used. The value of test voltage given in Table 8.2 for the AC voltage test after impulse voltage test is also recommended to be used. These values are recommended subject to agreement and consideration of the condition of the cable used for the test. After installation test: the test procedure as given in the main body of this document is recommended.

Appendix B Design Features, Performance and Necessity for Performing Type Tests for Transition Joints Transition joints might be either of innovative design, in which case the full scale development and type tests need to be performed or the joints might be constructed of well known and type tested components, in which case development and type tests are not necessary. This Appendix is a general description of constructional principles of some common types of transition joint (Cigré TB 89), with drawings and principal design

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features and is meant to give a general understanding and guidance to those studying this subject for the first time (See ▶ Chapter 1 of this book). In the last section of this Appendix a methodology is described which may be employed to assess the need for testing a particular new design.

B.1 Back-to-Back Transition Joint with Two Insulators The transition joint as shown in Fig. 8.1 comprises either: • Two GIS terminations in a common joint shell or • Two oil immersed terminations in a common joint shell. The terminations are in back-to-back arrangement and connected with a short length of busbar. In the case of GIS terminations the joint shell is filled by insulating gas (either with SF6 gas or mixture of SF6 gas and nitrogen). In the case of oil immersed terminations either cable oil, transformer oil or other insulating oil is used. Features Extruded and paper cable terminations are identical to terminations used in either SF6 switchgear or transformer applications and therefore further approval testing should not be necessary. Current carrying connection between two terminations may be specific to transition joint, in which case this may need separate evaluation. The electrical field design in the central region may be specific to the transition joint, however this may be assessed by electric field calculations if adequate test data is available. In the case of gas, a gas supply is required, and in case of oil the oil may be connected to the cable oil system or alternatively a header tank of some type could be used. Both systems would normally require some sort of fluid loss alarm. Joint shell

Gas or liquid immersed terminations

Fixing flange Insulated flange End metalwork

Plumb Insulator

Conductor stalk

Gas or insulating liquid Connector

Corona shields

Fig. 8.1 Single phase back-to-back transition joint with (2) insulators

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In the case of oil immersed terminations the chamber required may be very large, see EN 50299. In special cases, the busbar connection can be designed so that the two cables may be disconnected allowing independent high voltage commissioning tests on the two cables.

B.2 Back-to-Back Transition Joint with One Insulator Figure 8.2 shows example of the GIS or oil immersed type termination with rigid, epoxy or porcelain, insulator on the paper-insulated cable side. The insulator anchors cable conductors and centres the corona shield within the joint shell filled with insulating fluid (SF6 gas, mixture of SF6 and nitrogen or insulating oil). The insulator of the GIS/transformer termination is the barrier between insulating liquid of the paper-insulated cable and the insulating oil of the joint shell. The extruded cable end is terminated by a stress cone, which is directly immersed in the insulating fluid of the joint. It is necessary to seal the strands and sheath of the extruded cable conductor to prevent loss of insulating fluid. The arrangement of the joint with the rigid insulator at the extruded cable side can be utilized too. In this case the joint shell is filled with insulating liquid of the paper cable and the stress cone of the paper cable is directly immersed into this liquid. Features The terminations with and without rigid insulator are identical to terminations used in either SF6 switchgear or transformer applications. Current carrying connection between two terminations may be specific to transition joint. Corona shield (individual or collective) may be specific to transition joint. An oil reservoir might be required to control thermal expansion/compression of the insulating fluid. Joint shell

Gas or liquid immersed terminations Stress cone Conductor seal Fixing flange Insulated flange End metalwork

Plumb Insulator Gas or insulating liquid Connector Corona shields Conductor stalk

Fig. 8.2 Single phase back-to-back transition joint with one insulator

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B.3 Composite Type Transition Joint (Three-Core, Single Core) The composite type transition joint shown in Fig. 8.3 features central barrier, usually made of cast thermoset resin, which closely resembles the stop joint barrier employed to segregate pressure between single core liquid-filled cables. The barrier is cylindrical with an embedded metallic HV electrode, which is sealed to the conductor connection to form a seal between the two sides of the joint. The stress control on the paper cable is made of hand applied oil-impregnated paper tapes or a combination of hand applied paper tapes and thermoset resin stress cone. In the case of single core cables there is a channel at the connector to permit insulating liquid from the cable conductor duct to be fed into the joint shell on the paper cable side. For three core cables the oil feed comes directly from the core separation position near the end of the cable sheath. The extruded cable side of the transition joint is similar to a dry-type GIS termination, the stress cone and springs normally being identical to those used in GIS terminations. The elastomeric stress cone is sandwiched between the cable insulation and central barrier and the interface pressures are maintained by the springs. Features A common thermoset resin barrier with an embedded corona shield may be specific for use in the transition joint. The stress cone for the extruded cable may be identical to that used in dry-type terminations. The paper cable side may be identical to a stop joint in which case testing of the stop joint will be applicable. In the case of the extruded cable side, testing of similar dry type accessories may be considered in conjunction with electrical stress calculations.

Oil or gas filled paper cable

Fluid feed union Paper insulation Cast thermoset resin stress cone Cast thermoset resin barrier HV electrode Ferrule

Polymeric extruded cables

Plumb

Plumb Oil or gas

Joint shell Insulated flange

Compression device Semiconducting elastomer Insulated elastomer

Fig. 8.3 Single phase composite, fed-type transition joint

Elastomeric moulded stress cone

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The current carrying connection between two cables may be specific to the transition joint.

B.4 Single-Core or Three-Core Type with Bushing The same electrical design applies to single or three core versions of this joint. A central barrier plate with a single bushing or multiple bushings separates lapped cable joint side from the side with extruded cable. The barrier and bushings that are usually made of premoulded thermoset resin are designed to withstand operating and test pressures required for the lapped cable. The bushing is connected to the extruded cable in the form of extruded cable joint that can be of various designs, such as taped insulation, premoulded elastomeric body, heat shrink sleeve, etc. The joint on the paper cable side is usually insulated with either impregnated plain or crepe paper tapes. The joint shell on the paper cable side is filled with the insulating liquid of the paper cable. Features Thermoset resin bushing may be similar to one used in stop joints. On the paper cable side the design is usually identical to a stop joint and thus previous testing of a stop joint may be applicable. Interface of insulation with the bushing on the extruded side is specific to transition joint, however standard premoulded components or taping methods may be used. The current carrying connection between the bushing and paper cable is usually the same as in the stop joint. Likewise, current carrying connection between bushing and extruded cable may be the same as the connection in extruded cable joint. In these cases testing of these items will already have been carried out for the standard joints. Electrical testing of a single core can be considered valid for three core designs, only thermomechanical and pressure characteristics of the casings need to be considered when moving from single to three core versions (Fig. 8.4). Oil or gas filled cable

Paper insulation

Insulated conductor rod Cast thermoset resin bushing Insulation (tape etc.) Polymeric extruded cables

Plumb

Plumb Barrier plate Oil or gas

Spacer

Joint shell

Fig. 8.4 Three-core transition joint with the bushing

Insulated flange

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B.5 Methodology for Assessing Test Requirements In assessing the need for type and/or prequalification tests it is first necessary to consider the proposed design and to evaluate which parts of the accessory are similar to established accessories or parts of established accessories (for example the oil filled side of a transition joint might resemble an established stop joint). Then, based on available knowledge or lack of knowledge of performance of the particular design feature, the confidence in performance in certain test conditions should be evaluated. This can be done by considering the range of service and related test conditions which have to be met. The following table gives an example for this for a fictitious design. The need for test can then be judged based on the number of entries on the “Type Test Required” column.

Appendix C List of Type and Prequalification Tests of Cable Systems Type tests of transition joints are covered by paragraph 8.8 (Table 8.5). Table 8.6 gives a summary and references for type testing of transition joints. Prequalification tests of transition joints are covered by paragraph 8.9. Table 8.7 gives a summary and references for prequalification testing of these transition joints.

Appendix D Transition Joint Experience Data Part of the terms of reference of WG B1-24 was to review the range of transition joints currently available. To this end the WG has carried out a survey amongst its members to investigate the types of transition joint used, their availability and number in service in the members’ countries. The results of this survey are presented in the following table (Table 8.8):

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Table 8.5 Example of evaluation of need for performing certain type tests of novel transition joint Condition (examples – others might be chosen in practice) PD initiation Breakdown at AC withstand voltage Breakdown at DC withstand voltage Breakdown at impulse withstand, at ambient Breakdown at impulse withstand, hot Ionization initiation in paper insulation

Confidence Level High Low X X

Type Test Required? Yes No X X

Reason for confidence-level rank & remarks (typical comment shown as an example)

X

X

Thermoset resin barrier is new

X

X

X

X

X

X

Voltage breakdown in oil (or gas) in shell Voltage breakdown in termination Thermal runaway of centre connector Mechanical or thermal failure during short-time current test

X

X

X

X

X

X

X

X

Breakdown at AC withstand voltage after short-time current test Mechanical or thermal failure of ground connections during shorttime current test Voltage breakdown of the shield break during load cycling in water Pressure and leak test

X

X

Load cycling

Electrical stress in paper insulation is influenced by the stress cone design. Test is not required if PD level is acceptable. Thermoset resin barrier is new

Manufacturer to evaluate necessity of performing these tests as development tests based on past experience.

X

X

Metallic shield restoration has been individually tested

X

X

Jacket restoration and shield-break have been individually tested

X

X

Material and dimensions of the shell and the sealing system are critical for pressure and leak test. No test is required if previously tested on similar design

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Table 8.6 Type tests on transition joints Item a b c d e f g h i j k l

Test General Range of type approval Type test arrangement Test voltage values Tests and sequence of tests Partial discharge measurements Heating cycle voltage test Switching impulse voltage test (for Um 300 kV) Lightning impulse voltage test followed by power frequency voltage test Leak test Pressure test Examination

Clauses 8.8.1 8.8.2 8.8.3 8.8.4.1 8.8.4.2 8.8.4.3 8.8.4.4 8.8.4.5.1 8.8.4.5.2 8.8.4.7.1 8.8.4.7.2 8.8.4.7

Table 8.7 Prequalification tests on transition joints Item a b c d e e f

Test General and range of prequalification test approval Prequalification test arrangement Test voltage values Tests and sequence of tests Heating cycle voltage test Lightning impulse voltage test Examination

Clauses 8.9.1 8.9.2 8.9.3.1 8.9.3.2 8.9.3.3 8.9.3.4 8.9.3.5

Table 8.8

Yes Yes Yes No No No No Yes Yes No No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

3C HP Gas to SC XLPE SC LPOF to SC XLPE

3C LPOF to SC XLPE 3C IPG to 3C Pipe XLPE 3C GC to 3C Pipe XLPE 3C IPG Gas to SC XLPE 3C GC to SC XLPE SC LPOF to SC XLPE 3C LPOF to SC XLPE 3C HP Gas to 3C Pipe XLPE 3C HP Gas to SC XLPE 3C MIND to SC XLPE SC LPOF to SC XLPE/EPR SC LPOF to SC XLPE 3C LPOF to SC XLPE SC LPOF to SC XLPE 3C LPOF to 3 SC XLPE 3C MIND to 3 SC XLPE SC LPOF to SC XLPE 3C LPOF to SC XLPE SC LPOF to SC XLPE 3C LPOF to SC XLPE SC IPG to SC XLPE 3C IPG to SC XLPE SC LPOF to SC XLPE 3C HPOF to SC XLPE 3C IPG to SC XLPE

5 0 0 0 0 3 30 0 0 80 0 100 300 5 51 12 15 4 500 250 20 0 Unknown Unknown Unknown

1 0

0 0 0 0 0 0

No. in service

Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes No Yes No No Yes No Yes Yes Yes Yes Yes Yes Yes

No Yes

Yes Yes Yes Yes No No

110-219kV Available

5 7 2 40 40 0 1 2 1 0 27 30 0 51 0 0 10 0 65 75 30 0 10-20 20-30 At > Am n…number of A-spots

Fig. 10.8 Definition of the different contact areas in a real connection

Conductor 1

Area of homogeneous current-flown material  RM1

Constriction of the current flow line at the a-spot  RC

Conductor 2

Area of homogeneous current-flown material  RM2

R M1

RC

RJ

R M2

Fig. 10.9 Resistivity equivalent circuit of a connection (schematic drawing – microcontact)

mechanically loaded which is again larger than the area which is electrically conducting. Within the A-spots there are conducting areas, where the metal of the conductors is in true contact, quasi conducting areas where some surface impurity layer is still present, giving limited conductivity, and insulating areas which are in mechanical contact but provide no significant conduction. • Sphere and ellipsoid model [14]: this is a development of the A-spot model. Holm considered that the A-spots cause a “restriction” of current flow in the vicinity of the contact, see the red lines in Fig. 10.9, representing the current flow in a single A-spot contact between two cylinders. An increase in resistance compared to that for a solid rod is considered to be caused by the increase in current density close to the A-spots. Equivalent Network for Connections with Stranded Conductors Equivalent circuit with infinitesimal resistances: Fig. 10.10 shows a schematic of an element of current flow from a connector body, through a microcontact and then via the cable conductor. A simple electrical model of a connection is provided by Möcks

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Test Regimes for HV and EHV Cable Connectors

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Fig. 10.10 Electrical network of compression type connection [19–21]

[21]. The model divides the connection in infinitesimal elements of the length dx, each consisting of the material resistances R1 and R2 per unit length of the jointed conductors and a resistivity Rq. This resistivity is assumed to be equally distributed along the connection. The Kirchhoff’s laws are applied to the circuit and a differential equation is formed for the voltage ux. An expression for the joint resistance Rj is derived from the solution of the differential equation [21]: 
 R þ R cosh ðαsÞ ð R1  R2 Þ R1 αs 2 tanh R sþ þ 1 Rj ¼ α 2 R1 þ R2 2 α sinh ðαsÞ

ð10:1Þ

with rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R1 þ R 2 α¼ Rq

ð10:2Þ

Equation (10.4) may be rearranged so that an expression for the ratio k0 is obtained 0

1 pffiffiffiffiffi a 2 A Rq k0 ¼ @ þ c tanh pcffiffiffiffi c sinh pcffiffiffiffi Rq

Rq

ð10:3Þ

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M. Uzelac

with   R1 R2 þ R2 R1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi c ¼ s K F ð R1 þ R2 Þ a¼

If the resistivity Rq fulfills the condition in Eq. (10.4), Eq. (10.3) may be solved for resistivity Rq analytically: Rq  s2 ðR1 þ R2 Þ Rq ¼

h

c 0 k a

i2

ð10:4Þ ð10:5Þ

In case the condition in Eq. (10.4) is not fulfilled, Eq. (10.3) may only be solved numerically. In both cases, a solution for the characteristic value Rq of a connection may be calculated. For this purpose, the material resistances of conductor and sleeve and the geometry of the compression connection must be known. The resistivity Rq provides the opportunity to rate the electrical performance of a connection independently from the material resistances. Thus, the resistivity Rq is related to the contact resistance of a connection directly. Of course, due to the constraints of the electrical model it may only be applied for those types of connections that meet the following conditions. The electrical model implies the assumption that the distribution of the a-spots in the connection is almost homogenous in axial and radial direction. Of course, the resistivity Rq integrates the influence of all a-spots in a connection regardless of their actual size and location or whether there is more than one actual contact surface. Thus, it cannot be separated, whether a rise in the resistivity Rq stems from the a-spots between conductor and sleeve or from those a-spots between the single strands. Nevertheless, the value of Rq is a suitable criterion to analyze and compare the electrical behavior of different designs of compression connectors. However, the derived ratio k’ still contains a part of the material resistances and is therefore no autonomous criterion to rate the electrical performance of different connections. Further conclusions can be found in [21].

10.3.1.1 Aging of Electrical Connections Aging mechanisms (force reduction, chemical reactions, inter-diffusion, rubbing wear, electro-migration) are discussed in the following references: [16, 17, 22, 25] and the mechanisms and their effects are given in Table 10.3. The dominant aging mechanism depends on type of connection. 10.3.1.2 Comparison of Material Properties The Table 10.4 below lists basic properties of Al and Cu materials used in manufacturing cable conductors. – Oxide layers and their characteristics (Table 10.5)

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Test Regimes for HV and EHV Cable Connectors

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Table 10.3 Aging mechanisms of current-carrying connections [25] Mechanism Chemical reactions Chemical interaction between conductor material environment

Force reduction Change in texture of material according to temperature and mechanical stress

Rubbing wear Change surface properties by relative movement Interdiffusion Diffusion of different metals into each other

Electromigration Directional material transport due to high current density and current density gradient

Effect Growth of high resistivity oxide or impurity layers- galvanic corrosion at bimetal connections ➔Decrease of the conducting contact area, Rj" Especially under extreme environmental conditions (e.g., industrial atmosphere, high humidity, salt fog) Decrease of the joint force ➔Loss of the mechanical integrity of the a-spots, mechanical strength#, RV", especially at clamped or bolted connections (dominate force closure) Wear of coatings ➔Generation of rubbed-off particles which sediment in the contact area, Rj" Especially at plug-in connections Growth of intermetallic compounds (IMP) with different electrical and mechanical properties ➔Higher resistance at the contact area, Rj" Depending on material combination Formation of holes and accumulate of material at irregularities of crystal structures ➔ higher defect density ➔Decreasing of the a-spot ampacity, Rj" Especially for direct current devices but also at alternative current with high current density

Table 10.4 Material properties of copper and aluminum [24] Property/material κ in MS/m (20  C)

Cu-ETP CW004A Min. 57

Al 99.5 EN AW-1350A 34–36

αT in 1/K Melting temperature ϑs in  C ρ in g/cm3 E in kN/mm2 Rp0.2 in N/mm2

0.00381 1083 8.93 110 Min. 180

0.004 ca. 660 2.71 65 –

Rm in N/mm2

Min. 250

Min. 60

H in HBW

65–90

Typ. 20

αL in 106 K1 (20–200)  C λ in W/Km

17.7 394

23.8 215–235

AlMgSi alloy EN AW-6101B 30–34 (T6  30) (T7  32) 0.004 ca. 660 2.70 69 Min. 160 (T6) Min. 120 (T7) Min. 215 (T6) Min. 170 (T7) 70 (T6) 60 (T7) 23.4 215–225

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Table 10.5 Properties of oxide layers [22] Property/material ϑs/ C δ/g/cm3 κ/mm2/(Ω∙m) H/HV αL/1/(K∙106) (20–200) C λ/W/(m∙K)

Cu-ETP CW004A 1083 8.93 Min. 57 90 17.7 394

Al99.5 EN AW-1350A 660 2.71 34–36 21–48 23.8 215–235

Cu2O 1242 5.8–6.11 1.82∙1011 109–189 – 6.3–8

Al2O3 2050 3.97 1012–1014 1730–2060 6.2 40

Fig. 10.11 Typical compression connector for HV outdoor terminations

10.3.2 Connector Construction and Types for HV and EHV Extruded Cables Unlike connectors for medium voltage which are a commodity product, the connectors for high voltage applications are carefully selected for a particular application in the cable system. There are many designs of cable connectors for high voltage applications. Some of those are described in this section. Compression type connectors are used most commonly for HV cable accessories, but lately mechanical (shear-bolt) types are gaining ground due to the ease of installation which does not require any special tools.

10.3.2.1 Compression Type Connectors The most common cable connector in HV applications is compression type often referred to as crimp type. The connector features a hollow cylindrical section, called the ferrule as shown in Fig. 10.11 (see also ▶ Chap. 1, “Compendium of Accessory Types Used for AC HV Extruded Cables”) of this book, barrel or sleeve. The cable conductor is fed into the ferrule and the connection between connector and conductor is obtained by pressing the ferrule outer diameter with the appropriate compression (or crimping) tool. 10.3.2.1.1 Compression Connector Design The material of the connector matches that of conductor except that hardness of the connector material has to be appropriate for the method of crimping. For example, most aluminum crimp connectors are made from soft aluminum and are used on conductors made of either hard or soft aluminum. Softer material is easier to

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Test Regimes for HV and EHV Cable Connectors

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compress and will not crack while being crimped. The connector designer has to take into consideration the increase of material hardness during crimping operation. The length of the ferrule and its inner and outer diameters depend on many factors including conductor material, size, and crimping method. Typically, the inner ferrule diameter is selected to be a tight fit to the bare conductor diameter. The cross-sectional area of the connector does not necessarily need to match effective area of the conductor. In general, the cross-section of connectors for copper conductors is somewhat smaller than the effective area of the conductors while aluminum connectors have significantly larger area than the associated conductors. In the case of copper connectors, a smaller area is possible due to a higher heat dissipation from the connector surface. In the case of aluminum connectors, a larger area is required for mechanical reasons. The length of the ferrule is very critical. It establishes the length of engagement between the cable conductor and the connector. Some of the variables that influence selection of the length of the ferrule include the crimping method, the size of the crimping dies (if used), the number of crimps, the type of connector material, and the conductor size, material, and construction. The longer the ferrule the better connection between the conductor and connector may be achieved. On the other hand, due to constraints involving cable accessories, the goal of a connector manufacturer is to make the ferrule as short as possible.

10.3.2.1.2 Crimping Tools There is variety of crimping tools on the market, but it is crucial that only the crimping tool that has been used in connector testing is also used for the field installation. Many connector manufacturers make their own crimping tools. Some accessory manufacturers who make their own connectors also make crimping tools. There are different crimp tools for deep indent and circumferential die connector designs. The deep indent tool features either one, two or four rams (pins). The rams penetrate connector compressing it to the conductor. Figure 10.12 shows the indent crimping tool with four rams 90 apart. The distance between and number of successive sets of indents as well as the depth of indents are specified by the connector manufacturer. The depth of each indent is vital for the quality of the connection and must meet requirement specified by connector manufacturer. The depth of each indent is measured in the field after crimping. The distance between the tips of opposing rams in the crimping tool head is adjustable. This feature allows the tool to be used for a wide range of connector sizes. Due to its universal usage and relatively easy handling, this tool has gained popularity in the USA. The crimping tools that make circumferentially shaped crimps on large size connectors feature a crimping head with interchangeable dies for variety of connector sizes. There are also die less crimping tools which are typically for use with smaller size connectors. The size of the crimping tool is determined by the force (in Tonnes) that is imposed on the movable half of the crimping die. The “tonnage” of crimping tools

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Fig. 10.12 Four-ram, deep-indent press and crimp connector for 2000 mm2 Cu cable

Fig. 10.13 200 ton press with hex-dies and crimp connector on 2500 mm2 Cu cable

varies from few tons to few hundred tons. Figure 10.13 shows a 200 ton crimping tool used for the connector for a 2500 mm2 copper conductor. The pressure of the die on the connector depends on the force, the width of the crimping surface of the die, and shape of the die (hex, circular, oval. . .). A good crimp is achieved when die is fully closed. In that case the crimped connector under the die forms to the shape of the die. The die design usually allows for flow of the metal. Some connector manufacturers characterize crimp efficiency of specific die set by the crimp ratio, which is the ratio between the area of closed die set and sum of

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Test Regimes for HV and EHV Cable Connectors

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effective cross section area of the cable conductor and connector ferule. The following formula defines the crimp ratio. CR ¼

Adie  100 ACond þ Aferule

The procedure for connector installation should be provided for each project. The procedure should specify starting point of the crimp, the number and sequence of crimps, the distance between crimps, rotation of the crimping head, and the necessity for full closure of the dies. The procedure should also include preparation of the conductor for crimping, for example, cleaning the conductor and necessity for removing fillers and/or cleaning individual strands in case of strand filled or enameled conductors.

10.3.2.2 Mechanical Connectors (Shear-Bolt) The reliability of conductor connections has historically depended on the skill and experience of the jointer. The introduction of mechanical connectors has been primarily to reduce the skill required and hence to minimize variability in the connections and thus to improve reliability. Other features which are also beneficial: • There is a reduction of the time to install the connection • No special tools or equipment such as presses are required • No heat or flames are involved, improving safety The mechanical connectors are commonly referred to as “shear bolt” connectors, the name indicating the key feature of such connectors which makes them much less influenced by the skill of the jointer. The shear bolts (the number, size, and material depending on the connector design and conductor size) are designed to break at a specified torque value controlling the tightening torque of the bolts. Different types of shear bolt are used. In some, the torque is determined by a reduced diameter region of the bolt, where it breaks. In another type a special double bolt is used which has a steel screw inside a brass body that is internally and externally threaded. In this latter design the brass component breaks off flush with the surface of the connector body minimizing any work necessary to produce a flush profile (which facilitates the design of the joint body itself). The connector body is usually internally grooved or threaded so that the cable conductor is forced into contact with the crests of the thread giving a high local contact pressure and a good electrical contact. The connector body is frequently made of aluminum alloy, which guarantees high elasticity of the cylindrically shaped body and mechanical resilience of internal grooves or threats. These properties enable connector body to deform cable conductor and stabilize its movement in radial direction, thus achieving and maintaining maximum thrust-forces introduced by the shear bolts. The bore of the connector body is often tin-plated and lubricated and further oxidation of the aluminum material is thus prevented. For tin-plating, the surface

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oxide layer on the connector body is removed by dipping in a caustic bath. Tin is very ductile. During installation of the conductor, the tin is pushed aside while the peaks of the threads are cutting into the conductor surface. The lubricant remains in the “valleys” of the threads or grooves of the body preventing further oxidation of the just created metal contact areas between connector body and conductor. Tin-plating of the threated holes for the shear-bolts in the connector body controls the coefficient of friction during fastening of the shear bolts. It may be required that for HV and EHV joints the outer diameter of the body of the connector matches insulation diameter (see Fig. 10.6). Hence, the wall thickness of shear-bolt connector bodies for HV and EHV joints is typically much bigger comparing to MV-applications (Fig. 10.14). The thicker the connector wall the longer is the length of a threaded hole for the shear bolt, and higher force from shear bolt to conductor may be achieved. This increases design margin for the yield strength and decreases possibility of relaxation of compression force in service. It also has to be kept in mind, when designing shear bolt connectors, that the bigger the conductors are, the higher radial forces must be applied to break oxide layers of stranded aluminum conductors. A given mechanical connector design may be adopted to a specific slip-on joint on specific cables by modifying body of the connector and keeping the design and number of shear bolts constant for a specified conductor. Features of such connector are: • Outer diameter of the connector body closely matches the outer diameter of prepared cable insulation to minimize the slip-on step. By leaving almost no “air” between connector and the rubber body of the joint, the heat transfer from connection through the connector and the joint is maximized and Fig. 10.14 Four different types of mechanical connector

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Test Regimes for HV and EHV Cable Connectors

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Fig. 10.15 A mechanical connector on a 2500 mm2 Milliken conductor

By increasing the OD of connector body (to match cable insulation OD), the electrical resistance of the connection is lowered. • The inner diameter of the connector is tight fit to actual conductor in the project to minimize the gap between cable conductor and connector body. The design of the face of the shear bolt that presses on the cable conductor also varies; some designs avoid rotation of the face against the conductor to prevent strand damage, whereas others have direct contact from flat, curved, or conical ends. Mechanical connectors with aluminum bodies can be used on both copper and aluminum conductors although conclusions as to their long-term suitability for the latter are not yet generally available [26]. The above features can be seen in Fig. 10.14, which is provided courtesy of European Manufacturers. Mechanical connectors are widely used for LV and MV accessories, sizes suitable for conductors from 25 mm2 up to 630 or 1000 mm2 being widely available. Development of designs suitable for HV conductors up to 2500 mm2 is also in progress. To achieve satisfactory connection of such large conductors can require a large number of shear bolts in a relatively long connector body. An example of a 2500 mm2 mechanical connector is shown in Fig. 10.15. The picture depicts connector secured to cable conductors but the bolts need yet to be sheared off during final installation.

10.3.2.3 MIG and TIG Weld Connectors A creep phenomenon can take place as an effect of loading cycles when crimp-type connectors are used for large size cable conductors. Current-cycling tests with aluminum compression connectors with large aluminum conductors have shown an increase of electrical resistance between cable conductor and connector after several cycles. The creep leads to reduction of the contact pressure between connector and conductor, with a corresponding increase of the electrical resistance. This causes increased losses and hence unstable thermal behavior, ultimately resulting in thermal failure. As a result, the preferred method of joining large aluminum conductors is welding. The practice in the USA is to use welded connectors for aluminum

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conductors above approximately 1250 kcmil (~630 mm2) while for smaller aluminum conductors a crimp connection is utilized. Welded connections fuse two cable conductors (in case of cable joints) or cable conductor and connector (in case of cable terminations) by application of molten metal. Either the Metal Inert Gas (MIG) or Tungsten Inert Gas (TIG) welding process is adopted in these cases. The MIG welding process, which generates less heat during the welding operation, is commonly used in the USA for large aluminum cable conductors. Welded connections are also used for copper conductors with enameled strands. If crimp connectors were used on these conductors, the enamel coating would need to be removed from every strand, which is a quite elaborate and time-consuming process. As significant heat is generated during welding, care should be taken that cable insulation is not overheated. It is necessary to dissipate excess heat to keep temperature of the adjacent conductor and surrounding cable insulation below a specified value. Once the cable conductor reaches the maximum allowed temperature, the welding operation has to be interrupted to let conductor cool down. This is why installation of welded connections is more time consuming than for other connection types. Only trained, highly skilled operators using appropriate welding and cooling equipment can perform this task. Efficient cooling speeds up the process. Use of heat-sink clamps in combination with forced air or gas flow is the most common method. In some cases, water is used as coolant for extruded cables. In case of oil filled cables, the cooling system is more elaborate since it has to incorporate provision of vacuum application in order to remove the oil from the strands in the weld area. The strands of the cable conductor have to be thoroughly cleaned to remove any insulating tapes or cable conductor filler material before welding. If these are not removed, the weld is contaminated and becomes porous which results in reduced conductivity and low mechanical strength of connection. As shown, the procedure for welding power cable conductors is elaborate. That is one of the reasons why commercial installations of underground cable with large aluminum conductors only began after methods for field-welding the conductors were developed and proven. 10.3.2.3.1 MIG Welded Connector for HV Joints Two cable conductors can be welded to each other with or without use of a connecting sleeve. The weld itself provides an electrical connection that is equivalent to the conductor and is not subject to instability during heat cycles due to decrease of contact pressure. However, the tensile strength of the welded connector is significantly (50–60%) lower than the ultimate tensile strength of the conductor due to annealing of the conductor near the weld. If necessary, mainly for submarine cables, the tensile strength can be improved by round-compressing the conductor and the weld (hardening process) and/or using a connecting sleeve, which is crimped to both cable conductors to bridge the weld and provide mechanical strength.

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Fig. 10.16 Aluminium MIG-Weld Connector for HV cable terminations

The conductors to be joined are cut in a wedge shape, and then buttered with weld metal, and the two “V” grooves are filled until the conductor diameter is reached. As a result, this connection is flush with the conductor, which is necessary when the joint must have a minimum diameter, such as in laminated paper cable joints. 10.3.2.3.2 MIG Welded Connectors for HV Terminations Figure 10.16 shows a typical MIG weld connector for HV cable terminations where a high tensile strength of the connection is required. The connector features a ferrule to be crimped to the cable conductor and provide additional mechanical strength. Electrical connection is established with molten metal applied between conductor and connector through weld “windows” in the connector barrel. The connector stalk (stud) is threaded and secured with a threaded hood at the termination top plate, ensuring that the cable will not slip due to its own weight and the weight of other termination components. The cable conductor to be joined is cut in a wedge shape, and then buttered with weld metal as shown in Fig. 10.17. The connector is slipped over the prepared conductor and windows in the ferrule aligned with the wedge shaped and buttered conductor. The two windows are filled until the connector diameter is reached. As a result, this connection is flush with the connector. The weld provides an electrical connection that is equivalent to the conductor. For additional pull out strength, the ferrule is crimped to the conductor.

