Application of Tap changers to Transformers [1st ed.] 9789811539541, 9789811539558

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Power Systems

T. V. Sridhar

Application of Tap changers to Transformers

Power Systems

Electrical power has been the technological foundation of industrial societies for many years. Although the systems designed to provide and apply electrical energy have reached a high degree of maturity, unforeseen problems are constantly encountered, necessitating the design of more efficient and reliable systems based on novel technologies. The book series Power Systems is aimed at providing detailed, accurate and sound technical information about these new developments in electrical power engineering. It includes topics on power generation, storage and transmission as well as electrical machines. The monographs and advanced textbooks in this series address researchers, lecturers, industrial engineers and senior students in electrical engineering. **Power Systems is indexed in Scopus**

More information about this series at http://www.springer.com/series/4622

T. V. Sridhar

Application of Tap changers to Transformers

123

T. V. Sridhar Chennai, Tamil Nadu, India

T. V. Sridhar is Deceased ISSN 1612-1287 ISSN 1860-4676 (electronic) Power Systems ISBN 978-981-15-3954-1 ISBN 978-981-15-3955-8 (eBook) https://doi.org/10.1007/978-981-15-3955-8 © Springer Nature Singapore Pte Ltd. 2020 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 Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

This is an ambitious book. It has triple objectives—to introduce tapchangers at an appropriately elementary level to students, at the same time to give a background on tapchangers to the top hierarchy of organizations, and to help them arrive at the informed decisions in matters relating to tapchangers. At another level, the book aims to be a guide and reference work for practical professional engineers in the design office, manufacturing plant, and out in the field. No more background knowledge than a sophomore course in electrical engineering is assumed. Even though there are many manufacturers of tapchangers around the world, most electrical engineers are probably more familiar with the literature and product range of Maschinenfabrik Reinhausen (MR, Regensburg, Germany) and ABB, Sweden. For this reason, I have cited their published information extensively and used their specific products whenever illustrations were required. Having admitted that, I must confess the book is flooded with examples and illustrations of OLG, an organization for which I worked for more than two decades. OLG have very kindly permitted to use any and all of their information, irrespective of whether published or not, patented, copyrighted, or otherwise restricted. Over the years I have bowed to the wisdom that availability is a serious virtue surpassing many others. I have transgressed against the Standards in some contexts. The official designation of the subject of the book is On-Load Tap-Changers. I felt that too many capitals and hyphens appearing repeatedly and frequently, in a book on tapchangers, could be distracting, if indeed not irritating to the reader. I have used the simpler term tapchanger, with no breaks, hyphens, and capitals. I have also stuck to designating the phases R-Y-B, rather than the more official U-V-W. This comes out of my experience at trying to decipher handwritten reports from the maintenance staff from the field, where U and V are indistinguishable and both occasionally resemble W. Besides RYB conjures up a colourful picture, which apart from hinting at an allure of art has practical uses and which contrasts against the dour poesy-free UVW. Besides these, I have also griped at some unimaginative and inappropriate designations used by the IEC and suggested more lively alternatives. I hope the committee reviewing the IEC next time will take note.

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Preface

Unless otherwise specified the term tapchanger means oil type on-load tapchangers. I have requested a general permission to use published information of MR, ABB, and On Load Gears (OLG) for illustrative purposes. Wherever such information is quoted in the book, the source is acknowledged. Some information is provided from earlier manufacturers, like AEG-Siemens, Fuller Electric, NGEF, and others. No specific permission was obtained because the sources cannot be traced or are not clear at the time of writing. Such information, drawn from memory, may not accurately reflect the manufacturer's product. The quality of material sourced is sometimes circumscribed by the quality of the original. Unfortunately this could not be improved. Gratitude There are many people whose interest and inspiration are responsible for this book. The list is long, but I do not want to appear short on gratitude. 1. Professor James Bates, who at the Imperial College, London, in 1963, imbued me with an abiding interest in Electrical Engineering. Besides, on a personal level, “Prof.” Jim and his entire family have been a source of friendship and strength to me all these years. Thank you, Jim, and Isobel and the youngsters. 2. Likewise on our first meeting in 1968, Karl Stenzel of MR infected me with his enthusiasm for tapchangers. The happy infection has never been cured. Danke schön vielmal, Hr. Stenzel. 3. But for the support of my immediate family, my mother Kamala, and wife Raji, I could never have undertaken this work. In particular my mother-in-law, Lalitha Sivasubrahmanyan, had a greater faith in my knowledge and learning than me and kept urging me to “write it all down” for the future. 4. A special thanks to my old friend and colleague S. V. Gopalen who without the slightest demur was so generous in allowing me to use all and any material in possession of his Company OLG, whether published or not. The book is peppered and plastered with OLG data. Some of it is unpublished investigative information generated by the R&D of OLG. I wish to thank my R&D team for their ever enthusiastic cooperation: Balu, Venkatesh, “Kaptain” Perumal, Vasu, and Bhaskar. 5. What would I have done without the constant technical support of G. Balu, not only with the content, but also mundane formatting issues, advice on software, making drawings and sketches, and locating and supporting information from sundry sources. M. Nivedhidha was wonderfully patient in reviewing and correcting all the ACAD stuff.

Preface

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6. During the course of writing the book, I was felled by a serious and enormously expensive illness. But for the unstinting financial support of CTR Manufacturing Company Limited, I probably would have been so bogged down by financial issues that I may not have sustained the effort. Many thanks to Mr. Anil Kumar, CX, and Mr. Talegaonkar, Executive Director, for their kindness. Chennai, India

Dr. T. V. Sridhar

Contents

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Transformer and Tapchanger . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Chapter Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Tapchanging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Probable Historical Development of Ideas . . . . . . . . . . . . 1.5 Preliminary Idea of On-Load Tapchanging . . . . . . . . . . . 1.5.1 Circulating Current . . . . . . . . . . . . . . . . . . . . . 1.5.2 Solution to the Problem of High Circulating Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.3 Insertion and Removal of the Current Limiting Element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Tapchangers and Arcing . . . . . . . . . . . . . . . . . . . . . . . . 1.7 Essential Considerations on Arcs . . . . . . . . . . . . . . . . . . 1.8 Tappings and Windings . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 Physical Location of Taps . . . . . . . . . . . . . . . . . . . . . . . 1.10 Electrical Location of Taps . . . . . . . . . . . . . . . . . . . . . . 1.11 Tappings and Core Flux . . . . . . . . . . . . . . . . . . . . . . . . 1.12 Tap Numbering, Direction of Raise. Principal Ratio, Number of Different Voltages, Tapping Range . . . . . . . . 1.13 Variable Secondary Voltage Applications . . . . . . . . . . . . 1.14 Reactor and Resistance Switching Tapchangers . . . . . . . .

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Resistance Tapchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Chapter Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Advantages of Reactor as a Current Limiting Element . 2.2.1 Disadvantages of Reactor . . . . . . . . . . . . . . 2.3 Early Resistance Tapchangers . . . . . . . . . . . . . . . . . . 2.4 Dr. Jansen and His Patent . . . . . . . . . . . . . . . . . . . . . 2.4.1 Why High Speed? . . . . . . . . . . . . . . . . . . . 2.4.2 Internal Stored Energy Device . . . . . . . . . . .

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2.5 2.6 2.7 2.8 2.9 3

Main Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Disadvantages of Resistance Switching . . . . . . . . . . . . . . Reactor as Current Limiting Device . . . . . . . . . . . . . . . . How High a Speed Is “High Speed” . . . . . . . . . . . . . . . . Why Is a Tapchanger Not so Rugged as a Personal Car? .

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Switching in Resistance Tapchangers . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Functional Description of a Compartment Type Selector Switch Tapchanger with Single Resistance . . . . . . . . . . . . . 3.3 Transition from One Tap to the Next in the Single Resistance Tapchanger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Switching in the Reverse Direction . . . . . . . . . . . . . . . . . . . 3.5 Functions of Contact S . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Contact Interruption Duties . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Operating Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Direction of Power Flow and Contact Duties of a Single Resistance Selector Switch . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Reversing the Connections of S and T Contacts to Reverse Direction of Power Flow . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 Limited Reverse Power Flow Possible . . . . . . . . . . . . . . . . 3.11 Position of the T and S Contacts Relative to the Fixed . . . . 3.12 Approximate Magnitude of Transition Resistance . . . . . . . . 3.13 Pennant Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14 Recovery Voltage Is Not Always the Same as the Step Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.15 Phase Relationship Between the Recovery Voltage and the Interrupted Current . . . . . . . . . . . . . . . . . . . . . . . . 3.16 Maximum Magnitude of Contact Duties . . . . . . . . . . . . . . . 3.17 A Word of Caution Regarding Actual Measurement of Interrupted Current and Recovery Voltage . . . . . . . . . . . 3.18 Selector Switch with Double Transition Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.19 Features of Double Resistance Tap Selector Operating Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.20 Algebraic Expressions for Contact Duties . . . . . . . . . . . . . . 3.21 Maximum Interruption Duty . . . . . . . . . . . . . . . . . . . . . . . 3.22 Phase Relationship Between Interrupted Current and Recovery Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.23 Direction of Power Flow . . . . . . . . . . . . . . . . . . . . . . . . . . 3.24 Flag Feature of the Double Resistance Selector Switch . . . . 3.25 Positioning of the Contacts Relative to the Centre Line of the Fixed Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.26 Comparison of Single and Double Resistance Transition Contact Interruption Duties . . . . . . . . . . . . . . . . . . . . . . . .

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3.27 3.28

3.29 3.30 3.31 3.32 3.33 3.34 3.35 3.36 3.37 3.38 3.39 3.40 3.41 3.42 3.43 3.44 3.45

3.46 3.47 3.48 3.49 3.50 3.51

3.52 3.53

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Comparison of Size and Cost . . . . . . . . . . . . . . . . . . . . . . . Tapchanger for High Power . . . . . . . . . . . . . . . . . . . . . . . . 3.28.1 Limitation of the Selector Switch . . . . . . . . . . . . . 3.28.2 A Concept to Reduce the Tap Selector Size . . . . . Tapchanger with Diverter Switch . . . . . . . . . . . . . . . . . . . . Two Features of the Tap Selector Drive Are Noted . . . . . . . Comparison of Selector Switch and Diverter Switch Tapchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Switching Duties of Diverter Switch Contacts . . . . . . . . . . . Features of Diverter Flag Cycle Contact Switching Duties . . Magnitudes of Interrupted Current and Recovery Voltage . . Highest Magnitude of Interruption Duties . . . . . . . . . . . . . . The Importance of Being Able to Calculate the Actual and Maximum Interruption Duties . . . . . . . . . . . . . . . . . . . Direction of Power Flow . . . . . . . . . . . . . . . . . . . . . . . . . . Phase Relationship Between the Interrupted Current and Recovery Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Flag in Diverter Switching Cycle . . . . . . . . . . . . . . . . . Diverter Switch with Symmetrical Pennant Cycle . . . . . . . . Switching Duties in Symmetrical Pennant Diverter Switch . . Features of Diverter Symmetrical Pennant Contact Switching Duties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnitudes of Interrupted Current and Recovery Voltage . . Highest Magnitude of Interruption Duties . . . . . . . . . . . . . . The Importance of Being Able to Calculate the Actual and Maximum Interruption Duties . . . . . . . . . . . . . . . . . . . 3.45.1 Direction of Power Flow . . . . . . . . . . . . . . . . . . . 3.45.2 Phase Relationship Between the Interrupted Current and Recovery Voltage . . . . . . . . . . . . . . . The Symmetrical Pennant Feature in Diverter Switching Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Multi-resistance Transition . . . . . . . . . . . . . . . . . . . . . . . . . Four Resistance Diverter Switch Switching Sequence . . . . . Interruption Duties of Four Resistance Diverter Tapchanger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Circulating Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interruption Duties for the Tapchange Position 1 to 2 . . . . . 3.51.1 Interruption Duties of the Main Contact W . . . . . 3.51.2 Interruption Duties of Transition Contact X1 . . . . 3.51.3 Interruption Duties of the Transition Contact X2 . 3.51.4 Interruption Duties on Other Tapchanges . . . . . . . Interruption Duties for Transition 2 to 3 . . . . . . . . . . . . . . . Interruption Duties in the Reverse Direction . . . . . . . . . . . .

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3.54 3.55

Frequency of Contact Interruptions . . . . . . . . . . . . . . . Algebraic Expressions for Interruption Duties for Four Resistance Diverter Switch . . . . . . . . . . . . . . . . . . . . . 3.56 Main Contacts W and Z . . . . . . . . . . . . . . . . . . . . . . 3.57 Transition Contact Duties . . . . . . . . . . . . . . . . . . . . . 3.57.1 Interruption Duties of Current Contacts X1 and Y1 . . . . . . . . . . . . . . . . . . . . . . . . . 3.57.2 Interruption Duties of Contacts X2 and Y2 . . 3.58 Maximum Interrupted Currents and Recovery Voltages 3.59 Direction of Power Flow . . . . . . . . . . . . . . . . . . . . . . 3.60 Phase Relationship Between Interrupted Current and Recovery Voltage . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Constructional Aspects of Tapchangers . . . . . . . . . . . . . . . . . . 4.1 Chapter Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Construction of Selector Switches Type Tapchangers . . . . 4.2.1 Compartment Type Tapchangers Using Selector Switch Principle . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 General External Arrangement . . . . . . . . . . . . . . 4.2.3 Internal Construction (Live Part of the Tapchanger) . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4 Arcing at Contacts . . . . . . . . . . . . . . . . . . . . . . 4.2.5 Tapchangers for Star and Delta Applications . . . 4.2.6 Single Compartment Tapchanger . . . . . . . . . . . . 4.2.7 Terminal Barrier Board . . . . . . . . . . . . . . . . . . . 4.2.8 Terminal Barrier Board Mounted on the Tapchanger Tank . . . . . . . . . . . . . . . . . . 4.2.9 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.10 An Alternative Approach to the Construction of Selector Switch (OLG Type SS) . . . . . . . . . . 4.2.11 ATL Single Compartment Execution . . . . . . . . . 4.2.12 Provision for Oil Expansion in Compartment Tapchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Tapchanger in an Insulated Tank . . . . . . . . . . . . . . . . . . . 4.3.1 Which Construction Is Better? . . . . . . . . . . . . . . 4.3.2 Comparison of Compartment Type and Intank Executions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 In Favour of the Intank Tapchanger . . . . . . . . . . 4.3.4 Construction of Single Compartment Intank Tapchanger . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.5 Solution to the Problem of Access to Selector Switch in Intank Tapchangers . . . . . . . . . . . . . .

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4.4

4.5

4.6

4.7

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Diverter Switch Type Resistance Tapchangers . . . . . . . . . . 4.4.1 Constructional Aspects of Diverter Switch Type Intank Tapchangers . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Construction of the Diverter Switching Element . . 4.4.3 Cylindrical Construction . . . . . . . . . . . . . . . . . . . 4.4.4 AEG-Siemens Diverter Switches Arrangement . . . 4.4.5 The Diverter Switch NK Used by OLG Is Their Earlier Intank Tapchangers . . . . . . . . . . . 4.4.6 The OLG Cylindrical Diverter Switch . . . . . . . . . Alternative Methods of Mounting Intank Tapchanger on the Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1 Cover Mounting . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Bell-Shaped Transformer Tank Mounting . . . . . . . 4.5.3 Shelf Mounting . . . . . . . . . . . . . . . . . . . . . . . . . . General Construction of Tap Selectors for Intank Tapchangers with Diverter Switch . . . . . . . . . . . . . . . . . . . 4.6.1 Selector Drive . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.2 Application of the Geneva Intermittent Drive to Concentric Inner and Outer Selector Drive Shafts . 4.6.3 A Space Saving Alternative Arrangement of Tap Selector Drive . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.4 Tap Selector Execution in Cylinder . . . . . . . . . . . 4.6.5 An Alternate Legacy Selector Drive Arrangement Used in Fuller Electric Co’s. Type EHS Tapchanger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pre-selectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.1 Construction and Operation of the Pre-selector in Compartment Tapchangers . . . . . . . . . . . . . . . 4.7.2 Pre-selector for Intank Type Tapchangers . . . . . . . 4.7.3 Construction of the Multiple Coarse/Fine Selector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diverter Switch Construction for Line End Delta . . . . . . . . 4.8.1 Tapchanger for Delta Application . . . . . . . . . . . . 4.8.2 A Special Construction for 110 kV Delta . . . . . . . 4.8.3 The AEG-Siemens Construction for 110 kV Delta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8.4 The Problem of Spare Coil in Some Winding Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8.5 Problem of Selector Discharge in High-Voltage Delta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8.6 Tapchanger Using Diverter Switch in Sheet Metal Execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8.7 Diverter Switch on Top of the HV Bushing . . . . .

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4.9 4.10

5

A Further Development of the Sheet Metal Concept . . . . . . Energy Storage Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10.1 Spring Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10.2 Latched Spring Drive . . . . . . . . . . . . . . . . . . . . . 4.10.3 Direct Motor Drive . . . . . . . . . . . . . . . . . . . . . . . 4.10.4 Flywheel Drive . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10.5 Falling Weights . . . . . . . . . . . . . . . . . . . . . . . . . 4.11 Transition Resistances . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.11.1 Current Loading . . . . . . . . . . . . . . . . . . . . . . . . . 4.11.2 Magnitude of Resistance . . . . . . . . . . . . . . . . . . . 4.11.3 Thermal Design . . . . . . . . . . . . . . . . . . . . . . . . . 4.11.4 Estimating Temperature Rise . . . . . . . . . . . . . . . . 4.11.5 Measurement of Temperature of Resistance Wires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.11.6 Transition Resistance Under Short-Circuit Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.11.7 Transition Getting Stuck with the Resistance Carrying Current . . . . . . . . . . . . . . . . . . . . . . . . . 4.11.8 Transition Resistance Failure Due to Loss of Pre-selector Synchronism . . . . . . . . . . . . . . . . 4.11.9 Changing of Resistance Wire for Spare Tapchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.11.10 Some Typical Executions of Transition Resistances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.11.11 Typical Physical Arrangements . . . . . . . . . . . . . . 4.12 Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.12.1 Contacts for Selector Switches . . . . . . . . . . . . . . . 4.12.2 A Problem with Roller Contacts . . . . . . . . . . . . . 4.12.3 Difficulty of Current Transfer to the Central Pivot During Arcing Condition . . . . . . . . . . . . . . . . . . . 4.12.4 Edging Fixed Contacts with Tungsten Tips . . . . . 4.12.5 Contacts for Diverter Switches . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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140 141 144 144 146

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. . . .

147 147 148 148

Selection and Application of Tapchangers to Transformers 5.1 Chapter Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Which Winding Must Be Tapped? . . . . . . . . . . . . . . . 5.2.1 The US Practice . . . . . . . . . . . . . . . . . . . . . 5.2.2 Advantages of Tapping the H.V Winding . . . 5.2.3 Generator Transformers . . . . . . . . . . . . . . . . 5.2.4 Process Control Industries . . . . . . . . . . . . . . 5.3 Tapchanger Selection . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Current Rating . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Current and Contact Heating . . . . . . . . . . . .

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151 151 151 152 152 152 153 153 153 154

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130 131 131 134 135 135 135 136 136 136 137 137

. . 138 . . 138 . . 139 . . 139 . . 140

Contents

5.4

5.5

5.6 5.7

5.8

5.9 5.10

xv

5.3.3 5.3.4 5.3.5 5.3.6 5.3.7 Voltage 5.4.1 5.4.2

Raised Temperature of the Ambient . . . . . . . . . . . Short-Circuit Current Rating . . . . . . . . . . . . . . . . Magnetising Inrush Current . . . . . . . . . . . . . . . . . Overload Conditions . . . . . . . . . . . . . . . . . . . . . . Other Limitations for Overload . . . . . . . . . . . . . . Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Voltage Stresses . . . . . . . . . . . . . . . . . . . . . . . . . Considerations Which Affect the Voltages Generated by the Transformer . . . . . . . . . . . . . . . 5.4.3 Tapchanger at the Solidly Grounded Neutral . . . . 5.4.4 Voltage Stresses Due to Impulse Voltages . . . . . . Number of Taps and Tapping Range . . . . . . . . . . . . . . . . . 5.5.1 Linear Tappings . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.2 Insulation for Tap Breaks . . . . . . . . . . . . . . . . . . 5.5.3 Linear Taps for VFVV . . . . . . . . . . . . . . . . . . . . 5.5.4 Coarse/Fine Tappings . . . . . . . . . . . . . . . . . . . . . 5.5.5 Method of Obtaining the Maximum Different Voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.6 Selector Having One Pos. Less Than the Number of Leads from the Fine Tappings . . . . . . . . . . . . . 5.5.7 Insulation Requirement at Tap Breaks . . . . . . . . . 5.5.8 Multiple Coarse/Fine Selectors . . . . . . . . . . . . . . 5.5.9 Possible Difficulty with Transition Impedance Test, Sect. 5.2.5.1 of IEC 60 214 [12] . . . . . . . . . . . . . Reversing Tapping Arrangement . . . . . . . . . . . . . . . . . . . . Method of Connection and Operation . . . . . . . . . . . . . . . . . 5.7.1 Some Important Points of the Reverser-Tapping Arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.2 Method of Obtaining the Maximum Different Voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.3 Obtaining 17 Different Voltages from a Nine-Position Selector . . . . . . . . . . . . . . . Physical Location of Taps . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.1 Unbalanced Axial Ampere-Turns . . . . . . . . . . . . . 5.8.2 Location of Taps Along with the Winding Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.3 Neutral Taps in the Physical Middle of the Winding Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.4 Taps on a Separate Tapping Barrel . . . . . . . . . . . Voltage Stress in Tapping Barrel Applied to Taps at Neutral End . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tapchanger for Delta Transformers . . . . . . . . . . . . . . . . . . 5.10.1 Voltage Stresses Across Internal Insulating Distances in Delta Application . . . . . . . . . . . . . . .

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154 154 156 156 157 157 157

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159 160 162 165 165 165 167 167

. . 172 . . 173 . . 173 . . 174 . . 175 . . 176 . . 176 . . 178 . . 178 . . 179 . . 180 . . 180 . . 183 . . 183 . . 184 . . 184 . . 186 . . 187

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Contents

5.10.2

5.11

5.12

5.13

5.14

5.15 5.16 5.17

5.18

5.19

5.20

Separate Tapping Barrel with Delta-Connected Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10.3 Why Is It Difficult to Locate Taps in the Electrical Middle of Delta on a Tapping Barrel . . . . . . . . . . Clearance Over the Tapping Range (Gap Between First and Last Contacts) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.11.1 A Special Example of Increased Gap Between Selector-End Contacts: Diverter Switch Tapchanger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tapchangers for Autotransformers . . . . . . . . . . . . . . . . . . . 5.12.1 Taps at the Neutral End . . . . . . . . . . . . . . . . . . . 5.12.2 Taps at the End of the Series Coil . . . . . . . . . . . . 5.12.3 The Potentiometric Arrangement . . . . . . . . . . . . . 5.12.4 “Hanging Out” Taps or “Tee Taps” . . . . . . . . . . . 5.12.5 Physical Location of the Tapping Barrel in Autotransformers . . . . . . . . . . . . . . . . . . . . . . . . 5.12.6 Use of Series Boosters with Autotransformers . . . 5.12.7 Booster Transformer for High-Current L.V . . . . . . Use of Interpolating Transformer for High-Current Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.13.1 Irregular Variation of Output Voltage When a Pre-selector Is Used . . . . . . . . . . . . . . . . Tapchanger for Zig-Zag-Connected Transformer . . . . . . . . . 5.14.1 Need for the Same Number of Turns in the Zig and the Zag . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tapchanger for Scott Connection . . . . . . . . . . . . . . . . . . . . 5.15.1 Earthing Problems . . . . . . . . . . . . . . . . . . . . . . . Switching Capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Parallel Connection of Tapchangers . . . . . . . . . . . . . . . . . . 5.17.1 Paralleling Two Different Tapchangers with Individual Driving Mechanisms Is Not Practical . . 5.17.2 Enforced Current Sharing . . . . . . . . . . . . . . . . . . 5.17.3 Paralleling of Switching Element Directly in Parallel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phase Shift Between Interrupted Current and Recovery Voltage in Parallel-Connected Transformers with Circulating Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application to Furnace Transformer . . . . . . . . . . . . . . . . . . 5.19.1 Furnace Applications Pose Especially Onerous Conditions on the Tapchanger . . . . . . . . . . . . . . . Operating Temperature Considerations . . . . . . . . . . . . . . . . 5.20.1 Requirements Stipulated in the Mechanical Endurance Tests Cl. 5.6.2.2 of IEC 60 214 . . . . .

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191 191 192 192 193 194

. . 195 . . 196 . . 198 . . 199 . . 200 . . 200 . . . . .

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201 202 203 203 205

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Contents

xvii

5.20.2

A Special Problem for Intank Tapchanger with High-Temperature Conditions . . . . . . . 5.21 Tapchanger Operation When Immersed in Ester Fluids 5.22 Driving Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . Appendix: Commutation of Current Between Coarse and Fine Tapping Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

. . . . . . 214 . . . . . . 215 . . . . . . 216 . . . . . . 216 . . . . . . 223

Special Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Chapter Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1 Application to Shunt Reactors . . . . . . . . . . . . . . . 6.1.2 What Is the Special Problem? . . . . . . . . . . . . . . . 6.2 Tapchanging in Shunt Reactor . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Removing Turns . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Steps Corresponding to Removal of Turns . . . . . . 6.2.3 Steps Corresponding to Adding Turns . . . . . . . . . 6.3 Some Observations on Tapchanger Application for Reactor Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Tapchanger in a System . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 General Conclusions . . . . . . . . . . . . . . . . . . . . . . 6.4.2 Tapchanger During Abnormal System Operation . 6.5 Control of Real Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Some Features and Terminology for Phase Shifters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Single Core Phase Shifter Connections and Their Influence on Tapchanger Selection . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.1 Single Core Asymmetrical PST . . . . . . . . . . . . . . 6.6.2 Lack of Self Protecting Leakage Impedance . . . . . 6.6.3 Ratings of Tapchanger for the PST of Fig. 6.7 . . . 6.6.4 Voltage Over the Tapping Range . . . . . . . . . . . . . 6.7 Switching Capacity Per Tap as a Limitation in PST Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7.1 Use of Coarse/Fine with Advance/Retard Switch . 6.8 Symmetrical Arrangement of Twin PSTs on a Single Core . 6.9 Use of Two-Phase Tapchanger . . . . . . . . . . . . . . . . . . . . . . 6.10 Tapchanger Ratings for the PST of Fig. 6.9 . . . . . . . . . . . . 6.11 Variable Inline Flux During Phase Shift . . . . . . . . . . . . . . . 6.12 Delta Hexagonal Phase Shifter . . . . . . . . . . . . . . . . . . . . . . 6.12.1 Tapchanger Ratings for the PST of Fig. 6.15 . . . . 6.13 Limitation by Step Capacity . . . . . . . . . . . . . . . . . . . . . . . . 6.14 PST Implemented with Star-Connected Transformer . . . . . . 6.14.1 Tapchanger Ratings for the PST of Fig. 6.18 . . . . 6.15 Limitations of Single Core PSTs . . . . . . . . . . . . . . . . . . . . 6.16 Dual Core PSTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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225 225 226 226 227 227 227 229

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231 233 235 235 235

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238 239 240 241 241

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242 242 245 245 245 248 248 249 249 251 251 252 252

xviii

Contents

6.16.1 6.16.2 6.16.3 6.16.4 6.16.5

Tapchanger Current at Zero Injection Position Inline Voltage Injection . . . . . . . . . . . . . . . . . Tapchanger Ratings for the PST of Fig. 6.19 . Voltage Rating . . . . . . . . . . . . . . . . . . . . . . . Increasing the Number of Steps to Reduce Requirement of Step kVA . . . . . . . . . . . . . . . 6.17 Symmetrical Dual Core Phase Shifter Corresponding to Fig. 10 IEC 62 032 . . . . . . . . . . . . . . . . . . . . . . . . . 6.17.1 Tapchanger Ratings for the PST of Fig. 6.21 . 6.17.2 Other Equivalents . . . . . . . . . . . . . . . . . . . . . 6.18 Use of Coarse/Fine . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.19 Limitation of Single Core PSTs . . . . . . . . . . . . . . . . . . 6.19.1 Further Reading . . . . . . . . . . . . . . . . . . . . . . 6.20 Tapchanger Operating in Air Environment . . . . . . . . . . 6.21 Operational Problems Encountered with the Tapchanger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

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252 253 254 254

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256 257 257 258 258 258 259

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Problem of Capacitively Determined Potential . . . . . . . . . . . . . . 7.1 Chapter Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Description of the Problem . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Pre-selector Arcing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 General Methodology of Analysis for the Determination of the Arc Current and Recovery Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 A Realistic Perspective of the Effects of Pre-selector Arcing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Reversing Taps at Neutral End . . . . . . . . . . . . . . . . . . . . . . 7.5.1 Recovery Voltages . . . . . . . . . . . . . . . . . . . . . . . 7.5.2 Arc Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.3 Numerical Values . . . . . . . . . . . . . . . . . . . . . . . . 7.5.4 General “Ball Park” Values . . . . . . . . . . . . . . . . . 7.6 Application of Single Compartment Tapchangers at the Neutral End . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6.1 Taps at the Neutral Located in the Physical Middle of the Total Winding . . . . . . . . . . . . . . . . . . . . . 7.6.2 Arc Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6.3 Approximate Quantitative Evaluation . . . . . . . . . . 7.7 Coarse/Fine Taps as Separate Tapping Barrel at the Neutral . 7.7.1 Arc Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8 Coarse/Fine at the Neutral End for Taps in the Body of the Winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8.1 Recovery Voltages . . . . . . . . . . . . . . . . . . . . . . . 7.8.2 Arc Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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261 261 262 263

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265 265 266 267 267 268

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269 270 271 271 272

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xix

7.9

8

Reverser Switching at the Line End of Delta-Connected Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.9.1 Physical Arrangement . . . . . . . . . . . . . . . . . . . . . 7.10 Reversing Taps in the Middle of the Delta with Taps Out of the Body of the Main Winding . . . . . . . . . . . . . . . . . . . 7.10.1 Determination of Recovery Voltages . . . . . . . . . . 7.10.2 Arc Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.10.3 A Practical Numerical Example . . . . . . . . . . . . . . 7.11 Delta-Connected Transformer with Coarse/Fine Tapchanger at Line End . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.11.1 Recovery Voltages . . . . . . . . . . . . . . . . . . . . . . . 7.11.2 Arc Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.11.3 Illustrative Example . . . . . . . . . . . . . . . . . . . . . . 7.12 Reversing Taps in Autotransformers . . . . . . . . . . . . . . . . . . 7.12.1 Taps at the End of the Series Winding . . . . . . . . . 7.12.2 Taps in Series with the Common Winding of Autotransformer . . . . . . . . . . . . . . . . . . . . . . . 7.13 Application to Phase-Shifting Transformers . . . . . . . . . . . . 7.13.1 Symmetrical Single-Core Phase Shifter—Recovery Voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.14 Application of Pre-selector in Dual-Core Phase Shifter . . . . 7.15 Mitigation of High Recovery Voltage by Using Tie-in Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.15.1 Reversing Taps on a Tapping Barrel at the Neutral . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.16 Tie-in at Delta Line End . . . . . . . . . . . . . . . . . . . . . . . . . . 7.16.1 Recovery Voltages . . . . . . . . . . . . . . . . . . . . . . . 7.16.2 Arc Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.16.3 Illustrative Example . . . . . . . . . . . . . . . . . . . . . . 7.17 Other Mitigation Techniques . . . . . . . . . . . . . . . . . . . . . . . 7.17.1 Shielding to Influence Capacitive Coupling . . . . . 7.17.2 Use of the Double Reverser Switch . . . . . . . . . . . 7.17.3 Dissipation in the Tie-in During Changeover . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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294 297 298 299 299 299 299 302 303 305

Vaccum Tapchangers . . . . . . . . . . . . . . . . . . . . . . . 8.1 Chapter Content . . . . . . . . . . . . . . . . . . . . . . 8.1.1 A Word of Caution on Terminology 8.2 Basic Principle . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Tapchange in The Reverse Direction 8.3 Common Features Summarized . . . . . . . . . . .

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307 307 308 308 311 312

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278 280 281 281

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282 282 284 284 284 285

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Contents

8.4

Contact Bounce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.1 A Refinement of Operation . . . . . . . . . . . . . . . . . 8.4.2 Vacuum Tapchangers with Other Switching Cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 Why Is the Vacuum Interrupter So Good in Its Function . . . 8.6 Specialties in Interrupter Construction for Tapchanger Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6.1 Construction of Standard Vacuum Interrupters . . . 8.6.2 Modifications For Tapchanger Duty . . . . . . . . . . . 8.6.3 Operation Under Short-Circuit Conditions . . . . . . 8.6.4 Electromagnetic Force Generator . . . . . . . . . . . . . 8.6.5 Waukesha Contact Force Generator . . . . . . . . . . . 8.6.6 Use of Shunt Contact . . . . . . . . . . . . . . . . . . . . . 8.6.7 Problems with Moving Bottles . . . . . . . . . . . . . . 8.6.8 Non-sustained Disruptive Discharge in Vacuum . . 8.6.9 Consequences of Failure of Bottles . . . . . . . . . . . 8.6.10 Other Switching Sequences with Vacuum Tapchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7 IEC 60 214 Table 3 Ref [1] Circuit for Diverter with Two Bottles, Two Auxiliary Switches, and One Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8 Algebraic Expressions for Interruption Duties . . . . . . . . . . . 8.8.1 Main Interrupter Vm . . . . . . . . . . . . . . . . . . . . . . 8.8.2 Transition Interrupter Vt . . . . . . . . . . . . . . . . . . . 8.8.3 Maximum Interruption Duty . . . . . . . . . . . . . . . . 8.9 A Diverter with Two Interrupters and One Auxiliary Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.9.1 The IEC Table A3 Selector Switch with Single Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.10 The IEC Table A3 Diverter Switch with Three Interrupters and Two Transition Resistances . . . . . . . . . . . . . . . . . . . . . 8.10.1 The IEC Table A3 Selector Switch with Three Interrupters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.10.2 A Diverter Switch with Only One Vacuum Interrupter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.11 Vacuum Diverter for Refurbishment Purpose . . . . . . . . . . . 8.12 Switching Sequence in OLG Tapchanger T01 . . . . . . . . . . . 8.12.1 Realization of the Switching Scheme of Fig. 8.18 . 8.13 Conclusions on Vacuum Technology for Tapchangers . . . . . 8.14 Thyristor-Assisted Tapchanging . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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9

Reactor Tapchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Reactor as a Current Limiting Element . . . . . . . . . . . . . . . . 9.2 Introducing the Preventive Autotransformer (PAT) . . . . . . . 9.3 Circuit Arrangement and Switching Sequence in Reactor Tapchanger with Vacuum Technology [2] . . . . . . . . . . . . . 9.3.1 Output Voltages . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.2 Interruption Duties . . . . . . . . . . . . . . . . . . . . . . . 9.3.3 Algebraic Expressions for Interruption Duties . . . . 9.3.4 Phase Relationship of the Interrupted Current and Recovery Voltage . . . . . . . . . . . . . . . . . . . . . 9.3.5 Maximum Interruption Duty . . . . . . . . . . . . . . . . 9.3.6 Influence of the Direction of Power Flow . . . . . . . 9.3.7 Features of Reactor Tapchanger with Vacuum Interrupter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Some Constructional Features of Reactor Tapchangers with Vacuum Interrupter . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.1 The Tap Selector . . . . . . . . . . . . . . . . . . . . . . . . 9.5 Bypass Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Vacuum Interrupter Arrangement . . . . . . . . . . . . . . . . . . . . 9.7 Parameters of the PAT . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.8 Protection Against the Tap Selector Opening with Arcing . . 9.8.1 Tapchanger Not to Be Manually Operated When the Transformer Is on Load . . . . . . . . . . . . . . . . . 9.9 Connection Diagram with Reverser . . . . . . . . . . . . . . . . . . 9.10 Reactor Tapchanger with Arcing Diverter Switch . . . . . . . . 9.10.1 Interruption Duties of Arcing Diverter Switch . . . 9.10.2 Reverse Power Flow . . . . . . . . . . . . . . . . . . . . . . 9.11 Use of Equalizer Windings . . . . . . . . . . . . . . . . . . . . . . . . 9.11.1 Interruption Duties of Arcing Diverter Switch with Equalizer . . . . . . . . . . . . . . . . . . . . . . . . . . 9.11.2 Algebraic Expressions for Interruption Duties of Arcing Diverter Switch with Equalizer . . . . . . . 9.12 Reactor Tapchanger with Selector Switch . . . . . . . . . . . . . . 9.12.1 Execution of Voltage Regulators . . . . . . . . . . . . . 9.12.2 Tapchanger for Step Voltage Regulators . . . . . . . 9.12.3 Interruption Duties of Selector Switch Type Tapchanger with Arcing Contacts . . . . . . . . . . . . 9.12.4 Selector Switch with Equalizer . . . . . . . . . . . . . . 9.12.5 Interruption Duties of Selector Switch with Reactor and Equalizer . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.13 Regulating Transformers for Furnace Application . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xxi

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Contents

10 Drive 10.1 10.2 10.3 10.4 10.5 10.6

10.7 10.8 10.9 10.10 10.11 10.12 10.13 10.14 10.15

10.16

10.17 10.18 10.19 10.20 10.21 10.22

Mechanism and Controls . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Control Requirements . . . . . . . . . . . . . . . . . . . . . . The Power Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Single- and Three-Phase Motors . . . . . . . . . . . . . . . . . . . . . Number of Starts Per Hour . . . . . . . . . . . . . . . . . . . . . . . . Motor Overload Protection . . . . . . . . . . . . . . . . . . . . . . . . . 10.6.1 Motor “No Start” Protection . . . . . . . . . . . . . . . . 10.6.2 Thermal Protection . . . . . . . . . . . . . . . . . . . . . . . 10.6.3 Particularly Low Temperatures . . . . . . . . . . . . . . 10.6.4 Manual Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.6.5 Number of Turns of Handle Per Tapchange . . . . . 10.6.6 Electrical Limits . . . . . . . . . . . . . . . . . . . . . . . . . 10.6.7 Mechanical Limits . . . . . . . . . . . . . . . . . . . . . . . 10.6.8 Mechanical Limit by Solid Block . . . . . . . . . . . . 10.6.9 Differential Gear System . . . . . . . . . . . . . . . . . . . Integral and Dismountable Mechanisms . . . . . . . . . . . . . . . Demountable Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . Matching of the Drive to the Tapchanger . . . . . . . . . . . . . . Connecting the Drive Mechanism to Three Single-Pole Tapchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Peripheral Misalignment Tolerance . . . . . . . . . . . . . . . . . . . Motor Power Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Motor Control Logic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stopping the Motor Drive at the End of a Normal Tapchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Brake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.15.1 Using the Motor to Brake . . . . . . . . . . . . . . . . . . 10.15.2 Using the Spring Charging to Cause Braking Effect . . . . . . . . . . . . . . . . . . . . . . . . . . Termination of a Tapchange . . . . . . . . . . . . . . . . . . . . . . . 10.16.1 Snap Action Quick Acting Switch for Termination of Tapchange . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.16.2 Evaluation of Snap Action Switch . . . . . . . . . . . . 10.16.3 Cam Switch for Termination of Tapchange . . . . . 10.16.4 Stepping Function . . . . . . . . . . . . . . . . . . . . . . . . A Critical Problem with Limit Switch Setting . . . . . . . . . . . Controls for Parallel Operation . . . . . . . . . . . . . . . . . . . . . . Use of the Out-of-Step Relay . . . . . . . . . . . . . . . . . . . . . . . Possibilities of Contention in Interchange of Commands Between Master and Follower . . . . . . . . . . . . . . . . . . . . . . The Circulating Current Method of Paralleling . . . . . . . . . . Automatic Voltage Regulating Relays . . . . . . . . . . . . . . . . .

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Contents

10.23 Line Drop Compensation . . . . . . . . . . . . . . . . . . 10.24 Conventional Method (Refs. [8, 9] (Beckwith)) . . 10.25 Modern Line Drop Compensation . . . . . . . . . . . 10.26 The Load Bonus Feature . . . . . . . . . . . . . . . . . . 10.27 Indications Connected with Tapchanger Controls 10.28 Indications at the Remote Control Panel . . . . . . . 10.29 Blocking Under Transformer Short Circuit . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xxiii

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11 Operation, Maintenance, and Monitoring . . . . . . . . . . . . . . . . . . 11.1 Content of Chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Sundry Considerations for the Transformer Manufacturer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 Operation of Tapchanger Without Drive Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.2 Coupling of the Tapchanger to the Drive Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.3 Oil Filling the Suction Pipe . . . . . . . . . . . . . . . . . 11.2.4 Free-Level Operation . . . . . . . . . . . . . . . . . . . . . 11.2.5 Gas Accumulation and Venting . . . . . . . . . . . . . . 11.2.6 Gas Reaching Areas of High Electric Field . . . . . 11.3 Anxieties About Integrity of the Current Path . . . . . . . . . . . 11.3.1 Winding Resistance . . . . . . . . . . . . . . . . . . . . . . 11.4 Test to Indicate Problems in Transformer Current Path . . . . 11.4.1 Problems in Tapchanger Transition . . . . . . . . . . . 11.5 Polarization Index as a Measure of Insulation Quality . . . . . 11.5.1 Current Flow Through an Insulator During Resistance Measurement . . . . . . . . . . . . . . . . . . . 11.5.2 Polarization Index in Oil-Filled Equipment . . . . . . 11.6 Transient Current Due to Difference in Tap Positions in Three-Pole Tapchangers . . . . . . . . . . . . . . . . . . . . . . . . . 11.7 Contact Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.7.1 Loss of Operational Cycle Due to Contact Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.8 Operation of Transformers in Parallel . . . . . . . . . . . . . . . . . 11.8.1 Operating Transformers with Different Percentage Impedance in Parallel . . . . . . . . . . . . . . . . . . . . . 11.9 Influence of Circulating Current on Switching Duty . . . . . . 11.9.1 Parallel-Connected Transformers with Voltage Difference at No Load . . . . . . . . . . . . . . . . . . . . . 11.9.2 An Approximate Numerical Evaluation . . . . . . . . 11.9.3 Tapchanger Control with Circulating Current . . . . 11.10 Staggered Switching of Tapchangers in Parallel . . . . . . . . . 11.11 Protection Against Tapchanging Under Short Circuit . . . . . .

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Contents

11.12 Tapchanging with Magnetizing Inrush Current . . . . . . . 11.13 Contact Coking in Pre-selectors . . . . . . . . . . . . . . . . . . 11.14 Application of DGA to LTCs . . . . . . . . . . . . . . . . . . . . 11.14.1 Approach to Analysis . . . . . . . . . . . . . . . . . . 11.14.2 Types of Incipient Faults Identified by DGA Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.14.3 Construction of the Duval Triangle 2 . . . . . . . 11.15 Acoustic Signals as a Means of Estimating Tapchanger Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.16 Operation Monitoring by Extended Voltage Regulating Relays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.16.1 Features Offered by Monitoring and Data Acquisition Systems . . . . . . . . . . . . . . . . . . . 11.17 Tapchanger Failures . . . . . . . . . . . . . . . . . . . . . . . . . . 11.17.1 Failure of a Diverter, or Selector Switch to Complete a Tapchange . . . . . . . . . . . . . . . 11.17.2 Component Failure . . . . . . . . . . . . . . . . . . . . 11.18 Oil Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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451 452 452 452

About the Author

Dr. T. V. Sridhar obtained his PhD from Imperial College London in 1966, after finishing his Bachelors in Engineering from Bombay University, India in 1960. He had a vast experience of more than 40 years in the industry having worked in different capacities from Deputy Chief Design Engineer (Transformers) at Hackbridge Hewittic & Easun Limited to Director (R&D) at On Load Gears (OLG), Chennai. During his time at OLG, he had established a product line of external and internal tap changers, resistance and reactor type, dry and oil type with vacuum technology. Dr. Sridhar also briefly worked as a Lecturer at the University College, Nairobi, Kenya between 1966 and 1968. During his extensive career, he had published several research articles in international peer-reviewed journals.

xxv

Chapter 1

Transformer and Tapchanger

“Begin at the beginning,” the King said, very gravely, “and go on till you come to the end: then stop”.—Lewis Carroll, Alice in Wonderland Anfangen ist leicht, Beharren eine Kunst—German Proverb

1.1 Chapter Content The purpose of this chapter is to introduce some basic concepts and terminology. The ideas are elaborated in later chapters relating to practical tapchangers.

1.2 Transformer The most elementary transformer has a magnetic core, on which two coils, 1 and 2, are wound. The basic transformer equation, linking the number of turns to the terminal voltage, is very simple, namely V 1/V 2 = N 1/N 2 where V 1 and V 2 are the coils voltages at no load (there could be peak, or RMS), N1 and N2 are the respective numbers of turns of the coils. It follows that if we have a steady source of sinusoidal voltage with constant magnitude V 1 and if we need a voltage of constant magnitude V 2, we can have a transformer wound with a turns ratio of V 1/V 2. When V 1 is connected to coil 1, a voltage V 2 will result across coil 2. See Fig. 1.1. This will result in a satisfactory and harmonious situation, except for the fact that the two coils exhibit a property called leakage reactance, which acts as inductive coil in series with the transformer. The effective series reactance with the transformer results in a voltage drop, so that the output is not a steady and

© Springer Nature Singapore Pte Ltd. 2020 T. V. Sridhar, Application of Tap changers to Transformers, Power Systems, https://doi.org/10.1007/978-981-15-3955-8_1

1

2

1 Transformer and Tapchanger

(a)

(b)

Fig. 1.1 The transformer

constant V 2, but a variable voltage depending on the load current. Admittedly the voltage drop through the transformer leakage reactance is a small part of the output and one may be tempted to ignore it and not do anything about it. However we have to consider that the transformer derives power at voltage V 1 from some other transformer upstream. The upstream transformer will also have a voltage drop in its leakage reactance, causing V 1 itself to drop. The voltage V 2 further drops, as V 1 drops. In a power distribution system, we may have a cascade of transformers, each dropping voltage. Likewise, when loads are thrown off, voltage V 2 may rise considerably. The cumulative variation of voltage could be considerable. It seems desirable to install some measure of compensation. For instance, we could wind coil 2 in the above example with more turns than N2. We could connect a lead out at every few turns interval and terminate them on an insulated board. For good measure, we could even connect a lead at a few turns intervals at less than N2 turns. If such a coil is available, we could derive the output voltage V 2 between the top end of the coil and any one of the leads that we brought out. This arrangement is called a tapped coil, and the lead outs are called taps. See Fig. 1.2. If a drop in V 2 is noted, we can simply abandon that tap, and go to the next, where more turns are available, and the induced voltage is higher, providing a buffer against voltage drops. There is no reason why voltage correction should be done only this way. We could also tap the coil 1 in a similar way. In the most general case, both coils could be tapped. We may reserve the taps on the primary to correct for variations in the incoming voltage and use the taps on the secondary to secure against voltage changes due to the drop through the transformer.

1.3 Tapchanging

3

Fig. 1.2 Concept of tapped coil

1.3 Tapchanging In principle, we could select a new connection position of the bottom lead of coil 2 manually. The reconnection is justly termed “Tapchanging”. We would not do this when the transformer is live. So we would switch off the transformer for safety, before tapchanging. There is another reason why we must switch the transformer off. At any operating tap, the connections of coil are carrying current. We cannot simply take off the connection from the transformer, because there will be arcing and a flash. So out of two considerations, we need to switch the transformer off first.

1.4 Probable Historical Development of Ideas It is imaginable that the historically first tapchanging transformers actually functioned as described above. When there was a need, the transformer was stopped, and a reconnection of the operating tap was manually made. As system voltages went up, the tap terminals had to be terminated on an insulated board, which had to be under oil, for insulation. Accessing the board for tapchanging would have been quite a job. Soon enough the idea would have germinated that the “tappings” could be terminated in a multi-position switch which could be operated by an insulated shaft brought out from the transformer oil space into the external world. This would avoid the need to physically access the taps. The switch could take any of the forms shown in Fig. 1.3. In Fig. 1.3a we have linear switch, with a moving contact which could typically be mounted on a lead screw. The lead screw must be turned from the outside world to make tapchange. The switch could be circular, with the taps attached to fixed contacts on the periphery (Fig. 1.3b). A centrally rotating moving contact connects to the desired tap. The moving contact is driven by an insulated shaft from the external world, mechanically terminated in a wheel. In principle the arrangement of Fig. 1.3c

4

1 Transformer and Tapchanger

(a)

(b)

(c)

Fig. 1.3 Execution of tap switches

is also possible. Here it does not matter about the mechanical distribution of the taps: Each tap is connected to an “output” bar by a two-position switch, which could be electrically operated from the outside world. In one position the switch connects the tap to the output bus, and in the other it is open. The end output lead of the transformer is permanently connected to the “output” Bar. Incidentally this last idea was in fact never used but may stage a grand entry in future with thyristor operated tapchangers, intended for extreme high-speed operation (See Chap. 9). This icon is also useful in explaining some basic concepts, as the reader will see in Sect. 1.5.

1.5 Preliminary Idea of On-Load Tapchanging With the increased sophistication of the power user, such interruptions in power supply cannot be tolerated. There appears to be a very simple solution. In Fig. 1.4 we have a tapchanging transformer. Each tap is provided with a switch which can connect that tap to the transformer output. Let us say the transformer is operating at a position when tap 3 is connected (Fig. 1.4b). Suppose for some external operating reasons, the voltage falls. We must go to tap 4, where the number of turns is lower, with a lower induced voltage in the coil, which could offset the voltage rise. But we should do this without breaking the connection at 3, which would result in a break of service. Why not close the switch at 4 (Fig. 1.4b), then open the switch at 3 (Fig. 1.4c). There is no time when the current path is broken.

1.5.1 Circulating Current The problem with this simplistic solution is that there is an induced voltage between taps 3 and 4. With both taps closed to the output bus, this interstep voltage drives a high current in the closed loop. It looks that closing two switches to short contacts 3 and 4 will produce infinite current. The current is not infinite, due to the circuit

1.5 Preliminary Idea of On-Load Tapchanging

5

Fig. 1.4 Tapchanging on load

(a)

(b)

(c)

(d)

impedance, but can be very high. We cannot short a tap in a transformer even for a short while.

1.5.2 Solution to the Problem of High Circulating Current The solution is to introduce a current limiting device, like a reactance, or resistance in the 4 arm, before contemplating a tapchange. In Fig. 1.5a the operating tap is 3. In Fig. 1.5b a current limiting element Z is introduced in arm 4. With this invention, we can close the switch at 4, without disastrously high intertap circulating currents as shown in Fig. 1.5b. The through current to the load continues through switch 3. There is also a circulating current driven by the interstep voltage and limited by the current limiting element. We can open the switch at 3 (Fig. 1.5c). Current flow to the load now continues through switch 4. There is, of course, a voltage drop due to the flow of the load current through Z. Some further arrangements must be incorporated to remove Z from the circuit as shown in Fig. 1.5d. That would complete the tapchange. Throughout the tapchange, current to the load flows at all times continuously, as shown in Fig. 1.5.

6

1 Transformer and Tapchanger

(a)

(c)

(b)

(d)

Fig. 1.5 Transition with current limiting element

1.5.3 Insertion and Removal of the Current Limiting Element Figure 1.5 is not a practical tapchanger. There is no explanation of how the current limiting element suddenly appears and later disappears. It is only a fanciful way to highlight what is required. But this magic can be realized in practice by many electromechanical layouts. A practical tapchanger consists of fixed and moving contacts, and a sequence of switching, which inter alia introduces the current limiting element and later removes it. You will see more about switching sequence in Chap. 3. You will also come across tapchangers in which the current limiting element is not totally removed from circuit, but merely rearranged (Chap. 9 Reactor Tapchangers).

1.6 Tapchangers and Arcing

7

1.6 Tapchangers and Arcing There are some engineers who naively believe that since the load current is not interrupted in an on-load tapchanger, there should be no arcing. This is not correct. If there are two parallel current paths of identically zero impedance, opening of one path commutates the current instantly to the other available path, with no arcing. Except in this ideal situation, the reactance of the path which is switched off as well as that of the path where the current must double will resist these changes of current. This produces an arc at the opening contact, trying to maintain the status quo, the arc being sustained by the voltage difference that arises between the parting contacts. Such a voltage, called the recovery voltage, arises in all situations except ideal zero impedance of both paths. To take for instance Fig. 1.5b when both switches 3 and 4 are closed, the through current flows through switch 3. There also exists a circulating current through both switches, this current being driven by the interstep voltage, limited by the current limiting element. When switch 3 is opened, the voltage drop across the current limiting element applies cross the contact break and supplies the impetus for the arc. The tapchanger situation is quite different from opening one of two parallel current paths with zero impedance. We shall examine current switching activity, arcing, and quenching for different switching sequences in Chap. 3.

1.7 Essential Considerations on Arcs In Fig. 1.5a it is necessary for the contacts of the switch 3 to part sufficiently to create arc quenching conditions. The arc will eventually become unstable with increased length, and the current will cease at a current zero. When there is arcing between contacts, they are practically short-circuited, with only a low arc voltage between them. At current zero a voltage appears across the contacts, which we call the recovery voltage. In switchgear practice, they recognize this voltage as the restrike voltage, trying to reignite the contact gap. This is because in switchgear application there is generally a considerable distributed capacitance in the circuit, which oscillates with the circuit inductance. The final power frequency recovery voltage between contacts is reached through a “restrike” transient. In the case of tapchanger, the physical extent of the circuit involved is small, consisting only of the transformer tap, and the lead to the tapchanger. Therefore we can safely omit the transient voltage behaviour and take the sinusoidal power frequency recovery voltage. The main challenge in tapchanger design consists in arc quenching. Figure 1.6 schematically explains the tapchanger designer’s problem. In on-load tapchanging, contact openings draw an arc across the parting contacts. In Fig. 1.6a the moving contact has drawn an arc to the fixed contact 1 as it tries to break from it. The moving contact is travelling towards the fixed contact 2. It is essential that the arc to fixed contact 1 should go out (Fig. 1.6b), before the moving contact touches 2. Otherwise successive taps are shorted (Fig. 1.6c).

8

1 Transformer and Tapchanger

(b)

(a)

(c)

Fig. 1.6 Tapchanger mechanism of failure

This consideration underlies all issues and problems of tapchanging. The study of switching duties in Chap. 3 is guided by this concern.

1.8 Tappings and Windings If it is desired to provide taps on a winding, the winding lathe is stopped at the appropriate position, and a loop out of the winding wire is made, for eventual connection as a tap. Many such taps can be taken from a winding. Figure 1.7 is the electrical representation of the “plucked out” taps. In Fig. 1.7a taps can be selected as required by a “wander lead”, shown as W. W can select any of the taps. The circuit of the

(a)

Fig. 1.7 Winding and taps

(b)

1.8 Tappings and Windings

9

Fig. 1.8 Physical location of taps

(a)

(b)

winding is between terminal 1 and W. In Fig. 1.7b the winding circuit between terminals 1 and 2 is completed by bridging any two taps. For reasons that will become apparent in Chap. 3 when we consider switching duties, the bridging taps can only be used for De-energized tapchangers. On-load tapchangers need the wander point arrangement. The tables shown are an important communication link between the transformer manufacturer and the tapchanger supplier. Variously called phasor diagram or connection diagram, they correlate the tapped winding with the tapchanger. They may contain more information, e.g. nominal voltage.

1.9 Physical Location of Taps Tappings can be placed anywhere along the winding. The schematic of Fig. 1.8a shows the more frequently used arrangement of tapping in small transformers. Please note that Fig. 1.8 does not show the actual tapchanger, but only the transformer winding. This method is called tapping from the body of the winding. Another way of arranging the taps is to place the tapped section of the winding in a separate coil, and series connected with the untapped “main” section of the coil (See Fig. 1.8b). This arrangement is called separate tapping barrel and is used in larger transformers to reduce axial forces on the windings.

1.10 Electrical Location of Taps Figure 1.9 shows the manner in which the tapped zone relates to the rest of the winding. In Fig. 1.9a the taps are at the neutral end of a star-connected transformer. This location is preferred as the impulse voltage occurring is lower, with a milder wave

10

1 Transformer and Tapchanger

(a)

(d)

Fig. 1.9 Electrical location of taps

(b)

(e)

(c)

(f)

1.10 Electrical Location of Taps

11

front. In Fig. 1.9b the taps are in the electrical middle of the winding. This arrangement is usually chosen for delta-connected transformers. The impulse is slightly mitigated compared to the line end. We note that the untapped “main” coil is split into two parts. In the “all taps out” operating position shown in Fig. 1.9c the entire tapping range voltage occurs across the gap marked “tapping gap”. Under impulse conditions the “free end” of the tapping range oscillates producing rather high voltages. The tap gap must be suitably insulated for the highest occurring voltage. For mechanical reasons, taps in the electrical middle are not feasible, if they are on a separate tapping barrel (See Chap. 5 for more details). In that case the taps are at the line end, as shown in Fig. 1.9d. Taps at the line end experience high voltages, particularly impulse across adjacent taps. This is due to the uneven distribution of impulse voltages, concentrating on the first few sections of the winding. Intertap impulse withstand may become a limitation for this application.

1.11 Tappings and Core Flux With the exception of the USA, the philosophy all over the world is to have taps on the primary to take of variation of the incoming voltage. If the incoming voltage goes up, more turns are included in the primary and vice versa. Taps are used to regulate in a manner that the core flux is constant. This is called the constant flux voltage variation, or CFVV. The secondary voltage remains substantially constant, except for the internal impedance drop of the transformer. It must be understood that CFVV is a discipline. Technically the same taps can be used to vary the secondary voltage, with constant primary voltage. With CFVV the core operates at its optimal flux density. This is obviously desirable, as the core is the heaviest and most expensive part of transformer. In the USA they hold the view that the whole purpose of the distribution system is to deliver the correct voltage to the consumer, and this is best done by regulating the lower voltage secondary. Taps are therefore provided on the secondary to keep its voltage constant, taking care of the primary incoming voltage variation, and the variable impedance drop within the transformer. As there is no regulation of turns on the primary to cater for variation of the incoming voltage, the core flux varies with incoming voltage. The core use is less than optimum, because the working flux density must be lowered to allow a margin against saturation at higher excursions of the incoming voltage.

12

1 Transformer and Tapchanger

1.12 Tap Numbering, Direction of Raise. Principal Ratio, Number of Different Voltages, Tapping Range The operator usually does not know the relative number of turns at each tap. To assist him in the operation of the transformer, the taps are numbered. The tapchanger has an indication of what the operating tap number is. All transformers have a diagram plate fixed on the side wall showing the tap numbers, connections of the tapped winding at this position and the corresponding voltage. With this information at hand the operators know where they stand. The diagram plate tells the operator which is the tap nr, which gives the nominal voltage ratio. This position is the principal tap. The convention is that the tap position with the maximum number of turns of the tapped winding is called tap nr 1. This is a convention, not a standard, or a rule. Taps are numbered in sequence from here, till the other end of the tapped winding. When tapchanging, going towards a higher tap number is by convention called the “raise” direction. Even though the number of operating position and the voltage range covered by tappings are not standardized, the most usual tapping range covers 20% in 16 steps. The 20% could be ±10% of the principal tap or from ±5 to ∓15%, or any other limits, of the nominal voltage. In the USA the most usual tapping range is similar but covered in 32 steps, or 33 different voltages. Most leading tapchanger manufacturers offer tapchangers which can give up to 33 different voltages, as part of their standard production range. Special design tapchangers with 101 operating positions have been delivered by OLG to Andrew Yule and Co Limited. No doubt other manufacturers have the capacity to match this if the demand arises.

1.13 Variable Secondary Voltage Applications There are applications in which we may need a variable voltage output from a transformer, which is the fed by a reasonably constant Incoming supply. This requirement often arises in the case of process control application, electrolysis, and furnace. We can meet this requirement also by tapping the primary, with constant turns on the secondary. This method is called varying the secondary by taps on the primary. As the secondary coil is of fixed turns, we can vary its voltage only by changing the core flux density. This method of applying tappings is thus justifiably called “varying flux voltage variation (VFVV)”. The maximum core flux density will be reached at a tap position corresponding to the maximum secondary voltage. The transformer designer takes care that the transformer core does not saturate here. At all other taps, the core is underused, because the flux density is lower. This is in some ways wasteful of the core, which in power transformers are often the biggest, heaviest, and most expensive component of the transformer. This is a sacrifice at the altar of better control.

1.14 Reactor and Resistance Switching Tapchangers

13

1.14 Reactor and Resistance Switching Tapchangers On-load tapchangers which use a reactor to limit the current are naturally called reactor tapchanger. Chapter 9 discusses reactor tapchanging in detail. Reactor tapchangers are popular in the USA and areas of the world where the US technology is followed. When a resistance is used to limit the current, the tapchanger will, of course, be called resistance tapchanger. Resistance tapchanger is almost de rigour in the rest of the world. This whole book deals with various aspects of resistance tapchangers.

Chapter 2

Resistance Tapchangers

God said ‘Let Dr. Jansen be’, and there was Resistance. Modified from Pope’s Epitaph on Newton

2.1 Chapter Content In this book, tapchangers in which the current limiting element is a resistance is taken as the default tapchanger. There is however a specific chapter on Reactance limited Tapchanger (Chap. 9), as these are also an important class of Tapchangers which are used widely, particularly in the USA. It is useful to have a brief introduction to resistance tapchangers, and why this is the preferred technology in most parts of the world except the USA.

2.2 Advantages of Reactor as a Current Limiting Element It is possible to imagine that the earliest tapchangers incorporated a reactor as most suited for current limiting function. Reactors are used very often in this capacity, as high ohmic values can be built, without too much loss. Besides if a resistance was used in this function, it will get hot and cooling may be a difficult problem.

2.2.1 Disadvantages of Reactor The reactor as a current limiting element has a major disadvantage. As we saw in Chap. 1, currents through the element have to be interrupted during the stages of tapchange. In a reactor, the current lags the voltage by 90°. At current zero, © Springer Nature Singapore Pte Ltd. 2020 T. V. Sridhar, Application of Tap changers to Transformers, Power Systems, https://doi.org/10.1007/978-981-15-3955-8_2

15

16

2 Resistance Tapchangers

when a circuit can be interrupted, the recovery voltage is at the peak, contributing to difficult circuit breaking conditions. Interruption of reactor currents frequently involves multiple restrikes, longer arcs, much oil burning, and more contact wear. The early on-load tapchangers with reactor current limiting had a poor contact life and severe oil deterioration due to heavy arcing at the interrupting contacts. Modern reactor tapchangers are discussed in Chap. 9.

2.3 Early Resistance Tapchangers Resistance was used as current limiting element in some early tapchangers. They did not incorporate some of the key features of the modern resistance tapchangers, which are discussed shortly. The resistances were made large and worked at low current density to limit temperature rise. Typical commercial executions housed the resistance in a tank with fans, on top of the transformer for better cooling. Bushings were provided on both the transformer top and the resistance box for interconnection. In another commercial execution, Tapchanger Type EH of Fuller Electric Company, a tapchanger of 66 kV class manufactured in the 1950s, huge resistance mats served as current limiting resistances. The typical size of the mat was 2000 × 800 × 60, supported by insulators with a supporting metallic frame. The resistance also had a temperature sensor mounted on top, but separated from the resistance body for insulation. If the resistance became too hot, the temperature sensor tripped the transformer. Unfortunately photographs of these early executions are not available anymore.

2.4 Dr. Jansen and His Patent Currents through resistances are interrupted more easily, with shorter arc lengths, and relatively less contact erosion. Tapchangers become more compact, with less contact maintenance, and lower oil deterioration. For these reasons, today in most parts of the world outside the USA, resistance tapchangers are preferentially used. But it took a further step of tapchanger evolution to reduce the size of the resistances. It took the genius of Dr. Jansen to introduce the concept of high-speed resistance tapchanging. In a historic patent in 1939 (Fig. 2.1), Dr. Jansen set out the principle of resistance switching.

2.4.1 Why High Speed? The problem of resistance heating was resolved by the concept of high-speed transition. The idea was to perform a tapchange very quickly, in the order of milli seconds, so that the resistance is only exposed to a short current pulse. This limits the heating.

2.4 Dr. Jansen and His Patent

17

Fig. 2.1 Dr. Jansen’s Patentschrift

At such short times, a modern composition of resistance material can be worked at 60–70 A/mm2 . The resistance is then physically very small, compared to a reactor, with its magnetic core, and large coils. The whole tapchanger becomes smaller. In fact when rector switching is used, the reactors are not a part of the tapchanger. The transformer manufacturer built them, as part of his transformer. The tapchanger simply uses the reactor as an external current limiting device.

18

2 Resistance Tapchangers

2.4.2 Internal Stored Energy Device With high-speed resistance switching, it is imperative that if a tapchange commences, there should be no stop, till it is completed, and the resistance is safely out of the current path. It was realized that an external motive device, like a motor, could not be relied on for this. It could fail without completing the tapchange, leaving the resistance trapped in the current path for too long. The resistance would overheat and will be damaged. The solution was to use an internal energy storage device to drive the contacts. The external motive power was used to charge the internal energy storage device. Actual tapchanging was allowed to start only when the energy storage device was fully charged. If the external motive power failed, during the charging process, it did not matter, as actual tapchange would not start till the energy storage was completely charged. Once the switching components start moving, with the resistance in the current path, the tapchange is completed quickly, driven fully by the stored energy at the desired speed.

2.5 Main Features The main features of the modern high-speed resistance switching tapchanger enunciated by Dr. Jansen are 1. Resistance as the current limiting element 2. High-speed switching 3. Switch drive by internal stored energy. In Chap. 3 we shall see some executions of resistances tapchangers. These always incorporate an internal energy storage device and feature high speed of transition.

2.6 Disadvantages of Resistance Switching Tens of thousands of resistance tapchangers, possibly a total of 100,000 units per year, are manufactured today at various distributed locations of the world. There are possibly many hundred thousand tapchangers working in the field. To raise objections at this technology is perhaps a little unfair. Yet some built-in weaknesses do exist at the conceptual level, which can paradoxically be attributed to the very features that make the resistance tapchanger such a success. It would take excellent, high quality, error free, technology, and engineering to make a reliable resistance tapchanger. Some frequent modes of failures of resistance tapchangers are discussed in Chap. 11.

2.7 Reactor as Current Limiting Device

19

2.7 Reactor as Current Limiting Device There is an entire class of tapchangers, which do not use resistance as the current limiting element. They use a reactor instead. Most of the discussions in this book relate to resistance tapchangers. This is not always specifically mentioned each time, but is assumed. Reactor tapchangers are discussed separately in detail in Chap. 9.

2.8 How High a Speed Is “High Speed” There is a widespread incorrect impression, in which the manufacturers also have taken a hand in spreading, that a very high speed of tapchange will result in short arcs. This is completely incorrect. When the contacts part, there is arcing. The arc continues till the next current zero. At that point the current tries to go out, but the recovery voltage between the contacts tries to re-ignite the gap. If the contact separation is sufficiently large, there is the possibility that the gap will not re-strike and the current goes out. If the contact parting is just at current zero, the contact gap is too small, and the current will arc through for the next current zero. At that point, there is a large contact gap, provided that the speed is not too low. The large gap withstands the recovery voltage at current zero, and the arc goes out. This would be the situation even when the contacts part a little after current zero. The arc lasts for more than half a cycle. We may anticipate that if the contacts part after 4 ms after current zero, the arc may not go out at the first current zero because there is not enough contact separation. The arc burns for nearly 14 ms. When the contacts part just before current zero, the gap is again too small at the first current zero for the arc to go out. We may expect that at a contact parting at about 4 ms before current zero, the contacts may have a reasonable gap at current zero to go out. If the gap is not enough, the arc will burn for about 14 ms before the next current zero. In the analysis of arc currents in switching tests it is found that there is an occasional arcing time of about 16 ms. The speed of tapchange must be reasonably large to produce a respectable contact gap at the propitious next current zero, but a higher speed than that only results in a longer arc. A very high speed of contact parting may result in the arc being dragged between successive contacts and cause intertap shorting, as shown in Fig. 1.6.

2.9 Why Is a Tapchanger Not so Rugged as a Personal Car? Some surprise is often expressed that tapchangers seem delicate and prone to failure, in spite of being mollycoddled by well-trained and experienced operators, whereas other mechanical devices like the personal automobile account for themselves better. The question posed is why cannot tapchangers be made like automobiles, with a

20

2 Resistance Tapchangers

high degree of reliability, low maintenance, and capability of operation by relatively untrained and often uncaring operators. The following discussion, though applicable generally to all tapchangers, is specific to high-speed resistance tapchangers immersed in transformer fluid, particularly mineral oil. There are four basic reasons why tapchangers present a different picture to the seemingly rugged personal autos and domestic gadgets. 1. The most usual cause for failure of the high-speed resistance class of tapchangers is mechanical. The time interval for tap transition must be low. The contacts are stationary to start with. They have to be accelerated to a high speed at the start of the tapchange and braked to a halt when the tapchange is complete. To achieve the average speed required, in the size and geometry of a tapchanger calls for accelerations of the moving masses by a few “gs” (g is the acceleration due to Earth’s gravity, approx. 9.8 m/s2 ). Expressed in another way, this means that the components “weigh” several times more than their normal weight, when in transition between contacts. To get the tapchange off from standstill requires a big hammer blow and at the end of a tapchange, there is a crash at a head long speed to come to a halt. This is hardly a kind way to treat a mechanical system. We encounter a similar application in breakers, but as against a life of a few thousand operations of a breaker, most tapchanger manufacturers routinely offer more than half a million operations. This is far in excess of the demand of the Standard IEC 60 214. 2. The mechanical system cannot be an integral structural unity because the structure must necessarily be broken up by the need for electrical insulation, by relatively weak insulated components. The weak electrical components themselves are a liability in the mechanical structure, with sudden starts and stops, involving high “gs”. The many joints and couplings between the strong metal components and the weak insulation confer a degree of weakness on the entire construction. 3. The tapchanger has to face high dielectric stresses, as well as the effects of arcing. The arcing deteriorates the atmosphere in which the tapchanger is immersed, reducing dielectric, thermal, and mechanical properties. This calls for more frequent maintenance than other mechanical equipment. 4. The tapchanger is immersed in transformer oil and depends on it for insulation, cooling, and lubrication. The surroundings are controlled by the transformer, for instance, the temperature, pollution by arc products, and degradation products of the transformer itself. The working atmosphere of a tapchanger is indeed more strenuous than in other seemingly stressful mechanical devices.

Chapter 3

Switching in Resistance Tapchangers

Ordnung muß sein. German Proverb

3.1 Introduction Tapchangers are now made by several manufacturers around the world. Some designs are new, but some are decades old. Materials, designs, and physical arrangements differ. It is not possible, nor is it necessary to include in a book of this kind every execution. For the purpose of this chapter we consider two main classes: selector switch type and diverter switch type. In a selector switch, taps are selected by a tap selector and the very action of selection causes current to be switched on. In the diverter switch type of tapchanger, the tap selector only selects the tap, without switching the current on. Switching current on and off is done by a diverter switch. This chapter examines the interruption duties of contacts of both the selector switch type and diverter switch type tapchangers, employing a resistance as the current limiting element. The chapter mainly elucidates switching functions of tapchangers described in Annexure 1 to IEC 60 214 [1] and takes the excellent analysis of [2] a stage further.

3.2 Functional Description of a Compartment Type Selector Switch Tapchanger with Single Resistance The above is a simple construction used in relatively small rated tapchangers. This execution is described below in some details to introduce some common tapchanger concepts. Figure 3.1 shows the basic structure of a tapchanger with a selector switch.

© Springer Nature Singapore Pte Ltd. 2020 T. V. Sridhar, Application of Tap changers to Transformers, Power Systems, https://doi.org/10.1007/978-981-15-3955-8_3

21

22

3 Switching in Resistance Tapchangers

Fig. 3.1 Schematic arrangement of single resistance selector switch

The basic selector switch is built on a board made of insulating material, in which a number of contacts are distributed evenly around the periphery of a circle. The tap leads from the transformer winding are connected to the fixed contacts in sequence. The insulating board provides isolation between contacts. Figure 3.1 shows a rotatable central shaft, which carries two moving contacts, marked S and T, following the notation of IEC 60 214 Table A.1. In Fig. 3.1 the tapchanger is shown in an operating position. In the operating position, one of the two moving contacts, called the main contact, connects a fixed tap directly to the output of the tapchanger. The output is usually in the form of a slip ring, with a sliding or rotating output contact which is always connected to the slip ring, even during a transition from one tap to the other. IEC 60 214 designates the main contact by the symbol T. The moving contact T can be positioned against any fixed contact by rotating the shaft. The second moving contact S connects to the same output slip ring through a resistance. Except for this connection at the slip ring, the two moving contacts are isolated to withstand at least

3.2 Functional Description of a Compartment Type Selector …

(a)

(e)

(c)

(b)

(f)

23

(d)

(g)

Fig. 3.2 a–g Page 1: Interruption duties of single resistance selector switch. h–m Page 2: Interruption duties of single resistance selector switch

one tap voltage. The S contact need not necessarily be in touch with the fixed contact at operating positions. In some designs it may do, but even then it carries very little current due to the resistance connected in series with it. The function of the S contact and the resistance will be clear, when we consider a transition from one contact to the next, as shown in Fig. 3.2.

3.3 Transition from One Tap to the Next in the Single Resistance Tapchanger The following description is based on Annexure A, Table A.1 of IEC 60 214 [1]. Since the purpose of Fig. 3.2 is to study the interruption duties during the transition between two contacts, it shows only two fixed contacts, and the movements of contacts are shown linear and not rotational. This representation adds to clarity. The step voltage vector is shown double headed, as there is no restriction regarding its direction. In later discussion in this chapter, the ambiguity of the direction of the voltage vector will be resolved. The current vector is shown by a green arrow. Figure 3.2a shows the tapchanger in operating position against the fixed contact 1. The contact S is in physical touch with the fixed contact 1 at the starting position. No current

24

3 Switching in Resistance Tapchangers

(m)

(k)

(j)

(i)

(l)

(h)

Fig. 3.2 (continued)

flows through the contact S because of the transition resistance in series with it. A transition takes place, in the direction left to right in Fig. 3.2; the first stage is that the contact S makes contact with the fixed contact 1 (Fig. 3.2b). This stage is trivial, and obviously not needed, in designs where the S contact is already in touch with the fixed contact. Note that during this stage, the S contact still carries no current. In the second stage (Fig. 3.2c) the main contact T rolls off the edge of the fixed contact. The current through it, which is the through current of the transformer, cannot however be interrupted till the next current zero. It continues through the contact T in the form of an arc formed between the fixed contact and the moving contact. When the current is interrupted at a current zero (Fig. 3.2d), the transformer current seamlessly transfers to the contact S. The voltage drop RI appears as a recovery voltage across the contact gap. The recovery voltage tries to break down the gap between contact 1 and contact T and reignite the current. If there is no breakdown of the gap, the arc is successfully extinguished. We can say that the interrupting duty of the contact T is current I, at a recovery voltage of RI. The T contact travels the rest of the gap between fixed contacts 1 and 2 with no current flowing through it. The geometry of the contacts, the width of the fixed contact, the pitching of the fixed contacts, and the spacing of the moving contacts are such that while the T contact is between the fixed contacts, the S contact continues to stay in touch with the fixed contact 1, carrying the current. In the third stage (Fig. 3.2e), the contact T makes with the next fixed contact. When T makes with fixed contact 2, the through current resumes its path

3.3 Transition from One Tap to the Next in the Single …

25

through it. However, as the two successive fixed contacts are bridged by the moving contacts, a circulating current I c ensues, under the influence of the voltage of the tap connected between them (Fig. 3.2e). The circulating current is limited by the resistance, I c = E/R. In the fourth stage (Fig. 3.2f), the contact S rolls off the fixed contact, interrupting the circulating current. There is an arc. At current zero, the arc goes out, and the step voltage E appears across the contact gap, trying to break it down. This is the recovery voltage corresponding to this interruption. We can say the interrupting duty of contact S is current E/R at a recovery voltage of E. The last stage (Fig. 3.2g) is trivial, in that the contacts assume their operating positions at the next fixed contact, with no further switching event. The interrupting duties are marked up in Fig. 3.2.