10.3.2.4 Exothermic Welded Connections An exothermically welded connection is produced using a welding process that employs molten metal to permanently join the conductors. They are widely used in MV and some HV applications. The process employs an exothermic reaction of a thermite composition to heat a metal powder together with the conductor and conductor/ferrule to be joined. Thermite is a pyrotechnic composition of metal powder, which serves as fuel, and metal oxide. It requires no external source of heat or current. These connections can have excellent current-carrying and short circuit capacity, do not loosen up in service, and do not corrode. They form a permanent, low resistance connection, provide a molecular bond, and do not deteriorate with age.

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MIG weld Al connector

“V”-shape cut and “buttered” cable conductor

Cutting and “buttering” heat-sink jig

Welding heat-sink jig Weld

Thermocouple location

Fig. 10.17 Steps in installation of MIG weld connector for HV cable termination

The exothermically welded connection requires lesser length of exposed connector from that required by other connector types, for example, crimp or mechanical connector. Less than 100 mm of conductor needs to be exposed at each cable end. Fillers and nonmetallic materials must be removed before making the connection. Environmental conditions have to be considered when preparing and making a connection. The handling of exothermic weld powder must be treated with care and in accordance to manufacturer’s instructions. Exothermically welded connections are applicable for all types of HV cable conductors and materials: • Round and stranded Al or Cu conductors • Segmented Al or Cu conductors • Solid Al or Cu conductors Figure 10.18 shows different types and material combinations of exothermically welded connections for HV cable terminations. Inside of the connector the area should be melted homogenously without any cavities. Single wires should migrate into the melted area transition-free and seamlessly. Combinations and transition of different types of conductors and/or materials are also possible. Figure 10.19a shows connection of two aluminum stranded conductors of different size. After it is made, each exothermically welded connection must be visually inspected at the site. The condition of the outer surface must comply with visual inspection requirements of the connector manufacturer. Relationship between connector quality and condition of the outer surface of the connection has been established by manufacturer during development tests. Figure 10.19b shows cut through the connector that successfully passed

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Fig. 10.18 Aluminum and copper exothermically welded connector for HV cable terminations

Fig. 10.19 Aluminum and copper exothermically welded connections for HV cable joints

development tests. Notice that there are very few porosity holes inside the connector as well as at the outside holes. Each connector manufacturer has their own criteria in size and population of the porosity holes and depressions. The equipment for making exothermically welded connection consists of crucible made from graphite and unique ceramic fiber smoke filter system as shown in Fig. 10.20. The filter prevents sparks and reduces generation of dust and other emissions to acceptable level, even if used in unventilated manholes and cable tunnels. The installation equipment is portable with no external source of power required.

10.3.2.5 Copper Brazing Copper brazing is often used in submarine cable joints especially in factory joints and repair joints because they must have almost same diameter as the cable before the armor is applied. Compression type connector and mechanical (shear-bolt) connector are not used for factory joints because they would add up to the diameter and obstruct further manufacturing.

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Fig. 10.20 Equipment for making an exothermically welded connection

Like welding, brazing and soldering are important methods of thermally joining metals. Both welding and brazing/soldering lead to the formation of a metallic joint; however, the chemical composition of these joints differs. Whereas a welded joint has the same chemical composition as that of the two identical parent metals being joined, the use of a filler alloy in a brazing or soldering procedure means that the brazed or soldered joint has a different chemical composition to that of the parent materials. Brazing and soldering do not involve any melting of the parent material, that is, of the surfaces to be joined. Instead, the work pieces are joined by introducing an additional molten metal, the “filler metal,” possibly in combination with a flux and/or in a protective gas atmosphere. Some of the advantages of brazing or soldering compared to welding are: • As less heat is applied in the joining process, brazed or soldered parts tend to exhibit greater dimensional accuracy and less distortion. • Multiple brazed/soldered joints can be created on a single work piece in a single operation. • Intricate assemblies can be brazed/soldered without damage. • Brazed/soldered joints exhibit good thermal and electrical conductivity. • As brazing/soldering directs less heat into the joint than welding, there is less residual stress and distortion in the component. The following points should, however, be noted: the strength of a brazed or soldered joint is typically not as great as that of the parent material; the parent metal

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and the braze/solder metal have different chemical potentials; there is a risk of chemical corrosion due to the presence of flux residues; extensive preparatory and after-treatment procedures are often required, such as degreasing, etching, removal of flux residues. The related joining techniques of brazing and soldering are distinguished in the DIN ISO 857-2 standard by the liquid temperature of the filler metal used. In soldering, the liquid temperature of the filler metal is below 450  C; in brazing it is above 450  C. Brazing is used if the joint will be subjected to high mechanical and thermal stresses. When brazing copper, the filler metals of choice are brass brazing alloys, copper-phosphorus, and silver brazing alloys. Silver brazing filler metals have lower brazing temperatures, which reduces the risk of forming coarse grains and enables faster brazing speeds. Copper brazing procedure is shown in Fig. 10.21. In order to achieve a high-quality brazed joint between cable conductors, the mating surfaces need to be carefully prepared. Surface preparation can involve chemical, mechanical, or thermal cleaning procedures or a combination thereof. The parts to be brazed must be clean and free from any residues that might inhibit wetting, such as oxides, oil, grease, dirt, rust, paint, cutting fluids. In case of stranded conductor, inner wires as well as outer wires of near mating surface of the conductor should be cleaned carefully. The connecting mold can be mounted to position each end of conductor properly. In order to avoid overheating of cable insulation, heatsink or cooling blocks of water or forced air or cooling gas are usually set up near the insulation. The choice of an appropriate brazing process and suitable flux and filler materials is critically important to produce conductor joint with sufficient mechanical properties. Conductors are brazed with a single brazing seam across the entire diameter or layer by layer or segment by segment or wire by wire. The quality of a brazing process depends on the skill and experience of the operator. If the parent material and filler alloy are properly matched, and if the joint is properly designed and made, a brazed joint can provide as a reliable join as that achievable by welding. Brazing faults and defects such as flux burning, de-wetting, discontinuities, cracks, porosity, incomplete fusion, or penetration and nonmetallic inclusions must be avoided. Sometimes, in order to check brazing

Fig. 10.21 Copper brazing procedure

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quality, nondestructive testing such as X-ray or Ultrasonic can be executed according to the manufacturer’s instruction.

10.3.2.6 Clamp Connectors A clamp connector (Fig. 10.22) consists of two components, machined to fit bare cable conductor of specific size. In some cases, it may cover several conductor sizes. The inner surface of the connector often has grooves to increase number of connection points (“A”-spots). This design can be used for both aluminum and copper conductors. Bolts in different numbers and position are used to apply pressure on the conductor and are tightened to a specified torque. Tightening the bolts in sequence specified in installation will insure proper d forces and doing so the conductor will have a relatively even force from all sides. An advantage with this connector type is that they do not require special tools. It is absolute requirement that conductors are free of any foreign material. Segmental or strand filled conductors or conductors with individually insulated wires (enameled strands) must be thoroughly cleaned. The segments and then individual wires must be spread out, cleaned, and then put back together as in individual conductor (see Fig. 10.23). This is very time-consuming operation, requires extreme patience and concentration but needs to be properly done to insure lifetime performance of the clamp connectors. 10.3.2.7 Creuset Connector The Creuset “crucible” welding process is a reliable method of making electrical connections used to connect two cables with aluminum conductors. In this process, granular metals of aluminum are dispensed into a graphite crucible mold adapted to the cross section to connect and heated (Fig. 10.24). A gas flame as source of energy is used to cause the fusion inside the crucible. The mold is then removed and the weld is allowed to solidify. The process takes seconds to complete. Excessive heat is generated during welding process and care should be taken that cable insulation is not overheated. Appropriate cooling system is used to dissipate

Fig. 10.22 Clamp type connector

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Fig. 10.23 Cleaning of strand-fill segmental conductors

Fig. 10.24 Mold setup for Creuset connection

excess of heat and to maintain temperature of the conductor and surrounding cable insulation below specified value. Creuset welding produces connection with high performances as it is a perfect molecular bond, the conductor is not broken, and there are no contact surfaces. The integrity of the effective cross section of the conductor is unaltered. The final result is shown in Fig. 10.25. Only trained and qualified jointers using appropriate welding and cooling equipment can perform this kind of welding (see ▶ Chaps. 5, “Cable Accessory Workmanship on Extruded High Voltage Cables” and ▶ 6, “Guidelines for Maintaining the Integrity of Extruded Cable Accessories”) of this book.

10.3.2.8 Grounding Cable Connectors The good electrical contact between the metallic screen and the ground connection should be effective during the complete life of the cable and particularly in the accessories.

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Fig. 10.25 Mold removed and finished “Creuset” weld after final cleaning

Particular attention has to be paid to the endurance of the ground connection at the accessories, currents in the screen may be important, and the ability of the screen connection must not to overheat during cycling should be checked. The TB 446 (Advanced Design of Metal Laminated Coverings: Recommendations for Tests, Guide to Use, Operational Feedback) by WG B1.25 [35] describes the three successful designs that are today covering the market: Combined Design – combined mechanical and electrical properties Separate design – separated mechanical and electrical properties Separate semi-conductive design – separated electrical and water tightness properties with semi-conductive plastic-coated foil Following are the most commonly used cable sheaths: • • • • • • • •

Lead sheath Lead sheath with copper wires Aluminum corrugated sheath Aluminum smooth sheath Aluminum laminated sheath Copper wires + aluminum laminated Copper wires + Cu laminated Aluminum wires + aluminum laminated Ground connectors must satisfy the following electrical functions:

• • • • • • •

Satisfy the short-circuit conditions, transfers short circuit current Transfers currents circulating in cable screen Collects capacitive current and induced currents Fixation of earth potential Evacuates short circuit current and circulating currents with both ends earthed In case of disconnection from earth, electrical withstand between earth and screen Suppression of screen currents with single end earthing or special bonding

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Chemical functions: • No corrosion of contact points Thermal and mechanical properties: • No degradation of connection and surrounding with thermal short circuit In the case of a single phase short circuit, the current can flow through the metal screen, the metal screen/grounding connection, the ground lead of the joint or the termination. Existing short circuit tests as per TB 446 [35] have been designed to test metal screen/grounding connection in a simple way, electric parameters of this test have been chosen to be such that the test capabilities are available in many laboratories at an acceptable cost. Test Five short circuits shall be applied successively to the assembly: • Before the short circuit test, the cable conductor shall be heated and stabilized for at least 2 h at a temperature 90–95  C. • The short circuits are separated by an interval of time long enough to cool down the cable screen within 5 K of its initial temperature. Rating • The short circuit rating has to be determined by calculation. • The maximum short circuit duration is 5 s. • Asymmetry is free. Result Examination of the samples should reveal no cracks or separation of the metal foil of laminated protective coverings or damage to other parts of the cable. There shall be no sign of harmful deterioration of the cable/joint screen connection, neither at the cross-bonding leads nor at the grounding connections.

10.3.3 Diagnostics for Cable Connector Condition Assessment Regular check of the temperature of termination connections is done by infra-red (IR) cameras. Poor connection pressure due to improper installation or inappropriate surface protection and treatment of contact surfaces between termination cable connector and buss connector resulting in corrosion of contact surfaces are the issues that will result in overheating of the connection. If not detected on time, the issue may result in thermal runaway and physical damage of the connection resulting in costly repair.

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The temperature of the connection of the molded rubber joint may be controlled be fiber optic cable which may be laid over the joint body or over the housing in which the joint is installed. The CIGRE TB 247 on Optimization of Power Transmission Capability of Underground Cable Systems Using Thermal Monitoring shows current practice of laying FO cable over the joint in different countries [33]. CIGRE TB 756 “Thermal Monitoring of Cable Circuits and Grid Operators use of Dynamic Rating Systems [36] by CIGRE WG B1.45” provides an update and a comprehensive bibliography. The X-ray method is mostly used in MV applications. This method shows proper positioning of the cable connector inside the joint. Improper positioning of the connector may cause dielectric failure. This method is also used to periodically check for signs of cable distortion resulting from thermo-mechanical movement of cables and joints in the HV pipe-type cable systems, resulting in thermo-mechanical bending (TMB) of the cables, which eventually may cause electrical failure.

10.4

Cable Connectors in Accessories

10.4.1 General Accessories are an essential part of HV cable system. They need to operate under the same electrical conditions as the cable. In relation to the connector system of the accessories, we can distinguish between current load and mechanical load. The current load is enforced by the operation current of the cable and can be divided in normal load, emergency load (in some countries), and the load under short circuit conditions, that is, short circuit currents. The mechanical load or forces in the connector system can be divided in internal and external forces. The internal forces, also called thermo-mechanical forces, are generated by the thermal expansion of the cable due to operating and short circuit currents. The external forces are generated by the environment of the cable system and are, for example, the consequence of gravity, clamping, soil movements, vibrations, etc. Forces generated by gravity or thermo-mechanical movements of the system are the so-called static forces. These forces act continuously under the influence of gravity or have a cyclic behavior as a consequence of the load cycles, which usually appear in a daily sequence. The forces usually start as pushing forces, so called thrust forces, caused by the thermal expansion of the conductors, but will eventually also generate pulling forces during no load or low load of the system. Under the influence of the high thrust forces (in the order of tons) the conductor starts to deform, distributed over the entire cable, making it effectively shorter. Once the load decreases the thrust forces will go down and eventually generating pulling forces. For this reason, the connector system has to be able to withstand both pushing and pulling forces.

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The main cause for the dynamic load of the system is short circuit forces passing through the system. Due to the magnetic field, generated by the short circuit current, forces are generated perpendicular on the cable. These forces, the so-called Lorentz forces, can generate pushing and pulling forces and are usually of short duration (less than 1 s). Apart from the magnetic forces there is also the effect of the thermal expansion of the conductor under the short circuit conditions. In case of HV systems, with usually large conductor sizes, the temperature increase is low and the impact of the thermomechanical expansion negligible. Short circuits can also have a very local impact on the connector system, thermally and magnetically. High contact resistance will cause high local dissipation and increasing temperatures. Because of this the mechanical properties of the conductor material can change (annealing) lowering the contact pressure and increasing the contact resistance. The currents in the parallel conductor wires will cause a strong magnetic field and related magnetic forces pushing the wires more closely. Also, this effect may temporary lower the contact pressure with the connector, increasing the contact resistance. Detailed overview of the thermo-mechanical forces involved with large conductor XLPE cable system (1000 mm2 and above) is provided in CIGRE TB 669 “Mechanical Forces in Large Conductor Cross-Section XLPE Cables” [32]. Such forces can generate high axial thrust and tension and/or significant cycling movements in the installed cable system. The designer of connectors for these cables in HV and EHV applications has to take these forces into account and to incorporate into development tests.

10.4.2 Mechanical Loads The connectors for HV cables are exposed to significant mechanical forces during assembly process of cable accessories due to movement of the cable and accessory into their final position [32]. More importantly, connectors are exposed to thermomechanical forces in service generated by thermal expansion and contraction of the cable due to variations of the load current or exposure to the short circuit current [32]. In practice, the forces caused during the installation process are not problematical and are not usually limiting for an HV connector. However, the loads imposed on the conductor due to changes in conductor temperature can be very significant. After initial installation, the cable conductor is normally relaxed with little axial mechanical force present. However, as the temperature increases the conductor expands and, if the cable is rigidly installed [27], then axial thermal expansion is prevented and instead a compressive force develops in the conductor (which is evidenced as a thrust force from the conductor at the cable ends). If a rigidly installed cable is heated rapidly to its full operating temperature on the first load cycle, then relatively high forces can be developed. However, if expansion is fully prevented

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then the result is that relaxation of the maximum thrust occurs towards the maximum conductor temperature, reducing the compressive force produced. However, a consequence of this relaxation is that a tensile force develops in the conductor when it cools down to the initial installation temperature (or below). Typically, after a number of cycles the compressive and tensile forces developed in a conductor cycling over its full temperature range will be similar, and with values of approximately 50% of the maximum force that might be achievable in a single rapid cycle. As far as the connector is concerned the conductor tensile force is the most critical as it will tend to pull the conductor out of the connector. In the case of the conductor compressive force then this is normally equal and opposite to the force from the next cable length on the other side of a joint or in the case of a termination has to be resisted by the connection of the connector to the body of the termination itself. Thus, in terms of testing the performance of connectors only the tensile (pullout) force from the conductor has to be considered. In assessing the behavior under tension of a conductor to connector connection, the tensile load that can be resisted with minimum movement has to be assessed.

10.4.3 Environment Consideration of Environmental Conductor Size Optimization Over the years, energy utilities have considerably increased their requirements to consider the ecological environmental impact of electric cables in relation to their service conditions. Environmental considerations should be included in both design and redesign work with respect to the raw materials used, energy consumption and emissions during production, end of life disposal or recycling, and in-service performance. A specific guidance document has been published by IEC TC 20 in 2007 as the technical report IEC/TR 62125 “Environmental statement specific to IEC TC 20 – Electric cables.” This guidance will be replaced by IEC 62125 Ed. 1 showing a qualitative checklist based approach for environmental impact and quantitative approaches by using life cycle assessment (LCA) or an energy cost based conductor size optimization (ECSO). The results obtained by applying such methodologies can be used for external communication. As recommended by IEC Guide 109, the basis of the assessment of productrelated environmental impact is Life Cycle Thinking. Environmental impact of a product needs to be evaluated considering its whole life cycle and evaluating various environmental indicators. LCA is a tool covering all life cycle stages (cradle to grave). An LCA shall be carried out in accordance with the methodology of life cycle assessment (LCA) specified in ISO 14040 and ISO 14044. Environmental and energy cost based Conductor Size Optimization is taking into account the cable’s life phases’ costs and reduction in power loss costs during use phase and related costs of CO2 compared to the conventional sizing of highly loaded cables with significant energy losses. ECSO takes specifically into account:

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• Initial cost of investment including manufacturing, transportation, installation, and final disposal costs • Cost for CO2 emission during manufacturing, transportation, and installation and final disposal • Costs for Joule losses during anticipated life time • Costs for CO2 emission during anticipated life time There is an ecological and economical demand for more efficient operation of electric cables and some information is available on suitable cable design parameters to achieve lower losses. Unfortunately, diverse pressures from a number of interests usually result in the need to compromise in this area. Lower transmission losses, reduced heating effects, and, as a result, lower emission of greenhouse gases and hence reduced carbon footprint might be achieved. Energy losses during service are dominated either by the duration under load, which could be many decades for HV cables in transmission or distribution networks, or by a combination of time under load and length of the network. The current loads will create temperatures which might start ageing effects in insulation and inside connections. A certain end temperature is needed to initiate such aging effects and long lasting high temperatures might consume lifetime. The lower the current loads are, the lower the reached temperature exposure of cable systems and, respectively, the lower the risk of limiting service lifetime will be. Balancing total costs of ownership by additionally involving environmental aspects often shows that the conservative approach, running a cable system at 40–60% nominal load, will create optimal results and besides that limit the risk of connector failures. Cable system qualification tests including connector development tests will simulate some worst case conditions but will give only limited information about long time endurance and life time expectation when running a cable system close to its nominal loads. The Technical Brochure 689 “Life Cycle Assessment of Underground Cables” by CIGRE WG B1.36 [37] provides guidance for the implementation of an eco-design approach applied to underground cables, through the use of LCA methodology by giving principles of LCA (standards, guides, historical approach), a state of the art regarding studies and reports on this topic, a methodology to perform LCA on underground cables, and an LCA case study. In conclusion the main results, highlights, and limits are discussed.

10.4.4 Cable Connectors in Joints 10.4.4.1 General Joints are subjected to current load and mechanical forces. The main consequence of the current load is the heating of the cable and connector system, leading to elevated temperatures inside the joint. In continuous operating conditions, temperatures exceeding the maximum operating temperature of the extruded cable need to be avoided [30]. In this respect the conductivity of the connector system and the thermal properties of the joint have to be considered.

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The connector system needs to withstand the mechanical forces that do exist in cable systems, such as pushing (thrust), lateral and pulling forces. See Reference [32]. These forces should not lead to instabilities in the performance of the connector system.

10.4.4.2 Thermal Rating of the Joint The CIGRE TF 21 (B1)-10 reviewed if existing IEC Specifications for HV/EHV extruded cables and their accessories appropriately specify and verify the crucial thermal and thermo-mechanical characteristics of accessories. The thermal rating of an accessory is defined as “the maximum temperature of the conductor or conductor connector contained within the accessory (whichever is the higher) allowed in normal operation.” The report of TF 21-10 is given under ▶ Chap. 9, “Thermal Ratings of HV Cable Accessories” of this Book. TF B1-10 finished its work on schedule in 2003 with the following conclusions (see reference [30] and ▶ Chap. 9, “Thermal Ratings of HV Cable Accessories”): • Thermal ratings of accessories need not be specified separately from cables, as they are considered identical due to the presence of cable inside the accessory. • The successful completion of IEC thermal tests at a complete cable system can be considered as simultaneous verification of the adequate thermal design of both, cables and accessories, provided that comparable or higher conductor temperatures as rated for the cable are achieved inside joints. These test conditions shall be realized by applying only cable conductor current heating. • The thermal performance of terminations in normal operation is not considered critical; therefore, they do not have to reach the rated temperature for the cable during test. • External thermo-mechanical forces can be reproduced in the IEC prequalification test only for the specific installation conditions applied. • The thermal limits of accessories and external thermo-mechanical forces in service operation cannot be reproduced comprehensively by standardized tests, but have to be taken into account for each individual case by the systems design engineering. The Annex 1 of referenced document [30] shows thermal calculation for HV and EHV extruded cable joint. It is shown that in stationary conditions, the cable conductor in the joint reaches a higher temperature than in the cable, because of higher thermal resistance of the joint. In the first 6 h of the heating cycle, the temperature of the cable conductor in the joint is lower than in the cable conductor outside the joint, as shown in Fig. 10.26 and Fig. ▶ 9.3 in ▶ Chap. 9, “Thermal Ratings of HV Cable Accessories” of this book, due to the longer thermal time constant of the joint.

10.4.5 Cable Connectors in Outdoor Terminations In most of the termination designs, the connector is located in the top of the termination. The connectors in terminations are usually large, compared to the

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110 100 90 Temperature (˚C)

80 70 60 50 40 30 Joint

20

cable

10 0

0

1

2

3

4

5 6 Time (hours)

7

8

9

10

Fig. 10.26 Typical heating curve of 138 kV extruded cable and molded rubber joint

connectors in joints, and therefore usually not considered critical. Beside this, the termination connectors are exposed to environmental conditions, which usually represents effective cooling. Exposed section of connector has to be designed to operate under different environmental conditions. Important for termination connectors is its thermal stability and the resistance against thermo- mechanical forces. The conductor below and inside the termination will generate thrust forces during load, which might change to pulling forces in no load conditions. Depending on the conductor arrangement inside the terminations, the thrust force can result in cable bending causing cantilever loads. The connector systems have to be able to handle all these forces.

10.4.6 Cable Connectors in Equipment Type Terminations Equipment terminations, such as GIS and transformer terminations, can be divided in open and closed top design (see ▶ Chap. 11, “Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power Cables up to 145 kV”). In the open top design, the connector is mechanically fixed in the top of the termination after installing the cable. This is usually a screw connection. This allows the thermo-mechanical forces to the transferred to the termination insulator. Apart from the connection with to the cable conductor, there is also an electrical contact with the embedded electrode of the termination insulator. This contact is a metal/ metal contact based on adequate contact pressure. In the closed top design, a metal enclosed top electrode is integrated in the (epoxy) termination insulator. During installation, the cable is plugged into the termination. The conductor of the cable is provided with a plug and the plug is connected to the top electrode by means of multicontact system.

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For the connection with the conductor, similar conditions apply as for the joints and outdoor terminations. Plug-in contacts are based on a male/female connector system, connected through a multicontact system, achieving continuous contact pressure. In the equipment terminations, there are the locked and unlocked designs of the connector system. In the locked design, the plug-in connector is fixed in the termination, transferring the forces to the termination insulator. In the unlocked design, the cable is fixed below the termination by means of clamping. The clamping can be integrated in the termination design or installed externally.

10.4.7 Connections to the Cable Connectors 10.4.7.1 General Cable connectors are used for connecting two or more cable conductors to each other inside the joint or connecting cable conductor to the terminal of the overhead line or terminal of the equipment such as GIS, transformer. The construction, type, material, and installation methods of connecting cable conductors to the cable connectors are well explained other part of this report, so in this section, it will be described to connect cable connectors to the terminal of overhead line or the equipment. 10.4.7.2 Outdoor Terminations Outdoor termination connects the cable to either substation component or busbar or to the overhead line. The connection of the cable conductor to the substation component or overhead line is achieved. 10.4.7.3 Equipment Type Terminations (GIS and Oil Immersed) Cable connections for gas-insulated switchgear (GIS) are defined by IEC 62271-209 [7] and oil-immersed cable connections for transformers and reactors are defined by the relevant standard, EN 50299-1 [8] or EN 50299-2 [9]. Figures 10.27, 10.28, and 10.29 illustrate typical arrangements proposed in these documents. The main circuit end terminal of GIS or transformer is generally connected to the top conductor of the cable termination through screws at rated normal current up to 3150 A. In order to reduce electrical contact resistance, the normal current-carrying contact surfaces of the connection interface are silver- or copper-coated or solid copper and apply adequate screw torque during assembling according to the manufacture’s installation manual. The mechanical forces on the cable termination caused by short-circuit may be critical to the design of the cable connection system. Total dynamic forces generated during short-circuit conditions consist of those generated within the cabletermination and those coming from the main circuit of the switchgear. For a threephase connection, the maximum additional force applied from the switchgear to the connection interface transversely and then being transferred from the main circuit end terminal shall not exceed 5 kN. For single-phase connections, taking into

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Fig. 10.27 Fluid-filled cable connection assembly for GIS – typical arrangement ([7], Fig. 2)

Fig. 10.28 Dry-type cable connection assembly for GIS – typical arrangement ([7], Fig. 4)

account lack of symmetry, it is considered that this additional force is small. However, a total mechanical force of 2 kN applied to the connection interface transversely should be assumed. It is the responsibility of the manufacturer of the switchgear to ensure that the specified forces are not exceeded. For both single-phase and three-phase connections, additional forces and movements from the switchgear can be experienced due to temperature variations and vibrations in service. These forces can act on both switchgear and cable-termination

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Fig. 10.29 Cable connection assembly – transformer ([9], Fig. 1)

and depend largely on the switchgear layout, termination installation, cable design, and the methods of mechanical support. The design of any support structure shall take into account these forces and movements. It is particularly important that the support for the switchgear shall not be affixed to the insulator collar and/or clamping flange. The top conductor design of cable termination can be divided in open and closed system whether embedded top metal part of insulator is open or closed. Figure 10.30 shows typical open and closed top connector systems. In case of open top connector system, conductor connector is generally connected to the embedded top metal part of insulator by screw connection. In order to prevent gas (or oil) penetration from GIS (or transformer) to cable termination, additional sealing systems are required between conductor connector and embedded top metal part of the insulator. Because cable conductor, conductor connector, and embedded metal part are all connected directly, the thermo-mechanical force due to thermal expansion and extraction of the cable may affect to the insulator. To minimize this affection, suitable clamping system is required inside the cable termination or near the termination. During installation, cable with conductor connector inserted to the

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Embedded top insert

Cable connector

Cable conductor

Insulator

(a) Open top connector system

(b) Closed top connector system

Fig. 10.30 Typical open and closed top connector system of cable termination

insulator together first and then cable termination is assembled to the GIS or transformer generally. So, equipment (GIS or transformer) metal enclosure cannot be factory tested with the insulator. During the final assembly, both installers (equipment and cable termination) are needed and vacuuming and gas or oil filling must be performed after final assembly. In case of closed top connector system, conductor connector with cable is plugged-in to the embedded top metal part by multicontact, spring, or tulip contact system. The current-carrying surfaces of conductor connector, multicontact, and embedded top metal parts are generally silver-coated. The dimensions of outer diameter of multicontact and the inner diameter of the embedded top metal part are important in order to maintain suitable contact pressure. Because embedded top metal part is closed and normally leak test is performed as a routine test, additional sealing system is not required and more reliable structure in a viewpoint of gas or oil leakage compared with the open top connector system. There are two types of closed top connector system. One is locked plug-in type and the other is unlocked plug-in type. The plug of locked plug-in type cannot be removed without disassembling insulator from the equipment enclosure, whereas the plug of unlocked plug-in type can be removed without disassembling insulator from the equipment enclosure. The thermo-mechanical force of cable may affect directly to the insulator or partly be compensated according to the design of locked plug-in type; however, clamping system inside the termination or near the termination is needed to minimize this affection. Unlocked plug-in type can partly or fully be compensated according to

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the design; however, clamping is essential because plug can easily be removed without clamping during operation. In case of closed top connector system, insulator can be preinstalled in the equipment enclosure and the cable with plug inserted later on site. In this case, equipment metal enclosure can be factory tested including the insulator including voltage test. During the final assembly, both installers (equipment and cable termination) are not needed at the same time. More information will be found in ▶ Chap. 11, “Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power Cables up to 145 kV” of the book.

10.5

Installation of Connectors

Proper cable conductor preparation and connector installation are crucial for a long and successful field performance of cable systems. Many failures of medium voltage accessories have been caused by thermal runaway because of poor connector installation or/and poor conductor preparation. Most common mistakes are insufficient engagement of conductor into the connector ferrule, insufficient tightening of the bolts in mechanical connectors, use of inappropriate crimping die for a press-type connector, fail to remove water blocking material from filled conductors, when required, or fail to remove insulating coatings from individual strands, when present. Mandatory trainings in installation of HV/EHV cable accessories, including preparation of conductors and installation of connectors, increase awareness of importance of proper installation and provides opportunity to check the skills of the splicers. Hence, there are very few field failures of HV/EHV cable accessories due to connection failure.

10.5.1 Installation Instruction Manual The manuals for installation of cable accessories (joints and terminations) either describe steps in installation of connectors or refer to separate manual for connector installation. Selection of installation tools and strict minding of steps described in instructions is of essence. The CIGRE TB 476 Cable Accessory Workmanship on Extruded High Voltage Cables [31] which is the content of ▶ Chap. 5, “Cable Accessory Workmanship on Extruded High Voltage Cables” of this book describes in detail technical risks and required skills for preparation of conductors and installation of different types of connectors. Only competent and trained personnel familiar with cables, accessories, and safe operating practices should install accessories, both for testing and field assembly. Cleanliness during the whole installation is of great importance.

10.5.2 Cable Conductor Preparation Preparation of conductor is as important as installation of connector. It involves cutting the conductor (in most cases it is required that conductor cut is square), removing insulation and exposing cable conductor to specified length, removal of

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Fig. 10.31 Cleaning and crimping of filled 2500 mm2 Cu conductor

strand filling materials (powders, yarns, cloth. . .) if present, removal of coating from individual strands when strand insulating conductors are used and cleaning exposed conductor. The cable insulation must be protected from damage and metallic particles during all these steps. Figure 10.31a shows example of preparation of segmental, strand filled cable conductor. Individual strands from each segment must be flared out, cleaned from the filler, and then put back together to form the segments of the same shape as in original conductor. Considerable skill and training is required to perform this operation. If not done properly, the strands may be damaged or conductor shape and outer diameter changed such that connector would not fit.

10.5.3 Mechanical Connectors This connector type uses bolts to apply pressure to the underlying conductor. It can be used on both copper and aluminum conductors. Mechanical connectors do not require special tools for installation and the skill level is relatively easy to achieve in proper training, positioning of properly prepared conductor(s) into connector and tightening the bolts is spelled out in instructions. Most connector designs are of the shear-bolt type where the bolt is tightened until they shear. Other designs require tightening of the bolts to specified value. The tightening sequence must be followed as specified in instructions. Factory applied lubricants (if present) are not to be removed. The sharp points need to be removed and, if required, the holes filled after all the bolts are tightened or sheared.