3.4 Switching in the Reverse Direction The switching sequence in the single resistance tapchanger is unsymmetrical in the two directions. The reverse switching will therefore be described in detail. Figure 3.2h represents the contact positions at the end of transit from fixed contact 1 to 2. In the reverse direction, we must end up in a situation as shown in Fig. 3.2a. For this, the S contact moves to the left from Fig. 3.2h. In cases where the S contact is in touch with the fixed contact, it rolls off the edge of the fixed contact 2. As the S contact carries no current, there is no arc. The S contact makes with the fixed contact 1 after transiting the gap between the fixed contacts (Fig. 3.2i). The two successive fixed contacts, which have a step voltage between them, are bridged. A circulating current I c = E/R ensues. The T contact carries the through current from fixed contact 2, as well as the circulating current. The total current is I ± I c vectorially. The significance of the sign ± will be apparent in the next section, where we consider the direction of power flow through the transformer. In the next stage of transition, the T contact rolls off the edge of the fixed contact 2, forming an arc (Fig. 3.2j). At the next current zero, the current I ± I c is interrupted (Fig. 3.2k). The through current flows through the transition resistance and the contact S. A recovery voltage of E ± IR is established trying to breakdown the gap at T. If there is no restrike, the arc is successfully extinguished. In the next stage, the T contact reaches the fixed contact 1 (Fig. 3.2l). The through current now flows through the T contact. The contact set moves a little further to the left to assume the final operating position against fixed contact 1 (Fig. 3.2m). This completes the transition in the reverse direction. Table 3.1, extracted from Fig. 3.2, shows the current interrupted and the recovery voltage at all stages of transition. The last column needs an explanation. All possible switchings are performed in a double transition from 1 to 2 and back. During these two tapchanges, the contacts perform the interruption duties shown in the previous column once. Therefore in N tapchanges, they will perform the switchings N/2 times. This is indicated in the last column.

26

3 Switching in Resistance Tapchangers

Table 3.1 Interruption duties of a single resistance selector switch, ref. Fig. 3.2 Direction

Ref. Fig.

Left to right

Contact

Interrupted current

Recovery voltage

No. of interruptions

T

I

RI

N/2

S

E/R

E

N/2

S

–

–

N/2

T

E/R ± I

E ± RI

N/2

3.2b 3.2e Right to left 3.2i 3.2k

3.5 Functions of Contact S 1. It serves to carry the through current, when the main contact T is not in a position to do so. 2. It switches the resistance into the circuit and out of it. Note how the concepts of introducing a current limiting element when required and taking it off again, discussed in Fig. 1.5 of Chap. 1, are practically implemented. 3. As the contact S carries current only during the short period of transition, it is correctly called the transition contact. Likewise the resistance also carries current for a short while during transition and is referred to as transition resistance.

3.6 Contact Interruption Duties Though the through current is never interrupted, there are internal switchings among the paths of the tapchanger. These switchings result in interruption of current through the contacts which do the switching. This means arcing. The current switching must be completed in the time available, as the contacts are moving, to constitute a successful tapchange. These current interruptions are followed by a recovery voltage, when the contacts are open. The recovery voltage tries to cause a breakdown in the contact gap, and a restrike of current. The switched current, the recovery voltage, and the number of times a contact performs the switching function constitute the switching duty of the tapchanger.

3.7 Operating Cycle The whole of the activities described during the transition constitute the “operating cycle” of the tapchanger. What has been described is a simple operating cycle. In a

3.7 Operating Cycle

27

more complex tapchanger there may be more contacts and transition resistances. The operating cycle describes the orderly progression of the moving contacts between the fixed contacts, in either direction. The duties depend on the order of making and breaking of the contacts, the direction of movement, and may also depend on the direction of current vector in relation to the step voltage vector. This phase relationship defines the direction of power flow through the transformer. Every operating cycle shares two common features that (1) there is always a path for the through current and (2) adjacent taps are never shorted without an intervening current limiting resistance. We shall study other operating cycles with more contacts further down this chapter.

3.8 Direction of Power Flow and Contact Duties of a Single Resistance Selector Switch It is seen from Table 3.1 that the contact duties of the single resistance selector switch involve expressions like (E ± RI)/R (for the current) and E ± RI (for the recovery voltage). We have to recall that E and I are vectors. Figure 3.3 shows some more of the transformer winding and the direction of flow of the current through the rest of the winding. Taps and tapchanger connections are as shown in Fig. 3.2. In Fig. 3.3a the transformer and tap connections are shown. The terminal voltage vector and the current vector are pointing in the opposite directions. The transformer is therefore receiving power and pumping it out of the secondary. In Fig. 3.3b the voltage vector alone is reversed. This transformer is sending power out of the winding which carries the taps. Figure 3.3c, d correspond to Fig. 3.3a and show the duties of the T contact in the bridging position, when it breaks from fixed contact 2, towards fixed contact 1. The interrupted current is (E − RI)/R and the recovery voltage is E − RI. To emphasize what follows, we can take a special case, with power factor of unity, and choose a value of transition resistance such that E = RI in magnitude. Then the interrupted current and the recovery voltage are both 0. In the more general case, the interrupted current is [(E − RI cos ø) − j RI sin ø]/R and the restrike voltage is (E − RI cos ø) − j RI sin ø. This is an algebraic expression, and not vectorial. In Fig. 3.3b the same transformer, with the same connections of tapchanger, has its power flow reversed. This is represented by the reversal of the red arrow representing the voltage. The transformer is now pumping power into the primary system. It is seen that the currents I and I c add (Fig. 3.3e) and the step voltage E adds to the RI drop (see Fig. 3.3f). The current interrupted and the recovery voltage becomes (E + RI)/R and E + RI, respectively. Let us once again take the special case of unity power factor and E = RI in magnitude. The interrupted current becomes 2I and the restrike voltage becomes 2E. We notice the emphatic difference brought about by the direction of power flow. In the more general case, the interrupted current is [(E + RI cos ø) − j RI sin ø]/R and the restrike voltage is (E + RI cos ø) − j RI sin ø. While the transformer Fig. 3.3a enjoys a light duty (near zero in fact!), the duty

28

3 Switching in Resistance Tapchangers

(a)

(b)

(c)

(d)

(e)

(f)

a

b

Fig. 3.3 Single resistance selector switch direction of power flow

of the transformer Fig. 3.3b is heavy. For this reason, the single resistance selector switch is not suitable for full reverse power flow.

3.9 Reversing the Connections of S and T Contacts to Reverse Direction of Power Flow Figure 3.3 shows the correct connections to minimize contact duty for the direction of power flow in Fig. 3.3a. If the transition resistance was to be interchanged between the S and T contacts, the switching would be heavy. If a tapchanger is required for the power flow in Fig. 3.3b, the S and T contacts should be interchanged.

3.10 Limited Reverse Power Flow Possible

29

3.10 Limited Reverse Power Flow Possible It is not that a tapchanger as described above cannot be used at all if there is a possibility of reverse power flow. The maximum interruption duty under reverse power direction occurs at the T contact at the bridging condition. In this condition, if the interrupted current and recovery voltage are still within the permissible values for the tapchanger, due to a reduction in the current or the step voltage of the transformer, the tapchanger can of course still be used.

3.11 Position of the T and S Contacts Relative to the Fixed It is not necessary that the contact S should be in touch with the fixed contact in the operating position. Even if it touches the fixed contact it carries no current. It is an advantage to position the T contact at near the middle of the width of the fixed contact. This locates the T contact, which carries the current of the transformer in every operating position, in a region of the fixed contact which is clean of any arcing damage. The latter happens mostly at the edges of the fixed contacts. The S contact then takes any position, including that which leaves it “hanging” outside the width of the contact. However moving the S contact too much from the symmetrical position affects the clearance to the next fixed contact. Therefore the ideal position is to place the two contacts symmetrically about the centre line of the fixed contact.

3.12 Approximate Magnitude of Transition Resistance The contact duty of the T contact in the non-bridging condition consists of an interrupted current of I at a recovery voltage of RI. If the transition resistance is made small, the recovery voltage will be low, and the switching duty is also low. The duty of the S contact, which is an interrupted current of E/R at a recovery voltage of E, is always reduced with high transition resistance value. Because of this consideration the transition resistance can be neither too high, nor too small. A value of R = E/I, to 0.6E/I works as good compromise. When it is known that the transformer mostly operates at low loads in relation to the tapchanger rating, the use of a high transition resistance lowers the duty of the S contact, while not greatly increasing those of the T contact.

30

3 Switching in Resistance Tapchangers b

(a)

d

(b)

(c)

Fig. 3.4 The pennant trajectory

3.13 Pennant Cycle The operating cycle of the single resistance selector switch used to be called the “pennant cycle” in the older IEC. This designation derives from the locus of the tip of the output voltage during transition. Figure 3.4 shows this locus. The pennant is shown in Fig. 3.4c. The operating cycle according to the present designation is operating cycle 1 for the direction 1–2 in Fig. 3.2. In the opposite direction it is operating cycle 2. Operating cycle 1 is where the main current through is diverted to the resistance contact before the circulating current starts. In the operating cycle 2, the circulating current starts before the through current is diverted to the resistance contact. It looks as if pennant cycle was a better descriptive term.

3.14 Recovery Voltage Is Not Always the Same as the Step Voltage In the non-bridging condition, the T contact always interrupts a current of I at a recovery voltage of RI. This is not related to the step voltage. In the bridging condition the recovery voltage of the T contact is a combination of the step voltage and the transition resistance voltage drop. In its only interrupting duty the S contact has an interrupted current is E/R, at a recovery voltage of E.

3.15 Phase Relationship Between the Recovery Voltage …

31

3.15 Phase Relationship Between the Recovery Voltage and the Interrupted Current The interrupting duties are generally of the form of a voltage such as E ± RI (or RI for the T contact), and the current interrupted is the recovery voltage divided by R. Therefore the recovery voltage and the interrupted current are in phase. This phase relationship is not dependent on the load power factor. Thus at current zero, when the arc can be interrupted, the instantaneous recovery voltage is also zero. The possibility of the contact gap breaking down is reduced. This is a factor which contributes to the success of the resistance tapchanger. The interruption conditions are favourable. Even though we have so far examined only one operating cycle, this assertion is correct for all resistance tapchangers. A more general proof of this condition is found in [2].

3.16 Maximum Magnitude of Contact Duties In some interruptions during the operating cycle, interrupted currents like (E − RI)/R, with recovery voltage E − RI occur. These are vector expressions. The magnitude depends on the phase angle ø. For unity power factor, both these quantities are small. In fact both are zero, when R is selected to be E/I. The maximum occurs √ 2 when2 the power factor is zero, or ø = 90°. The maximum interrupted current is (E + RI )/R √ and the corresponding recovery voltage is (E 2 + RI 2 ). In real life, this is a trivial condition, as the transformer load is almost never at zero power factor. When the tapchanger is applied to a reactor, the load is at zero power factor. Therefore for the purpose of the type test (Chap. 6) the maximum interrupting duties will have to be proven.

3.17 A Word of Caution Regarding Actual Measurement of Interrupted Current and Recovery Voltage It is a well-known paradox in electrical switching that the interrupted current cannot be captured by any method. All currents however high are interrupted at zero. In switchgear test practice for instance, the current wave is captured oscillographically prior to interruption at the next current zero, and this is designated the current interrupted. If, due to the transient nature of the current, the captured segment of the wave form is too short, it is difficult to assign a value. In many tapchanger transition situations, the current and voltage last for a fraction of a cycle. It is difficult and may be erroneous to attribute a magnitude other than instantaneous, or phase angle to an entity of such short duration.

32

3 Switching in Resistance Tapchangers

3.18 Selector Switch with Double Transition Resistance The limitation on the direction of power flow is not always acceptable. To convert the single resistance selector switch into a version that ignores the direction of power flow, we add one more resistance as shown in Fig. 3.5. This restores symmetry. The central contact B carries the whole of the transformer current at operating positions.

(a)

(d)

(b)

(e)

(c)

(f)

Fig. 3.5 a–f Page 1: Switching duties two resistance tapchanger operating cycle 1 (earlier selector SW. Flag cycle). g–l Page 2: Switching duties two resistance tapchanger operating cycle 1 (earlier selector SW. Flag cycle)

3.18 Selector Switch with Double Transition Resistance

(l)

(i)

Fig. 3.5 (continued)

(k)

(h)

33

(j)

(g)

34

3 Switching in Resistance Tapchangers

Table 3.2 Interruption duties for double resistance tapchanger, ref. Fig. 3.5 Direction

Ref. Fig.

Left to right light switching

3.5c

Contact

Interrupted current

Recovery voltage

No. of interruptions

B

I

RI

N

A

[E/(R − I)]/2

E − RI

N/2

B

I

RI

N

C

E/(R + I)

E + RI

N/2

3.5e Right to left heavy switching

3.5i 3.5k

It is therefore designated as main contact. It will be seen shortly that the two contacts on the sides, A and C, carry current only during transition of the set of moving contacts from one tap to the other. Therefore, A and C are called transition contacts. Figure 3.5 shows the operating cycle of this tapchanger. As before, only two successive contacts are shown in a linearized form for clarity. The method of deriving the switching sequence and contact duties are similar to that described above for the single resistance selector switch. We shall therefore only draw the sequence and the duties, without describing the stages. Table 3.2 shows the contact interrupting duties for the double resistance tapchanger.

3.19 Features of Double Resistance Tap Selector Operating Cycle The following features are noteworthy. 1. There is always a path for the through current. The transformer current is never interrupted. 2. Two adjacent fixed contacts are never shorted, without two transition resistances in series to limit the circulating current. 3. The transition resistances are introduced into the circuit when required and finally removed. This meets the requirement stated in Sect. 1.5. 4. The main contact B always interrupts a current of I with a recovery voltage of RI. It does one such interruption every tapchange. Thus in N operations, there will be N operations. This is indicated in Table 3.2. 5. In Fig. 3.5 the transition contacts interrupt a current of (E − RI)/2R at a recovery voltage of E − RI, when the contacts move from left to right (Fig. 3.5d). In this direction, the transition contact C does not perform any current interruption. In the opposite direction of the movement of the contacts, the C contact interrupts a current of (E + RI)/2R, with a recovery voltage of E + RI (Fig. 3.5k). The transition contact A does no switching in this direction. Therefore in N tapchanges

3.19 Features of Double Resistance Tap Selector Operating Cycle

35

in total, which must on the average consist of N/2 operations in each direction, each transition contact does only N/2 interruption duties. This is indicated in Table 3.2. 6. The interruption function of the A contact is obviously lighter than the C contact. The A contact therefore performs a “light switching”. While the C contact does a “heavy switching”. This is marked up in Fig. 3.5. This situation is only reversed if the direction of power flow through the transformer is reversed. In this condition, the transition contacts merely interchange the heavy and light switching functions. 7. At no load, or very light loads on the transformer, the interruption duties of the transition contacts become about equal, each being approximately E/2R at a recovery voltage of E. The main contact does nil or very light switchings. There are instances where a transformer is mostly on no load or very light load. These are transformers in an industrial plant with practically 100% self generation. A transformer in such cases “floats” on an intertie with the local power supply. This is for security of power supply. In such an application, one finds that the transition contacts are well marked, and the main contact is relatively free of signs of wear, after a few thousand operations. If such an application is envisaged at the time of manufacture of the transformer, a higher than usual transition resistance value may be used to equalize all contact wears.

3.20 Algebraic Expressions for Contact Duties The expressions for the currents and voltages in Table 3.2 are in vector form. The arithmetic expressions are 1. B contact: Both directions: interrupted current I and recovery voltage RI. 2. A contact:    (E − R I cos ø)2 + R I sin ø2 Interrupted current = (3.1) 2R 3. Recovery voltage = 2R. Interrupted current. 4. C contact:  (E + R I cos ø)2 + (R I sin ø)2 Interrupted current = 2R

(3.2)

5. Recovery voltage = 2R. Interrupted current. The magnitude of the duties depends on the power factor of the current. In these expressions, the RMS values of the through current and step voltage must be entered.

36

3 Switching in Resistance Tapchangers

3.21 Maximum Interruption Duty The highest interruption duty occurs on the transition contact C at full load and unity power factor. The magnitude is (E + RI)/2R at a recovery voltage of E + RI. This is an algebraic expression, where the terms can be simply added, after substituting the RMS values of the current and step voltage. To have a “feel” of what this means, let us assume that the tapchanger manufacturer chooses a transition resistance of R = ½ E/I. The maximum interruption duty is then 1.5 I at a recovery voltage of 1.5 E. Since the heavy switching transition contact only switches in half the number of total operations, this high duty may not affect the contact wear. More important is the rupturing capacity proven by the “breaking capacity test”. We shall look at this again in Chap. 6.

3.22 Phase Relationship Between Interrupted Current and Recovery Voltage The main contact B always interrupts a current of I at a recovery voltage of RI. These two quantities are in the same phase. The transition contacts interrupt a current of (E ± RI)/2R at a recovery voltage of E ± RI. The interrupted current and the recovery voltage are in phase. This means at current zero, where an ongoing arc can be quenched, the instantaneous recovery voltage is also zero. The contact gap may be able to withstand the low instantaneous voltage, without breaking down again. This is favourable for arc extinction.

3.23 Direction of Power Flow The interruption duty of the B contact is a current of I at a recovery voltage of RI in both directions of switching. The duties are independent of the direction of power flow. We recall that the reversal of power flow means replacement of I by −I in the expressions of the duties. When we do this, we find that the duties of the A and C contacts are merely interchanged. Thus reversal of the direction of power flow does not change the overall contact duties. Therefore, the double resistance selector switch is not influenced by the direction of power flow. The tapchanger is fully bidirectional.

3.24 Flag Feature of the Double Resistance Selector Switch The operating cycle of the double resistance selector switch used to be called the “selector switch flag cycle” in the older IEC 60 214. As we saw with the single resistance selector switch, the term describes the locus of the tip of the output voltage

3.24 Flag Feature of the Double Resistance Selector Switch c

(a)

d

(b)

37

e

(c)

f

(d)

(e)

Fig. 3.6 Vector diagram selector switch flag cycle showing the flag trajectory of the output voltage

vector during the transition. This is shown in Fig. 3.6. The locus looks like a flag and hence the name. Figure 3.6 shows the flag for both directions of power flow. The present issue of IEC 60 214 calls the flag cycle “operating cycle 1”, which is perhaps less evocative than the old name.

3.25 Positioning of the Contacts Relative to the Centre Line of the Fixed Contacts There was some option regarding this for the single resistance tapchanger. The symmetry of contacts in the double resistance switch enforces the contacts to be placed centrally on the fixed contact in the double resistance switch.

3.26 Comparison of Single and Double Resistance Transition Contact Interruption Duties From Tables 3.1 and 3.2 we may compare of the interruption duties, on the basis of equal current and step voltage. The comparison is on the basis of unidirectional power flow and with the S contact on the “correct “side of the T contact. A strict comparison is not possible, as one of the tapchangers has only two contacts, while the other has three. However some interesting conclusions may be drawn. For the single resistance selector switch, one interruption duty of the main contact T is the same as

38

3 Switching in Resistance Tapchangers

the main contact of the double resistance. On reversal of the direction of switching of the selector switch, the duty of the main contact may be quite small, being (E − RI)/R. For the double resistance case both directions result in the same duty. The main contact of the double resistance selector switch is relatively more heavily loaded. For the heavy switching transition contact, C of the double resistance switch, the duty is always higher than the transition contact S. It is only at no load that the duty of C comes down to that of S contact. The light switching contact A to the interruption duties is generally higher, as the duty of the S contact in the corresponding direction is zero.

3.27 Comparison of Size and Cost It may appear at first sight that the double resistance switch, which has to accommodate three moving contacts, may occupy more space than the single resistance selector switch, which has only two moving contacts. This is not necessarily correct. It is necessary that in both cases the contact geometry allows bridging of two successive contacts during transition. This usually requires deliberate wide spacing of the moving contacts in the single resistance switch. It may be possible to arrange one more contact in between and modify the moving contact pitch and fixed contact width marginally to suit. The difference in the accommodation required is generally not large. Thus the tap selector sizes are about the same. However the double resistance tapchanger employs more material in the form of the extra moving contacts. In obedience to the business maxim that cost is a matter of fact, but price a matter of policy, the prices at which the two versions are available are not determined by the material content alone.

3.28 Tapchanger for High Power 3.28.1 Limitation of the Selector Switch A tapchanger applied in a high-power transformer has both high current and recovery voltage. A higher current, when interrupted, leads to higher formation of ionic debris in the contact gap. Taken together with the higher recovery voltage, the gap between the fixed contacts may have to be increased considerably when the power goes up. In an ideal tap selector design (as against a selector switch design), the fixed contacts will be as positioned just as far away from each other, as is necessary to meet the voltage withstand requirements (see Chap. 5). If we use this geometry in a selector switch, the gap between fixed contacts limits the switching capability. We can increase the latter by pitching the fixed contacts further apart, i.e. increase the gap to provide for the arc. But in order to maintain the requirement of bridging of two successive

3.28 Tapchanger for High Power

39

contacts, the pitching of the moving contacts must also be increased. What is gained in the swings is mostly lost in the roundabouts. To increase the effective arcing gap between fixed contacts is a game of diminishing returns. As arcing takes place at all the fixed contacts, all fixed contact gaps must be increased. The tapchanger quickly goes up in size.

3.28.2 A Concept to Reduce the Tap Selector Size Current interruption and arcing take place only in the gap between two adjacent fixed contacts at a time. In order to increase the arcing distance, it becomes necessary to increase all the intercontact gaps, even though there is no arcing everywhere. This seems a wasteful solution. This would surely set an intelligent designer to think that if only there is a method of removing the arcing from the fixed contacts, compactness can be regained. The actual current switching and therefore arcing should be removed to some other specialized “diverter switch”, with the necessary arcing gap. The multiple arcing gaps of the selector switch will be eliminated. A further advantage of this approach is that the tap selector does not need high-speed drive.

3.29 Tapchanger with Diverter Switch Figure 3.7 shows the operating scheme of a tapchanger with diverter switch. The transformer winding with tappings is also shown. The tap Fig. 3.7 selector (now only a tap selector, not a selector switch) is split into two. For ease of presentation and clarity of understanding, the tap selectors are shown as linear. There are two tap selector moving contacts, each sliding on its own slip ring. The tap selector on the left is mechanically so arranged that it can only connect to the odd-numbered taps. The tap selector on the right can only connect to the even-numbered taps. Figure 3.7a shows the tapchanger in the operating position 3. Taps 2 and 3 are connected to the slip rings. The two slip rings are connected to the diverter switch as shown. The diverter switch has a moving contact which can rotate about the point J. Four fixed contacts, named W, X, Y, and Z, are arranged in a circle about J as centre. A transition resistance R1 is connected between W and X. Another transition resistance R1 is connected between Z and Y. The diverter rotating moving contact can assume two end positions, one in which W and X are shorted, and another in which Z and y are shorted. In Fig. 3.7a the moving contact is on W and X. The connection W provides the completion of current path from tap 3 to the output. The path from tap 2 is broken at the diverter switch. No current flows through selected contact 2. The tapchanger is now operating at contact 3. The current path is shown by a thick line. As the diverter moving contact rotates, it makes contact in sequence with every adjacent fixed contact. The diverter moving contact can either be on one fixed contact, or bridge two adjacent contacts in sequence. The diverter switch moving

40

3 Switching in Resistance Tapchangers

(b)

(a)

(d)

(c)

(e)

Fig. 3.7 Operating schematic diverter switch

(f)

3.29 Tapchanger with Diverter Switch

41

contact is shown sitting on W and X in Fig. 3.7a Current flows only through the main contact W. Tap 3 is the current input point to the tapchanger and J is the output. If it is now desired to change tap to 4, the selector moving contact now at 2 moves off the fixed contact to reach 4 (Fig. 3.7b). Please note that this is an off current operation. As no transition resistance is inserted, nor any current interrupted, this motion can be slow. After the selector moving contact is firmly on selector fixed contact 4 and stops moving, the diverter switch starts to switch. Figure 3.7c shows the contact at W broken and the current flows through the contact X alone and the transition resistance R1. There will be at arc at W. Figure 3.7d shows the two slip rings bridged through X and Y. The through current splits through the paths X and Y to J. There is also a circulating current driven by the tap voltage and limited by the two transition resistances effectively in series. In Fig. 3.7e the contact at X is shown broken. There will be an arc at X. The through current flows through the contact Y alone, through the transition resistance R1. In Fig. 3.7f the main contact Z is bridged, and the resistance R1 is bypassed, and current flows only through Z. Figure 3.7f is the final position, now operating at tap 4. Four principle features of the transition are worth noting: 1. There is always a path for the output current to J, and that the through current is thus never interrupted. 2. In the position where adjacent taps are bridged (Fig. 3.11d), there are two transition resistances in series in the local loop to limit the circulating current. 3. Transition resistances are inserted into the current path as required and finally removed. Recall the discussion in Chap. 1, in Fig. 1.5. 4. There are two arcing situations at the contacts of the diverter switch. Therefore the oil of the tapchanger must be isolated from the oil of the transformer. If it is now desired to make a further tapchange in the same direction, the sequence described above repeats, except now it starts with the selector moving contact 3 moving to 5, in an off current manner. If, on the other hand, it is desired to revert to tap 3, please note that the selector moving contacts are already in their correct position, and that they need not move. Only the diverter switch moves, completing the tapchange in the reverse direction.

3.30 Two Features of the Tap Selector Drive Are Noted 1. When proceeding in the same direction of tapchange, the selector moving contacts move alternately. 2. When the direction of tapchange is reversed, no selector contact moves. This is mechanically achieved by the insertion of a “lost motion coupling” in the selector drive.

42

3 Switching in Resistance Tapchangers

3.31 Comparison of Selector Switch and Diverter Switch Tapchangers 1. The diverter switch eliminates the need for arcing distances in the selector and therefore enables a tighter packing of the fixed contacts. This reduces the tap selector dimension. 2. The tap selector no longer needs a high-speed drive. 3. In a selector switch, all the taps sit in oil contaminates by the operations of the switch. The voltage across the tapping range (see Chap. 5) can be quite high. This high voltage must be withstood under unfavourable oil conditions. The clearances must be higher than otherwise. 4. Features aimed at better current breaking can be incorporated in the diverter switch. For example, it may be possible to provide arc chutes in the specialized diverter switch to confine the path of the arc, so that it remains restricted to the space between the contacts and not spread everywhere, for instance to the conducting carriers of the contacts, and other metallic parts in the vicinity. The MR type M uses arc chutes on the diverter fixed contacts to confine the arc between the fixed and the radially retreating moving contacts. 5. It may be possible to employ concepts like double break. MR type F tapchanger had a double break diverter switch. 6. The diverter switch may be implemented in devices like vacuum interrupters, with high rupturing capacity. 7. The conclusion is that for high power, the selector switch must be replaced by a tap selector and a diverter switch. 8. As against these advantages, it may be mentioned that the selector switch is simpler. One switch performs all that is necessary to complete a tapchange. The function of tap selection and switching on the current is simultaneous. In the diverter switch concept, it is necessary to synchronize the movements of the tap selector and diverter switch. Loss of synchronism leads to failure.

3.32 Switching Duties of Diverter Switch Contacts Figure 3.8 shows two successive operations in the same direction and two in reverse. These cover all switchings. Part of the tap selector is also shown, as this plays a role in the diverter switching duties. Figure 3.8 shows three adjacent fixed contacts 1, 2, and 3, between which the switching is to take place. Table 3.3, derived from Fig. 3.8, summarizes the interruption duties.

3.33 Features of Diverter Flag Cycle Contact Switching Duties

(a)

(d)

43

(b)

(c)

(e)

(f)

Fig. 3.8 a–f Page 1: Interruption duties of diverter switch. g–n Page 2: Interruption duties of diverter switch. o–u Page 3: Interruption duties of diverter switch. v–ac Page 4: Interruption duties of diverter switch

3.33 Features of Diverter Flag Cycle Contact Switching Duties The main contacts W and Z always interrupt the through current I at a recovery voltage of RI. This interruption duty occurs only once in two tapchanges for each of these contacts. Thus in a total of N tapchange operations each contact does N/2 switchings. This is indicated in Table 3.3. With regard to the duties of the transition contacts, the following observations can be made: 1. Interrupted current (E + RI)/2R, at a recovery voltage of E + RI in one tapchange in one direction (Contact X in Fig. 3.8d). This is a heavy-duty interruption.

44

3 Switching in Resistance Tapchangers

(n)

(j)

(m)

(i)

(l)

(h)

(k)

(g)

Fig. 3.8 (continued)

2. During this tapchange, the other transition contact Y does not suffer any interruption. 3. In the next tapchange, in the same direction, the contact which did the heavy interruption in the first operation (X) takes a rest with no interruption duty. 4. But the other transition contact Y does a light-duty interruption of (E − RI)/2R, at a recovery voltage of E − RI. 5. Summarizing, one transition contact does one heavy switching and the other one light switching in two successive tapchanges in the same direction. They each have one zero switching duty during such a double transition. 6. In the reverse direction, the duties of the transition contacts are interchanged. The one that does the heavy duty now does the light duty and vice versa. Once again each contact has a zero interruption duty tapchange. 7. In N operations, for half the number, i.e. N/2 operations each transition contact does one heavy and one light duty, and two zero interruptions. The number of

3.33 Features of Diverter Flag Cycle Contact Switching Duties

(o)

(s)

(p)

(r)

(q)

(t)

45

(u)

Fig. 3.8 (continued)

heavy-duty operations performed is N/4, and the number of light-duty operations is also N/4. The other N/2 operations are with no interrupting duty. This is shown in Table 3.3.

3.34 Magnitudes of Interrupted Current and Recovery Voltage The expressions for the contact duties shown in Table 3.3 are vector equations. The interrupted current and recovery voltage of the main contacts W and Z are numerically I and RI, where I is the through current of the tapchanger. For the transition contacts, the interrupted currents alternate between (E + RI)/2R and (E − RI)/2R. The recovery

46

3 Switching in Resistance Tapchangers

(ac)

(ab)

(x)

(y)

(aa)

(w)

(z)

(v)

Fig. 3.8 (continued)

voltages alternate between (E + RI) and (E − RI). The arithmetic expressions for the calculations of magnitudes are Heavy-duty switching: current: 

(E + R I cos ø)2 + R I sin ø2 2R

(3.3)

Recovery voltage: current interrupted × 2R and Light-duty switch: current: 

(E − R I cos ø)2 + R I sin ø2 2R

Recovery voltage = current interrupted × 2R.

(3.4)

3.34 Magnitudes of Interrupted Current and Recovery Voltage

47

Table 3.3 Interruption duties of double resistance diverter switch, ref. Fig. 3.8 Direction

Ref. Fig.

Left to right heavy switching

3.8b

Contact

Interrupted current

Recovery voltage

No. of interruptions

W

I

RI

N/2

X

(E/R + I)/2

E + RI

N/4

Z

I

RI

N/2

Y

(E/R + I)/2

E + RI

N/4

W

I

RI

N/2

X

(E/R − I)/2

E − RI

N/4

Z

I

RI

N/2

Y

(E/R − I)/2

E − RI

N/4

3.8e Right to left light switching

3.8i 3.8l

Left to right light switching

3.8p 3.8s

Right to left heavy switching

3.8x 3.8aa

The magnitudes are derived by entering the values of the through current and step voltage. The magnitudes are dependent on the power factor.

3.35 Highest Magnitude of Interruption Duties This occurs in the heavy-duty switching, when the current is at unity power factor. The magnitude of the maximum interrupted current is (E + RI)/2R. The maximum recovery voltage is E + RI. The values of the current and step voltage must be substituted in these expressions.

3.36 The Importance of Being Able to Calculate the Actual and Maximum Interruption Duties The ability of a tapchanger to successfully perform the duties imposed by a through current and the relevant step voltage is proven by the switching tests of Sect. 5.2.3 of IEC 60 214. The ability to quench the arcs created during the transition of a tapchange depends on the magnitude of the interrupted current and the recovery voltage, as well the phase relationship between them. The through current is not always the interrupted current. The recovery voltage corresponding to a current interruption is not always the step voltage, even though the step voltage has a dominant contribution.

48

3 Switching in Resistance Tapchangers

In practice, there are combinations of current and step voltage. It is not possible to perform switching tests at all possible combinations of the two. Calculations relating the current and the step voltage to the interrupted current and recovery voltage would be helpful to provide a guidance regarding the suitability of a tapchanger to an application. When a successful switching test at a through current and relevant step voltage is completed, it is possible to generate the switching ability of the tapchanger in terms of the interrupted current and recovery voltage. If a different combination of current and step voltage is required, it is possible to calculate the interrupted currents and corresponding recovery voltages that would arise in the new requirement. If these are below the values for which a switching test has been successfully conducted, the tapchanger is capable of meeting the application. This is why relating the current and the step voltage to the interruption duties is important.

3.37 Direction of Power Flow Table 3.3 shows that the main contacts W and Z always interrupt I at a recovery voltage of RI. These contacts are therefore unmindful of the direction of power flow. Each transition contact interrupts a current of (E + RI)/2R (heavy duty) in N/4 operations and (E − RI)/2R (light duty) in another N/4 operations. A reversal of power flow means the current I is replaced by −I in the vector expressions for the contact duties. It is seen from the expression for the contact duties that this makes no difference. Only the light and heavy duties are interchanged. The tapchanger is therefore bidirectional.

3.38 Phase Relationship Between the Interrupted Current and Recovery Voltage From Table 3.3 it can be seen that each recovery voltage is equal to the interrupted current times R, or in some cases 2R. In either case the interrupted current and the corresponding recovery voltage are in phase. At current zero, the instantaneous recovery voltage is also zero. This is a favourable condition for arc interruption and contributes to the success of the diverter switch.

3.39 The Flag in Diverter Switching Cycle The operational sequence of the diverter switch was called the diverter switch flag cycle in the older IEC Standard. It is now called “operating cycle 1”. The older name

3.39 The Flag in Diverter Switching Cycle

49

derives from the shape of the locus of the tip of the output voltage. This locus is shown in Fig. 3.9. The flag is clearly seen.

(a)

(b)

(e)

(f)

(c)

(d)

(g)

(h)

(i)

Fig. 3.9 Vector diagram, diverter switch flag cycle showing the flag trajectory of the output voltage—tapchange 1 to 2

50

3 Switching in Resistance Tapchangers

3.40 Diverter Switch with Symmetrical Pennant Cycle Another common operating cycle for diverter switches is listed in Annexure A, Table A.1, of IEC 60 214. The transformer tapping arrangement, the arrangement of the tap selector and its movements are as with the diverter switch flag cycle. The diverter switch has a moving contact which can rotate about the contact N. Four fixed contacts J, K, L, and M are arranged in a circle about N as centre. A transition resistance R is connected between J and K. Another equal transition resistance R is connected between L and M. The diverter rotating moving contact can assume two end positions, J and M. These are tap operating positions, Fig. 3.1 shows the operating cycle of a tapchanger with symmetrical pennant cycle diverter switch, in which only either J or M carries current. These two are the main contacts. The other contacts K and L carry current only during transition and are transition contacts. In Fig. 3.10a the moving contact is on J and K. The connection N provides the completion of current path from tap 3 to the output. The path from tap 2 is broken at the diverter switch. No current flows through selected contact 2. The tapchanger is now operating at contact 3. The current path is shown by magenta dotted lines. As the diverter moving contact rotates, it makes contact in sequence with every adjacent fixed contact. The diverter moving contact can bridge two or three adjacent contacts in sequence. The diverter switch main moving contact is shown sitting on J in Fig. 3.10a. Current flows through J. Tap 3 is the current input point to the tapchanger and J is the output. If it is now desired to change tap to 4, the selector moving contact now at 2 moves off the fixed contact to reach 4 (Fig. 3.10b). Please note that this is an off current operation. As no transition resistance is inserted, nor any current interrupted, this motion can be slow. After the selector moving contact is firmly on selector fixed contact 4 and stops moving, the diverter switch starts to switch. Figure 3.10c shows the transition contact L also made, without J breaking. The result is that the tap section is bridged and a circulating current ensues driven by the tap voltage and limited by one transition resistance on the even side. In this respect, the symmetrical pennant diverter differs starkly from the flag cycle diverter where the main contact breaks before the circulating current is made. In Fig. 3.10d the main contact J breaks, with arcing. When the arc extinguishes, the true bridging position is attained with the tap section bridged by the transition resistances on the odd and even side in series. In Fig. 3.10e the main contact M on the even side makes, before the transition contact L breaks. The through current shifts to M. This again is a significant deviation from the flag cycle diverter, where the transition contact breaks first, before the final main makes. In Fig. 3.10f the transition contacts L break with arcing. Figure 3.10f is the final position, with the arc at L quenched, and the tapchanger operating at tap 4. Four principle features of the transition are worth noting: 1. There is always a path for the output current to N, and that the through current is thus never interrupted. 2. In the position where adjacent taps are bridged (Fig. 3.10c–e), there is at least one transition resistances in the local loop to limit the circulating current.

3.40 Diverter Switch with Symmetrical Pennant Cycle

51

(b)

(a)

(c)

(d)

(e)

(f)

Fig. 3.10 Operating cycle symmetrical pennant diverter switch operating cycle 2

3. Transition resistances are inserted into the current path as required and finally removed. Recall the discussion in Chap. 1, Fig. 1.5 4. There are two arcing situations at the contacts of the diverter switch. Therefore the oil of the tapchanger must be isolated from the oil of the transformer. If it is now desired to make a further tapchange in the same direction, the sequence described above repeats, except now it starts with the selector moving contact 3 moving to 5, in an off current manner. If, on the other hand, it is desired to revert to tap 3, the selector moving is already in position.

52

3 Switching in Resistance Tapchangers

3.41 Switching Duties in Symmetrical Pennant Diverter Switch Figure 3.11 shows the interruption duties of contacts in the two resistance symmetrical pennant cycle. Figure 3.11 shows two successive operations in the same direction and two in reverse. These cover all switchings. Part of the tap selector is also shown, as this plays a role in the diverter switching duties. Figure 3.11 shows three adjacent

(a)

(d)

(b) (c)

(e)

(f)

Fig. 3.11 a–f Page 1: Interruption duties of diverter switch symmetrical pennant cycle. g–m Page 2: Interruption duties of diverter switch symmetrical pennant cycle. n–s Page 3: Interruption duties of diverter switch symmetrical pennant cycle. t–z Page 4: Interruption duties of diverter switch symmetrical pennant cycle

3.41 Switching Duties in Symmetrical Pennant Diverter Switch

(i)

(h)

(g)

(j)

53

(k)

(l)

(m)

Fig. 3.11 (continued)

fixed contacts 1, 2, and 3 of the tap selector, between which the switching is to take place. Table 3.4, derived from Fig. 3.11, shows the interruption duties as they occur.

3.42 Features of Diverter Symmetrical Pennant Contact Switching Duties 1. The tapchanger works on the operating cycle 2. Circulating current is switched on before the main contact opens.

54

3 Switching in Resistance Tapchangers

(p)

(o) (n)

(q)

(r)

(s)

Fig. 3.11 (continued)

2. In the ascending tap direction, taps 1–2, 2–3 in Fig. 3.11, the main contacts J and M always interrupt a current of E + RI at a recovery voltage of (E + RI)/2. This interruption duty occurs only once in four (two ascending plus two descending) tapchanges for each of these contacts. Thus in a total of N tapchange operations each contact does N/4 switchings. This is indicated in Table 3.4. 3. In the descending tap direction, taps 3–2, 2–1 in Fig. 3.11 the main contact J and M always interrupt a current (E − RI)/2R, at a recovery voltage of (E − RI)/2. This interruption duty occurs only once in four (two ascending plus two descending) tapchanges for each of these contacts. Thus, in a total of N tapchange operations each contact does N/4 switchings. As E + RI is a larger quantity than E − RI, the ascending direction is the heavy switching direction, and the descending the light.

3.42 Features of Diverter Symmetrical Pennant Contact Switching …

(u)

55

(v)

(t)

(w)

(x)

(y)

(z)

Fig. 3.11 (continued)

4. In the ascending direction, the transition contact K interrupts a current of E/R at a recovery voltage of E. The transition contact L does not perform any switching in this direction. 5. In the descending direction, the transition contact L interrupts a current of E/R at a recovery voltage of E. The K contact does not perform any switching in this

56

3 Switching in Resistance Tapchangers

Table 3.4 Interruption duties of diverter SW symmetrical pennant cycle, ref. Fig. 3.11 Direction

Ref. Fig.

Left to right heavy switching

3.11c

Contact

Interrupted current

Recovery voltage

No. of Interruptions

J

E/R + I

(E + RI)/2

N/4

K

E/R

E

N/2

M

E/R + I

(E + RI)/2

N/4

L

E/R

E

N/2

J

E/R − I

(E − RI)/2

N/4

K

E/R

E

N/2

J

E/R − I

(E − RI)/2

N/4

K

E/R

E

N/2

3.11e Right to left heavy switching

3.11j 3.11l

Left to right light switching

3.8p 3.8r

Left to right light switching

3.8w 3.8y

Note

J, K perform switching only in direction left to right L, M perform switching only in direction right to left

direction. Therefore each transition contact does the same interruption duty of in half the number of total tapchanges. This is indicated in Table 3.4

3.43 Magnitudes of Interrupted Current and Recovery Voltage The expressions for the contact duties shown in Table 3.4 are vector equations. The arithmetic expressions for the calculations of magnitudes are  Heavy-duty switching current = 

(E + R I cos ø)2 + (R I sin ø)2 R

(E + R I cos ø)2 + (R I sin ø)2 2  (E + R I cos ø)2 − (R I sin ø)2 Light-duty switching current = R  (E + R I cos ø)2 − (R I sin ø)2 Recovery voltage = 2 Recovery voltage =

(3.5)

(3.6)

(3.7)

(3.8)

3.43 Magnitudes of Interrupted Current and Recovery Voltage

57

For the transition contacts, the interrupted current is always E/R and the recovery voltage E. The magnitudes are derived by entering the values of the through current and step voltage.

3.44 Highest Magnitude of Interruption Duties This occurs in the heavy-duty switching, when the current is at unity power factor. The magnitude of the maximum interrupted current is (E + RI)/R. The maximum recovery voltage is (E + RI)/2. The values of the current and step voltage must be substituted in these expressions.

3.45 The Importance of Being Able to Calculate the Actual and Maximum Interruption Duties 3.45.1 Direction of Power Flow Table 3.4 shows that the transition contacts K and L always interrupt I at a recovery voltage of RI. These contacts are therefore unmindful of the direction of power flow. Each main contact interrupts a current of (E + RI)/2R (heavy duty) in N/4 operations and (E − RI)/2R (light duty) in another N/4 operations. A reversal of power flow means the current I is replaced by −I in the vector expressions for the contact duties. It is seen from the expression for the contact duties that this makes no difference. Only the light and heavy duties are interchanged. The tapchanger is therefore bidirectional.

3.45.2 Phase Relationship Between the Interrupted Current and Recovery Voltage From Table 3.4 it can be seen that each recovery voltage is equal to the interrupted current times R, or in some cases 2R. In either case the interrupted current and the corresponding recovery voltage are in phase. At current zero, the instantaneous recovery voltage is also zero. This is a favourable condition for arc interruption and contributes to the success of the diverter switch.

58

3 Switching in Resistance Tapchangers

(a)

(b)

(c)

(d)

Fig. 3.12 Vector diagram for symmetrical pennant diverter switch

3.46 The Symmetrical Pennant Feature in Diverter Switching Cycle The operational sequence of the diverter switch was called the symmetrical pennant cycle in the older IEC Standard. It is now called “operating cycle 2”. The older name derives from the shape of the locus of the tip of the output voltage. This locus is shown in Fig. 3.12.

3.47 Multi-resistance Transition In tapchangers with diverter flag cycle (operating cycle 1), the main contacts perform an interrupted current of I at a recovery voltage of RI (see Table 3.3). The duty is obviously lighter if the resistance is lower. The transition contacts interrupt a heavyduty current of ½(E/R + I) at a recovery voltage of (E/R + I) (see Table 3.3). Clearly an increased value of the transition resistance reduces the circulating current, as well as ease up on the duties of the transition contacts. Ideally we must contrive to have a low transition resistance when switching with the main contacts, and a high transition resistance when switching with the transition contacts. The four resistance per phase diverter switch shown in Fig. 3.13 achieves these objectives. Historically the ACEC diverter switch tapchanger and the MR diverter type D 0466 used in their type D 400 A are good examples.

3.47 Multi-resistance Transition

59

(a)

(b)

(c)

(e)

(d)

(f)

Fig. 3.13 Operating sequence of four resistance diverter switch

(g)

60

3 Switching in Resistance Tapchangers

3.48 Four Resistance Diverter Switch Switching Sequence Figure 3.13 shows the operating scheme of a four resistance diverter switch tapchanger. The transformer winding with tappings is also shown. The tap selector arrangement is similar to the other diverter switches tapchanger with diverter switches and will not be described here again. The diverter switch has a moving contact which can rotate about the point J. Six fixed contacts, W, X1 , X2 , Y2 , Y1 , and Z, are arranged in a circle abut J as centre. A transition resistance R1 is connected between W and X1 . Another transition resistance R2 is connected between W and X2 . On the even side a transition resistance R1 is connected between Z and Y1 , and another transition resistance R2 is connected between Z and Y2 . The diverter rotating moving contact can assume two end positions, W and Z. These are the two operating positions of the tapchanger. At these positions current is carried either by W or Z. These are the two main contacts. X1 , X2 , Y2 , and Y1 are transition contacts, which carry current only when the tapchanger transits between taps. In Fig. 3.13a the moving contact is on W. The connection W provides the completion of current path from tap 3 to the output J. The path from tap 2 is broken at the diverter switch. No current flows through selected contact 2. The tapchanger is now operating at contact 3. The current path is shown by broken magenta lines. As the diverter moving contact rotates, it makes contact in sequence with every adjacent fixed contact. The diverter moving contact can bridge two or three adjacent contacts in sequence. The diverter switch moving contact is shown sitting on W in Fig. 3.13a. Current flows only through the main contact W. Tap 3 is the current input point to the tapchanger and J is the output. If it is now desired to change tap to 4, the selector moving contact now at 2 moves off the fixed contact to reach 4 (Fig. 3.13b). Please note that this is an off current operation. As no transition resistance is inserted, nor any current interrupted, this motion can be slow. After the selector moving contact is firmly on selector fixed contact 4 and stops moving, the diverter switch starts to switch. In Fig. 3.13c contact W breaks with arcing. When the arc is extinguished, the current flows through the contact X1 and X2 and the transition resistances R1 and R2 . There will be at arc at W. Figure 3.13d shows the two slip rings bridged through X2 and Y2 . The through current splits through the paths X and Y to J. There is also a circulating current driven by the tap voltage and limited by the two transition resistances R2 effectively in series. In Fig. 3.13e the contact at Y1 is also made. The currents split through the contacts X2, Y2 , and Y1 . In Fig. 3.13f X2 breaks with arcing. In Fig. 3.13g contact Z is made, and the transition resistances are bypassed. Current flows only through Z. Figure 3.13f is the final position, now operating at tap 4. Four principle features of the transition are worth noting: 1. There is always a path for the output current to J, and that the through current is thus never interrupted. 2. In the position where adjacent taps are bridged (Fig. 3.13c–f), there is at least one transition resistance in the local loop to limit the circulating current. 3. Transition resistances are inserted into the current path as required and finally removed. Recall the discussion in Chap. 1, Fig. 1.5.

3.48 Four Resistance Diverter Switch Switching Sequence

61

4. There are two arcing situations at the contacts of the diverter switch. Therefore, the oil of the tapchanger must be isolated from the oil of the transformer. If it is now desired to make a further tapchange in the same direction, the sequence described above repeats, except now it starts with the selector moving contact 3 moving to 5, in an off current manner. If, on the other hand, it is desired to revert to tap 3, the selector moving contacts are already in their correct position, and that they need not move. Only the diverter switch moves, completing the tapchange in the reverse direction.

3.49 Interruption Duties of Four Resistance Diverter Tapchanger Figure 3.14 shows the operating scheme of a tapchanger with diverter switch. Three positions of the tap selector are also shown, as they are relevant to the switching sequence of the diverter switch. Figure 3.14 shows two successive tapchange operations in either direction. This completes all possible switchings of the contacts.

3.50 Circulating Current In the four resistance diverter switching, there are three regimes of circulating current. 1. There is a circulating current I c1 in Fig. 3.14c. The circulating current is set up by the step voltage E and is limited by the resistance R2 on the even side, in series with the parallel combination of R1 and R2 on the odd side. The effective resistance is R1 + R1 ·R2 /(R1 + R2 ), The circulating current Ic1 =

E(R1 + R2 ) R2 (2R1 + R2 )

(3.9)

2. In the situation of Fig. 3.13d, the circulating current is limited by the resistances R2 on the odd and even side in series. The effective resistance is 2R2 . The circulating current Ic2 =

E 2R2

(3.10)

3. The third situation, as shown in Fig. 3.13e, is a mirror image of the circuit condition in Fig. 3.13c, The circulating current Ic3 =

E(R1 + R2 ) R2 (2R1 + R2 )

(3.11)

62

3 Switching in Resistance Tapchangers

The interrupted current at various stages is a combination of a fraction of the through current and the circulating current. The procedure for determining the interruption duties involves inter alia the determination of these current fractions. In the diverter under discussion, there are three interruptions per tapchange. These are represented by Fig. 3.14b, d, f. Figure 3.14 indicates the magnitude of the circulating current whenever there is one. The sharing of the through current by the available paths is not shown for better clarity. This sharing is discussed below.

3.51 Interruption Duties for the Tapchange Position 1 to 2 3.51.1 Interruption Duties of the Main Contact W This interruption occurs in Fig. 3.14b. The current carried by the contact W in Fig. 3.14a is interrupted. Interrupted current = I After interruption the through current is shared by the resistances R1 and R2 in parallel (Fig. 3.14b). The voltage drop across this parallel-connected network is the recovery voltage. The recovery voltage is therefore =

I R1 R2 R1 + R2

(3.12)

3.51.2 Interruption Duties of Transition Contact X1 1. In Fig. 3.14d transition contact X1 interrupts the current through it in Fig. 3.14c. The current consists of two parts: the contribution by the through current and the circulating current I c1 . The through current splits into the resistances of the odd side and the even side. The resistance on the odd side is the parallel combination of R1 and R2 and is R1 · R2 /(R1 + R2 ). The effective resistance on the even side is R2 . The odd side takes a share of the through current of Iodd =

R2 +

I R2 

R1 R2 R1 +R2

=

I (R1 + R2 ) (2R1 + R2 )

(3.13)

2. We are interested in the share of R1 on the odd side, as it is this which is interrupted at X1 next. This share R2 /(R1 + R2 ). I odd , because the parallel-connected resistances R1 and R2 on the odd side share the current.

3.51 Interruption Duties for the Tapchange Position 1 to 2

(b)

(a)

(c)

(e)

63

(d)

(f)

(g)

Fig. 3.14 a–g Page 1: Interruption duties of four resistance Diverter Switch. h–n Page 2: Interruption duties of four resistance Diverter Switch. o–u Page 3: Interruption duties of four resistance Diverter Switch. v–ab Page 4: Interruption duties of four resistance Diverter Switch

64

3 Switching in Resistance Tapchangers

(h)

(i)

(j)

(l)

Fig. 3.14 (continued)

(k)

(m)

(n)

3.51 Interruption Duties for the Tapchange Position 1 to 2

(o)

(p)

(q)

(s)

Fig. 3.14 (continued)

65

(r)

(t)

(u)

66

3 Switching in Resistance Tapchangers

(v)

(w)

(y)

(x)

(z)

Fig. 3.14 (continued)

(aa)

(ab)

3.51 Interruption Duties for the Tapchange Position 1 to 2

67

3. The share of the odd side current shared by R1 is R2 Iodd = I R1 = (R1 + R2 ) 4. Ic1 =



R2 R1 + R2



 R1 + R2 R2 I = I (3.14) 2R1 + R2 2R1 + R2

E(R1 + R2 ) R2 (2R1 + R2 )

(3.15)

5. The share of I c1 through R1 is =

Ic1 R2 E = R1 + R2 (2R1 + R2 )

(3.16)

6. The total current interrupted by X1 is E + I R1 (2R1 + R2 )

(3.17)

7. The recovery voltage corresponding to this interruption is the voltage drop in R2 after the interruption. The current through R2 after the interruption is ½(E/R2 + I) See Fig. 3.14e. The recovery voltage is =

(E + I R2 ) 2

(3.18)

We shall see below that the contact X1 does an interruption duty in the tapchange Pos 3 to Pos 2, where the expression (E + IR2 ) is replaced by (E − IR2 ) in the equations both for the interrupted current and the recovery voltage. The duties which involve the expression (E + IR2 ) are obviously heavier than those involving (E − IR2 ). Therefore, the contact X1 does one heavy-duty switching, when progressing from Pos 1 to Pos 2, and one light-duty switching when progressing from Pos 3 to Pos 2. It does no switching in the tapchanges from Pos 2 to Pos 3 and Pos 2 to Pos 1.

3.51.3 Interruption Duties of the Transition Contact X2 1. This interruption occurs at Fig. 3.14f. We must consider the current carried by this contact before interruption (Fig. 3.14e). 2. The through current splits between the resistance networks on the odd and even sides. The resistance on the odd side is R2 . The resistance on the even side is the parallel combination of R1 and R2 = R1 R2 /(R1 + R2 ). 3. The current carried on the odd side is

68

3 Switching in Resistance Tapchangers

R1 R2 /(R1 + R2 ) R1 I   I = R1 R2 2R1 + R2 + R2 R1 +R2

(3.19)

4. R2 also carries the circulating current I c3 =

E(R1 + R2 ) {R2 (2R1 + R2 )}

(3.20)

5. The total interrupted current

I =

E(R1 +R2 ) R2

+ R1 I

(2R1 + R2 )

 (3.21)

6. For the recovery voltage we consider the loop starting from the contact X2 , through the transformer tap and the voltage across the parallel-connected resistances R1 and R2 on the even side. The recovery voltage is =E+

I R1R2 (R1 + R2 )

(3.22)

7. We shall see below that the transition contact X2 does an interruption duty once again in the tapchange from Pos 3 to Pos 2. The addition sign + between the two terms added in the numerator of the equations for the duties is replaced by a subtractive sign—in the equations for the duties in tapchange from Pos 2 to Pos1. The duty with the + sign between the added terms is obviously heavier than the one with the—sign. The contact therefore does one heavy-duty switching and one light-duty switching in four tapchanges. The contact does not perform a switching action in the tapchanges from Pos 2 to 3 and Pos 2 to 1. Therefore the heavy-duty and light-duty functions are each done once in N/4 operations. This is shown in Fig. 3.14 and in Table 3.5.

3.51.4 Interruption Duties on Other Tapchanges Figure 3.14 examines three other tapchanges, from Pos 2 to Pos 3 (i.e. in the same direction), and from Pos 3 to Pos 2, and from Pos 2 to Pos 1 (both in the reverse direction). These cover all the interruption duties that occur on all the contacts. The interruption duties can be analyzed in the same manner as the transition Pos 1 to Pos 2, as done above. However a more elegant approach is available.

Right to left heavy duty

Left to right light duty

Right to left heavy duty

Left to right light duty

Direction

3.14aa

3.14y

3.14w

3.14t

3.14r

3.14p

3.14m

3.14k

3.14i

3.14f

3.14d

3.14b

Ref. Fig.

Y2

Y1

Z

X2

X1

W

Y2

Y1

Z

X2

X1

W

Contact

[E(R1 + R2 )/(R2 − R1 I)]/(2R1 + R2 )

(E − R2 I)/(2R1 + R2 )

I

[E − (R1 + R2 )/(R2 − R1 I)]/(2R1 + R2 )

(E − R2 I)/(2R1 + R2 )

I

[E(R1 + R2 )/(R2 + R1 I)]/(2R1 + R2 )

(E + R2 I)/(2R1 + R2 )

I

[E(R1 + R2 )/(R2 + R1 I)]/(2R1 + R2 )

(E + R2 I)/(2R1 + R2 )

I

Interrupted current

Table 3.5 Interruption duties of four resistance diverter tapchanger, ref. Fig. 3.14

E − [R1 R2 I/(R1 + R2 )]

½(E − R2 I)

R1 R2 I/(R1 + R2 )

E − [R1 R2 I/(R1 + R2 )]

½(E − R2 I)

R1 R2 I/(R1 + R2 )

E + [R1 R2 I/(R1 + R2 )]

½(E + R2 I)

R1 R2 I/(R1 + R2 )

E + [R1 R2 I/(R1 + R2 )]

N/4

N/4

N/2

N/4

N/4

N/2

N/4

N/4

N/2

N/4

N/4

N/2

R1 R2 I/(R1 + R2 ) ½(E + R2 I)

No. of interruptions

Recovery voltage

3.51 Interruption Duties for the Tapchange Position 1 to 2 69

70

3 Switching in Resistance Tapchangers

3.52 Interruption Duties for Transition 2 to 3 Comparing Fig. 3.14a–g with Fig. 3.14h–n, we see that there are essential differences. The action of the tap selector has reversed the direction of the circulating current relative to the through current vector. The sequence of contacts is reversed. These changes result in the same interruption duties as determined above for the movement tap 1 to tap 2, if we interchange Z for W and Y for X. The transition contacts Y1 and Y2 do a heavy-duty switching in the tapchange from Pos 2 to Pos 3.

3.53 Interruption Duties in the Reverse Direction The diagrams of Fig. 3.14o–u corresponding to the tapchange Pos 3 to Pos 2 are mirror image of the set Fig. 3.14h–n. Here the circulating current direction is not reversed, only order of contacts is. The interruption duties are thus the same in the tapchange from Pos 3 to Pos 2 and from Pos 2 to Pos 3, if the vector I is replaced − I. It is also necessary to interchange W for Z and X for Y in the vector expressions. The transition contacts X1 and X2 do a light-duty switching in this tapchange. In a likewise manner, the interruption duties for the tapchange Pos 2 to Pos 1 are mirror image of those of the tapchange Pos 1 to Pos 2. The duties are same, only with W and Z interchanged, and X and Y interchanged. The transition contacts Y1 and Y2 do a light-duty switching in this tapchange.

3.54 Frequency of Contact Interruptions Table 3.5 lists the duties performed by various contacts in the 4 tapchanges of Fig. 3.14. It is seen that each main contact W and Z perform N/2 switchings in N tapchanges. Each of the transition contacts does N/4 heavy switchings and N/4 light switchings in N tapchanges. These frequencies are incorporated in Table 3.5.

3.55 Algebraic Expressions for Interruption Duties for Four Resistance Diverter Switch The interruption duties shown in Table 3.5 are vector equations. These can be expanded into their algebraic expressions.

3.56 Main Contacts W and Z

71

3.56 Main Contacts W and Z The interrupted current is always I at a recovery voltage =

R1 R2 I R1 + R2

(3.23)

These are simple algebraic expressions into which the current value can be entered to work out the desired interrupted current and recovery voltages.

3.57 Transition Contact Duties 3.57.1 Interruption Duties of Current Contacts X1 and Y1 3.57.1.1

Heavy Interrupted Current  =

(E + R2 I cos ø)2 + (R2 I sin ø)2 (2R1 + R2 )

(3.24)

At a recovery voltage 

(E + R2 cos ø)2 + (R2 sin ø)2 2

3.57.1.2

(3.25)

Light Interruption Current  =

(E − R2 I cos ø)2 + (R2 I sin ø)2 (2R1 + R2 )

(3.26)

At a recovery voltage 

(E + R2 I cos ø)2 + (R2 I sin ø)2 2

(3.27)

72

3 Switching in Resistance Tapchangers

3.57.2 Interruption Duties of Contacts X2 and Y2 3.57.2.1

Heavy-Duty Interrupted Current

=

  2  2 E R1R+R + I cos ø) + (R1 I sin ø)2 (R 1 2 2R1 + R2

(3.28)

At a recovery voltage =E+

3.57.2.2

R1 R2 I R1 + R2

(3.29)

Light-Duty Interrupted Current  =

E(R1 +R2 ) R2

2 − R1 I cos ø + (R1 I sin ø)2 2R1 + R2

(3.30)

At a recovery voltage =E−

R1 R2 I R1 + R2

(3.31)

3.58 Maximum Interrupted Currents and Recovery Voltages The duties of the main contacts are not dependent on the load power factor. The maximum current is I at recovery voltage of R1 R2 I/(R1 + R2 ). For the transition contacts we need to consider only the heavy switching. For the contacts X1 and Y1 , the maximum interrupted current occurs at unity power factor and =E+ At a recovery voltage

R2 I (2R1 + R2 )

(3.32)

3.58 Maximum Interrupted Currents and Recovery Voltages

=

(E + R2 I ) 2

73

(3.33)

For the contacts X2 and Y2 the maximum interrupted current at unity power factor =

E(R1 +R2 ) R2

+ R2 I

(2R1 + R2 )

(3.34)

At a recovery voltage =E+

R1 R2 I R1 + R2

(3.35)

Even though these are vector equations, the vector is in phase. They can be used as scalar equations. The RMS values of the current and the step voltage need to be entered to get the maxima.

3.59 Direction of Power Flow It was noted that the tap selector effectively reverses the phase relationship between the step voltage and the through current on alternate tapchanges in the same direction. Therefore, such reversals are an intrinsic part of the operating cycle. A reversal of power flow, which means the reversal of the phase relationship between the step voltage and the through current, does not create a new switching demand. The tapchanger is therefore bidirectional.

3.60 Phase Relationship Between Interrupted Current and Recovery Voltage From Table 3.5 it can be seen that for the main contacts, the current interrupted I has a recovery voltage of R1 · R2 /(R1 + R2 )I. We can define a resistance Req = R1 R2 /(R1 + R2 ). The interrupted current is the recovery voltage divided by Req. The interrupted current and recovery voltage are therefore in phase. For the transition contacts X1 and Y1 , the interrupted current is of the form (E ± R1 I)/(2R1 + R2 ). The corresponding recovery voltages are ½(E ± R2). The current is double the recovery voltage divided by the resistance (2R1 + R2). The current and voltage are again in phase. For the transition contacts X2 and Y2 , the interrupted current is (E ± Req I )(R1 + R2 )/{R2 (2R1 + R2 )}

(3.36)

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3 Switching in Resistance Tapchangers

The corresponding recovery voltages of the form (E ± Req I). The expression multiplying the recovery voltage in the equation for the interrupted current is (R1 + R2 )/{R2 (2R1 + R2 )}. Irrespective of the magnitude, this multiplier has the dimension of 1/R , where R is a resistance. The interrupted current is therefore the recovery voltage/R . Once again the interrupted current and the recovery voltage are in phase. In all cases of current interruption, the interrupted current and the recovery voltage are in phase. At current zero, the instantaneous recovery voltage is also zero. This is a favourable condition for arc interruption and contributes to the success of the diverter switch. A more detailed analysis of the phase relationship between the interrupted current and recovery voltage can be found in [2].

References 1. IEC 60 214-Part 1:2014 2. Krämer A (2000) On Load tapchangers (Book): Sect. 4.2.1.1. Publication of MR 2000

Chapter 4

Constructional Aspects of Tapchangers

Anything not well made does not exist. Theophile Gautier, French Poet

4.1 Chapter Content It is not proposed to discuss the constructional details of different makes of tapchangers. This information to the extent that the manufacturer estimates the user needs to know is available in the technical information supplied by the manufacturers. It is only proposed to discuss some generic aspects of construction, which stamp individuality on the tapchanger. Individual manufacturer’s executions are only cited as illustrations. The illustrative examples are generally those of MR, ABB, or OLG from their published information. Variations of basic themes are highlighted to show that some executions offer advantages in some respects, while they may suffer from disadvantages too. Some aspects of tapchanger construction are such that they almost preclude their application in some specific cases. Older designs, which are hardly used anymore are described, mainly to introduce them to the reader, who may come across them (tapchangers have a long life!). These older designs may sometimes offer advantages (e.g. sheet metal clad double compartment tapchangers, Sect. 4.8.6) and may enjoy a rejuvenation in the future. From the point of view of the transformer manufacturer, some tapchanger executions can have restricted application, e.g. taps only at the middle of the winding.

© Springer Nature Singapore Pte Ltd. 2020 T. V. Sridhar, Application of Tap changers to Transformers, Power Systems, https://doi.org/10.1007/978-981-15-3955-8_4

75

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4 Constructional Aspects of Tapchangers

4.2 Construction of Selector Switches Type Tapchangers We saw in Chap. 3 that selector switches arc during transition. The switch is immersed in a quenching medium, for better arc quenching. The medium is mainly transformer oil in which the selector switch is immersed. We shall later see that ester fluids or SF6 can be also used. But for reasons of lack of arc quenching capabilities, silicone fluid is never used. The arcing cause deterioration of the oil, forming several gases and other degradation products, including conductive detritus. It is therefore important that the tapchanger oil is separated from the transformer oil. There are executions to meet this requirement. One is to house the entire live parts of the tapchanger in a sheet metal tank (Sect. 4.2.1). The other approach is to house the live parts of the tapchanger in an insulated enclosure (Sect. 4.3).

4.2.1 Compartment Type Tapchangers Using Selector Switch Principle Many tapchangers produced worldwide fall in this category. Figure 4.1 shows the schematic arrangement of compartment type tapchangers using the selector switch principle. Figure 4.2 is a photograph of a practical resistance type compartment

Fig. 4.1 Schematic arrangement of compartment tapchanger

4.2 Construction of Selector Switches Type Tapchangers

77

Fig. 4.2 Compartment tapchanger, OLG 33 kV

tapchanger (OLG) in a despatch-ready condition.

4.2.2 General External Arrangement All live parts of the tapchanger are enclosed in a sheet metal tank (Fig. 4.3). The tank is filled with oil. The tank has a flanged opening in one of the vertical tank walls. The flange can be bolted on or welded to a corresponding flange on the vertical wall of the transformer tank. Thus the tapchanger hangs from the transformer flange. Where bolted on construction is employed, there is an oil tight gasket between the flanges. An insulated terminal barrier board is mounted on the transformer tank wall and is enclosed by the common flanges. There is gasket between the barrier board and the transformer tank. The barrier board carries a number of oil tight stems, which connect to the transformer tap leads on the transformer side and the leads from the

Fig. 4.3 Compartment tapchanger, OLG 33 kV internal arrangement

78

4 Constructional Aspects of Tapchangers

fixed contacts on the tapchanger side. The transformer oil space and the tapchanger oil space are separated by the barrier board. See Sect. 4.2.7 for more on terminal boards.

4.2.3 Internal Construction (Live Part of the Tapchanger) The following is a functional description. The same functions as described for the compartment tapchanger may be achieved by variations of the basic concept. The selector switch tapchangers have a “Phase board” on which the selector switch is built (Fig. 4.4). The fixed contacts are mounted in a circle on the phase board. These contacts are connected to a terminal barrier board (see Sect. 4.2.7). The number of moving contacts can be two (single resistance selector switch) or three (double resistance selector switch). The fixed contacts are the input point of the selector switch. A central slip ring fixed on the phase board constitutes the output point of the selector switch. The main contact is connected to the output slip ring through a permanent slip ring contact, which maintains a contact with the slip ring at all times. A central drive shaft carries a rotor arm, which in turn carries the moving contacts. Figure 4.5 shows a typical moving contact assembly (OLG). The moving contact is positioned against the fixed contacts at all operating positions. It essentially bridges the fixed contact to the slip ring. The moving contacts are loaded by springs. These springs provide the required pressure between the contacts. The positioning accuracy arises from the use of geneva mechanism which is part of the selector drive. The geneva mechanism also locks the contacts in position. During transition, a step voltage occurs between the main and transition contacts. The transition contacts are insulated from the main contact to withstand this voltage. The transition resistance is also carried on the rotor arm. It is connected between the main and transition contacts Fig. 4.4 OLG type ABS phase board with contacts, pre-selector is at the bottom

TAP SELECTOR FIXED CONTACT

SLIP RING

PRE SELECTOR FIXED CONTACT

4.2 Construction of Selector Switches Type Tapchangers MOVING ROLLER CONTACT CONTACT SPRING

79 STOPPER

CAST ALU. ROTOR SLIDING CONTACT

Fig. 4.5 Rotor arm assembly OLG twin

(Fig. 4.5). In double resistance execution, a centre tapped transition resistance is provided. The ends connect to the two transition contacts, and the centre point to the main contact. During transition, the central shaft is rotated by a stored energy device from one tap position to the next, in a very short time (typically 40–100 ms).

4.2.4 Arcing at Contacts Arcing takes place at the “leaving edge” of selector fixed contacts. Often the contact edges are lined with arc resistant contact material such as tungsten copper (Fig. 4.70). The main moving contact sits on the fixed contact in a zone where there is normally no arcing damage. Silver plating to improve contact is not of much use, as the plating of the moving contact goes off soon due to arcing.

4.2.5 Tapchangers for Star and Delta Applications In a three-phase tapchanger, three-phase assemblies, consisting of the phase board assembly, with the fixed and moving contacts, and transition resistances are arranged within the tank in tandem. The phase boards are spaced apart by insulated spacer tubes. In some executions, the phase board assemblies are mounted on the tank floor. The phase board itself is extended downwards to provide the required ground clearance. In each case, the phases are spaced apart for the interphase voltage withstand requirements. Connections from the tap selector terminals are made to a terminal barrier board. In the ABB type UZD construction of the phase board, these connections are buried integrally with the phase + terminal board insulated casting (Fig. 4.6) [1]. The photograph shows the internal constructional details. The component was made transparent for development purposes and is not an actual tapchanger component.

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4 Constructional Aspects of Tapchangers

TRANSPARENT EPOXY CASTING

TRANSFORMER SIDE TERMINAL FLANGE FOR TANK FIXED CONTACT CENTRAL SHAFT LEADS

Fig. 4.6 Wauksha electric (SPX) experimental transparent phase board casting for tapchanger UZD for checking lead layout

This approach provides better insulation between leads and avoids accidental shorting. In low-voltage applications, up to 66 kV, the phases are fully isolated. There is no electrical connection between phases. These are eventually made in the transformer. These tapchangers are therefore usable for delta-connected transformers. When star application is required, a connection shorting the selected tap of the three phases is made at the terminal board to form the star point. The transformer manufacturer runs a lead from the star point on the terminal board to the neutral bushing on the transformer. There is otherwise no significant difference between tapchangers for star and delta application.

4.2.6 Single Compartment Tapchanger From the description above, it is seen that all the live parts are located in one enclosure. The tapchange action takes place within that enclosure. Such a tapchanger construction has rightly been called conventionally as single compartment tapchanger. Later (Sect. 4.3) we shall encounter a different execution, where the selection of taps and current switching during transition take place in two different chambers, with isolated oil systems. This type of tapchanger is conventionally recognized as double compartment tapchanger. It is pity that the new definition “compartment tapchanger” (Definition 3.56 of IEC 60 214) clashes with the old convention! Thus a diverter switch tapchanger of the Intank construction would be a double compartment tapchanger by well-established convention.

4.2 Construction of Selector Switches Type Tapchangers

81

4.2.7 Terminal Barrier Board The terminal barrier board serves to isolate the transformer oil system from the tapchanger oil. Figure 4.7 (OLG) is an example of the terminal barrier board. The board is made of cast epoxy, with the connector stems embedded in the casting. Most tapchanger manufacturers separately test the board for leak tightness as an acceptance procedure. This method of construction produces a leak tight barrier board. The more commonly adopted method of transformer construction is to receive the barrier board as a component of the transformer and mount it on the transformer port opening. This must be done by the transformer manufacturer. Connections on both the transformer side and the tapchanger side must also be carried out by the transformer manufacturer.

4.2.7.1

Some Practical Tips on Mounting the Terminal Barrier Board

The production process of the transformer port opening involves heavy gas cutting and welding. There may be some distortion. This is particularly true with studs welded to directly take on the barrier board, which is provided with holes. The situation is ameliorated when clamps are used to tighten the board, which does not have holes for receiving the studs directly. Before applying the terminal barrier board to the port opening on the transformer tank, it may be first applied without the gasket and lightly bolted on with only a few bolts. The gap should be checked for uniformity. If necessary, thin gasket shims may be added to ensure uniformity, as heavy tightening may crack the board. The shims must be tapered at either end. The barrier board is then applied to the port flange with a gasket, with adjusting shims as necessary. Gasket pressure should be applied uniformly in a manner that the board does not bend and crack. The tapchanger is mounted on the port over the barrier board, with TERMINAL BOARD CLAMPING CONTACT

Fig. 4.7 Cast epoxy terminal barrier board, OLG

82

4 Constructional Aspects of Tapchangers

a gasket. The gap between the board and the tapchanger flange is usually small. It is wise to use a sensitive crane, so that during mounting the tapchanger does not strike the board.

4.2.8 Terminal Barrier Board Mounted on the Tapchanger Tank Some compartment type tapchangers have the terminal barrier board fitted on the tapchanger tank. The ABB tapchanger-type UZE/UZF in which the phase board and the terminal board are combined must naturally be of this mode of construction. The connections from the fixed contacts on the phase board are made by the tapchanger manufacturer. The time needed to connect the terminals on the tapchanger side of the board by the transformer manufacturer is saved, leading to improved productivity. Further there is the advantage that the entire tapchanger is fully constructed at the tapchanger manufacturer’s works. As against these advantages, the barrier board mounted on the tapchanger calls for a temporary closure of the port opening of the transformer, during transformer “processing”. The transformer is processed in its own tank, or a vacuum autoclave, and filled with oil. After some soaking time, the oil is lowered, and the tapchanger is applied to the transformer tank. There is some exposure of the core and coil assembly to the atmosphere. This is not always acceptable. This extra step in transformer manufacture could be avoided if the tapchanger can be mounted on the transformer tank and both processed together. For this, it may be necessary to have a demountable drive mechanism. The drive mechanism contains components which do not tolerate transformer processing. For instance the lubricants applied on the gearing system will drip off, or otherwise damaged. This is particularly true if the processing is by vapour phase drying. Applying the lubricant again is a slow and expert procedure. Remounting the drive mechanism by the transformer manufacturer takes additional time. Further many medium size transformers are processed in a vacuum autoclave. The dimensions of the unit to be processed go up considerably if the tapchanger is already on the transformer tank. Autoclaves may not be large enough to accommodate the extra size. Because of these reasons, most transformer manufacturers prefer not to mount the tapchanger for processing. A further drawback of the barrier board mounted on the tapchanger is that in case of a tapchanger problem in the field, the tapchanger cannot be taken off. The ability to take the tapchanger off and fit with a spare is considered one of the advantages of the sheet metal construction.

4.2 Construction of Selector Switches Type Tapchangers

83

4.2.9 Nomenclature Even though this construction is dignified with the designation “compartment type” by the Standard IEC 60 214, the author would have preferred the term “Separate Tank Tapchanger”. This seems more appropriate. This distinguishes this construction from the so-called “Intank” type described below, where the tapchanger shares the tank with the main transformer. This reinforces the authors lament that the better designation for the compartment type would have been separate tank tapchanger.

4.2.10 An Alternative Approach to the Construction of Selector Switch (OLG Type SS) The OLG tapchanger type SS functions in a similar manner to the ones described above (Sect. 4.2.8), but has a different physical execution. In SS, there is a common phase board for all the three phases (Fig. 4.8). Fixed contacts are mounted in circles centred around three points forming an equilateral triangle. The moving contacts and rotor arm are attached to a gear wheel which rotates on a stub axle fixed at the centre of the contact circle. A common driving gear engages all the three-phase gears and turns them together by the required angle. The total thickness of the type SS is very small, which is an useful feature when applied to transformers of small rating. The SS tapchanger for 11 kV delta has only 95 L of oil. In order to manage proper engagement of the four gear wheels, the dimensions must be kept small. The tapchanger therefore could not be extended to higher voltage classes.

TOP COVERING PHASE BOARD

STIFFING BOX SHAFT

CENTRAL STEEL GEAR

CLAMPS

FIXED CONTACT

Fig. 4.8 Internal arrangement of OLG

DRIVE GEAR

84

4 Constructional Aspects of Tapchangers

Fig. 4.9 ATL contact system with variable pitch of moving contacts

OPERARTING ARM FIXED CONTACTS CONTACT SPRING ROTOR ARM

TRANSITION RESISTANCE

MOVING CONTACTS

4.2.11 ATL Single Compartment Execution The ATL single compartment selector switch tapchanger deserves particular attention, because of some special and unusual features. In a manner similar to OLG type SS, but differing slightly, the phases are arranged side by side in one plane. Each phase assembly has a central stub axle, which carries a toothed wheel. The drive input has a tooth wheel which engages with the individual drive wheels of the phases. Figure 4.9 shows the contact arrangement of an earlier ATL tapchanger. The moving contacts can assume a variable pitch. This is achieved by mounting the contacts on a “Lazy Tongs” linkage. It is not proposed to go into a detailed description of this linkage. It is sufficient to remark that this enables a “make before break” contact system in remarkably small dimensions. The collapse together of the moving contacts round the fixed stem produces the required electrical clearances, further contributing to the compactness. This construction results in small thickness of the tapchanger. https://www.youtube.com/watch?v=5jF-XwQJHzk, Video for ATL tapchanger May 17, 2013.

4.2.12 Provision for Oil Expansion in Compartment Tapchangers 1. Own tank The sheet metal clad tapchanger is filled with oil. In some executions, the tapchanger tank is not completely filled with oil, but some volume is left on the top. This forms the oil expansion space. A breather is provided for air exchange to the atmosphere. This route also provides for the gases formed during tapchange to escape. Though this construction is favoured in the USA, it has a drawback in that protection by Buchholz Relay is not possible. It is usual to use pressure relief device, and sudden

4.2 Construction of Selector Switches Type Tapchangers

85

Fig. 4.10 Partitioned conservator

pressure relief device suitable for air-filled compartments. The air space on the top may contain toxic and inflammable gases. Special precaution is necessary in venting this space. 2. Separate conservator A separate conservator is often provided with compartment as well as Intank tapchangers. This may be just a metallic partition of the main transformer conservator as in Fig. 4.10. A sector of the partition is cut off at the top, to provide equalization of pressure on the two sides. Gases produced in the tapchanger are vented through a breather provided on the tapchanger compartment. At the bottom of the partition, a filter consisting of felt material is often provided. This enables the oil level on the transformer and tapchanger side to be equal. Even though there is a small exchange of oil, the felt is enough to prevent oil contamination. However where DGA of the main transformer is planned, there could be false impressions due to the exchange of gas from the tapchanger side. Such admixture is likely to be small, as the gas would preferably went through the breathers. Even though gases would vent through the breathers, it is a good precaution to vent the air space on top, before any maintenance work. 3. Conservator at low level Sometimes a separate tapchanger conservator at a lower level, close to the top of the tapchanger is used (Fig. 4.11). This is particularly so in the case of Intank tapchangers. The difference in oil level between the main conservator and the tapchanger conservator provides a useful seal against tapchanger gases escaping into the main tank through weak seals. It also ensures that in case of leaks, the “good” oil of the transformer would leak into the tapchanger, but the reverse flow is prevented. This

86

4 Constructional Aspects of Tapchangers

Fig. 4.11 Conservator on tapchanger tank

CONSERVATOR BUCHHOLZ RELAY MECHANISM DOOR

MECHANISM

is a useful safeguard when DGA of the main tank is programmed. If there is a leaky communication between the main tank and the Intank tapchanger anywhere, main tank oil may slowly flow through and cause an overflow from the tapchanger breather. This however is a “fail safe” indication. 4. Totally sealed construction It would be unwise to completely seal off the tapchanger oil system from the atmosphere. This denies a path for the gases generated by the tapchanger to escape. There must be a tapchanger tank mounted breather, with or without a rubber bag.

4.3 Tapchanger in an Insulated Tank A different approach to keeping the oil systems separate is possible. It is interesting to conceptually generate the alternative in steps, to show two types are functionally alike, though startlingly different in appearance. For this purpose, Fig. 4.1 is redrawn in Fig. 4.12a that we now imagine that we take off all leads and connecting stems of the terminal barrier board (Fig. 4.12b). We further imagine that the terminal barrier board is flexible, so that it can be rolled into a cylinder as shown in Fig. 4.12c. Instead of connecting the fixed contact by leads to the stems of the terminal barrier board, we can reshape the fixed contacts on to the inner surface of the cylinder, with integral stems projecting through to the outside. These stems are fitted with oil seals. At this state, the tank does not seem to be doing anything, so we discard it (Fig. 4.12d). The contact cylinder is turned vertical, as this is more convenient. One end of the cylinder is closed with a blind flange in an oil tight manner. The other end also is fitted with an oil tight, but open flange. The open flange can be mounted on a tapchanger head plate, which carries other equipment necessary for the functioning of the tapchanger (Fig. 4.12d). The tapchanger head in turn is mounted on the top

4.3 Tapchanger in an Insulated Tank

87

(c) (a)

(b)

(d) (e)

Fig. 4.12 Equivalence of compartment and intank construction

cover of the tapchanger tank, with a gasket to seal oil (Fig. 4.12e). The blind flange and the head provide bearing support for the central rotating shaft. Note that the live parts are now encased in an insulated, oil tight cylinder. The cylinder and tapchanger head can be filled with oil. To suit its new function, we shall rename the cylinder as the tapchanger oil vessel. Oil in the vessel has no communication to the external surroundings. Finally we note that we have retained the original operating parts as before, as in Fig. 4.1. Thus the two tapchangers of Figs. 4.1 and 4.12e are identical in their operating mode, and only cosmetically different. The modifications resulting in Fig. 4.12e enable the tapchanger to be designated as an “Intank tapchanger”, in accordance with Sect. 3.55 of IEC 60 214. The author once again has some reservations about this designation. “Common Tank Tapchanger” may well have been a better designation, particularly as it fits well with his earlier suggestion of designation for the compartment type tapchangers as “Separate Tank Tapchanger”.

4.3.1 Which Construction Is Better? The conceptual tour de force of the previous section is justified, because there is a constant discussion amongst the tapchanger and transformer fraternity about which is better. They are essentially the same. Old wine in new bottle. However there

88

4 Constructional Aspects of Tapchangers

are features conferred by the method of construction, which may result in some differences. A comparison of the two types of construction, arising out of details in construction, is discussed below.

4.3.2 Comparison of Compartment Type and Intank Executions In favour of the compartment type 1. The accessibility is much better. The tapchanger is a separate identity. Opening one access cover on the tank leaves the whole live parts within easy access. There is no need to lower oil in the main transformer tank. No special tools are required to access the live parts. 2. In case of problems with the tapchanger, it can be removed and replaced with another healthy unit. This can be done without disturbing the transformer side. 3. If a new replacement unit is not available, the suspect tapchanger can still be taken off. A temporary connection can be made on a fixed tap on the terminal barrier board. A fabricated hood can be fixed on the transformer port, and operation of the transformer can be resumed at fixed tap. 4. In case of incipient faults, the operators have a better chance of observing difficulties, before they develop into major faults. 5. Transformer processing is usually carried out without the tapchanger. This reduces the size of the processing equipment. 6. In difficult transport situations, particularly in the less infrastructural developed parts of the world, the tapchanger can be taken off for reducing transport dimensions and weight. 7. As against all the favourable points, the fixed contacts are individually mounted on the flat phase board plate during initial assembly. Maintenance of alignment, particularly when the phase boards are large, as in higher voltage applications is no mean task. Misalignments can easily happen. Moving contacts may not easily mount the fixed contacts. In the worst case, the moving contacts can collide with the fixed contact, leading to a tapchanger stuck with tapchange incomplete.

4.3.3 In Favour of the Intank Tapchanger 1. Connections from the windings to the tapchanger are directly made within the transformer oil space. This is a great advantage and justifies this structure for high-voltage leads. No expensive terminal barrier board is required. 2. In case of a rise in internal pressure of the tapchanger, e.g. an explosion, the cylinder has a much higher strength than a flat terminal barrier board. However the flats ends may get damaged.

4.3 Tapchanger in an Insulated Tank

89

3. There is no need to mount the tapchanger externally on the transformer vertical walls, which are crowded with other equipment like radiators, cable boxes, marshalling kiosks. 4. The fixed contacts are well supported on a cylinder. Alignment of contacts is almost automatic. The cylinder is a stiffer support than a flat phase board. Contact vibration and consequent loss of contact are less likely. 5. Slow local mechanical creepage, yielding, and deformation of the support, particularly due to higher operating temperature, are unlikely. In a flat plate phase board construction, the localized contact force may cause local yielding, resulting in loss of contact force and loss of contact alignment.

4.3.4 Construction of Single Compartment Intank Tapchanger The construction is based on a single vertical cylinder, made of insulating material, e.g. glass fibre, cloth, or fleece reinforced epoxy. The entire power switch is enveloped in the cylinder, which is named the oil vessel. Figure 4.13 shows the general appearance. Thus the construction is of the single compartment type. The oil vessel is attached to a tapchanger head through the top flange. The head mounts with a gasket on the top cover, or a shelf on the transformer tank. Tap selector fixed

TOP FLANGE

OIL VESSEL

TAPSELECTOR CONTACT

PIPE CONNECTIONS

FIXED CONTACT

SELECTOR SLIP RING CONTACT

BOTTOM FLANGE Fig. 4.13 Single compartment selector switch tapchanger, MR OILTAP® V

90

4 Constructional Aspects of Tapchangers CENTRALIZED DRIVE SHAFT SHIELD RING ROLLER CONTACT

TRANSITION RESISTANCE CONTACT CARRIER

Fig. 4.14 Switching element intank single compartment, MR OILTYPE® V

contacts are arranged equally pitched peripherally. An extension of the fixed contact pierces the cylinder through oil seals, to form external connections to the tap leads. The tap leads lie in transformer oil space. Thus there is no need for a terminal barrier board. A circular slip ring fixed below the contact circle forms the tapchanger output. The bottom opening is closed by a cover with a gasket, which seals off the oil. The bottom cover also provides a central bearing support for the moving contact shaft. The moving contact assemblies, complete with transition resistance, of the phases are mounted on the central switching shaft (Fig. 4.14). The shaft is driven by an energy storage device incorporated in the drive and ensures correct positioning and locking. The moving contact assembly has a central main contact. The switching elements comprised of the moving contacts and transition resistance are mounted on the central switching shaft. The ends of the transition resistances connect to the transition contacts, while the mid-point connects to the main contact. The central shaft assembly is also called the switching column. A central suction tube is usually provided, to suck oil from the bottom of the cylinder, with the tapchanger in the assembled condition.

4.3.5 Solution to the Problem of Access to Selector Switch in Intank Tapchangers The most serious problem in Intank tapchangers is the relative lack of access. We shall first consider Intank tapchangers with selector switch. The problem is partially solved by incorporating all the moving parts of the selector switch, particularly all the contacts and transition resistances in a removable “Insert”. Figure 4.15 shows the removable Insert of OLG tapchanger type RMV which is an 11 kV delta unit, which explains the slender and tall construction. The insert can be pulled out through

4.3 Tapchanger in an Insulated Tank

91

Fig. 4.15 Switching column of single compartment intank tapchanger PRE-SELECTOR DRIVE PRE-SELECTOR SWITCHING

PRE-SELECTOR SLIP RING

SWITCHING ELEMENT

TRANSITION RESISTANCE

the hollow top flange of the tapchanger for inspection and repair. In some designs, a special tool may be required. However the fixed contacts cannot be removed, though they can be inspected through the top opening of the top flange. The removal of the insert needs a considerable degree of dismantling subassemblies and components as a preparatory action because they are in the way. This is often perceived by the transformer user as skilled work. Many of them feel that it would be best to take the assistance of the tapchanger manufacturer. This is in stark contrast to the relative ease of accessing the entire live parts of a compartment tapchanger.

4.3.5.1

Insert Removal Options

The insert of most tapchangers is heavy enough to call for a crane lift. Mobile cranes find difficulty in working with high-voltage overhead equipment in a substation environment. It may be necessary to shut down the entire or at least a part of the substation. This is not relished by the station operators. Realizing this, many tapchanger manufacturers offer specialized lifting gear which can be anchored to the transformer cover plate, or even the tapchanger head. These are helpful in lifting the insert out without the fuss of a mobile crane. The difficulty arises because for working on it, the insert needs to be brought down to ground level, which is 4 m or more down. The Immediate vicinity of the transformer is filled with many kinds of transformer equipment, e.g. radiators. To manoeuver the heavy insert down through all the obstructions is a procedure which remains a memorable experience for most transformer operators.

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4 Constructional Aspects of Tapchangers

4.4 Diverter Switch Type Resistance Tapchangers We saw in Chap. 3 that for high-power tapchangers a diverter switch is employed. The main difference between tapchangers employing a selector switch and diverter switch lies in that there is no arcing at the tap selector. The diverter switch removes all the current making and breaking, and therefore all arcing, from the selector. The tap selector may be directly immersed in the main transformer oil space anyway. Tap leads can then be directly routed from the core and coil assembly to the selector, instead of going through a barrier board mounted on the transformer tank. This is a great boon in high-voltage transformers, where taking out several H.V. leads from the tank into the tapchanger presents a considerable problem. As the voltage goes up, the barrier board becomes extremely large to endow sufficient electrical properties. It becomes very difficult to mount the tapchanger on the transformer. The highest voltage for which tapchangers are available with terminal barrier board is 650 kVp BIL. This is an ABB tapchanger type UZE/F [2]. For higher voltages, tap selectors are always immersed in the main transformer tank oil space.

4.4.1 Constructional Aspects of Diverter Switch Type Intank Tapchangers The most popular construction of the modern diverter switch tapchangers is based on the conceptual development described in the previous sections. This construction is exemplified by the MR type M and the ABB type UC. Other similar constructions are available among the range of manufacture of Huaming (China), Elin OLTC (Austria) (not functional anymore), Hyundai (Bulgaria), and OLG (India). Figure 4.16 shows the application of an Intank diverter switch type tapchanger (OLG tapchanger type PG). The tapchanger is mounted within the transformer oil space. The tapchanger hangs vertically from the transformer top cover plate. The diverter oil vessel, which contains the complete diverter switch, with its energy storage device, is at the top. The tap selector hangs below. Figure 4.16 shows the main components, omitting details such as shafts, bearings, retainers. An insulated diverter oil vessel is attached to the head through an interconnecting metal flange at the top. The oil vessel contains the diverter switch. The diverter oil vessel and the head are filled with transformer oil. A gasket between the top flange periphery of the oil vessel enable connection and the head provides oil sealing. Oil sealed contacts on the outer from the diverter switch to the tap selector. In a three-phase tapchanger, there are six of them, two per tap selector of each phase. A suction pipe meant for oil circulation in the diverter, without opening the cover plate runs down on the inner periphery of the diverter oil vessel. At the top, it connects to one of the flanges mounted on the head for external connection. A driving mechanism is mounted on the external vertical wall of the transformer. The drive shafts consist of a vertical and a horizontal component, connected at the top by a bevel gearbox. Figure 4.17 is a photograph of the tapchanger with diverter

4.4 Diverter Switch Type Resistance Tapchangers

93

Fig. 4.16 Showing diverter tapchanger in position within transformer oil space

switch. A common feature of tapchangers of this type is that they are mounted on the horizontal surface on the top plate, or similar, of the transformer tank and hang vertically in the transformer oil space. We shall later see some other ways in which the tapchanger can be mounted on the transformer (see Sect. 4.5). The head is the only component of the tapchanger visible to the outside. The head offers some standard facilities for mechanical connections, as shown in Fig. 4.18. A suction pipe running internally from the bottom to the top of the oil vessel connects to a flange on the head (The central flange in Fig. 4.18). The flange can be connected to an oil filter. The tap selector hangs below the diverter oil vessel. In most applications, the tapchanger can be lowered vertically into the transformer through a cut-out on the top plate. The tapchanger is supported by the head, which sits on the transformer top plate. The tapchanger does not need any other support. A gasket between the tapchanger head and the transformer seals the transformer oil. The output of the selector slip rings connect to the contacts on the diverter oil vessel, by means of leads which lie in the transformer oil space. The main drive shaft enters the head from the drive mechanism. The output shaft of the mechanism is connected to the head by vertical and horizontal drive shafts connected by a bevel gear. The bevel gearbox is mounted

94

4 Constructional Aspects of Tapchangers

HEAD

OIL VESSEL

OUTPUT LEAD

TAP SELECTOR TAP PRE-SELECTOR

Fig. 4.17 Diverter switch type intank tapchanger OLG

Fig. 4.18 Typical facilities available on the tapchanger head based on OLG tapchanger PG with diverter switch

4.4 Diverter Switch Type Resistance Tapchangers

95

on the transformer, aligned with the mechanism shaft on the vertical side, and the head shaft on the horizontal side. Mounting and aligning the bevel gearbox is done by the transformer manufacturer. A selector drive shaft emerges from the bottom flange of the diverter oil vessel (Fig. 4.16). It drives the tap selector through a gearbox housed between the diverter oil vessel and the tap selector.

4.4.2 Construction of the Diverter Switching Element It is but natural that the diverter switch housed in a cylindrical oil vessel takes a cylindrical shape as well. However, there are exceptions. ABB has a rectangular V Switch housed in the cylindrical oil vessel. The most usual number of switching elements is one per phase. This forms a three-phase tapchanger. The moving diverter contacts are driven from a central shaft from the energy storage device. The moving diverter contacts are all connected together with no insulation between them. This connection forms the neutral star output of the tapchanger. At this point, there is no insulation required between the live parts of the phases. The open contacts of the diverter switch and associated supporting and connecting live parts of phases need to be insulated only for a step voltage from the star neutral. This enables the three phases of the diverter switch to be incorporated in a compact physical unit, with minimum insulation where required. Connection of the selected contacts of the diverter switch is in fact the default execution mode of Intank diverter switches. In delta units, which will be discussed later, insulation between phases is provided by axially separating the complete switching elements of the phases. Naturally this makes for a much higher axial height. The height may rise to an extent that it is impractical to house the tapchanger under the transform cover. Besides a very high length makes the tapchanger column unstable. This is the reason why Intank diverter switch tapchangers are usually not available for H.V. application. When applied as a singlephase unit, say at the line end of transformers, the common connection of the diverter switch moving contacts serves as the output point. In other words, single-phase Intank tapchangers have a three-phase star switching element. The switching elements are connected in parallel. In some cases, some economy is affected by removing the contacts of one, or even two of switching elements. When one switching element is removed, the other two are arranged at 180°, rather than the customary 120°, for better balance. Both the selected and non-selected fixed contacts of the diverter switch are housed on the periphery of a common insulated cylinder (“switching cylinder”). The cylinder periphery provides the necessary insulation.

4.4.2.1

Kinematics of Diverter Contacts

The diverter switch moving contacts are driven at high speed by the energy storage device. The exact kinematics of the switching element is usually not among published

96

4 Constructional Aspects of Tapchangers

information of the manufacturer. This aspect of diverter construction will not be discussed for that reason.

4.4.3 Cylindrical Construction In most cylindrical Intank tapchangers, the diverter fixed contacts are secured to the inner periphery of a contact cylinder. In MR Type M, diverter switch is mounted on the inner periphery of three cylindrical shells (Fig. 4.19). The moving contacts projects form the inner core of the cylinder (Fig. 4.20). ABB and AEG-Siemens use a non-cylindrical construction for the diverter switch. In the MR type M tapchanger, Fig. 4.19 MR OILTAP® M tapchanger. From MR transform campus, UWE SELTSAM, 2013

MAIN INSULATOR

DIVERTER SWITCH

Fig. 4.20 MR type M diverter switching element [3]

4.4 Diverter Switch Type Resistance Tapchangers

97 FLEXIBLE BRIDE

CONTACT WITH SOLID GUIDE SLOTS

TOP GUIDE PLATE DIVERTER SHAFT

GUIDE SLOT

Fig. 4.21 Diverter moving contacts of MR type D [4]

the fixed contacts of the moving contacts have slotted guides to move radially. This is more clearly seen from the photograph of an earlier MR diverter type D similar to the type M (Fig. 4.21). Contact parting is radial. This has the advantage that the contact separation increases rapidly, enabling early arc quenching (Fig. 4.21). Further the entire contact surface is offered for arcing, making the best use of material and ensuring long life. The arc is mainly confined to the volume between the contact faces. The radial movement allows fitting of arc chutes on the fixed contacts, restricting the arc between the parting contact faces. The exact mechanism of coupling the moving contacts to the energy storage device, and the method by which they derive their movement is not available on the public domain and will not be described here.

4.4.4 AEG-Siemens Diverter Switches Arrangement This tapchanger was made by AEG-Siemens, who licensed the Indian company NGEF to produce them. Even long after AEG-Siemens ceased to manufacture tapchangers, NGEF continued to make it in sizeable numbers, amounting to possibly more than 1000 units, till they themselves went out of business. A vast majority are still in service after nearly 40 years. Figure 4.22 [5] shows the kinematics of the diverter switch. Only half a tapchange is shown, as the other half is a mirror image. The switch follows the operating cycle nr. 1. The following description is based on [5]. The fixed contacts are mounted on vertical flat insulated supporting plates. The main and transition moving contacts move parallel along an axis at right angles to the meeting surfaces. The transition contacts are equipped with tiny pantograph mechanisms to ensure parallel movement of contacts. This enables quick withdrawal and build-up of contact gap during arcing. The full contact surface is used in the arcing process, facilitating long life.

98

4 Constructional Aspects of Tapchangers

(a)

(b)

(c)

Fig. 4.22 Kinematics of the AEG NGEF Diverter switch

4.4.5 The Diverter Switch NK Used by OLG Is Their Earlier Intank Tapchangers The switch can be called a “V” switch, because of the disposition of contacts. The fixed contacts are on vertical insulated support plates, symmetrically positioned about the central rotating axis (Fig. 4.23a). Figure 4.23 omits details such as contact springs, mounting details, and actual shapes in the interest of clarity. The moving contacts are mechanically and electrically connected to the rotating arm which is made of conducting material, by brazing. The output is taken from a hub on the arm, by flexible connectors. There are two rotating arms one behind the other, of which the rear cannot be seen in Fig. 4.23a but indeed it can be in the representation of Fig. 4.23b where the front contact has swung over to the right. The drive is contrived to rotate the front arm first, and then the rear arm and vice versa on tapchange direction reversal. The third row of Fig. 4.23 shows the connections made by the diverter corresponding to its position in the top two rows. The fourth row shows the switching sequence, which is operating sequence nr. 1 (former flag cycle). The fourth row also shows the connections to the transition resistances. The V contact system allows practically full face-to-face meeting. It is theoretically possible to use the full face of the moving contacts for arcing. Contact parting is almost at right angles, so that rapid build-up of contact gap takes place. In Fig. 4.23a, the tapchanger is at tap 1, with the main contact W made. Figure 4.23d, g shows the connections made by the diverter switch in this position. During the tapchange the rotor arm rotates right, opening W, and making Y. In the mid position (Fig. 4.23b), the main contact W has broken, but the transition contacts X and Y are still made. This corresponds to the bridging condition shown in Fig. 4.23e, h. Shortly after this position the rear arm also rotates right, opening X and making Z (Fig. 4.23c, f, i). This completes the tapchange, with the diverter at rest at tap 2. The diverter follows the operating cycle nr. 1. The contacts move with their surfaces practically parallel, so that parting is rapid and the contact gap builds up facilitating arc quenching. Almost the entire contact surface takes part in arc formation so that the material is well used. OLG abandoned this diverter design

4.4 Diverter Switch Type Resistance Tapchangers

99

(b) (a)

(c)

(d)

(e)

(g)

(f) (b)

(a)

(h)

(c)

(i)

Fig. 4.23 Representation of “V” diverter switch (OLG)

because the three phases of the diverter had to be arranged vertically, and the height of the diverter switch could not be reduced to be in line with other commercial models available in the market. A few OLG tapchangers with the NK diverter switch are still in operation, thirty years after installation.

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4 Constructional Aspects of Tapchangers

4.4.6 The OLG Cylindrical Diverter Switch Figure 4.24 shows the principle of operation of the OLG cylindrical diverter switch. As mentioned earlier, this eventually replaced the NK diverter due to commercial considerations. The fixed contacts are disposed on the inner surface of the contact cylinder. The moving contacts are located in aluminium sector housing (see Fig. 4.25). The contacts can slide radially in slots, as well as rotate about a vertical axis within slots. The maximum outward radial travel is limited by a circular belt fixed on the aluminium housing.

4.4.6.1

Kinematics of OLG Diverter Switches PG

The aluminium sector housings (green coloured in Fig. 4.25) are driven by the energy storage device through the central rotating shaft. A roller fixed at the outer radius of the housing constrains it to move only radially in and out in a guide fixed to the contact cylinder (cyan coloured in Fig. 4.25). This results in the sectors partly rotating, and partly advancing radially. The contacts follow the requirements of operating sequence 1 of the IEC. The contacts do not part radially, but the movement is peripheral in part. Thus the contact gap does not build up at the maximum rate. The contact also do not experience arcing across full to face. Arcing wear will be uneven. Because of the wiping action of the moving contacts, it is not possible to fix arc chutes on the fixed contacts. As against these disadvantages, the contact movement does not

Fig. 4.24 Diverter switch fixed contact arrangement OLG diverter switch type PG shown closed on the Z side

4.4 Diverter Switch Type Resistance Tapchangers Fig. 4.25 Moving contact assembly diverter switch OLG tap changer type PG, shown upside down

101

INPUT DRIVE SHAFT GUIDE ROD FOR HOUSING

MOVING CONTACT

CONTACT HOUSING TOP FLANGE

GUIDE BLOCK FOR CONTACT

involve a cam, where high-pressure angles may cause jamming (for definition and discussion of pressure angle, see any introductory textbook on mechanics).

4.4.6.2

Diverter “Insert”

All the switching elements, transition resistances, energy storage drive, and other details which may need inspection and maintenance are mounted on a removable diverter insert. Figure 4.26 shows a typical insert (OLG type PG) cover plate. When necessary, the diverter insert can be pulled up vertically from the head. To remove the insert from the diverter oil vessel, the insert cover is first taken off, and then fasteners holding the insert in position, and perhaps one or two other details after being taken out. This is a simple procedure. The removable insert helps in maintenance, servicing, and repairs of the diverter switch. In some designs (MR type M, OLG type PG), the output contacts of the insert are spring loaded outwards. These slide against the diverter fixed contacts on the inner surface of the oil vessel (see Fig. 4.27). An extension of the diverter fixed contacts emerges on the outer periphery for connecting to the tap selector. These extensions are oil sealed. In some designs, e.g. Elin, Hyundai, the diverter output contacts are bolted down to the horizontal surface of corresponding fixed connectors, see Fig. 4.28. These connectors have extensions emerging from the oil vessel bottom flange. In the case of the sliding contact design, the insert can be pulled out without removing fastener for the contacts. In designs with bolted connection, a special tool reaches down the side of the insert, to disconnect from the fixed connector, before the insert can be pulled out. Some difficulties regarding removal of insert were discussed earlier.

102

4 Constructional Aspects of Tapchangers TAP NO. INDICATOR ASSEMBLY TOP SUPPORT

DRIVE SHAFT

SPRING CHARGE SYSYTEM

INSERT

Fig. 4.26 Diverter switch insert OLG tapchanger type PG

Fig. 4.27 Diverter oil vessel sliding contact assembly

4.5 Alternative Methods of Mounting Intank Tapchanger on the Transformer

103

Fig. 4.28 Example of diverter fixed on top of output bushings. Bushings go through bottom flange for selector connections

DIVERTER

OIL SEALS BOTTOM FLANGE

4.5 Alternative Methods of Mounting Intank Tapchanger on the Transformer 4.5.1 Cover Mounting The European practice is to connect up the transformer coils as required while holding the transformer cover plate over the core and coil assembly in approximately in its final position (Fig. 4.29). This construction offers a neat way to mount an Intank tapchanger on the transformer. The tapchanger is let down a cut-out on the cover plate, and the head is bolted down, with a gasket. The tapchanger is thus in its final position relative to the active parts. Access for connection around the tapchanger is almost total. Leads can be connected directly to the tapchanger. After all connection and other sundry manufacturing stages, the assembly is lifted up and lowered into the tank. The transformer can then be processed. Fig. 4.29 Example of cover mounting TRANSFORMER COVER PLATE TAPCHANGER

COIL

104

4 Constructional Aspects of Tapchangers

4.5.2 Bell-Shaped Transformer Tank Mounting For larger transformers, some clients specify a high degree of access to the live part assembly in the field, without the need for heavy lift. The transformer tank is then made in two parts. After the live part assembly is completed, it is placed on the bottom tank. The top tank is hollow. It is lowered vertically over the live parts and joined to the bottom tank by a flange. When needed, the top tank alone can be removed, with a relatively light lift, offering almost total access to the live parts. This construction lends itself to mounting the tapchanger in position relative to the live parts from the cover plate very well. However a special top flange is required. Such a bell mounting support flange is provided by the tapchanger manufacturer. Figure 4.30 (from Technical Brochure of Electroputere SA, Romania [6]) shows a large transformer employing the bell-shaped tank concept. Figure 4.31 shows the method of tapchanger in a bell-shaped tank. The diverter insert and the head of the tapchanger are removed, before assembling on the live parts. The tapchanger is supported by the bell mounting flange from the live part assembly by a support bracket. The tapchanger is held almost at its final position. It is dropped only a few millimetres. The tap leads are connected to tapchanger. The head is mounted on the top tank. After the connections and other work on the live parts are finished, the top tank is lowered to its position and joined up. The tapchanger head is now a few millimetres above the separating flange of the diverter oil vessel. Sector plate seen in is inserted through the oil vessel opening (Fig. 4.31a) and turned up to fit under the oil vessel flange (Fig. 4.31b). The device is lifted up a few millimetres, until the top flange can be drawn up and bolted to the head. The device is then removed. The insert can be lowered into the oil vessel, to complete the tapchanger. The construction needs some accurate work. When the top tank is lowered, the tapchanger head must reasonably align vertically with the diverter oil

COILS

CORE

BOTTOM TANK

Fig. 4.30 Large transformer with core and coil in bottom section of tank

4.5 Alternative Methods of Mounting Intank Tapchanger …

105

Fig. 4.31 Tapchanger in a bell shaped tank

vessel. The gap by which the tapchanger is pulled up to join with the head must be small. Otherwise the already made tap leads may exert excessive force on the tap selector (Fig. 4.32).

106

4 Constructional Aspects of Tapchangers

(a)

(b)

Fig. 4.32 Mounting in bell tank

4.5.3 Shelf Mounting The cover mounted construction of Sect. 4.5.1 above is not popular in some engineering traditions. There is a valid technical objection that when the entire live part assembly is lowered into the transformer tank, there is no access to see and verify that electrical clearances are not transgressed due any departure from design, or workmanship. There is a psychological factor that in some areas of the world, workers are not happy working in the cramped space under the suspended cover plate. In such a case, the tapchanger is mounted on a “shelf” on the transformer tank (Fig. 4.33). The entire tapchanger, without any special flange, or any partial dismantling can be lowered into the cut-out of the shelf, even prior to tanking of the live parts. The live parts are finished with all the tap leads connected at the coil end. The leads are temporarily folded back towards the live part, to allow tanking. The connectors get into the tank through manholes, or the still open top, and connect the tap leads to the tapchanger. In case of inspection of the core and coil, the tapchanger is disconnected using hand holes on the tank. The core and coil with the tap leads connected can be lifted out. This of course needs a heavy lift.

4.5.3.1

Tapchangers Which Cannot Be Lowered from the Top Through a Cutout Out in the Transformer Cover

The tapchanger head must be mounted on the transformer cover plate. If the tap selector foot print is larger than the head, it cannot be lowered from a top cut-out. For bell mounting, this is no problem, as the tapchanger is not lowered through the

4.5 Alternative Methods of Mounting Intank Tapchanger …

107

Fig. 4.33 Shelf mounting

cut-out. Tapchangers with pre-selectors often cannot be dropped through the cutout on the top plate. The MR type D tapchanger always had a tap selector larger than (Fig. 4.34) the cut-out. Most Intank diverter switch types with a coarse/fine selector share this problem too. Such tapchangers are easily tackled in bell mounting tanks. However, in early days of transformer manufacture, decapitating a new and expensive tapchanger to facilitate mounting, with the consequent uncertainty of its being reassembled correctly, was a considered a daunting problem. The manufacturer HHE who used more than 200 type D tapchangers in the sixties and seventies, all of them with a selector larger than the diverter found another solution demanding less skills. For the benefit of small transformer manufacturers who may lack the necessary Fig. 4.34 MR type D tapchanger with selector of larger diameter

HEAD DIVERTER OIL VESSEL

TAP SELECTOR

108

4 Constructional Aspects of Tapchangers

skills and whose usage of tapchangers is limited, the HHE method is described here. Figure 4.35 shows the HHE method. An intermediate plate of ordinary transformer tank plate was fitted under the head, before the selector was mounted at the bottom of the diverter oil vessel (see Fig. 4.35a, c). The intermediate mounting plate had leak-proof studs welded on the top surface, to eventually receive the transformer tank cover. The assembled tapchanger was supported by the intermediate mounting

(a)

(b)

(c)

Fig. 4.35 HHE method of MTG tapchanger with a selector larger than the cutout on cover

4.5 Alternative Methods of Mounting Intank Tapchanger on the Transformer

109

Fig. 4.36 Formation of gutter in HHE method of MTG

plate from a temporary support from the core and coil assembly. This gave complete success to the tapchanger for connections. After connections, the entire core and coil assembly, complete with the tapchanger was lifted and tanked. After the transformer cover plate was down in position, the tapchanger was lifted up, using its own lifting facilities on the head, till the welded studs on the intermediate plate could be tightened to the transformer cover, with a gasket. A problem with this method was that a gutter formed all round the tapchanger head (Fig. 4.36), where rainwater could gather, and attack the gasket. To resolve this, the gutter was filled in the earlier days with cable sealing compound, and later with epoxy. This is not a recommended method of manufacture, as the sealing compound eventually fissured and cracked in service, allowing rainwater access to the gasket. Changing the gasket of the intermediate plate was no mean task.

4.6 General Construction of Tap Selectors for Intank Tapchangers with Diverter Switch As mentioned, the tap selector is suspended from the bottom of the diverter oil vessel. A usual construction, for instance used by OLG, is in the form of a cage, with top and bottom metal rings (cage rings), between which insulated flat bars are attached (see Fig. 4.37). Tap selector fixed contacts are attached to the bars. The construction resembles a birdcage, and hence the name “cage rings” for the top and bottom metallic support members. In some designs, the flat bars are replaced by round tubes. Field shaping electrodes are fixed on the contacts for high-voltage tap selector to reduce electric field concentration. In three-phase tap selectors, the odd and even fixed contacts are arranged alternately on the same flat making six contacts per stick. The MR type D tap selector had only three contacts, all odd or all even,

110

4 Constructional Aspects of Tapchangers

Fig. 4.37 Tap selector tapchanger type PG OLG TOP CAGE RING

GEAR BOX DRIVE SHAFT FIXED CONTACT

BOTTOM CAGE RING

per selector stick. The number of tap selector “Sticks” doubles in this case, making the diameter of the tap selector larger. Against that disadvantage, the tap selector is shorter, which is an advantage in reducing the oil side height of the entire transformer.

4.6.1 Selector Drive In diverter switch type tapchangers there are two tap selectors which must be driven alternately. The modern and almost universal approach is shown in Fig. 4.38. The geneva mechanism turns the constantly rotating input drive to alternate intermittent drive for the odd and even selector shafts. It ensures correct positioning of the moving contacts centrally against the fixed contacts. It is also important that when the tap selector stops, the geneva must be locked, so that the contacts do not move under constant transformer vibration. Due to the requirements of free running without too much friction, there is a relatively large mechanical clearance between the locking surfaces. This allows the contacts to move peripherally somewhat, even when locked, as the lock is not very rigid. Further if the contact load is very high, the moving contact “under reaches” the central position in either direction. This degree of locking is sufficient for the purpose.

4.6.2 Application of the Geneva Intermittent Drive to Concentric Inner and Outer Selector Drive Shafts In Sect. 4.6, we saw that a common tap selector construction involves location of the fixed contacts in two layers per phase long the height of the tap selector. The moving contact structure must match this. This can be achieved by placing the moving contacts with matched axial separation on two independently rotating concentric

4.6 General Construction of Tap Selectors …

(a)

(c)

111

(b)

(d)

Fig. 4.38 Principle of the geneva mechanism

drive shafts, as shown in Fig. 4.39, which shows the application of the geneva mechanism to drive two selector drive shafts in tandem. The shafts are coaxial. The outer shaft is driven by the top geneva, while the inner by the bottom geneva. The cam is common. It is located between the two genevas and rotates one complete circle each tapchange, imparting alternate motion to the two tap selectors. The drive shafts are provided with bearings at the top and the bottom cage rings.

4.6.3 A Space Saving Alternative Arrangement of Tap Selector Drive In one interesting variation (Elin OLTC), the tap selector has only one central drive shaft. Both moving contacts are mounted on this shaft. The moving contacts do not locate centrally on the fixed at the operating positions (see Fig. 4.40). The moving contacts close on adjacent fixed contacts, which are at different levels in “position” (see Fig. 4.40a). In Fig. 4.40a, the left-hand diverter is closed and the right open.

112

4 Constructional Aspects of Tapchangers

(a)

(b)

(c)

Fig. 4.39 Schematic of geneva drive for inner and outer drive shaft arrangement for three phase tap selector (OLG tapchanger type PG)

4.6 General Construction of Tap Selectors …

(a)

(b)

113

(c)

Fig. 4.40 Single drive shaft selector (ELIN)

The current is therefore carried only by the top white selector moving contact. The dimensional details are such that for each movement, one moving contact which is carrying current at the start of the movement (top white in Fig. 4.40a) slides on the fixed contact, but remains connected to it. The other contact (bottom green in Fig. 4.40a) which is not carrying current, because its diverter is open breaks with the original fixed contact, and makes on to the next, with no current (Fig. 4.40b). In the position of Fig. 4.40b when the selector drive stops, the diverter switches over, causing bridging of adjacent taps. The diverter opens and extinguishes its arc, and the tapchange is complete (Fig. 4.40c). There is no arc making or breaking at the tap selector contacts. Figure 4.40c shows the moving contacts and the diverter aspect at the next tap position. This execution is enabled by making the moving contacts sit off centre on the fixed. This arrangement which may not look elegant, saves on tap selector dimensions.

4.6.4 Tap Selector Execution in Cylinder The cage construction is not ideal if the contact pressure is high, as needed in tap selectors of very high currents. The force exerted by the moving contacts as the climb the fixed exert considerable side-ward bending of the “cage bars”. In that case it would be advantageous to mount the fixed contacts on an insulated cylindrical tube. Figure 4.41 shows the ABB UCG tapchanger in such an execution [3]. Figure 4.42 shows an MR tapchanger with the tap selector in cylindrical execution for high current.

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4 Constructional Aspects of Tapchangers

Fig. 4.41 Tapselector execution in cylinder HEAD DIVERTER

DIVERTER TO SELECTOR CONNECTIONS

PRE-SELECTOR

SELECTOR

Fig. 4.42 Cylindrical selector for high currents, MR execution

HEAD DIVERTER

DIVERTER TO SELECTOR CONNECTIONS

SELECTOR

4.6.5 An Alternate Legacy Selector Drive Arrangement Used in Fuller Electric Co’s. Type EHS Tapchanger The EHS tapchanger described below is probably an original ASEA (later to be ABB) design. Figure 4.43 showing the execution of the tap selector drive avoids all unnecessary mechanical details, so as not to obfuscate the essential. The selector moving contacts are carried on two lead screws arranged in parallel. At the bottom, each shaft carries a driven pinion. The drive gear (green coloured in Fig. 4.43) rotates continuously half a turn per tapchange, making one complete turn in two tapchanges. It has teeth only on a sector of the internal surface, which can engage and drive the

4.6 General Construction of Tap Selectors …

115

Fig. 4.43 Schematic of fuller EHS tap selector drive

selector shaft pinions. After the required motion in one tapchange, the drive gear disengages, as it has no more teeth. About half a rotation later, the drive gear engages the other selector pinion. This results in the two tap selectors being driven alternately. This seemingly neat design had a fatal flaw. Re-engagement of the gears was only possible if the driven pinion stayed strictly at the place it was left behind at the time of disengagement. Any movement resulted in the teeth clashing. However, the arrangement did not provide for a positive location of the pinion, hoping for contact friction to maintain the shaft in position. During transformer vibration, the pinion moved sometimes, resulting in catastrophic failure to re-engage. The EHS had a few other design weaknesses, which could be re-engineered.

4.7 Pre-selectors The application of pre-selectors is treated in Chap. 5. For the present we consider only the constructional aspects. The pre-selector is used to extend the number of operating positions, using the contacts of the tap selector more than once. There are two types of pre-selectors: the reversing switch and the coarse/fine selector.

116

4 Constructional Aspects of Tapchangers

4.7.1 Construction and Operation of the Pre-selector in Compartment Tapchangers The pre-elector is a two-way change over switch. It is used either as a coarse/fine switch or a reverser. The three contacts are wired down to the transformer terminal board where the necessary connections are completed. Figure 4.44 shows the preselector of OLG compartment type tapchanger. The pre-selector is directly driven from the motor drive through appropriate gearing. It is important to maintain “synchronism” in the sense that the pre-selector operates only when it is not carrying current. Such positions are any case required to operate the pre-selector in the off current condition. The movement of the pre-selector contact is required to takes place during that one tapchanger interval. At all other times, the geneva is kept locked from moving. Figure 4.45 shows a typical pre-selector drive. In Fig. 4.45 the synchronized main drive comes from the drive mechanism shaft at the top (Fig. 4.45a). A roller (green colour) is attached to the lock (cyan colour), which is mounted on the drive shaft. As the lock turns with the shaft, the roller enters a slot in the geneva to drive it when required. In Fig. 4.45b the lock is about to release for counterclockwise rotation of the geneva because the drive roller has entered the slot to drive it. Figure 4.45c shows that during the entire course of movement, the relief slot cut on the lock avoids fouling of the pre-selector geneva with the lock. Figure 4.45d is the end of the preselector movement, after which the geneva is locked again. The pre-selector moving contacts are mounted on the geneva shaft and are driven by it (Fig. 4.45e–g). The geneva of which only one petal is needed in this case (a single petal geneva) is kept appropriately locked after it completes its motion (Fig. 4.45a). This ensures that the pre-selector contacts do not run off the fixed contacts due to transformer vibration. Fig. 4.44 Pre selector of OLG compartment type tapchanger

4.7 Pre-Selectors

117

(a)

(b)

(d) (c)

(e)

Fig. 4.45 Typical pre-selector drive

(f)

(g)

118

4 Constructional Aspects of Tapchangers

4.7.2 Pre-selector for Intank Type Tapchangers Pre-selectors of Intank selector switches are similar to the compartment type and will not be discussed again. It is important as will be seen from Chap. 5 that the pre-selector contacts move only at defined positions of the tap selector contacts. Mechanically, this is best achieved by building the pre-selector as an extension of the fine selector (Fig. 4.46). In Intank diverter switch type tapchangers it is convenient to have two different pre-selectors. The reverser can be built more tightly on the selector saving space. The coarse/fine is larger and needs a different mechanical construction, and drive. In either case the operation takes place when the contacts carry no current. This is realized by driving the pre-selector contacts through the motion of one of the main tap selector drive shafts. The pre-selector support plate is attached to the cage rings of the tap selector (see Fig. 4.46). The plate supports the pre-selector fixed contacts. The drive pin is attached to one of the selector drive genevas and rotates with it. It enters the pre-selector drive fork, rotating the contacts from one position to the other. After the movement is complete, the drive fork is locked in position by additional linkages and components, so that the moving contacts do not peripherally slide off due to transformer vibration. The locking system is deliberately not shown in Fig. 4.46, so as not to obfuscate the basic operation with too many details.

4.7.3 Construction of the Multiple Coarse/Fine Selector The application principle of the multiple coarse/fine selector is described in Sect. 5.5.8 of Chap. 5. In the multiple coarse/fine selector execution, the main tap selector moving contacts rotate several times round the circle. This enables the tap selector fixed contacts to be used over and over again, greatly aggrandizing the number of operating tap positions, resulting in a large number of different voltages. Figure 4.47 shows a 54 position tapchanger, with multiple coarse/fine, supplied by OLG for an aluminium pot line regulator (The yellow object is a stand not part of tapchanger). The principle is to attach a second selector with multiple fixed contacts to the basic tap selector structure. One of them is the regular fine selector (on the right in Fig. 4.47). The other, on the right, selects the “K” point where the pre-selector changes over (see Fig. 5.12). The need for selector switches and their timings will be made clear in Sect. 5.8.1.1 of Chap. 5.

4.8 Diverter Switch Construction for Line End Delta The standard three-phase star unit has no insulation at all at the selected tap, because this forms the neutral end of all phases. All phases are connected conductively together at the star point. In delta application, there is no point which is common to

4.8 Diverter Switch Construction for Line End Delta

(a)

(b)

(c)

Fig. 4.46 Coarse/fine selector arrangement and drive

119

120

4 Constructional Aspects of Tapchangers

Fig. 4.47 OLG tapchanger with 54 operating positions DIVERTER

GEAR BOX COARSE SELECTOR FINE SELECTOR STAND

all three phases. When the tapchanger is applied at the line end, or delta windings, a much higher degree of insulation between phases of the diverter switch becomes necessary. The diverter switching elements are then constructed as three physically separate units, insulated between each other. In Intank tapchangers the phases are located vertically separated from each other for the required insulation. The diverter length becomes high, due to the three switching elements, and their insulating separation. This makes for a significant difference in the size and complexity of the diverters of three-phase units. The length under oil of the diverter increases. This can be accommodated in diverter switches of relatively low voltage class. In high-voltage applications, other solutions are available as discussed below. These considerations apply specially to Intank diverter switch type construction.

4.8.1 Tapchanger for Delta Application As discussed in the preceding section, the height of the delta tapchanger rises rapidly with voltage class. Only a few Intank tapchanger manufacturers offer three-phase delta units. MR had a type D tapchanger arranged in a single column which was good for 20 kV application. For 66 kV the construction was already in two columns. One column housed the three diverter switches, and the second the three-phase tap selector. For higher voltage delta the solution was to use a tapchanger in three columns, each with one diverter switch at the top, and a single-phase tap selector at the bottom. The diverter could be of the standard three switching element type, with the phases connected in parallel, or one or two elements could be removed to make space for the transition resistance. This made the tapchanger compact. The tapchangers are driven by a common drive mechanism. The type D is not extant anymore, except for a special construction of D delta for 110 kV, described in section below.

4.8 Diverter Switch Construction for Line End Delta

121

4.8.2 A Special Construction for 110 kV Delta There was a demand for a large number of 110 kV delta tapchangers in South India, starting from about 1975. Three large institutional buyers embarked on an ambitious programme of installing every year several hundred intermediate level distribution transformers with 110 kV delta H.V. Their specification initially called for diverter switch type of tapchanger on the 110 kV side. This demand in their marketing bailiwick spurred MR to devise a tapchanger suitable for this application. Figure 4.48 shows a photograph of the application. The tap selector was implemented in two columns, each significantly shorter than the diverter column. One of the two tap selectors accommodated the odd taps, and the other the even taps. The diverter column accommodated the three diverter switches, stacked axially one over the other. The connectors from the diverter to the tap selectors lay in the transformer oil space. An angled insulated drive from the head drove the tap selectors alternately. The whole tapchanger was mounted on a common steel plate. While all the live parts were insulated for 110 kV test levels, the tap selector phases were insulated for about 80 kV test levels, so as to reduce the installation length under oil within the transformer tank. The tapchanger could therefore not be used at the line end of 110 kV delta, but could be applied at the electrical middle of the phases, where the required phase to phase insulation level is lower (see Sect. 5.9, Chap. 5). The tapchanger could be lowered into the transformer tank vertically through a large cut-out at the top. A drive mechanism mounted on the vertical wall of the transformer tank provided the drive at the tapchanger head, through the usual vertical and horizontal drive shafts and connecting bevel gear. The tapchanger was not exactly small, but smaller than a three single pole execution, which would otherwise be needed. The construction was so successful that over three decades several thousand units were manufactured and used. Fig. 4.48 MR tapchanger type D in special execution for 110 kV delta

HEAD DIVERTER MAJOR INSULATION FOR TAP SELECTOR

TAP SELECTOR

122

4 Constructional Aspects of Tapchangers

4.8.3 The AEG-Siemens Construction for 110 kV Delta AEG-Siemens was another active market participant in the geographic area mentioned in the previous section. Their solution was referred to as I + II; for the reason that the tapchanger consisted of a single-phase unit in one column, together with a two-phase unit in a second column. This solution is of course not unique, for all other tapchanger manufactures also had the same type of tapchangers to offer. The two-phase unit was an adaptation of a regular three-phase neutral end tapchanger, with one phase removed, and ground insulation increased. The ground insulation for both tapchangers corresponded to 110 kV test levels. Taps are placed at the line end of two of the phases. The two-phase tapchanger is applied at the vertex of the delta as shown in Fig. 4.49. The selected tap in each phase is connected together and forms the vertex of the delta. At the selected tap, the diverter phases do not need any Insulation. Non-selected diverter contacts of the phases are at one tap step voltage removed from the common point. The insulation between phases is now reduced to about one tap interval. Contacts of the tap selector far from the selected tap need moderate insulation between them. This could be provided as usual by the manner in which the tap selectors were constructed. A German Patent Application by K. Stenzel (of MR) for this application is shown in Fig. 4.50. It may be emphasized that the I + II solution absolutely needs taps at the line end, at least for two phases. The single-phase tapchanger could be applied anywhere on the third phase, but was usually applied also at the line end, to save on variety of transformer coils.

(a)

Fig. 4.49 Connection diagram tapchanger I + II

(b)

4.8 Diverter Switch Construction for Line End Delta

123

Fig. 4.50 Application of MR tapchanger MI + MII patent application 24 55 309 NOV 1974. K. STENZEL [8]

4.8.4 The Problem of Spare Coil in Some Winding Configurations Some transformer manufacturers were misled by the tapchanger connection diagram such as Fig. 4.49. They concluded that to comply with the tapchanger requirements they had to wind one of the three-phase coils in the opposite sense, i.e. right to left, if the other coils were left to right (see Fig. 4.51). It was usually the middle phase which was wound in the opposite sense. This resulted in the problem that they had to supply two spare coils, one wound in either direction, whenever spares coils were specified. The better solution is to wind all coils in the same direction and modify the interphase connections as shown in Fig. 4.52.

124

Fig. 4.51 Coil configurations for I + II

Fig. 4.52 Coil configurations for I + II

4 Constructional Aspects of Tapchangers

4.8 Diverter Switch Construction for Line End Delta

125

4.8.5 Problem of Selector Discharge in High-Voltage Delta When a set of three-phase tap selector moving contact leaves the fixed contact, the contacts lose electrically conductive connection to the windings. The potential of the contacts is determined by capacitive coupling of the transformer winding. Let us consider that the three odd selector moving contacts are in the current carrying condition. For the next tapchange the even contacts part with the fixed contacts (Fig. 4.53a) and so they lose the potential connection. In this illustration, the moving contacts of the RY and BR are in the operating position. Each of these lies equidistant, and similarly place with reference to the YB moving contact. The moving contacts of YB resistively and capacitive by symmetry pick up half the voltage between the fixed contacts of the RY and BR. Relative to the fixed contact of its own phase the voltage √ of the moving contacts is V · 3/4 (Fig. 4.53b). This is a very high-voltage just off the fixed contacts. It falls √ off when the contacts move away. The charging current can be estimated as (V · 3/4) divided by the capacitive impedance. Taking a 110 kV delta, the current is of the order of a few microamps. In spite of the high voltage the current is quite small. Compare with the capacitive currents calculated when the pre-selector breaks current (Chap. 7), which of the order of 50 mA. However this is enough current to make a discharge noise at the start and end of the selector contact movement. In practical cases the discharge is audible outside the transformer tank. In tapchanger configured as I + II this problem does not arise, because the moving contacts have to face only one tap voltage. In the special tapchanger design of MR shown in Fig. 4.48, all the odd contacts are in one column and all the even in the other. They have no chance of coupling to contacts of other phases.

(a)

Fig. 4.53 Capacitive pick up in H.V. delta tap selectors

(b)

126

4.8.5.1

4 Constructional Aspects of Tapchangers

Why Does This not Happen Even in Star Connect Three-Phase Selectors?

The suggestion of tap selector discharge causes a worry as to why the same mechanism does not cause a problem in ordinary three-phase star-connected tapchangers. In this case the potential acquired by the moving contacts is only half the step voltage. The problem is special to delta tapchangers.

4.8.6 Tapchanger Using Diverter Switch in Sheet Metal Execution Tapchangers in the most modern and accepted execution have a diverter switch encased in an insulated oil tight vessel, and the tap selector directly immersed in the transformer oil space. This construction has been commercially so successful, that it is possible to forget that other executions are possible. Some of the tapchangers described in the remaining sections of this chapter are not in use any more. The reason for presenting these ideas, even though they do not appear to be extant commercially any more, is that they are basically quite sound and have some advantages over the currently fashionable construction. Some early British designs, some of them possibly based on ASEA/ABB, were executed in sheet metal clad structure. These tapchangers were offered by AEI Metropolitan Vickers, as well as Fuller Electric Co. These covered applications up to 132 kV class. Figure 4.54 shows schematically the arrangement of the Fuller EHS tapchanger for 132 kV neutral applications.

Fig. 4.54 Fuller double compartment tapchanger type EHS in external sheet metal clad execution

4.8 Diverter Switch Construction for Line End Delta

127

Figure 4.54 is somewhat approximate. Details of Fuller tapchangers are no longer available. It is hoped that Fig. 4.54 covers most of the relevant essentials of the design. The most significant feature is the drop down tank, which can be manually ratcheted up or down by a built-in ratchet wheel, galvanized steel ropes, and guides. The tank engages in a U-shaped gasket in its upper position, sealing the diverter off from the atmosphere. A breather vents the gas produced by switching action to the atmosphere. When the tank is dropped down to its lower limit position, total access to the diverter is obtained. This is with no special tools or heavy lift!

4.8.6.1

The Tap Selector

The tap selector compartment was at the top. The diverter compartment hung below. The tap selector was housed in an outdoor sheet metal tank. The tap selector compartment had an open port, which could be mounted on a port opening on the transformer tank, with a gasket. Leads could pass from the transformer to the tapchanger through the opening. It was not necessary to provide for a terminal barrier board. However optionally a terminal barrier board, which was mountable on the transformer tank was provided. This enabled the tapchanger to be taken off for transport. Six Insulated Bushings were provided at the bottom of the tank, at the rate of two per phase, to terminate the odd and even slip rings. The tap selector drive shaft had an extension through oil seal, to drive the diverter switch, which hung below. The transformer manufacturer had to connect the tap leads on both sides of the terminal barrier board.

4.8.6.2

Diverter Switch

As stated above, the diverter switch is enclosed in its own oil tank. The switch used was a modified ABB V type switch (not to be confused with the MR Type V tapchanger), the type of which ABB and AEG-Siemens used for long even in their more modern Intank tapchangers.

4.8.6.3

Advantages of the Sheet Metal Construction

The sheet metal construction enjoyed the advantages of the compartment type tapchangers and at the same time offered the customer-friendly eases of inspection and maintenance, particularly of the diverter switch. Dropping a steel tank down, with a ratcheted winch, without any removal of a number of fasteners was a customer delight. The diverter could be “processed” like a transformer, in its own tank, to develop full electrical strength of the insulation. The tapchanger was delivered to the transformer manufacturer in a fully assembled and tested condition. This contrasts with the modern delivery, of the tap selector and diverter in separate packages to be assembled, and electrically connected, by the transformer manufacturer. Another advantage was that in cases of restricted transport dimensions, like often happens in

128

4 Constructional Aspects of Tapchangers

the relatively under developed infrastructure of some countries, the tapchanger can be taken off.

4.8.6.4

Insulation Levels

The sheet metal execution of double compartment tapchangers suits the neutral end application very well. The live parts have to be insulated from the grounded sheet metal enclosure. The test voltage levels to which the diverter switch, and the leads from the diverter switch to the tap selector are to be insulated are the ACSD test voltage of the transformer neutral end. These demands are low at the neutral end. The bushings provided to insulate the leads interconnecting the diverter switch and tap selector are quite small and compact. There are only six leads; the size of the separator plate is much smaller than a normal terminal barrier board. The live parts of the selector compartment will have to be insulated from ground for voltages only slightly more than the basic insulation level against ground.

4.8.6.5

Application to Line End Taps

If the taps are at the line end, or on a fully insulated delta winding, some difficulties arise. The insulation levels to which live parts have to be insulated to ground become quite high. In particular the six leads from the tap selector to the diverter switch need regular oil-to-oil condenser bushings, insulated for the line end test voltages of the transformer. Much of the charm of the sheet metal clad execution is lost.

4.8.7 Diverter Switch on Top of the HV Bushing A seemingly neat idea to avoid penetration of grounded sheet metal by H.V. leads was to mount the diverter switch on the H.V. bushing (Fig. 4.55). The diverter drive shaft, as well as the diverter to selector. Connectors are run inside the bushing mandrel. The connectors are at a maximum of one tap voltage from the mandrel, so that the insulation required from the mandrel, and from each other is moderate. The drive shaft and the leads must be taken into the diverter switch oil tank through oil sealed openings. The bevel gear housing is supported from the bushing mandrel and bottom shielding “bun”. It is at the line potential. The linking drive between the earthed shaft from the mechanism and between phases must be insulated for high voltages. Access to the diverter switch was excellent through the top cover plate. The concept however suffers from the following disadvantages: The mandrel had to be larger than standard bushing, which alters all the dimensions, e.g. the condenser layers.

4.8 Diverter Switch Construction for Line End Delta

129

Fig. 4.55 Diverter switch on top of H.V. bushing

1. Larger porcelain and bigger tank cut-out would be required. 2. It could easily be imagined that the H.V. porcelain manufacturers were not attracted by the prospect of making nonstandard bushings in small numbers. 3. The bushing is subject to higher mechanical stresses of diverter switch operation. 4. The H.V. bushings could be easily inclined outward from the transformer tank long axis. The bevel gear at the bottom allows this. But inclining the outer bushings away from the central, which is often required to obtain the air clearance required further complications in the drive. 5. The selector to diverter timing depended on a large number of linkages distributed over a wide volume. 6. A great deal of delicate work in aligning the common drive shaft of the phases, and proper bevel gear engagement, is left to the transformer manufacturer.

130

4 Constructional Aspects of Tapchangers

4.9 A Further Development of the Sheet Metal Concept There were two obvious disadvantages to the sheet metal execution of the tapchanger. The tapchanger tanks were very large, as they contained both the tap selector and diverter switch. The tapchanger manufacturer OLG in India improved on the concept in their prototypes. The tapchanger was mainly intended for 132 kV neutral end application. The entire tapchanger was built on a common steel plate, which would mount on the transformer flange. The diverter switch was mounted on one side, and the tap selector on the other side of a common plate (see Fig. 4.56). The diverter switch had a sheet metal tank and a built-in expansion chamber. The neutral bushing was mounted on the diverter switch tank. The star lead was made within the diverter switch and terminated in the tank mounted bushing. Six low voltage bushings were provided on the common plate, to connect the diverter and tap selector. The diverter could be easily taken off the common plate if required. The tap selector did not have a container. It was directly inserted into the transformer oil space. The tapchanger was intended for neutral end applications. This concept did not progress beyond the prototype stage, because market surveys showed no takers.

Fig. 4.56 Schematic of partly intank sheet metalclad tapchanger (OLG)

4.10 Energy Storage Devices

131

4.10 Energy Storage Devices Resistance tapchangers need an energy storage device to ensure completion of a tapchange without dependence on external drive means. This is to ensure that the transition resistance is not left in the current path for too long. The energy storage device could employ a charged spring, rotating flywheel, falling weight or pneumatic or hydraulic pressure.

4.10.1 Spring Drive Springs are the most popular stored energy sources in tapchanger mechanisms. Springs are compact, least expensive, and most reliable. The torque requirement of tapchangers is uneven. Considering rotary movement for example, a high initial torque is desirable, to accelerate the moving masses. Whenever contacts meet, the load increases suddenly. This may happen several times during a tapchange depending on the design and the number of contacts. It is not necessary that the drive torque should exceed the load torque at all points of movement.

4.10.1.1

Matching Load Torque to Drive Torque

Figure 4.57 shows as an example the drive and load torques for a spring energy accumulator. In the areas shown hatched, the energy already received from the drive and stored in the moving masses is used to keep the tapchange going, even though the drive torque is below the load torque. The movement merely slows down, but may not stop. At the start of the tapchange energy is absorbed from the storage device. A part of it is dissipated in overcoming frictional and other losses, but some is stored as kinetic energy. Due to both the very variable load and the dependence in part on the kinetic energy, the speed of transition is variable. It is only necessary that the tapchanger does not stop at any time due to lack of energy for continuing the drive. Completion of a tapchange can be ensured by packing sufficient energy in the drive before coupling the source of energy to the moving system. If too much energy is stored, the moving masses arrive at the end of a tapchange at high speed. This is also a problem, since the surplus unspent energy must be dissipated to bring the system to a halt. The energy required per tapchange may be variable due to: 1. Uneven contact alignment. This is particularly true of contacts mounted on flat insulating plate support. 2. Contact alignment also varies due to temperature, and the consequent yield of the supports. 3. A jump in load torque in executions if the pre-selector is driven by the stored energy. This happens for instance in the ABB tapchanger type UZDE/F. The

132

4 Constructional Aspects of Tapchangers

Fig. 4.57 Torque characteristics

energy storage drive does not provide sufficiently for this, the tapchanger may stop without completing the tapchange. 4. Variable friction because of the uncertain lubricity and variable viscosity of the surrounding oil. In particular at low temperatures, the high viscosity of the oil may slow down the movement considerably. 5. Development of higher torque demand due to change in dimensions of the insulating component with temperature and age. This expresses itself as a tightness of the bearings. 6. Lowering of energy demand at the initial stages of life due to the “running in” process. In spring operated mechanisms, the motor initially charges up the spring by rotating the charging crank (Fig. 4.58a). During this period the spring is not coupled to the contact system. When the spring is fully charged, it stores sufficient energy to take the contact system through one operation confidently. The mechanical arrangement is that the crank is rotated by the motor drive till full charge (Fig. 4.58b), after which the crank runs ahead. At this stage the mechanical linkages ensure that the charged crank is coupled to the geneva drive cam of the tapchanger drive. A little prior to the end of the tapchange the cam disengages from the contact drive geneva and locks it. Figure 4.58c shows the force diagram during spring discharge.

4.10 Energy Storage Devices

(a)

133

(c)

(b)

Fig. 4.58 Determination of torque characteristic

4.10.1.2

Driving Torque Produced by the Spring Mechanism

The torque equation becomes tractable if we assume that the spring is very long relative to the crank radius, i.e. the angle δ in Fig. 4.57 is nearly zero. Within this condition, at any value of φ the force of the spring is f = Sr (1 + cos φ) =

Fm r (1 + cos φ) 2

(4.1)

where S is the spring stiffness and F m the maximum spring force. The component of the force along the tangential direction is f sinφ, from Fig. 4.57c. This is the force which contributes to the torque. The radial component simply loads the bearings. The torque arm of the crank is rsinφ. The torque produced is therefore Fm 2 r (1 + cos φ) sin2 φ 2

(4.2)

Figure 4.59 shows the torque output. The following points can be noted. 1. The torque builds up very slowly. This is the period when we require high torque to get the moving masses going. 2. If near the start region, a big load occurs, due to say the moving contact climbing the fixed, the torque may be insufficient and the mechanism stalls. To get over this disaster we have only the option of increasing the maximum spring force, and consequently the energy content. This leads to the moving masses arriving at the termination point with a lot of unspent energy. This poses a problem of energy re-absorption.

134

4 Constructional Aspects of Tapchangers

Fig. 4.59 Variation of drive torque spring charged mechanism

3. If the motor drives the spring only just after the dead centre, a hesitant and slow start results. If the motor drives the spring well past the dead centre, the movement will start more briskly. 4. This results in variable timing of contact traverse. In particular if the cranking is manual, it is likely that the operator stops cranking just after the dead centre. Besides the speed of movement may be lower than motor cranking, so that less kinetic energy is available to get over the initial hesitancy. Due to this uncertainty it is inadvisable to manually operate a tapchanger when on power. 5. The contribution of torque, and therefore energy after mid travel point, is quite small. Nothing much is lost if the spring drive is de-coupled even at mid-travel.

4.10.2 Latched Spring Drive As a popular alternative to the rotary charging of the spring, and uncontrolled discharge at the dead centre, a latched drive is often used. Here an eccentric charges the spring by linear stretching. A latch mechanism from the diverter drive shaft holds one end of the spring from moving. The eccentric stretches the other end. When the spring is fully stretched, the latch is released by the charging arm, and the hitherto locked end of the spring collapses towards the other end, in the process driving the diverter shaft. The advantage of this system is that a high torque can be made available right at the start. The “firing” of target diverter shaft movement is precise, and repeatable, and independent of the spring charging speed. The downside is that the latch must always catch the load shaft at the end of its travel, and lock it. Because of the surplus energy available, the moving system hits some kind of end stop and rebounds with high velocity. The latch must be quick enough to catch the shaft. A failure of the latch

4.10 Energy Storage Devices

135

allows the load shaft to get past the open “latch gate”. On the next spring charging, the rear end of the spring is not restrained, but the entire spring moves slowly along with the load shaft at low speed, being directly driven by the motor or hand crank. The speed is not sufficient, and the transition resistances are likely to be damaged. There is no similar high speed criticality with the spring charged system described in the previous section.

4.10.3 Direct Motor Drive An advantage of the spring drive described in Fig. 4.58 is that if at any point the drive slows down or stops, there is a chance that the motor can catch up and give a further push. This results in a “slip stick” motion, which is slow. However the drive will not stop. There is a danger that the transition resistances heat up. A consistent and repeated failure of the spring to complete the tapchange is undesirable.

4.10.4 Flywheel Drive In very early tapchangers flywheels were sometimes used as the energy storage. The Fuller Electric tapchanger type EH is a typical example. Unfortunately no literature is available on this type of tapchangers anymore. The basic concept was that the motor speeds up a flywheel, with the tapchanger disconnected from it. When sufficient speed was reached a speed sensitive coupling engages the flywheel to the tapchanger drive. The advantage of the flywheel drive was that the system supplied torque “on demand” tidying over zones of high drive torque. Towards the end the coupling disengages, so that the extra energy of the flywheel is not a bother for the contact system. It is only necessary to reabsorb the kinetic energy of the contacts and the linkages. The flywheel has a smoothening effect on the velocity curve of the transit. It is used in conjunction with a spring store energy device in ABB UZD/E mechanism BUE, thereby combining the benefits of both systems of drive. Judging by the mass and speed of the flywheel of the earlier EH tapchanger, it looks as if too much energy was packed, so that both charging and discharging the flywheel took embarrassingly long, limiting the frequency of tapchange. It may be possible to reconsider the future in a more balanced and well informed way and use flywheels once again as the preferred drive system.

4.10.5 Falling Weights This system is a magnified version of the familiar grand-father clock drive. A weight is wound up to the top position by a motor, or by hand during manual operations.

136

4 Constructional Aspects of Tapchangers

The tapchanger is de-coupled at this time. After it is at the topmost position, it is held against a latch. When a tapchange is required, the weight is allowed to drop down, but now the tapchanger is coupled to the falling weight by a gear system. The weight mechanism produces a constant drive torque. Consequently the speed of contact transition is variable. The falling mass itself acts as inertia of the system to regulate speed and torque to a great extent. Towards the end of the tapchange, the weight comes to rest on a platform, leaving the moving contact system to freewheel to the end of its travel. This relieves the problem of re-absorption of excess energy from the contact system. Falling weight drives are not known to be in use any more with tapchangers.

4.11 Transition Resistances 4.11.1 Current Loading Transition resistances in tapchangers carry current only during a tapchange. In some operating sequences the resistance may carry two current pulses in one tapchange. The time interval between the pulses can be regarded as practically zero, as far as cooling is concerned.. The duration will depend on the speed of tapchange. Typical exposure to current is from 5 to 25 ms per pulse.

4.11.2 Magnitude of Resistance The magnitude is closely related to the switching duty of the contacts. If a very high resistance is chosen, the circulating current gets blunted, but the recovery voltage, which contains terms like IR and (E ± IR)/2 will become high. Here E is the step voltage, and I the tapchanger current. If the value is too low, the circulating current at bridging is high, with the consequent challenge of high current interruption. The value chosen by most manufacturers lies between E/I and E/2I. Transformers connected to “weak power supply”, i.e. high source impedance experience a flicker during tapchange, if the transition resistance value is small. Some users like to charge the transformer from LV and operate the tapchanger on no load as a procedural check. Such tests almost invariably cause a flicker, as the LV supply is weak. This is to be expected. During switching duty tests, with very high step voltage, it is advantageous to depress the transition resistance value to contain the restrike voltage and ensure arc quenching at first current zero.

4.11 Transition Resistances

137

4.11.3 Thermal Design The thermal design is practically guided by the requirements of cl. 5.2.5 of IEC 60 214. This clause requires measurement of the temperature rise of the transition resistance following a set procedure. 1. A number of tapchanges equal in number to one half of a cycle of operations, i.e. equal to travel from one end to the other of the tapping range. 2. A through current of 1.5 times the max. rated through current and at the relevant step voltage. 3. Such tapchanges must be performed at normal motor operating speed, and without intentional pause between tapchanges. The measured temperature rise above surroundings shall not exceed 400 °K for compartment type tapchangers and 350 °K for Intank.

4.11.4 Estimating Temperature Rise We can imagine the following picture during conditions of the transition impedance test according to cl. 5.2.5 of IEC 60 214. Each tapchange puts in a pulse of heat energy into the body of the transition resistance. It is reasonable to assume that because of the short duration of the current pulses, there is no cooling, and all the heat is stored in the body. In the period up to the next tapchange the resistance cools, and its temperature falls exponentially. If we restrict our attention to the periodicity of tapchanges in the transition impedance test, the fall in temperature can be taken as linear, as the time constant of temperature decay is in minutes, whereas the interval between operation is only about 3.5–6 s. If we ignore any change in the resistance with increasing temperature, the heat input for each tapchange is the same. The increase in (Fig. 4.60) temperature over the initial point of tapchange is the same for each tapchange. The cooling is however small at the beginning because of the low

Fig. 4.60 Temperature build up of transition resistance

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4 Constructional Aspects of Tapchangers

dissipation from the surface at low temperature rise. The temperature reached at the end of each pulse of current drops only slightly till the next pulse. Loss of heat from the surface increases with increasing temperature, so that the gain of temperature per tapchange decreases. Figure 4.60 shows the progress of temperature build up during a transition impedance test. It is possible to imagine that after sufficiently large number of tapchanges, equilibrium will be reached, where the temperature gain due to energy input is exactly compensated by drop in temperature during the cooling period. It must be emphasized that this equilibrium temperature may be too high for acceptance. In such a case it is important to restrict the number of continuous tapchange operations. It is not just the resistance wire, but the bobbins or other structures on which it is mounted also take part in the temperature cycle. It therefore not possible to make a exact calculation based on the resistance wire alone. A current density of the order of 60 A/mm2 survives the transition impedance test in normal tapchangers, i.e. ones with 16 operations during the test.

4.11.5 Measurement of Temperature of Resistance Wires The usual technique of estimating temperature rise by relating temperature with resistance, which is commonly used with transformer windings fails with resistance material. In the case of copper and aluminium the change of resistance with temperature (thermal coefficient of resistance) is substantial. This forms a convenient method of measuring resistance of copper or aluminium windings of transformers. The thermal coefficient of resistance wires is extremely low. Any attempt at measuring change of resistance and relating it to temperature change leads to substantial errors. Temperature of resistance wires is measured by application of thermo-couples directly on the wire. The variation of the output of thermo-couples with temperature is very small. Therefore thermo-couples are used with a corresponding amplifier and reader. The measurement system has a substantial time constant. Therefore thermocouples can not be used to dynamically measure fast changing temperatures, such as during a tapchange.

4.11.6 Transition Resistance Under Short-Circuit Current As mentioned the thermal design of the transition resistance is guided by the requirements of the transition impedance test Cl. 5.2 of IEC 60 214. Based on this it is possible to extrapolate what may happen if the transition resistance is involved in a short-circuit current. We take as an example a tapchanger designed to achieve a temperature rise above ambient of less than 400 °C after 16 tapchanges. The average temperature rise per tapchange is 25 °C. The preceding section discusses how the gain in temperature per tapchange falls from the initial value due to intermittent cooling. It is reasonable to take that the first shot may cause an increase of temperature

4.11 Transition Resistances

139

of about double the average, i.e. 50°. During short circuit, the current could be 8 (for large transformers) to 16 (small transformers) times the normal, depending on the transformer impedance. Taking the large transformer, and ignoring that the current through the transition resistance may be different from the transformer current during transition, the temperature rise during the first tapchange could be 82 × 50 = 3200 °C. The typical melting temperature of resistance wires is 1400 °C. Therefore the calculated temperature will not be reached, as the resistance will melt in the process. In smaller transformers, the situation is worse due to the higher short-circuit current relative to normal. Tapchanging under short-circuit current always damages the transition resistance.

4.11.7 Transition Getting Stuck with the Resistance Carrying Current We can use the same logic to estimate what may happen if the tapchanger gets stuck due to incomplete tapchange, carrying the full current. The current pulse lasts during normal tapchange for typically 30 ms, the time being adjusted for the component in the bridging condition on the basis of equal power input (equated I 2 R). The average temperature rise per tapchange in the first few operations is then is about 25 °C. The melting temperature of 1400 °C is reached if the resistance is left carrying current in 1400/25 × 30 ms or about 1.86 s. However measurements carried out by OLG on transition resistance assemblies designed at 60 A/mm2 show a survival of 4–6 s. The discrepancy is no doubt due to the heating up of the local structure which reduces the temperature rise of the wire. While the estimates quoted are not very conclusive, it may be estimated that the tapchanger may survive a considerable slowing down, but not outright stoppage.

4.11.8 Transition Resistance Failure Due to Loss of Pre-selector Synchronism The correct movement of the pre-selector with respect to selector position is set at the manufacturer’s works. Ordinarily this synchronism should not go wrong. In compartment type tapchangers there are two extraordinary situations when there could be a mishap, which turns out a catastrophic failure. One is outright mechanical failure of the drive to the pre-selector which is direct from the motor through its own gear train. A component failure here could lead to loss of drive. Secondly there could be circumstances when the factory setting is upset during maintenance work, e.g. changing of a leaky oil seal. Figure 4.61a shows the tap selector at the changeover point. The failure of the pre-selector drive does not affect operations till the tapchanger reaches the changeover point (Fig. 4.61a). Before commencement of

140

4 Constructional Aspects of Tapchangers

(b)

(c)

(a)

Fig. 4.61 Coarse/fine switch malfunction causes range voltage across transition resistance

switching between (Fig. 4.61a, b) the pre-selector should have changed over from 5 to 3 and 5 to 4. If the drive fails, the entire fine tap voltage gets applied across the transition resistance (Fig. 4.61c). In the bridging condition the current is high enough to cause resistance burning or serious damage. Similarly in the reverse direction the coarse tap would have been applied to the transition resistance.

4.11.9 Changing of Resistance Wire for Spare Tapchangers In small tapchangers, a situation often arises, when a tapchanger supplied on one transformer specification is desired to be used on another. If the ratings of the new transformer are lower, it should be possible to interchange the tapchanger. The result is as if the tapchanger is always used at partial load. No change of transition resistance is needed. If the new current rating is substantially higher, changing of transition resistance may be considered.

4.11.10 Some Typical Executions of Transition Resistances Transition resistances are made of metallic compositions of high specific resistance. In tapchangers Ni-chrome is almost always used, because of its high resistivity (typical composition has a resistivity of 106 µ-cm). Resistance materials such as Manganin or Konstantan which are preferred in some applications because of their solderability do not find application with transition resistances. Ni-chrome has the

4.11 Transition Resistances

141

disadvantage that it is not weldable nor solderable. Connections must be by mechanical means, such as crimping sockets. When resistance wires are terminated under a threaded termination, the high temperature of the wire is communicated to the thread. This may cause progressive distortion of threads and loss of contact pressure over a time. Development of poor contact at the termination is a frequent cause of transition resistance failure.

4.11.11 Typical Physical Arrangements 4.11.11.1

Resistance on Flat Plate Former

Figure 4.62 shows an arrangement in which the resistance wire is wound using an insulated plate as former. The plate is provided with ceramic edgings so that the wire is kept off the plate by oil gaps. This arrangement provides excellent cooling. It also prevents direct contact of the wire and former plate. The MR tapchanger Type C (now obsolete) and OLG ABS use this construction. Figure 4.63 shows an execution where the transition resistance wire is wound on a ceramic former. Several bobbins are connected in series to give the desired resistance value. This construction is safe even if the transition resistance temperature rises abnormally, as only ceramic is in the vicinity. The shape of the ceramic former maximises cooling.

Fig. 4.62 Transition resistance on plate former edged with ceramic

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4 Constructional Aspects of Tapchangers

Fig. 4.63 Transition resistance wound on ceramic former, OLG tapchanger RMV

4.11.11.2

Transformer Lamination as Resistance “Wire”

An interesting arrangement based on transformer core amination material as the conductor is employed by Hyundai tapchanger type RS. Figure 4.64 [9] shows the arrangement. In the picture at right, corresponding to a three-phase diverter switch, the resistance bobbins are mounted on top of the diverter. For the icon on the left, corresponding to a single-phase unit, the resistance bobbin is included within the

RESISTANCE WITH 3 PHASES

RESISTANCE WITH SINGLE PHASE SPRING

Fig. 4.64 Transition resistance execution in Hyundai RS 9 tapchanger

OUTPUT CONTACT SWITCHING CYLINDER

4.11 Transition Resistances

143

Fig. 4.65 Punched transformer lamination as conductor material for transition resistance

diverter cylinder. This execution has much to commend for it. Transformer lamination strips in long lengths, about 100–150 mm wide, are punched at regular intervals to produce a rise of about 1.5 mm on the surface (Fig. 4.65). When wound as a coil, these punchings provide for inter-turn oil flow. The very hard and thin insulated coating with which transformer laminations are supplied serves admirably for compact isolation. The large surface area and good cooling ducts between turns are a guarantee against very high temperature rise under abnormal operation. As transformer lamination scraps are available inexpensively, the whole construction is very economical.

4.11.11.3

Transition Resistance Made up of Coiled Coil Wire

In the Waukesha SPX tapchanger type UZD the resistance wire is used in a “coiled coil” form. It is wound on a moulded former and kept in place by a high temperature cord through the middle of the coils (see Fig. 4.66).