10.5.4 Crimp Connector The most common connectors in HV applications are still of the crimp type. Depending on the cable size and manufacturer, many crimping tool types are used. They may be either with or without crimping dies.

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The die-less crimping tools feature one, two, or four rams (indenters). Rams put deep indents in the ferrule and deform the ferrule and underlying conductor. The die-less crimping tools are adjustable to take larger span of connector/cable conductor sizes. The crimping tool with dies features the crimping head that accommodates dies for different connector sizes. The dies are usually of circular or hexagonal shape but other shapes, for example, ellipse, are also used. The width of the die differs substantially between manufacturers. Material of connector ferrule matches material of conductor: copper ferrules are used on copper conductors and aluminum ferrules on aluminum conductors. Since the cross-sectional area of aluminum ferrule is bigger than cross-sectional area of conductor, the crimping dies for copper and aluminum connectors are different. The aluminum ferrules tend to be longer than the copper ones, accommodating larger number of crimps. All the variables mentioned above are taken into account by cable accessory manufacturer when the connector is designed and the crimping tool, crimping dies, sequence, and number of crimps are specified. It is important that the cable accessory manufacturer is consulted if different crimping tool or set of dies are intended to be used.

10.5.5 Exothermic Welding Connector Exothermic welding uses chemical reagents in a reusable crucible, placed above a mold specifically designed for the conductors being welded. This is a special technique and includes placing the correct quantity of reactants in the crucible. It is necessary to take good care to avoid porosity and cavities in the welding mass. This can come from the presence of moisture or filler in the conductor. The setting of the gap between the conductors needs to be done carefully, then the crucible and mold assembly is installed and filled before firing the reactants so they drop into the mold, melting the ends of the conductors together. During this process, the presence of any porosity should be noticeable. It is important to follow the manufacturer’s instructions carefully and to be aware of the risks, for example, high temperatures and toxic gases evolved during the process.

10.5.6 MIG or TIG Welding Connector The installer must be qualified and experienced for this kind of installation and follow the instructions for the welding equipment. This technique involves using arc welding in an inert gas with a feed wire of copper or aluminum. The wire must be appropriate to the welding machine. The conductor ends are cut diagonally to form a V shape when placed in the welding jig.

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In a normal installation, overheating of insulation during welding must be avoided. In a test situation this might not be a critical point. Heat sinks or forced coolers are generally applied on both sides of the exposed conductor. Prior to welding all enamel must be removed from enameled copper wires before MIG or TIG welding is carried out. After welding is completed any sharp edges or points must be removed from the connection.

10.6

Experience

The working group spent substantial time in gathering information on experience with cable connectors in HV and EHV cable systems. The methods to get information were presentations by users at the WG meetings and world-wide survey.

10.6.1 Utility Presentations at WG Meetings 10.6.1.1 USA Two major utilities from the USA shared their experiences. One of the utilities addressed cable connectors in HV applications (69–345 kV) and the second one in MV applications (up to 46 kV). While there were only a few connector failures in 60 years history of HV laminated cable connectors, there were no reported failures for connectors in over 20 years’ experience with HV XLPE cable systems. Experience for MV cable connectors is not so good. The number of failures, and in particular connector failures, in joints increased with introduction of wind farms. Analysis of the failures indicated the cause generally to be the incorrect selection of connectors. 10.6.1.2 Germany The following is a short summary of the presentation by Christian Walter at the third CIGRE B1.46 meeting in Winterbach, Germany, on 26 Nov 2014. Mr. Christian Walter leads the technical team “HV cable systems” of “E.ON Centre of competence grid and distributed energy” in the legal entity “Bayernwerk AG” located in Bayreuth, Germany. As a global player, E.ON sources cable systems worldwide. Up to now their experience shows no connector failures. The load management on cable systems is conservative due to the n-1 criterion and is reaching up to 50% of the nominal rated load. For the implementation of new cable systems, measures have been taken to improve the quality management systems to be able to run cable systems at their calculated limits, adapted and verified for each single project, especially when connecting wind farms and renewables. Systems for distribution cables up to 110 kV are specified in a technical specification following in most parts test conditions for the next higher voltage level 132 kV according to Cenelec HD 632.

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For future operation, the ampacity calculation according to IEC 60287 is verified by DTS measurements taking into account the soil conditions while increasing loads. Even joints will be equipped with optical fibers to determine the connector temperatures. In particular there will be an evaluation of the behavior of new cable systems due to rapidly changing grid loads from nearly 0% up to 100% daily (in solar plants) or weekly (in wind farms). There will also be a lifecycle evaluation process with periodic PD and tan Delta measurements being made. Cable-system delivery for a project is only accepted by a single contractor representing the cable manufacturer, the accessory manufacturer, and the installation company. During the qualification process, two type tests according to HD 632 have to have been completed: One for a small aluminum conductor, for example, 630 mm2, and one for the biggest aluminum conductor, for example, 2500 mm2. The cable conductor has to be according to IEC 60228 with the following designrestrictions: conductors to be compacted stranded circular (class 2). Solid conductors made are not used. Conductors above 1200 mm2 shall be segmented (Milliken conductor). Milliken designs shall have at least five segments. In future only cables with aluminum conductors will be purchased. Due to increasing ampacity requirements, cross sections larger than 2500 mm2 may be needed in future projects. Longitudinal water tightness of stranded conductors is required. Materials used for that purpose shall not be toxic. A semi-conductive bonding tape shall be used to prevent the inner semi-conductive layer penetrating the gaps between the conductor wires. To reduce installation failures, only mechanical connectors equipped with shear bolts shall be used on stranded conductors. Due to a lack of test experience, no compression connectors are in use. Welding of connections will no longer be accepted due to an incomplete quality management system and for safety reasons on site. Because connector type tests for high voltage cable applications are not yet specified, available type tests similar to IEC 61238-1-3 are requested with a minimum of 1000 heat cycles showing temperature stability with a min. 2500 A for 2500 mm2 aluminum. Currently just a few type tests can be shown on connectors for big cross-sections. Short circuit withstand ability should be tested with a min. 40 kA/ 1 s (3 phase). A mechanical tensile test with a min. 100 kN for 2500 mm2 will be required. Aluminum should show no slippage and no elongation. As long as no standardized types and designs for conductors and connectors are available a qualification of each combination will be necessary. To get at least limited comparability, a common type test specification for connectors will be appreciated.

10.6.1.3 France As the transmission engineering and expertise center for the EDF Group, the CIST (Power System and Transmission Engineering Centre) shared its experience in HV/EHV cable systems. It is the focus for all the specialist disciplines involved in power transmission (power systems and transmission grids).

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CIST supervises the construction and maintenance of electricity supply installations, from the generating plant to the power transmission grid operated by RTE (French transmission system operator). In France, it negotiates the conditions for connection to the grid; it operates at voltages of 63 kV, 90 kV, 225 kV, and 400 kV. Regarding HV cable systems, there were no failures in accessories related to connectors in the last 10 years.

10.6.2 Worldwide Survey A survey was sent to utilities in order to assess their cable system and the type of problems encountered due to connector failures. The following tables summarize the data collected. Table 10.6 shows the number of utilities with the range of cable length per voltage range at the present time, and the additional number of utilities that will fall in that range. On the bottom of the table, we have the total length of cable per voltage range for the surveyed utilities. In voltages above 220 kV, we will see an increase of over 35% of the installed base within the next 5 years. Table 10.7 shows the number of utilities that use a range of conductor sizes at the present time, and the additional number of utilities that will use these sizes in the next 5 years. Large sizes up to 1600 mm2 and above are used at all voltage levels. The trend is to use larger size conductors in higher voltage systems. Table 10.8 shows the number of utilities that use copper and aluminum conductors at the present time and the additional number of utilities that will use them in the next 5 years. Copper is still mostly used for heavy loads, but aluminum is used as well. Table 10.9 shows the number of utilities that use the different types of outdoor terminations of different voltage class. Oil filled type terminations are still used, especially at higher voltages. Dry types are becoming more popular at up to 161 kV. Table 10.10 shows the number of utilities that use different types of termination vs. voltage class. Dry type terminations are mostly used nowadays, at other than the highest voltage range. Table 10.11 shows the number of utilities that use the different joint types vs. voltage class. An outer housing is widely used, especially at higher voltages. Table 10.12 shows the number of utilities that experienced connector failures on accessories vs. Voltage class. Table 10.13 shows the number of utilities that experienced connector failures attributed to different causes vs. voltage class. Failures were reported on terminations, but more so in joints (at all voltage levels). Generally, the failures were due to installation errors, but also from overheating. Conclusions • A good number of responses from the utilities (34 surveys from 12 countries) were received.

Range (km) 1–10 11–25 26–50 51–100 101–250 251–500 501–999 1000+

Nominal cable system voltage (kV) 45–70 110–161 220–287 Now Future Now Future Now 1 3 4 7 4 1 1 1 2 1 1 2 3 5 5 1 1 1 2 2 4 3 3 7 1 3 1 1 6 3 3 1 Total cable system length in service (km). Surveyed utilities only 12108 9456 1840 Future 2 2 2 1 1 1

777

315–500 Now 4 2 2 1 1 1 Future 3 2 1 1

101

1

500+ Now 1 1

Table 10.6 Number of surveyed utilities that own cable systems and planned in the next 5 years. Total installed length (at surveyed utilities only)

Future

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Range (mm2) 1600

Nominal cable system voltage (kV) 45–70 110–161 Now Future Now 8 5 16 12 4 23 9 2 18 8 2 13 Future 11 8 10 7

Table 10.7 Range of conductor sizes in service and in next 5 years 220–287 Now 4 7 12 12 Future 1 2 6 8

315–500 Now 1 1 3 6

1 3 8

Future

1

1

500+ Now

1

Future

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Range (mm2) Copper Aluminum

Nominal cable system voltage (kV) 45–70 110–161 Now Future Now 7 8 25 3 1 3 Future 17 6

Table 10.8 Conductor material currently used and planned for future 220–287 Now 16 2 Future 10 1

315–500 Now 11 2

Future 7 1

500+ Now

Future 1 1

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Table 10.9 Number of utilities that have outdoor terminations in service Outdoor termination type Oil filled Dry type, not-supporting Dry type with additional support insulator Dry-type self-supporting

Nominal cable system voltage (kV) 45–70 110–161 220–287 315–500 14 23 16 10 1 4 2 7 3 5 4 1

500 2

Table 10.10 Number of utilities that have equipment-type terminations in service Equipment termination type Oil filled Dry type, not plug in Dry type, plug in

Nominal cable system voltage (kV) 45–70 110–161 220–287 6 9 8 10 14 10 12 22 7

315–500 10 4 2

500

Table 10.11 Number of utilities that have joints in service Joint type Without housing for outer protection With Cu or fiberglass housing

Nominal cable system voltage (kV) 45–70 110–161 220–287 315–500 3 7 1 13 21 16 7

500 2

Table 10.12 Number of utilities that experienced connector failures in service Accessory type Outdoor terminations Equipment type terminations Joints

Nominal cable system voltage (kV) 45–70 110–161 220–287 10 no 20 no 13 no 2 yes 1 yes 17 no 23 no 10 no 1 yes 2 yes 10 no 14 no 11 no 1 yes 4 yes 2 yes

315–500 5 no

500 3 no

5 no 2 yes

3 no

315–500

500

Table 10.13 Cause of failures (number) Cause of connector failure Wrong design Installation error Overheating Fillers Push/pull force Outer corrosion

Nominal cable system voltage (kV) 45–70 110–161 220–287 1 3 8 4 2 1 1 1 1 1

2

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• A substantial increase in higher voltage installations (up to 35% of current base length) is expected within the next 5 years. • An increase in the use of shear bolt (mechanical) connectors is foreseen. • The failures reported are mainly in through connectors, due to installation errors. • Bigger sizes of conductors are expected to be used in future projects. • In high voltage cable systems, for copper conductors, the crimped or deep indent type connections are predominant, while for aluminum conductors welded connectors are widely used, with a trend towards the use of shear bolt connectors.

10.7

Existing Test Methods, Requirements, and Assessment in Cable Connector Testing

The principles for connecting connector to cable conductor are basically the same in HV, EHV, and MV systems. Since type tests are standardized only for MV connectors, it is important to understand the issues encountered in the world of MV connectors in order to specify test requirements for HV/EHV connectors. Some differences between MV connections and HV connectors are: • The cross-section range in MV is typically from 95 to 1000 mm2 and in HV from 185 to 3000 mm2 or even larger. • MV connectors may be purchased separately from cable accessories, as commodity item, which is not the case in HV/EHV applications. • Qualification of MV connectors is done per IEC 61238-1-3 standard. There is no standard for qualification of HV/EHV connectors as separate components, outside the cable accessory. • There is a difference in test requirements for components of cable systems between MV and HV/EHV applications. The Cenelec standard for MV cables HD620 (similar to IEC 60502-2) and Cenelec standard for MV cable accessories HD 629.1 (similar to IEC 60502-4) specify test requirements separately for cable components (cables and accessories), while the major focus in the IEC standards 60840 and 62067 is on cable systems. For example, the test setup for a PQ test of EHV cable systems is designed to pay special attention to thermomechanical aspects of cable accessories, while that is not the case in MV test setups. Sections 10.7.1 and 10.7.2 consider aspects of MV connectors. It summarizes test requirements per IEC 61238-1-3 and additional test requirements specified by some users in certain countries. Also, real-life examples of testing and use of MV connectors are provided in this section. Section 10.7.3 explains current practice in additional tests performed on connectors and accessories for HV/EHV cable systems.

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10.7.1 Medium Voltage Connectors 10.7.1.1 IEC 61238-1-3 Requirements MV connectors of the mechanical and compression type are qualified per IEC 61238-1-3 standard. When a design of connector meets the requirements of this standard, then it is expected that in service: 1. The resistance of the connection will remain stable. 2. The temperature of the connector will be of the same order or less than that of the conductor. 3. The mechanical strength will be fit for the purpose. 4. If the intended use demands it, application of short-circuit currents will not affect a) and b). There are three test criteria for evaluation of MV connectors per this standard: stability of temperature, stability of connection resistance, and mechanical strength. The statistical method of assessing test results described in this standard is mainly based on a compromise between the Italian Standard CEI 20-28 and the British Standard BS 4579: Part 3. The connection resistance and temperature stability are checked by performing 1000 heat cycles on the test loop consisting of six connectors and the corresponding reference conductor, which is identical to that used in the connectors. The influence of short circuit current on resistance and temperature stability is also checked. This is done by interrupting temperature cycling after 200 cycles and applying a short circuit current of certain intensity and duration. The connectors are installed on bare cable conductors. The test loop shall be installed in a location where the air is calm. The ambient temperature of the test location shall be between 15  C and 30  C. The heat is generated by circulating ac current in the test loop. The preferred method of measuring temperatures is using thermocouples. The temperature of the reference conductor, which is the control parameter of the test, is determined in the first cycle of the test. The current is adjusted to bring reference conductor to 120  C at equilibrium (the moment when the reference conductor and the connectors do not vary in temperature by more than 2 K for 15 min). If at that time the connector with the third highest temperature (median connector) is below 100  C, the current is further increased until the temperature of median connector reaches 100  C at equilibrium, subject to the reference conductor temperature not exceeding 140  C. Subsequent cycles are controlled by this reference conductor temperature. During the cooling period, the reference conductor has to be cooled down to 35  C or below. The criterion for temperature stability is that temperature of any of six connectors does not exceed temperature of reference conductor at any time during the test.

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The criterion for connection resistance is much more involved. Here, a statistical method of evaluating the trend of electrical resistances was adopted. It requires calculation of the connector resistance factor (k) for each connector each time the resistance is measured. The measurement is done before cycling, immediately before and after the short circuit test at cycle 200, after the 250th cycle and then after every 75 cycles (total 14 times from start to finish of heat cycling). A total 64 k values (6 connectors 14 resistance measurements) are then used in the statistical analysis to determine if the connector passed or failed the test. Statistical parameters for k, to which the connectors are evaluated, are set in the standard. The selection of assessment criteria and values was made after evaluating test results and experience from different laboratories and countries. In short, assessment criteria require that: • Six specimens shall be similar in resistance at the beginning of the test. This is assessed by calculating initial scatter δ between the six values of k before heat cycling and shall not exceed the value 0.3. • The resistance should not change extensively during test. This is assessed by calculating: – The mean scatter β between the six values of k averaged over the last 11 measurements which verifies that the connectors behave in the same way and that they belong to the same “family.” The mean scatter β shall not exceed the value 0.3. – The change in resistance factor D, which shows the change of the resistance factor k for each connector over the last 11 measurements. Statistical methods are used to assess the probability that the change of resistance will not exceed the specified value. The change in resistance factor D shall not exceed 0.15. Note: These 11 readings start at the 250th cycle point, and then every 75 cycles up to 1000 cycles. • The resistances shall not change excessively as a result of the short-circuit test. This is assessed by calculating the resistance factor ratio L, which shows the relationship between the resistance at any stage of the measurements and the initial resistance. The resistance factor ratio L shall not exceed 2.0. The connection resistance factor k is ratio between the resistance of a connector and an equivalent length of the conductor. The connection resistance itself cannot be measured. Instead, it is derived from measured resistance of known length of reference conductor and resistance measured between two points on the conductor with connector. Measuring points are on the cable conductor (or equalizer, where required) at each side of connector. By knowing the length between measuring points and lengths of cable at each side of the connector, the resistance of the connection is calculated by subtracting the resistance of the conductor at each side of the connector from the resistance of the equivalent length of conductor. The value obtained is then normalized to temperature of 20  C. All measurements of resistance are made with direct current (preferably 10% of the ac test current) and at ambient temperature.

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The measuring accuracy of connection resistance is critical. Measured values are very small, in the range of micro-ohms, which requires measuring equipment of high accuracy (within 1% or  0.5 μΩ, whichever is the greater) and experienced personnel. There are many resistance readings taken throughout the test (total 86) and any incorrect reading affects analysis and may result in not qualifying a connector, not because of its performance in the test, but due to erroneous measurements. Accurate readings are of particular importance for large-size connectors, 1000 mm2 and above. The electrical resistance of these conductors is even smaller and requires more precise readings. Measuring instrumentation should be calibrated and not changed for any of the readings. The same measuring points should be used throughout the test, since calculation always refers to the initial situation. Verification of the measuring points, especially after the short-circuit test, is advised. It is recommended that the same value of direct current is used throughout the test program. Bending or vibrations during transport and handling may give rise to mechanical forces, which affect the contact resistance of the test objects and should be avoided. In general, every effort should be made to avoid spurious readings. Equalizers at the measuring points of a stranded conductor are required to assure all strands are at the same potential. If the strands are not galvanically connected a potential between the strands develops during measurement of resistance resulting in a measuring error. In addition, equalizers in the reference conductor ensure uniform current distribution in conductor strands, Welded or soldered equalizers are most commonly used. It is very important that the equalizers are not affected by heat or movement and maintain acceptable stability throughout the test. There are 14 equalizers in a test loop for joint connectors and making all of them in acceptable quality is highly challenging, particularly for large-size stranded cables. One bad equalizer can cause a test failure. The short-circuit test is intended to reproduce the thermal effects of high currents only. In (6) short-circuit applications after 200th temperature cycle the current level shall be such that it raises the bare reference conductors to a temperature between 250  C and 270  C. The standard recognizes that for cross-sectional areas exceeding 630 mm2 copper or 1000 mm2 aluminum, the specified maximum parameters (45 kA and 5 s) are insufficient to reach 250  C. The mechanical test is performed on three additional connectors. The purpose of these tests is to ensure an acceptable mechanical strength for the connections to the conductors of power cables. The result of mechanical test does not give any reliable indication of the electrical quality of the connector. The rate of application of the tensile load shall not exceed 10 N per square millimeter of cross-sectional area and per second up to the value of tensile force in Newton that is equal to 20 times cross sectional area for aluminum conductors and 40 times for copper. For both aluminum and copper conductors the tensile force should not exceed 20,000 N. This force is then maintained for 1 min. The criterion for the mechanical test is that neither connector slip.

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Range of Approval In general, tests made on one type of connector/conductor combination apply to that arrangement only. However, to limit the number of tests, using the same conductor material, the following is permitted: • A connector which can be used on stranded round conductors or on stranded sector shaped conductors which have been rounded, is approved for both types if satisfactory results are obtained on a compacted round conductor. • A connector which covers a range of consecutive cross-sectional areas shall be approved, if satisfactory results are obtained on the smallest and the largest crosssectional area. • If a connector is a through connector for two conductors of different crosssectional areas, shapes, or materials, and if the jointing method and the connector barrels used have already been tested separately for each cross-sectional area, no additional test is necessary. If not, and if it is required for bimetallic through connectors, additional tests shall be made using the conductor having the highest temperature of the two conductors, as reference conductor. • If a type test for a range taking mechanical connector is passed on the biggest possible conductor cross-sectional area, this result is also valid for similar connector designs with the same material of the connector body but bigger outer diameter provided that the design of the conductor clamping channel (inner diameter, shape, etc.), quantity and design of clamping screws (torque, material, size, shear-off characteristic, etc.) are identical. • If a manufacturer can clearly demonstrate that common and relevant connector design criteria were used for a family of connectors, conformity to this document is achieved by successfully testing the largest, the smallest and two intermediate connector sizes. – Exception no.1: for a family of connectors consisting of five sizes, only the largest connector, the smallest connector, and one connector of a representative intermediate size need to be tested. – Exception no.2: for a family of connectors consisting of four sizes or less, only the largest connector and the smallest connector need to be tested. • If conformity to this document is achieved by successfully testing a connector on dry conductor then approval is achieved for the same conductor used in an impregnated paper insulated cable. • For connectors where one or both sides are designed for a range of cross-sectional areas, and a common clamping or crimping arrangement serves for the connection of the different cross-sectional areas, then mechanical tests on conductors with the largest and smallest cross-sectional areas shall be carried out according to Clause 7. • If conformity to this document is achieved by successfully testing a mechanical connector on round stranded aluminum conductors, this type test approval can be applied to solid aluminum conductors of the same cross-sectional area(s). • If conformity to this document is achieved by successful testing of a through connector, this type test approval can apply to the barrel of a termination which

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uses the same design criteria. Approval of the complete termination can be achieved if the termination connection does not influence the barrel performance, proven through design parameters, drawings or through thermal verification test. • If conformity to this document is achieved by successfully testing a connector on a conductor with water blocking, approval is achieved for the same conductor without any water blocking but not for the same conductor with different types of water blocking.

10.7.2 Additional Tests on MV Connectors/Accessories Some recent studies show an increase in the failure rate of MV joints in several countries. It seems to be related to the increase of the load and new cycle types (especially in the case of renewable generation, e.g., wind farms) and failures appear in unusual conditions. The main contributor to those failures was identified as thermo-mechanical force, which can induce an increase of the resistance of the contacts, a bending of the conductors and a loss of water tightness, and pure thermal effect. The above aspects, except for the thermal effect, are not addressed in current standards for MV connector and accessory testing. For example, in testing MV joint or joint connector, the cables are not required to be fixed; therefore, no mechanical forces are applied to the connector and the joint. The influence of the size of the connector on the thermal behavior of the accessory is also not considered. Axial force generated by temperature change of conductor may be calculated [32]. Calculated values are very high. The values of axial forces measured in laboratories are much lower from calculated but still significant. For example, a theoretical force of more than 60 kN is calculated when temperature of a 630 mm2 aluminum conductor is increased for 60  C, while in the lab the force was measured 16 kN on fixed cable. Another interesting observation from these tests is that type of cable conductor (stranded or solid) does not seem to influence the force. Additional Requirements in Some Countries In order to consider those aspects, some countries require additional tests for their accessories. • Test of the cable system (with the connector and cable as used by the company) with additional cycles at a higher temperature (Belgium). The requirement is 50 cycles at 110  C and 2.5 U0 with a thermal analysis during the last cycle. • Real condition test (France) – Robustness test. Evaluation of Thermal Behavior (Belgium) The goal of the test is to evaluate thermal behavior of the connector in real conditions, in the joint installed on particular cable (the cable system approach). The current is circulated in the test lop for cable conductor to reach different temperature levels at location of TC5: 80  C, 95  C, 110  C, and 125  C. The

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Fig. 10.32 Location of the thermocouples

temperatures of the cable conductor, cable connector, and the joint are measured at each temperature level at locations shown in Fig. 10.32. The requirements were set to: • Temperature TC1 of the joint connector not to exceed temperature TC5 of cable conductor at the point which is at least 1500 mm from the joint body. • Temperatures TC2 and TC3 of cable core in the joint 10 mm away from either side of connector not to exceed temperature TC5. The value of this test is to find out which joint/connector/conductor systems are suitable for the application. In some instances the system with a” large connector” failed the test while the same system with the same joint body with a smaller connector passed. The temperature of the outer surface of the joint TC4 and the hot spot temperature were recorded for engineering information. Simulation of Heat Losses with an Artificial Connector (Belgium) The goal of the test is to evaluate the thermal parameters of the joint body. The test rig is relatively small. The test is realized by injecting controlled losses inside an artificial connector by means of a resistive wire and temperature is measured at different locations of the joint body. Two metal shells are placed above an electric wire which is wound around a cable conductor in order to establish a thermal connection. The test setup is shown in Figs. 10.33 and 10.34.

10.7.2.1 Additional Studies Other tests are studied in order to check their ability to detect poor joints. 10.7.2.1.1 Mechanical Tests on Connectors In the tests made in the past on aluminum connectors, different forces/stresses (see Table 10.14) have been used and up to 200 push/pull cycles have been performed.

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Fig. 10.33 Principle of the injection of losses

Fig. 10.34 Mounting of the wire and shells

Table 10.14 Stresses/forces Lab measurement Lab measurement Applied force during testing IEC 61238-1-3 IEC 61238-1-3

Cross section 240 mm2 630 mm2 240 mm2 240 mm2 630 mm2

F/mm2 27.92 N/mm2 28.41 N/mm2 18.75 N/mm2 40.00 N/mm2 40.00 N/mm2

F 6700 N 17,900 N 4500 N 9600 N 25,200 N

To evaluate the behavior of the conductor connections, a dedicated test setup was built to allow for the application of higher forces and evaluation of the conductor slippage. The test specimen consists of two identical connectors installed in the same way pffiffiffiffi on the conductor with a small distance (e.g., A with A ¼ cross section of the conductor) between the edges of the connectors (Fig. 10.35). This assembly is mounted to the test fixture by appropriate connection parts installed on the barrel of the connectors. Several connector types – including hexagonal and deep indent crimped contacts and screw type connectors – installed on solid and stranded Al conductors of 240 mm2 and 630 mm2 were tested.

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Fig. 10.35 Setup for testing mechanical endurance of connectors

Fig. 10.36 Typical mechanical cycle and typical test record

They were subjected to alternating push-pull force applications. The magnitude of the applied forces was gradually increased in steps from about 25 to 40 N/mm2. Applied force (N) – measured with a load cell – and relative displacement of the contacts (mm) – measured with a miniature displacement sensor installed between the contacts – were logged continuously. Mechanical stress (N/mm2) and energy exchange (Nmm) were calculated from these data (see Fig. 10.36). On some tested objects, the resistance between the connectors was measured while cycled at about 20 N/mm2. A substantial resistance increase was observed at the relaxation phase during force reversal. Samples that had not been prestressed did not show this behavior indicates that an irreversible change had taken place during the cycles with nonelastic deformation of the samples (Figs. 10.37 and 10.38). This suggests that even momentary mechanical overloading – for example, by conductor displacements in non-elastic cable arrangements – affects the contact resistance, the effect of which remains after the stress has been removed. The temperature increase could not be studied in this test setup however. 10.7.2.1.2 Water Ingress in Joint The forces due to the expansion of the conductor may cause the joint to move and bend putting pressure on the water seals. This situation can lead to water ingress inside the joint. The test aims at reproducing the situation in the field when the cable is blocked by the surrounding soil.

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Fig. 10.37 Resistance of 240 mm2 connection

Fig. 10.38 Resistance of a nontested 630 mm2 connection mechanically tested before

The joint, with 5 m of cable connected to each side, is mounted in a fixture that blocks lateral and longitudinal forces of the cables. Cable cleats are installed every 30 cm for a length of 4 m on both sides of the joint. On each side of the joint 1 m cable section is not blocked such that lateral movement is possible in order to get a maximum stress on the water seals. The test setup is shown in Fig. 10.39 and the expected behavior is described in Fig. 10.40. There is a water filled plastic pipe installed around the cable joint with a diameter of at least three times the diameter of the joint. The pipe is water sealed at both ends, between the PVC pipe and the cable. Oversized silicon rubber (for flexibility under mechanical stress) will be utilized as the seals. These seals exist and are commonly used in current water penetration tests. Water will be entered using either an elevated water vessel (for pressure build-up) or directly to a water pressure vessel in order to exert a water pressure according to the desired standard or customer required pressure level. The length of the pipe is 2 m plus the length of the joint so that the water seals inside the pipe are located close to the cable cleats. This minimizes movement of the cable inside the artificial seals which in its turn minimizes the chance of leakage at

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Fig. 10.39 Test setup

Fig. 10.40 Expected deformation of the blocked cable

that location. Furthermore, this minimizes the effect of the plastic pipe on the cable movement. After the test setup has been fully built up at, the following testing procedure applies: 1. After the cable has been installed, the PVC pipe will be filled with water at the required pressure. As soon as the pipe is filled a first sheath test is performed. 2. The assembly is left to soak for 24 h. 3. A second sheath test is performed at 10 kV DC, for 5 min. 4. The cable is subjected to 10 heating cycles as prescribed by IEC 61442, clause 9 [6]. No test voltage shall be applied. The water temperature will not be controlled or measured. 5. A third sheath test is performed at 10 kV DC, for 5 min.

10.7.3 Existing Practice in Testing HV/EHV Connectors Separate tests on connectors are not required in existing IEC Standards for HV/EHV cable systems and accessories (62067 and 60840). However, details of connectors used in accessories must be provided, together with information concerning type test approval where applicable as per the following extract from IEC 62067 (or 60840): Under “Accessory Characteristics” clause 7: “b) conductor connections used within the accessories shall be correctly identified, where applicable, with respect to • assembly technique, • tooling, dies and necessary setting,

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• preparation of contact surfaces, • type, reference number and any other identification of the connector, • details of the type test approval of the connector if applicable;”

In practice, specific type testing of connectors per IEC61238-1-3 Standard has not been carried out, except in cases of a specific customer request. In the absence of IEC test requirements for HV/EHV connectors, manufacturers of HV/EHV cable systems and accessories have been performing tests at their discretion to evaluate performance of new connector designs. Different manufacturers have different test protocols but in essence those are combination of modified requirements from IEC 61238-1-3 (MV connectors) or other national Standards, for example, US Standard ANSI C119.4, Italian Standard CEI 20-28, and British Standard BS 4579: Part 3 (now withdrawn); the network requirements, user’s specifications, and manufacturers’ experience. After being “qualified” in development tests, those connectors were further checked in type and PQ tests of HV and EHV cable systems and accessories. These tests were performed per IEC 60840 and IEC 62067 Standards.

10.7.3.1 Development Tests on HV/EHV Connectors The temperature stability criteria without statistical evaluation of the connection resistance are adopted by most manufacturers of HV/EHV cable accessories/systems in development testing of HV/EHV connectors. This is particularly true for large size connectors, for example, 2500 mm2 and higher, where making equalizers and resistance measurements is extremely demanding and can have questionable end results. If performed, the resistance measurement is used for engineering information only and not as the qualification criterion. The temperature stability is checked in temperature a cycling test. The number of temperature cycles varies between manufacturers. The WG has made attempt to “standardize” number of cycles based on the rational supported by the current development testing of connectors, type and PQ tests of accessories and systems, and field experience. A cable system manufacturer’s test arrangement for temperature cycling testing of connectors on large size conductors is shown in Fig. 10.41. One of the tests that is often adopted by manufacturers is a pretension test. This test simulates the tensile forces acting on connectors in field installations of HV/EHV cable systems. This test is usually performed before heat cycling since tension forces may influence performance of connectors in heat cycling and short circuit testing. The tensile test in IEC 61238-1-3 is performed on separate connectors and not on those that are part of the test loop for temperature cycling. Figure 10.42 shows one of the setups for the pretension test. The short circuit current test is performed only on those connectors that may be thermally or mechanically challenged under short circuit conditions. Final verification of connector performance is achieved by performing type tests and PQ tests (where applicable) on the cable systems/accessories according to IEC 60840 and IEC 62067.

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Fig. 10.41 Preparation of the setup for temperature cycling in connector development test

Fig. 10.42 Pretension test on connectors

Some customers have proprietary system specifications, defining tests to be done and results to be achieved, for connectors to be used in their network. The WG was given access to some such system specifications from major utilities and their requirements were considered and implemented in the new development test proposal wherever possible.

10.7.3.2 Type and Prequalification Tests for HV/EHV Cable Systems and Accessories Type tests are defined in IEC 62067 as: “tests made before supplying, on a general commercial basis, a type of cable system covered by this standard, in order to

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demonstrate satisfactory performance characteristics to meet the intended application.” In IEC 62067 (and 60840) the range of approval for cable system type tests is given in clause 12.2 and specifies that the following condition is met with an explanation in note 3: “c): the cable and the accessories have the same or similar constructions as that of the tested cable system(s)”. “NOTE 3 cables and accessories of similar construction are those of the same type and manufacturing process of insulation and semi-conducting screens. Repetition of the electrical type tests is not necessary on account of the differences in the conductor or connector type or material or of the protective layers applied over the screened cores or over the main insulation part of the accessory, unless these are likely to have a significant effect on the results of the test. In some instances, it may be appropriate to repeat one or more of the type tests (e.g. bending test, heating cycle test and/or compatibility test)”.

10.7.3.3 Work of CIGRE WG B1.06 Concerning Connectors The WG has also considered the work of the CIGRE WG B1.06 on “Revision of Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems” [27] and ▶ Chap. 4, “Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems” of this Book. That WG proposed tests to requalify an already prequalified EHV cable system, in case of less significant changes/modifications of components without doing the full set of type and PQ tests according to the then active IEC Standard (edition 3 of IEC 60840 and edition 1 of IEC 62067). Both PQ (not included in IEC 60840 ed.3) and type tests were reviewed, although the PQ test received greatest attention, as it takes long time and is very costly. In their work, WG B1.06 considered the influence of changes in material, manufacturing process, design and stress level, of already qualified cable system components and proposed “Extension of Prequalification” (EQ) tests to be performed on such modified cable systems/components to qualify the changes. Consequently, IEC accepted the proposals and issued new editions of the Standards (edition 4 of IEC 60840 and edition 2 of IEC 62067). At their next major revision, both standards were changed, incorporating the WG B1.06 proposals: • IEC 60840 (edition 4, 2011): a PQ test was added, together with the EQ test, for cables with high conductor screen or insulation screen stress. • IEC 62067 (edition 2, 2011): the EQ test was added. Tables 2.4 (for EHV cable systems) and 3.4 (for HV cable systems) in TB 303 [27], which are the guides to the selection of tests due to modifications of cable system component in a prequalified HV cable system, do not give any recommendation for tests in the case of change of connector or connector/conductor combination and the current versions of IEC Standards (edition 4 of IEC 60840 and edition 2 of IEC 62067) do not specify any test to be performed in the case of such a modification.

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Annex 5.4 of TB 303 [27] though recommends the “Functional Analysis Method” as means for a systematic assessment of the significance of changes/ modifications in components of a cable system and the tests to prove functionality of the modified component. The analysis includes modifications of “Metallic connection and its eventual covering” for joints and “Metallic connection of conductor to network” for terminations. Tables 5.4.2 for joints and 5.4.3 for terminations from that Annex give a consideration of potential effects of the modification of critical components of the joints and terminations and recommended testing for specific changes within accessories. Excerpts from these two tables, relevant to connector/conductor combinations, are given in Tables 10.15 and 10.16 below. Note that the comments relating to specific IEC standards have been adjusted, where necessary, to reflect the current editions of the standards. The content of these two tables is quite similar. The significant comments are made as follows: • Heat cycles to IEC 61238-1-3 could be used as development tests in case of mechanical connectors and where appropriate. • Short circuit testing is considered, to at least at a level of the network requirement, as a development test. • In relation to demonstrating adequate mechanical properties, the comments indicate that PQ tests were considered as adequate for this.

10.8

Test Regimes for Cable Connector/Conductor Combinations in HV AND EHV Applications

10.8.1 General The WG has considered the test methods described in the current Standard for MV connectors (IEC 61238-1-3) and existing practice in testing and evaluation of HV/EHV connectors performed by cable system/accessory manufacturers. The general view is that test methods and test sequence from IEC 61238-1-3 should not be applied for evaluation of cable connectors for HV and EHV cable systems. During the maintenance cycle for the revision of IEC 61238-1-3 (MV cable connectors), it was decided to limit the scope of the standard to a maximum conductor size of 1200 mm2 mainly because verified test experience for larger size conductor/connector combinations is not available for the time being. Generally larger conductor sizes are in use in HV and EHV cable systems, currently up to 3500 mm2 with a tendency towards even larger sizes. The large cable sizes (>1200 mm2), variety of conductor designs, and variety of designs of connectors for HV/EHV cable accessories may lead to unrealistic test results and unnecessary expense when test requirements from IEC 61238-1-3 are fully followed. It is acknowledged by the WG that current practice adopted by manufacturers of HV/EHV cable systems/accessories in testing and evaluation of connectors

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Table 10.15 Updated and abbreviated excerpt of Table 5.4.2 on functional analysis when joint component is changed (CIGRE TB 303/Chap. 4) Function or Test to check the property Specification/threat functionality Metallic connection and its eventual covering Electrical Transports nominal Heat cycles on continuity/ current without connections. Use IEC electrical overheating 61238-1-3 when resistivity appropriate (¼ not welded): Suggested as Development test

Mechanical properties

Thermal function

Interface with joint semi-con

Supports short circuit current and temperature

Short circuit test following the network needs: (Such test is part of IEC 61238-1-3). Suggested as Development test

Supports longitudinal tension and thrust forces from cable conductor in service. Prevents twisting of conductor during heat cycles Dissipates correctly the heat generated in the connection and avoids overheating in the center of the joint

Heat cycles of cable loop with joint installed: Test requirements are specified in Type and PQ tests of IEC 60840 and 62067 Heat cycles of connections per system type tests in IEC 60840 and IEC 62067, but without voltage + Measurement of temperature of connector versus conductor + examination. Suggested as Development test Long- term test (suggested as development test or PQ + examination)

Compatibility of the possible used additives with the semi-con of the joint (grease, mastic, water sealant) Possible additives (grease, mastic, water sealant. . .) Electrical No negative influence Heat cycles on function on the conductivity of connections (see above) the contact Thermal Supports the Heat cycles of properties temperature of the connections: see above.

Comments Not required per IEC 60840 ed4 or 62067 ed2 The IEC 61238-1-3 presently applies only to connectors for cables 30 kV and below but could be useful for HV connections Short circuit test is not required per IEC 60840 ed4 or 62067 ed2 The short circuit temperature shall not be at 250  C as required per IEC 61238-1-3 but derived from short circuit currents selected from IEC 61443 Are 20 cycles enough to see the effects of longitudinal forces? See tensile strength of welded connectors If a reliable program to calculate the temperature profile in a joint is available, it may replace the test on cable system

Test is also possible on materials: semi-con plates in air oven exposed to the additives

(continued)

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Table 10.15 (continued) Function or property

Chemical properties

Specification/threat connection during service without degradation Gives some protection against electrical degradation of the contact of the connection

Test to check the functionality

Comments

Examination of the additive after cycling Heat cycles of connections: see above. Examination of the additive after cycling

Table 10.16 Updated and abbreviated excerpt of Table 5.4.2 on functional analysis when termination component is changed (CIGRE TB 303/Chap. 4) Function or Test to check the property Specification/threat functionality Metallic connection of conductor to network Electrical Transports nominal Heat cycles on continuity/ current without connections. Use IEC electrical overheating 61238-1-3 when resistivity appropriate (¼ not welded): Suggested as Development test Supports short circuit Short circuit test current and temperature following the network needs

Mechanical properties

Supports compression/ extension efforts during cycling of cable conductor Supports the thermal short circuit efforts

Chemical properties

Resistance to corrosion

Interface with network

Connection fits with terminal lugs of network interface (sliding contacts, bimetallic interfaces. . .)

Heat cycles per requirements specified in Type and PQ tests of IEC 60840 and 62067 Short circuit test following the needs of the network would be useful as a development test Humidity and pollution test as a development test

Comments Not required per IEC 60840 ed4 or 62067 ed2

Short circuit test is not required per IEC 60840 ed4 or 62067 ed2 Short circuit test values could be selected from the data of IEC 60859 (now 62271-209). System aspect Long term PQ tests missing in IEC 60840 ed3 (included in ed4) No such test in HV IEC specifications (for cable systems and accessories)

Matter for engineering of network

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(connector development tests), followed by type and PQ (where required) testing of the cable system/accessories per IEC 60840 and IEC 62067 has been successful. The survey of users of HV/EHV cable systems (see Sect. 10.6) shows positive field experience with existing HV/EHV connectors. Therefore, WG B1.46 has considered that the test methods and the assessment of results per IEC 61238-1-3 should not be mandatory for cable connectors in HV/EHV applications. Nevertheless, recommendations are given to cable system/accessory manufacturers for connector development tests to ensure that the specific conductor/ connector combination will pass a type and (when required) PQ tests according to the relevant HV/EHV standard and perform successfully in service. Based on positive experience from development tests performed by cable system/ accessory manufacturers and other laboratories, the WG proposes that temperature stability should be the criteria for passing connector development tests and omit resistance stability requirement required per IEC 61238-1-3. Most of connector development tests on large size cables, performed by HV/EHV cable system manufacturers, have been done without equalizers for resistance measurement. The test criterion was only temperature stability. As previously stated, such connectors have been successfully used in type and PQ testing of cable systems/accessories and have excellent field record. Some of the issues of implementing resistance stability criteria on large cable conductors are: It is very difficult to make functional equalizers which remain stable during the test on large cables (e.g., 2500 mm2 and above); the connection resistance values are very small and there is question of maintaining required measuring accuracy throughout the test; interpretation of resistance pass/fail criteria have been some of the issues that lab personnel faced during testing large size connectors per IEC 61238-1-3. On the other hand, measuring the temperature of connectors and conductors and assessing temperature stability may be performed for any connector type and any connector/conductor combination. In addition, temperature stability of any connection system to the cable connector, for example, on sliding contacts in plug-in cable terminations, may be evaluated, while the resistance stability criteria is strictly limited to non-movable mechanical and compression connectors. The short circuit withstand capability of connections in HV and EHV cables is so far not covered by existing HV and EHV cable system/accessory type and PQ test standards. In MV-applications test experience with IEC 61238-1-3 shows that application of short-circuit shots may have a significant impact on the temperature and/or resistance stability of the connection. In case of big conductor cross sections, the short circuit current carrying capacity of the cable conductor itself is far beyond the capability of available test facilities. Where short circuit testing is considered necessary WG B1.46 is recommending the application of short circuit levels (current and time) which are realistic, that is, related to actual service levels and future needs in intended new HV/EHV cable system applications using such conductor/connector combinations. As the following proposed recommendations of WG B1.46 for connector development tests, although based on combined practice of several cable system and

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connector manufacturers, are new and not yet tried on a large scale, they are presented for evaluation purposes at this stage.

10.8.2 WG Recommendations for Testing Connectors for HV and EHV Cables WG B1.46 concluded that current practice in verification of the performance of cable connectors (connector/conductor combinations) in HV and EHV applications resulted in positive field service. Existing practice is a combination of development tests, currently performed at the full discretion of cable system/accessory manufactures, and type and PQ (where required) tests of cable systems/accessories. Based on the collective experience of manufacturers of cable systems and accessories, connector manufacturers, and experience in testing both MV and HV connectors, the WG recommends following: • A separate type test for mechanical or compression connectors according to IEC 61238-1-3 is not required and not mandatory in HV/EHV applications for cable systems and accessories complying with IEC 60840 or IEC 62067. • Instead, a test regime as development tests for HV/EHV connector/conductor combinations is recommended as follows.

10.8.2.1 Development Tests for Conductor Sizes up to and Including 1200 mm2 It is recommended that for cables which have conductor sizes up to and including 1200 mm2 that development tests of new connector/conductor combinations shall be carried out per IEC 61238-1-3 but with modified short circuit requirements. If connectors should only be used in HV/EHV applications, the short circuit current, number of short circuits, and duration should be selected based on the short circuit rating required for the accessory in service. No additional development tests are required for connector/conductor combinations in HV/EHV applications that are already approved per IEC 61238-1-3 for MV applications and followed the criteria mentioned in the range of applicability as shown in the Sect. 10.8.3 for design-modifications of connector or conductor compared to the tested combination. 10.8.2.2 Development Tests for Conductor Sizes Above 1200 mm2 The existing standards IEC 60840 and IEC 62067 do not require any specific tests for change of connector type or conductor material or construction. Therefore, in cases where such changes are significant, such as a change of material, or change in conductor or connector design, then it is recommended that development tests are undertaken before these are used in type and PQ (where required) tests. The recommended development tests are described in the subsections below. They are not only limited to mechanical or compression connectors like in IEC 61238-1-3 and may also be used for other types of cable connectors described in Sect. 10.3.2. These

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tests can also be used for evaluation of other connections to cable connectors in cable accessories, for example, aerial lugs, sliding connections in equipment type terminations, and so on. See Sects. 10.8.4, 10.8.5, and 10.8.6 for proposed test loop, development test sequence, and test methods.

10.8.3 Range of Applicability of Development Tests In general, tests made on one type of HV/EHV connector/conductor combination apply to that arrangement only. However, to limit the large number of tests which would be necessary for the variety of existing HV/EHV cable conductor designs, it is essential to develop criteria to predict that combinations of the same connector with other conductors may produce comparable test results without performing tests for every combination. Based on current experience in testing and usage, the range of applicability for development tests of other connector/conductor combination in HV/EHV applications is proposed as follows:

10.8.3.1 Covered Range of Nominal Cross-Sectional Areas of Conductor If the test is carried out on a single cross section, the range covers 20% of the tested cross-section. For example, if connector is tested on 2500 mm2 cable conductor of a certain design and material, the connector may be used on a 2000 mm2 conductor, and not vice versa. If a manufacturer can clearly demonstrate that common and relevant connector design criteria are used for a range of connectors for specified conductors having different cross-sectional areas, then successfully performed development tests on the largest and the smallest nominal cross-sectional area will additionally cover all nominal cross-sectional areas in between for the same conductor design. For example, if connectors for 1200 mm2 and 2500 mm2 are tested, the connectors for 1600 mm2 and 2000 mm2 will also be covered without additional testing. In addition, if the same connector/conductor combination is tested per IEC 61238-13 at 1000 mm2, the design can be used from 1000 mm2 to 2500 mm2. 10.8.3.2 Covered Range Based on Cable Insulation Material: Extruded vs. Impregnated Paper If a connector successfully passed development tests on a conductor used in extruded insulation cables, then performed development test covers, additionally, applications for the same conductor design and material used in impregnated paper insulated cables. 10.8.3.3 Covered Range of Conductor Designs: Round Stranded and Compacted If a connector is successfully tested on a compacted round stranded conductor, then performed development test also covers applications for the use on any round

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stranded conductor with the same or less number of strands and the same material and class (e.g., within the same class according to IEC 60228, which is different for stranded, fine stranded and flexible) or for conductors to be rounded and where all nonconductive material(s) on and between strands are removed during installation of the connector.

10.8.3.4 Covered Range of Conductor Designs: Conductors with Insulated Segments or Strands or with Water-Blocking Material and Conductors Without Those Materials If a connector is successfully tested on a conductor using segment and/or strandinsulation materials and/or water-blocking materials without their removal, then applicability is restricted to the same type, amount, and distribution of materials in the conductor. Successful development tests performed on conductors with those materials are applicable for the same conductor designs without these materials or where these materials are removed during installation of the connector. 10.8.3.5 Covered Range of Conductor Designs: Segmented and Milliken Conductors If connectors are successfully tested on segmented conductors with four, five, or six equal segments, for example, with Milliken design according to IEC 60228 class 2, then performed development test also covers applications for segmented conductors, having less segments and for round stranded conductors of the same crosssectional area and material provided that all non-conductive material(s) on and between strands are removed during installation of the connector. 10.8.3.6 Covered Connection Applications: Through Connectors for the Joints for the Same and Different Size Cable Conductors If connectors are successfully tested for different cross-sectional areas for specified conductors, then performed development test also covers through connectors connecting two conductors of different cross-sectional area within the range, provided that the same connector barrel designs are used and the connector body is produced from one unjointed piece of metal. 10.8.3.7 Covered Connection Applications: Through Connectors and Terminal Lug If through connectors are successfully tested on specific conductor, then performed development test also covers terminal lugs utilizing the same design of the barrel as in tested through connector and on the same cable. 10.8.3.8 Covered Modifications of Mechanical Connectors in HV and EHV Applications Additional criteria have been worked out based on test experience with mechanical connectors on large conductors:

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• If a mechanical connector is successfully tested on round stranded aluminum conductors, performed development test also covers applications for solid aluminum conductors. • If mechanical connectors are successfully tested on stranded conductors, then performed development test also covers applications for conductors with the same cross-sectional area but with different diameter to which this connectors have to be adapted by decreasing the inner diameter of the connector barrel accordingly, as long as other relevant design parameter for the clamping channel (e.g., shape, grooves, surfaces), the clamping bolts (e.g., tightening torque, material, dimensions, tolerances, surface, shear-off characteristic), and connector body (e.g., material, tolerances, surface) are the same as in tested connector. • If mechanical connectors are successfully tested on conductors, then applicability might be assumed for cable applications using the same conductors where the outer diameter of the connector barrel should be increased to have approximately the same thickness as the actual used cable insulation, as long as other relevant design parameter for the clamping channel (e.g., shape, grooves, surfaces), the clamping bolts (e.g., tightening torque, material, dimensions, tolerances, surface, shear-off characteristic), and connector body (e.g., material, tolerances, surface) are the same as in tested connector.

10.8.3.9 Covered Short Circuit Current Withstand Capability If the connector/conductor combination successfully passed short circuit (SC) test, then this test covers applications for such combinations where the Joule-Integral (I2t) value is lower than or equal to that tested. The test setup should fix the conductors in such way to allow for thermal expansion of conductors and prevent radial movement in order to avoid any additional mechanical impact on the connectors due to flow of SC current. If it is necessary to check the behavior of a connector/conductor combination when exposed to dynamic forces, caused by asymmetric SC current, the precise layout of the test loop must be specified. This may be different from above case, where only the thermal effect of the SC is considered. Generally, SC tests are not considered necessary for copper connectors on “bare” copper conductors. Only in specific cases where the copper is not bare, for example, uncleaned, enameled, or oxidized conductors or other insulation material and water blocking material not to be removed during installation of the connector, are these tests recommended. For stranded aluminum conductors, SC tests are generally recommended in all cases due to the existence of a surface oxide layer. Other empirical verified criteria for comparing HV cable connector/conductor combinations should be developed in future to cover other possible cable applications by successfully performed development tests. As soon as more test results are available, shared for comparison, evaluated on a statistical basis, and agreed by technical experts, they may be integrated into the set of commonly applicable “rules” to widen the range of applicability given above.

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10.8.4 Test Loop for Heat Cycling and Temperature Stability Tests for Development Tests with Conductor Sizes Above 1200 mm2 For each series of tests, at least four connectors shall be fitted in accordance with the manufacturer’s instructions, on a bare conductor or on a conductor that has had the insulation removed before assembly, to form a test loop together with the corresponding reference conductor. The installation instructions and the tools recommended by the connector manufacturer must be used for the preparation of the test assembly. An example of the test loop is shown in Fig. 10.43. The figure indicates the minimum length of exposed conductor on either side of the test object and the minimum lateral distance between test objects and between conductors. In this example, individual legs of the test loop with connectors may be easily disassembled for transport to short circuit and mechanical test labs and reassembled quickly when needed. Connectors of the type to be tested are applied onto bare conductors of the appropriate size, material, and type for the connector/conductor combination under test. Disconnecting terminals may be additional test objects (e.g., terminal lug). All conductors of the same cross-sectional area in the test loop shall be taken from the same length. The test setup is applicable to all types of connector in combination with any conductor design and material, not restricted to mechanical and compression connectors.

10.8.5 Recommended Development Test Sequence with Conductor Sizes Above 1200 mm2 The development tests for connectors to be used on conductors above 1200 mm2 should be performed in the proposed test sequence acting on the same samples to

Fig. 10.43 Test loop setup

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accumulate stresses and verify performance of the connector/conductor combination. The development test is to be considered as successfully passed, if the test sequence in the given order is passed and all mentioned verification criteria are met. Note: Resistance stability assessment is not recommended to be used as evaluation criterion for connector/conductor combinations on conductors with crosssection areas exceeding 1200 mm2.

10.8.5.1 Prestress Preferably applied separately to each of the four legs of the test loop shown in Fig. 10.43. Each leg consists of: disconnecting terminal lug–conductor–test connector–conductor– disconnecting terminal lug. (a) Tensile load test per method described in Sect. 10.8.6.1. Each leg of the test loop should be subjected once to a mechanical tensile load which is related to the nominal cross-section area and the conductor material: The force value to be applied in axial direction is: (i) 30 N/mm2 for copper conductors and (ii) 20 N/mm2 for aluminum conductors The tensile force not to exceed 80 kN for copper conductors and 50 kN for aluminum. (b) Six short circuit shots based on the maximum short circuit level of the cable system in service per method described in Sect. 10.8.6.2. After passing tests, a and b above the test samples should be assembled for heat cycling as shown in Fig. 10.43. The test arrangement should be placed in one horizontal plane in a test area where the air is calm, minimizing variations in convection acting on the different specimens. Ambient temperature should be maintained below 35  C during cooling. The test loop may be of any shape, if it is arranged in such a way that there is no adverse effect from the floor, walls, and ceiling.

10.8.5.2 Constant High-Current Temperature Stability Test 200 h with reference conductor temperature between 120  C and 140  C and target temperatures of connectors 100  C should applied as described in Sect. 10.8.6.3. The control parameter during the test is the temperature of reference conductor. The temperature stability assessment should be performed as described in Sect. 10.8.6.3. 10.8.5.3 Heat Cycle Temperature Stability Test 200 heat cycles should be performed (no. of cycles per IEC 60840/62067, 20 for type test plus 180 for PQ test). The cycle regimes are with reference conductor temperature from 120  C to 140  C as described in Sect. 10.8.6.4 and target connector temperatures of 100  C. The control parameter during the test is the temperature of reference conductor.

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Fig. 10.44 Temperature regime of a cycle

The temperature regime of the cycle should be such to ensure that targeted connector temperatures are held stable within the target temperature range as shown in Fig. 10.44. At the end of the heating period, forced air cooling may be used. The reference conductor must cool to within 10 K of ambient temperature before the next heating cycle commences.

10.8.5.4 Tensile Strength Test on (3) New Connectors 60 N/mm2 for copper conductors and 40 N/mm2 for aluminum conductors (as per current IEC 61238-1-3). The tensile force not to exceed 80 kN for copper conductors and 50 kN for aluminum.

10.8.6 Test Methods Following sections describe methods, assessment, and verification criteria of each development test.

10.8.6.1 Tensile Load (Prestress) Test Method The test should be carried out at ambient temperature. Each leg of the test loop should be subjected once to a mechanical tensile load which is related to the nominal cross-section area and the conductor material as defined in Sect. 10.8.5.1 (a). The rate of application of the load should not exceed 10 N/mm2 of the nominal cross-sectional area per second and the maximum force will be maintained for 1 min.

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Test Regimes for HV and EHV Cable Connectors

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The test is considered as successfully passed if no visual slippage between conductor, connector, and disconnecting terminal lugs will occur after test. A stability verification by measurement of slippage is not recommended. The applied forces should be included in the test report.

10.8.6.2 Short Circuit Current Test Method Layout of the test loop: The recommended layout of the test setup for short-circuit current tests is to use single phase test arrangements with one leg out of the test loop from Fig. 10.43 with a concentric return as short as possible and/or a mechanically stiff fixing of the test specimen to minimize mechanical forces and movement induced by current flow through adjacent current carrying structures. Short circuit shots: The short circuits shall be applied to each connector/conductor combination when preheated to 90  C and allowing cooling to between 90  C and 95  C in between short circuits. Preheating of the connector/conductor combination can be done either by heating conductor by an a.c. current or by external heating elements. The short circuit current and duration defines the Joule-Integral to be applied and should be selected based on the maximum short circuit level of the cable system in service. The current and time can be adjusted to achieve the required thermal energy level given by the Joule-Integral, but the duration should not be greater than 5 s (in order to achieve adiabatic heating) and the short circuit current and duration should not be less than 25 kA, 1 s and not more than 45 kA, 5 s. The calculated end temperature of the reference conductor to be tested with the Joule-Integral, for example, according to IEC 61238-1-3 Annex D or IEC 60949, should not be above 250  C. Verification: The test is considered as successfully passed if no obvious signs of overheating can visually be detected. Temperature measurement of the connectors and reference conductor during short circuit tests is not recommended but might be recorded if measured. Starting temperatures of specimen, current and duration of each short circuit shot should be included in the test report. 10.8.6.3 Constant High-Current Temperature Stability Test Method Temperature measurement: The recommended method of temperature measurement is to use thermocouples. Temperature measurements should have device uncertainty within 2 K. The temperature of the reference conductor should be measured on the surface of the conductor or between the first layer of conductor strands halfway between the disconnecting terminal lugs. The temperature of each connector shall be measured on the connector surface in the middle of the connector. The test method according to IEC 61238-1-3 Chap. 6.3.2 is selected to determine the reference conductor temperature to be used and to identify the median connector at equilibrium for the specific test-loop. The median connector in this test setup is defined as the connector which during the first heating records the second highest temperature of the four connectors in the test loop. Heating: Current is circulated in the test loop, bringing the reference conductor to 120  C at equilibrium when temperatures of the reference conductor and the

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connectors do not vary, during application of current, by more than 2 K for 1 h. If at that time the temperature of the median connector (the one with the second highest temperature of all connectors in the test loop) is equal to or greater than 100  C, the 120  C of reference conductor temperature will be used as control parameter for duration of the test. The minimum heating period to maintain temperature stability is 1 h. If median conductor does not reach 100  C with reference conductor at 120  C, then the current shall be increased until the median connector temperature reaches 100  C at equilibrium, subject to the reference conductor temperature not exceeding 140  C. Such determined reference conductor temperature θR, (120  C < θR

140  C) will be used as control parameter for duration of the test. This temperature should be maintained within 5 K for 200 h. Temperature readings for each connector and the reference conductor should be taken 8 h after reaching equilibrium and then at least every 24 h. Verification: Every recorded maximum temperature of each connector in each measurement campaign should not exceed that of the reference conductor measured at the same time by more than 5 K. Additionally the arithmetic mean value calculated from all (four) connector temperatures in all (eight) measurement campaigns should not exceed the highest temperature value of the reference conductor out of this record. The test is considered as successfully passed if the verification criteria are met. The data and the evaluation should be included in the test report.

10.8.6.4 Heat Cycle Temperature Stability Test Method Temperature measurement: The same method for measuring temperature is used as described in Sect. 10.8.6.3. The temperature of the referenced conductor θR previously established in Sect. 10.8.6.3 is control parameter of the head cycle test. The median connector, the one with the second highest temperature, is also identified in Sect. 10.8.6.3. First cycle: The object of the first heat cycle is to determine the heat cycle duration and temperature regime which will be used on the test loop for all subsequent heat cycles. Current is circulated in the loop until the main reference conductor temperature reaches the value θR with a tolerance of 0, +6 K over 120 min period, and the median connector temperature is stable within 2 K over a 15 min period at the end of heating period t1 (see Fig. 10.44). Note: See Appendix D for discussion on justification of using the longer, 120 min, exposure time comparing to that in IEC 61238-1-3. At the beginning of the heat period t1, an elevated current up to 150% of the heating current at equilibrium may be used to reduce the heating period. The current shall thereafter be decreased or regulated to a value of the current at equilibrium to ensure stable conditions during the median-connector control period. It may be necessary to use more than one cycle to determine the temperature regime. The heating period t1 is followed by a cooling period t2 to bring the temperatures of all connectors and the reference conductor to values below ambient temperature. It may be necessary in subsequent heat cycles to adjust t2 to ensure that the temperature conditions are reached. If accelerated cooling is used, it shall act entire test loop, and use air within ambient temperature limits. The total period t1 + t2 constitutes a heat cycle.

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Subsequent cycles: The reference conductor temperature is the only control parameter, to reproduce the temperature regime for all subsequent cycles. In this way, the fluctuation of the ambient temperature will not affect the temperature profile of the reference conductor within the specified tolerances. The heating regime of the reference conductor containing the characteristics of temperatures in time is shown in Fig. 10.43 and should be reproduced in subsequent cycles, while the median connector will not be controlled anymore and may differ by more than 3 K compared to the initial situation. Instead of cooling down below 35  C as done in the first cycle, it is recommended to cool down at least to 10 K above ambient temperature before starting a new cycle. Temperature measurements should be taken by using the method described in Sect. 10.8.6.3. The first measurement campaign should be collected after the 10th cycle, the next campaigns then every 10 cycles. One measurement campaign consists in recording temperatures on each connector and the reference conductor taken every minute during the last 15 min. of the heating period t1. The maximum measured connector temperature in every measurement campaign should be recorded together with the reference conductor temperature measured at the same time. A set of 20 measurement campaigns with four pairs of connector/conductor combinations temperature values will then be available for the assessment of temperature stability. Heat-cycle temperature stability verification: Every recorded maximum temperature of each connector in each measurement campaign should not exceed that of the reference conductor measured at the same time by more than 5 K. Additionally the arithmetic mean value calculated from all (four) maximum connector temperatures in all (20) measurement campaigns should be below the highest temperature value of the reference conductor out of this record. The test is considered as successfully passed if the verification criteria are met. The data and the evaluation should be included in the test report.

10.8.6.5 Tensile Strength Test Method The test should be carried out at ambient temperature. The test shall be made on three additional connectors having the same combination of conductors and installation procedure as used for the electrical test. The recommended conductor length, between connectors or between the connector and the tensile test machine jaws, is 500 mm. The rate of application of the load shall not exceed 10 N per square millimeter of nominal cross-sectional area per second and then up to the value as defined in Sect. 10.8.5.4, which is then maintained for 1 min. The test is considered as successfully passed if not more than 3 mm slippage will occur during the last minute of the test. The applied forces should be included in the test report.