PHASE BOARD COMPLETE

PRESELECTOR FIXED CONTACT

ROTOR ARM COILED COIL TRANSITION RESISTANCE

Fig. 4.66 Waukesha SPX phase board assembly tapchanger UZD showing transition resistance made out of coiled coil wire

144

4.11.11.4

4 Constructional Aspects of Tapchangers

Fuller Electric Resistance Mats

An obsolete transition resistance was used in Fuller Electric tapchanger type EH. Unfortunately no published literature is extant. This was a huge resistance mat about 1.8 m × 0.6 m. Three such mats were supported at the port opening for mounting the tapchanger on the transformer. Even though the diverter used the high speed principle, using a rotating flywheel for stored energy, the resistances were designed to operate in the “tapchanger stuck” mode. Each phase resistance mat was provided with a temperature sensor mounted at a respectable separation, to take care of the high voltage. If the temperature became too high, due to the resistance remaining in the current path for too long, the transformer was tripped, This is not a situation acceptable today, when continuity of supply is paramount. O tempora, O mores (Cicero).

4.12 Contacts 4.12.1 Contacts for Selector Switches The contact drive is usually from one end through a geneva. At the end of the tapchange the geneva arrests and locks the shaft at the driving end. Due to the inertia of the moving masses, the contacts keep moving, twisting the shaft. This overshoot is mostly temporary, and the contact geometry reverts to normal by the untwisting of the shaft. A low contact mass and inertia are helpful. Such overshoots may be not be visible, as the contacts mostly return to the correct position after the tapchange. Figure 4.67 shows an illustrative example of contact overshoot in a two resistance selector switch pennant cycle tapchanger, and how to find it. The three signal resistances R are added to detect overshoot. Figure 4.67b shows the contacts stopping at their correct position without overshoot. The waveform of normal operation is shown in Fig. 4.67c. The intermediate changes of the waveform are not shown, as they are not relevant. In Fig. 4.67e there is an overshoot. Figure 4.67g captures the overshoot and the return to normal. In the condition of Fig. 4.67e the main contact opens the circuit, leaving the transition contact to carry the current. There will be arcing at the main contact with a current I at a recovery voltage of RI. As the shaft untwists there is a reversal to normal position, but the movement is relatively sluggish. All this does not lead to a tapchanger failure, but long arcing times and material loss. The situation is ameliorated by low contact mass, low speed, and high polar moment of inertia of the shaft. The last means a shaft of large diameter.

4.12 Contacts

145

(a)

(b)

(c)

(d)

(e)

(g)

Fig. 4.67 Contact overshoot in selector switch

(f)

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4 Constructional Aspects of Tapchangers

4.12.2 A Problem with Roller Contacts Roller contacts rotate on a central pin which is also made of high conducting material. Figure 4.68 shows a roller contact pressed against a flat fixed contact by a spring. The spring provides the normal contact force to enable current transfer. The current flows into the roller contact, and then transfers to the central pin under the same spring force, pressing the roller on the pin. The pin is rivetted to the contact carrier at the ends, and the current is finally taken off from the carrier by flexible braids (not shown). This arrangement has the advantage that the roller provides a great deal of surface for fresh contact as it rotates. The roller is rotated by the frictional force between the flat stationary contact and the roller. The rotation is opposed by the force of friction at the inner diameter of the roller. Under normal conditions the roller rotates. But if the roller develops high friction at the axis, rotation may cease. If the outer surface develops a flat, again rotation may stop, and the roller drags on the flat, behaving like a flat contact. Due to arcing on the roller outer surface, a small flatness may develop. In the course of time the flat area will widen and eventually block rotation altogether. There is no guarantee that a roller contact will rotate all its life.

Fig. 4.68 Current transfer in roller contact

4.12 Contacts

147

Fig. 4.69 OLG leaf spring arrangement for axial loading

4.12.3 Difficulty of Current Transfer to the Central Pivot During Arcing Condition The force between the roller and its axis of rotation is provided by the spring pressing the roller carrier assembly to the fixed contact. So long as the roller is on the fixed contact this force is effective. When the roller comes off the fixed contact it may still have to carry current through arcing between the contacts. At this time, there is no positive contact pressure for the current to transfer to the axial pin. This results in the development of several minor arcs between the inner diameter of the roller and the periphery of the pin. Extreme wear out of this interface can happen after a time. There are methods of putting on some axial pressure pressing the roller on to the carrier, so that the current may exit from the edge of the roller. Figure 4.68 shows a method. OLG however were not very happy with this arrangement as the spring cannot be seen. Their method of axial pressure, involving a leaf spring is shown in Fig. 4.69. This axial pressure once again reduces the tendency of the roller to rotate, and therefore must be carefully calibrated.

4.12.4 Edging Fixed Contacts with Tungsten Tips In rotary selector switches, arcing takes place as the moving contacts leave the fixed contact edge. In some tap selector executions the fixed contact edges are lined with tungsten copper inserts, as these are highly arc resistant. An example is seen in Fig. 4.70 (OLG tapchanger type ABS).

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4 Constructional Aspects of Tapchangers

Fig. 4.70 Tungsten edged fixed contacts of selector switch, OLG tapchanger type ABS

4.12.5 Contacts for Diverter Switches Arcing diverter swith contacts are almost always made of Tungsten or similar alloy to withstand erosion. Diverter switches almost always use non arcing shunt contacts in parallel with the main arcing contact. This allows for a fresh contact surface uninjured by arcing when tapchange is complete. Shunt contacts are often made of specially low resistance alloys. This reduces contact resistance in the operating position. The low resistance of shunt contacts is a very useful feature for carrying the through short-circuit current.

4.12.5.1

Contact Kinematics

In some diverter switches the contacts are driven by cams along guiding lots in such a way that their parting is parallel to the contact faces. In rotary switches this is achieved when the moving contacts part peripherally at the time of initial separation. In MR type M tapchanger, the moving contacts are driven along radial slots in top and bottom guide plates by a cam (Fig. 4.20). This method of drive achieves quickest separation and widest gap in the shortest time. This helps in arc quenching at first current zero. Both cases offer the possibility of arc chutes which can prevent the arc products from spreading peripherally.

References 1. SPX Transformer Solutions Inc. UZD-1211 (Rev.5/15) 2. ABB, On load tapchanger selection guide 1ZSE 5492-103En, Rev.4 3. MR Transform Campus (2013) Design, function, and operation of on-load tap-changers. Uwe Seltsam 4. Tech. Brochure “On-load tap changer type D: general manual”. Easun-MR Tapchangers (P) Limited 5. NGEF: on-load tap-changers operating and maintenance manual

References

149

6. SC ELECTROPUTERE SA Brochure. Power transformers and renewable energy. Romania 7. ABB Tech Brochure “On load tapchangers type UC and VUC”. ABB Publication 1ZSE 5492155.en/Rev 3 8. Application of MR tapchanger MI + MII. Patent Application Nr. 24 55 309 Nov 1974. K. Stenzel 9. Spare parts catalogue on-load tap-changers type RS 9.3. Hyundai Heavy Industries Co, Bulgaria. http://www.hhi-co.bg

Chapter 5

Selection and Application of Tapchangers to Transformers

Age cannot wither her, nor custom stale. Her infinite variety. W. Shakespeare, Antony & Cleopatra

5.1 Chapter Content In this chapter we consider the selection criteria and applications of tapchangers to transformers. The transformer is a flexible component in the power network. It can take many forms and shapes to satisfy the user’s requirements. Its companion the tapchanger follows suit. In this chapter we consider many “standard” applications, and some unusual executions of tapchangers. In some cases solutions for specific issues are found by unusual “out of the box” thinking. It is difficult to entirely separate constructional aspects from applications. There may thus be some overlap between Chaps. 4 and 5.

5.2 Which Winding Must Be Tapped? The most fundamental question in deciding on the tapchanger is “which winding must be tapped”? It is logical to provide taps on the winding which is connected to the incoming power, i.e. the primary winding. This allows the use of taps to regulate for varying incoming voltage, and keeps the core flux constant. This method of regulation is identified in the International Standard IE 60 214 as Constant Flux Voltage Variations, CFVV [1]. CFVV prevents the core from saturation when the incoming voltage rises. The output voltage of the transformer varies with load due to the leakage impedance of the transformer. There are further voltage drops in the transmission lines up to the next stage of transformation. Such voltage drops can be

© Springer Nature Singapore Pte Ltd. 2020 T. V. Sridhar, Application of Tap changers to Transformers, Power Systems, https://doi.org/10.1007/978-981-15-3955-8_5

151

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5 Selection and Application of Tapchangers to Transformers

corrected by providing taps on the primary of the next transformer. This scheme is followed in most parts of the world from the highest system voltage transformers down to the last distribution level.

5.2.1 The US Practice The US practice is to tap the secondary low-voltage output winding. There is a sound logic behind this concept. The purpose of the transformer is to produce a voltage that the user wants at its secondary terminals. Therefore taps must be on the secondary to produce, maintain, or vary this voltage according to the users need. The USA poses some extra challenges in tapchanger selection and application. These will be considered in the course of this chapter.

5.2.2 Advantages of Tapping the H.V Winding In most transmission, and distribution transformers, the primary is the H.V winding. Only generator transformers have the H.V on the output side, for connection by H.V transmission lines to the load centres. The advantage of tapping the H.V is that the current is smaller and more easily tackled by a tapchanger. Provision of taps on the LV has a problem in that the tap leads have to carry the high current. Making several heavy current connections to the low-voltage winding is an expensive and cumbersome process. Besides, the L.V winding has fewer turns. The taps are taken from a small percentage of the total winding to provide for a fine voltage regulation. For instance, if 1¼% of taps are needed, the tap turn interval is 1/80th of the total turns. If the L.V winding has 100 turns, tappings at such fine level are difficult. If a rounding off is used, the tap steps may not be uniform, which may not always be acceptable. Winding a coil, in which tappings occur very close to each other, presents a problem of winding a satisfactory coil. The coil has a natural tendency to unwind. It is necessary to anchor the taps to prevent such unwinding. These considerations are not important when the L.V. winding itself is of relatively high voltage, and provides a number of turns in the tap interval. In such transformers, it is not important which winding is tapped. This situation applies mainly to H.V autotransformers.

5.2.3 Generator Transformers For generator transformers, the secondary is the high-voltage side. The following remarks relate to modern high-rated generators, rather than small-scale generation, such a wind turbines, solar PV, microhydel, or other renewables. Generator transformers for bulk power from fossilised fuel, or atomic energy, have very high ratings.

5.2 Which Winding Must Be Tapped?

153

Tapping the L.V is almost impossible due to the very high current. Besides tapchangers for such high current may not be available. Taps, if at all provided, are at the neutral end of the star-connected H.V side. Taps cannot control the core flux.

5.2.4 Process Control Industries Many process control applications need high current, e.g. furnace and electrolysis. The number of turns on the L.V winding is then very small, and taking taps out of the L.V is a problem for a stable winding. In many applications, the H.V is tapped, and the tappings are used to control the output voltage. The flux in the core varies. Such regulation is defined in the standards as variable flux voltage variation (VFVV). The main disadvantage of VFVV is that the core is underused in many operating tap positions. The core is the heaviest and the most expensive component of the transformer. Under using the core makes the transformer expensive. We shall see some techniques employed to overcome the problems: Parallel connection of tapchangers is used to increase the current rating (Sect. 5.18), delta connection of the winding, and the use of a series transformer (Sect. 5.12.6).

5.3 Tapchanger Selection In selecting a tapchanger for an application to a transformer, the following parameters of the tapchanger need to be considered. These are discussed in detail in the sections of this chapter. 1. 2. 3. 4. 5. 6. 7. 8. 9.

Current. Voltage stresses. Number of steps and extent of the tapping range. Electrical location of taps in the winding. Interphase connection. Tapping arrangement. Physical location of taps. Diverter or selector switch switching capability. This is dealt with in Chap. 3. Operating temperature limits (see Sect. 5.21).

5.3.1 Current Rating Under constant kVA loading, the maximum current through a winding occurs at minimum voltage. This current forms the basis for selection of the tapchanger. In marginal cases it may be possible to exceed the current rating of the tapchanger,

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5 Selection and Application of Tapchangers to Transformers

subject to the switching capacity (Chap. 3), on the assumption that the tapchanger operates at the position of maximum current only for small part of its life.

5.3.2 Current and Contact Heating Current passing through contacts, and current paths, causes ohmic heating. The concern is to limit the temperature rise to acceptable values. Currents also cause forces. But, this is small at normal or slight overloads. The role of the short-circuit current in this respect is discussed in Sect. 5.3.4. When a tapchanger is connected to a winding, it naturally has to carry the current of the winding. The normal load current as well as any requirements of overload of the transformer has to be considered. Such currents primarily affect the temperature rise of components of the current path. The transformer Standard IEC 60 076-7 [2] specifies the over current duties of transformers. It is necessary for selecting a tapchanger for a transformer to ensure that all the loading requirements of the transformer are met by the tapchanger.

5.3.3 Raised Temperature of the Ambient When a tapchange is done, the heat produced by the transition resistances, by ohmic heating, increases the temperature of the surrounding oil, and consequently, also the contacts, and other parts of the current path. 1. Heat produced by the arcs. This heats up both the oil and the contacts directly. 2. When the contacts just close, the contact force is lower than at the operational point. As the transition proceeds, the contact force increases to the operating value, as the contact springs compress (or stretch, depending on the mechanical implementation), to the designed value. The contact resistance is, therefore, higher at the start of the current flow, which adds to local heating of the contacts. This effect is probably minor except at over currents. The effect of all these factors is only a short-time transient on the contacts and other current-carrying parts of the tapchanger, when the current is the normal current.

5.3.4 Short-Circuit Current Rating Short-circuit current-carrying capability is a main parameter in tapchanger selection. Short circuit on a transformer is a rare event. Tapchange transition, on the other hand, is a very quick event. The possibility of a short circuit occurring just as the selector switch or diverter switch is in the process of transition between taps is quite negligible. Therefore there is no requirement in the Tapchanger Standards to do a

5.3 Tapchanger Selection

155

Fig. 5.1 Transformer short circuit current waveform

tapchange when carrying short-circuit current. Chapter 4 in Sect. 4.11.5 shows that by accepted design practices, a tapchanger will not be able to complete a tapchange with the short-circuit current on. However the current has to be carried, without excessive overheating, possibly contact welding, and damage to the current paths by mechanical forces when the contacts are in “Position”. The current has a magnitude and duration. A transformer conforming to the International Standard IEC 60 076 [2] is usually required to carry the short-circuit current for two seconds. In some cases, the transformer user can make an exception and specify a three seconds shortcircuit current rating. This often is the case with furnace application. The short-circuit current is asymmetric for a short duration (usually a few cycles) following the onset of a short circuit. The standard peak asymmetric current has a value of 2, 5 times the RMS short-circuit current (Fig. 5.1) [3]. Without going into the details, it may be said that such peaking is in the nature of sudden short circuits in any inductive circuit, like a transformer. The peak current imposes an additional heating of the current paths, but equally importantly it causes higher mechanical forces on the components and supports structure. The contacts often fail due to thermal causes, by contact welding, or by severe erosion at the contact point. Mechanical forces on the member of the current path may cause permanent deformation. Tapchangers often use flexible connectors, typically copper braids, to facilitate current flow from moving parts to stationary parts. Braids can twist and distort due to mechanical forces exerted by the current. The forces may be high enough to cut the braids, at the point of entry to the end termination. It is important that the transformer, as well as the tapchanger connected to it is capable of withstanding the increased thermal and dynamic effects. IEC 60 214 requires that the tapchanger must have a short-circuit capability commensurate with its current rating (Fig. 1, Sect. 5.2.4, IEC 60 214).

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5 Selection and Application of Tapchangers to Transformers

Fig. 5.2 Transformer magnetising inrush current waveform [1]

The maximum short-circuit current of the transformer is limited by its own leakage impedance. Most usually, if a tapchanger is selected on the basis of the transformer current, it will meet the short-circuit requirement also. For transformers of small ratings, the short-circuit current may be higher.

5.3.5 Magnetising Inrush Current When a transformer is charged, there is an inrush current due to the core magnetisation process [3–10]. In some cases the magnetisation inrush current may have peaks comparable to the short-circuit current. The inrush current transient may last several seconds. However the current is highly non-sinusoidal with high instantaneous values in part of one half cycle and practically zero magnitude in the other half. Figure 5.2 shows a typical current waveform [5]. Thus the RMS value of Fig. 5.2 inrush is not as high as the peak value suggests, and the heating effects are mitigated. The forces would be high during the peaks of the magnetising inrush in the tapchanger. Fortunately, the very high peaks on inrush current only occur in transformers where the inner coil next to the core is charged. This is a rare, but not impossible case. Reference [6] discusses one such legitimate application. If such is the application envisaged, it may be prudent either to select a tapchanger with higher current rating and to consult the tapchanger manufacturer.

5.3.6 Overload Conditions The different kinds of permissible overloads on transformers are to be found in the IEC Application guide for oil-immersed power transformers IEC 60 076-7 Table 4 [6]. An overload factor of 1, 3 times the name-plate rating is applicable for larger transformers. The tapchanger must obviously meet the overload conditions of the transformer on which it is applied, without exceeding the limits imposed (e.g. temperature rise of contacts) by IEC 60 214 [1]. IEC 60 214 requires that the tapchanger should meet the temperature rise requirements of Sect. 5.2.2 when carrying current of

5.3 Tapchanger Selection

157

1.2 times the rated current. The conclusion is that if the rated current of the tapchanger matches or exceeds that the transformer name-plate rating, the overload requirements are nearly met, as far as the temperature rise is concerned. For smaller transformers, the overload current factor is higher. It is necessary to check that the tapchanger-rated current is adequate to meet the overload requirements of the connected transformer.

5.3.7 Other Limitations for Overload The tapchanger must be operable even on overload. Overload also affects the switching duties of the contacts. The duties are not determined totally by the current alone but the step voltage as well (see Chap. 3). As a number of factors are involved, such as the results of switching duty tests performed at different combinations of current and step voltage, reference to the tapchanger manufacturer is advised.

5.4 Voltage Ratings While considering the stresses caused by the voltage we have to consider the following types of voltages: 1. The normal frequency voltages that occur in service condition. 2. Voltage stresses during transformer testing. 3. Impulse voltages, caused by external events such as lightning. The Standards prescribe the waveform and magnitude of lightning impulse. 4. Switching impulse voltages. These are transient over voltages which occur due to switching in and out of equipment in the system to which the transformer is connected. The waveform and the magnitude are prescribed by the Standards.

5.4.1 Voltage Stresses Unlike a transformer where a winding has one voltage rating, we have to consider several voltages for a tapchanger. The voltage at which all the live parts are designated to operate is the voltage class of the tapchanger. The winding connected to the tapchanger throws voltage stresses to ground, and across several internal insulation distances between live parts which stand at different voltages to ground. Figure 5.3 Page 1 shows the internal insulation distances for a selector switch tapchanger. Figure 5.3 Page 2 shows the internal insulating distances for a diverter switch type tapchanger. Table 5.1 defines the internal insulating distances of Fig. 5.3.

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5 Selection and Application of Tapchangers to Transformers

(a)

(d)

(e)

(b)

(c)

(f)

Fig. 5.3 a–c Page 1: Internal insulating distances selector switch type tapchanger. d–f Page 2: Internal insulating distances diverter type tapchanger Table 5.1 Internal insulating distances of tapchanger, refer Fig. 5.3

Distance

Definition

f0

Between tapchanger output lead to ground

a1

Between adjacent selected tap in selector

a2

Between adjacent non-selected taps in selector

an

Over the fine tapping range

d0

Between open and closed contacts of diverter switch

b

Between output leads of different phases

C1

Over the coarse tapping range

C2

Between coarse and fine taps

5.4 Voltage Ratings

5.4.1.1

159

Some Notes on the Internal Electrical Withstand of Tapchangers

1. Where the tap selector contacts are uniformly pitched around the entire circle, the withstand between all adjacent contacts is the same. 2. In that case, the withstand over the range, distance an is the same as between adjacent contacts, distance a2 . 3. When a fixed contact is occupied by the moving contact, the withstand to adjacent contacts is usually reduced (distance a1 ). 4. In linear tapchangers, where there is no necessity to complete the contact circle, the end withstand is usually increased by omitting one or more contact, creating a larger gap. This is of course not possible when the selector moving contacts have to go round more than once. 5. In tap selector laid out linearly (e.g. MR. Type AVT) the withstand over the range is not compromised by the presence of contacts beyond the ends. 6. The withstand between adjacent occupied contacts of diverter switch type of tapchangers often is reduced by the diverter switch closed and open contacts (Distance d0 ). 7. In Pos 10 the entire fine range voltage appears across the fine range occurs between the changeover point K and contact 9. In Pos 17 the entire coarse voltage appears between K and 1. Thus the insulation must be sufficient to withstand the range voltage, and the corresponding impulse.

5.4.2 Considerations Which Affect the Voltages Generated by the Transformer It is essentially the transformer winding to which the tapchanger is connected which causes the voltage stresses across the insulating distances. These are influenced by the electrical and physical location of the taps. Other factors are winding arrangement, interleaving, and similar practices. Voltages occur to ground, and the internal insulating distances during normal operation, as well as during test conditions of the transformer. The withstand capabilities for these distances are declared for each model of tapchanger by the manufacturer in his published information. The published information covers the voltage withstand during the ACSD test Sect. 12.3, IEC 60 076 Part-3, and lightning impulse voltages. The ACSD test is also known colloquially as the induced over-potential (Overpot) test. The term Overpot is more evocative than the dreary acronym ACSD and will be used in this book. The tapchanger manufacturer usually does not declare normal operating voltage withstands, for the simple reason that the published voltage stresses determine tapchanger selection and application. The normal operating voltages are usually much lower. If a tapchanger is selected on the basis of the Overpot, it is deemed good enough. Transformer designers must estimate the voltage stresses of various kinds that the transformer throws

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5 Selection and Application of Tapchangers to Transformers

on the tapchanger and ensure the suitability. The operating voltages and Overpot levels are easily calculated, as they depend on turns ratio. Impulse voltages are more difficult to estimate. Sect. 5.4.4 elaborates further on impulse voltage stresses.

5.4.2.1

Exceptional Cases

In some cases of applications, the operating voltages may need to be considered. These are when the tapchanging mode is VFVV. Many instances occur when a tapchanger is used for furnace transformer regulation. These are sometimes small, low-voltage tapchangers, and operating on small furnaces. They may be of selector switch design. As an illustration we may consider an 11 kV tapchanger, which may be required to vary the untapped L.V from about 150–280 V by VFVV. It can be easily calculated that the taps contain 86.6% of the minimum turns. When the tapchangers is in the min number of turns position, the voltage induced across the tapping range is 9.53 kV. This is applied between the first and last contacts of the selector. The voltage may be lower than the Overpot withstand but is still cause for pause, since the voltage is not applied short time, but semi permanently. In general it may be prudent to consider the operating voltage also, and consult the tapchanger manufacturer if VFVV regulation is involved.

5.4.3 Tapchanger at the Solidly Grounded Neutral As mentioned, the connection and location of the tapchanger affect the voltages. A mild application is when the tapchanger is at the grounded neutral. The tapchanger can be connected to the neutral end of a winding only in star-connected transformers. Locations of taps for delta and autotransformers are discussed later (Sects. 5.2.1 and 5.4.3, respectively). The insulation level required of all parts to ground, i.e. the voltage class, of the tapchanger applied at the grounded neutral can be low. In highvoltage applications, exceeding 66 kV, it is permitted to use the concept of “Graded Insulation”. The line-end a.c. voltage steadily drops over the winding towards the neutral. It is enough if the tapchanger applied to the grounded neutral withstands the reduced voltage. Thus a 400 kV tapchanger is not required for a 400 kV transformer. The method of H.V testing of the transformer has an influence on the tapchanger voltage class. The manner in which the testing a transformer is a matter which relates to the transformer, and need not normally be detailed here, except for the fact that the test connections and voltage level influences the ground voltage and other voltage stresses of the tapchanger. For the solidly grounded star transformer, we shall confine the discussion to the most commonly used three-phase star-connected transformer connection shown in Fig. 5.4. This connection suggested by Sect. 12.3 of IEC 60 076 Part 3 (Refer also to Fig. 2 of Sect. 12.3 of IEC 60 076) enables the tested terminal of the winding to be raised to the appropriate test Level, without the voltage between

5.4 Voltage Ratings

161

Fig. 5.4 Overpot test connections for star

(a)

(b)

the phases exceeding the same value. This again is a stipulation of Sect. 12.3 (a) of the Standard.

5.4.3.1

An Illustration on the Calculations of Test Levels

As an illustration for determining the voltage distribution Fig. 5.4 is marked up for a 220 kV star-connected transformer. The Overpot level at the 220 kV line is 395 kV (Table 4 of IEC 60 076). The transformer is taken to have 16 equal steps of 1/4% turns in the tapping section. The neutral voltage is 131.67 kV. The voltage induced in the tested phase is 263.3 kV. If the test is conducted at the “all turns in” position of the winding, the farthest tap terminal of the tapchanger rises by 47.87 kV over the

162

5 Selection and Application of Tapchangers to Transformers

neutral for a 20% tapping range. The voltage to ground of this terminal is, therefore, 179.54 kV. It can be shown easily that this is the connection that gives the highest possible voltage of a tap terminal. The tapchanger must withstand this level. This is not a “Standard” test voltage for defined voltage classes of H.V transformers. So we select a 110 kV (System highest voltage 123 kV) class tapchanger for this application. The corresponding Overpot test level for this tapchanger is 230 kV. The ground voltage for other transformers with a different Overpot level, or tapping range, may be worked along the same lines. In general the voltage class of the tapchanger at the graded neutral end is lower than that of the transformer. Thus a 400 kV star-connected transformer does not use a 400 kV rated tapchanger.

5.4.3.2

Voltage Between Phases

Voltage between phases for tapchangers at the neutral is not usually a major concern. For the same transformer considered as an example in the calculation of the Overpot test voltage distribution in Sect. 5.4.3.1 above, the maximum interphase voltage is seen to be 71.8 kV. This is not very high but it is neither a negligible figure.

5.4.3.3

Single-Phase Transformer for Three-Phase Star-Connected Bank

The method of connection of Sect. 23.3 of IEC 60 076 Part 3 is not applicable in this case. The line end can be raised to the full Overpot level with the neutral grounded. The stipulation of Sect. 12.3 (a) is not transgressed, as there are no other phases. The Volts/Turn becomes much higher than in the three-phase case. Instead of the two-thirds Overpot voltage, the full voltage is induced in the tested leg. The volts per turn become 1.5 times. This puts a stress on the winding insulation. If interleaving is involved, the interstrand insulation can be very high. As far as the tapchanger is concerned the voltages per step, over the range, and other insulation distance become 1.5 times also. The neutral voltage can be raised to some level, by an auxiliary transformer as shown in Fig. 5.5 to alleviate the high volts per turn.

5.4.4 Voltage Stresses Due to Impulse Voltages Apart from the normal operating and Overpot voltages, we have to also consider the impulse voltages that the winding imposes on the tapchanger. The impulse voltages are due to lightning impulse test and switching impulse tests.

5.4 Voltage Ratings

163

Fig. 5.5 Overpot connection single phase for star bank

5.4.4.1

Lightning and Switching Impulse Voltages

It is easy to estimate the distribution of voltages through the winding for frequencies at Overpot test because the distribution is decided by the turn’s ratio. This is also approximately true of switching impulse, which has a relatively low rise time. For lightning impulse condition, the voltage distribution along a winding is determined by the distributed inductance and capacitance networks. These depend on the dimensions and the distribution of turns, the insulation, breaks or changes in the equivalent inductance-capacitance network, shielding of conductors, interleaving, and such factors. It is not possible to provide an exact formula or methodology as was done above with Overpot. The distribution has to be worked out in each case. Transformer designers are equipped with tools of analysis for such estimation, and will carry out such an analysis to match the declared values of the tapchanger withstands, against what may actually obtain in a particular case [3, 8–10] give more details.

5.4.4.2

Some Generalities

Even though the exact distribution of impulse voltages must be carried out on a case-to-case basis, some general conclusions are valid. 1. Impulse voltages concentrate at the line end. These greatly enhance the insulation requirement of a tapchanger applied at the line end. 2. Impulses applied at the line end of a winding are attenuated towards the neutral. This is particularly true of the rise time of the impulse. This moderates the impulse voltage stresses at the neutral. 3. If there is a break or change in the R-C ladder network associated with a winding, a high impulse may occur at the point of discontinuity. Examples of such changes are tap breaks, changes in winding conductor insulation, increase or decrease of spacer thickness between disks in a disk winding, termination of interleaving.

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5 Selection and Application of Tapchangers to Transformers

These are problems for the transformer but so long as such discontinuities are not within the tapping zone, these are not a concern for the tapchanger. 4. For a delta-connected transformer, extremely high-impulse stresses may occur at such breaks of continuity, e.g. an increased insulation at the tap break in the winding. These may affect the tapchanger. Taps in delta-connected transformers are discussed in more details below.

5.4.4.3

Results of Some Studies and Estimates from [11]

Reference [11] introduces the useful concept of impulse ratio. This is helpful in relating the impulse stress to the voltage occurring across an insulating distance under linear voltage distribution. Impulse ratio = Impulse voltage occurring across an insulating distance as percentage of line-end impulse applied/ Percentage line-end voltage occurring across the same distance under linear distribution. Reference [11], Sect. 4.1.2 covers impulse ratio for specific cases of tapchanger connections and location of taps. For a more detailed discussion, consult [11]. The impulse ratio for neutral-end taps, using a coarse/fine tapping arrangement is estimated to be 3.5–5 across different insulating distances. The highest is across the coarse and fine taps connected together in series by the coarse/fine switch. At a value of 5 this means for a 20% tapping range, this insulation distance is stressed at 100% of line-end impulse. For selector switches and tap selectors this distance is “an ” in Fig. 5.2. In compartment tapchangers this could be a severe stress area, as the contacts face each other, and the insulation distance consists of creepage over flat surface of the phase board. The impulse stress can be reduced by measures such as interleaved windings and other methods of increasing the series capacitance. For Intank taps selectors, this insulation distance consists mainly of an oil gap. This structure is better suited for such high stresses. Reference [11], Table 4.1.2 also gives the range of impulse stresses for a reversing regulating winding at the end of the series coil of an autotransformer. Here also the impulse ratio is comparable to the neutral-end coarse-/fine-tap selector. Unpublished studies of impulse distribution with RSO on 33 and 66 kV deltaconnected windings, with coarse/fine selection, conducted by Hackbridge-Hewittic & Easun Ltd., show that apart from similar stresses as mentioned above within the internal tapchanger insulating distances, the impulse to ground from the ends of the tappings can reach 120% of the line end.

5.4.4.4

Impulse Condition at Fully Insulated Neutral

When the neutral of the transformer is either not grounded or grounded through earthing impedance (Earthing Resistance/Reactor/Petersen Coil), the user often specifies a higher degree of Insulation level to ground. Most usually, the neutral is taken either by cable or in air over post insulators from the transformer terminal to the grounding impedance. This line is short, and lies within a relatively protected substation

5.4 Voltage Ratings

165

environment. Lightning impulse strokes are unlikely. But switching impulses may well occur. Many buyers call for impulse testing, both lightning and switching, at the neutral. This requirement turns the standard situation upside down, because the taps are now effectively at the line end. This application therefore calls for all considerations applicable for a line-end tapchanger, as far as impulse voltages are concerned. In particular, the impulse withstand between phases may turn out a critical decider.1

5.5 Number of Taps and Tapping Range The features which influence the selection of tapchangers were listed in Sect. 5.1. The number of tap steps can be achieved in the following three ways.

5.5.1 Linear Tappings Figure 5.6 shows tapping arrangement according to the linear scheme. A sixteenstep tapchanger is taken as an example. The table shows the connections made by the tapchanger at different-numbered taps. This drawing is vital for the transformer constructor. It tells him which transformer lead is connected, where in the tapchanger at each position, and how many taps are included at that position. The number of leads to be brought to the tapchanger from the winding is one more than the number of taps in the tapping range. In addition there is the connection from one common output point of the tapchanger, lead No. 20 in Fig. 5.6. Tap No. 1 is the position with the maximum number of turns in the winding. When the primary winding of the transformer is tapped this is the position for the maximum incoming voltage. The principal tap is the position where the nominal ratio of the transformer is obtained. Linear-tapping arrangement is simple to comprehend. The linear arrangement needs more tap leads than the other connection methods discussed below. It is more expensive in terms of the lead cost, and the time spent in connecting all the leads to the winding. Only linear taps can be used in some transformer connection schemes, e.g. the potentiometric arrangement for autotransformers. See Sect. 5.12.3.

5.5.2 Insulation for Tap Breaks Attention is drawn to the insulated gap marked “Tap Break” between the tapping zone and the untapped main zone of the winding. At any tap position, the voltage induced in the tapping zone between the wander lead and the free end appears across this gap. The voltage peaks when all the taps are left out of circuit. For industrial 1I

am grateful to Mr. M. L. Jain, Emco for bringing this matter to my notice.

166

5 Selection and Application of Tapchangers to Transformers

(a)

(b)

Fig. 5.6 16 step linear taps

5.5 Number of Taps and Tapping Range

167

frequency voltages the voltage across the tap gap can be calculated from the volts per turn of the transformer. Under impulse conditions the free ends oscillate and very high voltages may occur. These may be estimated from available data base, or by analytical methods available to the designer [3, 8–10].

5.5.3 Linear Taps for VFVV For VFVV, the tap turn interval changes. The number of turns N at the principal tap is determined from the voltage ratio. We assume that the VFVV is used to change the voltage of the untapped winding by equal % of the principal ratio. At any tap, where the L.V voltage needs to be p% more than at the principal tap, the number of turns in the tapped winding is N/(1 + p/100). This equation can be used to generate the tap turn interval at every tap. For most practical cases, the deviation from the linear case is not much. For instance, let us consider a L.V variation of +15 to − 5% of the nominal. For the +15% position, we need the min number of turns on the tapped winding given by the above formula as Nmin = N /1.15 Likewise the max turns is N max = N/0.95. The total turns in the tapping range are Nmax − Nmin /N as a fraction of the nominal number of turns = 0.183 N or 18.3% of N The number of turns in the tapping range is 18.3% of nominal with VFVV. It would have been 20% for CFVV. The difference is not great. If the total number of turns in the winding is small, ratio errors could creep in due to rounding off to the next integral number of turns. The problem is sharper for a wide range of LV as discussed in Sect. 5.4.2.1.

5.5.4 Coarse/Fine Tappings The coarse/fine switch is one of the two methods which enables the tap selector contacts to be used twice during a traverse from one end of the tapping range to the other. The second method is the “Reversing Taps” which is discussed in Sect. 5.6. Both these approaches enable the generation of about double the number of different voltages from a given-tapped winding. The tapchanger needs to be equipped with an additional switch, called the coarse/fine selector. The coarse/fine selector is electrically a part of the tap selector and is driven in proper synchronism with it. The

168

5 Selection and Application of Tapchangers to Transformers

physical execution of the coarse/fine switch is discussed in Chap. 4. As far as the mode of electrical functioning of the coarse/fine is concerned, selector switch and diverter switch tapchangers, as well as Intank and compartment are similar.

5.5.4.1

Connections and Operation of the Coarse/Fine Switch

Figure 5.7 shows the connections for use with the coarse/fine tapping mode. For illustration, an eight-tap fine-regulating winding with an eight coarse is shown. The principle is however applicable to any other number of steps in the winding. The coarse can have one more tap step than the fine if required, as will be seen shortly. The coarse/fine switch is shown schematically within the cyan-coloured rectangle in Fig. 5.7. It is a two-position break before make switch, which connects the end terminal 5 of the fine selector to either terminal 4 or 3 of the coarse. In Fig. 5.7, one end of the fine tapping range (terminal 5) connects to the end of the coarse at tap 4. The wander point 14 of the selector selects contact 13. This position includes the maximum number of turns of the tapped winding. During tapchange the selector arm rotates clockwise, dropping one tap at each position, till it reaches position 9A, when all the taps of the fine taps are omitted from the circuit. See last column of Table in Fig. 5.7. The entire coarse tap of 8 taps is in circuit. The positions 9A, 9B,

Fig. 5.7 Coarse fine tappings in middle of winding body

5.5 Number of Taps and Tapping Range

169

and 9C are shown in Fig. 5.8a–c for clearer explanation of the mechanism of the coarse/fine. In Fig. 5.8a, Pos 9A, all the turns of the coarse are included in the circuit, but none of the fine. There is no change of turns as we go to Pos 9B (Fig. 5.8b). But importantly, the fine tap wander lead of the fine selector gets into Pos. 4 where no current passes through the coarse fine switch. Between the positions 9B to 9C, (or

(a)

(b)

(c)

Fig. 5.8 Current paths in coarse fine

170

5 Selection and Application of Tapchangers to Transformers

9B to 9A in the reverse direction) the coarse/fine changes over at no current. The current switches from the coarse winding to the fine during the subsequent switching of the fine selector switch between 9B and 9C (Fig. 5.8b, c). In the reverse direction switching is effected by the fine selector switch between positions 9B and 9A. The coarse fine switch is thus aptly named, as it facilitates interchange of the coarse and fine tappings into the circuit. However the coarse fine switch does only an offcircuit selection. It does not switch current. There is no change in voltage in the three positions 9A, 9B, and 9C. There is a change in voltage at 10 again after this pause at 9. After position 10 the selector continues to rotate clockwise, once again dropping one tap at each position, till it reaches pos 17, where all taps are omitted. It is seen that the tapchanger reaches 19 positions, but gives only 17 different voltages. Therefore, there are two “extra” positions where the number of turns and, therefore, voltage are the same.

5.5.4.2

Some Important Points of Coarse/Fine Tapping Arrangement

The following points of importance may be noted. 1. From Fig. 5.8 it is seen that the range voltage occurs between the changeover contact number 4 and the contacts on either side. 2. Selector moving contact goes over the fine contacts twice, contacting the fine taps in turn. In a selector switch, the fixed contact pitch must allow bridging by the moving contacts. This means uniform pitching of the fine tap contacts around the circle. The first and last contacts of the selector switch have the same physical gap as any other pair. Voltages withstand between adjacent contacts and over the range are same. Arising out of point 1 above the voltages withstand between adjacent contacts, with one of them occupied by the moving contact must be the range voltage. This is unlike the linear selector, Sect. 5.5.1. 3. It is not possible to use an arrangement in which selector-fixed contacts lie physically in a line. The contact circle must close. 4. The tap selector reaches 19 positions in its travel, but the number of different voltage is smaller. This is because the positions 9A, 9B, and 9C are used to manipulate the coarse/fine switch, so that it switches no current. Either one or two positions may be “lost” as far as changing voltages is concerned. Such positions are referred to as “mid positions”. Surprisingly the Standard IEC 60 214 does not define such a term. The author feels that the most appropriate method of designating the connection scheme of Fig. 5.7 would be 10.17.2 G (G stands for Coarse/Fine Selector, German Grobwähler). The way in which to read the designation would be: 10 position selector, 17 different voltages, 2 extra positions, using a coarse/fine scheme. Others, including MR call this 10.19.3 G which leaves the user perplexed, as the number of different voltages obtained is not explicitly stated. 5. At some point of operation, the coarse and fine are interchanged into the active current path. This happens when the selector switch changes from Pos 9B to

5.5 Number of Taps and Tapping Range

171

Pos 9C. In the reverse direction it occurs between 9B and 9A. The transformer current shifts from the coarse to the fine. This will be opposed by the leakage reactance between the coarse and fine winding. It was remarked in Chap. 3 earlier that the interrupted current and the recovery voltage in a resistance tapchanger are in phase. Now there is a reactance in the loop, which produces difficult conditions of arc extinction. The tapchanger could possibly fail by the mechanism of Fig. 1.6. Tapchanger manufacturers request the transformer builder to supply them information on the leakage reactance to verify that the tapchanger selected can handle the application. A more detailed analysis of current commutation between coarse and fine is presented in Annexure 5.A. 6. There is a position during the transition of the moving contact of the coarse/fine switch, when the fine winding is not connected to the rest of the Transformer, see Fig. 5.9. Figure 5.9 is an extract from Fig. 5.8 showing only the condition under discussion. At this position, the fine tappings do not have a fixed potential. The effect of the loss of potential and the manner in which this can be combated is discussed more fully in Chap. 6.

Fig. 5.9 Loss of definite potential of fine winding

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5 Selection and Application of Tapchangers to Transformers

5.5.5 Method of Obtaining the Maximum Different Voltages In the examples so far a tap selector with ten-pitch contacts was used to get 17 different voltages. The same selector can be used to get a maximum of 18 different voltages. Figure 5.10 shows the arrangement required. A point worthy of cautious note is that the coarse section needs one more tap than the fine. This does not mean that the total number of turns in the winding has to be increased by one tap. The reasoning is as follows. The transformer designer calculates the nominal, maximum, and minimum turns of the tapped winding. The difference of turns between the maximum and minimum is divided into 17 equal tap turn intervals. Nine intervals are assigned to coarse, eight to fine. The reminder goes into the main winding. He does not change the maximum turns of the winding to provide for the extra turn interval in the coarse.

(b) (a)

Fig. 5.10 Coarse/fine selector with one extra tap in coarse

5.5 Number of Taps and Tapping Range

173

Fig. 5.11 9 pitch selector applied to 8 fine taps

5.5.6 Selector Having One Pos. Less Than the Number of Leads from the Fine Tappings Figure 5.11 illustrates the situation when the number of the fine-tap selector positions is equal to the number of fine-tap leads. This situation applies for instance to a 9 position selector applied to an eight-step fine winding. The coarse step contains not eight but nine taps. Initially it may be thought that this may mean that the winding contains on extra tap turn interval. This is not so. The extra tap interval in the coarse is merely “borrowed” from the main winding, and is returned at the last position, when one tap of the fine is retained in circuit. (Pos 17 contain one tap of the fine in circuit.) There are no extra turns in the winding.

5.5.7 Insulation Requirement at Tap Breaks The coarse/fine needs an insulated tap break on either side of the fine winding, due to voltage occurring across the gap. The voltage appearing across the tap break at the top in Fig. 5.8a is zero when the coarse/ fine switch connects 5–4 and is the voltage of the coarse tap section when 5 connects to 3. The tap break at the bottom has a

174

5 Selection and Application of Tapchangers to Transformers

maximum voltage across it at the “all taps out” position. The tap breaks must be sufficiently insulated for the voltages that may occur. These voltages are calculable for normal conditions from the volts per turn of the transformer. Under impulse conditions much higher voltages occur. These may be estimated from available data base, or by analytical methods available to the designer [3, 8–10].

5.5.8 Multiple Coarse/Fine Selectors It is possible to extend the principle of coarse/fine selection to a tapping scheme using many coarse tap sections, and one fine-tap section. There are industries like Aluminium Extraction Lines and Electronics manufacturing, which need a wide and fine variation of voltage where tapchangers with a large number of taps find application.

5.5.8.1

Principle of Operation

The principle of multiple coarse selectors can be deduced by an examination of Fig. 5.12. The requirements are 1. One end of the fine selector is connected a to multi-way switch (Lead No. 6). which can select one of the two terminals of each coarse section. This selection takes place through the switch PS1. This selection takes place when the fine selector moving contact directly connects to the coarse section at Pos K. Under

Fig. 5.12 Multi coarse selectors scheme

5.5 Number of Taps and Tapping Range

175

this condition, no current flows through the selector PS1. The changeover of the switch PS1 is an off current operation. 2. A terminal on the fine selector circle, designated as “K”, at which the selector moving contact makes directly on to the end of the coarse section. In this position current passes from the coarse to the output terminal. There is no current through the fine switch. 3. When more than one coarse section is used, one more switch (PS2 in Fig. 5.12) is required to connect the relevant terminal of the coarse section to contact K on the fine selector contact circle. PS2 does not carry current in any position other than when the fine selector is at K. Therefore PS2 can move and make the required connection at any point other than when the fine tap is at K. 4. Only two coarse taps are considered for illustration of the concept. Though a selector switch is shown in the illustration, the principle is applicable both to selector switches and tapchangers with diverter switch. Table in Fig. 5.12 shows the sequence of movements of the switches involved. The movements of both PS1 and PS2 take place when they are not carrying current. It is seen that 13 different voltages are achieved. There are two extra positions reached by the fine selector, at which the voltage does not change. The author would designate this scheme as 05.13.2 MG. MG stands for multiple coarse/fine selectors. This designation does not have universal acceptance. Photograph of Fig. 4.47 shows a diverter switch tapchanger of OLG make giving 54 different voltages. It is in use with a regulating transformer for aluminium pot line of the Madras Aluminium Co Ltd. The tapping scheme has four coarse sections. OLG also supplied in 1996 a 100 Pos. 33 kV tapchanger as a possible alternative for the Brentwood Regulator, to Andrew Yule and Co Ltd.

5.5.9 Possible Difficulty with Transition Impedance Test, Sect. 5.2.5.1 of IEC 60 214 [12] The transition impedance test is to establish the temperature rise of the transition resistance. Essentially the tapchanger is continuously run without intentional pause, for one half cycle of the range of taps, when carrying 1, 5 times the maximum-rated current, with the drive operating at normal speed. The temperature rise at the end of the test is limited by the Standard. For normal tapchangers without multiple coarse sections, this is not a difficult goal. The transition resistance would be dimensioned by the manufacturer to meet this test. But when multiple coarse sections are used, the number of continuous operations required may be quite large. It may be useful to discuss the issue with the tapchanger manufacturer.

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5 Selection and Application of Tapchangers to Transformers

5.6 Reversing Tapping Arrangement The reverser switch is one of the two methods which enable the tap selector contacts to be used twice during a traverse from one end of the tapping range to the other. The other is the coarse/fine switch, already discussed. Both these approaches enable the generation of about double the number of different voltages from a given-tapped winding. The tapchanger needs to be equipped with an additional switch called the reverser switch. The reverser switch is electrically a part of the tap selector and is driven in proper synchronism with it. The physical execution of the reverser switch is discussed in Chap. 4. As far as the mode of electrical functioning of the coarse/fine is concerned, selector switch and diverter switch tapchangers, as well as Intank and compartment tapchangers are similar.

5.7 Method of Connection and Operation The reverser can connect the tap winding to the main in boost or buck mode. Figure 5.13 is an illustration, with an eight-tap fine-reversing regulating winding. The principle is however applicable to any number of steps in the fine tap. It is shown schematically within the green-coloured rectangle in Fig. 5.13. It is a twoposition break before make switch. In Fig. 5.13b it connects in such a manner that the voltages of the main and taps add. When the selector switch is at 12, all turns of the winding are added in the same direction, giving the maximum voltage. Table of Fig. 5.13 shows this. From this position, the selector arm rotates clockwise, dropping one tap at each position, till it reaches position 9A, when all the taps of the fine selector are omitted from the circuit. Figures 5.13b–d would be helpful for clearer explanation of the mechanism of the reverser. In Pos 9A the entire fine tap is out of circuit (Fig. 5.13b). All the effective turns are from the main winding. As we proceed to Pos 9B there is no change in the number of effective turns. But more importantly, the fine selector reaches Pos 3 where the current flows directly from the main to the output, without using contacts 3 and 4 (Fig. 5.13c). At this position the reversing switch can changeover off current connecting 3–12 instead of 3–4 (Fig. 5.13d). current is switched into the fine-tap winding by the action of switching of the fine selector from Pos 3 to Pos 12. The reversing switch is aptly named because it reverses the connection of the tapped winding in relation to the main. However the reverser is only an off current selection. It does not switch any current. In all the three positions 9A, 9B, and 9C the number of effective turns, and therefore the output voltage remains unchanged. It is only at Pos 10 that a new output voltage is seen, Table of Fig. 5.13. This is one tap connected in the reverse direction. From this position the selector continues clockwise, gathering one tap more at each position in the reverse direction, till it reaches Pos 4 where all the fine taps are in circuit, in the bucking direction. There is no change in voltage in the positions 9A, 9B and 9C.

5.7 Method of Connection and Operation

177

(a)

(c)

(b)

(d)

Fig. 5.13 Reverser application

178

5 Selection and Application of Tapchangers to Transformers

They are referred to as “mid positions”. The tapchanger reaches 19 positions, but only 17 different voltages.

5.7.1 Some Important Points of the Reverser-Tapping Arrangement Following important aspects of reverser switching may be noted. 1. The selector moving contact goes over the fine contacts twice, contacting the fine taps in turn. In a selector switch, the fixed contact pitch must allow bridging by the moving contacts. This means uniform pitching of the fine-tap contacts around the circle. The first and last contacts of the selector switch have the same physical gap as any other pair. Voltages withstand between adjacent contacts and over the range are same. This is unlike the linear selector, Sect. 5.5.1. 2. From Fig. 5.13a it is obvious that the selector-fixed contact sees the full-range voltage to the adjacent contacts on either side at some time. Arising out of 1 above, the withstand voltage between all selector contacts, when one of them is occupied by the moving contact, must be the range voltage. 3. It is not possible to use an arrangement in which selector-fixed contacts lie physically in a line. The contact circle must close. 4. In Fig. 5.13 the tap selector reaches 19 positions in its travel but there are only 17 different voltages. This is because the positions 9A, 9B, and 9C give the same voltage, so that the number of different voltages is reduced by two to 17. The positions 9A, 9B, and 9C are referred to in literature as “mid positions”. Surprisingly the Standard IEC 60 214 does not define such a term. The author feels that the most appropriate designation the connection scheme would be 10.17.2 W (W stands for reverser, German Wender). The way in which to read the designation would be: Ten position selector, 17 different voltages, two extra positions, using a reverser. Others, MR for instance, call this 10.19.3 W which leaves most users at a loss, because the number of different voltages is not explicitly stated. 5. There is a position during the transition of the moving contact of the reverser, when the fine winding is not connected to the rest of the transformer, see Fig. 5.14. Figure 5.14 is an extract from Fig. 5.13, showing only the condition under discussion. At this position, the fine tappings do not have a fixed potential. The effect of the loss of potential and the manner in which this can be combated is discussed more fully in Chap. 7.

5.7.2 Method of Obtaining the Maximum Different Voltages In the illustrative example of Fig. 5.13 a ten-pitch selector switch is used to get only 17 different voltages. The same selector switch could be used for getting 18

5.7 Method of Connection and Operation

179

Fig. 5.14 Loss of definite potential of fine winding

different voltages that are possible. Figure 5.15 shows the arrangement required. The designation of this connection would be 9.18.1 W.

5.7.3 Obtaining 17 Different Voltages from a Nine-Position Selector Some manufacturers offer a Pos 9 selector with a pre-selector. So Fig. 5.16 illustrates the connections required to obtain 17 different voltages from a nine-pitch selector, using an eight-step fine-regulating winding. As one fine selector position is required for reverser operations, and does not take a tap lead from the fine directly, an eight-step reversing winding with nine leads cannot be applied. A nine-tap reversing winding overcomes this difficulty. Of the ten leads from a nine-tap reversing winding, two are connected to the reverser. The remaining eight are connected to the nine-pitch tap selector leaving the last pitch for the reverser operation. The connection yields ±8 tap regulation. The type designation would be 9.17.0 W.

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5 Selection and Application of Tapchangers to Transformers

(b) (a)

Fig. 5.15 Reverser for max. different voltages

5.8 Physical Location of Taps The physical location of taps influences the mechanical forces generated by the tapping arrangement. The subject of mechanical forces in transformers belongs to the discipline of transformer design. We shall not go into it in detail, except to the extent of the impact on tapping location in the transformer, which in turn affects the selection and application of the tapchanger.

5.8.1 Unbalanced Axial Ampere-Turns An economic method is to physically locate tappings in the body of the winding. By taps from the body of the winding, an execution likes Fig. 5.17 is meant. There is a

5.8 Physical Location of Taps

181

Fig. 5.16 18 different voltages from 9 step reversing

Fig. 5.17 Taps located in the body of the winding

strong downside to this approach. Mechanical forces on the windings are caused by unbalanced axial ampere-turn distribution [3, 8–10]. Two transformer windings with totally uniform turn distribution do not exert any net-axial force. The axial ampereturn distribution is variable when the taps are in the body of the winding. This is because some turns do not carry current depending on the tap position. A uniformly wound untapped secondary leads to high-cross fluxing at the tap zone. To minimise

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5 Selection and Application of Tapchangers to Transformers

(b)

(a)

(c)

(d)

(e)

Fig. 5.18 Effects of taps in the body of the winding

cross fluxing, the untapped winding is “thinned out” in axial turn density opposite to the tap location. Figure 5.18a shows taps in the middle of the coil. Figure 5.18b shows tap the line end in the body of the winding. Figure 5.18c shows schematically the thinning out for a tapping arrangement corresponding to Fig. 5.18a. The ampereturns of the untapped winding in the thinned zone is made equal to the ampere-turns of the current-carrying turns of the tapped winding at mean tap position, so that at mean tap there is no residual crossfluxing. Figure 5.18d shows the axial ampere-turn distribution at all taps out condition. Figure 5.18e shows the crossfluxing ampereturns. As crossfluxing is unavoidable when the taps are located in the body of the winding, this method is limited to small transformers, where the force generated can still be tackled, e.g. more block lines to the circle, thicker winding supports at the ends.

5.8 Physical Location of Taps

183

5.8.2 Location of Taps Along with the Winding Height When in the body, taps can be at the middle or one end of the leg height. Figure 5.18a, b show the two possibilities. The placement of taps in the physical middle is to be preferred, as it takes the crossfluxing away from the solid-unlaminated steel structure of the transformer, such as the core clamps, winding ring, and winding supports. It is also a convenient position for delta-connected windings. A problem arises with line-end taps, and taps at the neutral end of star transformers. For delta, if taps are electrically at the line end, but physically in the middle, a great deal of crossconnections will be needed. Getting the taps to the physical middle, but at the star end as is required in star transformers is also equally difficult. This difficulty is elegantly solved by a method of winding used by Hackbridge Hewittic & Easun Ltd. as discussed in the next section.

5.8.3 Neutral Taps in the Physical Middle of the Winding Height A method developed by Hackbridge & Hewittic Electric Co. of England offers a neat solution for getting the taps in the middle of the coil height, for neutral-end taps. Hackbridge Hewittic & Easun Ltd. in India inherited the method from Hackbridge UK under a license, and used it for many hundreds of 132 kV star transformers. Figure 5.19 shows the arrangement schematically. The top part of the tapped H.V is wound left to right, till the tap break is reached. At this point the winding former

Fig. 5.19 Neutral taps at the middle of coil height

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5 Selection and Application of Tapchangers to Transformers

is reversed on the winding lathe. The bottom coil is wound right to left, starting at the tap break, with the tappings. The external connections required are shown in Fig. 5.19. While elegant, this method causes a high build-up of impulse at the tapping gap, due to the strong discontinuity of the reactance and capacitance ladder network at the tap break in Fig. 5.19. Hackbridge found that the impulse here could reach 100% of the line end, though with a different rise time.

5.8.4 Taps on a Separate Tapping Barrel Figure 5.20 shows taps arranged in a full length “tapping barrel” by the side of the main coils. Remembering the extra radial gap between the tap barrel and the main winding, taps on a separate tapping barrel is more expensive solution than taps in the body. In the all taps in and out conditions, there are no residual axial ampere-turns. Transformer core window shown in Fig. 5.20b makes this an expensive real estate. Fig. 5.20c shows the axial ampere-turn distribution for the mean tap position. To the astute reader the ampere-turn distribution disturbingly resembles that of the taps in the body, except that the peak residual radial ampere-turns occur at mean tap, instead of the extremes. While admittedly the out of balance ampere-turns is a chief contributor to the force, we have to also consider the relevant current. Comparison of short-circuit currents at different taps is mire, due to the variation of impedance at different taps, further complicated by the auto-factor, in case of autotransformers. We shall therefore avoid this mine field and simply accept the transformer orthodoxy that a separate tapping barrel always reduces force. The axial unbalanced amp-turn for Fig. 5.20d is a three-dimensional figure, and no attempt is made to portray this. A separate tapping barrel is sine qua non for all higher rated transformers, e.g. over 200 MVA short-circuit level.

5.9 Voltage Stress in Tapping Barrel Applied to Taps at Neutral End For larger high-voltage transformers, an approach shown in Fig. 5.21a, where the main coil is wound in two parallel-connected halves is often used. This lowers the voltage at the end of the winding, and eases the problem of end insulation to the yokes. Taps are placed in a separate tapping barrel, most usually in two parallel-connected halves. The H.V line lead emerges from the middle height, through an insulating gap between the two halves of the tapping barrel. Very heavy line lead insulation is required. Insulation requirements at the top and bottom ends are nominal. Small transformers, with insufficient current for two parallel paths are wound as a single coil, starting at the top end with the line and finishing at the other. Taps are separately placed on a tapping barrel and connected to the low-voltage finish end of the main

5.9 Voltage Stress in Tapping Barrel Applied to Taps at Neutral End

185

(a)

(c)

(b)

(d) Fig. 5.20 Taps on separate tapping barrel

coil. The voltage stresses are across the top radial gap, and the creepage at the top to grounded structures. The maximum industrial frequency voltage is that due to the main winding. If the winding is in star, the taps are approximately at neutral potential. It may be noted that the extra insulation at the end is only required at the end closest to the main H.V lead. The other end shows low-voltage level between the main and the tapping barrel. Even though the insulation arrangement of windings belongs to the area of transformer design, Figs. 5.21 and 5.22 are included to show the massive insulation is a requirement if a tapping barrel arrangement is chosen.

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5 Selection and Application of Tapchangers to Transformers

(a)

(b)

Fig. 5.21 Insulation arrangement for mid height line lead exit for tapping barrel for star connection

5.10 Tapchanger for Delta Transformers Delta windings and tapchangers connected to them have to withstand the Separate Source A C Withstand Voltage Test, Sect. 11 of IEC 60 214. This can only be met by a tapchanger which is insulated to ground for the full-test level of the transformer winding. In other words, the voltage class of the tapchanger must be equal or higher than the transformer. Application in the middle of the winding as in Fig. 5.23 gives some relief as far as the interphase voltage is concerned. The voltage between taps of the phases is approximately half that at line end. This comment is applicable to the normal operating and Overpot test levels. The impulse voltage to ground can be close to the line-end value. Thus the reduced A.C. withstand between phases can be

5.10 Tapchanger for Delta Transformers

187

Fig. 5.22 End creepage insulation for separate tapping barrel for star. Single ended winding

Fig. 5.23 Phase voltage between taps at middle of winding

exploited only in tapchanger constructions where the ground and interphase clearance can be independently controlled. This is the case in intank type construction, where the insulation to ground is only that of the top phase and ground. In the compartment type, all three phases need increased ground clearance. In spite of the difficulties pointed out, ABB offers a Standard compartment type execution delta-tapchanger type UZF for 650 kV impulses [13].

5.10.1 Voltage Stresses Across Internal Insulating Distances in Delta Application Only the options of taps in the electrical middle of delta Fig. 5.18a and at the lineend Fig. 5.18b are available for delta. Transformer theory and experience is that

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5 Selection and Application of Tapchangers to Transformers

Fig. 5.24 Condition of max voltage over tapping range (all taps out)

(a)

(b)

the impulse voltage between adjacent taps and over the range can be very high in delta connection. Voltage peaking under impulse conditions also exists in starconnected transformers, but the location of taps at the neutral mitigates this effect. The maximum internal voltage stresses arise in the “All Taps Out” shown in Fig. 5.24. The free end of the tap winding oscillates under this condition and produces highimpulse voltage across the tapping range. For taps at the line end (Fig. 5.18b) and in the middle of the phase winding (Fig. 5.18a), this may approach 80–100% of the line-end value, unless controlled by other means, e.g. interleaving. The situation is less onerous for taps at the neutral end. The impulse voltage across the entire tapping winding does not distribute within the winding evenly either, as in industrial frequency. Very high maximum impulse voltage across taps can occur. For a tapping arrangement with an equal taps, the voltage across a tap may be as high as 3x voltage across the range/n, when no distribution control is employed. Measures such as delta application demands tap at line end for practical reasons (see Sect. 5.10.2 below). The placement of taps at the line end is an absolute requirement of extremely highvoltage autotransformers, where taps are routinely placed at the 400 kV or 220 kV line ends. The astute student may wonder where the special problem with delta is. If the delta tapchanger also comprises of three single-phase units, as is always the case with E.H.V autos, there is no problem with interphase voltages. The presence of other phases in a single-column delta poses a problem. It is often not possible to manage the required high interphase insulation in three-phase execution. For these reasons, tapchangers as three-phase unit for delta connection for voltages higher than 66 kV are usually not a Standard production unit for most manufacturers. However special executions have been made for tapchangers for 110 kV deltas (Chap. 4) because of the high and repeated annual demand in the south Indian grid. This special construction was discussed in Chap. 4.

5.10 Tapchanger for Delta Transformers

189

5.10.2 Separate Tapping Barrel with Delta-Connected Transformers For delta transformers, the end insulation for separate tapping barrel arrangement is similar to the star case of Fig. 5.21, except that it gets two further degrees of complexity. The insulation shown at the top must be repeated at the bottom also. In Fig. 5.21 there is not much creepage insulation from the taps to ground. In the delta case several angle washers will also have to be provided to meet the separate source test voltage. A figure showing theses changed features is not presented.

5.10.3 Why Is It Difficult to Locate Taps in the Electrical Middle of Delta on a Tapping Barrel If taps in the electrical middle of a delta are on a tapping barrel, this will require a connection schematically shown in Fig. 5.25. A break will have to be made in the main winding and two leads (coloured cyan and red in Fig. 5.25) taken out through the axial gap between the halves of the tapping barrel. The break between the two halves of the main winding disturbs the distributed parameter ladder network, causing enormously high-impulse concentration. This will demand a high degree of insulation, involving possibly three angle rings and angle washers at the break for 110 kV delta main winding. The two lead outs from the main lie in a zone of high impulse. At least one lead also has a tapping range voltage to the tapping barrel, in the most onerous tap position. Once again the impulse voltage between this lead and the tapping barrel could be considerable. Another set of angle rings and washers is probably needed to insulate between the leads and the tapping barrel. The insulation requirements are fearsome. While a transformer designer may not follow exactly the arrangement shown in Fig. 5.25 his solution would involve about as much complexity. For this reason, when a tapping barrel is used, the taps are logically at the line end. For delta the only option is, therefore, taps at the line end, if separate tapping barrel is required from other considerations.

5.11 Clearance Over the Tapping Range (Gap Between First and Last Contacts) The maximum voltage within the tapchanger internal insulation occurs over the tapping range. It would be an advantage if the clearance over this insulation distance could be raised, without generally increasing insulation distances all over. In the linear-tapping scheme, there is no functional compulsion that the fixed contacts should occupy the complete selector circle. The contacts can be packed closer together, leaving a bigger gap between the two contacts at the ends. This increases

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5 Selection and Application of Tapchangers to Transformers

(a)

(b)

Fig. 5.25 Insulation arrangement for taps in the electrical middle of delta and on tapping barrel

5.11 Clearance Over the Tapping Range …

191

the voltage withstand over the range. With the coarse/fine arrangement, or reverser arrangements, the contact pitch over the circle must be the same, as the moving contact goes over the contact circle, a second time. Therefore the electrical clearance between the first and last contacts of the tapping range is the same as any two adjacent contacts (Distances a1 and a2 in Fig. 5.3). The withstand over the range can only be increased by increasing the pitch of the contacts everywhere.

5.11.1 A Special Example of Increased Gap Between Selector-End Contacts: Diverter Switch Tapchanger It is convenient, but not necessary in tap selectors with diverter switches, that the gap between successive contacts is the same. In one execution (MR Type M) the angular pitch between the fixed contacts at the ends of the tapping range is increased to give higher clearance over the tapping range. The principle is illustrated in Fig. 5.26. This approach needs a positioning geneva mechanism capable of different throws, as in MR TYPE M. This is a matter of mechanical detail. This ingenious stratagem enables the moving contacts to be on successive-fixed taps, irrespective of the contact pitching … an absolute necessity for tapchangers. There appears to be no technical limitation in applying the principle to selector switches but no such tapchanger is available in the market. Reference [14] catalogues selector contact locations for various numbers of contacts of MR Type M tapchangers.

5.12 Tapchangers for Autotransformers In autotransformers, several electrical locations are possible for the taps. As will be apparent from the discussion below, location of taps is chosen from considerations of voltage stresses, as also the manner in which the impedance varies with the tap Fig. 5.26 Increased gap over end contacts MR type M

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5 Selection and Application of Tapchangers to Transformers

position. Many autos are meant for parallel operation with existing units. In that case it is important to choose a method of application which meets the impedance requirements. In some cases, the location is determined by the practice of the manufacturer as also the preference of the user.

5.12.1 Taps at the Neutral End Location of taps at the neutral end of autotransformers has some limitations. This arises out of the fact that when turns are added or removed from one winding, they are also added or removed from the total number of turns. Thus all neutral-tap regulation becomes VFVV. The core must be designed not to saturate under maximum flux. Varying core flux implies that the core is underused at some tap positions. The following is an illustration by a 220/132 kV Auto, with taps to vary the 132 kV by ±10% is informative. Figure 5.27 shows the two extreme tap conditions. At all tap out, we would need an output voltage of 0.9 × 132 kV. At all taps in, the voltage of the common should be 1.1 × 132 kV. A little arithmetic will show that of the total winding, the series will contain 34.1% turns, the common 40%, and the taps 25.88%. The tapping coil is very large in terms of relative eight turn. It is about 64% of the common. Even if the increased copper content and copper loss are accepted, considering the current is the lower common current, this very large number of turns causes high-voltage stress across the tapping range, in this case 43.9 kV max. It also results in a poor space factor for the active materials in the core window. The core flux at all taps out is 35% more than at all taps in. The tapping coil size increases dramatically as the autoratio closes on 1. This could possibly be the most important consideration against locating taps at the neutral. As against these considerations, the three-phase neutral tapchanger is much less expensive than the line-end tapchangers needed for common line end. Taps at the neutral can be seriously considered when the autoratio is small, and for a small range of taps.

5.12.2 Taps at the End of the Series Coil Figure 5.28 shows taps located at the end of the series coil. These taps can be used to keep the core flux constant in the face of varying incoming, i.e. CFVV. The current is relatively small, being that corresponding to the highest voltage. The voltage to ground is a little higher than the common coil voltage. Many transformer designers prefer one class higher voltage for the tapchanger. The internal voltages are high, particularly for impulse condition. In this and other line-end applications, a reverser type tapping arrangement is preferred (see Sect. 5.6). The tapchanger is mostly three single-phase units with common motor drive. They are of the same voltage class as the intermediate voltage the transformer or one class higher. The line-end

5.12 Tapchangers for Autotransformers

193

Fig. 5.27 Taps at neutral of auto transformer

(a)

(b)

application in H.V autos is regarded in the tapchanger world quite correctly as “High End Technology”.

5.12.3 The Potentiometric Arrangement Figure 5.29 shows this arrangement. This is also a line-end application. It will be noted that no turns are ever left out of circuit. Turns are either in the series or in the common winding. This results in some economy for the transformer and is therefore preferred by some designers. This arrangement enables varying the LV voltage in the CFVV mode. The live parts reach a voltage corresponding to the highest tap voltage. The voltage class of the tapchanger must suit this condition. The current rating must be that of the lowest tap. The main disadvantage is that the arrangement cannot take

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5 Selection and Application of Tapchangers to Transformers

Fig. 5.28 Taps at end of series in auto transformer

(a)

(b)

care of core saturation. If the H.V incoming voltage goes up, it does not provide for increasing the total number of turns. The transformer designer must work the core at moderate flux density to avoid saturation. This robs the idea of some of the economy. A further limitation is that a pre-selector type of tappings cannot be applied, which implies both technical and economic disadvantages.

5.12.4 “Hanging Out” Taps or “Tee Taps” Another line-end application is shown in Fig. 5.30. This shows a tapping arrangement at the LV line end in series with the output of the transformer. This is suitable if the L.V is intended to be varied with constant H.V. It cannot take care of core saturation due to high incoming voltage. Of the L.V line-end applications, this and the potentiometric taps are the ones which demand a tapchanger of the highest current rating.

5.12 Tapchangers for Autotransformers

195

Fig. 5.29 Potentiometric taps

5.12.5 Physical Location of the Tapping Barrel in Autotransformers Figure 5.31 shows the radial positions of locating the tapping barrel. The location of the tapping barrel with respect to the power windings is a decision to be made by the transformer designer. The position strongly influences the manner in which the impedance varies with the tapping position [9]. This is well known in transformer design practice. Often the designer has little choice. He has to follow the impedance variation in existing units so that the new and the old transformers can be paralleled at all taps. He would spare little thought to the tapchanger and choose a location which suits the application at hand. Other consequential issues must be tackled as best as possible. Besides impedance the location of the barrel affects the impulse voltage distribution as well as the eddy current loss in the conductors of the tapping winding. In the application of Fig. 5.31a a tapchanger of the next voltage class to the common line end has been found successful. This is a preferred location among transformer designers because the winding can be continuous disk. Taps are easily taken on the outside surface of the coil. In Fig. 5.31b the tap coil must necessarily be of the multi-start barrel type, in order to take the leads out at the top and bottom. Such a winding shows high series capacitive coupling between turns and prevents high internal voltage stresses during impulse. The location of Fig. 5.31c is sometimes used for taps at the end of common. It is not preferred because an earth boundary runs the whole height and cause highly nonlinear impulse voltage distribution. This tendency is combated to some extent by the multi-start winding.

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5 Selection and Application of Tapchangers to Transformers

(a)

(b)

Fig. 5.30 “Hanging out” taps

5.12.6 Use of Series Boosters with Autotransformers The connections considered for autotransformers in Figs. 5.28, 5.29 and 5.30 envisage the use of a high-voltage tapchanger of voltage class of the common line voltage or higher. This is not a serious problem for autotransformers of up to 220 kV intermediate voltages. Tapchangers are available in plenty on the market. For higher voltage autos, such as 735/400 kV, an alternative other than a 400 kV tapchanger may be considered. Such high-voltage transformers are likely to be of very high rating also. It may be prudent to avoid taps in such transformers to make for safer design. The solution consists of the use of series booster at the intermediate output point, as shown in Fig. 5.32. If the range of regulation is ±10%, the rating of the intermediate circuit is 10%. The big advantage is that the choice of tapchanger is freed from the shackles of high voltage. The choice of low-voltage circuit, consistent with the current capacity of the tapchanger, is available. This is well illustrated by taking a numerical example, which involves regulation by ±10% of the intermediate voltage of a 315

5.12 Tapchangers for Autotransformers

197

(a)

(b)

(c)

Fig. 5.31 Radial location of tapping barrel

MVA, 735/400 kV autotransformer. For the circuit of Fig. 5.32 the series transformer voltage is 23.09 kV. The maximum current, at the −10% output of the auto is 505 A. We may choose a series transformer of 1:1 turns ratio, with a three phase, 33 kV, 600 A, star, eight-step tapchanger with reverser on the primary side. This could be ABB UCG or MR Oiltap® Type M III tapchanger. The step voltage is 2886 V which falls within the capabilities of the intermediate circuit and tapchanger. We can expect a short-circuit current of about 3600 A on the 400 kV side, which due to the 1:1 turns ratio chosen reflects as a current of the same magnitude on the tapchanger side. The tapchanger chosen has adequate capacity in this respect. Such a tapchanger can be quite economical. The solution of regulating 400 kV with a 33 kV tapchanger is not

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5 Selection and Application of Tapchangers to Transformers

Fig. 5.32 Series booster for auto transformer

so audacious as it sounds at first sight! It is hoped that the illustrative example is sufficiently clear to consider to work out other similar applications.

5.12.7 Booster Transformer for High-Current L.V A kindred application to the above is the use of a series booster for managing a tapchanger for high current. A typical example occurs mostly in the USA where L.V regulation with CFVV is quite common. A practical example could be a 50 MVA, 15 kV ±10% star-connected winding. The maximum current is 2138 A at the −10% tap. There are three problems. 1. A 2000 + A tapchanger is expensive. 2. For such a transformer, the 15 kV L.V may have only 100 turns. For 1¼% taps we shall have to take a tap at every 1¼ turns. As tappings at fractional turns are not possible, we have to compromise to round off, introducing high ratio

5.12 Tapchangers for Autotransformers

199

errors. Besides, taking such a high-current tapping almost every turn is a winder’s nightmare. This problem is ameliorated to an extent in reactor type tapchangers. 3. The high-current tap leads add to the cost. 4. For these reasons, we may consider a series booster, as in Fig. 5.32. The maximum voltage required on the series side is 866 V. We may use a tapchanger of 33 kV class, with a 600 A rating, e.g. ABB UZE 33 kV. The ratio of the booster may be made 4 so that the maximum current at negative tap is 534.5 A. The step voltage will be 433 V using a ±8 tap regulating winding.

5.13 Use of Interpolating Transformer for High-Current Application A concept that is helpful with respect to tap size for high-current transformer is the use of an interpolating winding between tappings [11]. This application allows the main transformer to be wound with taps equal to two steps. This reduces the number of tap leads to half, thereby saving on expensive material and cost of connections. Figure 5.33a shows the connections. A diverter switch type tapchanger with a tap selector is used. Every two adjacent positions of the tap selector are bridged permanently. Otherwise the tapchanger is Standard. Only one tap lead from the transformer is connected to the “double” contact. The interpolating transformer winding is connected as shown in Fig. 5.33. It can be wound on the same core as the main winding, and have turns to induce one step voltage. It may be wound more conveniently as a separate core and coil assembly, maintaining however that the induced voltage of the interpolating transformer is in phase with the winding to be regulated. The taps on the main winding have two step voltages, thereby doubling the number of turns per tap, which eases up the problem of taking high-current taps in the winding process. Figures 5.33a–d show the sequence of operation. Table of Fig. 5.33 shows that nine different voltages are derived from a 4 × 2 step regulating winding and an interpolating transformer. Thus the numbers of output voltages are not increased…only the tap step of the transformer is doubled. Figure 5.33 shows a nine-pitch selector for illustration. Since 18 pitch tap selectors are available as Standard, the interpolating transformer allows realising 8 × 2 taps, 17 different voltages in linear configuration. Because of the cost of the interpolating transformer, its application is considered only when at least one of the following obtains: 1. The number of different voltages required is high say 10+. 2. The current is high, typically 2000 A or more. 3. Tap turn interval is small. It is not possible to use an interpolating transformer with selector switch type of tapchanger.

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5 Selection and Application of Tapchangers to Transformers

(b)

(c)

(a) (d) Fig. 5.33 Interpolating transformer

5.13.1 Irregular Variation of Output Voltage When a Pre-selector Is Used It is quite possible to use the concept of the interpolating transformer with tapchangers equipped with pre-selector. An irregular variation of voltage occurs near the zone of the changeover of the pre-selector unless the tap winding is made in a particular manner. Reference [11] discusses the details.

5.14 Tapchanger for Zig-Zag-Connected Transformer A transformer with zig–zag connection as shown in Fig. 5.34 is used as earthing transformer. This connection provides for earthing an unearthed system, and provides for unbalanced loading, including single phase to earth fault, as well as harmonic

5.14 Tapchanger for Zig-Zag-Connected Transformer

(a)

201

(b)

Fig. 5.34 Schematic layout and connections of zig zag

components of Fig. 5.34, here magnetisation current. If a transformer is not solely for providing earthing, but used as a power source for auxiliaries, it may need an on-load tapchanger. Some potential problems are discussed below.2

5.14.1 Need for the Same Number of Turns in the Zig and the Zag When unbalanced currents flow through the three phases, they can be analysed into their symmetrical components. The zero-sequence current of the three phases is equal and co-phasal. The main virtue of the zig–zag connection is that it allows the flow of unbalanced currents in the three phases, including the extreme case single line to ground fault. The total neutral current is split into three equal parts, with one part flowing through each half of each phase. This is shown in Fig. 5.34. Under this condition there is no additional magnetisation of the core, because equal currents flow in opposite direction in the two coils of each leg. The load MMF through each leg totals to zero. The impedance to the flow of current in each coil is only due to the leakage flux between the zig and the zag. This can be made what is desirable by the design of the transformer. To obtain this condition, it is essential that the zig and the zag have equal number of turns. Otherwise the exact cancellation of the load MMF in each leg will not occur. This is easily achieved by the design of the transformer, when neither the zig nor the zag is tapped. If the zig–zag winding is tapped, it must be equally tapped on both the zig and the zag. Otherwise, the MMF of the legs will not cancel, establishing a flux through the core. This will endow the coils with high reactance, and not just leakage reactance as discussed above. We have effectively six coils to control. This will need at least two three-phase tapchangers. 2I

wish to thank Mr. S. Sen for bringing this application to my notice.

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5 Selection and Application of Tapchangers to Transformers

One may be connected to all three phase of the zig and the other to all three phases of the zag. The two tapchangers may have a common drive mechanism. But two different tapchangers even when driven by a common mechanism will have intrinsic time differences of diverter switching of the order of 200 ms. During this period, the zig and the zag do not have equal number of turns. The core flux can build-up, lending the coils considerable reactance. This will cause heavy voltage flickering. The correct solution is to use three two-phase tapchangers, each one accommodating one zig and one zag. So long as the diverter of each tapchanger is driven by a common energy storage device, there is no observable flicker. The two diverters will have to be insulated from each other for a little more than half the phase voltage.

5.15 Tapchanger for Scott Connection Scott-connected transformers are used to interconnect balanced two-phase systems to balanced three phases. A problem similar to the Zig–Zag arises with the Scott connection. Figure 5.35 shows the Scott-connected transformer. The Scott connection employs two transformers. The “Main” is shown connected in Fig. 5.35a between the Y and B phases of the three-phase supply. The “Teaser” is connected between the mid-point of the YB phase and the R phase. Figure√5.35 omits to show the secondaries, which have equal turns. The teaser must have 3/2 turns of the main to produce equal voltages on the secondary side. For the Scott-connected transformers to function correctly, the following conditions must be fulfilled.

(a) Fig. 5.35 Tapchanging in Scott connected transformers

(b)

5.15 Tapchanger for Scott Connection

203

1. The teaser current splits into the two halves into the main equally. If the two halves are identical, the MMF due to teaser current cancels completely, and there is no core magnetisation. The only voltage drop is due to the leakage reactance between the two halves of the main. 2. It is important that the two halves must have the same number of turns to achieve condition 1. In the case of tapped windings, the tappings must be on each half and the tap position must be identical. 3. The tapping arrangement may be implemented by a three-phase tapchanger, the third accommodating the taps of the teaser. The tap turn interval of the teaser √ should be 3/2 times of the main. 4. Even with a common energy storage drive for the switching elements of the diverter, there is usually some time difference in the transition. During that period, the number of turns on either side of the main transformer is different. This will cause core magnetisation and a bigger voltage drop than mere leakage reactance between the halves. There will be a flicker.

5.15.1 Earthing Problems In untapped teasers the system neutral divides the teaser turns in the ratio of 2:1 from the R terminal. This turn can be grounded. If taps are provided either between R and neutral, or the lower half, a tapchange will shift the position of the system neutral along the teaser. There is no permanent grounding point. By providing taps of half the size in the lower half as well and employing a further tapchanger this problem may be avoided. Still due to time difference in switching of the two tapchangers applied to the teaser, there could be transient short periods when the grounded point shifts from the neutral. A heavy current will flow from the Y and B phases, being restricted only the leakage reactance which is low. The R-phase current is choked off by core magnetisation of the unbalanced current. The better solution may be not to earth the system at the teaser but provide a separate earthing transformer.

5.16 Switching Capability Briefly the switching capacity of a tapchanger is its ability to commutate currents from one tap to the next, without dragging an arc between contacts which does not extinguish before harm is done. This is a rating in which the transformer designer is not an expert, and may need to refer to the tapchanger manufacturer. There are two different but related considerations regarding the switching capability. 1. The breaking capacity of a tapchanger relates to its ability to successfully clear all arcing without damage. This ability is a design characteristic and will have to be supported by a type test. There are many influences which define this capability.

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5 Selection and Application of Tapchangers to Transformers

1.1 1.2 1.3 1.4 1.5 1.6

The current to be interrupted. The recovery voltage distances available between arcing contacts. Speed of contact parting. The operating cycle of the tapchanger. Extent of turbulence created in the surrounding medium. Material of contacts. Materials which ionize easily emanate a high cloud of conducting matter in the contact gap and hinder recovery. 1.7 The ability of the surrounding medium to absorb charged matter and annul the charges. For instance, in breaker applications, SF6 finds a special place for its ability to control ionisation. With such a complex interplay of factors, it is difficult to predict exact performance. A newly designed tapchanger proves its breaking capacity through a breaking capacity test according Cl. 5.2.2.3 of IEC 60 214. The breaking capacity test is at double the rated current and relevant step voltage. The switching capacity is not directly determined by the through current or the step voltage (see Chap. 3). A large number of operations are not required to prove the breaking capability. The IEC specifies 40 operations at a test current and a relevant step voltage. The breaking capacity relates to but does not prove the ability of the tapchanger to do a large number of switchings, without too much loss of contact material, and destroy the surrounding medium. This is again established by a type test, according to IEC 60 214 Cl. 5.2.3.2 (Service duty test). The service duty test is done at the rated current of the tapchanger, and at the same relevant recovery voltage. 50,000 operations are required according to the Standard to make an assessment. This number by no means represents the contact life. Most manufacturers claim a contact life in excess of six times. Figure 5.36 is a typical example of the switching characteristics of a tapchanger. The switching characteristics show the absolute maximum current and step voltage. It also shows the relationship between current rating and the relevant step voltage. The characteristic is based on breaking capacity and a reasonable contact life.

Fig. 5.36 Typical switching characteristics of tapchanger. ABB tapchanger type UC

5.17 Parallel Connection of Tapchangers

205

5.17 Parallel Connection of Tapchangers Parallel connection of tapchangers is often done to increase the switching capacity of the diverter switch. While it is possible to connect selector switches in parallel, this is not a practical application, because selector switches are applied to small transformers where the need for paralleling is unlikely. Problems that arise in paralleling tapchangers which are common to selector switches, and diverter switches are considered.

5.17.1 Paralleling Two Different Tapchangers with Individual Driving Mechanisms Is Not Practical In Fig. 5.37 two three-phase tapchangers are applied in parallel, in each phase of the transformer. The tapchangers are driven by their own individual mechanisms. There could be considerable time difference in their switching times, even if driven by a common mechanism. When well adjusted, there could still be a time difference up to 200 ms. Figure 5.37 shows a case when one tapchanger has switched completely, while the other persists on the original tap. This is quite possible if the time difference exceeds the transit time. The latter is about 40–50 ms. There is a direct short between the taps without a resistance in circuit to limit the current. The circulating current must be restricted by the winding impedances. In Fig. 5.37 the circulating currents do Fig. 5.37 Paralleling of two independant tapchangers

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5 Selection and Application of Tapchangers to Transformers

not produce a net MMF on the core, and, therefore, no balancing current flows from the external system into the transformer. The leakage impedance between the main and the tappings plays no part in limiting the current. The only effective reactance in limiting the circulating current is the leakage between the two tapping windings. This is very small. Due to this reason, paralleling two tapchangers driven by individual mechanisms is not a practical solution.

5.17.2 Enforced Current Sharing To overcome this problem, the connection shown in Fig. 5.38 could be used. The circulating current path now includes the main winding. If there is time difference between switching of the diverter, the resulting circulating current flows through the main sections as well. Each half of the current path offers limiting impedance equal to the leakage between the main- and short-circuited tap. The winding resistances are also effective. If the net winding resistance between the line and neutral end is R, the winding resistance limiting the circulating current is 4R.

5.17.2.1

Application with Enforced Current Sharing

This subject is fully analysed in [11]. The phase windings can be wound in two or three identical parallel paths to share currents equally. Transformer manufacturers are well aware of the techniques of winding to ensure current sharing between the Fig. 5.38 Modified paralleling involving main winding

5.17 Parallel Connection of Tapchangers

207

parallel windings. Enforced current splitting in three parallel paths is analysed in detail in [11], Sect. 4.2.4.1. The conclusion is that the switching-current departure from equality is strongly affected by the ratio of the leakage reactance of the paths to the transition resistance value. It falls from a maximum of three for condition of no leakage reactance to only a minor difference for X/R = 2 ([11], Fig. 4.2–6). Transformer manufacturers, when advised of the problem, can very easily achieve such a ratio.

5.17.3 Paralleling of Switching Element Directly in Parallel The difficulty of time difference is minimised when all the switching elements are driven by common energy storage. Switching differences between elements may be expected to be less than 1 ms. Either two contacts or all the three contacts may be shorted and paralleled. There is no enforcement of equal current splitting. Currents will split depending on the path resistance. But for identical switching elements differences between the elements is very small, so that nearly equal current sharing takes place. However the following issue may need pondering over.

5.17.3.1

Minor Differences in Timing of Contacts

Diverter switches are not constructed as “tightly” as machine tools or motor car engines. Considerable slack and deviation from dimensions are tolerable. Much of such slackness is necessary to accommodate insulating components, which do not maintain dimensional integrity as firmly as machined metallic components. The slackness is necessary to keep the switching elements working. The three elements may not switch absolutely simultaneously. There could be time differences up to 1 ms. In Fig. 5.39 three diverter-switching elements of the flag cycle are paralleled directly with no enforced current splitting. It is assumed that the main contact W1 opens first, followed by W2, and later by W3. Before any of the paralleled main contacts are open, the through current splits about equally (Fig. 5.39b). When W1 opens, the current shifts to the other two contacts (Fig. 5.39c) and not into the transition resistance connected to W1. This does not imply current chopping. When a contact gap is produced, with no recovery voltage to strike and maintain the arc, the current goes out. The current carried by the contacts W2 and W3 now becomes half the through current, instead of one-third. When W2 opens all the currents are diverted to W3 (Fig. 5.39d) once again without arcing at W2. W3 must now break the full current (Fig. 5.39e). There could be mitigating circumstances to this picture.

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5 Selection and Application of Tapchangers to Transformers

(a)

(b)

(c)

(d) Fig. 5.39 Time difference in main contact switching

(e)

5.17 Parallel Connection of Tapchangers

5.17.3.2

209

Sources of Recovery Voltage for the Contacts that Break Earlier

The inductance of the individual paths of the main contacts will supply a recovery voltage. The resistance drops across the other main contact also apply a recovery voltage across the contact gap. These voltages are very low and may not be enough to sustain the current through the gap for long. Finally when the current is broken by the last main contact W3, the full recovery voltage appears across all the three gaps. If the gaps are still very small, the recovery voltage may reignite them, and equalize the currents. This cannot be taken for granted, because while the last contact to break does so with arcing, the other virgin gaps are unsullied by ionised material to reignite easily. It is therefore a question of how long the delay between contacts is, and how much gap is produced. Direct paralleling of diverter elements appears best avoided.

5.17.3.3

Switching at the Transition Contact

At the next switching, which involves the transition contact breaking the bridging condition is not fraught with this problem. To start with, currents will be enforced equally in all the six contacts because any differences in the contact resistances will be swamped by the high value of the transition resistances. Figure 5.40 shows the first transition contacts at X1 about to break. A recovery voltage due to the current

Fig. 5.40 Switching at transition contacts

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5 Selection and Application of Tapchangers to Transformers

flow through the transition resistances in the other switching elements will maintain an arc till the next current zero. By that time the other transition contacts would have opened also considering that time differences of the phase driven by common energy storage are very small. There is no tendency for the current to crowd into the transition contacts that break last. Interruption duties would be equal in all three phases.

5.18 Phase Shift Between Interrupted Current and Recovery Voltage in Parallel-Connected Transformers with Circulating Current It has been emphasized throughout this book that the success of resistance tapchangers in arc quenching lies in the co-phasal relationship of the interrupted current and recovery voltage. A situation arises when transformers are paralleled with circulating current. There is a phase shift between the interrupted current and recovery voltage. Examination of this condition is deferred to Sect. 5.12.7.

5.19 Application to Furnace Transformer Furnace application requires a low-voltage variable voltage control. The current of the furnace is high, so that placing the tapchanger on the LV is not feasible. Such transformers have a nearly constant voltage input to the primary with a tapchanger operating in the VFVV mode. The desired furnace control may require a specific pattern of voltage variation which is best served by a linear-tapping arrangement. This application is characterised by variable step voltage. Sometimes the step voltage at some region of the tapping zone may become too high. In such cases it helps to increase the number of steps in the region of high-step voltage for better control. This approach may be limited by the availability of a suitable tap selector with very large-numbered operating positions. The VFVV requirement may preclude use of multi-coarse/fine arrangement. Two variations are possible to increase the number of operating positions. 1. Two tapchangers in series. Taps are placed on the winding in two sets, as shown in Fig. 5.41. This will need two tapchangers per phase. 2. Use of delta/star changeover switch.

5.19 Application to Furnace Transformer

211

Fig. 5.41 Taps in series to increase no. of steps

5.19.1 Furnace Applications Pose Especially Onerous Conditions on the Tapchanger 1. Frequent operations. It is not unknown for a furnace tapchanger to perform more than 100,000 operations in a year. 2. Large number of continuous operations. 3. Frequent short circuits in some types of furnaces, e.g. arc furnace for steel. 4. High-temperature surroundings. Poor cooling and ventilation.

5.19.1.1

Consequences of Frequent Operations

Obviously frequent operations have a cumulative effect in terms of contact wear, despoliation of oil, and poor dielectric surroundings. Apart from these, the frequent pulses of high temperature communicate the heat to the terminations of the transition

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5 Selection and Application of Tapchangers to Transformers

resistances. The resistance is terminated by Standard steel fasteners, to a copper conductor, which in turn connects to the rest of the circuit. The frequent pulses of high temperature and subsequent cooling lead to thread distortion. Pressure needed for current conduction is lost. It is very often the consequent loss of contact at the termination that causes failure.

5.19.1.2

Frequent Short Circuits

High frequency of short circuits is an operational hazard in furnace application, for example, steel-melting arc furnaces. Later in this section an example is cited on how the furnace operators cause deliberate short circuits. The solid furnace charge has a relatively poor conductivity. As melting proceeds, the charge may collapse shorting the electrodes. Molten metal, which has a much higher conductivity, may also short the electrodes, as it gathers in pools around the electrode as melting proceeds. In some submerged-arc furnaces (Ferro Chrome for instance), the conductivity of the charge before melt is very low. In order to initiate melting, operators often deliberately short the electrodes on top of the frozen charge surface, by building graphite bridges. The initial current through the graphite is not too high, and does not constitute a serious short circuit by itself. But enough current is drawn to generate heat to melt the charge immediately below. When the metal below the graphite melts, there is a short circuit. This procedure is sometimes repeated at every charge, with a charge cycle time of one and a half hours to two.3 The severity of the short circuits can be lessened by high reactance of the transformer, or series-current limiting reactors. These solutions cause a voltage drop which is undesirable under normal operation. Saturable reactors, which show their reactance only at elevated currents, may be a better solution.

5.19.1.3

Large Number of Continuous Operations

A tapchanger conforming to IEC 60 214 is capable of performing a number of tapchanges equal to running through half the tapping range continuously. This is assured by Cl. 5.2.5 of the Standard. However if the surroundings are already at elevated temperature as is often the case with furnace application, or if the tapchanger is frequently subjected to this duty, without adequate time for the transition resistances to cool, this capability may be jeopardised. In particular such large number of continuous operations may occur at over loads. Large number of continuous operations can be prevented by building logic in the motor drive controls. The logic could be in two parts: 1. Prevention of large number of continuous operations. 2. Prevention of operation at over load. 3I

wish to thank Mr. Raju Garu for providing me an opportunity to observe this procedure in his Ferro-Chrome Plant.

5.19 Application to Furnace Transformer

5.19.1.4

213

High-Temperature Surroundings

The secondary current in furnaces is high. It is economical to position the transformer very close to the furnace to reduce the cost of the high-current bus work. This leads to high-surrounding temperature and poor ventilation. Cooling of the transition resistance takes longer than usual. Frequent operations under such conditions may damage them.

5.19.1.5

Reactor Tapchanger for Furnace Application

The reactor tapchanger offers a good solution to many of these problems. Since the root cause of the problem is often the transition resistance, its elimination will certainly improve reliability. Reactor tapchangers are of the slow speed type. There is enough time to detect a high current, and stop and reverse a tapchange after initiation. The reactor tapchanger is in any case relatively immune to episodes of high current of short duration. See Chap. 9.

5.20 Operating Temperature Considerations IEC Standard 60 214 Cl. 4.1 Table 1 [12] requires that compartment type tapchanger should be able to operate in the temperature range of −25 to +40 °C. The temperatures refer in practice to the surrounding air temperature. For Intank tapchangers the limits are −25 to +105 °C. The higher temperature is due to the fact that the tapchanger is surrounded by hot transformer oil. The heat produced by the dissipation in the current path and occasional contribution by the transition resistances is easily managed by compartment type tapchangers where the tank provides an effective cooling surface. There could be a minimal problem in applications such as furnace application, where the transformer is located close to the furnace. Compartment tapchangers could face a more onerous condition as regards the lower limit. Unlike Intank types, there is no warm oil to buffer the tapchanger against the surrounding low-temperature atmosphere. If starting from cold, the tapchanger may take a long time to warm up from the feeble heat contributed by the resistance of the current paths. Oil becomes very viscous as the temperature goes down, making switching lethargic. In the extreme case the tapchanger may not be able to complete a tapchange. In order to safeguard against such failures, there is a requirement in IEC 60 214, Cl. 5.6.2.2 that during the mechanical endurance type test 100 operations shall be performed at −25 °C. Regardless, if reliable low-temperature operation is required, it may be desirable to put in external heaters to warm up the oil. These problems apply equally to Intank tapchangers, except that the transformer load assists in thawing conditions more quickly. Intank tapchangers may also experience difficulties at very low-surrounding atmospheric temperature, due to the exposure of components mounted outside the transformer tank. Many intank tapchangers have such important

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5 Selection and Application of Tapchangers to Transformers

components as the energy storage device in the tapchanger head, which is exposed. The moving parts in the drive would need low-temperature lubricants.

5.20.1 Requirements Stipulated in the Mechanical Endurance Tests Cl. 5.6.2.2 of IEC 60 214 Recognising the problem of extreme temperatures of the surroundings, the Standard IEC 60 214 provides for testing at representative conditions. The mechanical endurance type test stipulates some conditions to verify that a tapchanger is capable of operating under the conditions of temperature limits [15]. It is required for liquid-immersed intank tapchangers, that 100 operations shall be performed with the diverter switch at 115 °C and with the tap selector at 105 °C or with the selector switch at 115 °C to demonstrate the capability to withstand the mineral insulating oil temperatures during emergency loading as stated in cl. 4.3. For both intank and compartment type on-load tapchangers, 100 operations shall be performed at −25 °C with the diverter switch only or with a selector switch.

5.20.2 A Special Problem for Intank Tapchanger with High-Temperature Conditions The diverter switch and other components in the diverter oil vessel of Intank types have a difficult condition of cooling. The circuit resistance of a three-phase 500 A diverter switch is 0.6–0.8 m. The average steady dissipation (ignoring the surge dissipation during tapchange due to the transition resistances) of the diverter at full load is 525 W. The two sinks of dissipation are the head plate and the diverter oil vessel. The head mostly offers a horizontal cooling surface which is not most effective for convective air flow. The surface area of typical Intank tapchanger head is 500 in2 . At a temperature rise of 60° over the ambient a horizontal surface like the head can dissipate about 150 W. The remaining 375 W will have to pour out through the oil vessel. The oil vessel does not provide an easy cooling path either due to its thickness and low thermal conductivity. In most transformer constructions, the diverter is mounted high up the core and coil assembly where the surrounding oil is hot. However the large surface is a help in transferring heat from the tapchanger oil to the transformer oil. The typical cooling surface towards oil of the diverter oil vessel is 3000 in2 . To dissipate the remaining 445 W from this surface, the average temperature rise must be about 20°–30°. This does not appear much, except when taken in the context that the oil vessel is surrounded by hot oil from the transformer. The local temperature could possibly reach a high level. The temperature mentioned in the Standard only refers to the average surrounding oil temperature, and not to highest. Thus even when the Standard is met, local temperature conditions, particularly under

5.20 Operating Temperature Considerations

215

the head cover could be very adverse. The gaskets and oil seals for the dynamic shaft must be able to withstand such high temperatures.

5.21 Tapchanger Operation When Immersed in Ester Fluids A great deal of interest has been evinced on the possibility of using ester oils in transformer, as they are bio-degradable, and therefore environmentally friendlier than mineral oils. Naturally such transformers have to use tapchangers which would be compatible with ester oils. In the initial rush, in which the author also took part, consisted of a rough “suck and see” approach. Tapchangers intended for mineral oil were filled with ester fluids and tested. The author filled a 33 kV, 300 A OLG compartment tapchanger with the natural ester FR3 and conducted a breaking capacity test, as well as a service duty test in accordance with IEC 60 214. The result showed nothing particularly remarkable except that the average arcing time seemed a little elongated. Tapchanger arcing time is in any case statistically variable, because there is no control of the point of current wave at which the contacts part. The author concluded that the elongated time was not injurious, and that a more controlled test taking into account all the parameters could be conducted later. Such a study would prima facie not use a tapchanger, but two identical electrically operable switches, with point on wave switching. One would be filled with mineral oil and the other with the other fluid. The point of wave switching would be controlled, so that arcing times could justifiably be compared. DGA, fluid BDV, solid detritus could also be compared. This project was not carried out. But other authors have done considerable work with ester-filled tapchangers [16–20]. The reader may refer to these. In the meanwhile ester fluids have been widely applied with tapchangers and found acceptance in the field. The following issues remain to be cleared. 1. Operation at low temperatures, when the viscosity of the fluid goes up, making the movement of the diverter switch sluggish. 2. The electrical withstand behaviour of ester fluids under non-homogenous electric field seems to be lower than mineral oil [17]. 3. Ester fluids have to be protected from moisture contamination to higher degree than mineral oils to derive the full functionality. 4. Ester fluids are derived from agricultural products like Soya, Rapeseed, and Canola. Agricultural products are notoriously variable, making, for instance, the art of the sommelier so interesting. The base stock from which the insulating fluid is derived is of variable properties. To what extent this is a feature in their performance will be clearer in days to come.

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5 Selection and Application of Tapchangers to Transformers

5.22 Driving Mechanisms The operating requirements of driving mechanisms are discussed under Chap. 10.

Appendix: Commutation of Current Between Coarse and Fine Tapping Sections Loop Inductance The derivations in Chap. 3 of interrupted current and recovery voltage show they are in phase. This is because the inductance in the local commutating loop, contributed by one tap is very low. This phase relationship is propitious for arc extinction. A more difficult switching condition is encountered in the position where the coarse and fine are interchanged into the effective circuit of the transformer. Briefly, in this position, the leakage reactance between the coarse and fine is included in the commutating loop, and causes a phase shift between the interrupted current and recovery voltage. To examine if the diverter switch is capable of performing this duty we need to know the magnitude of the interrupted current, the recovery voltage, and the phase angle between.

Commutation When Coarse and Fine Have Equal Voltage To study the effect of commutating current between coarse and fine-tap sections, we shall consider a tapchanger with flag cycle diverter switch. We first analyse the case when the induced voltages of the fine and the coarse are equal. In Fig. 5.42 the tapchanger is about to switch from tap No. 13 to tap No. 4, i.e. transition contact X is about to break current. This commutates current from fine to coarse. To derive the switching current of contact X we need to find the current through it before the contact breaks. To derive the recovery voltage, we have to establish the voltage across the parted contacts after the current is interrupted. The two conditions are shown in Fig. 5.42a, b, respectively. The through current I splits into the parallel paths through the fine and the coarse in the inverse proportion of their impedances in Fig. 5.42a. Ignoring winding resistances, which are too small, the current through the fine section is If = I (R + j X c )/(2R + j (X f + X c ))

(5.1)

The phase angle of the interrupted current relative to the reference vector is ø1 = ø + arctan X c /R − arctan(X f + X C )/2R

(5.2)

Appendix: Commutation of Current Between Coarse and Fine …

(a)

(d)

217

(b)

(c)

(e)

(f)

Fig. 5.42 Commutating current between coarse and fine

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5 Selection and Application of Tapchangers to Transformers

Where R is the transition resistance, and X f and X c are the leakage reactances of the fine and coarse windings. From Fig. 5.42b, the recovery voltage is Vr = I (R + j X c )

(5.3)

ø2 = ø + arctan X c /R

(5.4)

The phase angle of V r is

The phase shift between the interrupted current and recovery voltage is ø1 − ø2 . While these are exact equations, the significance of the special switching involved is not transparent. To get realistic relative magnitudes of the parameters and to appreciate the problem, we consider the following numerical example. 1. The magnitude and the phase angle of the interrupted current and recovery voltage are affected only by the transition resistance, the leakage reactance of the coarse and fine, and the relationship between the current and the transition resistance. Here we take R = E/I fl where I fl is the full load current. 2. For further simplification the two leakage reactances are taken as equal. 3. In most practical applications, the transition resistance is about 4–8 times the individual leakage reactances. We shall assume R = 6X f or 6X c . Substituting the assumed figures in Eqs. 5.1–5.4 results in the following values: 1. Interrupted current = I/2 2. Recovery voltage = 1.01E 3. The phase angle 9.5° The interrupted current and the recovery voltage are within the capabilities of the tapchanger but there exists a high angle between them. This is unlike the situation when tapchanging within the fine, when the phase angle is taken as zero.

Possible Failure to Interrupt at Contact X The phase angle may mean that X fails to interrupt at the first or even the next few current zeros after contact parting. The arc may still persist until the Z contact also closes (Fig. 5.42e). This by itself is not a failure to switch. It is necessary that with the new conditions that prevail with the Z contact closed, the diverter should extinguish the arc at X. The current through the arc and the fine taps is now If = j X c I /[R + (X f + X c )]

(5.5)

Applying the same parameters as before to the new switching condition, from Eq. 5.5 the phase angle between the interrupted current and the through current is

Appendix: Commutation of Current Between Coarse and Fine …

219

90°-arctan (X f + X c )/R. When the arc is extinguished (Fig. 5.42e), the recovery voltage V r that appears across X and Y is jX c I. The phase angle from the through current vector is 90°. The phase angle between the interrupted current and recovery voltage is the difference, 18.5°. Applying the same parameters as before to the new switching condition, from Eq. 5.4 1. I f = I int = 0.158 I in magnitude 2. V r = 0.16 E in magnitude 3. The phase angle between them is about 18.5°. The conditions are much ameliorated as compared to the earlier break (Fig. 5.42b). It would be the tap changer manufacturer who should confirm the ability of the diverter to clear these duties. Commutation of current from coarse to fine is similar to fine to coarse due to the symmetry of the circuit.

Case where there is a step voltage difference between the coarse- and fine-induced voltages In this case the transition from fine to coarse and vice versa is performed in one step. The circuit is shown in Fig. 5.43a in the bridging condition. There is now also a circulating current Ic = (Vc − Vf )/[2R + j (X f + X c )]

(5.6)

A rigorous analysis of the circuit is presented in [7], Sect. 4.4. For the present purpose, we shall only consider the case when the arc does not go out in the situation corresponding to Fig. 5.43b but the situation proceeds to Fig. 5.43c, where the Z contact closes while the X contact is still arcing to the Y contact. This shorts the transition resistance on the coarse side. However there is no short circuit, as the circulating current driven by the voltage difference between the coarse and fine is limited by one transition resistance on the fine side together with the leakage reactances of the coarse and fine. The revise circulating current Ic = E/(R + j (X f + X c ))

(5.7)

Where E = (V b − V f ). The main current splits into the fine side as If = j X c I /(R + j (X f + X c ))

(5.8)

The circulating current subtracts from component of the main in the fine taps, so that I f − I c which is the current to be finally interrupted is Iint = [I (cos ø − j sin ø) j X c − E]/[R + j(X f + X c )]

(5.9)

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5 Selection and Application of Tapchangers to Transformers

(a)

(b)

(d)

(e)

(c)

(f)

Fig. 5.43 a–f Page 1: Commutating current from fine to coarse. g–l Page 2: Commutating current from coarse to fine

Appendix: Commutation of Current Between Coarse and Fine …

(g)

(h)

(j)

Fig. 5.43 (continued)

(k)

221

(i)

(l)

222

5 Selection and Application of Tapchangers to Transformers

Figure 5.43f shows the vector diagram. The recovery voltage after the arc clears is Vr = j X c I (cos ø − j sin ø) − E

(5.10)

Numerical Example Using the same parameters as in the previous section for a load power factor of 0.8 1. I int = 0.816I in magnitude. The high interrupted current is practically all the circulating current set up by the voltage difference within the loop. See the small other component I f shown in Fig. 5.43f. The phase angle of I int with the reference vector is approximately 53.2˚ 2. Recovery voltage = 1.103 E 3. The recovery voltage (Eq. 5.10) is the interrupted current (Eq. 5.9) times R + j(X f + X c ). Therefore the phase angle between the two is arctan (X f + X c )/R. The voltage leads the current by this angle. The magnitude is 8.7°.

Commutation of Current from Coarse to Fine Figure 5.43i–k show current commutation in the reverse direction, from coarse to fine. In Fig. 5.43j must clear the arc, with W closed. Frotcher current splits into the coarse side as Ico = I (cos ø − j sin ø) j X f /(R + j (X f + X c ))

(5.11)

The circulating current now adds to the component of the main. The interrupted current is now Iint = I (cos ø − j sin ø)X f + E/(R + j (X f + X c ))

(5.12)

The recovery voltage is Vr = j X f I + E Figure 5.43l shows the vector diagram.

Numerical Example From Eq. 5.11, the interrupted current in the numerical example is

(5.13)

Appendix: Commutation of Current Between Coarse and Fine …

223

Iint = 1.00I From Eq. 5.12, V r = 1.03E By the same reasoning as with the interruption of the fine side, the phase angle between the interrupted current and the recovery voltage is 18.5°.

Note on Current Commutation Commutation of current between the coarse and fine is rendered difficult due to the phase angle between the interrupted current and the recovery voltage. The closure of the main contact on the closing side helps in reducing the switching severity. There is no provision in the Standards to demonstrate the successful commutation between coarse and fine.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

19. 20.

IEC 60 214:2014-05 International Standard “Tap-changers” IEC 60 076:2000-4 International Standard “Power Transformers” Blume et al., Transformer engineering. Wiley, New York Transformer Inrush current reduction, Doug Taylor, University of Idaho Presentation Canada Transformers. https://canadatransformers.com/transformer-inrush/ Case study of transformer magnetizing inrush current as a serious issue: T.V. Sridhar. Unpublished internal Report. 2018. [email protected] IEC 60 076-7: Loading guide for oil immersed power transformer Chiplonkar AV (2008) Design, operation, maintenance of core type oil filled power transformers. Self Published. [email protected] Del Vecchio R et al (2001) Transformer design principles. CRC Press Kerenyi, Karsai, Kiss (1967) Large power transformers. Elsevier, New York. And P.O.Box 149, H-1389. Budapest, Hungary Kraemer A, On load tapchangers for power transformers. MR Publication IEC International Standard IEC 60 214-1 2014-05. International standard tap-changers: Cl. 4.1, Table 4.1 ABB Publication 1ZSE 5492-104 en, Rev/09: On load tapchangers Type UZ, Technical Guide MR on-load tap-changer OILTAP® M, Technical Data TD 50/References IEC 60 214 2014(05). International standard tap-changers. Sect 5.2.6.2, Mechanical endurance tests MR Knowledge base: PB 351 8312/01 En F031 0301-09/14 MR 2013: alternative liquid for tapchangers. Rainer Frotscher www.transformers.magazine.com. TM 10_20_27_Frotscher.pdf. Column Tap-changer knowhow. Insulating liquids: Part II: non mineral insulating liquids. Rainer Frotscher CIGRÉ D1-105 2012. Behaviour of ester liquids under dielectric and thermal stress—from laboratory testing to practical use. R. Frotscher, Maschinenfabrik Reinhausen D. Vukovi, M. Jovalekic, S. Tenbohlen, Germany Universität Stuttgart, Germany. J. Harthun, C. Perrier, Alstom Germany, M. Schäfer, EnBW Regional AG Germany/France Dohnal D, Frotscher R (2010) The importance of alternative insulating liquids for power “transformers and tap-changers”. CEPSI Contribution, Taipei www.midel.com

Chapter 6

Special Applications

I have no special talent. I am only passionately curious. Albert Einstein.

6.1 Chapter Content In this chapter we shall see applications which are not primarily concerned with voltage control. One such application arises with reference to reactors. The reactance of reactors is proportional to the square of the number of turns. A tapchanger which performs the function of changing the effective turns in a transformer can be applied to a reactor to do just that. Control of reactance may be required in series reactors, as well as shunt reactors. The control of shunt reactors has a large application in power systems, where they are used for regulating the flow of reactive power. Static reactive volt–ampere control is used in some cases for varying the reactive power within half a cycle. Such static VAR control systems use high-speed switching with electronic devices such as thyristors. Where system stability is not so critical, a variable reactor implemented by a reactor together with a tapchanger is a suitable solution. Tapchangers also find an application with phase-shifting transformers used for evacuating real power between nodes of a power system. These inject a voltage in quadrature with the phase voltage, as against a normal in line regulator. Phase shifters are some of the largest units in a power system. Application to phase shifters is therefore critical, and a proper evaluation of the loads and duties is of great importance.

© Springer Nature Singapore Pte Ltd. 2020 T. V. Sridhar, Application of Tap changers to Transformers, Power Systems, https://doi.org/10.1007/978-981-15-3955-8_6

225

226

6 Special Applications

6.1.1 Application to Shunt Reactors In physical execution, shunt reactors are coils with a magnetic core, or without a core but with magnetic shielding. Such a device shows the property of reactance, with low resistance. Shunt reactors are employed in power systems to compensate for the distributed capacitance of transmission lines. By compensating for the capacitive current, the load of the charging current is relieved, and thus the current-carrying capacity is increased. Shunt reactors also reduce the voltage rise at the receiving end, caused by the so-called Ferranti effect of the distributed line capacitance. When there is load shedding, or line trip, shunt reactors hold down the voltage rise. Shunt reactors also improve stability of interconnected power networks by injecting reactive power as required. In this last application, a variable reactor is required. The reactance of a coil is proportional to the square of the number of turns. By varying the number of turns, a variable reactor may be implemented. Changing the number of turns is precisely the function that a tapchanger performs in a transformer in its traditional role of voltage variation. Therefore tapchangers offer a convenient platform on which a variable shunt reactor may be built [1, 2]. Even though some shunt reactors may be small, those applied to large interconnected grids may have ratings in several hundreds of MVA. Besides, as system conditions change rapidly due to fluctuating loads, changing system configuration due to new added generation, tripped lines, and changing load patterns, frequent adjustments are required. Shunt reactors can also be part of static VAR control systems, in combination with switched capacitors [3] and controlled reactors. The reactors may be entirely thyristor controlled, or controlled by a combination of tapchanger and thyristors. These applications often are a matter of large capacity and high number of operations.

6.1.2 What Is the Special Problem? In transformer application, the leakage reactance of the tapped section is very low. To get an idea of the relative magnitudes in a power transformer, and a shunt reactor, we may consider the following lines of thought. The argument is not exact, but gives a first introductory idea, particularly for the impatient reader who does not want too much mathematics, why reactor tapchanging is different from the transformer. We take a shunt reactor and a power transformer of the same rating. The leakage reactance of a tap section of a transformer may be taken as proportional to (Turns of the Tap/Total turns)2 · X where X is the leakage reactance of the transformer. In the case of a shunt reactor, at the time of changing taps, we have a situation where the tap turns form the secondary of the rest of the untapped zone (Fig. 6.1e). This transformer by definition has 100%

6.1 Chapter Content

227

leakage reactance. The leakage reactance of a power transformer for regulating voltage is typically 8–12%. For the same tap size, the leakage reactance of a tap of a shunt reactor must be 12.5–8.3 times of the corresponding value of a voltage regulating transformer. This is quite high. The nonzero leakage reactance causes a phase shift between the interrupted current and the recovery voltage. At current zero, there is a nonzero recovery voltage, unlike a power transformer. This makes arc interruption more difficult. We need some analytical tools to estimate the interrupted current, recovery voltage, and the phase shift, to evaluate the adequacy of a tapchanger as a switching device in a reactor application.

6.2 Tapchanging in Shunt Reactor 6.2.1 Removing Turns Figure 6.1 Page 1 shows the process of removing active turns from (Fig. 6.1a–f) a reactor. Page 2 shows adding to (Fig. 6.1g–l) the active turns.

6.2.2 Steps Corresponding to Removal of Turns The first stage of transition in, Fig. 6.1, the switching direction for the removal of turns is shown in Fig. 6.1a–f. This is similar to any transformer switching, consisting of the W contact switching the main current I with a recovery voltage of RI. The fact that the application is for a reactor does not impact on this switching duty. In the second stage, shown in Fig. 6.1c the arc may quench at X after the bridging condition, before Z closes. In that case we have once again a switching condition similar to normal transformer tapchange in spite of the reactance in the local loop. But in all likelihood the reactance in the loop may prevent such arc quenching at X before Z closes. The arc must quench at X after Z closes (Fig. 6.1d), a situation represented by the equivalent circuit of Fig. 6.1e. In order to evaluate the switching duties of the tapchanger, we need to find the current I 2 in Fig. 6.1e. This current will be interrupted at X. The recovery voltage is the voltage across the terminals Z and X after arc quenching, as shown in Fig. 6.1f. To find the recovery voltage corresponding to Fig. 6.1f, we note that

Therefore,

Vs = I Z 1

(6.1)

V2 = I Z m

(6.2)

228

6 Special Applications

 V2 = Vs

Zm Z1

 = Vr

(6.3)

The recovery voltage is in phase with V s . For deriving the arc current I 2 we resort to the Thévenin theorem. The voltage across the open contacts Z and X is the Thévenin source voltage. This is the recovery voltage defined in Eq. 6.3. The source impedance is Z th , defined for the connections shown in Fig. 6.3f.

6.2.2.1

Determination of Zth

Z th is the impedance seen at the terminals X and Z with the primary shorted. As a step to finding Z th we define reactances X 2sc shown by Fig. 6.2d, e. These are akin to the leakage reactance in the case of the transformer. Tools [4–6] are available to estimate these quantities at the design stage. Alternatively the value can be determined by actual measurement according to Fig. 6.2. Direct measurement gives

R

R X

W

(a)

Y

R Z

OPERATION AT TAP

W

(b)

R X

Y

R Z

W OPENS WITH ARCING

I 1

R

R W

I2

X

Y

W

(c)

R Y

X

Z

X ARCS AND QUENCHES BEFORE Z CLOSING

I

Z X R

Z X R

Z

(d) Z CLOSES BEFORE ARC QUENCHING AT X

(e) EQUIVALENT CKT BEFORE ARC QUENCHING

(f) EQUIVALENT CKT AFTER ARC QUENCHING

DIRECTION OF MOVEMENT

Fig. 6.1 a–f Page 1: Reactor during tapchange direction removal of turns. g–l Page 2: Reactor during tapchange direction adding of turns

6.2 Tapchanging in Shunt Reactor

R

R

R Y

X

W

229

(g) OPERATION

Z

(h)

AT TAP

R X

W

Y

Z OPENS WITH ARCING

R Z

W

R X

Y

Z

(i) Y ARCS AND QUENCHES BEFORE W CLOSING

1

R

R

W

2

R

R X

Y

(j) W CLOSES BEFORE ARC QUENCHING AT Y

Y W

Y W

Z

(k) EQUIVALENT CKT BEFORE ARC QUENCHING

(l) EQUIVALENT CKT AFTER ARC QUENCHING

DIRECTION OF MOVEMENT

Fig. 6.1 (continued)

Z 2sc1 =

Iint =

Vs

V2sc1 I2   Zm Z1

R + j X 2sc1

(6.4)

(6.5)

The phase shift between the interrupted current and recovery voltage is arctan

X 2sc1 R

(6.6)

6.2.3 Steps Corresponding to Adding Turns Figure 6.1b Page 2 corresponds to Fig. 6.1 Page 1 but for the direction of adding turns. The first stage of transition in the switching direction for the adding of turns is

230

6 Special Applications

V1

V2

(b) M E A S U R E M E N T O F

(a) M E A S U R E M E N T O F

S E LF IN D U C TA N C E Z

S E LF IN D U C T A N C E Z

V

V

(d) C O N N E C T IO N S F O R

(e) C O N N E C T IO N S F O R

M E A S U R E M E N T O F X2sc1

V2

(c) M E A S U R E M E N T O F

MUTUAL REACTANCE Z

M E A S U R E M E N T O F X2sc2

Fig. 6.2 Measurement of parameters

shown in Fig. 6.1g. This is similar to any transformer switching, consisting of the Z contact switching the main current I with a recovery voltage of IR. The fact that the application is for a reactor does not impact this switching duty. In the second stage, as shown in Fig. 6.1i the arc may quench at Y before W closes. In that case we have once again a switching condition similar to normal transformer tapchange, in spite of the reactance in the loop. But in all likelihood the reactance in the loop may prevent such arc quenching at Y before W closes. The arc must quench at Y after W closes (Fig. 6.1j), a situation represented by the equivalent circuit of Fig. 6.1l. In order to evaluate the switching duties of the tapchanger, we need to find the current I 2 in Fig. 6.1k. This current will be interrupted at Y. The recovery voltage is the voltage across the terminals W and Y after arc quenching in Fig. 6.1l.

6.2 Tapchanging in Shunt Reactor

231

Vr2 = V2 =

Vs (Z 2 + Z m ) Z 1 + Z 2 + 2Z m

(6.7)

The recovery voltage is in phase with the supply voltage V s .

6.2.3.1

Arc Current

The arc current is the recovery voltage divided by the Thévenin source impedance Z 2sc2 in series with R. Z 2sc2 is defined in Fig. 6.2e. Ir2 =

Z2 + Zm Vs Z 1 + Z 2 + 2Z m R + j X 2sc2

(6.8)

The interrupted current lags the recovery voltage by arctan

X 2sc2 R

(6.9)

A different approach to the evaluation of the interrupted currents and recovery voltages is presented in [7].

6.3 Some Observations on Tapchanger Application for Reactor Service The conditions of service in reactor application are completely different to the transformer. The following remarks apply specifically to two resistance flag cycle tapchangers. 1. The effective reactance in the local loop during bridging is higher. This may prevent successful arc quenching at the transition contact after bridging. In most tapchanges, the arc may quench only after the remote main contact closes. The geometry of most tapchangers is such that the contact gap of the transition contacts is high in this condition, which should help quenching. It must be evaluated whether the tapchanger will quench the arc after the main contact on the closing side closes. 2. A similar situation exists when current commutates between the coarse and fine in a normal transformer application. Experience with switching duty tests conducted under transformer tapchanging conditions, i.e. in accordance with IEC 60 214. Cl. 5.2 may not be extendable, as the phase shift between the interrupted current and the recovery voltage is unique to reactor application. New database may have to be established.

232

6 Special Applications

3. The values of the Z 1 and Z m used in the above derivations change from tap to tap. It is therefore necessary to evaluate switching duties at all taps. 4. The transition resistance must both limit the circulating current and diminish the phase shift between the interrupted current and recovery voltage. A high resistance will reduce the phase shift. But the magnitude of the recovery voltage when W or Z interrupts goes up. 5. The coupling between the tapped coil and the rest of the winding depends on the construction of the reactor. Reactors executed as a single coil on a gapped magnetic core (Fig. 6.3a) exhibit a behaviour closest to a transformer. There is a reasonable coupling between the taps and the rest of the winding. 6. If the winding consists of two series-connected coils on two limbs (Fig. 6.3c), taps located on only one limb will switch at a time. Almost the entire self inductance Fig. 6.3 Types of reactor construction

(b) (a)

(d) (c)

(f) (e)

6.3 Some Observations on Tapchanger Application for Reactor Service

233

of the tapped zone acts as the leakage. In other words there is hardly any coupling between the tapped coil and the rest. 7. Air cored, magnetically shielded reactors (Fig. 6.3c) behave the same way. The high leakage reactance helps in lowering the circulating current as compared to the transformer service, but causes high phase shift between the interrupted current and the recovery voltage. For application in HV shunt reactors the inductance in the commutating loop is high. This leads to high phase shifts. It may be wise to consider tapchangers with vacuum switching, to cater for the enhanced switching demand.

6.4 Tapchanger in a System The most frequent application of a tapchanger is its connection to a transformer, feeding a radial line, with no further interconnections, as shown in Fig. 6.4a. A tapchange creates an incremental-induced voltage equal to the tap, but the incremental terminal voltage differs due to the impedance drop in the transformer. The impedance drop is usually quite small, so that the incremental terminal voltage is almost equal to the tap voltage and is in phase. Figure 6.4e shows the vector diagram. The situation is quite different when the transformer is part of a large interconnected system. Figure 6.4c shows two large systems interconnected by a transmission line. By “large system” we mean one in which the short-circuit level is much higher than the rating of the transformer. For instance the 100 MVA transformers of Fig. 6.4c could be backed up by systems of 1000 MVA short-circuit level. On a 100 MVA base, the systems could be represented by a generator with 0.1 PU impedance. It is more difficult to put a figure on the transmission line reactance, as it depends on the length of the line and the voltage class. A typical 100-km-long 220 kV line has a reactance of about 0.012 PU, a 400 kV line 0.0012 PU and a 765 kV line 0.0002 PU [7, p 32] on 100 MVA base. We consider a 220 kV line of 100 km and take a line reactance of 0.012 PU for this illustrative example. The circuit can be reduced to the configuration of Fig. 6.4d. If in this system transformer T 1 makes a tapchange, injecting a step voltage E as shown, the resulting current though T 1 is I = E/j0.165. Of this current the part that flows into system 1 is I1 = j0.1/j0.167I = 0.6I. The remaining 0.4 I flows into system 2 over the transmission line. The resulting bus voltage increment in system 1 is E 1 = j0.1 × 0.6 I, for system bus 1, or about 0.36 E. For the bus voltage in system 2 the change in voltage is

234

6 Special Applications

(b)

(a)

(d)

(c)

(e)

Fig. 6.4 Tapchanger in a system

E 2 = j0.067 × 0.4 I, or about 0.162 E. Figure 6.4b shows that the incremental current is at quadrature to E. In a more realistic case of a power exchange of 1PU at near unity power factor from system 1 to system 2, the incremental voltage will be much smaller.

6.4 Tapchanger in a System

235

6.4.1 General Conclusions The conclusions of these estimates for a typical case are as follows: 1. In a large interconnected system, a tapchanger fails to produce significant changes in the terminal voltages, even though the incremental-induced voltage of the transformer is the tap voltage. 2. A tapchange produces an incremental current which is fully reactive when resistances are negligible. 3. A larger part of the reactive kVA flows into the local system. A smaller part transfers to the far end system. 4. The real power flow from one system to the other remains unaffected.

6.4.2 Tapchanger During Abnormal System Operation Ref [8] discusses in detail and cites other references to the role of tapchangers when abnormal system operating conditions prevail. These include voltage swings threatening interconnections at nodes of a grid network. Under such conditions tapchangers have a useful role in maintaining swings under control and preserving system stability. The subject however delves deep into system stability which is outside the scope of this book.

6.5 Control of Real Power The most important conclusion from the above is that a tapchanger which produces a voltage change in phase cannot control the flow of real power. There are situations where control of real power is required. One instance with reference to Fig. 6.5 could be that there is a surplus generation in system 1, which needs to be evacuated to system 2. Let us suppose that we can produce and inject a voltage E, which is in quadrature leading to the local phase vector V s at the sending end as in Fig. 6.5a. In order to simplify the issue of power transfer at first, we shall ignore all resistances and leakage reactances associated with the injected voltage. The injected voltage sets up an incremental current substantially in quadrature lagging to E (See Fig. 6.5c). This is because the network has only the line reactance. When E is reversed, as in Fig. 6.5b the injected current reverses also, thereby delivering power from the receiving end into the sending end (See Fig. 6.5d). There are methods of producing and injecting a voltage such as E. We shall examine these shortly. The voltage injection will be required to be reversible to enable the transfer of power in either direction. A method of transformer connection which yields a quadrature voltage such as ±E constitutes a phase shifter (PST). For the present we consider the

236

6 Special Applications

SYSTEM 1 TIE LINE

SYSTEM 2

SYSTEM 1 TIE LINE

jX

E

jX

E

(a) RETARD OPERATION

SYSTEM 2

(b) ADVANCE OPERATION

E Vs1

Vs1

a0

b

b

f

a

E

Vr

a

E 0

Vr

a

f

Vs

Vs

(c) ADVANCE VECTOR DIAGRAM

(d) RETARD VECTOR DIAGRAM IDEAL CONDITIONS: ONLY LINE HAS A REACTANCE

SYSTEM 1 TIE LINE

E

Rs+jXs

SYSTEM 2

RL+jXL

Rr+jXr

SYSTEM 1 TIE LINE

E

Rs+jXs

SYSTEM 2

RL+jXL

Rr+jXr

(f) ADVANCE OPERATION

(e) RETARD OPERATION

-

Vs

XL Vs1

a

a f

-

b

Xr

XL

Vs6.5

Vr1

0

E

E

b

Xr RL

Vr

Vs6.5

Vr1

f

Rr

0

Vr Rr

E RL Vs Vs1

E

Rs

(g) RETARD VECTOR DIAGRAM

(h)

PRACTICAL CONDITIONS: ALL IMPEDANCES INCLUDED

Fig. 6.5 a–d Page 1: Advance and retard control. e–h Page 2: Advance and retard control

Xs

6.5 Control of Real Power

237

effect of injecting such a voltage in a practical case, including resistance and leakage inductance of the phase shifter. In Fig. 6.5e, f the actual connections, including the resistances and leakage reactances, are shown. The receiving end voltage V r is the reference vector. The injected voltage E is in quadrature lead to the sending end voltage in Fig. 6.5e. The vector diagram in Fig. 6.5g corresponding to Fig. 6.5e shows that the receiving end voltage V r leads the sending end voltage V s . This connection is referred to as advance condition. The vector diagram in Fig. 6.5h corresponding to Fig. 6.5f shows that the receiving end voltage V r lags the sending end voltage V s . Under the retard condition the phase shift is increased by β, while on advance condition it is reduced by the same angle. Since the maximum phase shift is defined at the no load condition, it is not possible to achieve this angle on load in the advance operation, as the internal regulation reduces it.

6.5.1 Some Features and Terminology for Phase Shifters The following features implicit in Fig. 6.5 are noteworthy. 1. The phase shifter determines the angle α at no load. This depends on the amount of injection E. It is easy to see that the no load phase shift

α=

tan−1 E VS

(6.10)

The maximum phase shift at no load α m determines the value of the voltage requirement of the PST. PSTs are rated on the basis of the maximum no load shift. E max =Vs tanαm

(6.11)

2. E could be fixed, but in most practical cases is variable to offer better control. This is achieved by fitting the shifter with a tapchanger. It will be seen shortly that the requirement may be for more than just one physical tapchanging device. (See also [7, Table 1, page 22]). The no load phase shift α depends on the relative magnitudes of E and V s according to Eq. 6.12. 3. When the voltage injection is in the direction to cause a transfer of power from the sending end to the receiving end, it results in the receiving end voltage vector V r leading the sending end voltage vector V s (Fig. 6.5a, c). This is referred to as the advance mode. In the advance mode the phase shift on load between the sending and receiving ends is less than the no load angle α (See Fig. 6.5c) by the angle β. Phase shifters are rated by the phase shift in the no load condition. In the advance mode, it is impossible to operate on load at the max rated phase shift α m , due to internal regulation. The situation is analogous to the “normal” non-phase shifter transformer application, where the maximum no load voltage

238

4.

5.

6.

7.

6 Special Applications

cannot be achieved on load due to internal reactance drops. Since the purpose of the phase shifter is to achieve designed phase shifts between the two ends, this reduction must be taken into account while designing the phase shifter. This has an implication on the voltage across the range of the tapchanger. When the voltage injection is in the direction to cause a transfer of power from the receiving end to the sending end, it results in the receiving end voltage vector V r lagging the sending end voltage vector V s (Fig. 6.5b, d). This is called the retard mode. In the retard mode when on load, the phase shift β between the sending and receiving ends is greater than the no load angle α (See Fig. 6.5d). The voltage applied to the phase shifter is the vector difference between V s and V r . With increased β the voltage gets higher. There may be core saturation. The voltage across the tapping range stands increased compared to the no load condition. It is necessary to verify the maximum internal tapchanger voltages and satisfy that the capacity is not exceeded. At light loads, and small values of α (implying low injection), the problems brought out in 3 and 4 above may not exist. At or close to maximum no load shift α m , and high loads, these problems surface. In particular, attention must be paid to overload conditions. Under system short-circuit conditions the transformer may well saturate in the retard mode. This however is a matter for transformer design rather than tapchanger application. It may be pointed out, as a matter of academic interest, that some of these problems arise because the phase shifter is a series device. It does not have total control over the voltages that it sees, unlike traditional applications. In traditional applications, the voltage applied to the transformer drops with load due to internal regulation, barring the extraordinary case of capacitive loads. This may not always be so in a phase shifter. Further in conventional application, the short-circuit current is limited by the leakage impedance of the transformer. The through short-circuit current in a PST is not much affected by the PST, unless a leakage impedance is deliberately built in by design of the PST. Even then positions of the tapchanger may occur in some modes of connection, where the PST reactance is ineffective in limiting the through current. See also [7, 9].

6.6 Single Core Phase Shifter Connections and Their Influence on Tapchanger Selection Figure 6.6 which is in accordance with Fig. 7a of [7] (IEC) shows a transformer which is capable of injecting a voltage in quadrature with the phase voltage, and therefore is a PST. All windings are on one core. The triangle RYB in Fig. 6.6c represents a delta-connected exciting transformer. The PST windings 1 and 2 are wound on the same core, but on the leg of YB. The voltage induced in the PST coils is thus at quadrature with V rn . The PSTs are provided with two tapchangers per phase to vary

6.6 Single Core Phase Shifter Connections and Their Influence …

(a)

(b)

(c)

(d)

(e)

(f)

239

Fig. 6.6 Symmetrical single core phase shifter according to Fig. 7a of IEC 62 032

the injected voltage. In Fig. 6.6a the injection is maximum. This gives a phase shift of 2α m as seen in the corresponding vector diagram in Fig. 6.6b. In Fig. 6.6c the injection is zero, and the voltages V s , V r , and V rn are equal. This is shown in the vector diagram in Fig. 6.6d. In Fig. 6.6e the injected voltage is again at the maximum, but in the opposite direction.

6.6.1 Single Core Asymmetrical PST The transformer in Fig. 6.7 is a simpler version of Fig. 6.6. This connection is used for smaller rated PSTs with limited angle excursion. The connection is based on Fig. 7b of IEC 62 032 [7]. It has only one PST coil. There is only one tapchanger with reverser per phase. Otherwise the construction is same as in Fig. 6.6. Figure 6.7b is the vector diagram corresponding to Fig. 6.7a. All vector diagrams depict no load condition. Figure 6.7a shows the retard mode. Figure 6.7c where the reverser has

240

6 Special Applications

(b) (a)

(c)

(d)

Fig. 6.7 Single core asymmetrical PST-connection according to Fig. 7b of IEC. 62 032

operated shows the advance mode. Figure 6.7d is the corresponding vector diagram. The injection of the PST coil voltage causes a difference in the phases as well as absolute magnitudes of V r and V s . In other words the voltage injected by the PST has a component in phase with the sending end voltage. We should keep in mind that the vector diagrams are for no load. The in line component of the injected voltage will cause a reactive current flow in the system when closed. A winding with tapchanger may have to be provided to compensate for the in line variation if such a reactive power generation is not desired.

6.6.2 Lack of Self Protecting Leakage Impedance In the connections of Figs. 6.7 and 6.11 there is a position at which the PST winding is not in circuit, but the leads and the contacts of the tapchanger are (Fig. 6.8). A through fault is not limited by the leakage impedance of the winding. The situation is nearly same even one or two tap positions away from those depicted. See [10]. It

6.6 Single Core Phase Shifter Connections and Their Influence …

Vr

K

V1

R PH Vs

R PH

A

Vr V1 FINE

A

Vs

Vrn

C

K V1 COARSE

241

Vrn

B

(a) WITH REVERSER ONLY ONE PHASE SHOWN. LOW IMPEDANCE POSITION.

C

B

(b) WITH COARSE FINE ONLY ONE PHASE SHOWN LOW IMPEDANCE POSITION

Fig. 6.8 Low impedance positions

may be necessary to include a series reactor to provide succour for the transformer, which otherwise has to withstand much higher short-circuit current. Further below an elegant way is presented which solves the low through the impedance.

6.6.3 Ratings of Tapchanger for the PST of Fig. 6.7 We concern ourselves here only with the ratings which are special for PST applications, namely the through current and the tap range voltage. The PST is connected in series with the line interconnecting the sending and receiving ends. The current rating is therefore that which corresponds to the transfer of power between the two systems. If the rated power of the interconnecting tie is P, the current is P I =√ 3V

(6.12)

where V is the line-to-line voltage of the system.

6.6.4 Voltage Over the Tapping Range It is customary to define the shift angle α produce by the PST at no√load. Looking at Fig. 6.7b we note that V 1 for maximum no load α m = V tan α m / 3. Here V is the system line-to-line voltage. This magnitude is the voltage across the tapping range. But we recall from Fig. 6.5 that the internal reactive drop and the line regulation cause a lower shift when loaded in the advance mode. The designed α m may only be reached with a higher voltage across the tapping range to compensate for the series regulation. The voltage per step is E = V 1 /N, where N is the number of tappings.

242

6 Special Applications

6.7 Switching Capacity Per Tap as a Limitation in PST Application The step kVA required for the PST begins to worry about the largest commercial available tapchangers. Figure 6.9 shows the throughput for a tapchanger with a very large step kVA. In Fig. 6.10 the maximum step capacities mentioned, shows that the PST applications could reach the maximum capacity of tapchangers. It becomes necessary either to aggrandize the capacity by paralleling switching elements, or increasing the number of steps.

6.7.1 Use of Coarse/Fine with Advance/Retard Switch PSTs are usually very large throughput transformers. Throughput sizes like 800 MVA+ are not uncommon. With such an application, the step rating of the tapchanger can be exceeded [8]. One method of tackling this limitation is to use a coarse/fine switch with the tapchanger, to increase the number of steps (See Fig. 6.11). With increased number of steps, the kVA per step can be brought down. A coarse/fine can take the shape of a multiple coarse switch which can give very large number of steps. Throughput power versus no-load phase angle step capacity 5000 - 6000 kVA, +/ 32 steps

90

no-load phase angle (degree)

80 70 60 50 40 30 20 10 0 200

400

600

800

1000

1200

1400

1600

throughput power (MVA) 5500 kVA

Fig. 6.9 Step kVA and throughput

5000 kVA

6000 kVA

1800

6.7 Switching Capacity Per Tap as a Limitation in PST Application

Fig. 6.10 Step capacity of various ABB tapchangers

243

244

6 Special Applications

(a)

(b)

(c)

(d)

Fig. 6.11 Coarse fine application

Figure 6.11 shows the use of a single coarse step switch applied to a phase shifter. When a reversing type tapchanger is used (Fig. 6.7) with a phase shifter, the reversing switch enables advance and retard operations by reversing the polarity of the injected voltage. As such a switch is not available with a coarse/fine, an advance/retard switch (ARS Switch) needs to be added. The ARS is essentially an “off current” operation. It reverses the winding terminal connections between the “in” and “out” lines when the winding does not carry current, but allows a path for the through current at all times. The way in which the advance/retard switch operates is shown in Fig. 6.11d. In the course of its operation, the ARS switch shorts all terminals. This is only possible if they are otherwise equipotential. Such a condition occurs only when the tapchanger is in a specific position, for example, as shown in Fig. 6.11c.

6.8 Symmetrical Arrangement of Twin PSTs on a Single Core

245

6.8 Symmetrical Arrangement of Twin PSTs on a Single Core A study of the sequence of switching shows that it may be possible to drive all four tapchangers by one mechanism. Usually two drive mechanisms are used to achieve a useful participation of at least one-phase shifter in the short-circuit path, contributing some impedance. See Fig. 6.12. The tapchangers are directly connected into the HV line, and therefore, will have to be rated for high voltage. In practice, they will be single pole tapchangers. The step capacity required is reduced as there are now two PSTs and thus each contributes to the total number of operating positions. The step capacity can be halved, or the throughput doubled for the same step capacity. Each PST provides for a maximum no load shift of α m . The connection enables both advance and retard operations using the reverser of the tapchanger. The maximum phase shift in either operation is 2α m . Figure 6.13 shows a scheme of operating the tapchangers in a planned manner to avoid low impedance. While achieving the phase shift of ±2α m , the impedance of at least one PST remains in the current path. The mechanism controls must be programmed to achieve the correct sequence.

6.9 Use of Two-Phase Tapchanger An inexpensive solution, using one two-phase tapchanger per transformer phase is available, when the step capacity is not a limitation. Figure 6.14, which is based on Fig. 8 of [7], shows how the diverter switches of the two phases are connected together. As the selectors are driven by a common mechanism, they can be arranged so that the selector contacts are always driven in the opposite sense in Fig. 6.14, i.e. the selectors are always positioned symmetrically to the centre. The vector diagram of Fig. 6.14b shows that the sending and receiving end voltages are always equal in magnitude. No load phase angles of 0 through 2α m can be realized in both advance and retard operation as can be seen from the vector diagrams, and the corresponding tapchanger positions are shown in Fig. 6.13.

6.10 Tapchanger Ratings for the PST of Fig. 6.9 The PST is in series with the system. It must therefore be rated to carry a current corresponding to the designed power transfer P between the ends, at the rated voltage V. Therefore P I =√ 3V

(6.13)

246

6 Special Applications Vs R PH

Vr R PH

KK V1

V1

R Vs Y PH

Vrn

V1

Vr B PH

V1

Y

V1

B

V1

Vs R PH KK

Vr Vs Y PH B PH (a) SYMMETRICAL TWIN PSTS CONNECTIONS Vr 2V1 Vs R PH Vr Vs SENDING

m

m

Vr RECEIVING

m

m

V1

V1

2V1

R

(b) MAX. SHIFT

(c) VECTOR DIAGRAM

POS RETARD

Vs R PH

KK V1

(d) SENDING AND RECEIVING END VOLTAGE TRIANGLES

Vs Vr

Vs=Vr

Vr R PH V1

R

(e) ZERO SHIFT

(f) VECTOR DIAGRAM

POSITION

Vs R PH

KK V1

Vs

Vr

Vr R PH

m

(g) SENDING AND RECEIVING END VOLTAGES

Vr RECEIVING

2V1 m

Vs

SENDING

m

m

V1

R

(h) MAX. SHIFT

POS ADVANCE

(i) VECTOR DIAGRAM

(j) SENDING AND RECEIVING END VOLTAGE TRIANGLES

Fig. 6.12 Symmetrical twin PSTS

The voltage over the range must be enough to cause the maximum no load shift 2α m in Fig. 6.9. Then  Vs V1 = √ sinαm 3 

(6.14)

6.10 Tapchanger Ratings for the PST of Fig. 6.9 Vs

K

Vs

Vr

K

V1

K

K

Vs

(d) POSITION OF PHASE SHIFT BETWEEN m AND ZERO RETARD Vs Vr K K

K

V1

(g) POSITION OF PHASE m

RETARD

Vr

K

K V1

Vr

Vs

V1

Vr

K

K

V1

V1

V1

(h) POSITION OF PHASE SHIFT BETWEEN - m AND - 2 m ADVANCE

ADVANCE

m

(f) POSITION OF PHASE SHIFT BETEEN 0 AND - m ADVANCE

K

V1

V1

SHIFT Vs

V1

K

V1

SHIFT -

K

Vr

K

(c) POSITION OF PHASE m

(e) POSITION OF ZERO PHASE SHIFT Vs

K V1

Vr

V1

V1

Vs

V1

(b) POSITION OF PHASE SHIFT BETWEEN 2 m AND 1

Vr

V1

Vr

K

V1

V1

(a) POSITION OF MAX PHASE SHIFT 2 m RETARD Vs

K

247

(i) POSITION OF PHASE SHIFT - 2 m ADVANCE

Fig. 6.13 Sequence of operation of tapchangers Vr

Vs K

V1

V1

A

Vr

K

V1

V1

A

Vr

K

V1

A

K

(e) POS OF MAXIMUM INJECTION

Vrn

K

V1

(h)VECTOR DIAGRAM

V1

A

(i) POS OF ZERO INJECTION

(f) VECTOR DIAGRAM

Vrn

(j) VECTOR DIAGRAM Vr

2V1 Vs

Vr

K

m

Vs = Vr

K

Vr m

Vs

Vrn

Vr

Vs

2V1 Vs

V1

V1

A

(g) RANDOM POS

K

Vrn

(d) VECTOR DIAGRAM

Vs

V1

Vs

Vs = Vr

K

(c) POS OF ZERO INJECTION

Vr

K

(b) VECTOR DIAGRAM

(a) RANDOM POS Vs

2V

Vr

Vrn

K

Vr

Vs

2V

Vs

V1

V1

A

(k) POS OF MAXIMUM INJECTION

RETARD

K

Vrn

m

m

(l) VECTOR DIAGRAM

ADVANCE

Fig. 6.14 Application of two phase tapchanger

Here V s is the line-to-line voltage at the configuration of α m . The √ value of V s is dependent on the injection angle. V s at any injection angle α is V rn 3/cosα, where V rn is the phase to neutral voltage at zero shift. Thus V1 = Vrn tanα

(6.15)

248

6 Special Applications

6.11 Variable Inline Flux During Phase Shift The actual increase of the volts per turn between extreme taps is 1/cosα m = 1.035 for a 30° total shift (α m = 15°). At 48° total phase shift, the change in voltage/turn is about 8.7%. If the required phase shift is still higher, the mater may have to be looked into.

6.12 Delta Hexagonal Phase Shifter The concept of the delta hexagonal phase shifter is shown in Fig. 9 of [7]. Figure 6.15 is an elaboration of this connection. As is common in single core designs, the desired phase-shifted voltage to be connected to the R phase is obtained from a coil wound on the same core but on the leg connected to the Y and B phase. Two linear tapchangers are used per phase. The tapchangers are operated in opposite directions, i.e. one goes from tap 1 to tap maximum, while the other starts at maximum tap and runs towards tap 1 position. The injected series voltage is the difference between their selected taps. Figures 6.15b–g show the positions of the two tapchangers and the corresponding vector diagrams. In the position corresponding to Fig. 6.15d corresponding to zerophase shift the PST does not offer any series leakage impedance to restrict the through current. It may be damaged by the much larger through fault current. It will be R PH Vs

R PHVr

OLTC 1

V1 VsVr

V1

m

m

OLTC 2 R PH

Vs

(c) VECTOR

(b) MAX POS

OLTC 1 V1

R PH

DIAGRAM

Vs

R PHVr

OLTC 2

OLTC 1

Vr

Vs=Vr

V1

OLTC 2

(e) VECTOR

(d) ZERO PHASE SHIFT (a) CONNECTION

R PH Vs

R PH

OLTC 1

Vr

V1 VrVs

V1

OLTC 2

(f) MAX

ADVANCE POS

Fig. 6.15 Delta hexagonal PST

DIAGRAM

m

m

(g) VECTOR DIAGRAM

6.12 Delta Hexagonal Phase Shifter

249

necessary to include a series reactor to limit the through fault current. This of course increases the total reactance drop between V s and V r under load conditions and further reduces the phase shift in advance operation. The current limiting reactance increases the load phase shift β and may cause over excitation in the retard condition.

6.12.1 Tapchanger Ratings for the PST of Fig. 6.15 The PST is in series with the system. It must therefore be rated to carry a current corresponding to the designed power transfer P between the ends, at the rated voltage V. Therefore P I =√ 3V

(6.16)

Here V is the system line-to-line voltage. The voltage over the range must be enough to cause the maximum no load shift 2α m in Fig. 6.15c. Then V1 = 2Vrn tanαm

(6.17)

The step voltage is V 1 /n, where n is the number of taps.

6.13 Limitation by Step Capacity The use of a linear tapchanger in the delta hexagonal scheme of Fig. 6.15 implies smaller number of steps. This may limit the application to smaller PSTs with lower no load phase shift. One way in which this can be compensated is shown in Fig. 6.16. Here, there are two PST coils in series, each of which is connected to two tapchangers. This will need four tapchangers per phase. The total no load phase shift could be twice that of each PST. The number of steps is doubled, so that for the same step kVA of the tapchanger, the throughput can be doubled. Further by using a switching sequence of the tapchanger similar to that discussed in Fig. 6.13 it is possible to retain a minimum leakage impedance of at least one PST at all times. This will obviate the necessity of a series reactor.

250

6 Special Applications

R PH Vs OLTC 1

OLTC 3 V1

V1 OLTC 2

OLTC 4

R PH Vr

DELTA HEXAGON

Fig. 6.16 Two PSTS in series Vs

Vs

Vr B PH K

K

K

Vs

V1

Vr

V1

Vs

R PH

R PH

R PH K

Vr

Vr

V1

V1

(b) TAPCHANGER POS FOR MAX ADVANCE

V1

V1

(d) TAPCHANGER POS FOR MAX RETARD

V1

Vs

Vr

Vs

K Vr

(a) CONNECTION ALTERNATIVE 1 FOR PST

Vrn

(c) VECTOR DIAGRAM

(e) VECTOR DIAGRAM

FOR MAX ADVANCE

FOR MAX RETARD

Vs

Vs

Vs Vr

R PH Vr

Vrn

Vr

Y PH Vs

K

K

R PH

R PH Y PH

V1 V1

Vr

K

K Vr

V1

Vs

(g) TAPCHANGER POS FOR MAX RETARD V1 Vs K Vs

Vrn Vr

V1

V1

(i) TAPCHANGER POS FOR MAX ADVANCE Vr

V1

Vs Vr

Vrn

B PH

(f) CONNECTION ALTERNATIVE 2 FOR PST

Fig. 6.17 PST in star execution

(h) VECTOR DIAGRAM

(j) VECTOR DIAGRAM

FOR MAX RETARD

FOR MAX ADVANCE

6.14 PST Implemented with Star-Connected Transformer

251

6.14 PST Implemented with Star-Connected Transformer Figure 6.17 shows a PST implemented with an exciter connected in star. The R phase exciter is connected in series with a PST coil wound either on the Y leg or B leg. Figure 6.17a, f show the two alternative connections. The reversing switch enables advance or retard operation. The vector diagrams are valid for no load condition. The following features are implied by the connections and the corresponding vector diagrams. 1. The injected voltage shows a component in phase with the phase to neutral voltage. This is unlike the PSTs of above. The in phase voltage will cause a reactive power flow when the circuit is closed. This may not be desirable. If so, a further tapchanger to regulate the in line component may be introduced. For the connection of Fig. 6.17a, i.e. injection of B phase voltage into R phase the no load advance phase shift is smaller than the retard phase shift. The opposite holds for the connection of Fig. 6.17e. 2. At zero injection, no series leakage impedance is offered by the PST coils. It may be necessary to provide series reactors to limit the short-circuit current.

6.14.1 Tapchanger Ratings for the PST of Fig. 6.18 As is common with all series-connected PSTs the current rating of the tapchanger is P I =√ 3Vs

(6.18)

where P is the through power rating. Figure 6.18 shows the derivation of the tap range voltage V 1 . The tap step voltage is V 1 /n where n is the number of taps. Fig. 6.18 Tap range voltage

Vr

V1

Vr 120 -

a

B

60 °

A Vs Vrn

Vs A Vrn

60 -

12 0°

V1

r

B

a r

AB = Vs SIN V1 = Vs SIN

a = V1 SIN (120- a) a / SIN (120- a )

AB = Vs SIN V1 = Vs SIN

r = V1 SIN (60 r)

r / SIN (60 -

(a) VECTOR DIAGRAM

(b) VECTOR DIAGRAM

FOR MAX ADVANCE

FOR MAX RETARD

r)

252

6 Special Applications

6.15 Limitations of Single Core PSTs 1. The tapped winding and tapchanger are directly connected in the line between systems. They have to face the system short-circuit currents and voltage transients. 2. The current rating is determined by the through current of the system. The maximum injection voltage is determined by the phase shift required. These parameters cannot be changed. This makes it impossible to always use the full switching capacity per tap of the tapchanger. 3. In some connections, there could be a position of the tapchanger where there is no effective series leakage reactance to limit the short-circuit current, for example, as shown in Fig. 6.8. This is of course not always the case.

6.16 Dual Core PSTs Most of these deficiencies are removed by the use of a dual core construction (See Sect. 6.16.3). This is analogous to the use of series booster with the main transformer in conventional regulation Cl. 4.5.3, Page 19, of [9] introduces the basics of dual core design. The basic principle is that the PST is constructed on two separate magnetic cores. The windings on one of these are connected in series with the line connecting the sending and receiving ends connecting two systems (See Fig. 6.19). The transformer corresponding to this core is the series unit. The primary side of the series unit is connected in delta. It is fed from a variable source provided by the secondary of the other core. This core is the shunt unit. In Fig. 6.19, the R phase series winding (terminals R1.1 and R1.2) is excited by the primary, which in turn is fed from the Y and B phases of the star-connected secondary of the shunt unit. The primary of the shunt unit is also connected in star. Therefore the output from the Y and B phases of the secondary has a voltage at quadrature to the phase to line voltage of the primary (see Fig. 6.19b). When this voltage is applied to the primary of the series unit, the corresponding secondary injects a quadrature voltage into the R phase line. This satisfies the phase relationship of a PST. The reversing switch enables interchanging retard and advance (Fig. 6.19a, d). This PST can be implemented with one star-connected three-phase tapchanger, or in case higher step capacity is required by three single pole tapchangers in parallel. Other solutions are discussed below.

6.16.1 Tapchanger Current at Zero Injection Position It may be noted that at the position of zero injection the tapchanger shorts the primary of the series unit as shown in Fig. 6.19. It does not however short the tapped winding! The current is only the transformed series secondary line current. The tapchanger

6.16 Dual Core PSTs

253

V1

V1

V1

(b)

(a)

(c)

(d)

V1

V1

V1

(e)

(f)

Fig. 6.19 Asymmetrical dual core PST

carries the same constant current at all tap positions. At the zero injection position, the tapped secondary of the shunt unit and therefore the primary carry no current.

6.16.2 Inline Voltage Injection The vector diagrams in Fig. 6.19c, e show that the receiving end voltage increases with the phase shift, reaching a maximum at the maximum shift. This in effect introduces an in line component of voltage, leading to reactive power flow, which may not be

254

6 Special Applications

desirable. Where necessary it may be required to compensate the increased in line component. In this respect the operation of the dual core PST of Fig. 6.19 is similar to the asymmetrical phase shifter of Fig. 6.7.

6.16.3 Tapchanger Ratings for the PST of Fig. 6.19 One of the main advantages of the dual core construction is that it gives a choice regarding ratings of tapchanger. This choice lies in the selection of the transformer ratio r = nr. of turns in the primary/nr of turns in the secondary of the series unit. The series unit secondary connected with the line must carry the rated power transfer between sending and receiving ends. This as in the cases considered so far is P Ip = √ 3V

(6.19)

The current in the primary winding is I p /r. Because of the delta connection of the primary √ coils, the tapchanger connection is the current carried by the delta leads, which is 3 times the coil current. The tapchanger current rating is therefore I =

P rV

(6.20)

The selection of r is in the hands of the designer, who would optimize the ratings.

6.16.4 Voltage Rating The voltage that needs to be injected into the secondary of the series coil to achieve the required no load phase, looking at Fig. 6.22c, e shift is V1 = Vrn tanαm

(6.21)

Because of the delta star connection of the series unit, the primary voltage, which is equal to the tap range voltage, is √ Vrange = 3r Vrn tanαm

(6.22)

The step voltage will be 1/NV range for N equal steps. It may be noted that this expression already takes into account the variation of the voltage triangle (i.e. the size of the green and white deltas) in Fig. 6.22 with the shift angle.

6.16 Dual Core PSTs

255

6.16.5 Increasing the Number of Steps to Reduce Requirement of Step kVA

V1

V1

V1

V1

Figure 6.20 shows a connection using two reverser switches, with two tapped windings on the shunt unit secondary. The windings are connected in series. Two tapchangers will be needed per phase. One of them could be a star-connected unit, but the other must be single phase, or delta, with high withstand to ground. The use of the reverser restores the ability to interchange advance and retard operations, without an ARS switch. Also by employing the operating mode of the two tapchangers as discussed in Fig. 6.13 the reactance of at least one of the two series-connected windings remains in circuit at all tap positions. This will help in restricting the short-circuit current.

V1

V1

V1

V1

V1

(a)

(c)

Fig. 6.20 Two reversers in series in shunt unit

V1

(b)

(d)

V1

V1

256

6 Special Applications

6.17 Symmetrical Dual Core Phase Shifter Corresponding to Fig. 10 IEC 62 032 A dual core phase shifter similar to the single core PST of Fig. 6.9 is shown in Fig. 6.21. Figure 6.21 is in line with Fig. 10 of IEC 60 032. The significant difference from Fig. 6.19 is that the PST windings in series with the lines are centre tapped, and the corresponding line end of the shunt primary winding connects to the mid-point. The injected voltage is again in quadrature with the line to neutral voltage. The vector diagram in Fig. 6.21a shows that the sending and receiving end voltages are equal in magnitude though separated from each other by twice the injection angle α. There is however no net introduction of in line voltage component, since both the sending and receiving end vectors increment equally with respect to the in line component and cancel in the loop, leaving only the quadrature injected voltage effective. Thus the reactive power flow is not disturbed. In the configuration of Fig. 6.19 there is a net line voltage component leading to reactive power flow. In this respect the PST of Fig. 6.22 has an advantage over that of Fig. 6.19. As in the single-ended PST of Fig. 6.19 the execution of Fig. 6.21 needs one three-phase star tapchanger. If the step kVA is not adequate, two or three single-phase tapchangers may be connected

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 6.21 Dual core symmetrical PST

6.17 Symmetrical Dual Core Phase Shifter Corresponding to Fig. 10 …

(a) INTERNAL

257

(b) EXTERNAL

Fig. 6.22 Air insulated 11 kV 65 A 17 POS tapchanger

in parallel for each phase of the PST. The reversing switch enables advance or retard operation.