10.9

Conclusions

WG B1.46 has taken into account current practice in testing cable connectors for HV/EHV cables that included: development testing on connectors currently performed by cable system/accessory manufacturers at their own discretion; system/accessory type

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and PQ tests per IEC 60840 and 62067 Standards; existing positive experience with HV/EHV cable connectors in service; existing requirements and experience in testing MV connectors per IEC 61238-1-3 for MV connectors; and work of the CIGRE WG B1.06 on Revision of Qualification Procedures for HV/EHV AC Cable Systems. The WG came to the following conclusions: • A separate type test for mechanical or compression connectors per IEC 61238-1-3 is not required and not mandatory in HV/EHV applications for cable systems and accessories complying with IEC 60840 or IEC 62067. • A separate development tests for HV/EHV connector/conductor combinations followed by type/PQ tests on cable system/accessory is recommended. • The connectors for HV/EHV cables that have been included in type and PQ tests (where applicable) and have been used historically should continue to be used without further separate component testing being required. • Additional testing is also not required for connector/conductor combinations that have not been used in service yet but successfully passed proposed development test for connectors and type and PQ tests (where applicable) for cable systems/accessories. • If separate connector development or type tests have been made according to other specifications these tests are not required to be repeated according to this recommendation. • Recommendations have been made for development tests and the range of approval for new connector/conductor combinations in accordance with proposals made in Annex 5.4 of TB 303, taking into account the existing IEC standard for MV connectors and experience from manufacturers and users. • The WG proposes a sequence of development tests for HV/EHV connector/ conductor combinations based on current practice in development testing of HV/EHV connector and type testing of MV connectors. • To avoid the need to test every size and type of connector, details are given of the range of approval that is valid for different test scenarios (e.g., large and small sizes tested to also cover intermediate sizes). • Involved parties are invited to collect and share experience with the here proposed development tests of new connector/conductor combinations to verify practical use of proposed procedure and assessment before a further standardization of type testing of HV and EHV cable connectors is considered.

10.10 References 1. IEC 61238-1-3:2018 Compression and mechanical connectors for power cables – part 1-3: test methods and requirements for compression and mechanical connectors for power cables for rated voltages above 1 kV (Um ¼ 1.2 kV) up to 30 kV (Um ¼ 36 kV) tested on non-insulated conductors 2. IEC 60840 Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um ¼ 36 kV) up to 150 kV (Um ¼ 170 kV) – Test methods and requirements

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505

3. IEC 62067 Power cables with extruded insulation and their accessories for rated voltages above 150 kV (Um ¼ 170 kV) up to 500 kV (Um ¼ 550 kV) – Test methods and requirements 4. IEC 60050 (461) International Electrotechnical Vocabulary (IEV) – Chap. 461: Electric cables 5. IEC 60228 Conductors of insulated cables 6. IEC 61442 Test methods for accessories for power cables with rated voltages from 6 kV (Um ¼ 7.2 kV) up to 36 kV (Um ¼ 42 kV) 7. IEC 62271-209, 2007: High-voltage switchgear and controlgear – Part 209: Cable connections for gas-insulated switchgear for rated voltages above 52 kV – Fluid filled and extruded insulation cables – Fluid filled and dry-type cable-terminations 8. EN 50299-1, 2014: Oil-immersed cable connection assemblies for transformers and reactors having highest voltage for equipment Um from 72.5 kV to 550 kV. Fluid-filled cable terminations 9. EN 50299-2, 2014: Oil-immersed cable connection assemblies for transformers and reactors having highest voltage for equipment Um from 72.5 kV to 550 kV. Dry-type cable terminations 10. IEC TR 62125 Environmental statement specific to TC 20. Electric cables 11. IUPAC Periodic Table of the Elements 22/11/16. https://iupac.org/what-we-do/ periodic-table-of-elements 12. IEC 60287-3-2 Ed.2: Electric cables – calculation of the current rating – part 3-2: sections on operating conditions – economic optimization of power cable size 13. BOONE Wim, KACKER Arnav, BAL Remco “Copper or aluminium cable conductors, broadly compared in a life-cycle perspective” JiCable Conference 2015 14. Holm R.: Electric Contacts-Theory and Applications. Springer-Verlag 2000. ISBN 3-540-03875-2. 15. Vinaricky, E.: Elektrische Kontakte, Werkstoffe und Anwendungen. 2. Auflage: Springer-Verlag 2002. ISBN 3-540-42431-8. 16. Böhme, H.: Mittelspannungstechnik-Schaltanlagen berechnen und entwerfen. 2. stark bearbeitete Auflage Berlin: Verlag Technik 2005. ISBN 3-341-01495-0. 17. Hildmann, C. Schlegel, S.; Lücke, N.; Großmann, S.: Vergleich genormter elektrischer Alterungsprüfungen für Verbindungen der Elektroenergietechnik mit Erkenntnissen aktueller wissenschaftlicher Untersuchungen. Connectors Symposium, Lemgo, 2015 18. Braunovic, M.: Aluminium connections: legacies of the past. Tagungsbd. 40th IEEE Holm Conference on Electrical Contacts. 17.10-19.10.1994, Chicago, S. 1–31. 19. Naybour, R. D.; Farrell, T.: Degradation mechanisms of mechanical connectors on aluminium conductors. Proceedings of the Institution of Electrical Engineers, 1973, vol. 120, no. 2, S. 273–280. 20. Braunovic, M.: Effect of Current Cycling on Contact Resistance, Force, and Temperature of Bolted Aluminium-to-Aluminium Connectors of High

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23.

24.

25.

26. 27. 28.

29. 30. 31. 32. 33. 34. 35.

36. 37.

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Ampacity. IEEE Transactions on Components, Hybrids, and Manufacturing Technology, 1981, vol. 4, issue 1, S. 57–69. Möcks, L.: Die Stromverteilung in der Starkstromklemme. Elektrie, 1995, vol. 49, no. 8/9, S. 299–303 Hildmann, C.; Schlegel, S.; Großmann, S.; Dockhorn, T.: Investigations on the long-term behaviour of current carrying fittings for high temperature low sag conductors. 23rd International Conference on Electricity Distribution (CIRED), Lyon, 2015 Hildmann, C.: Zum elektrischen Kontakt- und Langzeitverhalten von Pressverbindungen mit konventionellen und Hochtemperatur-Leiterseilen mit geringem Durchhang. Dissertation, TU Dresden, 2016 (http://nbn-resolving. de/urn:nbn:de:bsz:14-qucosa-222889). Hildmann, C.; Grossmann, S.; Dockhorn, T.: The initial contact stress in Aluminium compression connections with high temperature low sag conductors. Tagungsbd. 27th International Conference on Electrical Contacts (ICEC), 22.26.06.2014, Dresden, S. 557562. Pfeifer, S.: Einfluss intermetallischer Phasen der Systeme Al-Cu und Al-Ag auf den Widerstand von elektrischen Verbindungen im Temperaturbereich von 90  C bis 200  C, Dissertation, TU Dresden, in press. Europacable Services Ltd., Great Britain, MECHANICAL SHEAR-BOLT CONNECTORS: A “Best Fit” Solution for Jointing Cable Conductors, 2016 CIGRE TB 194 Construction, Laying and Installation Techniques for Extruded and Self-Contained Fluid Filled Cable Systems, WG 21.17, October 2001 Lücke, N.; Schlegel, S.; Grossmann, S.: Vergleich von Werkstoffen auf Basis von Cu und Al sowie Trends bei deren Anwendung in der Elektroenergietechnik. Metall, 67. Jahrgang, 11/2013. CIGRE TB 303 Revision of Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems., WG B1.06, August 2006 Electra 212_4 Thermal Ratings of HV Cable Accessories, TF 21(B1)-10 CIGRE TB 476 Cable Accessory Workmanship on Extruded High Voltage Cables, WG B1.22, October 2011 CIGRE TB 669 Mechanical Forces in Large Conductor Cross-Section XLPE Cables, WG B1.34 CIGRE TB 247 Optimization of Power Transmission Capability of Underground Cable Systems Using Thermal Monitoring, WG B1.02, February 2004 Quaggia, D at all, Mechanical Connectors used inside M.V. Accessories: a system approach, Paper E9.5, Jicable 2015 CIGRE TB 446 Advanced Design of Metal Laminated Coverings: Recommendation for Tests, Guide for Use, Operational Feedback, WG B1.25, February 2011. CIGRE TB 756 Thermal Monitoring of Cable Circuits and grid Operators’ Use of Dynamic rating Systems, WG B1.45, February 2019. CIGRE TB 689 Life Cycle Assessment of Underground Cables, WG B1.36, May 2017

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Terms of Reference Scope, Deliverables, and Proposed Time Schedule of the Group Background During the meeting held in San Francisco on March 5 and 6, 2012, the SAG (Strategic Advisory Group) of SCB1 discussed what could be the next items to be studied by the Study Committee. One of the topics was: “Conductor joint strength (mechanical for long HV cables) + thermal test for connectors”. Current IEC 61238-1-3 standard applies to connectors for medium voltage cables. There is no IEC standard for connectors for HV cables. The procedures from IEC 61238-1-3 along with manufacturer and user specifications have been used to type test HV cable connectors. The thermal, mechanical, and resistance stability tests specified in current standard are applicable to HV but some requirements are specific to high voltage applications. These include dimensional and functional requirements of connectors within HV cable accessories, typically larger cable sizes, versatility of the conductor constructions as well as different circuit load patterns, short circuit levels and mechanical stresses due to tensile and thrust loads. The IEC WG16 of the TC20 commenced work on revision of current IEC612381-3 standard. During this work, some members of WG16 expressed interest that the scope of this standard is extended to high voltage cable application. The TF in charge of the revision believes this work needs to be done by a dedicated group of high voltage experts. At the Study Committee B1 meeting held in Paris on August 28 and 29, 2012, it was agreed that a task force be established to consider if further guidance was needed on the testing of connectors for HV cable accessories. It was also decided in the meeting that the topics should be expended to cover mechanical loads, not only thermal to include all connectors, not just mechanical and to include termination connectors, not only for the joints. Scope To review • The range and types of connectors currently available • Existing international standards and the extent to which they cover the testing of connectors • Any work been done by CIGRE, CIRED, JICABLE. . . • Extent of service experience so far for different connector types • Customer needs To analyze • Operation on high loaded systems where conductors are approaching or temporarily exceeding maximum conductor operating temperature • Thermo-mechanical performance of connectors under cycling loads

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• Performance of connectors in short circuit conditions, taking into account thermal and dynamic forces and actual network ratings • Performance of connectors installed in cable joints and terminations To propose thermal and mechanical test regimes for connectors for HV and EHV cables with special attention be given to connectors for large size cables. • Type, routine, and sample tests including mechanical, cycling, and resistance stability tests • Consider practicality of the short circuit test for large-size conductors and test loop arrangement • WG should be free to consider mechanical tests (e.g., tensile, thrust forces. . .) in order to evaluate mechanical strength of connection and physical properties of connector itself • WG should be free to consider separate or integral test sequences combining mechanical, cycling, short-circuit, and resistance stability (assessment) acting on the same samples • Extent of connector type test experience so far (for different connector types) • Evaluate necessity of performing type tests on connectors that already successfully passed qualification tests per IEC 60840 • WG should consider range of type test approval The WG should consider the tests that reflect mutual impact between connectors, cable conductors and accessories. The conductor connectors for HV and EHV applications are to be considered. The WG will make recommendation to include or not connectors for MV applications. Deliverables Technical Brochure with summary in Electra and Tutorial

Comparison of IEC and IEEE Type Test Requirements for Extruded Cables and Accessories for Voltages up to 245 kV See Table 10.17

Comparison of IEC and IEEE Type Test Requirements for Extruded Cables and Accessories for Voltages 245 kV and above See Table 10.18

Partial discharge test at high temperature Partial discharge test at ambient temperature (after final cycle or after lightning impulse voltage test in item i) Lightning impulse voltage test followed by power frequency voltage test

AC voltage 1 min dry withstand AC voltage 10 s wet withstand DC voltage 15 min dry withstand Heating cycle voltage test

Test Bending test Partial discharge test at ambient temperature AC withstand voltage

650 kV, 10+/ 10, at Tcmin ¼ 95  C

20 cycles at Tcmin ¼ 95  C, 2Uo 5 pC at 1.5Uo, at Tcmin ¼ 95  C 5 pC at 1.5Uo 12.4.4

– 12.4.4

12.4.7

12.4.4 12.4.4

12.4.7

12.4.7

12.4.4

12.4.6

12.4.6

12.4.6

Clauses IEC 60840 up to 245 kV Type tests on cable systems, cables, accessories Test values (applicable to Cable accessories) systems Cables Accessories 5 pC at 1.5Uo 12.4.3 12.4.3 – 12.4.4 12.4.4 12.4.4

650 kV, 10+/10, at Tcmin ¼ 105  C

30 cycles at Tcmin ¼ 105  C, 2Uo 5 pC at 1.5Uo (120 kV) 5 pC at 1.5Uo (120 kV)

315 kV, 15 min

3Uo (240 kV), 15 min

5 pC at 1.5Uo (120 kV)

7.7.3

7.6.1

7.6.1

7.9.2

7.7.2

7.7.1

Joints 7.6.1

IEEE Std 404-1993 for joints 138 kV

650 kV, 10+/10, at Tcmin ¼ 105  C

30 cycles at Tcmin ¼ 105  C, 2Uo 5 pC at 1.5Uo (120 kV) 5 pC at 1.5Uo (120 kV)

315 kV, 15 min

275 kV, 10 sec

310 kV, 1 min

5 pC at 1.5Uo (120 kV)

Test Regimes for HV and EHV Cable Connectors (continued)

8.4.1.6

8.4.1.1

8.4.1.1

8.4.2 item c

8.4.1.5

8.4.1.2/ 8.4.1.4 8.4.1.3

Terminations 8.4.1.1

IEEE Std 48-1990 for terminations 138 kV

Table 10.17 Comparison of IEC and IEEE Type Test Requirements for Extruded Cables and Accessories for Voltages up to 245 kV

10 509

Test Partial discharge test at high temperature (if not carried out after item g) above) AC voltage 6 h dry withstand Partial discharge test at ambient temperature (if not carried out after item g) above) Tests of outer protection for joints

Table 10.17 (continued)

12.4.4

Annex G

5 pC at 1.5Uo

1m watercolumn, 20 cycles, DC and BIL test

12.4.4

Annex G

12.4.4

–

Clauses IEC 60840 up to 245 kV Type tests on cable systems, cables, accessories Test values (applicable to Cable accessories) systems Cables Accessories – 12.4.4 5 pC at 1.5Uo, at 12.4.4 Tcmin ¼ 95  C

Sectionalizer (if applicable)

2.5Uo (200 kV), 6h 5 pC at 1.5Uo (120 kV)

5 pC at 1.5Uo (120 kV)

7.11

7.6.1

7.1

Joints 7.6.1

IEEE Std 404-1993 for joints 138 kV

2.5Uo (200 kV), 6h 5 pC at 1.5Uo (120 kV)

5 pC at 1.5Uo (120 kV)

8.4.1.1

8.4.1.7

Terminations 8.4.1.1

IEEE Std 48-1990 for terminations 138 kV

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Partial discharge test at ambient temperature (after final cycle or after lightning impulse voltage test in item i) Lightning impulse voltage test followed by power frequency voltage test

Partial discharge test at high temperature

1050 kV, 10+/10, at Tcmin ¼ 95  C

12.4.7.2

12.4.4

12.4.4

12.4.5 12.4.6

1050 kV, 10+/ 10, at Tcmin ¼ 105  C 5 pC at 1.5Uo (200 kV)

30 cycles at Tcmin ¼ 105  C, 2Uo 5 pC at 1.5Uo (120 kV) 5 pC at 1.5Uo (200 kV)

1050 kV, 10+/ 10, at Tcmin ¼ 105  C 5 pC at 1.5Uo (200 kV)

7.7.3

7.6.1

7.6.1

7.6.1

30 cycles at Tcmin ¼ 105  C, 2Uo 5 pC at 1.5Uo (120 kV) 5 pC at 1.5Uo (200 kV)

7.9.2

7.7.2

445 kV, 10 sec 525 kV, 15 min

525 kV, 15 min

5 pC at 1.5Uo (200 kV)

AC voltage 10 s wet withstand DC voltage 15 min dry withstand Tan δ measurement Heating cycle voltage test

7.7.1

Joints 7.6.1

460 kV, 1 min

5 pC at 1.5Uo (200 kV) 3Uo (400 kV), 15 min

AC voltage 1 min dry withstand

Bending test Partial discharge test at ambient temperature AC withstand voltage

20 cycles at Tcmin ¼ 95  C, 2Uo 5 pC at 1.5Uo, at Tcmin ¼ 95  C 5 pC at 1.5Uo

IEC 62067 230 kV Type tests on cable systems only Clauses Cable systems 5 pC at 1.5Uo 12.4.3 12.4.4

(continued)

8.4.1.1

8.4.1.6

8.4.1.1

8.4.1.1

8.4.2 item c

8.4.1.2/ 8.4.1.4 8.4.1.3 8.4.1.5

Terminations 8.4.1.1

IEEE Std 48-1990 for terminations Clauses

Table 10.18 Comparison of IEC and IEEE Type Test Requirements for Extruded Cables and Accessories for Voltages 245 kV and above

IEEE Std 404-1993 for joints Clauses

Test Regimes for HV and EHV Cable Connectors

Test

10 511

Partial discharge test at high temperature (if not carried out after item h above) Tests of outer protection of buried joints

Test

Table 10.18 (continued)

5 pC at 1.5Uo, at Tcmin ¼ 95  C 1 m watercolumn, 20 cycles, DC and BIL test Annex G

12.4.4

IEC 62067 230 kV Type tests on cable systems only Clauses Cable systems 2.5Uo (332 kV), 6h 5 pC at 1.5Uo (200 kV) Sectionalizer (if applicable)

7.11

7.6.1

Joints 7.1

IEEE Std 404-1993 for joints Clauses

2.5Uo (332 kV), 6h 5 pC at 1.5Uo (200 kV)

8.4.1.1

Terminations 8.4.1.7

IEEE Std 48-1990 for terminations Clauses

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Background Behind Range of APPLICABility and Proposed Development Tests The following table gives a brief explanation of the background behind some recommendations for covering other applications by a successfully performed development test. 8.1 General and 8.3 Range of applicability of development tests

8.3.2 Covered range based on cable insulation material: extruded vs. impregnated paper insulation

Collected experience show that an interpolation is possible between the smallest and the biggest positive test results of a connector/conductor combination as long as principle parameter of the connector design and the conductor design are equal and only the nominal conductor cross-sectional area is changed As the tests in IEC 61238-1-3 are more severe than that recommended for connections in HV/EHV applications and the smallest a member of the connector family have been successfully tested according this standard, then it is assumed that the design is suitable for sizes below 1200 mm2 and interpolation down to that tested size might be possible Tests on “fluid filled” conductors produced comparable test results as on “dry” conductors. Wet material may drop out of stranded conductors during heating in bare conductors. The fluid will prevent additional oxidation on the conductor strands. Therefore it was decided to test only in “dry” conditions to cover worst case. In contradiction some tests on impregnated aluminum conductors taken out after decades of service did not pass the test. After further investigations it was decided to restrict the tests to new conductors made out of one continuously produced conductor length. This observation leads to the conclusion that connections to “historic” cables and replacement of

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8.3.3 Covered range of conductor designs: round stranded and compacted conductors

M. Uzelac

joints might not have the same performance level as in complete new cable installations. The unknown degradation of aluminum conductor material will not be a reproducible test criterion A compacted round stranded conductor is considered to be the most critical conductor, because the strands are shaped and strain-hardened during manufacturing and all air in between is removed. Therefore, a connector needs to apply higher radial forces to create radial and axial deformation of strands to remove oxide layers on each surface and create the necessary contact pressure between the metallic parts for achieving a low and stable electric contact resistance. The more layers and strands, the higher this deformation should be to get an impact on most of the strands fixed inside the connector barrel. Where nonmetallic material will not be removed prior to installation, these layers have additionally to be pierced and pushed aside while installing the connector to get a pure metallic contact between the strands of the conductor. Where all strands are brushed and cleaned in the area of a connection, the conductor has to be rebuilt in a round shape which allows the correct placement inside the connector without overstressing the strands by bending and reducing the effective conductor cross-sectional area Experience show that mechanical connectors which are tested on stranded conductors will never have a problem to pass a test on solid conductors of that nominal size which fits into the connector barrel. Intrinsic watertight solid conductors are available in aluminum for the use in power cables,

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Test Regimes for HV and EHV Cable Connectors

515

for example, up to 1600 mm2. Solid conductors of copper of big sizes are not in use for power cables in typical distribution networks. Mechanical connectors are assumed to apply the same radial force via torque limited screws, no matter how the dimension, shape, and hardness of a used conductor will be. But compression connectors may not fulfill this rule, as usually depending on the dimension and hardness of the conductor inside the fixed deformation of the connector/ conductor combination introduced by the tool and die. The dominating phenomenon in tests of connectors on aluminum conductors is the penetration of the oxide layers on its surfaces. Besides the solid conductor, which has its surface only outside, the stranded conductor has additional oxide-layers on each strand. The hardness (or softness) of aluminum used in solid conductors allows axial mechanical tensile strength up to 40 N/mm2 multiplied by the size in mm2 as the maximum applicable value in Newton. At approximately 50 N/mm2 applied axial tensile load value an irreversible elongation of the solid conductor will start, making it thinner and longer which might have an influence on the partial discharge level due to insufficient coupling to the inner semi-conductive layer of the cable insulation. The usual hardness of aluminum material used for solid conductors seems to be “soft” enough for the bending and laying of the cable and “hard” enough that no further “cold flow” is starting inside installed connectors. Test experience show that in case of compression connector different sleeves and/or different compression layout has to be used for stranded and

516

8.3.4 Water blocking material

8.3.5 Use of segment- or strand insulation in the conductor

M. Uzelac

for solid conductors of the same nominal cross-sectional area The use of water blocking material is normally not specified or standardized in material, and distribution inside a connector. It is usually made of an electrically nonconductive or low-conductive material and has to be pushed aside by the connector during installation to guarantee a stable contact behavior to be investigated in a connection test. Similar considerations are applicable to “contact grease” normally applied inside compression connectors for aluminum conductors. Although there might be metallic ingredients of these compounds, it has to be pushed aside to guarantee direct metallic contact between conductor strands and to the connector. Compounds are primarily used to prevent the contacts from further oxidation after installation and are not able to carry the required currents The use of nonconductive materials inside conductors might be advantageous to achieve low AC-resistance values and handle skinand proximity effects of current distribution inside conductors with large cross-sectional areas. At cable ends and in conductor connections it is necessary to inject the current almost homogeneously in each conductive structure. This can be done either by manually removing all nonconductive material from the contact zone or by pushing aside during closing of the connector. Conductors of the same size and material as tested might be used with one connector as long as less or no nonconductive materials are used. This statement is applicable for compression and for mechanical connectors

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Test Regimes for HV and EHV Cable Connectors

8.3.6 Through connectors in joints for connecting different cable sizes

517

The most common approach for using a mechanical or compression connector family for the widest range of application is to remove all nonconductive material on and inside a conductor at least at the zone to be placed inside the connector barrel as to be described in the installation procedure of the connector. But this installation procedure might be time consuming and expected results might be depending on the working skills of fitters and the quality of their work. Rebuilding a segmented, stranded conductor to a round shape of almost the same diameter to fit into the connector barrel with parallel strands, not overstressed by bending and rearranging by not losing too much material, is hardly reproducible So there might be economic and quality assurance considerations to use connectors which are able to perform stable without removing nonconductive materials of the conductor during installation of the connector. The disadvantage is that each change in combination using a different conductor design has to be verified by tests Almost all connector barrels, no matter if used for compression connector or for mechanical connector families are made from one specific material. For connecting two of these barrels in a through connector jointing two equal or different conductors, usually one connector body is formed out of the same material without an additional join-patch in between. This will guarantee that the connector itself will have no additional internal transition resistance which might need a “risk assessment” due to production, installation or expected deterioration

518

8.3.7 Termination lug connectors

M. Uzelac

during service. Most of the “reduction connectors” jointing two different conductors are built in this way. In some cases, it might be necessary to joint two connector barrels made of different material, for example, like in compression connectors for jointing aluminum conductors to copper conductors. For jointing such different materials, for example, friction welding is used and the manufacturing method in this special application can be regarded as a family and might therefore be tested if significant influence of this internal, prefabricated joint in the connector body might be expected. As most of the jointed different conductors should be serial loaded during this test, the “weakest” conductor is limiting test currents There are also cases where two connector barrels in one connector body of the same material are separable by a mechanical connection, for example, to ease installation in specific accessories. The test performance of this additional internal connector joint besides the conductor connections might also be investigated like bimetallic through connectors The thermal situation at cable termination ends is normally less critical than inside joints of HV/EHV-cable accessories. Therefore, additional connector tests at termination bolts or on cable lugs are not necessary, as long as the same connector design criteria at connector barrels are used as successfully tested for through connectors used for joints on the same conductors. Some users are requesting combined tests including the clamping arrangement to the overhead line connection. But in most of the

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Test Regimes for HV and EHV Cable Connectors

8.3.8 Mechanical connectors – adjustment of dimensions to suit application

519

applications the current carrying capacity of the equipment-connection is higher than the nominal current of the connected cable and there is no need for additional tests. Metal parts and their coatings as well as means and arrangements to secure the termination end to other equipment should be able to withstand specified corrosive atmospheres without losing contact pressure. Universal applicable tests are not defined up to now, due to the variety of interfaces and environmental conditions at terminations The diameter of a stranded conductor of a cable is seldom a known and guaranteed characteristic when selecting a cable for a specific project and designing the accessories. For example, in case of aluminum conductors of class 2 according to IEC 60228 there is an informal guidance for minimum and maximum diameter for sizes up to 630 mm2 with a variation of 3.8 mm for this nominal cross-sectional area. A connector should be able to handle such dimensional differences without loss of performance. As in some HV/EHVaccessories it is advantageous to control eccentricity of installed connectors, it might be necessary to modify the inner diameter of a mechanical or compression connector to handle this nonstandardized dimensional difference of conductors within one nominal crosssectional area. To be covered by previous test approval of connector and conductor, it is allowed to decrease the inner diameter. Therefore, a test of the mechanical connector/conductor combination should cover worst case condition with the smallest volume of the connector body or the largest inner diameter of the connector barrel

520

8.3.9 Thermal short circuit tests

M. Uzelac

For example, in slip-on joints it is advantageous to have almost no diameter difference or “step” between prepared cable insulation and connector outer surface to avoid damage at the expanded insulation part while axially moved during installation and to have a good heat transmission by direct and intense connection between installed connector and elastic insulation part of the joint. To achieve this, a connector should be adapted to have the same wall-thickness as the primary cable insulation in each application. For a test of a connector on a conductor, the worst case should be selected, which is given by the lowest heat dissipation in the test setup in bare condition, respectively, the smallest surface of the connector body, the lowest mass and, respectively, the smallest outer diameter The thermal impact of short-circuit current tests on connections is adequately represented by the JouleIntegral applied during adiabatic tests by avoiding any additional dynamic and/or mechanical impact due to current-flow in the specific test setup Mainly the initial temperature shock between “hot” conductor and “cold” connector creates a mechanical stress inside the connector due to different thermal expansion of involved materials, which might shift the “microcontact-spots” to other regions of the connection. This is considered to be the major effect on connections to be checked in adiabatic single phase shortcircuit tests Experience shows that the integrated short-circuit tests in type tests, for example, according to IEC 61238-1-3, will have a clear impact on test results. But for bigger sizes the created

10

Test Regimes for HV and EHV Cable Connectors

8.3.9 Dynamic short circuit test validity is limited to the test-setup

521

temperature rise of short-circuit tests is very limited and created additional stresses due to different thermal expansion inside the connector/ conductor combination will be limited accordingly. Especially for big copper conductors less effect can be seen on the test results of already qualified connectors. So it might be allowed to skip this test procedure, mostly done in a different test laboratory with additional mechanical stress for test setup by dismantling, moving, and rebuilding after tests Although calculation with a shortcircuit current of 45 kA applied for 5 s show that the overall temperature rise will be not more than 30 K when starting from 90  C, which is the same or less as applied during heat cycling, the heat created in the single contact spots between conductor layers and connector body during short-circuit tests is much higher than during heat cycles. This cannot be measured from outside, but it might have an influence on poor designed connectors Experience shows that short-circuit tests shall be mandatory for all connectors for stranded aluminum conductors. The more strands and layers, the higher the risk of insufficient deformation during connector installation and the higher the risk of local overheating of single contact spots due to higher current densities of the remaining strands in the current path will be Some users prefer to apply higher currents for shorter times to get the same energy-input by the Joule-Integral or use an asymmetric test current specifying a certain peak-value or peak-factor. But as higher currents will lead to additional mechanical forces created from the

522

8.5.1 (a) Tensile load test

M. Uzelac

magnetic field of all current carrying parts in the test-setup, test results are specific for this test arrangement and its mechanical fixations. The validity of the test is limited to the used test arrangement only. Users have to check whether expected worst case conditions in practical cable installations will be covered The purpose of the test is to ensure an acceptable basic mechanical strength to stresses which may occur during the erection of a cable system by handling already installed connectors on cable conductors There is no mechanical type test specified in this development test recommendation to be performed separately on additional new samples like described in IEC 61238-1-3 Chap. 7. Mechanical type tests and electrical type tests are strictly separated in IEC 61238-1-3 while the mechanical load test specified in Sect. 8.2.5 is an integral part of each development test sequence. Therefore, the applied values for this combined mechanical and electrical test are lower. The aim is to create a realistic prestress to simulate usual installation conditions for a cable system before any current will be applied The mechanical performance of a connector/conductor combination is usually verified with a single axial pulling test in each assigned combination separately on new samples. The breaking load value of the combination should be sufficiently high above the withstand ability value of this connection, which is 40 N/mm2 for aluminum and 60 N/mm2 for copper multiplied by the nominal size of the conductor in mm2. It is assumed that

10

Test Regimes for HV and EHV Cable Connectors

8.5.1 (b) Short circuit tests

523

usual occurring push-pull-forces in cable installations, created by changing thermal expansion due to fluctuant currents, can be handled without changing the performance of this connector/conductor combination. It is also assumed that the cable installation will avoid nonaxial bending forces acting on the bearing point of the connection. Tests in MV installations simulating such bending forces in cable accessory tests show that the electrical performance is sufficient, if electrical and mechanical tests of connections according IEC 61238-1-3 are passed and installation is done properly The purpose of this test is to ensure basic short circuit current withstand ability to usual HV and EHV cable network service conditions for the intended application Advantage is that only “realistic” conditions are tested in available test facilities in case of conductors exceeding 1200 mm2 and there is up to now no other international recognized standard requiring short circuit tests in HV and EHV cable systems and/or with their accessories Disadvantage is that the complete test sequence should be repeated in case of higher required short circuit values occurring in service The purpose of the test is not to test connectors at 250  C conductor end temperature with max. 45 kA, 5 s for comparing performance-limits of different connector/conductor combinations among each other like done in IEC 61238-1-3 A passed dynamic short circuit test using asymmetric short circuit peak current covers, besides the thermal criteria additionally all applications

524

8.5 2, 8.6.3 Constant high-current temperature stability test

M. Uzelac

where the same or lower maximum Lorentz-forces may occur. If the layout of the test loop and current flow through adjacent parts is specified by dimensions, the same configuration is covered in practical applications The purpose of this test is to create additional ageing stress on the already pre-stressed samples by applying maximum allowed temperatures to the test loop during a long time, created by an almost constant heating current, far beyond nominal currents in cable systems. Before applying the materialexpansions and -relaxations created by heat cycling, the constant application of high temperatures and high currents creates worst case conditions following Arrhenius’ law. This stress accumulation and the temperature limits below 140  C are selected to avoid changes in involved material characteristics, especially for aluminum During 200 h thus determined equilibrium should be maintained within 5 K using the reference conductor as control parameter, in order to keep the temperature constant. In this way, the fluctuation of the ambient temperature will not affect the temperature of the reference conductor within the specified tolerances Temperature measurements should be taken by using the method described in e). The first measurement campaign should be collected 8 h after reaching equilibrium, the next campaigns then every 24 h. One measurement campaign consists in recording temperatures on each connector and the reference conductor taken every minute. If the temperature readings during 15 min do not vary by more than 2 K, the measurement campaign can be stopped.