6.17.1 Tapchanger Ratings for the PST of Fig. 6.21 There is obviously no change in the current rating, which remains as in Eq. 6.22 above. The maximum voltage injected into the primary side of the series PST by the shunt secondary is V1 = 2Vrn tan αm

(6.23)

Taking into consideration the delta star connection of the series unit, the primary voltage which is equal to the tap range voltage is √ Vrange = 3r 2Vrn tan αm

(6.24)

The step voltage is 1/N of the range voltage for N equal steps. It is to be noted that α m is only half the maximum phase shift at no load.

6.17.2 Other Equivalents By now it must be obvious that every execution of the single core PST can be replicated by a dual core equivalent. The approach is to transfer the injected voltage and current in a coil connected directly in series to a secondary transformer circuit, supplied by transformer on a separate core.

258

6 Special Applications

6.18 Use of Coarse/Fine The most recognized crisis with PSTs is that in practical case the step switching capacity of the tapchanger limits the through rating of the entire unit. By increasing the number of taps, the step capacity for the same throughput can be reduced. A well-known method of increasing the number of taps is to use a coarse/fine. This method has been discussed in detail through the chapter.

6.19 Limitation of Single Core PSTs In Sect. 6.15 some of the limitations of single core PSTs are mentioned. We shall now examine how the dual core eliminates or softens these. 1. Removal from direct line connection. The tyranny of having to carry the shortcircuit current of the line is removed. However, the PST is still connected to the system, and isolated from the line only by a one transformation ratio, that of the series unit. It still has to carry the transformed ratio of the current, but the relief is that the ratio of transformation could be judiciously chosen to suit the rating of the tapchanger. The tapchanger is now buffered to some extent from line voltage transients. The tapchanger now faces transferred transients, which are not so severe as the direct hits. 2. Dual core design confers better flexibility. We consider for example a tapchanger with a step capacity of 2800 kVA, a maximum current of 800 A, and maximum step voltage of 4000 V. If we have a PST requirement of 600 A and 4500 V, the step capacity is only 2700 kVA. Yet in single core application, we cannot use this tapchanger since the step voltage exceeds the limit value. In dual core application we can bring the step voltage down by employing a ratio of 0.88. The step voltage is now 4000, and the current is 675 A. Both parameters are within the capability of the tapchanger. 3. In single core PSTs, there could be situations where the series injector coil is short-circuited by the tapchanger (Fig. 6.8). At this position the coil in series with the line offers no effective leakage reactance to limit the short-circuit current. In dual core executions, even at zero injection position where the primary of the series unit is shorted by the tapchangers, the leakage reactance of the series unit offers a limitation on the short-circuit current.

6.19.1 Further Reading For further reading on phase-shifting transformers see [10–13]. In particular [11] has an extensive treatment of calculation of short-circuit currents.

6.20 Tapchanger Operating in Air Environment

259

6.20 Tapchanger Operating in Air Environment With the development of commercial complexes, with high-energy demand, it became a Standard architectural practice to house a supply transformer in the basement. Fire safety regulations mandated that no inflammable oil could be used. This lead to the proliferation of small 500 kVA to 1000 kVA, 11 and 22 kV to 415 V cast resin, and other relatively flame proof, non-liquid filled transformers. Naturally these needed tapchangers without oil. Based on early experiments with their oil-filled 33 kV transformers, OLG developed a tapchanger for this specific application by modifying their tapchanger to work in air. The photo shows the internal arrangement with the terminal board removed. The terminal board is mounted on the open flange. The tapchanger is mounted on an extension platform from the core and coil assembly. A common dust, moisture and vermin-proof enclosure, usually IP 44, and cooling air flow from fans are shared with the transformer. In order to protect from unfiltered cooling air, a minimum of IP 44 enclosure of the tapchanger alone is recommended. Connections between the tapchanger and the transformer are run in air, using highvoltage flame retardant and low smoke emission insulated cables. Humans entering the enclosure must always ensure that high-voltage power is definitely switched off to the transformer. All closing plates and hand holes are gasketed to deny ingress to vermin. Ventilation is by two sets of louvres provide diagonally opposite on the tank. For indication of any internal failure, a smoke detector with annunciation contacts is incorporated. It was found that up to about 65 A (about 1250 kVA at 11 kV) such tapchangers gave reasonable life expectancy and manageable maintenance at low cost. The absence of oil and easy access to the live parts made replacement of contacts a minor undertaking. The commercially successful 35 A tapchanger later spawned a 65 A enhancement using fixed contacts on both sides of the phase board, arranged back to back, with a parallel connection directly on the phase board. This latter uses a current splitting reactor to ensure the sharing of current between the parallel paths. The metallic detritus from arcing collects at the bottom of the tank. The manufacturer advised cleaning out every 10,000 operations. Because of frank arcing in air, this tapchanger cannot be applied to explosive surroundings, such as mines or gasoline refineries.

6.21 Operational Problems Encountered with the Tapchanger In operations up to 1250 kVA, 11 kV, with an average of 4000 or less operations a year, contact wear was not an issue raised by clients, over a total of 12,000 units supplied in a period of 15 years. It was found that high humidity raises copper to copper contact stiction. In extreme cases the movement slows down to such an extent that tapchange becomes incomplete, and the transition resistance is damaged. After a few initial failures of this type OLG verified this mechanism of failure by placing a beaker of water at the bottom of a new tapchanger. The tapchanger transition time increased

260

6 Special Applications

notably after a short time. It is important that the user is aware of the problem and ensures the maintenance of the space heater to maintain dryness. Ingress of vermin through openings in the tank or hand holes left open after inspection endangers the equipment, particularly in tropical areas. Maintenance crews have occasionally reported nesting of potentially dangerous vermin on the tank. A cautious approach to clear these before beginning maintenance work is advisable.

References 1. MR Technical Brochure: The solution for variable shunt reactors Vacutap® VRX for the highest requirements for the regulating range 2. Dohnal D, Kirchner L (2016) Variable shunt reactors with on-load tap-changers for reactive power compensation in high voltage power grids. CEPSI 2016 Transmission & Distribution. FP_B.121th Conference of Electric Power Supply Industry 23–27 October 2016, Centara Grand and Bangkok Convention Centre at Central World, Bangkok, Thailand 3. Parallel Compensation 263-160 391-WS-parallel compensation-US-02e-1611. Brochure of Siemens on compensation. www.siemens.com/static-var/compensator 4. MR Tech Data Type R 5. MR Tech Data Type M 6. Chiplonkar AV (2008) Design, operation and maintenance of core type oil filled transformers. Self published 7. Krämer A (2000) On-load tap-changers for power transformers: Book. MR publication 8. Manual on transmission line planning criteria, Central Electricity Authority, New Delhi, 2013, Page 32 9. IEC 62 032:2005 Guide for the application, specification, and testing of phase-shifting transformers 10. Leci G, Kulis IG, Benovic J (2014) CIGRE B5-104 2014. Automatic voltage control of OLTC power transformers between substations 11. McIver J. Phase Shifting transformers—principles, design aspects and operation. SIEMENS Energy, Inc. SIEMENS Transformers Austria, Weiz 12. Del Vecchio RM et al (2001) Transformer design principles, Chapter 13. CRC Press 13. Verboomen J, Van Hertem D, Schavemaker PH, Kling WL, Belmans R (2005) Phase shifting transformers: principles and applications. IEEE Conference Paper, December 2005. https:// doi.org/10.1109/fps.2005.204302. Source: IEEE Xplore

Chapter 7

Problem of Capacitively Determined Potential

One who performs his duty without attachment, surrendering the results unto the Supreme Lord, is unaffected by sinful action, as the lotus is untouched by water. —Bhagavat Gita 5.10.

7.1 Chapter Content Situations arise with tapchangers equipped with pre-selectors, when some part of the transformer windings, as well as contacts of the pre-selector are transiently disconnected from well-defined potential. These “floating” objects then acquire a potential determined by the distributed capacitances of the disconnected winding. An important manifestation of the potential acquired is the discharge that takes place between contacts of the pre-selector during its operation. This discharge produces audible noise and dissolved gases. The gases are similar in nature to those produced by partial discharges, but include acetylene in addition. The volume of gases per pre-selector operation is small compared with the normal insulation degradation processes within the transformer. The main difference is the presence of small quantities of acetylene, which is an “arcing gas”, not normally produced by degradation of transformer insulation. Of particular concern is when a moving contact between two fixed can cause breakdown of the gap between contacts. The ionized gas can also be a hazard. This chapter examines such phenomena and remedies. Readers who would be happy with a cursory briefing on the subject, without too much detail, may consider skipping the chapter after Sect. 7.4.

© Springer Nature Singapore Pte Ltd. 2020 T. V. Sridhar, Application of Tap changers to Transformers, Power Systems, https://doi.org/10.1007/978-981-15-3955-8_7

261

262

7 Problem of Capacitively Determined Potential

7.2 Description of the Problem Figure 7.1 shows a moving contact which can select between two fixed contacts. It is a “break before make” type of switch, for instance those of a pre-selector. The two end fixed contacts are connected to the transformer winding, and thus their potential is galvanically fixed. The moving contact is connected to a winding, which has no other direct connection to the transformer. In Fig. 7.1a, the moving contact sits on the left fixed contact. The potential of the contact is thus fixed, and so also that of the winding connected to it. In Fig. 7.1b, the moving contact breaks with the fixed contact. Now the moving contact and the winding have no definite potential, and they “float”. The winding, being situated in the midst of other windings will acquire a potential determined by the capacitance distribution amongst the windings. This potential could be very different from that of the fixed contact from which the moving contact has just parted. At low contact separation, there could be enough voltage difference to initiate an arc, as shown in Fig. 7.1b. As the contacts separate further, the arc may go out at a future current zero. A recovery voltage between the previously

(a)

(b)

(c)

(d)

Fig. 7.1 Arcing due to loss of defined potential the nature of the problem

7.2 Description of the Problem

263

arcing contacts ensues (Fig. 7.1c). If the recovery voltage is insufficient to cause reignition of the arc at any point of the travel, the arc goes out permanently, and the contact movement can be completed safely (Fig. 7.1c). If on the other hand, the recovery voltage is too high, the arc may be dragged between the end fixed contacts (Fig. 7.1d), causing total failure. The arc current is related to the discharge noise. It is therefore necessary to estimate the arc current and recovery voltage that may arise in any application. As the process of arc extinction also depends on the phase shift between the interrupted current and the recovery voltage, this quantity must also be derived.

7.3 Pre-selector Arcing The phenomenon described generally in the above section occurs whenever a preselector is used in a tapchanger. A position of the tapchanger arises at which the fine selector is not galvanically connected to the rest to the transformer winding. Figure 7.2 shows a typical instance, one with a reverser, and another with a coarse/fine selector, where the fine winding and the pre-selector pole contact are not directly connected to the rest of the transformer. The potentials of the fine winding and the contact are undefined in this position.

7.3.1 General Methodology of Analysis for the Determination of the Arc Current and Recovery Voltage To determine the arc current, the recovery voltage, and their phase relationship, it is necessary to analyse the potential acquired by the associated winding due to the capacitance network to the other windings. Even though the capacitances are distributed, for the purpose of analysis, a lumped equivalent capacitance connected between the mid-points of the winding may be used. From the physical layout, a connection diagram can be made out, where the lumped capacitances, the connections of the contacts, and the well-defined sources of voltage can be represented. Such an equivalent circuit helps in determining the magnitude of the arc current and the recovery voltage when the current is interrupted. Initially the arc serves to bridge the contacts between which arcing takes place. The magnitude of the arc current can be determined from the equivalent circuit, assuming the relevant contacts to be still closed. When the arc is finally interrupted, the recovery voltage arises between the contacts. If the recovery voltage is too high, the contact gap may never recover. It is possible to correlate the calculated recovery voltage with the withstand voltage between the contacts to ascertain the suitability of the tapchanger to the application. The tapchanger manufacturer declares a safe withstand voltage, and arc current (Fig. 7.3, from MR [1]) for the pre-selector. As there would be some limits of recovery

264

7 Problem of Capacitively Determined Potential

(a)

(b)

Fig. 7.2 Loss of potential of fine tapping winding

voltage and interrupted current for several types of MR OLTC from previous arcing between contacts in the recovery process under discussion, the declared values would presumably have taken into account the effect of the presence of arc products in the gap.

7.4 A Realistic Perspective of the Effects of Pre-selector Arcing

265

Fig. 7.3 Limits of recovery voltage and interrupted current of MR Oltc’s

7.4 A Realistic Perspective of the Effects of Pre-selector Arcing Pre-selectors have been used with tapchangers for very long now. There is a considerable fifty-year-old volume of field experience with transformers at least up to 220 kV. Until about thirty years back, the problems caused by pre-selector arcing did not draw anything more than a cursory complaint on noise by a few particularly astute operators. A large number of tapchangers have been working in H.V. transformers for over half a century, without any deliberate measures to control the arcing. There has been no spate of pre-selector failures arising out of insulation issues. It is only more recently, with the advent of DGA, and other monitoring techniques, sometimes based on tiny third-order effects, that anything so gross as audible noises cannot be accepted as normal. For 400 kV and higher story may not be so clement.

7.5 Reversing Taps at Neutral End The general methodology discussed in Sect. 7.3 will now be applied to the case of a reverser at the neutral end. Taps are located in a separate tapping barrel. Figure 7.4a shows the circuit connections, and Fig. 7.4b shows the distributed capacitances represented by discrete capacitance connected at the mid-point of the respective winding. When the moving contact is still arcing to the end contact, there is a connection between the two contacts through the arc as in Fig. 7.4c. When the arc clears, the recovery voltages appear in Fig. 7.4d.

266

7 Problem of Capacitively Determined Potential

(b)

(a)

(c)

(d)

(e)

Fig. 7.4 Reverser at neutral

7.5.1 Recovery Voltages Taking first Fig. 7.4d, in loop 1, the potential rise of the point B over ground is Vb =

1 Vm C1 2 (C1 + C2 )

(7.1)

The recovery voltages, taken as the potential rise of the contacts 1 and n over the pole which is grounded, are

7.5 Reversing Taps at Neutral End

Vrn =

267

  C1 1 + Vt Vm 2 C1 + C2

(7.2)

Under the condition that contact 1 opens, and   1 Vm C1 − Vt Vrn = 2 C1 + C2

(7.3)

when contact n opens. Figure 7.4e shows the vector diagram corresponding to the above derivations.

7.5.2 Arc Currents For determining the arc currents, we can use the Thévenin theorem. The recovery voltages worked out are indeed the Thévenin source voltages for the arc currents flowing through the contacts and the pole of the reverser in Fig. 7.4c. The source impedance consists of C 1 and C 2 in parallel, i.e. 1/jω(C 1 + C 2 )   1 Vm C1 + Vt (C1 + C2 ) jω 2 C1 + C2   1 Vm C1 − Vt (C1 + C2 ) Irn = jω 2 C1 + C2 Ir1 =

(7.4) (7.5)

interrupted arc currents and the corresponding recovery voltage are in quadrature.

7.5.3 Numerical Values The values V m and V t are phase quantities. For a line-to-line voltage of V, and noting √ that in the tap position under consideration Vm = V / 3 and V t = r · V m where r is the tapping range,   V C1 +r Vr = √ 2 3 C1 + C2   V C1 −r Vr1 = √ 2 3 C1 + C2

(7.6) (7.7)

and 1 Ir1 = √ jωVm [C1 + r (C1 + C2 )] 2 3

(7.8)

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7 Problem of Capacitively Determined Potential

1 Irn = √ jωVm [C1 − r (C1 + C2 )] 2 3

(7.9)

7.5.4 General “Ball Park” Values Reference [2] illustrates the calculation with a numerical example of a typical 240 kV transformer with ±12.5% taps at the neutral end, by tapchanger with reverser. The recovery voltages are worked out to be 64.95 and 47.63 kV. The arc currents are 48.97 and 35.91 mA. It is possible to generalize with these estimates. C 2 represents the capacitance of the outer tap winding to the grounded tank. For practical purposes, the tank which is the only earth boundary for the tap winding does not tightly hug the winding, but is generally far removed from it. This makes for a small value of C 2 , of the order of 1/4th of C 1 . In the example of [1], the ratio C 2 /C 1 is 0.23. We may expect this ratio to be in the range of 0.2–0.25 in most practical cases. Taking r as 0.1 we can draw up Table 7.1. It can be concluded that for 132 kV, it is not essential to take steps to reduce the recovery voltages. 220 kV is marginal and could do with some mitigating measures. The reason why many older pre-1980s 220 kV transformers have survived without specific measures to ameliorate the recovery voltages may lie in the fact that the voltages are not too high. 400 kV absolutely needs reduction techniques.

7.6 Application of Single Compartment Tapchangers at the Neutral End Tapchangers of the single compartment type (e.g. MR type V and ABB type UBB and UZE/F) have lower admissible recovery voltages over the pre-selector. It may be expected that they may need application of reduction techniques even at 132 kV level. Table 7.1 Recovery voltages

System voltage

Recovery voltages (kV)

kV

Contact 1

n

C 2 /C 1 = 0.2

C 2 /C 1 = 0.25

132

35.6

34.2

220

59.3

57.0

400

107.9

103.7

7.6 Application of Single Compartment Tapchangers at the Neutral …

269

It is however an interesting observation that these single compartment tapchangers are seldom fitted with measures to reduce the pre-selector recovery voltages. The next section examines the probable reason for their success.

7.6.1 Taps at the Neutral Located in the Physical Middle of the Total Winding Transformers which use single compartment tapchangers are usually of smaller rating. When the short-circuit MVA is less than 200 MVA, taps can be placed in the body of the winding. Taps in the body of the winding cause a high unbalanced axial ampere-turn distribution. This leads to higher axial mechanical forces. But for a transformer of low short-circuit MVA, this can be managed. A transformer with taps in the body of the winding, rather than a separate tapping barrel, is more economical. Further for a small transformer, stretching a tap winding with typically 10% of the turns over the whole axial length results in a thin coil of about 1/10th the radial thickness of the main winding, which in small transformers means a tapping barrel of about 8 mm radial thickness. Such a coil is mechanically unstable. For these reasons, a construction such as that shown in Fig. 7.5 is more suitable and was used by the English company Hack bridge and Hewittic Electric Co. Their licensee in India, Hack bridge Hewittic and Easun Limited used this construction over hundreds of 132 kV star-connected transformers.

7.6.1.1

Determination of Recovery Voltages

Figure 7.5a shows the disposition of the windings and the distributed capacitance connections. Each winding shows a capacitive coupling to ground. The capacitive coupling between the tapping winding and the main coil arises mainly through the radial thickness. Coupling between the cylindrical surfaces stacked axially is negligible. Five capacitances are shown in Fig. 7.5a, b shows the voltages, capacitors, and the circuit connection at the changeover point. Figure 7.5c is the equivalent circuit during arcing. The arc currents are calculated on the basis of Fig. 7.5c, d shows the condition after arc extinction. The pole is grounded at K, and therefore, the potential to ground of the contacts 1 and n of the reverser is the recovery voltages. In Fig. 7.5d, I =

1 (V + Vt )C3 C4 1 V + Vt = jω 2 Z3 + Z4 2 C3 + C4

The potential of B relative to ground = Vrn =

1 (V + Vt )C4 Vt + 2 C3 + C4 2

1 (V + Vt )C4 2 C3 + C4

(7.10) (7.11) (7.12)

270

7 Problem of Capacitively Determined Potential

(b) (a)

(c)

(d)

Fig. 7.5 Taps at the neutral in the body of the winding

The recovery voltage of 1 =

1 (V + Vt )C4 Vt − 2 C3 + C4 2

(7.13)

7.6.2 Arc Currents The arc currents can be calculated from Fig. 7.5c using the Thévenin theorem. The voltage between the pole and the contacts when the contacts are open is the Thévenin source voltage. This is the same as the recovery voltages determined in the last section. The source impedance is the parallel combination of C 3 and C 4 , i.e. 1/jω(C 3 + C 4 ) for contact 1. It is 1/jω(C 3 + C 5 ) for contact n. The arc current for the condition that n is closed is thus   1 (V + Vt )C4 + Vt (C3 + C5 ) (7.14) Irn = jω 2 C3 + C4

7.6 Application of Single Compartment Tapchangers at the Neutral End

271

  1 (V + Vt )C4 − Vt (C3 + C4 ) jω 2 C3 + C4

(7.15)

I1 =

Since each arc current is derived as the recovery voltage divided by a capacitive impedance, the interrupted current leads the recovery voltage by 90°. When working √ out numerical values, with a line-to-line system of V L , the value of V is VL / 3 and V t must be expressed as phase value.

7.6.3 Approximate Quantitative Evaluation The measured values of C 3 /C 4 for a transformer in the range 10–25 MVA, where the tapping arrangement described can be applied, is of the order of 3.5 to 5. Taking a typical 132 kV star transformer, with a tapping range of ±10%, C 3 /C 4 = 4, the recovery voltage V rn is about 12.2 kV and V r1 about 4.57 kV. These are very moderate values for a 132 kV star tapchanger. This explains why transformers with this application have not been noted to cause problems of discharge or DGA in service. In a qualitative manner, the low recovery voltages and arc currents can be explained by the weak coupling of the fine winding to the H V through small (high impedance) distributed capacitance. In the light of this, the application of coarse/fine at the neutral and located in the middle of the phase winding will not be worked out in detail.

7.7 Coarse/Fine Taps as Separate Tapping Barrel at the Neutral Before entering the detailed calculations, it may be noted that the coarse tap forms a shield for the fine taps from the main winding. It may be expected that the recovery voltages will be very small. Figure 7.6a shows the physical layout, and the coupling capacitances. Figure 7.6b shows the capacitor and ground connections at the changeover position. Figure 7.6c shows the equivalent circuit in the arcing condition. The currents through the respective reverser contacts are therefore the arc currents. Figure 7.6d shows the same circuit after arc clearance. In Fig. 7.6d, the voltage of point B relative to ground is   1 C1 Vb = Vc 2 C1 + C2   C1 Vt Vc + The potential of the pole = 2 C1 + C2 2

(7.16) (7.17)

n is grounded. The recovery voltage of n taken as its potential rise over the pole

272

7 Problem of Capacitively Determined Potential

(a)

(b)

(d)

(c)

Fig. 7.6 Coarse fine tapping barrel at neutral

Vrn = −

    1 C1 Vc + Vt 2 C1 + C2

(7.18)

The contact 1 is at +V c over n. The recovery voltage of contact 1 is Vr1 = −

    1 C1 Vc + Vt + Vc 2 C1 + C2

(7.19)

7.7.1 Arc Currents Figure 7.6c shows the current through the arc with contact 1 and N when they close on the pole. To calculate the arc currents, we can use the Thévenin theorem. The

7.7 Coarse/Fine Taps as Separate Tapping Barrel at the Neutral

273

Thévenin voltage in each case is the same as the recovery voltages worked out in the last section. The equivalent source impedance is that of C 1 and C 2 in parallel, i.e. 1/jω(C 1 + C 2 ). The arc currents therefore 1 In = − jω [Vc C1 + Vt (C1 + C2 )] 2

(7.20)

1 I1 = − jω [Vc C1 + Vt (C1 + C2 )] + Vc (C1 + C2 ) 2

(7.21)

Reference [2] gives a worked example of a typical transformer application. The recovery voltages are very small (0.5–5 kV). The maximum arc current is 3.84 mA. As anticipated earlier at the beginning of the section, the shielding off of the main winding from the fine taps by the coarse winding produces mild conditions of preselector switching.

7.8 Coarse/Fine at the Neutral End for Taps in the Body of the Winding For smaller transformers, coarse/fine taps may be located in the body of the winding, rather than as a tapping barrel. Figure 7.7 shows the application of coarse/fine tappings at the neutral end. Taps are physically in the body of the winding. Figure 7.7a shows the physical layout of the windings and Fig. 7.7b shows the connections, including the capacitive coupling at the changeover point. Figure 7.7c shows the equivalent circuit before the arc current is broken. This circuit is useful in calculating the arc currents. Figure 7.7d shows the condition when the arc is extinguished.

7.8.1 Recovery Voltages The contact n of the coarse/fine switch is grounded. The contact 1 is V c above n. We need the potential of the pole to determine the recovery voltages. For this, we calculate the current I 2 from the two loops shown in Fig. 7.7d. The potential of the pole is then I 2 Z 6 . V Vt + = I1 Z 5 + (I1 + I2 )Z 3 2 2

(7.20)

Vt = −I1 Z 3 − I2 (Z 3 + Z 6 ) 2

(7.21)

274

7 Problem of Capacitively Determined Potential

(b) (a)

(c)

(d)

Fig. 7.7 Coarse fine neutral taps in body of winding

7.8 Coarse/Fine at the Neutral End for Taps in the Body of the Winding

275

Resolving I2 = −

jω

1 C6 [V C3 + Vt (C3 + 2C5 )] 2 C3 + C5 + C6

The potential of the pole = −

1 [V C3 + Vt (C3 + 2C5 )] 2 C3 + C5 + C6

(7.22) (7.23)

The recovery voltage of n taken as the potential rise of n over the pole Vr n =

1 [V C3 + Vt (C3 + 2C5 )] 2 C3 + C5 + C6

The recovery voltage of contact 1 =

1 [V C3 + Vt (C3 + 2C5 )] + Vc 2 C3 + C5 + C6

(7.24) (7.25)

7.8.2 Arc Currents The arc currents can be derived from Fig. 7.7c by applying the Thévenin theorem. The Thévenin source voltages are respectively the recovery voltages. The source impedance consists of the capacitances C 3 and C 5 and C 6 in parallel. Thus the Thévenin source impedance is Z th =

1 jω(C3 + C5 + C6 )

1 Irn = jω [V C3 + Vt (C3 + 2C5 )] 2

(7.26) (7.27)

7.9 Reverser Switching at the Line End of Delta-Connected Transformers 7.9.1 Physical Arrangement Figure 7.8 shows reverser taps at the line end of a delta-connected transformer. Figure 7.8a shows the physical arrangement of windings, with the distributed capacitances shown lumped at the mid-point of each winding. Figure 7.8b shows the capacitive circuits and the connections made by the tapchanger at the changeover point. Comparing Fig. 7.8b with Fig. 7.3b which is applicable for reversing taps at the neutral, the similarity is so striking that one may wonder whether a separate

276

7 Problem of Capacitively Determined Potential

(a)

(c)

(e)

(b)

(d)

(f)

Fig. 7.8 Reverser at delta line end

analysis for the delta case is at all required. In the star case, the capacitive currents flowing from windings return through the grounded neutral. In the case of delta, the ground currents return through the system neutral to the phase, encountering a potential difference between the two points. This makes for a subtle, but substantial difference between the two cases.

7.9.1.1

Development of the Equivalent Circuit

Figure 7.8c shows the equivalent circuit for the condition of arcing. Please note the additional phase to neutral voltage between ground and R. The corresponding points are shorted in Fig. 7.4c. Figure 7.8d shows the same circuit after the arc is extinguished. The pole is at the potential of R. We can find the potential rise of B over R, and add to it ±V t /2 to get recovery voltages.

7.9 Reverser Switching at the Line End of Delta-Connected …

7.9.1.2

277

Recovery Voltages

We first take Fig. 7.8d for recovery voltages. The procedure is to find the potential of point B in Fig. 7.8d from the capacitance network. In loop 1, potential rise of B relative to ground =  Vr + V2m C1 Vb = C1 + C2 

(7.28)

The potential of point B is marked on this vector with B in Fig. 7.8f. The potentials of the reverser contacts, which are removed from B by ±V t /2 are also shown in Fig. 7.8f. The potentials of 1 and n of the reverser relative to R where the pole is located are the recovery voltages.   Vm + V2r C1 Vt Vt 2 − Vr + = Vrn = Vb − Vr + 2 C1 + C2 2  Vm  C1 − Vr C2 Vt Vrn = 2 + C1 + C2 2 Vr1 =

− Vr C2 Vt − C1 + C2 2

Vm C1 2

(7.29) (7.30) (7.31)

The recovery voltage is marked up in Fig. 7.8f. When using these relationships for a calculation, it must be kept in mind that these are vector equations. Taking √ the vector V m = line-to-line voltage as the reference vector, V r = V m (−½ +j/2 3). V t must be expressed as the tapping range voltage and is in phase with the reference vector.

7.9.1.3

Arc Currents

For the arc currents, we may invoke Thévenins theorem. For this purpose, we imagine opening the terminals of the reverser switch and evaluate the Thévenin source voltage across the opening. This is precisely what we did in the last section, to work out the recovery voltage. The Thévenin source impedance is the parallel combination of C 1 and C 2 , which is 1/jω (C 1 + C 2 ). The current through the arc by Thévenin’s theorem is the recovery voltage divided by the source impedance. Irn =

Vm Vr (C1 − Vr C2 ) + (C1 + C2 ) 2 2

(7.32)

Ir1 =

Vm Vr (C1 − Vr C2 ) − (C1 + C2 ) 2 2

(7.33)

278

7 Problem of Capacitively Determined Potential

It may be noted that the arc currents are derived by dividing the recovery voltage by capacitive impedance. Therefore, the interrupted arc currents lead the recovery voltage by 90°.

7.9.1.4

Estimation of Recovery Voltage for Typical Transformer Applications

Reference [1] gives an example of a typical 132 kV delta transformer with ±10% taps, with a reversing type tapchanger. The example shows recovery voltages of 73.78 and 60.33 kV. The currents are 63.97 and 52.75 mA. These values probably hold within a small band for most transformers with the same basic structure and voltage class. In any case, 132 kV is probably the highest practical voltage for a delta-connected transformer. According to the example, a technique for reducing the recovery voltage will be required for most 132 kV class tapchangers. As against that, there has been an annual production of 150–300 Nrs 110 kV delta transformers (not 132 kV as in [1]) for the past 40 years in the southern region of India. Some of these are equipped with reversing tapchangers and others with coarse/fine tapchangers covering ±10% or ±12½% range. No reduction technique for recovery voltage has been applied in most cases. Neither serial failure by pre-selector flashover nor adverse DGA has been reported. This success is probably because 1. Most of these transformers small enough rated to have taps from the body of the winding rather than a separate tapping barrel. 2. When using a tapping barrel, a coarse/fine arrangement, rather than a reverser was used. It will be seen later that the coarse/fine results in reduced recovery voltages. This is because then coarse winding acts as a shield for electrically removing the fine winding away from H V. 3. A small number of 50 MVAs were supplied with a tapping barrel. These had the I + II tapchanger. The analysis shows that the recovery voltages are low in such an application.

7.10 Reversing Taps in the Middle of the Delta with Taps Out of the Body of the Main Winding A large number of sub-distribution transformers around the world are delta connected in the primary. They are relatively small rated, 10–25 MVA, and have taps in the body of the winding, electrically located in the middle of the phase. In terms of numbers, this is an important application. Figure 7.9a shows the layout of the windings, with the capacitive distribution represented by equivalent lumped capacitors at the mid-point of the windings. In this case, the axial coupling capacitance of the fine winding to the rest is significant. There are two such capacitances represented

7.10 Reversing Taps in the Middle of the Delta …

279

by C 4 and C 5 (Fig. 7.9). All windings exhibit distributed capacitance to ground (C 1 , C 2 , C 3 in Fig. 7.9a). But direct capacitive coupling along the vertical cylindrical surface is negligible. Figure 7.9b shows the connections at the changeover point. Figure 7.9c shows the equivalent circuit during arcing conditions, with the pole of

(b)

(a)

(c)

(d)

Fig. 7.9 a–d Page1: Reversing taps in the middle of delta. e, f Page 2: Vector diagram reversing taps in the middle of delta

280

7 Problem of Capacitively Determined Potential

(e)

(f)

Fig. 7.9 (continued)

the reverser closed to the other contacts. Figure 7.9d shows the equivalent circuit after arc extinction. Figure 7.9 Page 2 shows the corresponding vector diagram.

7.10.1 Determination of Recovery Voltages Figure 7.9d is used to determine the recovery voltages. The green arrow shows the current through C 4 and the magenta arrow shows the current through C 5 . These currents share a path through C 3. The voltage drops across C 5 produced by the current I 2 are the recovery voltage of the contact n. The recovery voltage of the contact 1 is the voltage drop across C 4 . In order to solve for the currents I 1 and I 2 , we consider Vr + V /2 − Vt /2 = I1 Z 4 + (I1 + I2 )Z 3

(7.34)

Vr + V /2 + Vt /2 = I2 Z 5 + (I1 + I2 )Z 3

(7.35)

 Resolving I2 = jωC5

 Vr + V2 + V2t C3 + Vt C4 C3 + C4 + C5

(7.36)

7.10 Reversing Taps in the Middle of the Delta …

 I1 = jωC4

 Vr + V2 − V2t C3 − Vt C5 C3 + C4 + C5

281

(7.37)

The voltage drop across C 5 is the negative of the recovery voltage of contact 1.  Vr + V2 + V2t C3 + Vt C4 Vr1 = − C3 + C4 + C5 

(7.38)

The recovery voltage of the contact n  Vr + V2 − V2t C3 − Vt C5 Vrn = − C3 + C4 + C5 

(7.39)

7.10.2 Arc Currents Arc currents flow through the closed contacts 1 or n in Fig. 7.9c. We may derive the currents by application of the Thévenin theorem. The source voltage for the currents is the recovery voltage calculated already. The source impedance consists of C 3, C 4, and C 5 in parallel, i.e. 1/jω(C 3 + C 4 + C 5 ) 

 V Vt C3 + Vt C4 Ir1 = jω Vr + + 2 2   V Vt C3 − Vt C5 Irn = jω Vr + − 2 2

(7.40) (7.41)

When evaluating numerical values, it must be taken into account that the expressions for the voltages and currents are vectors. √If the line to line voltage V m is taken as the reference vector, V r = V m (−1/2 + j/2 3). The tap range voltage is in phase with the reference vector.

7.10.3 A Practical Numerical Example For a practical 33 kV delta transformer, the capacitance C 3 is about 3.5 to 5 times C 4 and C 5. For the following illustrative example, we shall take C 3 to be 3.5 times, and a tapping range of ±10%. Substitution in Eq. 7.30 yields Vrn = Vm (0.027 + j0.052) = 7734 V

(7.42)

282

7 Problem of Capacitively Determined Potential

For 33 kV, where such an application is very common, the recovery voltage is small, confirming the operation of many hundreds of transformers with no noticeable drastic discharge noise or gas evolution.

7.11 Delta-Connected Transformer with Coarse/Fine Tapchanger at Line End The application of coarse/fine regulation at H V Line end for delta-connected transformers is very common. This is implemented either with three single-pole line end tapchangers or with the I + II system popularized by AEG-Siemens, and NGEF. Figure 7.10 shows this application. Figure 7.10a shows the physical disposition of coils, with the distributed capacitance shown lumped at the mid-point. As discussed, the coarse winding shields off the fine taps from the main winding. There is no coupling to any high voltage source. Therefore we may expect even without a detailed analysis, that the recovery voltages and arc currents would be low. For the sake of completeness, we go through the formal analysis. Figure 7.10b shows the connections at the point of changeover, i.e. when the fine taps are disconnected from the rest of the winding. Figure 7.10c shows the equivalent circuit while the pre-selector is still arcing. Figure 7.10d shows the equivalent circuit after arc clearance. The difference in voltage between the pole and the other contacts in Fig. 7.10d is the recovery voltages. Figure 7.10e shows the vector diagram corresponding to Fig. 7.10d.

7.11.1 Recovery Voltages Taking first Fig. 7.10d, point B divides the voltage V r + V c /2 in loop 1 in the proportion C 1 /(C 1 + C 2 ). The potential of B above ground  Vr + V2c C1 Vb = C1 + C2 

(7.43)

Figure 7.10e shows the potential of the point B on the vector diagram.  Vr + V2c C1 Vt Vb + Vt /2 = + C1 + C2 2 

(7.44)

7.11 Delta-Connected Transformer with Coarse/Fine Tapchanger …

283

(b)

(a)

(c)

(d)

(e)

Fig. 7.10 Change over for coarse/fine at line end of delta

The potential rise of the pole of the pre-selector is equal to potential rise of the contact n above ground, that is V r . The recovery voltage is the potential rise of contact n over the pole and since the contact 1 is above contact n by V c , The recovery voltage of the contact 1 is Vr C2 − V2c C1 Vt − C1 + C2 2  Vr C2 − V2c C1 Vt Vr1 = − + Vc C1 + C2 2 Vrn =

(7.45) (7.46)

284

7 Problem of Capacitively Determined Potential

The two recovery voltage vectors are shown in Fig. 7.10e. When using these equations, it must be remembered that the quantities are vectors and therefore both magnitude and phase must be taken into account.

7.11.2 Arc Currents As in the earlier cases, we resort to Thévenins Theorem to derive the arcing currents. The Thévenin source voltage driving the currents is the recovery voltage. The equivalent source impedance is of the two capacitors C 1 and C 2 in parallel, i.e. 1/jω(C 1 + C 2 ). Therefore, the currents are   Vc Vt Irn = jω Vr C2 − C1 − (C1 + C2 ) 2 2     Vc Vt − Vc (C1 + C2 ) Ir1 = jω Vr C2 − C1 − 2 2

(7.47) (7.48)

Obviously the arc currents and recovery voltages are in quadrature.

7.11.3 Illustrative Example Reference [2] works out the actual values of the recovery voltage and arc currents for a typical 110 kV delta transformer with a tapping range of 20%. The recovery voltages work out to 21.14 and 11.03 kV. The arc currents are 15.94 and 8.31 mA. As anticipated, these values are small, due to the shielding effect of the coarse winding.

7.12 Reversing Taps in Autotransformers Application to autotransformers has the following prominent variants. 1. Taps at the end of the series winding. This is used for CFVV control for HV variation. The tapping barrel is most usually on the outside. 2. Tapping barrel at the end of the common winding. This is used when the LV voltage needs to be controlled. The tapping barrel may be accommodated between series and common or on the outside. If the tapping barrel is on the outside, the HV winding is sandwiched between “Earthy” boundaries on both sides during impulse condition. This leads to an unfavourable impulse distribution within the HV winding. For this reason, many manufactures prefer to place the taps between the common and series windings. In the example taken in Sect. 7.12.2 below, the taps are in the middle.

7.12 Reversing Taps in Autotransformers

285

7.12.1 Taps at the End of the Series Winding Figure 7.11a shows the physical arrangement of the windings, with the distributed capacitances lumped at the middle of the coils. Figure 7.11b shows the connections at the changeover point. Figure 7.11c shows the equivalent circuit with arcing. Figure 7.11d is the equivalent circuit after arc quenching. Figure 7.11e shows the corresponding vector diagram.

(a)

(b)

(d)

(c)

(e) Fig. 7.11 Auto transformer with reversing taps in series with the common winding

286

7.12.1.1

7 Problem of Capacitively Determined Potential

Recovery Voltages

We first take Fig. 7.11d. Point B divides the voltage difference between A and ground in the ratio C 1 /(C 1 + C 2 ). The voltage of point B is 

 Vc + V2s C1 C1 + C2

(7.49)

The potentials of the two contacts n and 1 are V b ± V t /2, respectively. The potential rise of the pole from ground is V c . Therefore the recovery voltages of the contacts are Vrn = Vr1 =

7.12.1.2

− Vc C2 Vt + C1 + C2 2

(7.50)

Vs C1 − Vc C2 Vt − C1 + C2 2

(7.51)

Vs C 2 1

Arc Currents

Applying Thévenins theorem, the arc currents are driven by a Thévenin source voltage equal to the recovery voltages, with a source impedance of 1/jω(C 1 + C 2 ).

 Vs Vt C1 − Vc C2 + (C1 + C2 ) Irn = jω 2 2

 Vs Vt C1 − Vc C2 − (C1 + C2 ) Ir1 = jω 2 2

(7.52) (7.53)

Although the last four equations involve vectors, all vectors are in phase. Therefore the respective values may be inserted directly for working out the final result.

7.12.1.3

Illustrative Example

We may make a real-life estimate of the recovery voltages and arc currents by making C 1 , V t = 0.1 (V s √ + V c ). For a a few realistic assumptions. We take C 2 = 0.25 √ 220√kV/132 kV auto, remembering that V s is 88/ 3 kV, V c = 132/ 3 kV, V t = 22/ 3, Vrn = 11.43 kV and Vr1 = −1.27 kV. These are low values.

7.12 Reversing Taps in Autotransformers

287

7.12.2 Taps in Series with the Common Winding of Autotransformer Figure 7.12a shows the winding arrangement with the distributed capacitances lumped at the mid-point of the windings. Figure 7.12b shows the connections at the changeover point. Figure 7.12c shows the equivalent circuit under arcing conditions. Figure 7.12d shows the equivalent circuit after arc quenching. Figure 7.12e is the corresponding vector diagram.

(a)

(b)

(d)

(c)

(e)

Fig. 7.12 Auto transformer with reversing taps in series with the common winding

288

7.12.2.1

7 Problem of Capacitively Determined Potential

Recovery Voltages

In loop 1 of Fig. 7.12d, the potential rise of B over C is  + V2c C2 C1 + C2

 Vs Vb =

2

(7.54)

The potential rise of the reverser terminals 1 and n overB are, respectively, ±V t /2. The pole of the reverser is at V c /2 over C. Therefore the recovery voltage of contact 1, which is its potential rise over the pole is  Vs Vr1 = Vrn =

7.12.2.2

2

 C2 − V2c C1 Vt + C1 + C2 2

(7.55)

− V2c C1 Vt − C1 + C2 2

(7.56)

Vs C 2 2

Arc Currents

Harking back to the Thévenin approach, the equivalent source voltages are the recovery voltages. The source impedance is that of the two capacitors in parallel, i.e. jω(C 1 + C 2 ). The arc currents are thus Ir1 =

Vs Vc Vt C2 − C1 + (C1 + C2 ) 2 2 2

(7.57)

Irn =

Vs Vc C2 − C1 − Vt (C1 + C2 ) 2 2

(7.58)

The arc currents are in quadrature with the recovery voltages.

7.12.2.3

Illustrative Example

Reference [2] gives an example of a practical 167 MVA, 550/245 kV, autotransformer with ±12% reversing taps in series with the common winding. The recovery voltages work out to 8.32 and 25.29 kV. The arc currents are 10.19 and 30.99 mA. The recovery voltages are moderate, even with taps at the 245 kV line end. There may not be a need for steps to be undertaken for reduction.

7.13 Application to Phase-Shifting Transformers

289

7.13 Application to Phase-Shifting Transformers In Chap. 6, many connections for phase-shifting transformers were presented. References [3] and [4] may be consulted for a description of phase shifters. It is proposed to consider the pre-selector issue only in respect of a representative phase shifter. The single-core symmetrical phase shifter of Fig. 6.6 is taken for illustration of the method involved, because this is not only a common phase shifter application, but the approach to the calculations can be applied on other connections easily. Phase shifters of this kind often give very high recovery voltages and reduction techniques may be required in each case. For an analysis of recovery voltages in different phase shifters, see [2].

7.13.1 Symmetrical Single-Core Phase Shifter—Recovery Voltages Figure 7.13a shows the physical arrangement of the windings, with the distributed capacitances lumped at the mid-points. There are two right separately, in Fig. 7.13 Page 1 and Fig. 7.13 Page 2, respectively. It is hoped that this adds to clarity. In Fig. 7.13d, which corresponds to the tapchanger on the left, the windings are superposed on the vector diagram in their relative electrical positions. The capacitive couplings are also shown.

7.13.1.1

Recovery Voltages for the Tapchanger on Left Side in Fig. 7.13

√ The pole of the reverser is at the potential of R. This is at V 1 / 3 above ground. The two other contacts are at V b ± V t /2, where V b is the potential of the point B relative to ground. We shall now work out the potentials of point A, B, C. We observe the following: √ 1. The potential of C is −V 2 /2 3 relative to ground, by virtue of its being at the mid-point of the delta-connected LV√phase. V 2 is the LV line-to-line voltage. 2. In a likewise manner A is at −V 1 /2 3, where V 1 is the line-to-line voltage of the larger delta. √ √ 3. The potential rise of A above C is −V 1 /2 3 + V 2 /2 3. 4. In Fig. 7.13c, point B divides the voltage between A and C in the ratio C 1 /C 1 + C 2 . The potential rise of B relative to A is

1 (−V1 + V2 )C1 − √ 2 3 C1 + C2

(7.59)

290

7 Problem of Capacitively Determined Potential

The potential of B relative to ground is V1 1 (−V1 + V2 )C1 Vb = − √ − √ 2 3 2 3 C1 + C2     √ C1 V2 C1 = 1/2 3 V1 −1 + − C1 + C2 C1 + C2 1 (V1 C2 + V2 C1 ) =− √ C1 + C2 2 3

(7.60) (7.61) (7.62)

5. The voltage of the contacts n and 1 from ground is V b ± jVt/2. 6. The√recovery voltages are derived from subtracting the potential of the pole V 1 / 3.

(a)

(c)

(b)

(d)

(e) (f)

Fig. 7.13 a–f Page 1: Loss of potential of fine winding in symmetrical phase shifter application. g–k Page 2: Loss of potential of fine winding in symmetrical phase shifter application

7.13 Application to Phase-Shifting Transformers

291

(h)

(g)

(j)

(i)

(k)

Fig. 7.13 (continued)

7.

1 (V1 (2C1 + 3C2 ) + V2 C1 ) Vt Vrn = − √ + C1 + C2 2 2 3

(7.63)

8.

1 (V1 (2C1 + 3C2 ) + V2 C1 ) Vt Vr1 = − √ − C1 + C2 2 2 3

(7.64)

292

7.13.1.2

7 Problem of Capacitively Determined Potential

Arc Currents for the Tapchanger on the Left Side of Fig. 7.13c

Figure 7.13f is the equivalent circuit after arc extinction. The recovery voltages are the Thévenin driving source for the arc currents. The source impedance, from Fig. 7.13f, is the parallel combination of C 1 and C 2 , i.e. jω(C 1 + C 2 ). The currents are

7.13.1.3

1 Vt Irn = − jω √ (V1 (2C2 + 3C2 ) + V2 C1 ) + (C1 + C2 ) 2 2 3

(7.64)

Vt 1 Ir1 = − jω √ [V1 (2C2 + 3C2 ) + V2 C1 ] − 2(C1 + C2 ) 2 3

(7.65)

Recovery Voltages for the Tapchanger on the Right in Fig. 7.13 Page 2

We may proceed along the same lines as with the tapchanger on the left. However a simpler solution suggests itself. It is noted that Fig. 7.13 Page 1 and Page 2 look similar. In fact they are the same if the potential shift of point C below neutral in Fig. 7.13b is set to zero. In other words, V 2 is set to zero in all the formulae derived so far. The capacitors C 1 and C 4 and C 2 and C 3 must also be interchanged. On that basis, the recovery voltages are + 3C3 )

Vt 2

(7.66)

1 V1 (2C4 + 3C3 ) Vt Vr1 = − √ + C3 + C4 2 2 3

(7.67)

Vrn = −

7.13.1.4

1 √ V (2C4 2 3 1

C3 + C4

−

Arc Currents for the Tapchanger on the Right in Fig. 7.13c

The currents are  Irn = − jω  Ir1 = − jω



1 Vt √ V1 (2C4 + 3C3 ) − (C3 + C4 ) 2 2 3

(7.68)

1 Vt √ V1 (2C4 + 3C3 ) + (C3 + C4 ) 2 2 3

(7.69)



7.13 Application to Phase-Shifting Transformers

7.13.1.5

293

Illustrative Example

Reference [2] provides calculations on a 675 MVA 240 kV ±40° phase shifter. The recovery voltage for the tapchanger on the left is 185.78 kV, and for the tapchanger on the right, it is 187.95 kV. The arc currents are 300.46 mA and 370.58 mA, respectively. It may be expected that most phase shifters, will have a ratio of capacitances and tap range voltages similar to the illustration. This means for phase shifter duty according to Fig. 6.6, we may expect very high recovery voltage stresses on the pre-selector. They will need moderating efforts.

7.14 Application of Pre-selector in Dual-Core Phase Shifter In dual-core phase shifters, the tapchanger is applied at the star point of the L V of the shunt unit. This is therefore equal to any other star point application, such as in Sect. 7.6.

7.15 Mitigation of High Recovery Voltage by Using Tie-in Resistance The general principle of application of the tie-in is illustrated in Fig. 7.14. For those who do not wish to go through the mathematical drudgery all over again, Fig. 7.14 shows the principle. In all pre-selector applications, either the pre-selector pole, or one of the contacts is fixed in potential. Without loss of generality, in Fig. 7.14a, the pole is taken to have a fixed potential. The fine winding is floating and is coupled capacitively to the pole potential. We need that the floating winding acquires potential

(a)

Fig. 7.14 Principle of tie-in

(b)

294

7 Problem of Capacitively Determined Potential

close to that of the pole, so that the recovery voltages will be low. If the capacitive network couples the winding poorly to the potential of the pole, meaning thereby a high impedance path, the result is high recovery voltages. We can create a path of low impedance in shunt, to charge the winding to the potential of the pole. This is most conveniently done by tie-in resistor, connected galvanically between the pole and the mid-point of the winding (Fig. 7.14b). The ohmic impedance of the tie-in must be low relative to the effective capacitance. The tie-in provides a relatively low impedance path so that the arc currents are higher. Discharge noise is associated with the arc current. Thus there is scope here for optimization of the recovery voltage and arc current.

7.15.1 Reversing Taps on a Tapping Barrel at the Neutral Figure 7.15 for a tapping arrangement at the neutral end. The tie-in can be connected between the end of the main and the mid-point of the fine taps. In practice, however the tie-in is connected between the mid-tap and the wander take-off. This reduces the thermal load of the tie-in which would otherwise be due to half the range voltage impressed on it at all tap positions. Figure 7.14 is a repeat of Fig. 7.4 except the former shows the tie-in connected.

7.15.1.1

Recovery Voltages

The recovery voltages can be calculated from Fig. 7.15d. The recovery voltages are the potential rise across Rt due to the flow of current through it, ±V t /2. To find the current through Rt , we imagine making a break in the circuit at the point denoted as Z in Fig. 7.15d. The voltage appearing across the break is the Thévenin voltage driving the current through Rt when the break is again closed. The source voltage is Vth =

Vm C1 2 C1 + C2

(7.70)

The impedance limiting the current through Rt is Rt + jω

1 C1 + C2

(7.71)

If we now make the simplifying assumption that the capacitive impedance is much larger than Rt. Irt = jω

Vm C1 2

(7.72)

7.15 Mitigation of High Recovery Voltage by Using …

295

(b)

(a)

(d)

(c)

(e)

Fig. 7.15 Reverser at neutral with tie-in resistance

The recovery voltages are Vrn = jω.

Vm Vt C 1 Rt + 2 2

(7.73)

Vr1 = jω.

Vm Vt C 1 Rt − 2 2

(7.74)

296

7 Problem of Capacitively Determined Potential

Figure 7.15e shows a vector diagram. The two terms of Eqs. 7.60 and 7.61 are at quadrature. In magnitude, 

 2  2 

Vt V m + C 1 Rt Vrn = Vr1 =  2 2

7.15.1.2

(7.75)

Arc Currents

Arcing currents are calculated from Fig. 7.15c. Arcing condition is equal to the pole being in contact with the respective arcing contact. The recovery voltage corresponds to the Thévenin source voltage for the arc currents. The source impedance consists of Rt , 1/jωC 1 and 1/jωC 2 in parallel. In order to simplify the expressions for the current without much loss of accuracy, we regard the capacitive impedances like 1/jωC 1 to be large as compared to Rt . They can be omitted when taking the parallel combination with Rt . The arc currents after this simplifying assumption are  Irn = jω  Ir1 = jω

C1 Vm Vt + 2 2Rt C1 Vm Vt − 2 2Rt

 (7.76)  (7.77)

The above are vector equations. In magnitude, both currents are 

 2  2 

V V t m  + C1 2Rt 2

7.15.1.3

(7.78)

Illustrative Example

To obtain an idea of how well the tie-in works, we shall review the example in Sect. 7.5.3 for a 240 kV transformer. The particulars relevant for the evaluation are found on P 128 of [2]. To recollect, C 1 = 1950 pf, and C 2 = 450 pf. We shall take Rt √ = 300 k. Vm /2 = 240/2 3 = 69.28 kV, and V t /2 = 6.92 kV. Using Eq. 7.62, the recovery voltages are 6.37 kV. This may be compared with V rn = 64.95 kV and V r1 = 47.63 kV calculated by Ref. [2] for the same transformer, when there is no tie-in.

7.16 Tie-in at Delta Line End

297

7.16 Tie-in at Delta Line End An important application of tie-in arises when taps are at the line end of delta. Figure 7.16 shows how these high voltages can be controlled by a tie-in resistance. Figure 7.16 is a repeat of Fig. 7.8 with the tie-in resistance added appropriately. Figure 7.16a shows the physical layout of the windings, with the distributed capacitances shown lumped at the mid-point. Figure 7.16b is the connection diagram at the point of changeover, where the fine winding has no conductive connection with the rest of the phase. The equivalent circuit of Fig. 7.16c shows the condition under

(a)

(b)

(c)

(d)

(e)

Fig. 7.16 Change over of reverser at delta line end with tie-in resistance

298

7 Problem of Capacitively Determined Potential

arcing conditions. This is used to find the arc currents. When the arc is quenched, the conditions are represented by Fig. 7.16d from which the recovery voltages can be established.

7.16.1 Recovery Voltages The recovery voltages are worked out from Fig. 7.15d. The recovery voltages are the voltage rise across Rt , caused by the flow of current through it, ±V t /2. To find the current through Rt , we may resort to the Thévenin theorem, by making a conceptual break at the point Z in Fig. 7.16d and establish the Thévenin source voltage driving the current through R when Z is closed. The potential of B over R with Z open is the Thévenin source voltage. In loop 1 (Fig. 7.15d), the    Vm C1 Vb1 = Vr + 2 C1 + C2

(7.79)

The source impedance is that of the capacitances C 1 and C 2 in parallel, i.e. 1/jω(C 1 + C 2 ). The impedance limiting the current is Rt +

1 jω(C1 + C2 )

(7.80)

To simplify, we assume that the capacitive impedance is much larger than Rt Then the current through Rt in Fig. 7.16d is   Vm C1 jω Vr + 2

(7.81)

The recovery voltages are 

 Vm C 1 Rt + 2   Vm C 1 Rt − Vr1 = jω Vr + 2 Vrn = jω

Vr +

Vt 2 Vt 2

 (7.82)  (7.83)

The two terms are at quadrature. In magnitude 

 2   2 

Vt V m + Vr + C 1 Rt Vrn = Vr1 =  2 2

(7.84)

7.16 Tie-in at Delta Line End

299

7.16.2 Arc Currents The arc currents are worked out from Fig. 7.16c. The voltage across the contacts when they open, i.e. the recovery voltages are the Thévenin source voltage. The impedance limiting the current is Rt shunted by the parallel combination of C 1 and C 2 . Since Rt is much smaller than the capacitive impedances, this can be taken Rt . Thus    Vm Vt C1 + (7.85) Irn = jω Vr + 2 2Rt    Vm Vt C1 − (7.86) Ir1 = jω Vr + 2 2Rt The two components of current in Eqs. 7.85 and 7.86 are at quadrature. In magnitude 

  2  2 

V V t m + Vr + C1 Irn = Ir1 =  2Rt 2

(7.87)

Figure 7.16e shows the vector diagram for the recovery voltages.

7.16.3 Illustrative Example Reference [2], Page 132, works out the recovery voltages of a 132 kV Reversing tap transformer with ±10% taps at the delta line end. The capacitances are C 1 = 1810 pf and C 2 = 950 pf. We now rework this case with a 300 k tie-in. using Eq. 7.45, V rN works out to 6, 6 + j20, 68 kV and V r1 works out to −6, 6 + j20, 68 kV. These are comfortable recovery voltages for a 132 kV line end tapchanger of most makes. The value can be compared to V rN = 73.78 kV and V r1 = 60.83 kV without tie-in in [2], Page 132. The steady dissipation at mid-tap is 145 W which is reasonable.

7.17 Other Mitigation Techniques 7.17.1 Shielding to Influence Capacitive Coupling An unfavourable capacitive coupling of the fine tap winding can lead to high recovery voltages. Suitably located and connected shields can influence the capacitive coupling and the voltage distribution in favour of reduced recovery voltages and arc currents.

300

7.17.1.1

7 Problem of Capacitively Determined Potential

Shielding of Reversing Taps at the Delta Line End

In Sect. 7.9, it was shown that when taps are located at the line end of delta the recovery voltages are high. The pole is retained at the potential of one corner of the delta due to direct connection (see Fig. 7.8). Qualitatively we can argue that the fine winding which is left “floating” can fall too far towards ground if it has a high capacitive coupling to it. On the other hand, if it couples closely to the remaining winding of the phase, its potential is pulled up towards half of line-to-line voltage. In either case, the result is a voltage very different from that of the pole. Since the reverser contacts derive their capacitively picked up voltage on the fine winding, they experience a high recovery voltage. If the fine winding can maintain its potential close to the pole, by high capacitance to a shield connected to the pole potential, the recovery voltages can be lowered.

7.17.1.2

Recovery Voltages

Figure 7.17 illustrates the principle of shielding. Figure 7.17a shows the physical arrangement of the windings and the position of the shield. The distributed capacitances are shown lumped at the mid-point of the windings. In Fig. 7.17b which shows

(a)

(c)

Fig. 7.17 Shielded taps at delta line end

(b)

(d)

7.17 Other Mitigation Techniques

301

the connections, R is the pole of the reverser. The tapping winding couples through the distributed capacitance C 11 to the shield connected to R. The tapping winding also couples to the main winding through the capacitance C 1 . The tapping winding is shielded off from the earth. Figure 7.17c shows the equivalent circuit. In loop 1 of Fig. 7.17c, the potential rise of B over R is Vbr C1 C1 + C2

(7.88)

Vrn =

1 Vm C1 Vt + 2 C1 + C2 2

(7.89)

Vr1 =

1 Vm C1 Vt − 2 C1 + C2 2

(7.90)

The recovery voltages are

Figure 7.17d shows the vector diagram corresponding to Eqs. 7.77 and 7.78.

7.17.1.3

Illustrative Example

The normal operating voltage between the shield and the fine winding is only the tapping range voltage. The fine winding has a higher voltage to the main. The radial gap between the fine winding and the shield can be much smaller that the gap between the fine winding and the rest of the phase winding. Capacitances C 1 and C 11 are roughly in the inverse ratio of the radial gaps. We can take C 11 /C 1 to be 2–4. For a 132 kV delta transformer, with 10% reversing taps and a ratio of C 11 /C 1 of 2, the higher recovery voltage is 28.6 kV. This may be compared to the recovery voltage of 73.78 kV worked out in a typical 132 kV delta transformer without shielding in [2].

7.17.1.4

Practical Difficulties with Shielding

Shielding imposes demands on the core and coil of the transformer. The shield may be at high voltage relative to the shielded winding or ground. For instance, in Fig. 7.17a the shield connected to the R terminal lies next to the ground. Provision of extra insulation in the core window is very expensive. The edges of the shields are sharp and prone to PD or corona discharge. Rounding off is possible, but this will again increase the clearance. The same effect may be obtained by the use of discrete external capacitances. In this alternative, very high voltage capacitance is required and will have to be fabricated on case-to-case basis. Insulating the connection leads may present a problem, in cases where the connection needs to be made to at mid

302

7 Problem of Capacitively Determined Potential

height of an inner coil. The lead must then run down the axial gap, in a region of high electrical stress. Use of discrete external capacitances appears to only a theoretical solution.

7.17.2 Use of the Double Reverser Switch The advance/retard switch of Sect. 6.7.1 and Fig. 6.8 reverses the direction of connection of the tapping winding between the sending and receiving terminals. During such a reversal operation, the through power is maintained. The transformer need not be de-energized. The same switch can be used to reverse connect the tappings in a transformer while the transformer is on load. Figure 7.18 shows the reversal operation of the tapping winding. When applied to tapchanger, the switch is often recognized as a “Double Reverser” on account of both terminals of the winding being reversed. It will be observed that the regulating winding is always connected to a defined potential. Throughout the operation, there is no break in the current path, current only commutates between paths. No tap is ever shorted.

7.17.2.1

Thermal Stress of Tie-in

The tie-in is connected between the pole of the pre-selector, and the wander point current takes off. The voltage applied to it is depended on the operating tap position. When on mid-tap, there is no voltage applied, and at each end tap, half the range voltage appears across it. The heat produced must be dissipated to maintain the temperature rise to safe limits. For an eight-step regulation, even at the next tap from the middle, the dissipation falls to about 50% of the maximum. As an illustration, we may look at the heat dissipation in the mid position in an application to a 220 kV transformer with ±10% taps. The maximum voltage applied is 6.35 kV. Taking the resistance to be 200 k, the dissipation is 202 W per phase. In order to get sufficient surface area to cool this dissipation, the resistance is wound in several cylindrical bobbins. The bobbins are enveloped in a solid insulation medium, which helps in removing the heat from the wire to the surface, where it can be dissipated to the surrounding oil. These are mounted either at the bottom of the selector (Fig. 7.19a) or separately elsewhere on the tapchanger structure (Fig. 7.19b). Many manufacturers prefer mounting the tie-in separately within the transformer tank. In the latter alternative, it is possible to locate the tie-in in a more accessible position on the transformer tank. This is a help in case of repairs or replacement. The life of the tie-in must match that of the tapchanger. This is quite a problem of technical management. The wire is often of very small gauge to get the high resistance. There is no practical way of applying much flexible insulation on such a wire. Yet the voltage applied is of the order of a few kV. The practice of winding the tie-in in several series-connected bobbins is helpful in resolving the voltage problem also, since the voltage between layers and turns is reduced. In the example considered, if there are

7.17 Other Mitigation Techniques

(a)

(d)

303

(b)

(a)

(e)

(c)

(b)

(f)

(c)

Fig. 7.18 Reversing operation with double reverser

six bobbins, we have about 1000 V between the first and last layer. Attention must be paid to the surface breakdown which may occur along the top and bottom edges. No doubt techniques are available to take care of this problem, e.g. progressively shortening the outer layers.

7.17.3 Dissipation in the Tie-in During Changeover In Sect. 7.17.2.1, the dissipation in the tie-in due to the application of half the tap range voltage at extreme tap position was considered. We also have to take care of a surge in dissipation which occurs during the transition of the pre-selector. The method of determining the extra surge is illustrated by the example of a reverser

304

7 Problem of Capacitively Determined Potential

Fig. 7.19 a Tie-in resistance mounted at the bottom of the selector, b Tie-in resistance mounted at the top of the selector

tapchanger at the neutral Fig. 7.15 is applicable. We can recognize two regimes of the changeover. 1. The pre-selector opens with arcing. So long as the arcing persists, the situation is as represented by Fig. 7.15c. This is the conductive regime, where the voltage applied to the tie-in is half the range voltage. 2. The arc is quenched finally. The situation enters the capacitive regime as shown in Fig. 7.15d. The current through the tie-in is determined by Eq. 7.59. This

7.17 Other Mitigation Techniques

305

condition starts immediately after arc between the pole and the parting fixed contact, say n is quenched, and continues till the pole makes to the contact 1. This lasts the most part of the time taken for the pre-selector movement, amounting to 1–1.5 s. The energy dissipated can be large in this long duration. The tie-in must be dimensioned to withstand the enhanced dissipation also.

7.17.3.1

Calculation of Current Through Tie-in During the Capacitive Regime

The current through the tie-in during the capacitive regime was already derived. The magnitude is shown in Eq. 7.59. The vector diagram for the condition is shown in Fig. 7.15e.

7.17.3.2

Illustrative Values

In order to derive some useful benchmark figures, we shall revisit the illustrative example. C 1, the case was taken s 1950 pf and the tie-in 300 k. Eq. 7.59 is applicable for the capacitive regime. The current through the resistance is 41.3 mA with a consequent dissipation of 511 W. This may be compared with the continuous maximum dissipation at extreme tap of 162 W. This condition may last typically for 1.2–1.6 s. The energy dissipated, which will largely go into heating up the tie-in will be around 613–817 J. This is not very large. Unless the pre-selector gets stuck in the capacitive regime due to drive failure, the additional heating is marginal.

References 1. 2. 3. 4.

Technical Data-General Section. TD 61. MR Technical Brochure On-Load Tap-Changers for Power Transformers (2000) Book. Axel Krämer. MR Publication IEC 62 320 Guide for the application, specification, and testing of phase-shifting Transformers Transformer Design Principles. Book. Robert del Vecchio et al. 2001. Chapter 13. Pub: Taylor and Francis. CRC Press

Chapter 8

Vaccum Tapchangers

When you look at a vacuum in a quantum theory of fields, it isn’t exactly nothing. Peter Higgs, Meet Peter Higgs video by CERN (July 2004).

8.1 Chapter Content In conventional tapchangers mechanical contacts make and break current in the surrounding medium, which is in most cases mineral oil. This leads to arcing at the contacts, contact damage, and despoliation of oil. The arc quenching ability of mineral oil has been acceptable for long. It has been found adequate to rupture arcs of a few thousands of kVA in tapchanger service. However contact wear and the need to maintain oil quality have been a problem associated with tapchangers with this technology. In vacuum tapchangers all current making and breaking duty is handled by a vacuum interrupter. The interrupter is enveloped in a hermetically sealed vacuum bottle. The arc products are completely isolated from the surrounding oil. The vacuum interrupter shows a very high rupturing capacity, as compared to an arcing contact in oil. The limitation in arc rupturing capacity, which was always a matter of extensive discussion in conventional oil based technology, is removed. This enables larger switching capacities in smaller volume. The life in terms of number of switching goes up. The oil is not contaminated with arc products and needs only as much routine conditioning as the transformer itself. For reasons which we shall see in this chapter, the contact wear of vacuum bottles is much lower than exposed metallic contacts in oil. Hundreds of thousands of interruptions are possible, replacing the older tens of thousands. With many of the limitations, and deleterious effects of contacts directly in oil removed, the vacuum technology is rightly hailed as a right step in the progress of tapchanger technology. At the minimum, the demand for maintenance of tapchangers due to contact condition and poor oil has come drastically down. This chapter essentially amplifies the switching sequences of vacuum tapchangers listed in Table A.3 of IEC 60 214 Ref. [1]. It is possible to devise other operating schemes © Springer Nature Singapore Pte Ltd. 2020 T. V. Sridhar, Application of Tap changers to Transformers, Power Systems, https://doi.org/10.1007/978-981-15-3955-8_8

307

308

8 Vaccum Tapchangers

besides those described in the IEC (See Sects. 8.10.2, 8.12). The IEC schemes allow the manufacturer to adopt one most suitable for his needs. This may include preexisting modules, such as bypass switches to be incorporated. Manufacturers also have the option of using from 1 (not an IEC scheme) to 4 vacuum interrupters in the diverter switch. There is no intrinsic superiority of one as opposed to the rest. It a question of the manufacturers choice as to what he considers optimum with reference to the number of interrupters, mechanical switches, and the overall complexity of the drive to the diverter switch.

8.1.1 A Word of Caution on Terminology Strictly, the so-called vacuum tapchanger of today does not deserve that moniker. It does not work in vacuum, any more than a vacuum breaker does. The tapchanger still needs oil for all functions other than arc quenching. It is still immersed in oil, atmospheric air, or ester fluids, or SF6 to provide insulation, cooling, keeping out the atmosphere, and to act as a messenger of the overall condition. So the vacuum tapchanger of today is an oil tapchanger with a vacuum interrupter to take care of one of its functions. However the term has been accepted for long, including the IEC Standard 60 214, and it would rival the redoubtable but sad Don Quixote to tilt at windmills.

8.2 Basic Principle We shall at first take one example of vacuum tapchanger as in Fig. 8.1 to establish the main features of vacuum switching. This circuit is also cited in IEC 60 214, Table A3. Other switching schemes will be deferred for later in the chapter. The basic principle is to ensure that all current making and breaking are done inside the vacuum bottle. The moving mechanical contacts only select the fixed contact on which the tapchanger must operate. A vacuum contactor is connected in series and is closed by mechanical linkage to switch the current on. Likewise when an interruption is required, the vacuum contact opens first. The series-connected mechanical contact then opens “Off Current”. These concepts are illustrated in Fig. 8.1 for a selector switch type tapchanger. Figure 8.1a shows the arrangement of mechanical contacts and the seriesconnected vacuum interrupters. This scheme employs two mechanical contacts and two interrupters. S m is the main mechanical contact and S t the transition mechanical contact. These are connected in series with the main and transition interrupter V m and V t, respectively. The contacts of the bottle V m are closed in this position. In Fig. 8.1a the tapchanger is operating on tap no.1. Current flow is from the fixed contact 1 through the mechanical contact S m and the vacuum bottle V m . The status of the vacuum contact V t connected in series with the transition contact S t is immaterial at this stage, but for reasons which will be apparent shortly, it can be closed. No current

8.2 Basic Principle

309

however flows through the contact S t on account of the series-connected transition resistance. It is now desired that a tapchange must be made towards fixed contact no.2. V t closes first, if not already closed at the end of the previous tapchange. This action provides an alternate path for the current when contact V m opens in Fig. 8.1b. There will be arcing inside the bottle, as the main current is interrupted. After the arc

(a)

(c)

(e)

(g)

(i)

(b)

(d)

(f)

(h)

(j)

Fig. 8.1 a–j Page 1: Double transition for VAC tap changer tap 1 to 2. k–t Page 2: Double transition for VAC tap changer tap 2 to 1

310

8 Vaccum Tapchangers

(t)

(s)

(r)

(q)

(p)

(n)

(o)

(m)

(l)

Fig. 8.1 (continued)

(k)

8.2 Basic Principle

311

is quenched the current through the mechanical contact becomes zero Fig. 8.1c. The width of the fixed contact is so designed that S m is still on 1 till the arc is quenched. S m exits the edge of contact 1 only after this point Fig. 8.1d. Therefore at the point of contact parting, there is no current in the mechanical contact and no arcing. Current in the meanwhile is carried on the transition path. When S m makes with contact 2, V m is still open Fig. 8.1e. The contact-making is at zero current. After S m makes with contact 2, V m closes (Fig. 8.1f). This bridges the two adjacent contacts, with the establishment of a circulating current. It is important that the transition contact S t remains on fixed contact 1 with its vacuum contact V t closed until this point. Now V t opens to interrupt the circulating current with arcing (Fig. 8.1g). The width of the fixed contact is such that till the arc in the transition bottle is quenched, the mechanical contact S t remains on it Fig. 8.1h. When the mechanical contact St rolls off the edge of the fixed contact Fig. 8.1i it carries no current, so that contact parting is without arc. When the contact S t rides on to contact 2, it is a current less contact-making anyway, irrespective of the status of V t . After S t is on 2, V t can be closed, ready for the next tapchange in the same direction Fig. 8.1j. This completes the tapchange from 1 to 2.

8.2.1 Tapchange in The Reverse Direction If it is desired to reverse the tapchange, the contact St can be allowed to break with the fixed contact 2 with its bottle open or closed, as in either case the contact S t carries no current (Fig. 8.1k). V t must open at any time before S t makes with contact 1, for instance at Fig. 8.1l. After V t opens S t rolls off contact 2 (Fig. 8.1m). When S t makes on contact 1, there is no current through it (Fig. 8.1n). Thereafter V t is switched on, establishing the bridging condition shown in Fig. 8.1o. At this point V m is opened, interrupting the current through the contact S m with arcing in the bottle (Fig. 8.1p). Once again the width of the fixed contact allows S m to be in touch with it, till the arc in the bottle is quenched (Fig. 8.1q). After that point, S m can roll off the edge of the fixed contact 2, carrying no current (Fig. 8.1r). The transformer current flows through the transition path and the transition resistance. S m makes the fixed contact 1 with V m still open, so that the contact-making is at zero current (Fig. 8.1s). Next V m closes (Fig. 8.1t). Current transfers to the main path, completing the tapchange in the reverse direction. V t may remain open, or it can close ready for the next tapchange. Whether V t should open or not is optional. This completes the tapchange in the reverse direction. The described switching cycle is used in OLG UZDvac, a highspeed resistance transition vacuum tapchanger. Table 8.1 summarizes the interruption duties.

312

8 Vaccum Tapchangers

Table 8.1 interruption duties of two interrupter selector switch. Ref Fig. 8.1 Direction

Figure references

Interrupter

Int current

Left to right

8.1b

Vm

I

RI

N/2

8.1g

Vt

E/R

E

N/2

8.1p

Vm

E/R ± I

E ± RI

N/2

Right to left

Recovery voltage

NR of operations

8.3 Common Features Summarized Even though the above description relates to a selector switch tapchanger, it incorporates some features common to all well designed and executed vacuum tapchangers. These can be summarized as follows. 1. The mechanical contacts do not break or make current. 2. All switching activity is confined to the bottles. Therefore there is no contamination of oil by arcing. 3. A transition resistance is required. This point needs particular emphasis because there is a widespread incorrect impression among the industry that transition resistance is not required with vacuum technology. 4. The tapchanger must operate on Dr. Jansen high-speed resistance principle (see Chap. 2). That is a stored energy drive for the contacts required. 5. The width of the fixed contacts as well as the spacing between them is increased relative to a non-vacuum execution. Therefore the tapchanger will be bigger. 6. The speed of transition can be reduced to advantage, in view of the several intended periods of inactivity with the progress of switching, during which the contacts are however moving. 7. The timing requirements are more complex than a non-vacuum tapchanger. 8. The question will no doubt spring in the mind of the discerning reader whether the selector switch operation described above is truly the same as the mechanical contact type selector switch operation. The mechanical contacts select the position of operation, and then the vacuum interrupter switches the current on like a conventional diverter switch. Is it then not appropriate to designate the vacuum switch as a diverter switch?

8.4 Contact Bounce In high-speed selector switches some kind of entry taper is provided on the edges of the fixed contacts to make it easier for the moving contacts to climb on, without a high impact. After the first impact, whether the moving contact will follow the taper and remain in contact depends on the contact mass, the restoring spring force, and the peripheral velocity of the impact. In general there may be loss of contact

8.4 Contact Bounce

313

for a short while, before the moving contact settles on the fixed. This is the socalled contact bounce. The contact bounce is more pronounced when mechanical contact is made. But it can also occur to a smaller extent at contact break, due to the inability of the moving contact to follow the taper due to its inertia. This is exacerbated by contact roughness and impact marks due to mechanical causes at the contact edges. Thus we may expect bounces both on making and on breaking. When a current-carrying contact bounces, an arc is drawn between the fixed and moving contacts. These are typically short arcs both in length and in time. The bounce arc adds to the contact erosion and oil damage. In non-vacuum tapchangers this minor phenomenon is overwhelmed by arcs of normal interruptions. In vacuum technology there must be no arcing at the mechanical contacts. The effect of contact bounce over a large number of operations will discolour the oil and produce discernible dissolved gasses. This will not be acceptable to the user. If nothing else he is likely to worry if the tapchanger is occasionally arcing at the mechanical contacts. With vacuum technology we are presented with the option of largely eliminating contact bounce when carrying current. This is described in what follows.

8.4.1 A Refinement of Operation We face the following three situations: 1. Contact bounce at the entry point. 2. Contact bounce at the exit point. 3. Contact bounce during arcing in the bottle, if the moving contact in series approaches too close the contact edge. We shall re-examine Fig. 8.1 from this point of view, considering only those situations when one of the above conditions is involved. In Fig. 8.1b the bottle is arcing with the through current. The contact S m is approaching the contact edge. If it gets too close to the edge, with its taper and roughened surface, S m may bounce. A similar situation arises with Figs. 8.1h, q. How much is too close will be judged by the designer from his experience and testing. He would want to make sure that the contact is at least two or three mm away from the taper when arcing ceases in the worst case. He would accordingly extend the contact width at both ends to take care of bouncing. In extending the fixed contact in this manner he would be aware that he is adding material for safety, and that the extension may not take part in the switching action otherwise. In Fig. 8.1f he may allow for S m to reach further down from the entry edge before V m closes, to avoid V m carrying current when S m may be bouncing. This allowance may be more than at the leaving edge. This consideration applies also in Fig. 8.1o, s. Once again some part of the fixed contact width does not take part in the actual switching activity. These “bouncing allowances” on the contact width contribute to the increased fixed contact width and hence the total size. But well-judged allowances will ensure that there is no contact bounce when a contact carries current.

314

8.4.1.1

8 Vaccum Tapchangers

Interruption Duties of Contacts

The interruption duties of the vacuum devices are exactly the same as those of the mechanical contacts of the single resistance transition tapchanger with mechanical contacts, described in Sect. 3.3. The duties of the main contact T are those of V m in Fig. 8.1 and that of the transition contact S are those of V t . The vector expressions for duties, the duty table, the algebra expressions, condition of maximum interruption duty, and the directionality of power are as described in the corresponding sections of Chap. 3. These will not be discussed further here.

8.4.2 Vacuum Tapchangers with Other Switching Cycles We shall examine some other switching cycles commonly employed with vacuum tapchangers in Sect. 8.6.10 onwards later. We shall first examine some issues with integrating vacuum interrupters into tapchangers.

8.5 Why Is the Vacuum Interrupter So Good in Its Function There is a large body of knowledge consisting of books, reviews, thesis, research papers, learned articles on vacuum interrupters from its first practical version developed by Royal Sorensen Ref. [2] in 1926, for the next 90 years. The following can only be a sketchy, but relevant summary of the reason why the vacuum interrupter is so good at current interruption. For details see Refs. [2–10]. 1. During interruption, current is carried by a diffuse metal vapour arc. 2. The electrical resistance of the arc is very low. This reduces the energy input into the arc by the flow of current. The temperature is low, so that melting of metal at the contacts at the roots of the arc is low. This minimizes contact surface damage. 3. The dielectric strength of the vacuum interrupter is very high at its operating level of vacuum. The Paschen’s curve (Fig. 8.2) shows how the breakdown value depends on the pressure Ref. [10]. 4. The vacuum interrupter maintains a pressure in the order of 10−5 torr, so that the breakdown value is high. This enables interruption at short contact gaps. 5. At current zero, with no instantaneous energy input to sustain it, the arc condenses rapidly. The breakdown strength of the contact gap has a very short time constant for recovery. This rapid build-up of strength prevents restrikes even with very fast transient restrike voltages. 6. The metal vapour arc has a high thermal conductivity. Heat is quickly transferred to the electrodes at the ends, and the condensation shields, so that condensation is hastened. As a result of these properties, the vacuum interrupter is able to quench

8.5 Why Is the Vacuum Interrupter So Good in Its Function

315

Fig. 8.2 Dielectric strength of vacuum-the Paschen curve

currents mostly in the first current zero, with minimum loss of contact material. Contact life is enormously increased.

8.6 Specialties in Interrupter Construction for Tapchanger Applications We shall look at the interrupter construction only from the point of view of its application to tapchangers. Refs. [3–6] may be consulted on normal vacuum interrupters. Vacuum interrupters are manufactured in large numbers around the world for breaker and contactor applications. Most interrupters are designed for operation in air. The number of operations required in conventional duty is not very high. As against this, interrupters for tapchanger duty are immersed in oil, and the life expectation in terms of number of operations is higher. It becomes necessary that almost all the constructional features of a standard vacuum interrupter have to be modified to develop one for tapchanger duty. Each tapchanger manufacturer goes through a long and expensive process of development. For instance OLG spent three years of cooperative research and development with Crompton Greaves Limited (who were the bottle manufacturers) before a satisfactory bottle was available. Naturally the manufacturers zealously guard their technology. For reasons of intellectual property rights and confidentiality it is not possible to enter into details of interrupters for tapchanger duty in this book. Only the general nature of the issues involved can be discussed without details.

316

8 Vaccum Tapchangers

8.6.1 Construction of Standard Vacuum Interrupters Figure 8.3 shows the basic structure of a “standard” vacuum interrupter. The interrupter is enclosed in a tubular ceramic or glass envelope. The ceramic envelope has metallized edges to which the two end metal caps are welded in a vacuum tight manner. The upper contact is fixed to the top end cap. The lower contact can move. An expanding metal bellows welded to the bottom end cap, and the moving contact stem allows vacuum to be retained in the bottle. A guide for the moving stem is fixed on the bottom end cap. The annular space formed by the moving contact stem, the bellows, and the guide is filled with air in most interrupter applications. Figure 8.4 shows a practical vacuum interrupter, with some details of contrite contact shape, condensation shields, bellows. One of the greatest advantages of the vacuum interrupter is that the contact separation required is very small. Such small strokes are only possible with flat butt contacts. Conduction of current during arcing in a vacuum interrupter is through a metal vapour arc occupying the space between the contacts. Force is applied on the arc column by creating a magnetic field in the axial or radial direction Ref. [10]. The contact shape known as “Contrate” contacts (Fig. 8.5a. Ref. [2]) produces a radial magnetic field. A straight flat butt contact produces an axial radial magnetic field (Fig. 8.5b Ref. [10]). Both types of magnetic fields prevent the arc roots from concentrating at a few cathode spots. This is an absolute necessity to achieve the rupturing capacity of vacuum interrupter Refs. [8, 9]. The metal of the arc condensate has a very small time constant and very quickly condenses on the contacts as well as the condensation shields. For effectiveness the condensation shield must be close to the arc. The low time constant is the main feature contributing Fig. 8.3 Construction of vacuum interrupter

8.6 Specialties in Interrupter Construction for Tapchanger …

317

Fig. 8.4 Practical interrupter

(a) CONTACT FOR

RADIAL MAGNETIC FIELD

(b) CONTACT FOR AXIAL MAGNETIC FIELD

(REF. 10)

(REF. 10)

Fig. 8.5 a, b Contact shapes for magnetic fields

to the interrupting prowess of the bottle, since a current zero very little metallic “gas” is available to contribute to restrike. From the point of view of tapchanger application the most noteworthy feature of the vacuum interrupter is that the contacts close by external atmospheric pressure on the bellows. Some commercial vacuum interrupters are shown in Fig. 8.6.

8.6.2 Modifications For Tapchanger Duty 1. Fatigue of the bellows with repeated extension and contraction of length limits the life of the interrupter. In tapchanger application where a large number of operations are required, the bellow life can be increased by minimizing the stroke, and using a long bellow length. Further at the point of installation the initial stretch or compression must be kept minimal. 2. When the annular space between the stem and the bellows space is filled by oil, it tends to be trapped when the stem moves quickly, compressing the oil column.

318

8 Vaccum Tapchangers

Fig. 8.6 Some commercial vacuum interrupters

Oil being incompressible, the force can be very high leading to bellows damage. The guide has to be modified in tapchanger applications to allow quick expulsion of oil to reduce compressive forces. 3. The metal vapour condenses on to the cool shield electrodes after arc quench. This helps in quickly cooling and condensing the arc, so that current interruption can occur mostly at the first current zero. If metal vapour condensate deposits on the bellows a hard spot results, and the bellows tear at that point soon. The design of the vapour condensation shields is very important to prevent condensation on the bellows. The shield structure must also be so designed in tapchanger application that the vapour does not condense on the insulated ceramic envelope in a manner as to reduce the electrical withstand. The metal condensate does not adhere too well to the shield surface. If the build-up is high, due to a large number of operations, the outer layers of the condensate tend to peel off, and the metal flake can short the shield with the contacts. The selection of the shield material, its compatibility with the contact material, and the clearances are critical in preventing flaking off. The location and geometry of the shields must be managed in a manner that condensation is confined to the areas where there would be no harm. 4. It is superfluous to add that the contact materials should be well researched, to ensure rapid arc quenching, without current chopping. The materials should be able to withstand a large number of operations with acceptable level of wear. The metal vapour formed should not be so high as to build thick condensate layers on the shield surfaces. 5. The vacuum interrupter is so good at current interruption, and some distress is experienced during current making. There is a tendency for the contacts to weld. This could happen even at low currents. The opening force must be sufficient, with adequate “Snatch Gap” to wrench the contacts apart if they are welded. (See Fig. 8.8). The Snatch Gap is a measure of the movement of the opening system before engagement of the actual opening of the contacts. This allows the opening

8.6 Specialties in Interrupter Construction for Tapchanger …

319

mechanism to gather momentum at the point of contact break and deliver a blow of kinetic energy, which is useful to ensure that light contact welds, if any, during the previous operation can be broken off. As the contact-making current duties are important in this regard, Table A3 of IEC 60 214 also lists the making duties for vacuum type tapchangers. The tearing off of welds naturally contributes to contact wear. The fact that the energy for breaking contacts must be stored in the hold-open spring complicates the closing requirements. The contact material must be chosen not to forms weld at all, and if such welds are formed they must be brittle and easily broken.

8.6.3 Operation Under Short-Circuit Conditions It has already been mentioned that vacuum interrupters have butt contacts. In such contacts current does not transfer uniformly over the entire mating surface, but through a small number of high points. Currents entering the contact from the stem at the end opposite the mating surface will trace a path towards these high points. For illustration Fig. 8.7 shows one high point and the current paths towards it in the two contacts. It will be noticed that just under the surface, currents in the two contacts flow in parallel streams in opposite directions. This causes a rejection force, which tries to separate the contacts. This “contacts throw off” force can be quite large under short-circuit conditions. Extra contact closing force may be required to keep the contacts in touch against the throw off force. The added force can be larger than that required from the point of view of contact resistance. This requires a heavier stored energy mechanism to drive the tapchange. To maintain life at the same level, the complexity of a heavy mechanism is much higher. Besides the stored energy

Fig. 8.7 Contact throw off forces

320

8 Vaccum Tapchangers

device arrives at the end of the tapchange with a larger energy, which must somehow be dissipated. Alternative solutions to just increasing the contact force seem worth looking for (Sects. 8.6.4 and 8.6.6). More details on contact throw off can be found in Ref [4], Sect. 10.4. An approximate expression for the minimum contact force to counter the throw off forces in butt contacts in high vacuum is given in Ref [4], Sec.10.4.2, Eq. 10.26 as Fmin = 4.45 × 10−7 I 2 Newtons

(8.1)

8.6.4 Electromagnetic Force Generator It is possible to attach a solenoid either directly axially, or through linkage to the contact stem to increase the closing force only under short circuit. Figure 8.8 shows schematically how this can be done. A facsimile of the AC short-circuit current through a CT, or even a rectified current can be used to energize the solenoid. With this arrangement, there is no need to increase the closing mechanical spring force of the contacts. The energy storage device to drive the contacts can remain small. If using AC directly, the stem and the solenoid magnet must be laminated. Otherwise the eddy currents will result in a substantial phase shift between current and flux. As the force depends on the instantaneous flux, the effectiveness is sacrificed between

Fig. 8.8 Electromagnetic force generator

8.6 Specialties in Interrupter Construction for Tapchanger …

321

Fig. 8.9 Waukesha UZD contact force generator

current and flux. As the force depends on the instantaneous flux, the effectiveness is sacrificed to an extent.

8.6.5 Waukesha Contact Force Generator Figure 8.9 shows a contact arrangement used by Waukesha Electric in their UZD tapchanger. This is probably an original ABB tapchanger UZD. The contacts are shod with a U-shaped steel shoe each. The current carried by the contact magnetizes the steel shoes, and they exert an attractive force on each other. Under short circuit the increase in contact closing force can be of comfort. The Waukesha shoe is not laminated.

8.6.6 Use of Shunt Contact The problem of the bottle contact opening under short circuit can be solved by bypassing it in the operating position by a shunt contact. The principle is shown in Fig. 8.10. There are now two parallel paths for the current: one through the main roller and bottle and the other through the shunt roller. The shunt contact is made of lower resistance than through the main arcing roller contact, so that most of the current take

322

8 Vaccum Tapchangers

Fig. 8.10 Principle of shunt contact

this parallel path. This bypasses most the short-circuit current away from the bottles in the operating position. When a tapchange is made, the shunt roller breaks first, and current commutates to the main roller. The very small resistance of the path through the roller and bottle offers a small recovery voltage as the shunt breaks and causes a minor sparking. In a simulated study conducted at OLG, it was found that at 600 A current, and a resistance of main current path (the one through the main roller and bottle) of 3 m, and a shunt path of approximately 0.6 m a slight discolouration of the oil could be observed after about 7000 operations. The total oil quantity in which the contacts were immersed was 150 lits. The dissolved gas content was not remarkable. This could be due to the fact that the experimental set up was open at the top of the atmosphere.

8.6.7 Problems with Moving Bottles In tapchangers operating as selector switches, it is convenient to house the bottles on a moving platform along with the mechanical contacts. In linear selector switches like MR Type AVT the bottles are mounted on a moving housing, along with the moving contacts and energy storage device. A fixed cam plate operates the bottles at the correct spatial position. In rotary selector switches similar to MR type VV and ABB VUBB, the bottles are mounted on the rotating central shaft of the contact movement. The movement of the contacts and the bottles driven by stored energy is jerky and causes considerable inertial forces on the components, including the bottle stems. If the bottles are mounted horizontally on a vertical drive shaft (Fig. 8.11a), the centrifugal force can add substantially to the contact spring force. This will load the

8.6 Specialties in Interrupter Construction for Tapchanger …

(a)

323

(b)

Fig. 8.11 Forces on stem and bellows of rotating bottle

stored energy drive, when the contacts have to be operated. If the bottles are mounted with their axis parallel to the drive shaft (Fig. 8.11b), there is a Coriolis force which presses the stem on to the guides increasing frictional resistance to motion. This force also acts on the bellows also causing bending. These are issues that the tapchanger designer must take care of.

8.6.8 Non-sustained Disruptive Discharge in Vacuum This phenomenon was discovered during testing of vacuum switchgear Refs. [11– 13]. When the supply to the test circuit was maintained for a while clearance of the test current, a short high-frequency discharge was sometimes observed through the bottle. The recovery voltage fell for a short period of the discharge. This discharge was not sustained by a through current flow under the influence of the industrial frequency applied voltage. This discharge is known as non-sustained disruptive discharge (NSDD). Some early vacuum tapchangers used two bottles in series, to avoid possible consequences of NSDD. However the consensus later was that NSDD will not cause power frequency follow through. Modern vacuum breakers and tapchangers do not incorporate any specific defence against NSDD.

8.6.9 Consequences of Failure of Bottles The extra complexity of switching in vacuum tapchangers brings in train extra modes of failure. The description in Sect. 8.2 of the operating cycle of a vacuum tapchanger indicates that a close and tight control of the operation of the bottles in relation to the position of the contacts is required. The word synchronism is a misnomer in this

324

8 Vaccum Tapchangers

context, but it will be used with the understanding that what is meant is the operation of the bottles relative to the position of contacts. A problem will arise if: 1. Synchronism is not maintained. This is not necessarily a bottle fault, but could be due to the mechanical malfunction of the linkages. The vacuum device may fail to be effective. The mechanical contacts will have to perform the switching function. 2. A bottle may fail to close. With vacuum inside, vacuum bottles close due to the atmospheric pressure acting on the bellows. In most cases there may also be an added contact closing force, applied externally by a spring, to combat contact throw off force in the event of a short circuit (Sect. 8.6.3). Thus a failure of the bottle to close can only be due to mechanical hindrance. Failure of the main bottle to close will result in interruption of the main current, with the phase voltage appearing across the contacts. Since the contact gap required to sustain even the phase voltage is small for an interrupter, the interrupter gap may not break down. Failure by voltage flashover will take place between the wander lead and other live parts in the tap selector. This is a major failure, most probably with catastrophic results. Similar problems arise due to the non-availability of a path for the current if the transition bottle does not close at the correct time. This is also a potentially catastrophic failure mode. 3. A bottle may physically fail to open or even if open may lose most of its ability to interrupt currents due to loss of vacuum. Vacuum interrupters need positive external force to open contacts against the atmospheric closing force. Therefore failure to open in the physical sense is an external mechanical failure. 4. If the bottle loses vacuum, it would be through a leak. Oil would fill the envelope quickly due to the pressure differential. Contacts now open in an oil medium. Since the gap is small, in all probability, the switching would not be properly executed. The build-up of pressure may damage the integrity of the bottle. If this does not happen, there would be another attempt at interruption by the mechanical contacts acting as a non-vacuum tapchanger. The success of such interruption depends on the load and individual design. It is likely that the tapchanger may perform several operations acting as a non-vacuum tapchanger. There will be accumulation of gas and discolouration of oil. This may alert the operators and result in a rescue. 5. Unfortunately there appears to be is no information on the public domain regarding consequences of loss of vacuum. A wonderful opportunity presented itself to the author at OLG to investigate and establish some basic data. Unfortunately at that time he failed to grasp the complexities of the issue and omitted to make a complete investigation. 6. It may be concluded that a bottle failure is a serious situation. The vacuum tapchanger becomes more complex than the non-vacuum. New modes of failure come into being. The technological perfection needed will be of a higher order than in old technology. In spite of the technical excellence, if there are failures, it will signify the price that technology pays at the altar of progress.

8.6 Specialties in Interrupter Construction for Tapchanger Applications

325

8.6.10 Other Switching Sequences with Vacuum Tapchangers IEC 60 214-2014 Table A3 lists four methods of applying vacuum technology to tapchangers. Note 1 to the Table admonishes that these are basic, and other more complex (or maybe simpler!) circuits involving more transition resistances and interrupter units may be possible. We shall examine in the following Sections the four IEC circuits, and other commercially or technically important applications.

8.7 IEC 60 214 Table 3 Ref [1] Circuit for Diverter with Two Bottles, Two Auxiliary Switches, and One Resistance Figure 8.12 shows the circuit and its transition stages. Breaking duties are shown whenever applicable. Four tapchanges in all, two in the forward, and two reversing are shown. These cover all the switching possibilities. The MR tapchanger type VR Ref. [19], and Huaming HWV Ref. [20] use this switching cycle. The features of this switching scheme can be summarized as follows: 1. The tapchanger follows the operating sequences 1 and 2 of IEC 60 214 in alternate tapchanges. 2. There is always a path open for the through current without interruption. 3. The tap is not short-circuited at any time without the transition resistance in position to limit the current. 4. The transfer switches Am and At always operate off current. 5. Interrupter Vm does two interruptions of current I at a recovery voltage of RI (Fig. 8.12b, p). This duty is thus performed N/2 times in N operations. It does an interruption of E+RI once in four tapchanges (Fig. 8.12z) and E-RI once in four tapchanges (Fig. 8.12l). These duties are therefore performed N/4 times each in N operations. 6. The E+RI is the heavy switching duty, and the E−RI the light switching duty. 7. The transition interrupter V t does an interruption of current of E/R twice in four operations (Fig. 8.12e, s), and 0 interruption when it opens in two other operations (Fig. 8.12i, w). The breaking duty is thus performed in N/2 operations in a total of N. 8. Even though the expressions for the interrupted current and recovery voltage include terms like E± RI, reversal of power flow (i.e. replacement of the vector I by—I) only shifts the duties between contacts, but does not affect the total duty in N operations. The tapchanger is unmindful of the direction of power flow. 9. Table 8.2 is an extract from Fig. 8.12. It summarizes the interruption duties of the contacts.

326

8 Vaccum Tapchangers

(b) (a)

(d)

(c)

(e)

(i)

(h)

(g)

(f)

(j)

(l) (k)

(m)

(n)

Fig. 8.12 a–n Page 1: Diverter with two bottles and one transition resistance two tapchanges in same direction 1 to 2 and 2 to 3. o–ab Page 2: Diverter with two bottles and one transition resistance two tapchanges in reverse direction

8.8 Algebraic Expressions for Interruption Duties

327

(p) (q)

(o)

(r)

(s)

(z)

(u)

(x)

(w)

(v)

(y)

(t)

(aa)

(ab)

Fig. 8.12 (continued)

8.8 Algebraic Expressions for Interruption Duties The expressions in Table 8.1 for the interruption duties are vector equations. The magnitudes depend on the power factor angle ø. The following are the corresponding algebraic expressions.

328

8 Vaccum Tapchangers

Table 8.2 Interruption duties of diverter switch with two interrupters and two transfer switches.Ref. Fig. 8.12 Direction

Figure references

Contact

Int current

Left to right

8.12b

Vm

I

RI

N/2

8.12e

Vt

E/R

E

N/2

8.12i

Vt

0

0

N/2

8.12l

Vm

E/R−I

E−RI

N/4

8.12p

Vm

I

RI

N/2

8.12s

Vt

E/R

E

N/2

8.12w

Vt

0

0

N/2

8.12z

Vm

E/R + I

E + RI

N/4

Right to left

Recovery voltage

NR of operations Light duty

Heavy duty

Table 8.3 Interruption duties of diverter switch two interrupters and one resistance. Ref Fig. 8.13 Direction

Figure references

Contact

Int current

Left to right

8.13c

Vm

I

RI

N/2

8.13f

Vt

E/R

E

N/4

8.13i

Vm

E/R + I

E + RI

N/4 N/4

Right to left

Recovery voltage

NR of operations

8.13l

Vt

0

0

8.13o

Vm

I

RI

N/2

8.13r

Vt

E/R

E

N/4

8.13u

Vm

E/R−I

E−RI

N/4

8.13x

Vt

0

0

N/4

Heavy duty

Light duty

8.8.1 Main Interrupter Vm Interrupted current = I

(8.2)

At a recovery voltage = RI

(8.3)

 Heavy duty interruption current = At a recovery voltage =

E + I cos ø R

2 + (I sin ø)2

 (E + RI cos ø)2 + (RI sin ø)2  

Light duty interrupted current =

2 E − I cos ø + (I sin ø)2 R

(8.4) (8.5)

(8.6)

8.8 Algebraic Expressions for Interruption Duties

At a recovery voltage =

329

 (E − RI cos ø)2 + (RI sin ø)2

(8.7)

8.8.2 Transition Interrupter Vt Interrupted current = E/R

(8.8)

At a recovery voltage = E

(8.9)

8.8.3 Maximum Interruption Duty The numerical values of the interruption duties depend on the angle ø. The maximum heavy interruption for the main interrupter V m occurs at unity power factor and is of magnitude  (E/R + I )2

(8.10)

 At recovery voltage =

(E + RI)2

(8.11)

8.9 A Diverter with Two Interrupters and One Auxiliary Switch Figure 8.13 is a variation of the diverter switch of Fig. 8.12, where only one transfer switch is used. This circuit is not listed in table A.3 of IEC 60 214. There are two interrupters, and one transfer switch. The earliest reference to the usage of this circuit appears to be an animation in a Toshiba Brochure “Toshiba Gas-insulated GV Series Type D” Ref. [14], where it is stated that over 260 transformers were delivered since 1980 with these tapchangers. The circuit is also used in OLG T03 and MR AVT Ref. [21]. The interruption duties are shown in Fig. 8.13. As in Fig. 8.12 two forward and two reverse tapchanges are shown. Table 8.3 shows the interrupted quantities. Therefore, algebraic expressions for the interrupted quantities are not repeated here again. The features of this switching scheme are the same as that of Fig. 8.12.

330

8 Vaccum Tapchangers

8.9.1 The IEC Table A3 Selector Switch with Single Resistance This circuit was already analysed in Fig. 8.1.

(a)

(b)

(c)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(d)

(l)

Fig. 8.13 a–l Page 1: Switching duties of single resistance transition diverter switch. m–x Page 2: Switching duties of single resistance transition diverter switch

8.10 The IEC Table A3 Diverter …

(m)

(p)

331

(n)

(q)

(o)

(r)

(s)

(t)

(u)

(v)

(w)

(x)

Fig. 8.13 (continued)

8.10 The IEC Table A3 Diverter Switch with Three Interrupters and Two Transition Resistances Figure 8.14 shows the connections and switching sequence of this circuit. This scheme uses three interrupters and one off current transfer switch. The switching duties are indicated whenever they occur in Fig. 8.14. Other features of the circuit are similar to Fig. 8.12. Table 8.4 summarizes the interruption duties. These are similar to Fig. 8.12. They will therefore not be discussed further.

332

8 Vaccum Tapchangers

Table 8.4 interruption duties of diverter switch with three interrupters and one aux switch. Ref Fig. 8.14 Direction Inc. Tap no

Dec. Tap no

Figure references

Contact

Int current

Recovery voltage

NR of operations

8.14b

Vm

I

RI

N

8.14f

V t1

(E/R−I)/2

E−RI

N/4

8.14j

Vm

I

RI

N

8.14n

V t2

(E/R−I)/2

E−RI

N/4

8.14q

Vm

I

RI

N

8.14u

V t1

(E/R + I)/2

E + RI

N/4

8.14y

Vm

I

RI

N

8.14ac

V t2

(E/R + I)/2

E + RI

N/4

Light Duty

Heavy Duty

8.10.1 The IEC Table A3 Selector Switch with Three Interrupters The IEC table A.3 selector switch with three interrupters is the vacuum technology enhancement of the selector switch flag cycle (operating cycle no. 1) of table A1. Figure 8.15 shows the arrangement of contacts and interrupters. Each contact of the circuit in table A.1 selector switch with three mechanical contacts is provided with a series interrupter. The mechanical contacts do not make or break current. All current breaking is achieved in the interrupters. The switching duties are identical to the non-vacuum selector switch. Therefore the switching sequence and interruption duties will not be discussed again.

8.10.2 A Diverter Switch with Only One Vacuum Interrupter Refs. [15, 16] describe a scheme originally patented Ref. [15] for use with thyristorassisted diverter switch. The arrangement can well be used with one vacuum interrupter replacing the thyristor. No published information is available if such a tapchanger is commercially manufactured. Figure 8.16 shows the arrangement of the contacts and the interrupter, and the switching sequence. The scheme uses one vacuum interrupter, two shunt contacts, one series switch S m , and two change over transfer switches Am and At , besides to shunt contact S H1 and S H2 for the entire diverter in operating positions. Of these only the interrupter does a breaking function. All other devices open at no current. The operational cycle is no. 1 of IEC 60 214. The traditional flag and pennant switching does not seem appropriate in this context. The interrupter breaks a current of I at a recovery voltage of RI once, and a current of E/R at a recovery voltage of E once in the progression from tap 1 to tap 2. The interrupter does only these interruptions in all tapchanges in both directions. No other device interrupts any current. Table 8.5 summarizes the interruptions. During bridging the

8.10 The IEC Table A3 Diverter …

333

(a)

(b)

(d)

(e)

(c)

(f)

(g)

Fig. 8.14 a–g Page 1: Interruption duties of diverter with three vaccum interrupters and one AUX switch. h–o Page 2: Interruption duties of diverter with three vaccum interrupters and one AUX switch. p–v Page 3: Interruption duties of diverter with three vaccum interrupters and one AUX switch. w–ad Page 4: Interruption duties of diverter with three vaccum interrupters and one AUX switch

334

8 Vaccum Tapchangers

(h)

(k)

(i)

(l)

(n) (o)

Fig. 8.14 (continued)

(j)

(m)

8.10 The IEC Table A3 Diverter …

(p)

(s)

(v)

Fig. 8.14 (continued)

335

(q)

(t)

(r)

(u)

336

8 Vaccum Tapchangers

(w)

(z)

(ac)

Fig. 8.14 (continued)

(x)

(aa)

(ad)

(y)

(ab)

8.10 The IEC Table A3 Diverter …

337

Fig. 8.15 IEC circuit for selector switch with three interrupters

interrupter carries a current of I + I c but it does not break this current. The main current is commutated to the parallel shunt contact, and only the circulating current is interrupted. In the next tapchange, during bridging, the interrupter current would be I−I c , as the tap selector effectively reverses the voltage applied to the circuit. Again this current is not interrupted, but the main current is commutated off to the parallel shunt contact. The tapchanger is fully bidirectional as the relative directions of E and I do not affect the switching duty. During the tap operation from 2 to 3 or 2 to 1, the devices open and close in the reverse order. This fits the requirement of the mechanical linkages driving the diverter switch. Because of the inclusion of the resistance in all switches tapchanges, the diverter has to operate at high speed through stored energy device. A separate table of interruptions is not shown in the text for this case. The interruption duties are not influenced by the load power factor.

8.11 Vacuum Diverter for Refurbishment Purpose The perceived success of vacuum technology has spawned a demand for “one-toone” replacement of existing oil diverters in service by vacuum diverters. Most manufacturers, for instance MR, ABB, and OLG, offer a refurbishment unit, where the old diverter insert can be taken out, and a new vacuum diverter inserted, with other changes being cosmetic. Not much technical details are available in the public domain

338

8 Vaccum Tapchangers

(a)

(c)

(e)

(b)

(d)

(f)

Fig. 8.16 a–f Page 1: Diverter with one interrupter and three changeover switches. g–k Page 2: Diverter with one interrupter and three changeover switches

8.11 Vacuum Diverter for Refurbishment Purpose

339

(g)

(h)

(i)

(j)

(k)

Fig. 8.16 (continued)

340

8 Vaccum Tapchangers

Table 8.5 Interruption duties of diverter switch with one interrupter. Ref Fig. 8.16 Direction

Figure references

Interprupter

Int current

Recovery voltage

NR of operation

Tap 1 to tap 2

8.16e

Vm

I

RI

N

Tap 2 to tap 1

8.16j

Vm

E/R

E

N

Fig. 8.17 Vaccum diverter for refurbishment (OLG)

regarding such replacements. OLG have kindly permitted the author to schematically present their version. This is shown in Fig. 8.17. All the mechanical contacts of the diverter are removed, along with their drive linkages. The space created is used to fit four interrupters, which together perform the sequential flag cycle operations of the old technology. They are driven by a rotating cam from the central shaft, which is driven by a stored energy device.

8.12 Switching Sequence in OLG Tapchanger T01 Figure 8.18 walks through the operational sequence of the OLG tapchanger T 01. In

8.12 Switching Sequence in OLG Tapchanger T01

341

(a)

(b)

(c)

(d)

(e)

(j)

(i)

(h)

(g)

(f)

Fig. 8.18 Operating sequence of tapchanger OLG T01

the top row, proceeding from tap 1 to 2, the contact S always breaks first, and this happens with no current breaking. There is one arcing situation in Fig. 8.15c, with arc the contact T. The sequence of the top ends at tap 2, with the resistance contact to the right of the main contact. The second row starts from tap 2 towards tap 1. If the reader would suspend his credulity for a while, it is the S contact that leaves the fixed contact 2 first. This is again without arc. The rest of the sequence is ordinary, except that at the end at tap 1, the S contact is again at the right of the T contact. Such a switching sequence spares any vacuum interrupter in series with S, as it never breaks current. Only T needs a vacuum interrupter in series.

8.12.1 Realization of the Switching Scheme of Fig. 8.18 At first there appears to be sleight of hand. How can the order of the contact movement be reversed! This cannot be done if the two contacts are mounted on the same shaft. On the other hand consider a drive similar to the tap selector drive of any diverter switch type tapchanger. A similar drive, with two shafts, each driven by a Geneva mechanism will achieve the requirement. The resistance contact always moves first and breaks with the fixed contact. Figure 8.19a shows in principle the contact arrangement and drive of OLG’s tapchanger type T01. The inner shaft carries the transition contact, and the outer carries the main contact. Each shaft is driven by its own Geneva. The fixed contacts are distributed in two layers on the periphery of a fixed contact cylinder. The Geneva’s and their operating cams can be so manipulated that in each direction, the inner shaft carrying the transition contact moves first. A peripheral slot cut in the outer

342

8 Vaccum Tapchangers

(a)

(c)

(b)

(d)

Fig. 8.19 OLG tapchanger drive scheme

shaft allows the inner contact to rotate to the required extent without interference. In Fig. 8.19b the transition contact has broken with the fixed contact at no current. In Fig. 8.19c the transition contact reaches its settling position on the next fixed contact. The vacuum switch in series with the main contact is now switched off. Figure 8.19d shows the main contact on its way to the next fixed, having broken with the first at no current. This manipulation achieves the operational sequence of Fig. 8.18. OLG T 01 uses this to realize a tapchanger with only one vacuum interrupter. The MR Ecotap VPD probably follows this principle, but published confirmation is not available. The tapchanger uses only one interrupter per phase, and no mechanical transfer switches. It follows a modified form of the asymmetrical pennant cycle.

8.13 Conclusions on Vacuum Technology for Tapchangers 1.

The vacuum technology removes arcing directly in oil. This greatly reduces maintenance requirements in service.

8.13 Conclusions on Vacuum Technology for Tapchangers

343

2.

Contact erosion in vacuum interrupters is very low as compared to mechanical contacts arcing in oil. This vastly increases contact life. However, tapchangers with oil technology also claim impressive contact life of nearly 500 000 operations (Ref. [17] ABB, Figs. 23, 24) and (Ref. [18] MR Fig. 12). In the light of this, further increased contact life seems an academic issue. 3. The much superior arc interrupting capacities make interruption duties in tapchanger application easier to manage. 4. Vacuum interrupters for tapchanger application are a little different to those for breaker application. Tapchanger manufacturers manage this issue in the light of their experience and perception. Small volume of requirement makes it difficult to source interrupters of such special design. 5. A transition resistance is required. This automatically means high-speed drive through stored energy device. Thus mechanical challenges of oil design remain. 6. No authoritative information has been published to show that a vacuum tapchanger will revert to satisfactory oil technology, in the event of failure of the interrupter. 7. Contact throw off forces under short circuit can be a dangerous threat. Several options such as direct increase of contact spring force, magnetic force generator, and shunt contacts are available. 8. When using a shunt contact, light discolouration of oil after many operations can be expected due to light sparking at shunt contacts. This would not be at all noticeable in oil technology, in which oil degradation products are plenty. In vacuum technology oil discolouration may arouse suspicion. 9. Transition resistances run at high temperatures for a short time, during tapchange. This also contributes to oil degradation and gas production. 10. The concept of using interrupters leads to increased complexity of timing. A failure of timing, due to whatever issue, may lead to catastrophic failure. 11. A failure of the interrupter to break current, by loss of vacuum is a serious issue, which is likely to lead to a catastrophic failure. There is no equivalent mode of failure in oil technology.

8.14 Thyristor-Assisted Tapchanging Even though this topic does not fully relate to vacuum tapchangers, the use of thyristors in place of vacuum interrupters is a similar application, of a superior switching element. We shall not go into the details of the thyristor, except to admit that this electronic switch can interrupt within half a cycle of current. The application to tapchangers is fraught with the following issues and considerations. See Ref. [19] for details of thyristors. 1.

Thyristors have very little heat storage capacity. The crucial junction can heat up within a few milliseconds beyond allowable temperature rise. For practical

344

8 Vaccum Tapchangers

purposes, it is difficult to provide for withstanding transformer short-circuit current. 2. This problem has to be overcome by providing shunt contacts. The issues with shunt contact sparking have been mentioned in the previous section. 3. Thyristors conduct in one direction only. To use a thyristor valve in AC each thyristor must be a back to back connected pair to enable current flow in each direction. 4. Thyristors need gate pulses from low power electronics to trigger them into a conducting mode. Carrying such low power pulses to a thyristor connected into a high-voltage transformer winding is a problem. In this context it may be recalled that even a neutral end tapchanger can rise to several kV relative to earth during the ACSD test. 5. Thyristors connected to different high voltages, e.g. three single-pole line end units, will need corresponding insulation for the low power electronics, and gate pulse leads. 6. Thyristors typically have 0.8–1 V forward conduction voltage drop. The heat generated is considerable. This combined with the low thermal mass of the junction calls for large heat sinks. The heat sinks also need insulation for the corresponding voltage. 7. Even though thyristors are small, the heat sinks and snubbers (see below) make up a large volume. Thyristor tapchangers are impressively large. It is difficult to resist quoting Mark Twain who wrote “Thunder is good, thunder is impressive. But it is the lightning that does the work”. 8. Thyristors need protection against both very rapid rise of reverse voltage, and forward current. Normally this is achieved with the so-called Snubber circuits connected across the device. These typically consist of electronic sized resistances and capacitors. In a tapchanger application, the voltage across the snubbers in the nonconducting mode can be from 1 to 4 kV. The snubbers need high-voltage elements and are under considerable duress. They are no longer small electronic devices. When shunt contacts are used to eliminate the thyristor from the circuit except while changing taps, snubbers are not a problem. 9. Theoretically, it is not necessary to tapchange step by step with thyristors. If thyristor pairs are connected to all taps, it should be possible to fire any tap and skip the intermediate taps. This however is only a theoretical thought. To realize such a scheme, most thyristor valves must be rated for half to full range voltage. This would normally mean series connection of several thyristor pairs. This is accompanied by problems of gate pulse distribution and reverse voltage sharing. 10. In practical applications, thyristor tapchanging is limited to step-by-step operation, with a transition resistance. The necessary speed of transition is achieved with thyristor firing within half a cycle, relieving the system of the need for mechanical high-speed transition. A mechanical tap selector is required. OLG demonstrated a single-phase thyristor tapchanger for 400 V, 100 A in 1995. The circuit arrangement and operational sequence were substantially that of Fig. 8.1.

8.14 Thyristor-Assisted Tapchanging

345

It works! It had two pairs of back to back thyristors (i.e. four thyristors in all per phase), no snubber, two moderate-sized (200 wide × 80 thick × 600 long) natural air convection cooled extruded aluminium cooling fins, a transition resistance, a highspeed energy storage selector switch drive. OLG did not pursue this development in view of the difficulties mentioned above for scaling up to high voltage.

References 1. IEC 60 214 -1.2014-05. Tap-changers 2. Sorensen RW, Mendenhall HE (1926) Vacuum switching experiments at California Institute of Technology, Trans. AIEE, 45:1102–1105 3. Falkingham L (2016) Tutorial: Leslie Falkingham: IEE Switchgear Committee, Pittsburg, Autumn, Vacuum Interrupters Limited. [email protected] 4. Slade PG (2008) The vacuum interrupter: theory, design and application. CRC Press 5. Vacuum interrupters, Tech Brochure, ABB 6. Eaton Vacuum Interrupter (EVI) Technology Product Guide 7. Vacuum Switchgear (1994) Book: Allan Greenwood, IEE Power Series 10 8. Circuit interruption in high vacuum (1959) Thesis, University of London. M.P.Reece 9. Vacuum switching II, extinction of A C vacuum arc at current zero. M.P.Reece. ERA Report G/XTI 67 1959 10. IEEE switchgear committee—Autumn (2016) Pittsburgh. Presentation by VIL (Dr. Falkingham) p 5, Paschen Curve 11. Smeets RPP, Lathouwers AGA (2002) Non-sustained disruptive discharges: test experiences, standardization status and network consequences. Published in: IEEE Trans Dielectr Electr Insul 9(2) 12. Schlaug M, Falkingham LT (2000) Non sustained disruptive discharges (NSDD)-new investigation method leading to increased understanding of this phenomenon. In: Proceedings. ISDEIV XIXth international symposium published in: discharges and electrical insulation in vacuum 13. Smeets RPP, Lathouwers AGA, Falkingham LT, Montillet GF (2005) A summary of nonsustained disruptive discharges (NSDD) in Vacuum Switchgear. In: IEEE conference power engineering Society general meeting 14. Toshiba Brochure Gas Insulated On-load Tap Changers, GV series type D for gas-insulated transformer. 1 resister 2 vacuum bottle system 15. Ainetter J, Brauaner G, Hauer H, Strof T, Kalininchenko A (1999) Thyristor Assisted Diverter switch (TADS). A progressive concept for the prolongation of maintenance-free intervals of on-load tap-changers of transformers. www.cired.net/publications/cired1999/papers/1/1_1.pdf 16. Stroff T et al TADS for thyristor aided making and breaking. European Patent EP 0641482 (ROMAT) 17. On-load tap-changer type UC Technical data. ABB 1ZC000562-AAW en Rev 2 18. On-load tap-changer OILTAP M. Technocal Data TD 050/030. MR Tech Brochure 19. MR Transform Campus. Design, Function and Operation of On-Load Tap-Changers: Uwe Seltsam. Chapter 2, Vacutap type VR. 07.06.2013. www.reinhausen.com 20. HWV External Resistance Vacuum Type On Load Tap Changer. Shanghai Huaming Power Equipment Co Ltd. 2008. www.huaming.com 21. MR Technical Brochure MR Vacutap® AVT: On-Load Tap-Changer for regulating transformers. www.reinhausen.com

Chapter 9

Reactor Tapchangers

If you really understand mutual reactance, you are ready for a University Degree. Prof. P.W. Dharap, VJTI, Bombay University, 1958, to Electrical Engineering sophomores.

9.1 Reactor as a Current Limiting Element It was seen in Chap. 1 that a current limiting element is required to limit the circulating current during bridging of two taps in a transition. This element could well be a reactor than a resistor. Tapchangers using a reactor are quite common in the USA. We shall see in this chapter the advantages and disadvantages, as well as the construction and operation of reactor type tapchangers.

9.2 Introducing the Preventive Autotransformer (PAT) The PAT is defined in Cl. 3.12 of IEC 60 214 Part 1 2014–05 [1]. It is the current limiting device, in place of the familiar resistance in resistance tapchangers. The current limiting reactor could well take the same form as the transition resistance, i.e. a two terminal element. But in tapchanger application it takes on a different avtar. The reactor used in tapchanger application is centre tapped, which offers the great advantage of doubling the number of output voltages per tap, as will be seen shortly. In such an execution it is called a preventive autotransformer, PAT. The construction of the basic PAT is shown in Fig. 9.1. The figure shows two equal coils wound on one core (Fig. 9.1a). The coil arrangement and interconnections ensure that the two halves of the PAT occupy exactly the same positions relative to the core, so that their reactances are equal. The method of coil placement on the core ensures practically zero leakage reactance between the two halves. Other executions of coils and cores may achieve the same properties, namely © Springer Nature Singapore Pte Ltd. 2020 T. V. Sridhar, Application of Tap changers to Transformers, Power Systems, https://doi.org/10.1007/978-981-15-3955-8_9

347

348

9 Reactor Tapchangers

(a)

(b)

(c)

Fig. 9.1 Construction principle of PAT

1. The two halves of the reactor are identical. 2. There is practically zero leakage reactance between the two halves. But various other methods of achieving the result lie in the field of transformer and reactor design and are outside the scope of this book. Figure 9.1a shows a prominent feature resulting from the properties 1 and 2. When equal currents flow in opposite directions from the mid-point, as shown in Fig. 9.1b, the core is not magnetized. There is no voltage induced in either half of the reactor. If the current flows from end to end as in Fig. 9.1c, the core is magnetized, and there is an equal voltage drop in each half. The mid-point reaches half the voltage applied to the ends. The next section discusses how this property is used in a tapchanger to derive an extra tap position.

9.3 Circuit Arrangement and Switching Sequence in Reactor …

349

9.3 Circuit Arrangement and Switching Sequence in Reactor Tapchanger with Vacuum Technology [2] Figure 9.2 shows the circuit arrangement of reactor tapchanger with vacuum technology. There are two mechanical moving contacts M 1 and M 2 . They move one at time and in alternate tapchanges. The switch geometry is such that they can occupy the same fixed contact or span adjacent fixed contacts. The vacuum interrupter V connected to the mechanical contacts M 1 and M 2 is bypassed by the mechanical contacts R and S in the operating condition. J is the output terminal. There are two normal operating positions, shown in Fig. 9.2a, b. In Fig. 9.2a both the selector contacts sit on the same fixed contact. In Fig. 9.2b they bridge adjacent fixed contacts.

9.3.1 Output Voltages The output voltage in Fig. 9.2a corresponds to the voltage of tap A. This is because the two equal currents flowing through the two halves of the PAT fail to magnetize the core and induce any voltage. In Fig. 9.2b the output voltage corresponds to the mean between taps A and B. This needs a little explanation. It looks as if two different currents flow in the two halves of the PAT and as they are similar, the voltage drops cannot be the same. It should be remembered that the two halves of the PAT are

(a)

(b)

Fig. 9.2 Operating positions

350

9 Reactor Tapchangers

closely coupled so that the voltage in each half consists of two components, one due to its own reactance and the second due to the mutual reactance of other half. The two halves therefore have the same voltage induced in them. A simpler way of visualizing the situation is that the two halves of the main current flowing in opposite directions through the PAT do not produce any flux, and therefore no voltage drops due to the main current. The two halves of the PAT act like a potential divider for the step voltage. The output voltage in the bridged position is thus the mean of the two ends irrespective of the through current and its phase. Therefore each tap gives two output voltages. This can be used to generate fine tap voltages or alternatively reduce the number of tappings. We shall see later that by using a reversing tapping arrangement, the number of different voltages obtained out of the same winding can be still doubled. The PAT is designed to carry the currents in the bridged condition forever, without temperature problems. In fact operation at the bridged condition is a legitimate position. This is not a great fear, as reactors exhibit resistance, and heating only as a “by the way” departure from the ideal. Any heat produced is minor. This removes the ever present dread of resistance tapchangers of contact drive failure with the transition resistance still in circuit. In the reactor tapchanger, the drive need not include a stored energy device. The motor can directly drive the contacts slowly. If the external drive fails at any point, no problem! Figure 9.3 shows the operating sequence of the tapchanger.

9.3.2 Interruption Duties Figure 9.4 shows two forward and two reverse tapchanges starting from one end. This w-excursion covers all the interruption duties that occur. As usual we have to derive the current interrupted by a contact from the current it carries just before interruption and the recovery voltage from the circuit conditions just after arc quenching. Figure 9.4 presents this information. Figure 9.5 shows the vector diagram of interruption and will be useful in deriving the mathematical expressions. In Fig. 9.4a V carries a current of I/2, so that the interrupted current. Iint = I /2

(9.1)

After arc quench, Fig. 9.4c, the current I flows through the left half of the PAT. This has half the number of the total turns so that the reactive impedance is Z/4. The voltage drop is Z/4 I. But the core flux established by the current flow induces an equal voltage in the other half of the PAT even though it does not carry current. The recovery voltage round the loop appearing across the open contact of V is Vr =

ZI 2

(9.2)

9.3 Circuit Arrangement and Switching Sequence in Reactor …

(a)

(b)

(d)

(e)

(g)

351

(c)

(f)

(h)

Fig. 9.3 a–h Page 1: Operating sequence. i–p Page 2: Operating sequence

352

9 Reactor Tapchangers

(i)

(j)

(k)

(m)

(l)

(o)

Fig. 9.3 (continued)

(p)

(n)

9.3 Circuit Arrangement and Switching Sequence in Reactor …

(a)

(d)

(b)

353

(c)

(f)

(e)

Fig. 9.4 a–f Page 1: Interruption duties of reactor tapchanger. g–l Page 2: Interruption duties of reactor tapchanger. m–r Page 3: Interruption duties of reactor tapchanger. s–x Page 4: Interruption duties of reactor tapchanger

V once again does the same duty when the direction of tapchange is reversed from tap 2 to mid-tap (Fig. 9.4o). Thus in four tapchanges, this duty is performed twice. In N tapchanges in total the frequency of this duty is N/2. Table 9.1 shows this in the last column. V does an interruption duty in the next tapchange in the same direction. In Fig. 9.4i, just before it opens, V carries a current of I/2 + I c = I/2 + E/Z. This is interrupted in Fig. 9.4i; thus the interrupted current is Iint =

I E + Z 2

(9.3)

After arc quenching, the load current flows through the right half of the PAT (Fig. 9.4f). This causes a voltage of ZI/4 in both halves of the PAT as discussed above. The voltage round the loop, appearing across the open contacts of V is the recovery voltage

354

9 Reactor Tapchangers

(g)

(h)

(i)

(l) (j)

(k)

Fig. 9.4 (continued)

Vr = E +

ZI 2

(9.4)

In the reverse direction of tapchange, in Fig. 9.4u the interrupted current is =

I E − Z 2

(9.5)

ZI 2

(9.6)

The recovery voltage =E−

9.3 Circuit Arrangement and Switching Sequence in Reactor …

(m)

(p)

(n)

355

(o)

(r)

(q)

Fig. 9.4 (continued)

Looking at Eqs. 9.3 and 9.5 we can say the former represents a “heavy switching” and the latter a “light switching”. This is graphically shown in Fig. 9.5. Table 9.1 summarizes the interruption duties. It must be recalled that all expressions for interrupted current and recovery voltage are vectors.

9.3.3 Algebraic Expressions for Interruption Duties In the following, it has been taken that the impedance of the PAT is a pure reactance. The non-bridging interruption is not dependent on the load power factor. The interrupted current =

I 2

(9.7)

356

9 Reactor Tapchangers

(s)

(t)

(v)

(w)

(u)

(x)

Fig. 9.4 (continued)

At a recovery voltage =

ZI 2

(9.8)

For interruption from bridging condition, the interrupted current for heavy-duty switching  = At a recovery voltage

I cos ø 2

2

 +

2 I E + sin ø Z 2

(9.9)

9.3 Circuit Arrangement and Switching Sequence in Reactor …

357

Fig. 9.5 Vector diagram interruption

Table 9.1 Interruption duties of vacuum reactor tapchanger. Ref. Fig. 9.4 Direction

See Fig.

Raise

Heavy duty

Contact

Interrupted current

Recovery voltage

Nr of operations

V

I/2

ZI/2

N/2

E/Z + I/2

E + ZI/2

N/4

I/2

ZI/2

N/4

E/Z − I/2

E − ZI/2

N/4

9.4b 9.4i Lower

Light duty 9.4o 9.4u

 =

2  2 ZI ZI cos ø + E + sin ø 2 2

(9.10)

For the light switching duty, the interrupted current  =

I cos ø 2

2

 +

2 I E − sin ø Z 2

(9.11)

At a recovery voltage  =

2  2 ZI ZI cos ø + E − sin ø 2 2

(9.12)

358

9 Reactor Tapchangers

9.3.4 Phase Relationship of the Interrupted Current and Recovery Voltage In each case it will be observed that the recovery voltage is Z times the interrupted current. The interrupted current and the recovery voltage are at quadrature. At current zero, when the arc can quench, the instantaneous recovery voltage is at its maximum. This may lead to several restrikes before the arc quenches. The adverse phase relationship is a feature of all reactor tapchangers discussed in further tapchanger executions below. This is why the earlier reactor tapchangers with mechanical arcing contacts had issues of contact wear and oil contamination. Without the very high interrupting capacity of the vacuum device this is a difficult switching condition. The vacuum interrupter however manages to rupture the current mostly at the first current zero.

9.3.5 Maximum Interruption Duty The maximum magnitude of the interruption duty occurs for heavy-duty interruption at a power factor of 0 (cos ø = 0). See Fig. 9.5b. The maximum interrupted current =

I E + Z 2

(9.13)

IZ 2

(9.14)

At a recovery voltage =E+

9.3.6 Influence of the Direction of Power Flow If the direction of power through the transformer is reversed, this is equivalent to replacing the vector I by -I in all the expressions in Table 9.1. The interruption duty at the non-bridging position remains unaffected. The light and heavy duties of the interrupter are interchanged between the two directions. Therefore for the tapchanger, the reversal of power does not make a difference, as far as interruption duties are concerned. The tapchanger is fully bidirectional.

9.3 Circuit Arrangement and Switching Sequence in Reactor …

359

9.3.7 Features of Reactor Tapchanger with Vacuum Interrupter The features of the reactor tapchanger with vacuum technology exhibit the following features. 1. There is always a path for the through current. It is never interrupted. 2. The tap is never short-circuited without an effective reactance to limit the circulating current. 3. The mechanical contacts never break or make current. Indeed current is broken off and later switched on when the contacts are stationary. Therefore there is no chance of even minor arcing due to contact bounces. 4. All current making and breaking are done in the vacuum interrupter. This ensures total absence of current interruption and arcing at the mechanical contacts. 5. In the operating position, the interrupter is bypassed. It is not exposed to transformer short circuit currents. 6. There is no oil contamination, or contact erosion, so that long and maintenance free life is ensured. 7. The vacuum interrupter contacts have high interruption capacity, low contact wear, and assured long life. 8. There is no need for an energy storage drive. If the tapchanger gets stuck between taps, it will not be damaged. Yet it is necessary to make a provision that the interrupter does not stall open with a small gap, which is insufficient to interrupt. This may happen if the external drive fails just at an unlucky juncture. Besides, for optimum interruption capability, the interrupter contacts must achieve a parting velocity specified by the interrupter manufacturer. As is well known, vacuum interrupter contacts close automatically, due to the external atmospheric pressure. The contacts must be held open against this tendency, by external means, which is most generally a “hold open” spring. The snap opening of the interrupter is achieved in the OLG tapchanger T 05 by a stratagem shown in Fig. 9.6. The motor-driven cam profile is made in such a way that the profile supports the opening load, preventing the opening process, till at the correct position the profile is cut off, enabling quick release. This achieves a snap opening. By appropriate design of the opening force, the specified contact parting speed is ensured. The interrupter drive is the crux of the design, and therefore manufacturers are jealous of their intellectual property right. Thus even Fig. 9.6 does not show some intimate details of how the snap action is actually achieved. The snap opening construction ensures that the opening spring is enabled to complete its job, so that the interrupter opening, once commenced, is independent of themotor. Fig. 9.7 shows the live part assembly of OLG T 05.

360

9 Reactor Tapchangers

(a)

(b) Fig. 9.6 Schematic of snap opening of interrupter (OLG)

INTERRUPTER

CONTACT CYLINDER CONNECTING LEAD

DIVERTER GEAR PRE-SELECTOR DRIVE FRONT ASSEMBLY PLATE

Fig. 9.7 Live part assembly ready for tanking OLG T 05

9.4 Some Constructional Features of Reactor Tapchangers with Vacuum Interrupter The main functional subassemblies of the reactor tapchanger with vacuum interrupter are: 1. The tap selector.

9.4 Some Constructional Features of Reactor Tapchangers …

2. 3. 4. 5.

361

The bypass switch. The vacuum interrupter with its drive. PAT. The driving mechanism.

Tapchangers based on vacuum technology have been built in the USA for a very long time. There are a variety of implementations of tap selectors. For illustrative purpose we shall consider the construction of MR type RMV-II [3] and OLG T 05. Both are compartment type executions. Reference [3] Sect. 2.3.2 shows mechanical details of MR tap selectors executed on the terminal barrier board and on a structure standing on the tank floor.

9.4.1 The Tap Selector Tap selector can be built on a flat phase board with fixed contacts positioned along a circle. The MR tapchanger type RMV-II [3] and ABB tapchanger VRLTC [4] have a tap selector which is built directly on the terminal barrier board separating the transformer and tapchanger oil systems. This has a great advantage for high current tapchangers of 1600 and 2500 A rating in eliminating high current bus work. Many reactor tapchangers are high current, as the practice in the USA is to tap the LV. The selector fixed contacts are embedded in the epoxy casting and penetrate the terminal board and enter the transformer oil space. The OLG T 05 on the other hand has a tap selector executed on a horizontal insulated cylinder aligned with the axis of the tank. Fixed contacts are mounted on the inner periphery of the cylinder. The moving contact system is supported from end flanges of the cylinder. The tap selector is thus almost free of the tank and terminal board and maintains alignment in the face of distortions. This construction avoids misalignment of the selector drive, in case of minor distortions of the flat terminal board due to temperature, errors in flatness of the transformer port flange, or vacuuming the transformer tank. Further the selector contacts are accessible for repairs or replacement, whereas contacts mounted on the terminal board cannot be easily accessed. As against this it becomes necessary to interconnect the selector terminals to the terminal board with heavy copper connections. The T 05 for 1 600 A rating has almost 500 kg of contacts and copper bus work!

9.4.1.1

Tap Selector Moving Contacts

To cater for the high current, tap selector moving contacts are often built with multiple individually sprung finger contacts in parallel (Fig. 9.8). The design must take care of heating by resistance dissipation when both moving contacts are on the same fixed contact. When operating in the bridged condition, one moving contact carries a current of I/2 + Ic and the other I/2 − Ic. If the PAT impedance is such as to restrict

362 Fig. 9.8 Multi finger contact of OLG T 05 tap selector

9 Reactor Tapchangers

CONTACT CYLINDER MULTI FINGER SELECTOR MOVING CONTACT SLIP RING DRIVE SHAFT SELECTOR FIXED CONTACT

the circulating current to half the load current, the total current through the fixed contact can be 90% of the full current, at a load power factor of 0.8.

9.5 Bypass Switch Figure 9.9 shows a typical bypass switch (OLG). As with the tap selector, there are a number of individually sprung finger contacts. The bypass switch does not break current, but as it opens on load, the current must commutate to the interrupter. This could cause a minor sparking due to the nonzero resistance of the interrupter circuit. In order that this does not mark the finger contacts, one of them is usually made of arc resistant material, such as tungsten–copper. This contact is arranged mechanically to open last, so that any sparking takes place over the arcing contact. After many thousands of operation, there could be some discolouration of oil. This would be noticeable as otherwise there is no scope for oil blackening in a reactor tapchanger with interrupter. A small amount of gas is generated which should migrate, so that no serious issues arise in practice over DGA. It is advisable that during routine inspection, the arcing contact finger is examined for unusual wear. If necessary the arcing tip must be replaced. Fig. 9.9 Bypass switch OLG T 05

INTERRUPTER BYPASS SWITCH SUPPORT ASSEMBLY BYPASS SWITCH DRIVE BYPASS SWITCH SHAFT

9.6 Vacuum Interrupter Arrangement

363

9.6 Vacuum Interrupter Arrangement The interrupter is opened after the bypass pole opens by a cam driven by the motor drive. There may not be a stored energy drive in the conventional sense. But as discussed, a device is incorporated in the bottle opening drive to ensure that it does not stall with very small contact separations. It is necessary to provide that if there is a failure to close because of loss of vacuum, the interrupter should still close fully. A defective closing of the interrupter leaves current flowing in the tap selector moving contact and will cause heavy arcing as it tries to break. That would be catastrophic. For this reason a closing spring is also provided on the interrupter drive. This of course increases the load on the opening spring.

9.7 Parameters of the PAT The PAT is not generally supplied along with the tapchanger by the tapchanger manufacturer. It is designed and built into the main transformer tank by the transformer manufacturer. The PAT must have sufficient reactance to limit the circulating current to an acceptable value in the bridging condition. The nearest guide to the value of the reactance of the PAT comes from Cl. B.1.1 and B.1.2 of IEC 60 214-Part 1 2014–05 [1]. The suggestion is that the circulating current in the bridging condition could be 50% of the rated current. This is however not mandatory. According to the IEC service duty test is carried out with a load current power factor of 0.8, while the breaking capacity test at 0 p.f. The current rating of the PAT can be determined from vector diagram Fig. 9.10. We assume full load at power factor 0.8 and a circulating current 0.5 I, The more highly loaded half carries a current of I + Ic = I (0.4 − j0.3) − j0.5 = I (0.4 − j0.8) 2

(9.15)

The RMS value is 0.894 I. If it is likely that the transformer will operate at full/frequent overload and lower power factors for extended periods, it would be safe to design the thermal rating of the PAT for the higher loading, using the equation Imax = I [(cos ø − j sin ø) − j0.5]

(9.16)

The worst loading occurs at zero power factor. Figure 9.10b shows the PAT loading at zero power factor. It is understandable that the Standards specify zero power factor for breaking capacity test, as it causes very high current interruption. Figure 9.10c represents the breaking test, when the current is doubled. When the transformer suffers a short circuit, both halves of the PAT carry substantially half the current, ignoring the circulating current in the bridged condition. This determines the short time rating of the PAT.

364

9 Reactor Tapchangers

(a)

(b)

(c)

Fig. 9.10 Vector diagram for PAT current loading

9.8 Protection Against the Tap Selector Opening with Arcing Tap selector contact opens after the opening of the interrupter. If the interrupter opens correctly, there is no current through the tap selector moving contact (See Fig. 9.3d for instance). If the bottle is still closed, or otherwise incapacitated to interrupt the current (e.g. bottle vacuum failure), the tap selector ends up breaking the current. The tap selector is not designed to break current, and there will be a catastrophic failure. Fortunately the slow action of the directly motor-driven tapchanger allows for a remedy. It is possible to check if the interrupter is still carrying current when it is expected to be open. This can be done by CTS mounted in series with the interrupter

9.8 Protection Against the Tap Selector Opening with Arcing

365

feeding a detection circuit or directly by current sensors on the interrupter bus. If a current is detected through the interrupter, the motor is cut off by the control, reversed, and brought back to the starting point, before the tap selector actually breaks. There is time enough for this corrective measure. The tapchanger is locked out from further operations, and a visual indication warns operators not to perform any tapchanges till the event is investigated, and matters set right. Most modern reactor tapchangers (G.E. LRT 200, MR RMV II [3], ABB [4], OLG T05) incorporate this feature.

9.8.1 Tapchanger Not to Be Manually Operated When the Transformer Is on Load The safety feature against the tap selector breaking current can be implemented only when the tapchanger is driven by the motor, with the full controls in position. For this reason, it is advisable that the tapchanger should not be manually operated when the transformer is on load.

9.9 Connection Diagram with Reverser Figure 9.11 shows the connection diagram for 33 position regulation, with reverser. It will be noted that a tapping winding which provides only eight taps is required. Of course it is possible to use the connection scheme for a different number of different voltages or without the reverser, with appropriate changes in the winding arrangement.

9.10 Reactor Tapchanger with Arcing Diverter Switch Table B.3 of IEC 60 214 describes a reactor tapchanger without vacuum interrupter. Instead an arcing diverter switch is used in combination with a non-arcing tap selector. This mode of operation would have no doubt been used when vacuum technology was yet to be applied to tapchangers. Reference [5] cites such tapchangers were made at one time or the other in the USA by all major manufacturers, GE, Westinghouse, Siemens-Allis, McGraw Edison. Figure 9.12 shows the connections of such a tapchanger and operational sequence, based on Fig. B.5 of the Standard IEC 60 214 is shown in Fig. 9.12 four operations, two in the forward, and two in the reverse are shown. These cover all the interruptions that occur.

366

9 Reactor Tapchangers

(a)

(b) Fig. 9.11 Connection diagram

9.10.1 Interruption Duties of Arcing Diverter Switch The interruption duties of the diverter with arcing switch are shown in Fig. 9.13. Figure 9.13 is essentially an extract from Fig. 9.12 showing stages at which an interruption occurs. We also need the conditions just before interruption to determine what current will be interrupted next. The recovery voltage can be determined after arc quenches. Figure 9.13 shows this condition also. The diverter switch pole H

9.10 Reactor Tapchanger with Arcing Diverter Switch

367

(b)

(a)

(c)

(e)

(d)

(f)

Fig. 9.12 a–f Page 1: Operational sequence of arcing diverter switch. g–l Page 2: Operational sequence of arcing diverter switch. m–r Page 3: Operational sequence of arcing diverter switch. s–x Page 4: Operational sequence of arcing diverter switch

368

9 Reactor Tapchangers

(g)

(h)

(i)

(k)

Fig. 9.12 (continued)

(j)

(l)

9.10 Reactor Tapchanger with Arcing Diverter Switch

369

(q)

(r)

(o)

(p)

(n)

Fig. 9.12 (continued)

(m)

370

9 Reactor Tapchangers

(x)

(w)

(u)

(v)

(t)

Fig. 9.12 (continued)

(s)

9.10 Reactor Tapchanger with Arcing Diverter Switch

(a)

(d)

371

(b)

(c)

(e)

(f)

Fig. 9.13 a–f Page 1: Interruption duties for arcing diverter switch. g–l Page 2: Interruption duties for arcing diverter switch

372

(l)

(i)

Fig. 9.13 (continued)

9 Reactor Tapchangers

(k)

(h)

(j)

(g)

9.10 Reactor Tapchanger with Arcing Diverter Switch

373

Table 9.2 Interruption duties of tapchanger with arcing diverter. See Fig. 9.12 See Fig. Non-bridging position

By contact

Direction

Interrupted current

Recovery voltage

Nr of operations

H

Don’t care

I/2

IZ/2

N/4

G

Don’t care

I/2

IZ/2

N/4

G

Light duty

1/2 (I − E/Z)

1/2 (IZ − E)

N/4

H

Heavy duty

1/2 (I + E/Z)

1/2 (IZ + E)

N/4

9.12b 9.12m

Bridging position

9.12g 9.12r

interrupts a current of I/2 in Fig. 9.13b. The current transfers in Fig. 9.12c to the left half of the PAT. We recall that the reactance of a coil is proportional to the square number of turns. If Z is the total reactance of the PAT with all turns in series, the voltage across the current-carrying half is IZ/4, as there are only half the total turns. The mutual flux induces the same voltage across the non-current-carrying half also. These voltages with relative polarity are marked up on the PAT Fig. 9.3c. Thus the recovery voltage is 2.I.Z/4 = IZ/2. In the four tapchanges shown in Fig. 9.12, H does this interruption only once. Therefore in N operations, the nr of interruptions is N/4. This is shown in Table 9.2 which extracts the interruption duties of the arcing diverter switch. The recovery voltage is in quadrature to the interrupted current. In Fig. 9.13e the diverter switch pole G interrupts a current of I/2–Ic = I/2 − E/Z. After interruption, the full current flows through right hand half of the PAT in Fig. 9.13f. This causes a voltage drop across both halves together of IZ/2 as discussed above. The net voltage in the loop appearing across the open contact G, i.e. the recovery voltage, is E − IZ/2. The recovery voltage is at quadrature to the interrupted current. This constitutes the “light switching direction”, because it will be seen shortly that the interruption duties are higher in the reverse direction. In the reverse direction, in Fig. 9.13h the diverter switch pole G interrupts current of I/2 at a recovery voltage of IZ/2. The pole H breaks current at Fig. 9.13k. The interrupted current is I/2 + Ic = I/2 + E/Z. The recovery voltage is E + ZI/2. This is the “heavy switching direction”. Table 9.2 summarizes the interruption duties. In the two forward and two reverse tapchanges analysed, each contact of the diverter switch performs an interruption of I/2 once. In N tapchanges, each will perform this duty N/4 times. The contact G performs the “light switching” function of I/2 − E/Z once in one direction. It does not perform any other bridging interruption in the two forward and two reverse tapchanges discussed. The contact H performs the heavy-duty bridging interruption once in these four tapchanges. The light and heavy bridging interruptions are each therefore carried out N/4 times in N tapchange operations. Table 9.2 shows the frequency of interruptions.

374

9 Reactor Tapchangers

9.10.2 Reverse Power Flow If the direction of power through the transformer is reversed, this is equivalent to replacing the vector I by –I in all the expressions in Table 9.2. The duties of the diverter contacts H and G are merely interchanged. Therefore for the tapchanger, the reversal of power does not make a difference, as far as interruption duties are concerned. The tapchanger is fully bidirectional.

9.11 Use of Equalizer Windings In the switching sequence described, when both moving contacts are on the same fixed contact, there is no circulating current. In the bridging condition, both diverter contacts contain a component due to the circulating current. This situation can be rationalized by the introduction of an equalizer winding into the contact loop, as shown in Fig. 9.14. Figure 9.14 is based on Fig. B.3. of IEC 60 214. The equalizer is a centre tapped winding, wound on the main core, and has only half the turns of the tapped coil. It is connected into the loop in such a manner that its voltage opposes the step voltage (Fig.9.14a). The induced voltage across the full equalizer is half the step voltage E/2. Across each half of the equalizer, there is an induced voltage of E/4. The equalizer reduces the circulating current component to half as compared to the bridging position but introduces that current when operating in the same tap. The operating sequence is shown in Fig. 9.14. Two consecutive raise and two immediate reverse tapchanges are shown. These cover all the switchings that can occur.

9.11.1 Interruption Duties of Arcing Diverter Switch with Equalizer Figure 9.15 would be helpful in arriving at the interruption duties of the arcing diverter switch with equalizer. It is an extract from Fig. 9.14, showing only the positions where there is an interruption. It is also necessary to consider the condition just before interruption to determine what current is going to be interrupted. Figure 9.15 extracts this information also. Finally the situation just after arc quench is also shown to derive the recovery voltage. In Fig. 9.15a the equalizer sets up a circulating current I c = 1/2 E/Z. The directions of connections are such that the circulating current adds vectorially to the current in diverter switch contact G and opposes the current in contact H. Therefore the current interrupted in Fig. 9.15b by the contact H   1 E −I = 2 Z

(9.17)

9.11 Use of Equalizer Windings

(a)

375

(b)

(c)

(d)

(f) (e)

Fig. 9.14 a–f Page 1: Operating sequence for arcing diverter switch with equalizer winding. g– l Page 2: Operating sequence for arcing diverter switch with equalizer winding. m–r Page 3: Operating sequence for arcing diverter switch with equalizer winding

376

9 Reactor Tapchangers

(g)

(h)

(i)

(k)

Fig. 9.14 (continued)

(j)

(l)

9.11 Use of Equalizer Windings

377

(r) (q)

(o)

(p)

(n)

Fig. 9.14 (continued)

(m)

378

9 Reactor Tapchangers

(a)

(b)

(c)

Fig. 9.15 a–c Page 1: Interruption duties for arcing diverter switch with equalizer. d–f Page 2: Interruption duties for arcing diverter switch with equalizer. g–i Page 3: Interruption duties for arcing diverter switch with equalizer. j–l Page 4: Interruption duties for arcing diverter switch with equalizer

9.11 Use of Equalizer Windings

379

(d)

(e)

(f)

Fig. 9.15 (continued)

380

9 Reactor Tapchangers

(i)

(h)

Fig. 9.15 (continued)

(g)

9.11 Use of Equalizer Windings

381

(l)

(k)

(j)

Fig. 9.15 (continued)

After interruption in Fig. 9.15b the current I flows in the left half of the PAT. The voltage across the current-carrying half is IZ/4. The voltage induced by the mutual flux in the non-current half is also IZ/4. The total voltage across the PAT is IZ/2. The voltages round the loop =

1 (E − Z I ) 2

(9.18)

382

9 Reactor Tapchangers

Table 9.3 Interruption duties of tapchanger with arcing diverter and equalizer windings. Ref. Fig. 9.14 See Fig. Non-bridging position

By contact

Direction

Interrupted current

Recovery voltage

Operations

H

Light duty

1/2 (I − E/Z)

1/2(IZ − E)

N/4

G

Heavy duty

1/2(I + E/Z)

1/2(IZ + E)

N/4

G

Light duty

1/2(I − E/Z)

1/2(IZ − E)

N/4

H

Heavy duty

1/2(I + E/Z)

1/2(IZ + E)

N/4

9.14b 9.14m

Bridging position

9.14g 9.14r

which appears cross the contact H, constituting the recovery voltage. The recovery voltage is the current interrupted times Z. Therefore it is in quadrature to the interrupted current. In the same direction of tapchange from mid-tap to tap 2 there is another interruption, this time by the diverter contact G in Fig. 9.15e. The current interrupted is I c − I/2 = ½(E/Z − I) from Fig. 9.15d. Figure 9.15f shows conditions just after arc quench. Current I now flows only the right half of the PAT. The voltage drops of the PAT are marked up in Fig. 9.15f. The recovery voltage is ½ (ZE − ZI). The recovery voltage is the interrupted current times Z. It is therefore in quadrature to the interrupted current. This direction of tapchange leads to “light-duty” interruptions. In the reverse direction in the tapchange tap 2 to mid-tap G breaks at Fig. 9.15h. The current interrupted is I c + I/2 = ½ (IE/Z + I). This is therefore the “heavy-duty” switching direction. Figure 9.15i shows the voltages of the PAT just after arc quench. The recovery voltage is ½ (E + ZI). The recovery voltage is the interrupted current times Z. It is therefore in quadrature to the interrupted current. Finally the contact H in the transit from mid-tap to tap 1 interrupts a current of I c + I/2 = ½ (E/Z + I) in Fig. 9.15k. The recovery voltage marked up in Fig. 9.15l just after arc quench is ½ (E + ZI). Each contact does one light duty in one direction and one heavy duty in the other. The frequency of such interruptions for each contact is therefore N/4 in N tapchanges. Table 9.3 summarizes the interruption duties.

9.11.2 Algebraic Expressions for Interruption Duties of Arcing Diverter Switch with Equalizer The expressions shown in Table 9.3 are vector equations. When working out numerical values, the phase relationship must be considered. Considering Table 9.3 with Table 9.1 it is seen that the contacts now do only the light and heavy duties, with the term E replace by E/2. Thus Eqs. 9.7–9.12 can be used by replacing E by E/2 in the equations. The phase relationship between interrupted current and recovery voltage and the maximum values are already discussed.

9.12 Reactor Tapchanger with Selector Switch

383

9.12 Reactor Tapchanger with Selector Switch A very high number of reactor tapchangers using the selector switch concept is manufactured and used for the US market each year for voltage regulators. In terms of numbers, this could well be the largest single type of application for tapchangers in the world. This is the application of step voltage regulators covered by IEC 60 076-21 Ed 1 2011–12. At the traditional level, step voltage regulators are transformers with selector switch with PAT current limiting, without vacuum technology [5–12]. They are used in large numbers to regulate the downstream voltage to the final user. They are most often single-phase, pole-mounted types. This means the transformer is not easy to access to operate manually and for routine maintenance. Tapchangers with extremely long life, as compared to other applications, are regarded as the correct solution. They are also constructed in a very “self contained” manner including all the controls, auxiliary power supplies, protection, and indications. Interchangeability is a prime virtue. The application is so important in terms of numbers that IEEE has a separate governing Standard [6]. IEC Standard IEC 60 076 Part 21 [7] is the corresponding European International Standard. The reactor tapchanger and the PAT are built as part of the core and coil assembly in one tank. This is a case of the tapchanger hanged by its own petard, having to operate with poor oil produced by its action. From the point of view of transformer engineering, step voltage regulators present some unique features (Reference [8], Chap. 8, Craig A. Colopy).

9.12.1 Execution of Voltage Regulators The voltage regulator has been so successfully used in the USA for many years that certain features, particularly relating to interchangeability, are mandatory. The tapchanger, for instance, is always reactor type, with a reversing switch, so that an eight-step regulating winding provides for 33 different voltages. The core and coil assembly, the tapchanger, the PAT, the drive mechanism with motor, and other live parts are immersed in the same oil space. Some voltage regulators do not use a stored energy device in the switch drive, because it does not matter if the tapchange is stuck in between. Paradoxically when very short timing for the coverage of the tapping range is required, a direct motor drive is found better suited. Eaton Cooper Power Systems offer a coverage time end to end (33 positions) of only 10 s, using direct motor drive [9]. Most voltage regulators are pole mounted. For this reason they must be self contained. Operation, control, data logging, and other monitory functions are taken care by a monitoring controller, which includes automatic voltage regulating relay. There is a mechanical position indicator mounted on the tank, at an inclination to the vertical, so that the tap number is easily read from ground. This is in addition to the electronic position record at control unit. The drive is by flexible shaft from the motor gearbox within the tank.

384

9 Reactor Tapchangers

9.12.2 Tapchanger for Step Voltage Regulators In this book, we shall restrict ourselves to only the tapchanger. Figure 9.16 shows

(a)

(c)

(e)

(b)

(d)

(f)

Fig. 9.16 a–f Page 1: Operational sequence of selector switch with reactor tapchanging. g–l Page 2: Operational sequence of selector switch with reactor tapchanging

9.12 Reactor Tapchanger with Selector Switch

385

(k)

(l)

(j)

(h)

(i)

(g)

Fig. 9.16 (continued)

the connections and operational sequence. The operational sequence is based on Fig. B1 of IEC 60 214. Figure 9.17 shows the interruption duties of the contacts. Current distribution before contact parting and just after arc quenching is shown, to evaluate the interrupted current and the recovery voltage. In Fig. 9.17b the contact H interrupts a current of I/2. The load current transfers to one half of the PAT. The induced voltage in the half carrying current is Z/4I. This induces an equal voltage in the non-currentcarrying half, by mutual inductance. The recovery voltage across contacts is E −

386

9 Reactor Tapchangers

IZ/2. In the next tapchange, in the same direction, contact G interrupts a bridging condition as shown in Fig. 9.17e. In the reverse direction, contact G interrupts a current of I/2 in Fig. 9.17h. In the same reverse direction, the contact H interrupts the bridging condition corresponding to Fig. 9.17k. These constitute all the four types of interruption duties performed.

9.12.3 Interruption Duties of Selector Switch Type Tapchanger with Arcing Contacts Figure 9.17 is an extract from Fig. 9.16 to help determine the interruption duties of the selector switch with reactor. The condition just before current interruption and just after arc quench is also shown to clearly decide the interrupted current and the

(b) (a)

(c)

(e) (d)

(f)

Fig. 9.17 a–f Page 1: Interruption duties of selector switch with reactor. g–l Page 2: Interruption duties of selector switch with reactor

9.12 Reactor Tapchanger with Selector Switch

387

(k) (l)

(j)

(h) (i)

(g)

Fig. 9.17 (continued)

recovery voltage. There are four interruptions, all different. Thus each interruption is carried out N/4 times in N operations. Comparing Table 9.4 with Table 9.1 it is seen that the interruption duties are the same. Therefore for the algebraic expressions, phase relationship between interrupted current and recovery voltage, maximum duties, directionality please refer to Sect. 9.3.3.

9.12.4 Selector Switch with Equalizer Equalizer winding is already introduced. Figure 9.18 shows the connections for a selector switch with equalizer. The operational sequence is also shown in Fig. 9.18. Operational sequence and interruption duties are similar to those of selector switch

388

9 Reactor Tapchangers

Table 9.4 Interruption duties of selector switch with reactor. Ref. Fig. 9.17 See Fig. Non-bridging condition

Contact

Direction

Interrupted current

Recovery voltage

Nr of operations

H

Light duty

I/2

ZI/2

N/4

G

Heavy duty

E/Z + I/2

E + ZI/2

N/4

G

Light duty

E/Z − I/2

E − ZI/2

N/4

H

Heavy duty

I/2

ZI/2

N/4

9.17b 9.17h

Bridging condition

9.17e 9.17k

with reactor, but without equalizer (Figs. 9.16, 9.17). Figure 9.18 shows four interruptions, all different.