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Test Regimes for HV and EHV Cable Connectors

8.5.3, 8.6.4 Heat cycle temperature stability test

525

If not, the measurement campaign should be continued until the 2 K-band during 15 min is achieved for every connector. The maximum measured connector temperature in every measurement campaign should be recorded together with the reference conductor temperature measured at the same time. A set of eight measurement campaigns with four pairs of connector/ conductor temperature values will then be available for the assessment of temperature stability The purpose of this final test in the sequence is to verify, if the such pre-stressed conductor/connector combination will be able to pass a cable system qualification including 20 heat cycles for type test plus 180 heat cycles for PQ test with 8 h at rated operating current followed by at least 16 h without current per IEC 60840/62067. The heatcycle temperature-time regime recommended here is much shorter, because performed on a test loop with uninsulated conductors, similar to the test loop and method used in IEC 61238-1-3. This allows to use higher heating currents and end temperatures (because the cable insulation will not limit any more) and speed up heating and cooling times by keeping a high temperature stability phase on the reference conductor of at least 120 min. In IEC 61238-1-3 this high temperature stability phase is around 15 min while the connector temperature should be hold for 10 min at the maximum level. The accumulated exposure time at elevated temperature stage is balanced in this test recommendation to be comparable as well with IEC 60840/ 62067 (for 200 cycles on longer duration but lower temperatures) and

526

8.5.2 and 8.5.3 Temperature stability criteria

M. Uzelac

with IEC 61238-1-3 (for 1000 cycles with much shorter duration) Regarding the test procedure this proposed development test follows strictly IEC 61238-1-3 in order to use the same requirements for performing the test and by using the same concept of “reference method”. That means, that the test can be done by using different parameter if adequate, as long as it is provided that the test will create the same or comparable results. Only in case of doubt, the reference method should be used for comparison The first aim of the high-current temperature stability and heat-cycle temperature stability tests is to show that the temperatures of the connectors are always below the temperature of the reference conductor measured at the same time during current flow in “constant conditions” assuming no changes in the heating current acting as heat source, in heat transfer in involved materials, in the radiation on material surfaces of the test setup, in convection of air flow in the test chamber and in the ambient temperature during these relatively long tests. The second aim is to show that there is no detectable increase of temperatures when comparing initial situation to the situation at the end of test, to prove that possible connector resistance increase inside will have no impact on the measured temperatures on the surface of connectors. But these assumed “constant conditions” can hardly be reproduced and it depends on the skills of lab stuff to balance stability between “hot” test setup and “cold” chamber while differences in natural convection might influence temperature of specimen in different positions inside

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Test Regimes for HV and EHV Cable Connectors

8.5 2, 8.5.3, 8.6.3, 8.6.4 Temperature measurement

527

the chamber. Therefore a tolerance for individual readings of 5 K is introduced and all connectors of the same design are “pooled” by the mean values to verify stability compared to the reference conductor to avoid, that the connectors will be designed to be too close to the limit where they might be warmer as the reference conductor. Nevertheless, the set temperature criteria in this recommendation for development tests seem to be demanding and future shared test experience will show if selected criteria should be kept Due to skin-effects in conductors above 1200 mm2 created when using a.c. heating currents, there are a higher surface temperatures than inside the conductor and due to proximity effects in the test loop setup, there might be higher temperatures at surfaces adjacent to other current carrying parts. This can be observed when using d.c. heating currents, which will create lower temperatures when applying the same value of current. When using a.c. current heating these effects cannot be avoided and are minimized as long as surface temperature measuring spots are at conductor and connector surfaces with adequate distance to each other

528

M. Uzelac

Milan Uzelac graduated from Electro-Technical University of Belgrade, Serbia, and joined Minel-Elektrooprema, Belgrade, as a design engineer, senior design engineer, and the head of R&D Department. Milan relocated to United States in 1989 and joined G&W Electric Company, Chicago, as an R&D engineer and product manager. Currently, he is chief R&D engineer. His responsibility has been the development and design of accessories for extruded and laminated high and extra-high voltage cables. Milan is active in IEEE working groups for developing industry standards for high voltage cable accessories (IEEE 48 and 404). He chaired IEEE WG for IEEE 1300 Standard for cable connections in gas-insulated switchgear. He has served as a member of several CIGRE working groups considering HV cable systems and was convener of CIGRE WG on connectors for HV cables. He was author of several papers and tutorials on high voltage cable accessories and is co-author of EPRI Underground Transmission Systems Reference Book.

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power Cables up to 145 kV

11

Pierre Mirebeau

Contents 11.1 11.2 11.3

11.4

11.5

11.6

11.7

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Terms of Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Definitions and Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.2 Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Criteria for Interface Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4.1 Number of Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4.2 Technical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4.3 Impact of Short Circuit Time Going to 40 kA 3 s . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4.4 Interface Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cable Library Dimensions State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5.1 Voltage Class 72.5 kV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5.2 Voltage Class 123 kV and 145 kV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inner and Outer Cone Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.6.1 General Evaluation of Inner and Outer Cone Technologies . . . . . . . . . . . . . . . . . 11.6.2 Evaluation of Inner and Outer Cone Technologies per Voltage Class . . . . . . . 11.6.3 Evaluation of Conductor Locking Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.6.4 Conclusion on Interface Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.5 kV Insulator Design and Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.7.1 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.7.2 Type Tests and Routine Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.7.3 Examples of Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

532 532 533 533 535 536 536 537 537 538 538 538 538 539 539 541 543 543 543 544 547 547

Pierre Mirebeau has retired. Electronic supplementary material: The online version of this chapter (https://doi.org/10.1007/ 978-3-030-39466-0_11) contains supplementary material, which is available to authorized users. P. Mirebeau (*) Villebon sur Yvette, France e-mail: [email protected] © Springer Nature Switzerland AG 2021 P. Argaut (ed.), Accessories for HV and EHV Extruded Cables, CIGRE Green Books, https://doi.org/10.1007/978-3-030-39466-0_11

529

530

P. Mirebeau

11.8

548 548 552 552 552 555 564 564 566 569 571 571 571 589 590 590 591 591 591 591 594 594

123 kV and 145 kV Insulator Design and Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.8.1 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.8.2 Type Tests and Routine Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.8.3 Example of Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.9 Pressure Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.10 Risk Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.11 Common Insulator Design Credibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.11.1 72.5 kV Insulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.11.2 145 kV Insulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.12 Qualification Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.13 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Evaluation of Inner and Outer Cone Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reference of Available Tests for Common Interface Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Routine Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Type Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prequalification Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Development Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tests After Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Information on the Selection of 145 kV Interface Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principles of Use of the Current Connection Areas of the Common Interface . . . . . . . . . . . . . . . . Principle of use of the Current Transmission Above the Lock-in System Area . . . . . . . . . . . Principle of use of the Current Transmission Below the Lock-in System Area ............................................................................................... Qualification Process Experts Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

595 596 597

Executive Summary In many countries, the market trends are towards a commoditization of the high voltage cables lower or equal to 145 kV. IEC TC 17, in charge of the maintenance of IEC 62271-209 for “Cable connections for gas-insulated metal-enclosed switchgear for rated voltages above 52 kV” asked CIGRE to evaluate technically the feasibility of a common interface. A first joint Working Group B1–B3 was set up in 2010 and concluded that it is feasible to have a common interface for voltages up to 145 kV and a rated current
1000 A. See TB 605. A second Working Group was set up in 2015 to propose standardized dimensions of the interface(s). The work of this Working Group B1–B3.49 is presented in CIGRE TB 784 and in this chapter of the book. After evaluation of the requirements such as: • • • • •

GIS cable enclosure dimensions Cables and accessories portfolio Feedback from the maintenance team of IEC 62271-209 Test requirements from the IEC 60840 Market trends

The Working Group recommends two different designs: • Outer cone design for Um ¼ 72.5 kV • Inner cone design for Um ¼ 123 kV and 145 kV

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In some countries, there are other maximum voltage levels Um such as 82.5 kV (Belgium), 84 kV (Japan), 100 kV (France), 126 kV (China). The common interfaces design can be used for these voltage levels; however, the test voltages need to be defined by national standards or specification. It is recommended to use the common design with Um values where test voltages are higher than the national level test voltages. There is only one design applicable for one voltage level. This is a consequence of the common interface concept. The Working Group is aware that other designs are existing as inner cone for 72.5 kV and outer cone for 123 kV and 145 kV. The designs are based on the manufacturers’ field and qualification experience, electrical field calculations, known material properties. • For 72.5 kV, the design is based on the on-going standard EN 50673. It has been validated by existing qualifications and installations. • For 123 and 145 kV, the proposed design is new, with larger design margins in reference to existing designs. No prototype has been made nor tests performed during the Working Group progress. The proposed designs of the inner and outer cone are described in Figs. ▶ 5.1 and ▶ 6.1, paragraphs 5.1 and 5.3. In addition to the drawings in written form, a .dxf file is included in the downloadable document, which can be used with any standard drawing software. For the range of products as defined by the terms of reference, the GIS manufacturer will get the possibility to order the cable termination interface and complete the GIS manufacturing up to and including the cable termination insulator, independent from the supplier of the cable and cable termination. With the common interface, the responsibilities will change as compared to the division of supply that is specified in IEC 62271-209. The scope of supply of the GIS manufacturer includes the insulator. In this new context, a risk assessment has been performed. It is available in Sect. 11.10. The proposed designs allow more compact cable termination envelopes which may be developed by GIS manufacturers. The qualification process, based on IEC 60840 requirements, has been studied. Three different cases were identified: • Initial qualification • Cross qualification in case of available initial qualification for both insulator and cable/stress cone assembly • Cross qualification without initial qualification of either insulator or cable/stress cone assembly The qualification process is available in Sect. 11.12. If these proposed interfaces are accepted by the market, they can be used for standardization.

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Background

Taking into account the market trend in some countries towards a commoditization of the High Voltage cables lower or equal to 145 kV, and as per a request of IEC TC 17 who is in charge of the maintenance of IEC 62271-209, the working group B1– B3.33 (TB 605) had concluded that there is room in these voltage levels for a standard design in parallel with the present designs. The common interface does not replace the cable termination standard IEC 62271-209; it consists basically in an alternate choice of a design which is given to the GIS manufacturer.

11.2

Terms of Reference

The goal of the JWG is to recommend a functional design of an insulator with a common interface. 1. Current is
1000 A. Short circuit is
40 kA 1 s. Cross sections are
1000 mm2 Cu or 1600 mm2 Al. 2. Technology has to be defined (inner or outer cone), with a detailed evaluation of technical advantages/disadvantages of the two technologies. 3. The number of sizes has to be defined; the short circuit current can be altered for the smallest sizes. Dimensions of insulator components have to be defined (current connection, electric design and properties, mechanical design and properties). The type and dimension of the main current connection have to be defined. 4. Consideration to be given to the consequence of a termination failure, the upgrading of the cable link for higher current loads, and installation constraints, with a special focus on the basement dimensions. 5. The design has to meet the requirements of IEC 62271-209 and IEC 60840 and there is a need to define the initial and cross qualification processes. 6. The stress cone design and material, the lubricant, and the design of the compression device should be left to the discretion of the accessory manufacturer within the limits of the standardized insulator properties. Cigre TB 303 (▶ Chap. 4, “Qualification Procedures for HVand EHVAC Extruded Underground Cable Systems” of this book) and the work of WG B1.44 (induced voltage) and WG B1.46 (connectors, TB 758 and ▶ Chap. 10, “Test Regimes for HV and EHV Cable Connectors” of this book) should be taken into account.

Note: The common interface proposed in this chapter is situated between the insulator assembly and the cable stress cone assembly as defined in Sect. 11.3.1.

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11.3

533

Definitions and Units

The definitions and terms of the different components of the dry type GIS cable terminations as described in IEC 62271-209 and CIGRE TB 605 (Chap. 7) are shown in Tables 11.1 and 11.2.

11.3.1 Definitions 11.3.1.1 Cable-Termination (IEC 62271-209) Equipment fitted to the end of a cable to ensure electrical connection with other parts of the system and to maintain the insulation up to the point of connection. Two types are described in this standard. 11.3.1.1.1 Fluid-Filled Cable-Termination (IEC 62271-209) Cable-termination which comprises a separating insulating barrier between the cable insulation and the gas insulation of switchgear. The cable-termination includes an insulating fluid as part of the cable connection assembly. 11.3.1.1.2 Dry-Type Cable-Termination (IEC 62271-209) Cable-termination which comprises an elastomeric electrical stress control component in intimate contact with a separating insulating barrier (insulator) between the cable insulation and the gas insulation of the switchgear. The cable-termination does not require any insulating fluid. Table 11.1 Identification of the different parts of GIS termination, inner cone type design GIS main circuit end terminal Connection interface Plug-in connector of insulator (integrated electrode) Insulator assembly Insulator Cable connection enclosure Flange (if needed) .

Sectionalising insulation

Plug-in connector of cable

Stress cone Cable/stress cone assembly Cable gland

Cable .

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Table 11.2 Identification of the different parts of GIS termination, outer cone type design Connection interface

Cable connection enclosure Insulator assembly Insulator Sectionalising insulation Plug-in connector of insulator

Stress cone

Plug-in connector of cable

Cable/stress cone assembly

Cable gland

Cable

11.3.1.2 Plug-in Cable Termination (IEC 62271-209) Cable termination where cable/stress cone assembly can be engaged into the insulator assembly that is already installed into sealed GIS enclosure. 11.3.1.2.1 Locked Plug-in Type Cable Termination (TB 605/Chap. 7) Plug-in cable termination where conductor of the cable is interlocked with the insulator assembly and cannot be removed without disassembling insulator assembly from the GIS enclosure. 11.3.1.2.2 Plug-in, Plug-out Type Cable Termination (TB 605/Chap. 7) Plug-in cable termination where the plug-in assembly may be removed from the barrier insulator assembly without disassembling the insulator assembly from the GIS enclosure.

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11.3.1.2.3

Locking Plug-in, Plug-out Type Cable Termination (TB 605/Chap. 7) Plug-in cable termination where conductor of the cable is interlocked with the insulator assembly and can be removed without disassembling the insulator assembly from the GIS enclosure.

11.3.1.3 Insulator Assembly (TB 605/Chap. 7) Assembly of insulator, plug-in connector of insulator, and flange if needed. 11.3.1.4 Insulator (TB 605/Chap. 7) Separates insulating gas (SF6) of GIS enclosure from the cable/stress cone assembly. 11.3.1.5 Plug-in Connector of Insulator (TB 605/Chap. 7) Provides connection to GIS main circuit end terminal and to plug-in connector of cable. 11.3.1.6 Plug-in Connector of Cable (TB 605/Chap. 7) Provides connection between cable conductor and plug-in connector of insulator. 11.3.1.7 Main-Circuit End Terminal (IEC 62271-209 and Compliant with IEEE 1300) Part of the main circuit of a gas-insulated metal enclosed switchgear forming part of the connection interface. 11.3.1.8 Cable Connection Enclosure (IEC 62271-209 and Compliant with IEEE 1300) Part of the gas-insulated metal-enclosed switchgear which houses the cabletermination and the main-circuit end terminal. 11.3.1.9 Cable Connection Assembly (IEC 62271-209 and Compliant with IEEE 1300) Combination of a cable termination, a cable connection enclosure, and a main circuit end terminal, which mechanically and electrically connects the cable to the gas-insulated metal enclosed switchgear. 11.3.1.10 Cable System (IEC 62271-209) Cable with installed accessories. 11.3.1.11 Sectionalizing Insulation (IEC 60840–2019) Insulating portion of the termination separating cable system screen from GIS enclosure.

11.3.2 Units 11.3.2.1 Pressure All pressure values in this document are given in bar as relative pressure. 11.3.2.2 Rated Voltages (IEC 60840) The symbols U0, U, and Um are used to designate the rated voltages of cables and accessories where these symbols have the meanings given in IEC 60183:

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U0 ¼ the rated r.m.s. power-frequency voltage between each conductor and screen or sheath for which cables and accessories are designed. U ¼ the rated r.m.s. power-frequency voltage between any two conductors for which cables and accessories are designed. Um ¼ the maximum r.m.s. power-frequency voltage between any two conductors for which cables and accessories are designed. It is the highest voltage that can be sustained under normal operating conditions at any time and at any point in a system. It excludes temporary voltage variations due to fault conditions and the sudden disconnection of large loads. Unless specified differently all voltages mentioned in this brochure are considering Um values. IEC 62271-209 includes definition of rated voltages and insulation levels taking into account both standards IEC 60840 and IEC 62271-203.

11.4

Criteria for Interface Selection

This chapter details the different requirements that were set by the Terms of Reference regarding the interface selection. • • • •

Number of interfaces Compliance to standards Technical considerations Interface design

11.4.1 Number of Interfaces The purpose of the common interface is to provide an option to GIS manufacturers that insulator supply is independent from cable termination manufacturer. It will not be the case if there are two common interfaces as all cable termination manufacturers will not be intending to develop a solution for both interface designs (i.e., inner cone and outer cone for the same voltage level). Hence, there will be one single common interface per GIS type. Regarding our scope of work, above 52 kV to including 145 kV, IEC 62271-209 defines two GIS termination types: • One type above 52 kV up to 100 kV • One type 123–170 kV Therefore, there will be maximum two interfaces. The reference voltages as per IEC 60840 are the following (Uo, range of U, (Um)): • 36, 60–69, (72.5) kV • 64, 110–115, (123) kV • 76, 132–138, (145) kV

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Hence there is a total of three voltage levels: 72.5, 123, and 145 kV. In some countries, there are other maximum voltage levels such as 82.5 kV (Belgium), 84 kV (Japan), 100 kV (France), 126 kV (China). The common interfaces design can be used for these voltage levels; the test voltages need to be defined by national standards or specification. It is recommended to use the common design with Um values where test voltages are higher than the national level test voltages.

11.4.2 Technical Considerations To anticipate the future needs, the following properties of the GIS termination were selected as relevant factors for the interface selection. • Feasibility of capacitive couplers for power conductor voltage and partial discharges measurements • Current access for DC routine test resistance measurement of GIS (100 A and voltage drop), without the cable – stress cone assembly • Minimum parts of the further installed cable – stress cone assembly • Clear responsibility of supply • Feasibility to be used as an interface to a test bushing for GIS after installation test • Feasibility to be used as an interface to a test bushing for cable after installation test • Feasibility to be used as an interface to Lightning Arrester, or to a Voltage Transformer at the same time as cable connection • Cable oversheath testing to be simple • Qualification to be simple • Measurement of cable screen voltage • Possibility of dimension decrease of the cable termination enclosure of the GIS • Conductor lockable/unlockable • Anticipate future developments to larger cross section and current • Safety

11.4.3 Impact of Short Circuit Time Going to 40 kA 3 s In IEC 62271-1, paragraph 4.7 the standard time is 1 s. But lower and higher values up to 3 s. are allowed (recommended values are 0.5 s, 2 s, and 3 s). Most TSO (Transmission System Operators) use the 1 s. short circuit time; UK TSO is the only utility that specifies the Icc 40 kA 3 s. An enquiry had been made to UK TSO: it seems that UK TSO has taken a worstcase position when determining the short circuit current levels and clearance times. At 132 kV, 40 kA is a practical value and will not be considered for change; however, the clearance time appears to be something that can be looked at further. The feeling is that this may be able to be reduced to 1 s. Following discussion of the matter it was agreed in the WG that for the standard design of insulator (for 145 kV), we should consider 40 kA/1 s and not 40 kA/3 s.

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11.4.4 Interface Designs To allow for a fair and unbiased selection of the common interface, the design will be selected • Either in the available standardized designs • Either as a new design not commercially available at the time of selection Selected designs have been checked as freely available to all manufacturers (no patent included). CIGRE Patent Policy

“CIGRE publications including brochures are non-binding: their objective is to ensure dissemination of information and understanding on a worldwide basis. These documents in respect of their use and applications need to be accessible to everybody. It follows therefore, that a patent embodied fully or partly in a CIGRE publication must be accessible to everybody without undue constraints. To meet this requirement in general is the sole objective of the code of practice. The detailed arrangements arising from patents (licensing, royalties, etc.) are left to the parties concerned, as these arrangements might differ from case to case.”

11.5

Cable Library Dimensions State of the Art

The cable catalogue was constructed taking into account all cable models that were collected by the group members. It applies only to extruded cables, XLPE and EPR. The purpose is not to build a comprehensive catalogue; it is intended to give the diameter range that the interface must accommodate.

11.5.1 Voltage Class 72.5 kV The cable characteristics that are dimensioning the GIS interface are mentioned in Table 11.3.

11.5.2 Voltage Class 123 kV and 145 kV The cable characteristics that are dimensioning the GIS interface are mentioned in Table 11.4.

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Table 11.3 72.5 kV cable characteristics Voltage class (kV)

Cross section Aluminium and Copper (mm²)

72.5 72.5

Diameter over insulation (mm)

Insulation thickness (mm)

Minimum

Maximum

Minimum

Maximum

150

32.2

46.8

7.15

13.65

185

33.8

47.3

7

13.5

72.5

240

35.9

47.7

7.15

13.5

72.5

300

38.8

49.8

7.25

13.5

72.5

400

41.8

51.4

7.35

13.5

72.5

500

45.1

55

7.5

13

72.5

630

49.2

59

7.6

13

72.5

800

53.6

62.9

7.7

13

72.5

1000

56.6

69.3

7.8

13

72.5

1200

61.3

72.4

7.85

13

72.5

1400

65.6

75.9

7.95

13

72.5

1600

72.6

81.4

8.05

13

72.5

1800

79.5

82.9

11.5

12.1

72.5

2000

76.3

86

9.75

12.1

72.5

2500

89

92.4

11.5

12.1

The grey cells are given for information as they are not in the TOR. Some cable models are coming from a single source

11.6

Inner and Outer Cone Evaluation

The choice of the interface technology (inner or outer cone) has been performed • By performing an evaluation as compared to the selection criteria of Chap. 2 • By the experts analysis and recommendation

11.6.1 General Evaluation of Inner and Outer Cone Technologies Inner cone and outer cone technologies have been evaluated as a function of the selection criteria (refer to Sect. 11.2). This did not bring any obvious advantage to one technology. Details of the comparison can be found in Appendix General Evaluation of Inner and Outer Cone Technologies; first lines are reported in Tables 11.5 and 11.6.

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Table 11.4 123 kV and 145 kV cable characteristics Voltage class (kV)

Cross section Aluminium and Copper (mm²)

123 123

Diameter over insulation (mm)

Insulation thickness (mm)

Minimum

Maximum

Minimum

Maximum

240

47.5

58.4

13

19

300

48.6

59.6

12.5

18.5

123

400

51.3

60.7

12.5

17.5

123

500

54.8

64

12.5

17.25

123

630

59.8

71.7

13

20

123

800

63.7

71.3

13

17.15

123

1000

71

82.5

13.5

20.75

123

1200

74.1

77.7

13.5

16

123

1400

77.6

81.2

13.5

16

123

1600

80.7

84.3

13.5

16

123

1800

84.5

88.1

14

14.7

123

2000

87.6

91.2

14

14.7

123

2500

94

97.6

14

14.7

145

185

53.7

53.7

17.1

17.1

145

240

53.5

57.1

16

16.8

145

300

54.6

60.9

15.5

18

145

400

55.3

63.7

14.5

18

145

500

57.8

61.3

14

14.7

145

630

62.8

70.7

14.5

18

145

800

66.7

74.6

14.5

18

145

1000

73.1

77.7

15

15.75

145

1200

77.1

84.2

15

18

145

1400

81.6

85.4

15.5

16.3

145

1600

83.1

92.3

15.5

18.6

145

1800

87.5

91.3

15.5

16.3

145

2000

90.6

94.4

15.5

16.3

145

2500

98

101.8

16

16.8

The grey cells are given for information as they are not in the TOR. Some cable models are coming from a single source.

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Table 11.5 Inner cone evaluation Inner cone model

Aspects

Dimensions of insulator and connection

Dimensions/size of insulator

... a

Remarks

Length inside GIS tank is more than for outer, but still shorter than traditionala Shortening of enclosure is possible yet not practiced for now Diameter is adapted to diameter of enclosure Length of connector outside of enclosure Larger above the base plate, smaller below Smaller size of plug in connector of insulator, less metal with same installation length (given by IEC 62271-209) ...

Rating –, -, 0, +, ++ 72.5 kV 123 kV and 145 kV + +

0 + 0 +

0 + 0 ++

“Traditional” refers to fluid filled termination

11.6.2 Evaluation of Inner and Outer Cone Technologies per Voltage Class A detailed analysis was performed by all experts. The possibility of having a single interface, inner cone, for the complete voltage range 72.5–145 kV was considered. This was not retained because the insulator base plate diameter and its length at the 72.5 kV level were not compatible with the dielectric requirements for the 145 kV larger cables.

11.6.2.1 Recommendation for the 72.5 kV Voltage Level The outer and inner cone models were considered as flexible regarding installation and implementation of new functionalities (cable connector configuration straight or elbow, parallel connections, installation of arresters). A standardization work was in progress at CENELEC for an extension to 72.5 kV of the type F outer cone and type 3 inner cone system from EN 50181. The new EN

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Table 11.6 Outer cone evaluation Outer cone model

Aspects

Dimensions of insulator and connection

Dimensions/size of insulator

...

Remarks

Length inside GIS enclosure could be much smaller with outer cone design than with inner cone but dimensions fixed by IEC 62271-209 usually does not allow this possible length reduction Allows smaller diameters of insulator and of the GIS since neither the cable nor the deflector need to engage Needs more space below the enclosure once installed Allows compact T-connector system Can be shorter above the base plate, but longer below the base plate Heavier entire design assumed (caused by heavier metal electrode though entire insulator) ...

Rating –, -, 0, +, ++ 72.5 kV 123 kV and 145 kV ++ ++

+

+

-

-

++ 0

++ 0

0

-

Mirror groups were held in several countries to enlarge the vision outside our experts, including TSO representatives. This did not bring strong elements in favor of a specific technology

50673 standard “Plug-in type bushings for 72.5 kV with 630 A and 1250 A for electrical equipment” is in its finalization stage at the time of publishing this brochure. A tendency to move from inner to outer cone was recognized and implemented in some projects of offshore windfarms at that voltage level. The outer cone design would allow a more compact GIS equipment compared to the dimensions specified in IEC 62271-209.

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Impact of a cable assembly failure on the GIS enclosure is minimized in case of outer cone insulator assemblies compared to inner cone insulator assemblies. On the other hand, there was a limited experience, and the insulator may be more exposed to mechanical damage than the inner cone during transportation. As a result, by 11 against 4 a majority of experts recommended the outer cone design at the 72.5 kV level, specially referring to EN 50181 – F.

11.6.2.2 Recommendation for the 123 kV and 145 kV Voltage Levels The inner cone was considered as more adequate due to the proven experience, with well-known materials, a minimized R&D work to make the new insulator, as compared to the outer cone design. It was recommended to add improvements as compared to the current practice where possible. The inner cone market acceptance is already established since it is implemented in many substations at that voltage level. Finally, experts unanimously recommended the inner cone design at the 123 kV and 145 kV levels, designing a single new insulator, to avoid competition distortion, where every accessory manufacturer can develop a cable termination.

11.6.3 Evaluation of Conductor Locking Connector Due to large thermomechanical forces (refer to TB 669), the discussion has led to give the possibility to lock the conductor connection inside the common interface. This feature is already implemented by some manufacturers.

11.6.4 Conclusion on Interface Technology For the 72.5 kV voltage level, the outer cone is chosen. It is according to the F cone as per EN 50181 that is currently being standardized at the 72.5 kV level in EN 50673. For the 123 kV and 145 kV voltage levels, the inner cone is chosen. It is a new insulator where dimensions have to be specified, see Sect. 11.8. The conductor connection design must allow to lock the conductor.

11.7

72.5 kV Insulator Design and Specification

The interface is specified in EN 50673 “Plug-in type bushings for 72.5 kV with 630 A and 1 250 A for electrical equipment,” which is derived from EN 50180 and EN 50181. The selected interface type is “F5.”

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11.7.1 Design 11.7.1.1 Geometrical Parameters Taken from EN 50673 The general parameters of the interface are shown Fig. 11.1

11.7.1.2 Additional Geometrical Requirements An electrode insert (screen in Fig. 11.1) at ground potential is necessary to screen the GIS bottom plate. The top of the insert must be at 92  2 mm from the top of insulating resin. Its minimum distance to the epoxy surface is 1 mm. A minimum clearance of 25 mm is needed between the bottom of the cone and the fixing elements of the interface to give enough insulating length to the screen interruption. The interface from insulator assembly to GIS is so far not completely specified to allow smaller dimensions of GIS compartment than the dimensions given in the current IEC 62271-209 (Fig. 11.2). A principle DXF file of the insulator with incorporated additional requirements is included as a downloadable document in the online version of this chapter. Table 11.7 72.5 kV common interface dimension (from EN 50673)

Interface type F5

Fig. 11.1 72.5 kV common interface

I

ØD

A 1250

mm 32

11

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . .

545

Fig. 11.2 Design of the 72.5 kV interface

Table 11.8 Dielectric requirements of 72.5 kV common interface Property Dielectric properties of insulator material (permittivity, tanδ) Dielectric performance requirement of completed insulator (electrical withstand level, PD level) Insulation shield break ring electric withstand level (of the complete termination) Resistance to SF6 by-products, if applicable

Requirement None, these properties are validated by the electric type test of the whole assembly In accordance with IEC 62271-209 paragraph 7.2.4 In accordance with IEC 60840 Appendix H.4 Depending on GIS design, to be defined by the GIS manufacturer

11.7.1.3 Dielectric Parameters There is no specific requirement on the dielectric constant neither tan delta. The validity of the chosen material properties and assembly is demonstrated during the qualification process.

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11.7.1.4 Mechanical Parameters

Table 11.9 Mechanical parameters of 72.5 kV common interface Property Contact surfaces material

Shape of conductor connection

Connection to ground shielding electrode

Design outside pressure

Resistance to cantilever force according to paragraph 6.104 of IEC 62271-209 Mechanical properties of epoxy and hardness, elongation, tensile strength, modulus, maximum permissible temperature, type of test, and test level Smoothness of the epoxy/outer cone interface Test at limit temperatures according to paragraph 6.106.2 of IEC 62271-203 (insulator thermal performance) and resistance to axial force on insulator test

Requirement The current-carrying contact surfaces shall be either bare copper or silver-coated or tin-coated copper In the connection of separable connectors to bushings, care shall be taken in the matching of the materials of the cable conductors, the cable lugs, and the bushings conductors. Where dissimilar metals are joined, appropriate precautions shall be taken to avoid electrochemical corrosion To be proposed by the cable termination manufacturer in compliance with the interface area of the insulator Vibrations of the connection must be considered The ground shielding electrode inside the plugin type bushing shall be established with a connection point to be grounded The design maximum SF6 outside pressure is defined by IEC 62271-209 paragraph 6.1: 8.5 bar abs. and the type test level is according to paragraph 6.104 of IEC 62271203 (pressure test on partitions) Note: It is recommended to make the pressure test at a higher design pressure (e.g., 11 bar absolute) to be able to match alternate gases requirements, provided other tests are fulfilled, such as compatibility and interface to gas dielectric strength tests 5 kN, the test being performed in accordance with IEC 60137, paragraph 8.10 Similar to partitions type test based on materials for class A according to IEC 60085, temperature 105  C and IEC 62271-1 Chap. 7.5.6 Table 14 Ra ¼ 0.4 μm On six different insulators, ten thermal cycles (a) 4 h at 30  C (b) 2 h at room temperature (c) 4 h at 105  C (d) 2 h at room temperature After the thermal cycles, each insulator shall be subjected to an axial force of 2.5 kN 1 min, (continued)

11

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . .

547

Table 11.9 (continued) Property

Tightness test according to paragraph 7.4 of IEC 62271-209 Vacuum test

Sealing surface finish

Requirement pulling and pushing on the metal electrode connection at room temperature followed by: A pressure test at twice the design pressure for 1 min For the pressure test the insulator shall be secured in exactly the same manner as in service The insulators shall not show any sign of overstress or leakage, then they are submitted the high voltage test with PD in accordance with IEC 62271-209 paragraph 7.2.4
107 pa  m3/s at minimal functional pressure for insulation pme Not relevant as a test, but in practical application the insulator may be for a short time exposed to a vacuum stress. This stress is covered by the pressure type and routine tests Rt
6.3

11.7.2 Type Tests and Routine Tests 11.7.2.1 Type Tests Type tests consist in the application of the relevant tests listed in paragraph 7.1. 11.7.2.2 Routine Tests Routine tests on insulator are in accordance with IEC 62271-209 clause 8. They consist in: • AC test 140 kV – 1 min • PD measurements are recommended • Pressure test with leakage detection at two times the design pressure, that is, 15 bars • Visual inspection

11.7.3 Examples of Implementation Examples are shown to illustrate the benefit of this interface (Figs. 11.3 and 11.4). These examples show principles of assembly. They do not fulfill all the specifications of this brochure. Each accessories manufacturer will develop his own termination.

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Fig. 11.3 Examples of implementation of the common 72.5 kV GIS interface: (a) in straight arrangement in accordance with IEC 62271-209; (b) in elbow arrangement

11.8

123 kV and 145 kV Insulator Design and Specification

11.8.1 Design 11.8.1.1 Geometrical Parameters After discussing the possibility of a compact solution (more compact than required by IEC 62271-209), and considering that most customers require the extension to fit in IEC standard dimension, it was decided to comply to IEC dimensions without evaluating a compact solution which most of the time will be globally more expensive because of the need for an additional extension.

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549

Fig. 11.4 Example of arrangement of two elbow 72.5 kV connections in series

The recommended design is the inside of the inner cone, the outer shape of the epoxy is drawn for information, and recommendations are given here under. The selected interface is shown in Fig. 11.1. The dashed lines give an example of possible outer shape design. A principle DXF file of the insulator is included as a downloadable document in the online version of this chapter. The angle of the conical dielectric part has been chosen to have one single insulator for the complete cable range. If a sharper angle was chosen, the epoxy/ stress cone interface pressure would have been higher, but the acceptance of different cable diameters lowered. If a wider angle was chosen, there would have been a higher electrical field inside the enclosure and the increase of epoxy/stress cone interface tangential stress would have restricted the design of stress cones.

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The metal part has been designed so that all cable termination manufacturers can implement the conductor connection inside the dedicated space. It includes (there is room for) an interface to a sliding connection, a lock-in area, a second interface to a sliding connection. Each manufacturer can use the allocated interface(s) (Fig. 11.5). The length and shape of the electrode over the conductor connector has been specified for field control purpose. Its cross section is enough to carry the full specified nominal current of 1000 A and short circuit current of 40 kA 1 s. Details about the selection of different parameters (dimensions, angles. . .) can be found in appendix Information on the Selection of 145 kV Interface Selection. 11.8.1.1.1 Recommendations for the Outer Shape of the Insulator The outer shape of the insulator shall not present sharp angles. Insulator insulation should be departing from the metal electrode with an alpha angle larger than 90 (to avoid being in the direction of electric field lines). The thickness of insulator wall must be large enough to pass the pressure test and avoid any break during plug in operation, for safety reason. The outer surface should be smooth enough to avoid local electric field enhancement. The outer shape of the 123/145 kV insulator does not have any influence relating to the cross qualification of the insulator with any cable/stress cone assembly.

11.8.1.2 Dielectric Parameters Table 11.10 Dielectric parameters Property Dielectric properties of insulator material (permittivity, tanδ) Dielectric performance requirement of completed insulator (electrical withstand level, PD level) Insulation shield break ring electric withstand level (of the complete termination) Resistance to SF6 by-products, if applicable

Requirement None; these properties are validated by the electric type test of the whole assembly In accordance with IEC 62271-209 paragraph 7.2.4 In accordance with IEC 60840 Appendix H.4 Depending on GIS design, to be defined by the GIS manufacturer

11.8.1.3 Mechanical Parameters

Table 11.11 Mechanical parameters Property Shape of conductor connection

Requirement To be decided by the cable termination manufacturer inside the silver coated dedicated space. See Principles of Use of the Current Connection Areas of the Common Interface Vibrations of the connection must be considered (continued)

11

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . .

551

Table 11.11 (continued) Property Design outside pressure

Resistance to cantilever force according to paragraph 6.104 of IEC 62271-209 Mechanical properties of epoxy and hardness, elongation, tensile strength, modulus, maximum permissible temperature, type of test, and test level Test at limit temperatures according to paragraph 6.106.2 of IEC 62271-203 (insulator thermal performance) and resistance to axial force on insulator test

Smoothness of the epoxy/stress cone interface Material of electrode and of connection interfaces Tightness test according to paragraph 7.4 of IEC 62271-209 Vacuum test

Sealing surface finish of insulator flange Insulator bottom surface finish

Requirement The design maximum SF6 outside pressure is defined by IEC 62271-209 paragraph 6.1: 8.5 bar abs. and the type test level is according to paragraph 6.104 of IEC 62271203 (pressure test on partitions) Note: It is recommended to make the pressure test at a higher design pressure (e.g., 11 bar absolute) to be able to match alternate gases requirements, provided other tests are fulfilled, such as compatibility and interface to gas dielectric strength tests 5 kN, the test being performed in accordance with IEC 60137, paragraph 8.10 Similar to partitions type test based on materials for class A according to IEC 60085, temperature 105  C and IEC 62271-1 Chap. 7.5.6 Table 14 On six different insulators, ten thermal cycles (a) 4 h at 30  C (b) 2 h at room temperature (c) 4 h at 105  C (d) 2 h at room temperature After the thermal cycles, each insulator shall be subjected to an axial force of 2.5 kN 1 min, pulling and pushing on the metal electrode connection at room temperature followed by: a pressure test at twice the design pressure for 1 min. For the pressure test, the insulator shall be secured in exactly the same manner as in service The insulators shall not show any sign of overstress or leakage, then they are submitted the high voltage test with PD in accordance with IEC 62271-209 paragraph 7.2.4 Ra ¼ 0.4 μm The electrode shall be made of aluminum or copper, and all current-carrying contact surfaces shall be silver coated
107 pa  m3/s at minimal functional pressure for insulation pme Not relevant as a test, but in practical application the insulator may be for a short time exposed to a vacuum stress. This stress is covered by the pressure type and routine tests Rt
6.3 μm Ra
6.3 μm (continued)

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Table 11.11 (continued) Property Tensile force of the inserts in the epoxy

Requirement Fix a closing plate using bolts tighten at specified torque (by epoxy manufacturer) in the inserts Apply the following thermal cycles: acc. to IEC 60068-2-14, edition 6 (January 2009) – Test Nb, with 4 heating cycles, TA ¼ 25  C, TB ¼ +100  C, t1 ¼ 1 h, tcycle ¼ 12 h Then perform a pressure test at 22.5 bars applied inside the insulator, with the closing plate being left free to be pushed out The closing plate must not move back. No visual degradation shall be observed

11.8.2 Type Tests and Routine Tests 11.8.2.1 Type Tests Type tests consist in the application of the relevant tests listed in paragraph 8.1. 11.8.2.2 Routine Tests Routine tests on insulator are in accordance with IEC 62271-209 clause 8. They consist in: • AC test 275 kV – 1 min • PD measurements are recommended • Pressure test with leakage detection at two times the design pressure, that is, 15 bars • Visual inspection

11.8.3 Example of Implementation Example is shown in Fig. 11.6 to illustrate the benefit of this interface. This example shows principle of assembly. It does not fulfill all the specifications of this brochure. Each accessories manufacturer will develop their own termination.

11.9

Pressure Management

Excerpt from the CIGRE TB 605 (Chap. 7) that is quoted here as it is fully applicable to the common interface insulators. “During GIS manufacturing, installation and delivery:

Ø60-0,00

+0,05

Ø80±0,30

Ra 0,8 μm

35

A-A

Ra 0,8 μm

104 +0,05

Ø62±0,5

Ø82±0,5

Ø100 (min)

Ø110 (max)

Ø300 (min)

4,85 Ra 0,8 μm

10 36±0,2

27

49±0,1

,5 ±0 ,05 ,00

+0

,1 ±0

VIEW A-A

0-0 Ø6

62

Ø

R7

204±0,2

Ø255 0,00

+0,50

Ra 0,4 μm

354±1,0

Ra 0,4 μm

41

Rt ≤ 6,3 μm

M12 n.12 298±0,3

150±1,0

50 max

Rt ≤ 6,3 μm

6061/t6 or 6082/t6 aluminium magnesium alloy

M12 x 25mm (useful threaded length)

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . .

Fig. 11.5 Design of the 145 kV interface

Cigré WG B1-B3.49 Status: 04.02.2019

17 5

Ø85-0,00

R5

Ø100±1,0

226±0

,5

65

123/145 kV COMMON INTERFACE

>9 0°

(40)

in

BORES M10 (n.4)

1,0°

20

(14°0')

Ø8 +0,05 Ø8 5-0,00 2±0

°m

150±1,0

0,00

204±0,2

+0,2

Ø185-0,0

Metallic inserts n.6 at 60°

Ø190-1,00

55 min

,0 76

8,4

Ø136±0,2

470±1,00

MAX Ø198

Silver plating thickness 10 μm min

Ø280±0,5

320±0,5

MIN Ø224

Ø278±0,5

Ø245±0,5

*measures in orange comply with IEC62271-209.

Ø 350 max

(250 max)

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Fig. 11.6 Example of inner cone solution

All insulators are pretested, tested in modules or complete in factory, and on site after installation. This requires adjusting the compartment pressures several times during the different phases of the GIS delivery. • • • •

The design pressure is 7.5 bars relative (reference to IEC 62271-209). During transportation, the pressure is decreased to 0.5 bar. During works, the pressure in adjacent compartments is decreased to 0.5 bar. The customer or an authorized third party can perform the pressure decrease and refill. There may be legal regulation regarding authorized persons. • Supervision performed by experienced people or GIS manufacturer is recommended. • SF6 maintenance equipment is available at the customer premises. • Decrease of pressure is specified in the operating/maintenance manual. During the plug-in of the cable termination: Uncontrolled forces or mistakes during the plug-in operation are more dangerous with high gas pressure. Similar to the work practice on the GIS compartments, the CIGRE WG recommends decreasing the pressure of the cable box to 0.5 bars relative for the above safety reason during the termination installation. Note that during the manufacturing of insulator and termination stress cone, it is common practice to first perform the pressure test of the insulator according to IEC 62271-203 and the maximum pressure of IEC 62271-209, then to perform the dielectric test of the stress cone. During the installation of the stress cone, the pressure in the cable box must be reduced to 0.5 bar.”

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11.10 Risk Assessment The limit of supply of GIS and cable termination manufacturer is specified in IEC 62271-209 for both inner and outer cone designs of cable interface to GIS. The epoxy insulator assembly is in the scope of termination manufacturer per this standard. A full design of epoxy insulator assembly is not provided. Only interface dimensions between the GIS housing and termination are specified. The task of CIGRE WG is to propose design of the common insulator assembly for plug-in terminations to enable interchangeability between the insulator assembly and cable stress cone assembly. Once standardized, the common insulator assembly will not be in the scope of termination manufacturer supply and will become responsibility of the GIS manufacturer, even if the insulator assembly is manufactured by a third party. This section discusses possible failure mechanisms of both designs of cable/GIS interface (outer cone for 72.5 kV and inner cone for 145 kV). It provides estimate of the risk that failure occurs in particular failing mechanism, lists possible root causes for each failing mechanism, and provides assessment of difficulty to allocate responsibility for failure to either GIS or cable termination manufacturer for each root cause. Five failing mechanisms are discussed (see Table 11.12): – Failure due to dielectric breakdown of insulating material either of epoxy insulator, stress cone, or cable – Failure due to dielectric breakdown of interface between the epoxy insulator and the stress cone – Failure due to dielectric breakdown of interface between the cable and the stress cone – Failure due to dielectric breakdown of SF6 gas along epoxy insulator – Failure due to thermal runaway of connection The level of the risk of failure for each failing mechanism is estimated and categorized. Levels of risk of failure are: – Low. It is very unlikely that the failure will occur due to this failing mechanism. – Moderate. It is unlikely for failure to occur. – High. Failure may occur. The difficulties in allocation of responsibility to either GIS or termination manufacturer are categorized as: – Easy to allocate when there is no doubt what caused the failure (white symbol is used ). – Somewhat questionable when there is high level of certainty in what caused the failure (black and white symbol is used ). – Very difficult to allocate when there is high level of uncertainty what caused the failure (black symbol is used ).

Outer Cone

Comment on responsibility allocation Responsible

Failing mechanism #1: Dielectric breakdown initiated within insulating material

Inner Cone

Assesment of responsibility allocation Mitigation

Stress cone (voids, impurities, electrode imperfections)

Inner Cone: Due to extend of damage it would be difficult to prove that insulator did not cause failure Outer Cone: There may be sufficient evidence to allocate cause of failure to stress cone with high certainty

Manufacturer of cable accessory

Routine tests on the stress cone

Risk: LOW since factory routine tests of each component are designed to discover imperfections in dielectric Inner Cone: Due to extend of damage it may be difficult to prove that insulator dielectric failed Outer Cone: If failure is within Manufacturer of GIS (epoxy Routine tests on epoxy , , Epoxy Insulator (voids, impurities) switchgear section insulator is insulator) insulator and GIS faulty. If insulator fails within stress cone section there may be sufficient evidence to allocate responsibility to the insulator with high certainty

Root cause of failure

Table 11.12 Risk assessment evaluation

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Certified, skilled installation crew. Installation manual and check-sheet are clear

Insulator manufacturer (improper packaging), third party (careless handling in transport) or installation crew Termination manufacturer (improper packaging), or installation crew

It may be difficult to pinpoint the cause of failure since the evidence may be destroyed during failure

It may be difficult to pinpoint the cause of failure since the evidence may be destroyed during failure

Crack in the insulator or insulator surface chipped

Cut in the stress cone outer surface

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . . (continued)

Certified, skilled installation crew. Installation manual and check-sheet are clear

Routine tests on the cable

Mitigation

Manufacturer of HV cable

Outer Cone

Responsible

Cable (voids, impurities, protrusions in extruded semi-con screens)

Inner Cone

Comment on responsibility allocation

No doubt that cable failed if failure is outside the stress cone. If failure is within stress cone area: Inner Cone: it would be difficult to exclude insulator as cause of failure Outer Cone: There may be sufficient evidence to allocate cause of failure to cable with high certainty

Root cause of failure

Assesment of responsibility allocation

11 557

Inner Cone

Outer Cone

Assesment of responsibility allocation Comment on responsibility allocation Responsible

Mitigation

Failing mechanism#2: Dielectric breakdown of interface between the epoxy insulator and the stress cone Risk: LOW - Installation is done by trained and skilled jointers Springs are used only in the inner cone design. Insufficient stress cone/insulator Certified, skilled fitters. It may be possible to prove that Accessory installation Installation manual and interface pressure due to improper N.A. the failure was caused by check-sheet are clear setting of spring tension insufficient spring tension and to exclude insulator Easy to check quality of spring material. Incoming inspection check Insufficient stress cone/insulator Accessory supplier if the It may be possible to prove that (material properties) and springs of inferior quality interface pressures due to N.A. the failure was caused by the certification received are used reduction of spring tension in service insufficient spring tension and to from spring manufacturer exclude insulator Accessory supplier if the Insufficient stress cone/insulator wrong stress cone size is In-house quality There may be sufficient evidence interface pressures due to provided. procedure. to pin-point failure to improper Accessory installation if the skilled fitters. incorrect stress cone dimension or N.A. stress cone size or cable insulation outer diameter of prepared Certified, Installation manual and smaller then specified cable insulation diameter and exclude insulator cable is not checked at the check-sheet are clear outer diameter site Accessory supplier if the Applies only to the outer cone In-house quality Insufficient stress cone/insulator wrong size stress cone is design. procedure. interface pressures due to provided. There may be sufficient evidence Certified, skilled fitters. N.A. Accessory installation if improper stress cone stretch, e.g. to pin-point failure to insufficient manual and dimensions are not checked Installation incorrect stress cone dimension stretch of the stress cone check-sheet are clear at the site Insufficient stress cone/insulator In-house quality check of Easy to check GIS (insulator) the certification received interface pressures due to wrong from spring manufacturer insulator dimensions

Root cause of failure

Table 11.12 (continued)

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Accessory supplier if the wrong lubricant is provided. Installation crew if other lubricant is used Installation (almost impossible to detect) Installation (almost impossible to detect) Cable system engineering or installation Cable system engineering or/and installation crew

It may be possible to exclude insulator as cause of failure Evidence of the failure due to contamination will be destroyed by the breakdown Geometrical distortion should be easy to demonstrate Movement of cable may cause failure. Evidence of cable not being supported per requirements specified by the termination manufacturer is evident

Lubricant not applied per instructions

Contamination of the stress cone insulator interface during installation.

Misalignment of cable and insulator

Cable not properly secured

Outer Cone

Responsible

Easy to allocate responsibility. Chemical analysis may be performed to check composition of lubricant

Inner Cone

Comment on responsibility allocation

Wrong lubricant used

Root cause of failure

Assesment of responsibility allocation

(continued)

Certified, skilled installation crew. Installation manual and check-sheet are clear

In-house quality procedure. Certified, skilled fitters. Installation manual and check-sheet are clear Certified, skilled fitters. Installation manual and check-sheet are clear Certified, skilled fitters; proper installation conditions. Installation manual and check-sheet are clear (TB installation) Initial alignment by certified skilled fitters and proper cable laying

Mitigation

11 Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . . 559

Inner Cone

Outer Cone

Assesment of responsibility allocation Comment on responsibility allocation Responsible

Mitigation

Failing mechanism#3: Dielectric breakdown of interface between the cable and the stress cone Risk: LOW - Installation is done by trained and skilled jointers Springs are used only in the inner cone design. Certified, skilled fitters. Insufficient cable/stress cone It may be possible to prove that Accessory installation Installation manual and interface pressure due to improper N.A. the failure was caused by check-sheet are clear setting of spring tension insufficient spring tension and to exclude insulator Easy to check quality of spring material. In-house quality check of Accessory supplier if the Insufficient cable/stress cone It may be possible to prove that the certification received springs of inferior quality interface pressures due to N.A. the failure was caused by from spring manufacturer are used reduction of spring tension in service insufficient spring tension and to exclude insulator Inner-cone: There may be Accessory supplier if the sufficient evidence to pin-point In-house quality Insufficient cable/stress cone wrong stress cone size is failure to improper stress cone procedure. interface pressures due to provided. size or cable insulation diameter skilled fitters. incorrect stress cone size or smaller Accessory installation if the Certified, and exclude insulator Installation manual and then specified cable insulation OD of prepared cable is not Outer-cone: It would be possible check-sheet are clear outer diameter checked at the site to exclude insulator since the failure is on cable section In-house quality Easy to allocate responsibility. Accessory supplier if the Chemical analysis may be wrong lubricant is provided. procedure. Certified, skilled fitters. Wrong lubricant used performed to check composition of Installation crew if other Installation manual and lubricant lubricant is used check-sheet are clear

Root cause of failure

Table 11.12 (continued)

560 P. Mirebeau

Cable system engineering or installation

Geometrical distortion should be easy to demonstrate

Installation crew

Installation (almost impossible to prove)

Installation (almost impossible to prove)

Misalignment of cable and insulator

Inner-cone: There may be sufficient evidence to exclude insulator Outer-cone: It would be possible to exclude insulator since the failure is on cable section Evidence of contamination failure will be destroyed by the breakdown Inner-cone: There may be sufficient evidence to exclude insulator Outer-cone: It would be possible to exclude insulator since the failure is on cable section Inner Cone: It may be possible to prove that insulator did not cause failure Outer Cone: It is obvious that insulator did not cause failure Cable system engineering or installation

Outer Cone

Responsible

Geometrical distortion is easy to verify

Inner Cone

Comment on responsibility allocation

Improper engagement of the cable in the insulator

Poor workmanship in cable preparation

Contamination of the stress cone insulator interface during installation.

Lubricant not applied per instructions

Root cause of failure

Assesment of responsibility allocation

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . . (continued)

Initial alignment by certified skilled fitters and proper cable laying. Check point in the installation instructions

Check point in the installation instructions

Certified, skilled fitters. Installation manual and check-sheet are clear

Certified, skilled fitters; Installation manual and check-sheet are clear.

Certified, skilled fitters. Installation manual and check-sheet are clear

Mitigation

11 561

Inner Cone

Outer Cone

Assesment of responsibility allocation

Movement of cable may cause failure. Evidence of cable not being supported per requirements specified by the termination manufacturer is evident

Comment on responsibility allocation

Cable system engineering or/and installation crew

Responsible

Failing mechanism#4: Dielectric breakdown of SF6 gas along epoxy insulator Risk: Low due to GIS routine factory and field tests (e.g. AC withstand and PD), even lower when insulator installed into Pressure monitoring system will If failure still occurs due to show if the pressure in the pressure responsibility Reduced SF6 gas pressure termination gas compartment was low is on maintenance crew reduced prior to failure If failure still occurs due to Chemical analysis of the SF6 gas of moisture in SF6 Presence of moisture in SF6 gas breakdown byproducts will show if presence gas responsibility is on there was moisture in the gas maintenance crew It would be difficult to exclude insulator as cause of failure. Contamination deposit at the Allocation of failure is more Crew that installed GIS insulator surface difficult when failure energy is higher Interface plate not chamfered or It is obvious that the interface GIS manufacturer rounded per IEC Standard plate does not meet IEC Standard

Cable not properly clamped

Root cause of failure

Table 11.12 (continued)

In-house quality procedure

Certified, skilled installation crew. Installation manual and check-sheet are clear

GIS maintenance manual to be followed by trained maintenance crew

GIS maintenance manual to be followed by trained maintenance crew

GIS at the factory

Certified, skilled installation crew. Installation manual and check-sheet are clear

Mitigation

562 P. Mirebeau

Installation crew

Installation crew

Evidence of improper connector installation or improper connection surface preparation may exist

Connection interface failed

Failing mechanism #5: Thermal runaway Risk: LOW - Installation is done by trained and skilled jointers

Outer Cone

Responsible

Evidence of improper connector installation may exist

Inner Cone

Comment on responsibility allocation

Cable connector not properly installed (e.g. insufficient crimp or not tightened bolts)

Root cause of failure

Assesment of responsibility allocation

Certified, skilled installation crew. Installation manual and check-sheet are clear Certified, skilled installation crew. Installation manual and check-sheet are clear

Mitigation

11 Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . . 563

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The table 11.12 is based on assumption that epoxy insulator is provided and installed into GIS housing by the GIS manufacturer. Risk assessment of failure is based on following assumptions: – The geometrical design parameters, dielectric parameters, and mechanical parameters of the insulator assembly are according to Sects. 11.5 and 11.6, and the qualification (initial qualification and/or cross qualification) is according to Sect. 11.10. Type tests on the insulator assembly have been performed per IEC 62271-209. – Routine tests on the insulator assembly have been performed per IEC 62271-209, this TB and additional routine test procedure of the manufacturer (responsibility of the manufacturer of insulator assembly).

The routine tests of the stress cone have been performed per IEC 60840 and additional routine test procedure of the manufacturer (responsibility of cable accessory manufacturer). It is to be noted that after-installation tests including Partial Discharge (PD) measurements can show up defects before the operation phase. PD monitoring during operation may allow to pinpoint some defect types before breakdown.

11.11 Common Insulator Design Credibility This section describes the technical parameters, tests, and/or the field experience that give the supporting arguments that proposed insulator designs will work and provide a lifetime in agreement with the cable systems design.

11.11.1 72.5 kV Insulator The proposed insulator design concept is based on a long and positive experience at the medium voltage level up to 52 kV. It has been already submitted to tests and actual installations at the 72.5 kV level. It is being standardized.

11.11.1.1 Service Experience with “Outer Cone” Connections, All Types The Table 11.13 below is referring to the knowledge of the WG experts. The total installed number of terminations is likely higher.

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Table 11.13 Outer cone service experience Voltage class MV (12–36 kV) MV (24 kV) MV (24–36 kV) MV (24–36 kV) HV (170 kV) HV (72.5 kV) HV (123– 170 kV)

Cable section Up to 630 mm2, Al or Cu 630 mm2 Al 400–630 mm2 Al or Cu 800–1000 mm2 Al or Cu 1000–2000 mm2 Al or Cu 240–1600 mm2 Al or Cu 240–2500 mm2 Al or Cu

Qty parts in service 100,000,000, worldwide 20,000 23,000 1000

Years in service of oldest 50 40 7 4

400

12

1000

15

3500

15

11.11.1.2 Service Experience with F Type Cone (as Defined in EN 50673)

Table 11.14 F type cone service experience Voltage class 36–52 kV 72.5 kV

Cable section 630–1000 mm2 95–240 mm2

Qty parts in service 150 150

Years in service of oldest 10 1

11.11.1.3 Type Testing According to IEC 60840 of Cable Systems with F-Cone Type Connectors Four manufacturers have reported 72.5 kV type tests of cable system with F-cone type termination (Table 11.15).

Table 11.15 72.5 kV type tests experience Test specification IEC 60840 IEC 60840 IEC 60840 IEC 60840 200 daily load cycles

Cable type 95 mm2 Cu flex EPR 630 mm2 Cu XLPE 1000 mm2 Al XLPE 400 mm2 Cu XLPE 1000 mm2 Al XLPE

Heating current 400 A 1600 A 1600 A 1000 A 1600 A

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11.11.1.4 Examples of Installation

Fig. 11.7 Example of 72.5 kV installed links using the common insulator during tests (three different GIS)

11.11.1.5 Electric Stress Electric stress is low. Under Uo, electric field module is less than 3 kV/mm at the interface and in the epoxy, tangential field is less than 2 kV/mm. The stress in the insulator does not depend on the stress in the cable. Each manufacturer should perform electric field evaluations during development of the insulator assembly (Fig. 11.8).

11.11.1.6 Conclusion The credibility shows that there is a limited field experience for the selected interface; however, it has been type tested for a set of heating current and cross sections that give confidence to this design. It is to be noted that the stress in the insulator is independent from the stress and dimension of the cable.

11.11.2 145 kV Insulator The common insulator is new, but proposed by the experts of the WG, based on proven design parameters, with long and positive field experience. These designs have been type tested and are in service for over 30 years.

11.11.2.1 Dielectric Parameters The dielectric credibility is based on the analysis of the dielectric stress. All permittivity parameters of different epoxy resins have been considered from εr ¼ 4–6 at ambient temperature. Figure 11.9 shows an example of the electric field magnitude in the stress cone, the insulator, and the cable termination enclosure; this depends on the complete stress cone and insulator design. Each manufacturer should perform electric field evaluations during development of the insulator assembly (Figs. 11.9 and 11.10).

11

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . .

567

Fig. 11.8 Example of electrical field calculation on 72.5 kV outer cone termination

The material hypothesis is the following: • • • •

εr ¼ 5 for epoxy resin εr ¼ 2.8 for the stress cone rubber insulation εr ¼ 2.3 for the cable XLPE insulation The cable is the smallest 1600 mm2 of our cable catalogue, which gives the maximum stress at the stress cone/cable insulation interface

The hereunder electric fields are calculated for a reference impulse voltage of 650 kV (or U0 ¼ 76 kV). Along the surface of conductive electrode, inside epoxy-resin insulator the calculation shows acceptable values up to: Emax. ¼23.5 kV/mm (2.75 kV/mm).

568

P. Mirebeau

Fig. 11.9 Example of electrical field calculation on 145 kV inner cone termination

Along the surface of conductive deflector inside rubber insulation part, the calculation shows acceptable values up to: Emax. ¼38.5 kV/mm (4.50 kV/mm). The tangential field plot along interface between rubber and epoxy-resin insulator starting from HV side has a maximum at 9 kV/mm (1.05 kV/mm). Along the insulator interface surface to gas starting from HV side, the calculation shows acceptable values up to: Emax. ¼13.7 kV/mm (1.60 kV/mm).

11

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . .

569

2.5e+007 4e+007 2e+007

3.5e+007

1.5e+007 1e+007

3e+007

2

2.5e+007

1

2e+007

5e+006

1.5e+007 0 1e+007

9e+006 8e+006 7e+006 6e+006 5e+006 4e+006 3e+006 2e+006 1e+006 0

1e+007

3

4

5e+006

0

Fig. 11.10 Stresses as per Fig. 11.9 (example as illustration): (1) along the surface of conductive electrode; (2) along the surface of conductive deflector inside rubber insulation part; (3) along interface between rubber and epoxy-resin insulator; (4) along the insulator interface surface

11.11.2.2 Connection The principle design and material of the current carrying path have been used in the field for more than 30 years for dry and for fluid filled GIS cable terminations. 11.11.2.3 Conclusion The dielectric design of the 123–145 kV interface is safe and achievable. There is design margin available when compared to the various manufacturers’ designs. A more compact insulator could be specified or the application range extended after there will be a market harmonization and field experience of the proposed current connection solutions.

11.12 Qualification Process The cases of cable and accessory qualification are specified in IEC 60840. The related extensions of qualification are recommended in CIGRE TB 303 (▶ Chap. 4, “Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems” of the book). IEC 60840 requires to qualify each cable together with its accessories and the validity of type test result is limited to the tested design provided from a single source of supply. Up to now it is not foreseen in high voltage applications to “mix and match” components. According to this Technical Brochure, preinstalled

570

P. Mirebeau

Table 11.16 Qualification and cross qualification process Legend:

insulator assembly never qualified

Initial qualification

cable/stress cone assembly never qualified

insulator assembly qualified

cable/stress cone assembly qualified

Type test according to IEC 60840 with the following amendment: 20 heating cycles with 2 hours between 95°C and 100°C , leaving the heating and cooling time free. Prequalification test according to IEC 60840 when applicable. Tests of sectionalising insulation according to IEC 60840. The range of approval of IEC 60840 chapter 12.2 subclause g remains applicable.

Cross qualification in case of available initial qualification for both insulator assembly and cable/stress cone assembly

Sample Test according to IEC 60840 chapter 11.2 i.e. a) partial discharge test b) voltage test Lightning impulse test: at ambient temperature Tests of sectionalising insulation according to IEC 60840

Cross qualification without initial qualification of either insulator assembly or cable/stress cone assembly or

Type test according to IEC 60840 with the following amendment: 20 heating cycles with 2 hours between 95°C and 100°C , leaving the heating and cooling time free. Prequalification test according to IEC 60840 when applicable. Tests of sectionalising insulation according to IEC 60840. No range extension limitation on cable stresses when the cable/stress cone assembly has been qualified with a cable of higher stress. These cross qualification tests are in accordance with TB 303 chapter 2.3.1.3, table 2.4 "Change of insulator material for indoor or outdoor terminations".

insulators assemblies from different manufacturers should work together with cable/ stress cone assemblies coming from different suppliers. Rules are necessary to deal with combinations of materials which might have been qualified separately before. The table 11.16 focuses on the qualification and cross qualification of insulator assembly and cable/stress cone assembly as defined in Sect. 11.3.1 representing the complete termination. Three different cases were identified and evaluated (see detail of experts views in Appendix E). Table 11.16 applies to insulators that already comply with the requirements of previous sections, regarding geometrical, dielectric, and mechanical parameters.

11

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . .

571

11.13 Conclusion In accordance with the terms of reference of CIGRE B1.B3-49 common interfaces were designed for • Um ¼ 72.5 kV • Um ¼ 123 kV and 145 kV • Current
1000 A These designs meet the requirement of IEC 62271-209 and IEC 60840. • For 72.5 kV, the outer cone design is based on the on-going standard EN 50673. • For the 145 kV, the proposed inner cone design is new, with larger design margins in reference to existing designs. The 72.5 kV design allows for an optimization of the switchgear dimensions. To meet the IEC 62271-209 dimensions, adaptor elements may be needed. Consideration was given to the manufacturers’ field and qualification experience, electrical field calculations, known material properties. The requirements as well as a .dxf file are included in the downloadable document, which can be used with any standard drawing software files to include in the book. If these proposed interfaces are accepted by the market, experience with interchangeability of components will be collected as soon as more suppliers will provide solutions. Accordingly they might then be used for standardization.

General Evaluation of Inner and Outer Cone Technologies Table 11.17 is a collection of the evaluation made by each expert. These evaluations were taking into consideration the 2 solutions (inner or outer cones) and the 2 voltage class ranges (72.5–100 kV and 123–145 kV). Each expert evaluation is identified by a single color.

Reference of Available Tests for Common Interface Evaluation Table 11.18 and 11.19 following tests lists are determined considering different types of characteristics to control. These lists are for information. These characteristics are grouped according to the following classification table. Some of these tests were excluded as they were considered not relevant.

Remarks

0

+

Length of connector outside of tank

Dimensions/size of insulator Larger above the base plate, smaller above

0

+

72.5 kV to 100 kV

0

123 kV to 145 kV

Rating --, -, 0, +, ++

Diameter is adapted to tank diameter

*"traditional" refers to fluid filled termination

Dimensions of insulator and Length inside GIS tank is more than for connection Outer, but still shorter than traditional*. Shortening of tank is possible yet not practiced for now

Aspects

Inner Cone Model

Table 11.17 Pros and cons evaluation of each technology

Can be shorter above the base plate, but longer below the base plate

Length inside GIS tank could be much smaller with outer cone design than with inner cone but dimensions fixed by IEC standard usually doesn’t allow this possible length reduction. Allows smaller diameters of insulator and of the GIS since the cable nor the deflector need to penetrate Needs more space below the tank once installed Allows compact T-connector system

Remarks

Outer Cone Model

0

++

-

+

0

123 kV to 145 kV

++

72.5 kV to 100 kV

Rating --, -, 0, +, ++

572 P. Mirebeau

Outer diameter of stress-cone is more depending on the cable diameter and influences the installation difficulty and position of the connection Need for a large number of sizes Position of deflector inside the insulator is variable and difficult to verify (is defined rather by pressure of the spring packet)

-

Dimensions/size of insulator Insulator is larger than outer cone model due to insert inner cone inside insulator

Mismatch risk insulator assembly/cable - stress cone assembly

+

+

Dimensions/size of insulator More defined insulator interface due to higher mechanical pressure (springs)

Outside of the compartment

0

Dimensions/size of insulator Inside of the compartment

72.5 kV to 100 kV

-

-

-

+

+

0

++

123 kV to 145 kV

Rating --, -, 0, +, ++

+

Remarks

Dimensions/size of insulator Smaller size of top connector, less metal with same installation length (given by IEC 62271-209)

Aspects

Inner Cone Model

Positioning of connector is verifiable externally and independent of the cable

Insulator is smaller than inner cone model due to almost same diameter with outer cone if there is no necessary to set up metal covering Can accommodate a larger range of cable diameters per size without influence on the position

Heavier entire design assumed (caused by heavier metal electrode though entire insulator)

Remarks

Outer Cone Model

0

0

0

+

0

+

+

72.5 kV to 100 kV

0

0

0

+

-

123 kV to 145 kV

Rating --, -, 0, +, ++

(continued)

11 Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . . 573

0 0

0

Mismatch risk cable dimensions

Mismatch risk insulator/stress cone assembly

Mismatch risk cable dimensions

0

-

+

72.5 kV to 100 kV

0

0

0

0

-

-

123 kV to 145 kV

Rating --, -, 0, +, ++

0

Dependency between cable preparation (shape and diameter) and outer shape of the stress cone, possibly affecting the interface between epoxy and rubber.

Pressure applied on the system automatically center the cable and stress-cone. Inner shape of cone and outer shape stress cone need to be dimensioned very carefully

Remarks

Inner Cone Model

Mismatch risk insulator/stress cone assembly

Mismatch risk cable dimensions

Mismatch risk insulator/stress cone assembly

Aspects

Table 11.17 (continued)

Interface epoxy/rubber mould is independent from cable preparation

Higher risk of inhomogeneous pressure distribution at stress-cone interfaces with de-centered cable. Less dependency on the cone shape and angle

Remarks

Outer Cone Model

0

+

0

0

+

+

72.5 kV to 100 kV

-

0

+

0

0

+

+

123 kV to 145 kV

Rating --, -, 0, +, ++

574 P. Mirebeau

0 -

No difference to install insulator

Needs more space to move the cable during assembly of the connector

0

Easier installation

Mismatch risk cable dimensions

0

Depended on maker’s design, but applicable range is limited due to more risky change of interface pressure in case rubber deformation by different diameter between rubber and cable Depended on maker’s design

Mismatch risk insulator/stress cone assembly

+

72.5 kV to 100 kV

0

0

0

+

123 kV to 145 kV

Rating --, -, 0, +, ++

0

More defined insulator interface due to higher mechanical pressure (springs)

Remarks

Mismatch risk cable dimensions

Mismatch risk insulator/stress cone assembly

Aspects

Inner Cone Model

If T- connector no need for large cable displacement

Depended on maker’s design

Depended on maker’s design, but applicable range is comparably wider due to generate interface pressure by rubber deformation

Remarks

Outer Cone Model

0

0

0

0

+

0

0

0

0

0

(continued)

123 kV to 145 kV

++

72.5 kV to 100 kV

Rating --, -, 0, +, ++

11 Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . . 575

+ +

Cable side

Plugging to insulator

Ease of installation

-

Failure in interface insulator-stress-cone -> risk of bursting of insulator

Failure in interface cable-stress-cone → risk of bursting of insulator

Failure of cable under stress-cone → less risk of bursting of insulator

Consequence of failure

0

Much time to set up spring device, Must insert cable and inner cone and be cable off-set process

Ease of installation

+

Easier for transportation und handling

Ease of installation

+

concentric positioning during plug-in process can be controlled

Ease of installation

0

72.5 kV to 100 kV

0

+

+

+

+

0

123 kV to 145 kV

Rating --, -, 0, +, ++

Plug: cable, stress cone and connector

Remarks

Inner Cone Model

Ease of installation

Aspects

Table 11.17 (continued)

Does not affect GIS side

Does not affect GIS side

Does not affect GIS side

Easy handling cable without overlap to insulator

concentric positioning during plug-in process cannot be controlled during entire process

Plug: cable with connector

Remarks

Outer Cone Model

+

0

0

0

-

+

72.5 kV to 100 kV

+

+

+

+

0

0

0

-

+

123 kV to 145 kV

Rating --, -, 0, +, ++

576 P. Mirebeau

Unlikely, low stresses

Failure in interface will affect the barrier between GIS and cable

Failure in cable part can affect the GIS

no difference in the consequence

Consequence of failure In the stress cone and interface stress cone/epoxy

Consequence of failure In the cable related part

Consequence of failure In the epoxy

Insulator can burst due to overpressure from mechanical or thermal effects Low probability of electrical failure of insulator Direct impact on the GIS casing in case of stress-cone failure Lower electrical stress in epoxy insulator

Remarks

Consequence of failure In the epoxy

Aspects

Inner Cone Model

0

-

-

0

0

-

-

+

Cable in cable will not affect the GIS

Failure in cable/mould part outside GIS

0

++

+

0

-

+

No impact on the GIS casing in case of stress-cone failure Higher electrical stress in epoxy insulator

+

-

Electrical failure of bushing insulation

+

0

++

+

-

(continued)

123 kV to 145 kV

++

72.5 kV to 100 kV

Rating --, -, 0, +, ++

Mechanically more robust

123 kV to 145 kV

Remarks

-

72.5 kV to 100 kV

Rating --, -, 0, +, ++

Outer Cone Model

11 Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . . 577

0

0

0 0

Consequence of failure In the stress cone and interface stress cone/epoxy

Consequence of failure In the cable related part

Consequence of failure in the epoxy

Consequence of failure in the stress cone and interface stress cone/epoxy

0

0

same effect for both designs

Consequence of failure In the cable related part

0

72.5 kV to 100 kV

0

0

0

0

0

0

0

123 kV to 145 kV

Rating --, -, 0, +, ++

Consequence of failure In the epoxy

same effect for both designs

Remarks

Inner Cone Model

Consequence of failure In the stress cone and interface stress cone/epoxy

Aspects

Table 11.17 (continued)

Potential pressure release in case of failure (no vessel damage)

Remarks

Outer Cone Model

+

0

0

0

0

0

0

72.5 kV to 100 kV

+

0

0

0

0

0

0

123 kV to 145 kV

Rating --, -, 0, +, ++

578 P. Mirebeau

0

-

Much damage of epoxy insulator with high inner pressure by breakdown but much experience

In case of failure most likely all components (epoxy/stress cone/cable) will be affected

Consequence of failure in the cable related part

Separation of responsibilities/liabilities in case of failure 0

0

Higher interface field but much experience

Consequence of failure in the stress cone and interface stress cone/epoxy

Separation of responsibilities/liabilities in case of failure

0

Higher electric field in epoxy but much experience Must open GIS enclosure

Consequence of failure in the epoxy

72.5 kV to 100 kV

0

-

0

0

0

0

123 kV to 145 kV

Rating --, -, 0, +, ++

0

Remarks

Consequence of failure in the cable related part

Aspects

Inner Cone Model

‘can’ be easier

Estimated less damage of epoxy insulator but less mechanical experience for cable transient reaction during short circuit phenomenon etc. Interface epoxy/rubber stress cone and interface rubber/cable responsibility of termination/cable supplier

Higher field in rubber electrode embedded in cone

Lower electric field in epoxy but less experience Must open GIS enclosure

Remarks

Outer Cone Model

0

+

0

0

0

0

72.5 kV to 100 kV

0

+

0

0

0

0

(continued)

123 kV to 145 kV

Rating --, -, 0, +, ++

11 Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . . 579

Easier to upgrade/uprate the current carrying capacities, more space for the multi-contact system

no difference with common interface

Uprating possibility (e.g. change of cable)

Uprating possibility (e.g. change of cable)

Upgrading facility

0

+

0

Separation of responsibilities/liabilities in case of failure

Simple interface between gas and insulator, longer interface of epoxy insulator

0

Separation of responsibilities/liabilities in case of failure

72.5 kV to 100 kV

0

+

0

0

0

123 kV to 145 kV

Rating --, -, 0, +, ++

0

Remarks

Inner Cone Model

Separation of responsibilities/liabilities in case of failure

Aspects

Table 11.17 (continued)

Would be easier with T-connector: e.g. adding another cable in parallel, test connector, surge arrestor Cast in rod limits the current capacity

Simple interface between gas and insulator, shorter interface of epoxy insulator

Remarks

Outer Cone Model

0

-

0

0

0

0

-

0

0

0

123 kV to 145 kV

++

72.5 kV to 100 kV

Rating --, -, 0, +, ++

580 P. Mirebeau

Components can be factory tested

Testing of epoxy and rubber stress cone separately

no difference

Quality assurance Components

Quality assurance Installation

Quality assurance Components

0

-

0

0

Limited range for cable insulator Can change cable with exchange of full components It must be depended on maker design Routine testing is possible

Uprating possibility (e.g. change of cable)

Quality assurance

0

Uprating possibility (e.g. change of cable)

72.5 kV to 100 kV

0

0

-

0

0

0

+

123 kV to 145 kV

Rating --, -, 0, +, ++

+

Remarks

Uprating possibility (e.g. change of cable)

Aspects

Inner Cone Model

Interface epoxy/rubber mould can be pre-assembled and pretested

Components can be factory tested

Wider range Can change cable without exchange of epoxy insulator It must be depended on maker design idem

Higher flexibility

Remarks

Outer Cone Model

0

+

0

+

+

+

72.5 kV to 100 kV

0

0

+

0

+

+

0

(continued)

123 kV to 145 kV

Rating --, -, 0, +, ++

11 Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . . 581

+

0 0 +

Complete establishment for quality assurance in factory

Much experience

Quality assurance Installation

Quality assurance Components

Quality assurance Installation

Quality assurance Components

Quality assurance Installation

+

72.5 kV to 100 kV

+

0

0

+

++

+

123 kV to 145 kV

Rating --, -, 0, +, ++

++

plug-in process can be controlled visually (better as outer cone designs)

Remarks

Inner Cone Model

Quality assurance Components

Quality assurance Installation

Aspects

Table 11.17 (continued)

Less experience for HV/EHV class

Not establish electrical routine test Must need additional equipment

Better visibility of insulator surface (can be easier cleaned)

Remarks

Outer Cone Model

-

+

0

+

+

0

72.5 kV to 100 kV

-

+

0

+

+

0

123 kV to 145 kV

Rating --, -, 0, +, ++

582 P. Mirebeau

Economic considerations + -

Hardware (springs etc) more expensive than outer cone

++

Insulator and stress-cone probably less expensive than outer cone

+

Experience/commonly used

Experience/commonly used

++

+

Experience/commonly used

90% (for HV) For MV the opposite

Experience/commonly used

++

Most systems are inner-cone concept in HV

Experience/commonly used

72.5 kV to 100 kV

+

++

++

++

123 kV to 145 kV

Rating --, -, 0, +, ++

Experience/commonly used

Remarks

Aspects

Inner Cone Model

10% (for HV) For MV much experience

Outer cone has limited application in HV to date

Remarks

Outer Cone Model

0

-

+

+

-

--

72.5 kV to 100 kV

--

-

--

123 kV to 145 kV

Rating --, -, 0, +, ++

(continued)

11 Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . . 583

0

Economic considerations Economic considerations

Handling (including testing) Testing of GIS with insulator installed is probably simpler than with outer cone

0

Economic considerations

0

-

+

72.5 kV to 100 kV

0

0

0

0

123 kV to 145 kV

Rating --, -, 0, +, ++

0

Metal springs needed

Remarks

Inner Cone Model

Economic considerations

Economic considerations

Aspects

Table 11.17 (continued)

more material, heavier metal electrode

More expensive rubber mould, no springs needed

Possibility to reduce GIS casing dimensions and reduce SF6 volume

Remarks

Outer Cone Model

0

0

-

0

72.5 kV to 100 kV

-

+

0

-

-

0

123 kV to 145 kV

Rating --, -, 0, +, ++

584 P. Mirebeau

+

Testing of insulator in GIS

No housing needed for HV test (only cover)

+

+

Testing of insulator in GIS

considering a field control element, no advantages which will be installed during testing (not very easy handling for both designs); considering a gas filled volume with cover plate → inner cone design has easier handling

Testing of insulator in GIS

0

+

Cable jacket testing is possible

Testing possible with adapter or closing plate and SF6

-

72.5 kV to 100 kV

+

+

+

0

123 kV to 145 kV

Rating --, -, 0, +, ++

After laying test is not possible unless via another entrance into the GIS

Remarks

Testing of insulator in GIS

Aspects

Inner Cone Model

Testing with test adapter

idem

If T-connector, after laying test is possible

Remarks

Outer Cone Model

0

0

0

0

+

+

72.5 kV to 100 kV

0

-

0

0

123 kV to 145 kV

Rating --, -, 0, +, ++

(continued)

11 Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . . 585

Inner cone needs more space during installation

Substation basement dimensions, plug-in (pull back) space needed Substation basement dimensions, plug-in (pull back) space needed

0

+

Substation basement dimensions, plug-in (pull back) space needed

Substation basement dimensions, plug-in (pull back) space needed

Less space required

0

0

+

72.5 kV to 100 kV

-

+

0

0

-

+

123 kV to 145 kV

Rating --, -, 0, +, ++

Substation basement dimensions, plug-in (pull back) space needed

Shorter endbell, but longer plug

Test with gas instead of inner cone

Remarks

Inner Cone Model

Testing of insulator in GIS

Aspects

Table 11.17 (continued)

entire installation length of sealing end incl. outer parts are longer before cable bending can be applied considering few space in current basement designs

Longer end bell, mould exceeding the base-plate, but shorter plug requiring less space to pull back

Outer cone needs more space under GIS casing after installation

In case pre-set of insulator, cable and outer cone shall be prepared

Remarks

Outer Cone Model

0

+

--

+

-

+

72.5 kV to 100 kV

0

0

--

+

-

123 kV to 145 kV

Rating --, -, 0, +, ++

586 P. Mirebeau

Larger dimension due to insulator with large diameter

Many experience, reliable design

Many experience, reliable design

Many experience, reliable design

Sheath break application

Placement of SVL

Metal covering (Protection)

Remarks

Substation basement dimensions, plug-in (pull back) space needed

Aspects

Inner Cone Model

++

++

++

0

72.5 kV to 100 kV

++

++

++

0

123 kV to 145 kV

Rating --, -, 0, +, ++

No metal covering for MV class But need metal covering to protect water penetration??

No experience?? Where to set?

Developing aspect (positioning, insulating, evaluation, qualification) due to few experience

Smaller dimension due to insulator with small diameter

Remarks

Outer Cone Model

0

0

0

+

72.5 kV to 100 kV

0

0

0

+

123 kV to 145 kV

Rating --, -, 0, +, ++

11 Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . . 587

588

P. Mirebeau

Table 11.18 Evaluation classification 1

Mechanical properties of epoxy

2

Smoothness of the epoxy/stress cone interface

3

Quality of main circuit end terminal interface

4

Type test of female plug-in connector interface (not relevant)

5

The lubricant should be compatible with the epoxy (not relevant)

6

Resistance to decomposition products (GIS design related)

7

Resistance to cable/stress cone assembly pressure

8

Resistance to operation pressure

9

Thermal dissipation property

10

Dielectric losses

11

Acceptance testing of Female plug in connector

12

Additional

11

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . .

589

Routine Tests Table 11.19 Routine tests Description

Existing test protocols, methodes or standards

Visual Inspection

EN 50089 (§ 7.2.1)

Radiography and /or ultra-sonic Non-destructive examination

EN 50089 (§ 7.2.2)

Pressure Test

EN 50089 (§ 7.2.3)

Records

EN 50089 (§ 7.2.4)

Production consistency pressure test

EN 50089 (§ 7.2.5)

Classification

8

Pressure Test

IEC 62271-203 (§ 7.104)

8

Partial discharge test

IEC 60840 (§ 9.2)

12

AC Voltage Test

IEC 60840 (§ 9.3)

12

AC Voltage Test

IEC 62271-203 (§ 7.1.101)

PD Measurement

IEC 62271-203 (§ 7.1.102)

Tightness test

IEC 62271-203 (§ 7.4) IEC 62271-1 (§ 7.4)

8

Vacuum test

TERNA standard UX LK 208 Rev. 01 (§ 7.15)

8

Tightness test

TERNA standard UX LK 208 Rev. 01 (§ 7.16)

8

Tightness test at the flange or other fixing device

IEC 60137 (§ 9.9)

8

Measurement of dielectric dissipation factor (tan δ) and capacitance at ambient temperature

IEC 60137 (§ 9.1)

10

Dry power-frequency voltage withstand test

IEC 60137 (§ 9.3)

12

Measurement of partial discharge quantity

IEC 60137 (§ 9.4)

12

Tests of tap insulation

IEC 60137 (§ 9.5)

12

Visual inspection and dimensional check

IEC 60137 (§ 9.10)

12

Design and visual checks

IEC 62271-209 (Figure 5)

12

590

P. Mirebeau

Type Tests Table 11.20 Type tests Existing test protocols, methodes or standards

Description

Classification

Overpressure withstand test

IEC 62271-203 (§ 6.106.3)

8

Measurement of the resistance of the main circuit

IEC 62271-203 (§ 6.4.1) IEC 62271-1 (§ 6.4.1)

3

Tightness tests

IEC 62271-203 (§ 6.8) resp. IEC 62271-1 (§ 6.8)

8

Pressure test on partitions

IEC 62271-203 (§ 6.104)

8

Burst test a pressure test at 3 x design pressure for 1 minute proposed

EN 50089 (§ 7.1)

8

External pressure test on partly or IEC 60137 (§ 8.12) completely gas-immersed bushings

8

Thermal stability test

IEC 60137 (§ 8.5)

9

Temperature rise test

IEC 60137 (§ 8.7)

9

Dry lightning impulse voltage withstand test (BIL)

IEC 60137 (§ 8.3)

12

Verification of thermal short-time current withstand

IEC 60137 (§ 8.8)

12

Cantilever load withstand test

IEC 60137 (§ 8.9)

1

Verification of dimensions

IEC 60137 (§ 8.13)

12

Thermal performance (Test at limit IEC 62271-203 (§ 6.106.2) temperatures)

9

Cantilever test

Based on requirement of IEC62271-209 (§ 6.2)

1

Type tests on cable systems

IEC 60840 (§ 12)

12

Type tests on accessories

IEC 60840 (§ 15)

12

Sample Tests

Table 11.21 Sample tests Description Mechanical properties: Heat distortion temperatures (ISO R75) Tensile strength (ISO 527) Impact strength (ASTM D256) Density Sample tests on accessories

Existing test protocols, methods, or standards EN 50089 (§ 6.4) ISO R75 ISO 527 ASTM D256

IEC 60840 (§ 11)

Classification 1

12

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Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . .

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Prequalification Tests Table 11.22 Prequalification tests Existing test protocols, methods, or standards IEC 60840 (§ 13)

Description Prequalification test of the cable system

Classification 12

Development Tests Table 11.23 Development tests Description Mechanical forces on cable-terminations Pressure withstand requirements Design of partitions Normal and special service conditions Rated normal current and temperature rise Rated short-time and peak withstand currents and rated duration of short circuit Rated filling pressure of insulating gas in the cable connection enclosure Pressure during plug-in of the cable termination Design and visual checks Test after installation

Existing test protocols, methods, or standards IEC 62271-209 (§ 6.2) IEC 62271-209 (§ 6.1) IEC 62271-203 (§ 5.104.1) IEC 62271-203 (§ 2) IEC 62271-1 (§ 2) IEC 62271-209 (§ 5.4) IEEE 1300 (§ 6.4) IEC 62271-209 (§ 5.5) IEEE 1300 (§ 6.5) IEC 62271-209 (§ 5.6) IEEE 1300 (§ 6.6) CIGRE TB 605 (§ 5.2.2) IEC 62271-209 (Fig. 11.5) IEC 62271-209 (§ 8.1)

Classification 1 8 8 9 3 12 12 7 12 12

Tests After Installation Table 11.24 Tests after installation Description Test after installation Electrical tests after installation

Existing test protocols, methods, or standards IEC 62271-209 (§ 8.1) IEC 60840 (§ 16)

Classification 12 12

Information on the Selection of 145 kV Interface Selection Setting of the dimensions of the 145 kV inner cone. All dimensions defined in IEC 62271-209 (Figure 5 of this intenational standard) remain applicable.

592

Fig. 11.11 Identification of the different dimensions to determine

P. Mirebeau

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Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . .

593

Table 11.25 Dimensional values Identification Top connector (electrode) A Top contact surface diameter (mm) Locking groove height (mm) B Locking groove depth (mm) C Top contact surface height (mm) D Length and shape of the connection screen W Lower contact surface height (mm) F Lower contact surface diameter (mm) G Stopper diameter (mm)

Screened area below the stopper flange (mm) Epoxy/stress cone interface Min cone diameter (higher diameter) (mm) H Max cone diameter (lower diameter) (mm) Cone angle ( )

Range

Target

50–70

50

10–20

15

5–15

10

75– 100

Cone height (mm)

Insulator bottom design J Distance between bottom of electrode and sealing surface (mm)

Final 60

If higher ¼ more robust, but more costly

17.5 11

65

Includes safety margin for short circuit current

65

110

Safety margin chosen, of which 100 mm cylindrical

104

80 100

Y

I

Comments

40

85 Vote: 4 for a stopper 4 neutral, defines the position of end of insulated core, prevents the stress cone from moving outside insulated core Including 10 mm conical

100

49

90– 120

100

120– 200 5–15

185 14

Experiences are between 4 and 16 , larger means shorter epoxy, less sensitive to cable dimension, larger means higher electrical field inside the enclosure

100– 200

14

204

150

Larger means larger dimensions and cost, less dielectric stress

155

(continued)

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Table 11.25 (continued)

E

Identification Thickness of insulator flange (mm)

Range 20

Target 40

Distance from sealing surface to bottom of the insulator (mm) Metallic inserts

100– 150

150

Comments Top of flange in coincidence with the lower end of the cone, space needed for tightness and inserts With screen interruption withstanding 50 kV impulse

Final 41

Six inserts to stand the force due to the stress cone compression mechanism

M12

150

Principles of Use of the Current Connection Areas of the Common Interface Principle of use of the Current Transmission Above the Lock-in System Area Sliding current connection

Lock-in system area (optional)

Cable conductor connection

Cable conductor

Fig. 11.12 Example of connection application: contact above lock-in system area

11

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . .

Principle of use of the Current Transmission Below the Lock-in System Area

Lock-in system area (optional)

Cable conductor connection, and sliding current connection

Cable conductor

Fig. 11.13 Example of connection application: contact below lock-in system area

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Qualification Process Experts Views Table 11.26 Expert views on test programs Legend:

insulator assembly never qualified

cable/stress cone assembly never qualified

Initial qualification

insulator assembly qualified

Cross qualification in case of available initial qualification for both insulator and cable/stress cone assembly

cable/stress cone assembly qualified

Cross qualification without initial qualification of either insulator or cable/stress cone assembly

or Sample Test according to IEC 60840 / § 11.2 i.e. a) partial discharge test b) voltage test + impulse test at ambient temperature Stick with existing standards. Sample Test according to IEC 60840 / § 11.2 i.e. Type Test according to IEC a) partial discharge test 60840 (with 20 heating b) voltage test + impulse cycles) + Prequalification test at ambient temperature Test in case of high stress Voltage and impulse tests Type Test according to HD 629.1 (with 120 heating cycles) but test values taken + 10 heating cycles in case from IEC 60840. Cycles: 5 h of smaller cross section heating + 3 h cooling with a maximum cross section of 630 mm² AL or 500 mm² Cu Type Test according to IEC Type Test according to IEC 60840 (with 20 heating 60840 (with 20 heating cycles) on any conductor size cycles) on maximum - i.e. follow TB 303 conductor size. Stick with existing standards. Sample Test according to IEC 60840 / § 11.2 i.e. Type Test according to IEC a) partial discharge test 60840 (with 20 heating b) voltage test + impulse cycles) + Prequalification test Test in case of high stress Sample Test according to IEC Type Test according to IEC 60840 / § 11.2 i.e. 60840 (with 20 heating a) partial discharge test cycles) + Prequalification b) voltage test + impulse Test in case of high stress. test at ambient temperature With no range extension limitation on the cable stresses. Type Test according to IEC 60840 (with 20 heating cycles) + Prequalification Test in case of high stress

Expert's views

Type Test according to IEC 60840 (with 20 heating cycles) + Prequalification Test in case of high stress, i.e. follow TB 303 Type Test according to IEC 60840 (with 20 heating cycles) + Prequalification Test in case of high stress, i.e. follow TB 303 Type Test according to HD 629.1 (with 120 heating cycles) but test values taken from IEC 60840. Cycles: 5 h heating + 3 h cooling with a maximum cross section of 630 mm² AL or 500 mm² Cu Type Test according to IEC 60840 (with 20 heating cycles) on any conductor size. i.e. follow 303. Type Test according to IEC 60840 (with 20 heating cycles) + Prequalification Test in case of high stress, i.e. follow TB 303 Type Test according to IEC 60840 (with 20 heating cycles) + Prequalification Test in case of high stress, i.e. follow TB 303. With no range extension limitation on the cable stresses.

(continued)

11

Standard Design of a Common, Dry Type Plug-in Interface for GIS and Power. . .

597

Table 11.26 (continued)

Initial qualification

Cross qualification in case of available initial qualification for both insulator and cable/stress cone assembly

Cross qualification without initial qualification of either insulator or cable/stress cone assembly

or Type Test according to HD 629.1 (with 120 heating cycles) but test values taken from IEC 60840. Cycles: 5 h heating + 3 h cooling with a maximum cross section of 630 mm² AL or 500 mm² Cu

Conclusion Type test with voltage level of IEC 60840. 20 heating cycles with 2 hours between 95°C and 100°C and leave the heating and cooling time free + prequalification test in cas of high stress. Tests of sectionalising insulation according to IEC 60840 The range of approval of IEC 60840 remains applicable

Voltage and impulse tests + 10 heat cycles in case of smaller cross section

Sample Test according to IEC 60840 / § 11.2 i.e. a) partial discharge test b) voltage test Lightning impulse test: at ambient temperature Tests of sectionalising insulation according to IEC 60840

Type Test HD 629.1 (120LC) but test values from 60840 with 5 h Heat 3h Cooling with maximum cross section 630mm² AL or 500mm² CU

Type test with voltage level of IEC 60840. 20 cycles with 2 hours between 95°C and 100°C and leave heating and cooling times free + prequalification test in cas of high stress. Tests of sectionalising insulation according to IEC 60840. Cross qualification is in accordance with TB 303, with no range extension limitation on cable stresses as recently introduced in IEC 60840.

References EN 50180:2010 – Bushings above 1 kV up to 52 kV from 250 A to 3,15 kA for liquid filled transformers EN 50181:2010 – Plug-in type bushings above 1 kV up to 52 kV from 250 A to 2,5 kA for equipment other than liquid filled transformers EN 50673:2019 – Plug-in type bushings for 72.5 kV with 630 A and 1 250 A for -electrical equipment HD 629.1 S3:2019 – Test requirements for accessories for use on power cables of rated voltage from 3,6/6(7,2) kV up to 20,8/36(42) kV. Accessories for cables with extruded insulation IEC 60068-2-14. Edition 6 – environmental testing – part 2–14: tests – test N: change of temperature IEC 60085. Edition 4 – Electrical insulation – thermal evaluation and designation IEC 60137. Edition 7 – Insulated bushings for alternating voltages above 1000 V IEC 60183. Edition 3 – Guidance for the selection of high-voltage A.C. cable systems

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IEC 60840. Edition 5 – Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um ¼ 36 kV) up to 150 kV (Um ¼ 170 kV) – test methods and requirements IEC 62271-1. Edition 2 – High-voltage switchgear and controlgear – part 1: common specifications for alternating current switchgear and controlgear IEC 62271-203. Edition 2 – High-voltage switchgear and controlgear – part 203: Gas-insulated metal-enclosed switchgear for rated voltages above 52 kV IEC 62271-209. Edition 2 – Cable connections for gas-insulated metal-enclosed switchgear for rated voltages above 52 kV – fluid-filled and extruded insulation cables – fluid-filled and dry-type cable terminations TB 303: Revision of qualification procedures for HV and EHV AC extruded underground cable systems, WG B1.06 (2006) TB 605: Feasibility of a common, dry type plug-in interface for GIS and power cables above 52 KV, WG B1/B3.33 (2015) TB 669: Mechanical forces in large cross section cables systems, WG B1.34, WG B1.44 – Work under induced voltages and induced currents, to come (2016) TB 758: Test regimes for HV and EHV cable connectors, WG B1.46 (2019)

Pierre Mirebeau, who graduated from the “École Supérieure de Physique et Chimie Industrielles” (Paris), has headed high-voltage R&D for Nexans over the past 25 years. As a Member of Cigré since 2005, he contributed to a variety of subjects, including testing of DC extruded cables, life management of buried AC lines, advanced designs of laminated metallic coverings, dry type interfaces for Gas-Insulated Switchgear and power cables, and the environmental impact of cable links. In recognition to this work, he was granted the Technical Committee Award for 2011. He is an Active Member of the International Electrotechnical Commission (IEC) standardization body, and the Institute of Electrical and Electronics Engineers (IEEE), where his presentations on development techniques for HVDC Links with synthetic insulation in 2001 and his collaborative (IEEE + IEC) presentation on cable terminations for gas insulated switchgears in 2006 were awarded “best presentation.” He also holds several important patents relating to lead-alloy composition, cable designs, and polymer material composition. He is the Liaison Member between IEC TC 20 and CigréB1 (both regarding insulated cables), and between CIBRE B1 and Cigré B3 (substations and electrical installations).