9.12.5 Interruption Duties of Selector Switch with Reactor and Equalizer Fig. 9.18 shows the stages of interruption, the stage just before, and after arc quench. The currents interrupted are derived from the stage just before. The mark up of voltages on the PAT after arc quench helps in arriving at the recovery voltages. There are four different interruptions in the two up and down tapchanges. These cover all the possible interruptions. Therefore each interruption is performed N/4 times in N tapchanges. Table 9.5 shows the interruption duties of the selector switch with reactor and equalizer. Comparing with Table 9.3 it is seen that the vector expressions are same in the two cases. Table 9.5 interruption duties of selector switch with reactor and equalizer. Ref. Fig. 9.18 Direction

See Fig.

Left to right

Contact

Interrupted current

Recovery voltage

Nr of operations

M2

E/2Z − I/2

E/2 − ZI/2

N/4

M1

E/2Z − I/2

E/2 − ZI/2

N/4

M1

E/2Z + I/2

E/2 + ZI/2

N/4

M2

E/2Z + I/2

E/2 + ZI/2

N/4

9.18b 9.18e Right to left 9.18i 9.18l

9.13 Regulating Transformers for Furnace Application

389

9.13 Regulating Transformers for Furnace Application The standard “off the shelf” US voltage regulator is eminently suitable for furnace duty. Three single-phase units can be used for almost up to 25 MVA. The advantages are

(b)

(a)

(e)

(d)

(c)

(f)

(g)

Fig. 9.18 a–g Page 1: Switching in selector switch with equalizer. h–n Page 2: Switching in selector switch with equalizer

390

9 Reactor Tapchangers

(m)

(n)

(k)

(j)

(l)

(i)

(h)

Fig. 9.18 (continued)

1. Practically short-circuit proof, except possibly at the time of tapchange. 2. Operates successfully under highly contaminated oil. 3. No high speed, stored energy drive required. Slow reliable direct motor drive possible. 4. It is not absolutely necessary that a tapchange once initiated must be completed. Tapchanger can work between taps indefinitely. 5. No transition resistance that could be damaged. 6. Standard 33 positions.

9.13 Regulating Transformers for Furnace Application

391

7. Availability of versions with quick tapchange [9]. 8. High contact life in standard execution. Extremely high contact life when used with vacuum technology.

References 1. IEC 60 214-1:2014-05 Tap-changers—Part 1: performance requirements and test methods 2. On-load tap-changers for power transformers. Book. Axel Krämer. MR Publication 2000. Sect. 2.2.2.3 3. MR VACUTAP® RMV-II https://www.reinhausen.com/…/vacutap_rmvii-F0325800_EN_ RMV_II_web.pdfs 4. ABB Technical Guide, Type VRLTC ™ on load tap changer 5. Waukesha SPX Training presentation (.ppt) Wahkesha Electric Systems, Energy Solutions 6. IEEE C57.15-2017—IEEE approved draft standard requirements, terminology, and Test code for step-voltage regulators. https://standards.ieee.org/findstds/standard/C57.15-1986.html 7. IEC 60 076 international standard power transformers. Part 21: standard requirements, terminology, and test code for step voltage regulators. https://webstore.iec.ch/publication/600 8. Electric power transformer engineering. Book. Pub. Taylor and Francis, 2007.https://www. taylorfrancis.com/books/…/chapters/10.1201%2F9781420008715-8 9. Eaton cooper power systems catalogue “Smarter voltage regulation”. Eaton publication Nr. 8225-14 038 10. GE Shreveport, Louisiana, USA. Catalogue DEA 270 300 SHR. Year 2000 11. Siemens voltage regulator Type JFR Website: http://www.usa.siemens.com/energy/ voltageregulators. E T TR, voltage regulator business segment. P.O. Box 6289, Jackson, MS 39288-3289 12. Howard industries utility products division single phase voltage step regulator SVR-1. Cat. 28-10. Document: 1029-03. Revision: 03. Issued: 8/2/04. Howard Industries, Inc. Laurel, Mississippi. Web: howardtransformers.com

Chapter 10

Drive Mechanism and Controls

According to their [Newton and his followers] doctrine, God Almighty wants to wind up his watch from time to time: otherwise it would cease to move. Gottfried Leibniz

10.1 Chapter Content Tapchangers are driven in normal operation by a drive mechanism. The mechanism may be integrally built as a part of the tapchanger or may be a separate physical entity. In the case of many compartment type tapchangers, the mechanism is built in. This is not a functional necessity. The OLG UZDvac Vacuum Resistance Tapchanger and the ABB UZE/F of the compartment type have a demountable drive mechanism. The mechanism as a separate physical entity is obviously sine qua non for Intank tapchangers. A common mechanism is often used for driving three poles of tapchangers. This situation is common in H.V. Delta-connected and three-phase autotransformers. In this chapter we shall review the basic functions, some constructional aspects, and operation of drive mechanisms. Tapchanger mechanism requirements and testing procedures are covered by Ref. [1] (IEC 60 214-1 2014-5), Sect. 6.

10.2 General Control Requirements We shall first layout the minimum functional and control requirements of a mechanism. In the rest of the chapter we shall examine how these are commonly fulfilled. The minimum features of a motor drive are: 1. 2. 3.

Electric motor. Reduction gear train. Motor protection.

© Springer Nature Singapore Pte Ltd. 2020 T. V. Sridhar, Application of Tap changers to Transformers, Power Systems, https://doi.org/10.1007/978-981-15-3955-8_10

393

394

4. 5. 6.

7. 8. 9. 10. 11. 12.

13. 14. 15. 16.

10 Drive Mechanism and Controls

Manual operation option. Usually used for initial commissioning and troubleshooting. Electrical limits at end positions. Mechanical limits, which operate after electrical limits, in case these fail. The mechanical limits prevent overrunning at the end position, which may cause damage. Motor control circuitry. Manual raise and lower option, from local, remote or supervisory off-premises operation. These must be mutually exclusive. Facility for automatic operation by motor drive controller. Automatic maintenance of operation till one tapchange is over, once initiated. Termination of tapchange at the end of each tapchange. Limiting operation to only one tapchange, even if the initiating pulse is maintained. This is called the stepping function. This feature is not common in US practice. Electrical limits at end positions. Provision for parallel operation. Mechanical tap position indicator. Number of operations counter.

10.3 The Power Drive Tapchangers are driven by three-phase or single-phase motors. In this case the motors are quite small, mostly in the Fractional-horsepower range. The torque is more important than the power. In normal operation, the torque and power requirements are small. The motor must however be designed to start under more onerous conditions than normal. Such conditions arise when the tapchange has to start from halfway when a previous operation is curtailed for any reason, e.g. power failure. When the power supply returns, or if an attempt is made to start the motor drive from a previous incomplete tapchange, the motor must be able to start. The most onerous conditions are (they may occur together!). 1. The incoming voltage is low. It is reasonable to expect that the drive mechanism should be able to manage its functions at 15% lower voltage. It may be kept in mind that the torque goes down by 30%. 2. The incoming voltage may be very high. This will heat up the motor. If the windings are already hot, they may be damaged. 3. Maximum load. This occurs when the drive mechanism is coupled to three tapchangers, one for each phase. If the tapchangers are equipped with a preselector, the worst time to start off is when the pre-selector contacts are about to climb on to the fixed contacts. The load in terms of torque is high. 4. High frequency of the incoming supply, which lowers the motor flux and causes loss of torque. This is limited to 1 or max 2%.

10.3 The Power Drive

395

5. Very low temperatures. The oil is viscous and offers a high load. The bearings shrink and become tighter. The rotor resistance is low, causing low starting torque by the nature of induction motors. If all the factors are taken together, the motor may become unnecessarily large. Many designers use their discretion in this regard.

10.4 Single- and Three-Phase Motors Single-phase motors for tapchanger application have some problems. Single-phase motors of the size used in tapchanger application are usually capacitor start motors. These do not have a very good starting torque. As most standard single-phase motors are made for unidirectional run, motors for tapchanger application with its bidirectionality become a special execution. Unfortunately capacitors have a reputation among electrical engineers as being unreliable. For the same torque and power, single-phase motors require a higher frame size than three-phase machines. For these reasons it is usual to fit drive mechanisms with three-phase motors. However single-phase motors avoid a frequent problem faced in tapchangers. This is motor burn out, due to loss of one phase supply to a three-phase motor. This happens often in distribution networks. Besides bad contacts, non-closing of all phases in a motor contactor, blown or faulty fuses contribute to this problem.

10.5 Number of Starts Per Hour This is probably more the nemesis for tapchanger motors than other causes. Typically there is no control, or restriction of the number of tapchanges performed in a short time. Induction motors take a higher current at start. This leads to high heating during the starting period. The number of starts per hour is usually restricted to avoid extreme overheating and possible burn out. In selecting a motor most engineers would not pay attention to this detail, as it is only in a few application that frequent start is a requirement. It is not difficult to build in a protection logic in the controls to prevent high-frequency operation. It is probable that motor damage in tapchanger drive mechanisms may be due to excessively frequent starts than overloads.

396

10 Drive Mechanism and Controls

10.6 Motor Overload Protection 10.6.1 Motor “No Start” Protection A motor may fail to start due to many reasons. Single phasing of three-phase motors and loss of capacitor in single-phase motors are prime causes. The tapchanger or the drive transmission could be jammed. In single-phase motors, there is no damage to the motor if the phase supply is lost, whereas in three-phase motors a locked rotor condition with full voltage occurs, which may cause severe damage. In tapchanger application, it is difficult to verify that a motor has indeed started to run after a start command. The tapchanger may be far from the control room for physical observation. Some clients ask for a protection which trips motor power if the tapchange is not completed within a set time, something like 1.3 to 1.5 times the normal tapchange time.

10.6.2 Thermal Protection A bimetal element-based thermal relay is the device most frequently used as motor overload protection in tapchangers. A little consideration will show that even conceptually this is not a reliable protection. 1. Bimetal-based protection does not work very well, because the actual temperature is not measured. The signal is derived from the current, but this is not a good and consistent indicator of how hot the motor is. 2. The time constant of the bimetal is not matched to the motor. In a situation of frequent starts of the motor, the protective element rapidly heats up with the pulses of current, but cools down nearly completely in the intervening periods. The motor on the other hand accumulates heat, thus rendering protection ineffective. 3. In a cyclical loading situation the motor and protective element temperatures may drift apart quite a lot. This will lead both to unnecessary false trippings and failure to protect. 4. In FHP motors such as those in tapchanger drives, the motor current hardly changes with the load. The no load current is high. The power factor is low. When loaded, the power required comes more out of the improvement in the power factor than a rise of current. It is difficult to distinguish between normal and overload conditions. Overload protection by bimetal protection is not reliable. 5. Bimetallic relays are variable in their performance. The surrounding atmospheric temperature affects the function. A setting made on a moderate day may well cause false trippings on a hot day. 6. The setting of the bimetal can be easily meddled with by unauthorized or uninformed personnel.

10.6 Motor Overload Protection

397

10.6.3 Particularly Low Temperatures IEC 60 214 requires tapchangers to operate down to −25 °C. Cl. 8.2.1 of the IEC requires that the drive mechanism should perform 50 operations with the temperature inside the mechanism at −25 °C. This would preclude warming the local space with heaters. The following problems may arise. 1. The slides of the control relays and contactors get stuck due to icing. Contacts do not close properly. 2. With very low running clearance in bearings, differential contraction of the housing and the bearing may cause seizure. Bearing manufacturers offer bearings with different ranges of running clearances [2] (SKF) and [3] (NMB Bearings). It is necessary to choose an appropriate clearance. SKF recommend clearance class C3 or even C4. (This remark applies to all bearings in the tapchanger.) Such high clearances are noisy, but as the tapchanger motor only runs occasionally for short time this may not be a problem. 3. If oil-filled reduction gearboxes are used, their lubricating oil must be capable of operating at the low temperature. 4. Low voltage insulation of the wiring may become brittle and break off. This is true with ordinary PVC, and many common “magnet wires” which are used for motor coils. 5. The starting torque of induction motors goes down as the rotor resistance falls due to low temperature. The motor may stall, unless specially chosen.

10.6.4 Manual Drive Tapchangers can be driven manually, mainly for initial settings, e.g. matching the motor drive to the tapchanger, and other checks. It is essential to incorporate cutoff switches to ensure that the motor drive cannot be engaged when the manual crank handle is inserted. The cut-off switch should not re-engage till the handle is completely out. Many utilities discourage hand operation with the transformer on power. This is particularly true of some reactor tapchangers, where some safety features are defeated in hand drive. Because of the high gearing, it is possible to damage mechanical blocks at end positions during hand drive. There is a very small possibility that a fault may develop during a tapchanger operation, and it is not wise to have an operator close to the installation. Hand drive is a useful facility to check if there is some mechanical hindrance to the operation of the tapchanger. This is a “feel” of the operator. It must be tried with the transformer off power.

398

10 Drive Mechanism and Controls

10.6.5 Number of Turns of Handle Per Tapchange If the number of turns of the handle required to complete a tapchange is high, the load on the arm of the operator is low. However, human psychological perception is that too many turns of the handle is a “load’. On the other hand, if the turns of the handle required are too small, like 1, 2, or 3, the force required to turn the handle is high, and this would be felt as tiring. Besides if a mechanical block is used to prevent overdrive of the tapchanger at end positions, such a block may be damaged if the gearing is too high, i.e. very small number of turns of the handle. It seems a requirement of 8–16 turns is the best deal.

10.6.6 Electrical Limits At each end position, the motor drive is cut off by electrical limit switches. Electrical limits may only cut off the motor reversing contactors, but in some case event the power supply to the motor is cut off by including series limit contacts in the motor power supply. These must be set to operate after the last tapchange is complete. This is not a trivial problem in some cases, as described below.

10.6.7 Mechanical Limits The motor drive is protected from overrunning in the end position by electrical limit switches. These can fail, if the phase rotation of the supply is incorrect. In such a case, the wrong limit switch opens at the end positions, e.g. raise limit at the lower end position. Correct phase rotation of the supply must be checked at the time of first commissioning. Some manufacturers incorporate a check for the correctness in the drive schematic itself, so that incorrect phase sequence shorts the supply after the motor starts up and trips the power (e.g. MR Motorantrieb MA7). As a further protection, mechanisms are equipped with mechanical limits, set to operate if the electrical limits fail. To maximize this protection, it is essential that the mechanism should be coupled to the tapchanger correctly. Most commissioning instructions provided by the manufacturer give details of correct coupling.

10.6.8 Mechanical Limit by Solid Block A simpler mechanical limit operates by solidly blocking the motor output shaft. In one system, this jams the entire drive, including the motor. The motor protective

10.6 Motor Overload Protection

399

device may trip after some time. In another system, a belt in the drive usually the first component in the motor speed reduction train slips.

10.6.9 Differential Gear System In some mechanisms (OLG Mechanism VB, MR Motorantrieb MA4) a differential gear system forms part of the gear train. This is similar to the motor car differential drive. If the mechanical limit is reached, the differential lock is released by the limit linkage, so that the output alone gets cut off. The rest of the drive including the motor can spin around without damage indefinitely. This is obviously a superior system of protection. But the differential gearing occupies a lot of pace and is expensive. Further with a number of gears, it tends to be noisy. It is also difficult to lubricate the gears in an air-filled mechanism box construction. The differential mechanical slip device is almost obsolete now (Fig. 10.1).

10.7 Integral and Dismountable Mechanisms Mechanisms which are integral as one unit have the obvious advantage that no further matching and adjustment are required at the transformer works. The tapchanger is completely set, aligned, and checked by the tapchanger manufacturer. There could be a problem, not very major, in a small number of cases. The drive shaft from the mechanism enters the oil-filled power switch compartment through a dynamic oil seal. This must naturally be located behind the mechanism. If there is a faulty oil seal, to access it, the entire mechanism may have to come off. This could be a skilled operation, needing assistance from the tapchanger manufacturer. The integral mechanism does not offer the facility of being taken off for transport if needed. Figure 10.2 shows a photograph of an integral drive mechanism (OLG).

10.8 Demountable Mechanisms As mentioned earlier, there cannot be an integral mechanism for an Intank tapchanger. The mechanism comes as a separate physical entity (Fig. 10.3 both MR and ABB). It must be mounted on the external vertical wall of the transformer by the transformer manufacturer. The drive is communicated by vertical and horizontal shafts, which need to be coupled through a bevel or worm gear, which is again mounted on the transformer by its manufacturer (Fig. 10.3). The shafts of the tapchanger head, the bevel gear, and the mechanism must be aligned by the transformer manufacturer. In

400

10 Drive Mechanism and Controls

Fig. 10.1 Differential Gearing for mechanical release at end positions. OLG drive mechanism type VB. Cover removed and without oil filling

some executions the speed of these components is deliberately made low, so that even if the alignment is not very good, the noise and vibration are tolerable. Many tapchanger drive shafts operate at 60–120 RPM, but there are designs which operate at over 600 RPM.

10.9 Matching of the Drive to the Tapchanger

401

Fig. 10.2 Integral drive mechanism OLG tapchanger ABS

(a) DEMOUNTABLE DRIVE OF MR.

(b) DEMOUNTABLE DRIVE OF ABB.

Fig. 10.3 Example of demountable drive mechanism of both MR and ABB [2]

10.9 Matching of the Drive to the Tapchanger Apart from the mechanical alignment the following need to be matched.

402

10 Drive Mechanism and Controls

Fig. 10.4 Connecting three pole tapchanger

1. The tap position indicated in the mechanism must match the actual tap position of the tapchanger. 2. In the operating position both the tapchanger and the mechanism must be in their respective neutral positions. The tapchanger manufacturer usually gives detailed instructions on this setting. When carried out meticulously, the limit controls of the mechanism and the tapchanger are matched, and the limits are effective. 3. It is a good practice to check the effectiveness of the proper working of the limits immediately after the matching is completed.

10.10 Connecting the Drive Mechanism to Three Single-Pole Tapchangers In some applications the tapchanger consists of two three-separate columns driven by a common drive. It is important that the poles of the tapchanger are all matched to the drive mechanism and switch together. Total synchronism is often not possible. A difference of timing between the poles of up to 200 m s is to be expected and is acceptable. Some manufacturers (OLG) supply vernier couplings between the connected components, which makes alignment easier (Fig. 10.4).

10.11 Peripheral Misalignment Tolerance It is almost impossible to make sure that all three diverter switches operate simultaneously. There may exist a difference up to 200 m s. When there is a time difference,

10.11 Peripheral Misalignment Tolerance

403

the volts/turn of the three phases may differ for the duration till all diverters complete their switching. Under this condition, the fluxes in the three legs do not add to zero, and some flux completes the circuit outside the core. There is a surge in magnetizing current. The transformer protective relays should be set so as not to operate within the short period. Differential relays, which do not have a time delay, may need higher bias. If operating in the manual mode care should be taken not to stop till all three diverters are switched. This is not a problem in five limbed cores.

10.12 Motor Power Circuit Figure 10.5 shows a motor power circuit. While control circuitry differs a lot from manufacturer to manufacturer, the motor power circuit is practically the same. The motor reversing contactors C1 and C2 are mechanically interlocked to prevent both closing at the same time. They are energized from the control circuits. Figure 10.6 shows a slightly different scheme, with braking by motor. The braking is achieved by shorting the motor terminals at the end of the energization period.

Fig. 10.5 Motor power circuit

404

10 Drive Mechanism and Controls

Fig. 10.6 Braking by motor

10.13 Motor Control Logic The functional logic of most tapchanger controls is described below. However, the manner in which the functions are implemented differs from manufacturer to manufacturer. Much of the control logic is implemented in modern tapchangers through microprocessor, or PLC logic. Therefore actual circuitry is not described. The examples of circuits taken are for the purpose of illustration of the functions. The actual circuitry is not mandatory. 1.

2.

3.

4.

The motor can be energized by actuating the “raise” contactor C1 , or the “lower” contactor C2 . These two are often mechanically interlocked to prevent closing at the same time. They are also electrically interlocked in the logic so that when one contactor is active, the other cannot be energized. The motor contactors can be energized by manual push-buttons. They can also be energized by the automatic voltage regulating relay. The two are mutually excluded in the control logic. The operation could be from the local station at the drive mechanism, or from anyone of several remote stations. Control logic prevents more than one station being active at the same time. Insertion of the manual crank inhibits energization of both contactors.

10.13 Motor Control Logic

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5.

At each limit position a mechanically operated limit switch provides for prevention of energization of the respective motor contactor. 6. There is always a provision for terminating a tapchange at the correct position. The initiating signal is provided by a switch in the mechanism, or even the tapchanger itself. 7. A scheme whereby only one tapchange is made per initiation through pushbuttons or automatic voltage regulating relay. Even if the initiating signal is maintained, the motor drive stops after completion of one tapchange. To start the next tapchange it is necessary to discontinue the initiating signal, e.g. pushbutton and start again. This scheme is referred to as the “stepping function”. Stepping function is often not used in US practice. 8. Sometimes a logic is included to oversee that the motor starts and runs for about time required for completing a tapchange. After this time the motor is de-energized. This prevents motor damage by stalling, e.g. single phasing. 9. A mechanically driven number of operations counter is provided. 10. A provision is made to display the tap number. This could be a high geared mechanical wheel or could be generated in logic, by a set of cam-driven contacts.

10.14 Stopping the Motor Drive at the End of a Normal Tapchange The signal for termination of a tapchange must be internally generated by the drive mechanism at the appropriate position. We shall see later in this chapter (Sect. 10.16.1) that there are two basically different approaches to motor drive controls. One of these operates without any cam switches. We shall see the details in Sect. 10.16.1. The other involves cam-operated control, which reconfigures the control circuit at different stages of progress of the tapchange (Sect. 10.16.3). In cam-controlled drives it is important that the motor must completely stop within a short distance of the end position. If there is a considerable over drive, there is a possibility that the cam switches re-engage, and start the next tapchange. This will lead to a runaway situation till the limit position.

10.15 Mechanical Brake In this approach there is a mechanical brake, where brake shoes operate on a flywheel forming part of the drive. The flywheel has a raised position on the rim on which the brakes sit in the start, and end positions, applying the brake (Fig. 10.7, Waukesha UZD Mechanism). After the drive starts up, the cam projection is relieved, so that the brake is off. At the end of the tapchange the brake applies automatically again, quickly stopping the rotating inertia. The brake needs resetting to compensate for

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Fig. 10.7 Mechanical brake, Waukesha SPX UZD

the wear of the shoes after many thousands of operations. There could be some reservations in using a brake of this kind, particularly in cold temperature. There could be ice formation in the narrow gap between the flywheel and the brake, which could seize the brake. As against that, ABB have successfully used this brake in many thousands of tapchangers, and Sweden is as cold as cold can be!

10.15.1 Using the Motor to Brake At the end of the tapchange, the motor contactors have switched off and the terminals of the three-phase motor are short-circuited by a contactor (Fig. 10.6). The eddy currents of the motor act as quick brake, dissipating the energy of the rotational masses in the rotor.

10.15.2 Using the Spring Charging to Cause Braking Effect In mechanisms where there is no cam switch control, over driving is not particularly critical. The motor power is cut off by the control circuitry at the end of the tapchange. There is no particular effort at braking. The rotational inertia takes the motor further towards the next tapchange, but as the energy storage springs charge up, they slow down and stop the motor.

10.16 Termination of a Tapchange

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10.16 Termination of a Tapchange 10.16.1 Snap Action Quick Acting Switch for Termination of Tapchange Figure 10.8 shows a control scheme where a tapchange is terminated at its end by a snap action switch marked b9. Only components involved in the termination are shown, out of the entire control schematic. The snap action switch is driven by the energy stored in a spring, which is charged during the tapchange. The spring is

(a)

(b)

Fig. 10.8 Snap action switch for termination of tapchange

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restrained by a latch so as not to discharge. At the end of the tapchange, the latch is released, causing the spring to discharge quickly, and this motion is used to drive the snap action switch. The speed is so high that for all practical purposes, there is hardly any movement of the gear train driven by the motor. This leads to accurate positioning of the termination of a tapchange. There is no danger of restarting the next tapchange due to overrun, as there are no cam switches.

10.16.2 Evaluation of Snap Action Switch The snap action switch is a precise way of terminating the tapchange. The snap action approach saves manufacturing time as compared to cam-operated switches. The latter need painstaking adjustments. The objection is the cost and the complexity of the switch. It must be fast enough to be precise, but not so fast that it does not allow enough time for the motor contactor to drop off. In the latter case, the motor contactor can reengage again, leading to a runaway to the end position. In compartment tapchangers which most usually have built-in drive mechanism, the main energy storage drive for the diverter itself can be used as a snap action drive for the terminating switch.

10.16.3 Cam Switch for Termination of Tapchange Figure 10.9 shows the manner in which a tapchange is terminated by a scheme employing cam switches. Only components relevant to tapchanger run and termination are shown. Essentially, tapchange is initiated by pressing a manual push-button say b1. The motor contactor C1 picks up. Sealing contact of C1 closes, so that the motor runs even if the b1 button is released. After an initial period, the motor contactor is “maintained” by a cam-operated contact (C4-1 in Fig. 10.9). C4-2 opens, rendering the sealing contact C1 ineffective. At the end of the tapchange the camoperated maintenance switch C4 opens first, which drops off C1 , and motor begins to run down. C4-2 closes reverting the circuit to its initial position. The disadvantage of the cam switches is that their adjustment is critical (See below). A problem which arises with limit switch setting, particularly when the number of operating positions is large is discussed below.

10.16.3.1

Criticalities in Cam Settings

It is important that in Fig. 10.9c C4-1 closes before C4-2 opens. If both C4-1 and C4-2 are open, the motor may stop as there is no circuit through the maintaining contact C1 . In the final stages C4-2 can close only after C1 definitely drops out; otherwise, the motor can pick up for another tapchange run. At the end of the tapchange, the motor should not run on its inertia for long enough for C4-1 to close.

10.16 Termination of a Tapchange

(a)

(c)

Fig. 10.9 CAM controlled opreration

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(b)

(d)

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10.16.4 Stepping Function It is desirable that a tapchange initiating action results in only one tapchange, irrespective of how long the initiation signal is maintained, e.g. how long the push-button is kept pressed. The initiating action must reset, before the next tapchange can be made. This is called the stepping function. Stepping is universal in the tapchanger world, except in the USA. Figure 10.10 shows a basic hard-wired scheme for stepping. It is used with snap action control of tapchange termination. The stepping function is implemented by the snap action switch b9 , together with the contactors C3 and C4 . On charging the control supply, one of the contactors C3 or C4 will pickup through the dotted cyan current path, as at this stage all push-buttons and contactors are in the un-operated condition. In Fig. 10.10 C4 picks up. This provides a path through its normally open contact at b9 , for the push-buttons to initiate a tapchange. When C4 has picked up, it seals through a contact C4 in series. At the end of the tapchange the snap action switch b9 flips over, cutting off the motor contactors C1 or C2 , C3 and C4 , and resetting the maintenance contact C4 as well. If all the push-buttons are released, and the motor contactors reset, the other contactor of the pair C3 picks up along the dotted cyan current path. If a push-button is not released, C3 will not pick up, and this opens the circuit for the motor contactors. Therefore the initiating push-button must be released before another tapchange can be initiated. Even though only one set of manual push-buttons are shown, the scheme can be expanded to include remote push-buttons and automatic voltage regulating relays.

10.16.4.1

Stepping for Cam-Controlled Drives

It is possible to contrive a scheme to use the above-described method of stepping for cam-controlled drives as well. However the scheme shown in Fig. 10.11 is more usual. Figure 10.11 is similar to Fig. 10.9, except that the stepping function is added at the top of Fig. 10.11a. In Fig. 10.11b a raise tap operation is started by pushbutton b1 . The raise motor contactor C1 picks up and its contacts change over. If the push-button b1 has not been released, the stepping contactor ST picks up. The coil is maintained by its self sealing contact. The normally closed contact of ST in series with the power circuit for the push-button opens. It is no longer possible to initiate a new tapchange from the push-buttons, until b1 is released.

10.17 A Critical Problem with Limit Switch Setting A problem can arise with reference to the setting of the cam-operated type of electrical limit switches. We shall consider a case where the limit switch has to open to disable further tapchanges in the same direction after the limit position is reached. The limit switch should open quite close to the termination point of the last tapchange before

10.17 A Critical Problem with Limit Switch Setting

Fig. 10.10 Stepping for snap action

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(a)

(b)

Fig. 10.11 Stepping for cam control

the limit is reached. If it operates too late there is a possibility that the tapchange is completed, with the limit switch till open. This will allow a further tapchange after the limit position is reached. To avoid this, the limit switch may be set to open a little earlier. But if it opens too early, the tapchange may be terminated before the final position is reached. This calls for a delicate adjustment of the limit switch. The difficulty increases with the number of tap positions, because the available movement of the cam per tapchange reduces, and the uncertainty of the point of operation takes a larger fraction of the tapchange. This problem is obviated in the control scheme shown in Fig. 10.12. The limit switches b4 and b5 are bypassed by a cam-operated maintenance contacts b9B /b10B for each direction. These contacts are open at the start of a tapchange (Fig. 10.12a), but are closed by cam soon after the tapchange starts. They are opened by the cam just as the tapchange is complete. This happens every

10.17 A Critical Problem with Limit Switch Setting

(a)

(c)

413

(b)

(d)

Fig. 10.12 Circuit to avoid criticality in limit switch setting

tapchange. In the last raise operation before raise limit, b9B closes soon after the start of tapchange, as usual (Fig. 10.12b). The raise limit switch b4 opens after b9B closes without waiting for the end position to be reached (Fig. 10.12c). Tapchange continues through the maintenance contact b9B . The maintenance contacts finally open at the end of the tapchange to terminate the tapchange (Fig. 10.12d). This ensures that the tapchanger arrives at the end position with the limit switch open. A further tapchange cannot be initiated in the same direction, as there is no circuit to C1 .

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10.18 Controls for Parallel Operation The so-called master/follower protocol is used widely for parallel control. This is a simple method of controlling several tapchangers to remain at the same tap number. Therefore it is suitable when the transformers are nearly identical in terms of voltage ratio at all taps and of matched impedance. Transformer theory shows that if such transformers are placed in parallel at the same tap, they will share the common load in proportion to their rating and there will be no circulating current within the paralleled loop. For parallel operating transformers which do not match these requirements may also be operated in parallel using the circulating current method or the negative reactance method. These methods are discussed in Sect. 10.21. Control for master/follower parallel operation involves collective implementation over several tapchangers which run in parallel. The modern implementation is through tapchanger supervisory controllers, through logic (Refs. [4–7]). In the following, the principles and requirements of parallel control with hard-wired logic are discussed, because several thousand older transformers are still working with this type of control. The following are the common features. 1. A group of several tapchangers are controlled together. 2. One unit is selected as master. Commands for raise and lower can only be issued by the master. Others will follow the commands of the master. All panels use the auxiliary power supply of the master. Their own individual power supplies are deactivated. 3. In case for any reason a follower falls out of step by one tap, an “out-of-step” condition is said to result. All tapchanges for the group from the master are suspended. A warning is issued to the operator of the failure of the group. This ensures that no tapchanger in the paralleled group can be more than one tap out with the rest. The situation must be reset manually to bring the group to the same tap. This restores further operation. 4. Even though the hard-wiring includes all the panels in a group, any panel can be opted out of the group to work independently. The independent tapchanger does not obey the master control. The independent tapchanger operates on its own power supply. It is also excluded from the out-of-step monitoring function. Some of these functions and options are implemented in the control circuits through a multi-pole parallel control switch (or sequence selector switch…surely a pretentious name!). 5. Any tapchanger in the group can be taken completely out, by selecting the “off” mode on the control selection option. Till recently, a forest of hard-wiring was required to implement the features of the master/follower scheme. With the advent of microprocessor and PLC logic the system has been much simplified.

10.19 Use of the Out-of-Step Relay

415

10.19 Use of the Out-of-Step Relay The master/follower scheme starts out with the premise that at the time a tapchanger is connected for parallel operation, it is set in the same position as the others in the paralleled group. Thereafter it is wired to follow a master from the paralleled group and maintain the same tap position as the rest. Each tapchanger in the group has one mechanically driven two-position switch, called the odd/even switch, and a relay called the out-of-step relay. The connection scheme shown in Fig. 10.13 allows operation of any tapchanger in the group only when all the units continue to remain at the same tap. Commands at the master are simultaneously communicated to the followers through the paralleling buses (Fig. 10.13b). Thus all tapchangers run together and maintain the same tap position. The master unit energizes two lines at the bottom of Fig. 10.13a in alternate taps. The followers can pick up power and complete the circuit to their coils, if they are at the same parity. When all the OSR relays of all the followers are energized, the series-connected line of their contacts

(a)

(b)

Fig. 10.13 Schematic of out of step lock out

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allows the OSR of the master to be charged also. If anyone follower is out of step, the master OSR drops off. A contact in series with the motor contact coil operating line locks out any further tapchange, so that the discrepancy does not increase. The master OSR also gives an audio-visual indication of the out-of-step condition. There could be exceptional circumstances when the transformers in a paralleled group are not at the same tap. This could happen for instance when a follower is unable to follow the master due to problems in the drive, e.g. blown fuse in one phase. In this circumstance, the power supply to the OSR of the faltering followers fails, and its OSR de-energizes. This breaks the circuit for the master OSR as well. A contact provided in the master unit motor relay activating circuits prevents any further tapchange of the master. Thus if a discrepancy occurs, the master/follower scheme is designed to prevent further tapchange of all the tapchangers in the group. In other words there is a lockout. An indication is given to the operator of the occurrence. The out-of-step lockout is implemented by a hard-wired circuit where the control is by conventional methods, not involving computational capability. In case of intelligent controls, the out-of-step condition can be deduced directly from the input of the operating tap numbers of the members of the group.

10.20 Possibilities of Contention in Interchange of Commands Between Master and Follower We consider a situation where one of the units is faster and takes a little shorter time to complete a tapchange. At the point of termination at the faster unit, the others are still running. The corresponding paralleling bus is charged. When the terminating switch b9 of the faster unit resets, the faster unit will start running again, picking up from the paralleling bus. A similar condition arises more particularly after a hand crank operation left partly incomplete bestowing the ability on that tapchanger to complete the tapchange ahead of the others. This process leads to a runaway to the end position of both tapchangers. This condition is prevented by the OSR contacts in the lines connecting to the paralleling buses in the followers (Fig. 10.13b). When the faster unit completes a tapchange, its OSR opens the connection to the paralleling buses. The time taken by the OSR to drop out must be shorter than the time taken by the b9 switch to reset. Otherwise bus contention will result, and the scheme defeated. This emphasizes the challenge of the mechanical design of the b9 switch.

10.21 The Circulating Current Method of Paralleling Many a times it may be desired to operate transformers in parallel, even though they may not share load in proportion to their ratings. A deviation from this ideal sharing is regarded as the presence of a circulating current between transformers.

10.21 The Circulating Current Method of Paralleling

417

The inequality of current sharing and the additional losses must be accepted by the user. Control systems in this case only ensure that the circulating current is minimum. Chapter 11 looks at the so-called circulating current method of parallel control.

10.22 Automatic Voltage Regulating Relays Tapchangers are not necessarily controlled automatically by a voltage sensing device, to maintain the voltage constant. They can be operated manually, where required an automatic voltage regulator can be used. The device monitors the transformer voltage which is to be maintained. For this purpose a PT is needed to give the actual voltage as input. A reference voltage is set up manually. The reference voltage is the transformer voltage which is desired to be maintained. If there is a deviation from the reference value, device operates the tapchanger, to bring the voltage back to the reference. Most automatic regulating relays allow field setting of the following parameters (Ref. [8] (Beckwith)). 1. It is undesirable that short deviations of voltage cause a tapchange. These may be due to transient changes of load which last for a short time, after which the voltage conditions return to normal even without any corrective action (Fig. 10.14 case at T1 ). To take care of this, a field settable time delay is provided. If the voltage disturbance persists for longer, only then a tapchange takes place. In Fig. 10.14, there is no tapchange at T1 because the disturbance does not persist long enough. The delay may be fixed or may follow a pattern similar to normal protective relays, i.e. IDMT, which act quicker for bigger faults. The time delay may follow a fixed, inverse, or very inverse pattern. This selection can be set in the field.

Fig. 10.14 Action of voltage regulating relay

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2. For stability the control must have dead band, within which the voltages can lie without initiation of a tapchange. If not even a tiny voltage deviation, say in the upper direction will initiate a tapchange to take the voltage down. A tapchange causes a change of voltage drop equal to one step. The new voltage will now be too low. A raise command is issued, which changes the voltage up again by a tap step. A process of “hunting” arises, when the tapchanger continuously operates, never finding a position of stable operation. To prevent this there must be a “dead band” of at little more than 1.3 times the step voltage. The dead band need not be the same on the positive and negative sides. The actual dead band can be set by the client. Figure 10.14 shows operation of an automatic voltage regulating relay with adequate dead bands and time delays. 3. If it is desirable to reduce the frequency of operations, the dead band may be set higher. Voltage regulating relays provide for client’s choice of the dead band. This is a field settable parameter. 4. The reference point itself may be set over a band of voltage about the nominal by the client. This helps the operator to take care of PT errors, as well as to choose a different set voltage, to take care of voltage conditions further “downstream” (See Sect. 10.23 Line drop compensation).

10.23 Line Drop Compensation The setup described above is good enough to control the voltage at the terminals of the transformer. In many cases the load centres are somewhat removed from the transformer. There is a voltage drop in the connecting cable or transmission line. The voltage at the load centre is lower and fluctuates with the load. The idea of the line drop compensation is to include this drop of voltage in the reckoning, so that the load centre voltage is maintained.

10.24 Conventional Method (Refs. [8, 9] (Beckwith)) An older method which is no longer used is helpful in explaining the principle. Voltage at the load centre is Vr = Vt − I (cos ∅ − j sin ∅)(R + j X )

(10.1)

See Fig. 10.15 for notation. In the older method, the voltage drop in the connecting line was reproduced to scale, by variable resistances and coils representing the line parameters. The CT secondary output was passed through the resistance and reactor connection (Fig. 10.12). The resistances and reactors were adjusted so that at any desired load current in the power line, the voltages out of the representational

10.24 Conventional Method (Refs. [8, 9] (Beckwith))

419

Fig. 10.15 Principle of line drop compensation

resistance and reactance were the same as the line/PT ratio. The voltage drop in the representational line was deducted from the PT input to the voltage measuring element. Now it is the load centre voltage that the regulating relay keeps constant.

10.25 Modern Line Drop Compensation Modern voltage regulating relays include line drop compensation as a feature. Because of their computational ability it is no longer needed to physically generate the line voltage drop as was done in Sect. 10.24 above. The values of the line resistance and reactance drops at 100% line current are calculated, and input as volts to the relay, after taking into account the PT ratio. The relay internally generates the value of V r at actual load conditions. Modern voltage regulating relays [5–8] are much more than mere voltage control with line drop compensation. They include a plethora of transformer monitoring tools, event log, dissolved gas analysis, and other facilities for overall transformer supervision.

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10.26 The Load Bonus Feature Many voltage regulators in the US employ a setting implemented by the tap position indicator to overload the regulator at close to 1:1 ratio position. This a somewhat old technology with hard-wired limit positions for the tapchanger. Such feature can be implemented easily in modern “intelligent” tapchanger controllers through software. The current through a series booster winding is a smaller fraction of the through current, when the ratio is close to 1:1. This would permit increasing the though current, provided that the range of operation of the tapchanger is restricted close to 1:1. The so-called Load Bonus Feature resets the limit switches of the tapchanger to allow increasing the through current [9].

10.27 Indications Connected with Tapchanger Controls The following facilities are required in the drive mechanism. Most are vital for the proper and safe functioning of the drive. Some are optional. 1. Mechanical tap position indicator. It is most usual that a wheel geared to the mechanism and marked up with tap numbers is used. 2. A number of operations counter. This could be mechanical or may involve electrically counting of the number of times the motor runs. 3. An interlock to prevent the motor from being energized when in the manual hand crank operation mode. 4. Electrical and mechanical limit switches. It is important that the mechanical limit operates only after the electrical.

10.28 Indications at the Remote Control Panel At the remote control panel the following indications may be desired. Some are essential, but others optional. 1. 2. 3. 4. 5. 6. 7. 8. 9.

Remote tap position indication. Tapchange in progress annunciation. This could consist of only indicating lamps. Tapchanger locked out due to out-of-step condition. Tapchanger in master/follower/independent/off mode. Tapchanger in automode or manual mode of control. Tapchanger set for local or remote operation. Tapchanger in limit position. Tapchange incomplete indication. Transformer voltage.

10.28 Indications at the Remote Control Panel

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In case of hard-wired panels, items 4–6 can be read of the bezel of the corresponding setting switches. 7–9 are optional.

10.29 Blocking Under Transformer Short Circuit The occurrence of a short circuit on the transformer just when the transition resistances are in circuit has a small probability for an individual transformer. This is because the duration of transition of the diverter switch, or selector switch is very short and the probable number of tapchanges in a given period is low. The exposure time of the transition resistances is low. The transition resistances are generally not capable of taking a transformer short-circuit current, even for the short transition time. A method of blocking the diverter/selector switch during a short circuit seems desirable. However there is no method of stopping the diverter or selector switch once it is in motion under stored energy. If a short circuit happens when the transition resistances are in circuit, it would be a statistical misfortune. In the case of a large system with several thousands of tapchanging transformers, the probability of a system wide misfortune is not so low as an individual transformer. For this reason, a precaution is often included to at least block the motor of a tapchanger when a short circuit is detected. This will at least prevent a tapchanger “walking into” a short circuit. A contact of the transformer protective relay can be used to break the motor contactor circuit.

References 1. IEC 60 214-2015-05. Part 1: Tap-changers Performance requirements and test methods 2. MR Technical Brochure TAPMOTION ® ED. https://www.reinhausen.com/desktopdefault. aspx/tabid-38/172_read-242/ SKF information Sheet “Bearing Internal Clearance” International Standard. https://www.skf.com/in/products/bearings-unitshousings/roller-bearings/principles/ bearing-data-general/bearing-internal-clearance/index.html 3. https://www.nmbtc.com/bearings/engineering/free-internal-clearance/ (NMB Bearings) 4. Beckwith Technical Brochure M 2001 D 5. MR Technical Brochure Tapguard 260 6. ABB Tech Brochure Voltage regulating relay for tapchanger control “RAYA” 7. A. Eberle Brochure REG-D Relay for Voltage control and Transformer monitoring 8. Beckwith App Tip #2: Line drop compensation for Substation application. 2003 9. Cooper Power Systems VR -32 Voltage regulator technical brochure SS 225–10.05

Chapter 11

Operation, Maintenance, and Monitoring

Über allen Gipfeln Ist Ruh, In allen Wipfeln Spürest du Kaum einen Hauch; Die Vögelein schweigen im Walde. Warte nur, balde Ruhest du auch. J.W.Goethe

11.1 Content of Chapter The reader who avidly takes up this chapter in the hope of picking up nuggets of useful information on the issue of operation and maintenance of specific tapchanger makes and models will be disappointed. The best information on operation and maintenance of specific tapchangers is to be found in the technical information published by the manufacturer. This chapter does not offer detailed advice on these issues, in futile competition with the manufacturer’s technical literature. General information on such details as routine maintenance, contact life, oil change, lubrication, when given would be too general to be of value. Therefore this chapter concentrates on special issues connected with operation and maintenance. Operation and maintenance are greatly helped by the availability of several transformers an tapchanger monitoring devices. Some basic monitoring approaches are discussed, but not devices of specific makes.

© Springer Nature Singapore Pte Ltd. 2020 T. V. Sridhar, Application of Tap changers to Transformers, Power Systems, https://doi.org/10.1007/978-981-15-3955-8_11

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11.2 Sundry Considerations for the Transformer Manufacturer Many tapchanger manufacturers incorporate a laudable routine of operating a newly manufactured tapchanger for many thousands, typically five to ten thousand, of mechanical operations as a part of the manufacturing process. This protocol averts the possibility of assembly errors and early component failures. Besides if a tapchanger is operated for a considerable number of operations initially, a lot of metallic detritus is formed. When the oil is drained, and the assembly hosed down with fresh oil, it carries away most of the metallic particles. After this settling, the rate of metallic erosion due to mechanical action goes down.

11.2.1 Operation of Tapchanger Without Drive Mechanism The need for operating the tapchanger arises during transformer construction. In cases where the drive mechanism is mounted separately on the transformer tank, it will not be available to operate the tapchanger. The tapchanger may need to be operated by other means, for instance for verifying the ratios after connection. It is important that the limits are not exceeded during such operation without the drive mechanism. In some executions all protection against over drive beyond the limits is provided only in the drive mechanism. The tap position indicator on the tapchanger must be referred all the time when operating by other means, so as not to endanger the tapchanger by over driving.

11.2.2 Coupling of the Tapchanger to the Drive Mechanism Where the tapchanger is driven by an externally mounted drive mechanism, it is important to reasonably align the drive shafts to maximize life and reduce noise. The coupling components often include small details, all which are functionally important. The manufacturer’s instruction for coupling, particularly with reference to maintaining the same tap position at the drive and the tapchanger, and the point of termination of the mechanism drive at the end of a tapchange must be strictly followed. This ensures that the limiting devices generally mounted on the drive mechanism are fully effective. It is a good practice to perform a full ratio test after the mechanism is finally coupled to the tapchanger. This shows up errors in tap numbers, and termination of the mechanical drive prematurely, or too late after the actual completion of tapchange. This check is also useful in verifying proper operation of the electrical and mechanical limits. These checks must be done without the transformer energized.

11.2 Sundry Considerations for the Transformer Manufacturer

425

11.2.3 Oil Filling the Suction Pipe When the tapchanger is oil filled prior to commissioning, the inbuilt suction pipe must be air released to fill the pipe completely. This is a detail easily missed. Unfortunately there is no other indication for an unfilled suction pipe. The suction pipe generally travels through regions of high electrical fields and will fail if not properly filled and air released.

11.2.4 Free-Level Operation The US practice, where compartment tapchangers are more the norm, is to operate tapchangers as free-level units, i.e. without a conservator. The top part of the tank is oil-free gas space and is vented to the atmosphere through a breather. Gases produced by the tapchanger diffuse into the atmosphere. The main disadvantage is that a Buchholz relay cannot be used. This lack is obviated by the use of a sudden pressure relief device such as the ABB Sudden Pressure Relay or its equivalent [1, 2]. The device is mounted in the gas space above the oil. It responds to sudden increase of pressure in the enclosure. An electrical contact is provided for alarm or trip. The relay is not sensitive to slow evolving faults with gas evolution. In some applications, the gas space is maintained under a nitrogen blanket under positive pressure. The relay senses sudden increase in gas pressure above the normal and thus is effective in this application also. There must be a venting path for the tapchanger gases to prevent gas accumulation.

11.2.5 Gas Accumulation and Venting Diverter and selector switches operate with evolution of gas produced by the arc. If a large number of continuous operations are performed there could be a considerable flux of gas. This could also happen if a number of tapchanges take place at over load, this condition often met in furnace application. In case of tapchangers with gas cushion on top of the oil, the problem of venting the gases is minimal. Gas accumulates with a slight rise of pressure. Compartment tapchangers under conservator provide for gas dissipation through a breather. If the breather is clogged with powdery desiccant, there could be substantial restriction of the escape path, and pressure could build up. In the worst case a false trip by the Buchholz may result. The problem is a little more severe in Intank tapchangers, where the gas escape path is restricted by a number of mechanical components mounted at the top. But even here, while the possibility of gas accumulation must be kept in mind, it is not a serious operational problem. Accumulated gases could be inflammable and toxic. In all cases it is necessary to safely vent the gases before any maintenance approach.

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11.2.6 Gas Reaching Areas of High Electric Field Gases produced by the arc travel up and may have to pass through areas of high electric stress. If the gases are still hot and ionized, there could be problems of encroachment of insulation distances. This is a particularly important issue in H.V. Delta-connected Intank selector switch tapchangers constructed as single column (e.g. OLG-type RMV). Gases from the lower phases pass through high fields in the upper-phase selectors. The most severe stress occurs in three-phase units, with the pre-selectors mounted above the selector switches. The field in the pre-selector zone can be very high in high-voltage delta applications. Deposition of arc material on the insulation components around the live parts has been observed, even though no arc takes place in the pre-selector itself (Fig. 11.1). This has been the experience of many manufacturers in the Southern Grid of India, where several thousands of such tapchangers have been in service for over 25 years. There have been cases of tapchanger failure due to this cause. In this context it is noteworthy that some manufacturers have removed the pre-selectors from the top position and mounted them external to the oil vessel. Even very high-voltage Intank diverter switches arranged in one plane have not shown a similar phenomenon.

11.3 Anxieties About Integrity of the Current Path Most commissioning protocols include verification that the current path though the transformer is intact. This involves primarily measurement of winding resistance at all taps. Such measurements instead of assuaging fears more often fan up anxieties. Fig. 11.1 Deposition around fixed contacts of pre-selector mounted on top of selector level, 110 kV delta applications

11.3 Anxieties About Integrity of the Current Path

427

11.3.1 Winding Resistance This vexing hardy annual is the bane of the industry. Most users wish to record and analyse the winding resistance at various times of manufacture, testing, commissioning, and during periodic inspection of a transformer. Most often the measurements are variable and not always easy to explain. The problem is that copper-to-copper or copper alloy contact resistance is very variable [3–5]. This is due to the formation of high resistance surface tarnish films of metal oxide, polymeric material [3], and sulphur compounds [4], with some carbonaceous content, when the contacts are not carrying current and are stationary. Dissolved oxygen and corrosive sulphur content in oil promote the formation of high resistance film on copper, copper alloy, and silver surface [5]. Reference [3] suggests that the formation of tarnish layers is very quick. A 2 nm film can form in about 9 s at a temperature of 100° C in the absence of current and relative movement on a copper to copper interface. These films will invariably form during the period of pre-commissioning of the transformer and frustrate resistance measurement. So long as these films are intact, they prevent intimate low resistance metal to metal contact. Contact resistance is high and variable and confusing!

11.3.1.1

A Practical Illustration on a Large Scale

Variation of contact resistance shows up sharply in low voltage, high current tapped windings, when resistance measurement at all taps, using a low DC current source, is taken. This can be illustrated by experience in the South Indian grid, where a number, possibly exceeding 500, of 16 MVA, 33/11 kV transformers power the intermediate distribution system. The tapchanger is on the 33 kV side to regulate by CFVV for the 33 kV variation, with 16 taps. The winding resistance is about 480 m. With 1 ¼ % taps, one would anticipate a resistance variation from tap to tap of 6 m. In a number of cases the variation is up to 12 m and in rare cases as high as 20 m. In the same system there are almost an equal number of 8 MVA transformers, where the winding resistance is about 1600 m. Often the same tapchanger is used in both ratings. It may be anticipated that the actual ohmic fluctuations are similar, but the high winding resistance of the 8 MVA camouflages the effect. There are usually no adverse reports, and the transformers are quietly commissioned. It is unfortunate that the regulations and culture of the governing bodies do not encourage publication.

11.3.1.2

Solutions to the Problem of Resistance Variability

In order to stabilize contact resistance, it is necessary that the surface films must be disrupted, and true metal-to-metal contact established. One brutal method is mechanical abrasion and cleansing. If the moving contacts are run over the fixed contacts repeatedly, the films may breakdown. This is quite effective in wiping contacts such

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11 Operation, Maintenance, and Monitoring

as commonly used in tap selectors and selector switches. In situations employing butt contacts, such as diverter switches the effectiveness is not too satisfactory. When this approach results in no solace, a direct contact cleansing operation may be tried in the case of compartment tapchangers, where the contacts are accessible with some effort. In Intank tapchanges, it will be necessary to resort to “fritting” described by Holm [6]. Fritting is the process of electrostatic microscale disruption of the films by the intense electric field between the contacts. Reference [5] suggests that fritting in copper-to-copper or copper alloy contacts may require a voltage drop from 0.7 to 1 V across the tarnish layer. In the usual method of resistance measurement the current passed is of the order of 20 A. If the contact resistance is 5 m, the voltage drop is too low for fritting. For proper fritting about a 100 A will be required. This incidentally is the current used in switchgear practice for contact resistance measurement [7]. It is not very practical to pass 100 A DC through a transformer winding, as a DC source of 10 s up to the low 100 s V may be required. Apart from this there is the danger of high voltage when the current is broken. Current must be gradually reduced, which will preferably require a variable voltage supply. High magnetizing currents may be anticipated when the transformer is charged again, due to high remnant flux density. On the whole not a desirable procedure!

11.4 Test to Indicate Problems in Transformer Current Path On the basis of the wild variations of winding resistance, the commissioning engineer suspects that the tapchanger contacts are not making properly. There could be genuine high contact resistance which would lead to high temperature on loading. An effective, easily interpreted, and unambiguous method to monitor the transformer circuit at every tap, without special equipment, consists of a two-part test. This test gives the results that are easily interpreted. 1. At each tap, apply a low voltage DC excitation to the transformer phase and monitor the current. An oscilloscope is optional, an ammeter with low damping will do. If the current is not steady, there is an indication of a bad contact in the circuit. This may not necessarily be in the tapchanger. 2. Apply a low voltage DC excitation to the transformer phase, do a tapchange while monitoring the current during tapchanges oscillographically. A complete break of current at any point of time is not necessarily indicative of a problem. It may be a contact bounce. If such a break is detected, repeat the test at that tap. Unless a consistent and repeated break is detected, there is no problem.

11.4 Test to Indicate Problems in Transformer Current Path

429

11.4.1 Problems in Tapchanger Transition The issue that the tapchanger may be breaking the circuit during transition is a constant nightmare. The following disturbing thoughts arise 1. The tapchanger is by design not maintaining the “make before break” geometry. This is an unworthy and dishonourable suspicion. No tapchanger manufacturer can get away with this neglect. His product will pass no tests and will not survive in the market. 2. The transition resistance is disconnected or has a poor connection. 3. The contact resistance is high. 4. There is contact bounce.

11.4.1.1

Futility of High-Speed Detectors for Zero Current at Low DC Excitation

A very rough and ready, but futile, method to try and detect failure of the “make before break” concept, and other continuity problems consists in monitoring the continuity of the transformer current under low DC excitation, including an analogue ammeter/ohmmeter in series. Sometimes resort is made to high-speed electronic bistable devices. This approach is bereft of any scientific basis. Firstly the cutting in of the transition resistance at some point of transition causes the current to drop and the analogue meter to dip. Besides most tapchangers will show a contact bounce on open circuit or at very low currents, this is totally immaterial. The correct way of checking is described in the following section.

11.4.1.2

Missing Connection to Transition Resistance

There is no reason why the connection between the transition resistance and the main contact should break. If such a breakage occurs, the current loses a path when the main contact if off during transition. This would indeed be a catastrophic failure.

Waveform of One Transition Figure 11.2 shows a useful connection which yields a lot of information on the tapchanger (See also [7]). It can be used when the transformer is de-energized on all kinds of tapchangers. Even though a two resistance diverter switch is shown, the method is applicable to all tapchangers. Figure 11.2a shows the circuit. An ordinary 12 V battery serves the purpose. The signal resistance can be electronic sized and about 10% of the transition resistance in value. Nothing happens till the transition resistance cuts in at A, when the current drops. The fall is strictly exponential, due to

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11 Operation, Maintenance, and Monitoring

(a)

(c)

(b)

(d)

Fig. 11.2 Test for transition

the transformer winding inductance, but at the time scale shown only a line will be seen. It rises a little at B when the second transition resistance cuts into the circuit. Figure 11.2b shows an orderly transition in a healthy tapchanger, with a neat line trace, without bumps, breaks, and shakiness. The waveform may be slightly different in a practical case. 1. Short breaks in vertical line (Fig. 11.2c). Probable contact bounces. This is not material. 2. A relatively long duration bounce on making contact make, as shown in Fig. 11.2d. This is also will not seriously affect the functioning. 3. If the top line goes through straight, with no steps, the transition resistances are short-circuited or disconnected. A serious matter.

11.4 Test to Indicate Problems in Transformer Current Path

431

4. An indeterminate, shaky, non-repeatable trace indicates an uncertain contact in this tap position. This may not be in the tapchanger. This is not a desirable condition. 5. Trace is zero during the whole period, including non-transit time. Leaving apart the trivial case of the flat battery, the transformer circuit is open at this tap. This is not necessarily a tapchanger problem. Again a serious matter.

Dynamic Resistance Measurement Application of a DC excitation at any tap position coveys information of the healthiness of the circuit through the transformer at that tap, but fails to detect problems with the transition resistance circuit. A method designated as dynamic resistance measurement (DRM) has been suggested in References [8, 9]. This test may only be done on a de-energized transformer. The transformer phase is excited by a low voltage DC; see Fig. 5 of Reference [8]. The test records the current variations through the transformer, when the tapchanger is continuously run from one end to the other of the tapping range. The current is monitored throughout the activity. During the quiescent period, when the tapchanger is at any one tap, the current builds up exponentially towards the final value in the transformer inductance. During the tapchange a relatively high transition resistance cuts into the circuit, suddenly dropping the current. The current once again builds up during the next rest period. A typical waveform of current is shown in Fig. 11.3. The DRM waveform is basically a signature. Any difference from the typical pattern is indicative of a problem at that transition, e.g. cut-off of current altogether due to a loss of connection to the fixed contact.

11.5 Polarization Index as a Measure of Insulation Quality Polarization index relates more to the transformer as a whole, than the tapchanger. However as the tapchanger is a large component of the transformer, it is necessary to review the role of the tapchanger in polarization index. Polarization index is defined as the insulation resistance measured at 10 min of the start of measurement to the value at 1 min. In order to appreciate what this means, it becomes necessary to delve into the mechanism current flow during measurement.

11.5.1 Current Flow Through an Insulator During Resistance Measurement When the insulation resistance of an oil-filled equipment, such as a transformer or reactor, is sought to be measured, a DC voltage is applied between a winding terminal and earth. The resulting current is measured. The ratio of the voltage to the current

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11 Operation, Maintenance, and Monitoring

Fig. 11.3 DRM waveform Dd [8, 9]

is taken as the resistance value. Instruments such as the Megger directly read this ratio on the dial (analogue instruments), or as a digital read-out. In the process of measurement, there are actually four components of current flow [10]. See Fig. 11.4. 1. The insulation between the transformer bushing and earth forms a capacitor, with the entire insulation structure acting as the dielectric. As with all capacitors, there is a high charging current initially, which decays exponentially. The time constant in a typical transformer is less than 1 min. This is the significance of the 1 min reading. The capacitance current is eliminated by that time. 2. A component Ir flows through the bulk of the insulation. This depends on the geometry of the insulation structure, the volume, surface, intrinsic resistivity, and the temperature. This component is independent of time, provided that the time frame is not so high that the flow of current increases the temperature. 3. A surface current component Is flows through the interface between different kinds of insulation, but mainly the solid and liquid interfaces. This current is colloquially referred to as the creepage. Creepage is affected by the length of the creepage path, dirt accumulation, surface roughness, and other factors. This component is also reasonably time independent. 4. The insulation has some polar components, meaning they form dipoles. Water is a strongly polar material, so that wet insulation is highly polar. In the unexcited state, the axes of the dipoles are randomly distributed through the volume of the

11.5 Polarization Index as a Measure of Insulation Quality

433

Fig. 11.4 Components of current while measuring insulation resistance [10]

insulation. When a voltage is applied, the dipoles move and rotate to align their axes with the external electric field. This process is slow and needs energy. The energy is supplied by a component of the current from the voltage source. The polarization current I p is high initially as there are a larger number of un-aligned dipoles. As more and more of these are aligned the current decreases. Typical time constant is ten mins. At about ten mins the polarizing current drops practically to zero. High-voltage engineering textbooks such as Zängl Kuffel [11] suggest that it in fact takes infinite time to go down to zero, during which time the polarization current may continue at the nano-amp level. But for practical purposes 10 min is a good time to consider that the polarization current has dropped to zero at 10 min. Then, both the capacitive currents and the polarizing current are very small, and the instrument indicates the “true” resistance of the insulation.

11.5.1.1

Calculation of Polarization Index

1. Insulation resistance at ten mins,

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11 Operation, Maintenance, and Monitoring

V Ir + Is

(11.1)

V Ir + Is + Ip

(11.2)

R10 = 2. Insulation resistance at one min, R1 = 3. Polarization ratio =

Ip R10 =1+ R1 Ir + Is

(11.3)

If the “intrinsic” currents I r and I s are large as compared to I p, the insulation is of a poor quality. The second term in Eq. 11.3 is small, and the polarization ratio is small. Thus low polarization index does not necessarily reflect wetness, but on the otherwise poor condition of the insulation. If on the other hand the insulation is of good quality, I r + I s is small as compared to I p , and the polarization index is high. 4. The currents I r and I s are not entirely determined by moisture, as I p is. Hard drying of the insulation will remove moisture and reduce I p . Unless the other two components reduce also, there is a likelihood of the PI falling even further! 5. In modern high-voltage transformers processed fully (may be kerosene vapour drying) and vacuum impregnated with naphthenic high resistance mineral oils, there are hardly any polar components left in the insulation. The value of I p is low, and the polarization index remains close to 1, despite excellent insulation. 6. From the above analysis, the role of tapchangers, which are constructed of components with practically no polarized material, seems to be minimal.

11.5.2 Polarization Index in Oil-Filled Equipment It is superfluous to add to the following quote from the Megger Brochure [10]. However it may be added that the IEC 60 076 [12] does not mention polarization index as a measure of insulation condition in oil-filled transformers. Cl. 7.2.13.4 of IEEE C57.152-2013 does mention PI, but in the note attached says “The Polarization Index for insulation liquid is always close to 1. Therefore the Polarization Index for transformers with low conductivity liquids (e.g. new mineral oil) may be low in spite of good insulation condition” [13]. Quote from Reference [10] It is also interesting to note that many people have tried to use the PI test on oil-filled transformer and cannot understand why a known good transformer gives them results close to 1. The answer is simple. PI testing is not appropriate for oil-filled transformers. The concept depends on the relatively rigid structures of sol insulating materials, where absorption energy is required to reconfigure the electronic structure of comparatively fixed molecules against the applied voltage field. Because this process can go to a theoretical state of completion

11.5 Polarization Index as a Measure of Insulation Quality

435

(at “infinite time,” which obviously cannot be achieved in the practical field, but can be reasonably approximated), the result is a steady diminution of current as molecules reach their “final” alignment. Because the PI test is defined by this phenomenon, it cannot be successfully applied to fluid materials since the passage of test current through an oil-filled sample creates convection currents that continually swirl the oil, resulting in a chaotic lack of structure that opposes the basic premise upon which the PI test rests. Unquote

11.6 Transient Current Due to Difference in Tap Positions in Three-Pole Tapchangers This problem may occur in transformers with three single-pole tapchangers driven by a common drive mechanism, e.g. H.V. Autos. A difference in diverter switching timing of up to 300 m sec may occur, even with careful alignment. When the firstphase switches, there is a different number of effective turns, and therefore volts/turn. The difference flux will have to complete the path outside the core in a three-phase three limbed core construction. There is a rise of magnetizing current, which would include a DC transient component. It may be expected that overcurrent relays would be indifferent, but differential relays may pick up, as in magnetizing inrush. A trip is a possible risk. Where frequent tripping with tapchange is observed, the sensitivity of the differential relays can be lowered by increasing the bias. No time setting option is available with differential relays. The problem does not exist with five limbed construction, or single-phase three limb cores with one wound limb, because the surplus or deficient-phase flux has a high permeability path, and current will not rise. Even three-phase units driven by the same energy storage device show some difference in diverter switching times, but the duration is very short.

11.7 Contact Life IEC 60 214 Cl. 5.2.3.2 requires type testing for contact life of 50,000 operations, at the rated current and relevant step voltage. Modern oil tapchangers claim a contact life one order of magnitude higher, and vacuum tapchangers still higher. The statistics of Reference [14] Table 1.1 estimate 20,000 operations per year for power generation, transmission and distribution industry, and a more robust 300,000 for furnace application. In this context it may be remembered that the test conditions of IEC 60 214, Cl. 5.2.3.1 forbid changing of contacts, or oil during the test. But this restriction would not apply to real-life situations. It must also be remembered that the claimed contact life is estimated on the basis of remaining contact material, and the need to maintain the operational sequence (See Sect. 11.7.1 below). It is probable that before these end of life criteria area reached, continuing with eroded contacts may become untenable from the point of view of contact surface condition, leading

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11 Operation, Maintenance, and Monitoring

to extreme contact bouncing and increased resistance due to lowered spring force. Taken in all it looks that with timely maintenance, contact erosion is not a limiting factor in tapchanger life.

11.7.1 Loss of Operational Cycle Due to Contact Erosion The comforting conclusion of the last section is disturbed by some exceptional situations. Contact erosion can proceed to such an extent that the operational cycle of the tapchanger cannot be maintained. This is explained in Fig. 11.5 in the context of a selector switch pennant cycle. Figure 11.5a shows a transition from the bridging condition with new contacts, where the trailing transition contact is still riding on the fixed contact when the main contact makes on to the next fixed contact. As erosion of contacts proceeds, the trailing transition contact breaks off earlier, but the main contact makes later Fig. 11.5b. A situation could arise when neither contact is made and there is a threat that the main current is denied a path, i.e. the make before break principle is not maintained. The current arc is driven by the phase voltage. In such situations the arc is not necessarily confined to the gap between contacts, but may become more general, causing collateral damage to neighbouring components which normally do not take part in arcing. The illustrative example shows a pennant cycle transition, and a similar situation could arise with other cycles of transition. This shows that contact wear is not merely a source of a relatively minor problem as contact resistance. It can lead to catastrophic failure if not attended to in time. The manufacturer’s instruction regarding wear limits should be diligently followed.

(a)

(b)

Fig. 11.5 Loss of operational sequence due to contact wear

11.8 Operation of Transformers in Parallel

437

11.8 Operation of Transformers in Parallel When two or more transformers, each with the same voltage ratio, and percentage impedance are connected in parallel, they will share the load in the proportion of their individual ratings. When new transformers are added to a fleet, it is possible to specify the desired characteristics for ideal paralleling. It is necessary to charge the transformers at the same tap and thereafter maintain the taps synchronized. This purpose is well served by the master/follower control scheme (Sect. 10.18). However the requirements stated are unnecessarily stringent. It may not always be possible in a fleet that such matched transformers are available at a location for parallel operation. Paralleling of transformers with slightly different ratios, or tap sizes, or percentage impedance is possible, at some loss of combined capacity, and extra losses. Ideal sharing of the load will not be obtained. In case of voltage difference, there could also be a circulating current leading to extra load losses. Except in the rare case of all transformers involved being at their maximum rating, they can be parallel operated regardless, conferring operational flexibility at a relatively small price. In such cases of parallel operation with circulating current, methods are available to maintain minimum circulating currents, as discussed below.

11.8.1 Operating Transformers with Different Percentage Impedance in Parallel Even though this subject relates more to transformer rather than tapchanger operation, it will be discussed here for two reasons. First, there is a widespread incorrect understanding that paralleling of transformers with different ratios or impedances is likely to damage them. Second, paralleling has a relevance to tapchanger control.

11.8.1.1

An Illustrative Example

In Fig. 11.6 we consider two transformers T 1 and T 2 , rated 10 and 12.5 MVA, respectively. The open-circuit voltage of T 1 at the principal tap is taken to exceed that of T 2 by 3%. The impedance of T 1 is 8% and that of T 2 is 9%. Thus the example defies the generally held assumptions regarding impedance and open-circuit voltage. It will be shown that such paralleling is possible, without any danger to the equipment, though some loss of rating and extra copper losses result. Figure 11.6b is redrawn with impedances adjusted to 10 MVA base. There is no external load, but there is a circulating current. The impedance limiting the current is the series sum of the two transformer impedances, 8% + 7.2% = 15.2%. The current is

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11 Operation, Maintenance, and Monitoring

(b) (a)

(d)

(c)

Fig. 11.6 Unequal transformers in parallel

Ic = (3/15.2) × 100% = 19.73% on 10 MVA base

(11.4)

The current in the 12.5 MVA as %age of its own rating is (10/12.5) × 19.73 = 15.78%. The copper loss in the 10 MVA due to the circulating current is 0.1973ˆ2 × its full load copper loss = 3.89% of full load. The copper loss in the 12.5 MVA is 0.1578ˆ2 × its full load copper loss = 2.49% of full load. The result shows that in spite of being paralleled with a difference in voltage, the circulating current is not at a harmful level. As regards the copper loss, at no load it is very small. Interestingly, the current in neither transformer exceeds its rating till the difference in the open-circuit voltage reaches 15.20%, at which point the circulating current is 100% of the current of T 1 . This is nearly the full range voltage. These calculations are typical, and the method can be applied to other ratings and impedances.

11.8.1.2

Load Sharing

In order to demonstrate the effect of the difference in the open-circuit voltage, we shall first calculate the load sharing with equal voltages. Load on T 1 = 7.2/15.2 ×

11.8 Operation of Transformers in Parallel

439

Total Load. T 1 reaches its rated current when the load is 21.11 MVA. There is a loss of 1.39 MVA of the total capacity of 22.5 MVA.

11.8.1.3

Effect of Circulating Current

We shall assume that the load power factor is 0.85. The share of the load current taken by T 1 is 7.2/15.2 I L . The current through T 1 IL (7.2/15.2)(0.85 − j0.526) − j0.1973 = IL (0.402 − j0.4464)

(11.5)

The current taken by T 2 is 8/15.2 I L . The current through T 2 IL (8/15.2)(0.85 − j0.526) + j0.1973 = IL (0.447 − j0.07985)

(11.6)

When the total load is 16.7 MVA, T 1 reaches its full current. Figure 11.6d shows the vectors. There is a loss of 5.8 MVA load capacity. Comparing with the previous section it is seen that the circulating current has a drastic effect. If parallel control is by master/follower, the situation can be alleviated by operating the tapchangers an even number of tap positions apart, reducing the difference in open-circuit voltage. Alternatively the transformers can be operated in parallel using a circulating current relay, such as Beckwith M 2001 C (See Sect. 11.9.3 below). We may conclude as follows: 1. The example takes arbitrary values for rating, impedance, and difference in opencircuit voltages. The numerical conclusions are not universally valid. It is necessary to work out the details whenever a real situation arises. However the following general conclusions are valid. 2. Paralleling of transformers with different percentage impedances and slightly different voltage ratios is possible, without damage to the transformers. 3. At no load, the circulating current cause extra losses, but remains well below the rating of the transformers. 4. The reduction in combined rating due to differences in percentage impedance is moderate in most practical cases. 5. The circulating current due to differences in tapping positions can have a significant reduction in the combined rating.

11.9 Influence of Circulating Current on Switching Duty When a tapchange is made, at some point the transition resistance is inserted in the current path. We shall see shortly that the circulating current between two or more parallel-connected transformers can change by the insertion of the transition resistance. This causes a restrike voltage which is no longer in phase with the interrupted

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11 Operation, Maintenance, and Monitoring

current. Such a change of through current as a result of the transition resistance insertion is not important in isolated, i.e. non-paralleled transformers. Figures 11.7a, b show the conditions before, and after the transition resistance is inserted in an isolated transformer, using a diverter switch flag cycle. The current I 1 with W made I1 =

V [(RL + j X L )+(Rt + j X t )]

(11.7)

And after W breaks =

V [(RL + j X L )+(Rt + j X t ) + R]

(11.8)

In all practical cases the transformer leakage impedance and the load impedance are much larger than the transition resistance. Besides which these are largely reactances, whereas the transition resistance is of course a resistance, so that the circuit impedance hardly changes (taking the square, adding and taking the square root!). The assumption that the transformer current shifts from the W contact to the X contact without a change in magnitude and phase angle is reasonable. The current interrupted by the main contacts is the through current I and the recovery voltage is RI, as discussed in Chap. 3. The interrupted current and recovery voltage are in phase.

(a)

(b)

Fig. 11.7 Effect of switching transition resistance into current path

11.9 Influence of Circulating Current on Switching Duty

441

11.9.1 Parallel-Connected Transformers with Voltage Difference at No Load In the case of transformers in parallel there may exist a phase shift between the interrupted current and recovery voltage. In order to simplify the interpretation, we shall first analyse the issue of interruption without load. We shall later take into account the load current. We consider two transformers in parallel, but with no external load (Fig. 11.8a). A circulating current exists in the local loop if the transformer voltages V 1 and V 2 are not equal. This situation could arise when the transformers are not in the same tap, or transiently when a faster tapchanger finishes an operation before the slower one starts. The circulating current is Ic1 =

V1 − V2 [(R1 + R2 ) + j(X 1 + X 2 )]

(11.9)

A tapchange is made. The first contact to open say W1 will interrupt this current. In Fig. 11.8b W 1 has interrupted the circulating current. The current now transfers to the transition resistance. The circulating current I c2 now is Ic2 =

(a)

(c)

V1 − V2 [(R + R1 + R2 ) + j(X 1 + X 2 )]

(b)

(d)

Fig. 11.8 Interuption duties of diverter in parallel connected transformers

(11.10)

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11 Operation, Maintenance, and Monitoring

The recovery voltage is RI c2 . The phase shift between the interrupted current and the recovery voltage is the phase angle between I c1 and I c2 . This is Arctan(X 1 + X 2 )/(R1 + R2 ) − Arctan(X 1 + X 2 )/(R + R1 + R2 )

(11.11)

We can gain an indicative magnitude of the phase shift by considering two relatively small transformers like 25 MVA in parallel. 1. The transformers may have a reactance in the region of X = 8% of V /I 2. The transition resistance is more likely to be R = 1 ¼ % V /I = 15% of the leakage reactance. 3. The winding resistance for a relatively small transformer like 25 MVA is about 4% of the leakage reactance. The angle of Eq. 11.11 works out to about 4.24°. This gives us a practical view of the order of the phase shift under no load conditions. For larger transformers all X/R ratios go up, and therefore the phase shift will go down.

11.9.1.1

Under Load Conditions

Reference [14] (Sect. 5.5.2) gives a detailed analysis of diverter switching duty in transformers connected in parallel. The following is not as mathematically rigourous, but simplifies the outcome based on a justifiable assumption mentioned later. The currents in the two parallel-connected transformers in Fig. 11.8c can be determined as follows. When the main contact W1 is still made, with the situation corresponding to Fig. 11.8c, the load current splits into the paralleled transformers in the inverse ratio of the impedances I1.1 = IL (R2 + j X 2 )/[(R1 + R2 ) + j(X 1 + X 2 )]

(11.12)

I2.1 = IL (R1 + j X 1 )/[(R1 + R2 ) + j (X 1 + X 2 )]

(11.13)

In addition to the load component there is a circulating current component Ic1 =

(V1 − V2 ) (R1 + R2 ) + j (X 1 + X 2 )

(11.14)

The current which will be interrupted when W 1 opens is I 1.1 + I c1 . See Fig. 11.9b. After W 1 breaks, with the assumption that the load current does not change significantly by the introduction of the transition resistance, the following equations hold

11.9 Influence of Circulating Current on Switching Duty

(a)

443

(b)

(C)

Fig. 11.9 Vector diagrams of interruption in parallel connected transformers

IL (R2 + j X 2 ) [(R + R1 + R2 ) + j(X 1 + X 2 )]   IL (R + R1 + R2 ) + j X 1 = [(R + R1 + R2 ) + j(X 1 + X 2 )]

I2.1 =

(11.15)

I2.2

(11.16)

Ic2 = (V1 − V2 )/[(R + R1 + R2 ) + j (X 1 + X 2 )]

(11.17)

The recovery voltage is R (I 1.2 + I c2 ). As this voltage is in phase with (I 1.2 + I c2 ) the required phase angle between the interrupted current and recovery voltage is the angle between (I 1.1 + I c1 ) and (I 1.2 + I c2 ). This is ø1 − ø2 in Fig. 11.9 showing the vector diagrams.

11.9.2 An Approximate Numerical Evaluation The above equations are difficult to interpret because of both the involvement of the circulating current and load component. The two components are not related in anyway. An attempt could be made by delinking the two. We have already seen in the previous section the treatment of zero load component, i.e. on open circuit. We can apply the same concept by taking the circulating current to be zero in the other extreme. This is when the voltages of the two paralleled transformers are equal,

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11 Operation, Maintenance, and Monitoring

or when the load current is much higher than the circulating current. Under this condition, the phase shift between the interrupted current and the recovery voltage is that between I 1.1 and I 1.2 . For the 25 MVA taken as example, substituting numerical values in Eqs. 11.12 and 11.15 results in a phase shift of 4.3°. The illustrative example of Reference [14] shows a phase shift of about 6°.

11.9.3 Tapchanger Control with Circulating Current When transformers with different terminal voltages and varying percentage impedances are paralleled, the main moorings of the master/follower control stand undocked. Instead it is necessary to replace that scheme with one which minimizes the circulating current. There are a number of tapchanger control relays which manage this function. The method used in the Beckwith M 2001 C [15] will be taken for illustration. The Beckwith application note [16] shows how by a clever concatenation of CTs; it is possible to separate the transformer through current and the circulating current. The circulating current is then used to bias the voltage sensor element of the controller. In Fig. 11.10 there is already a circulating current. The biasing is in such a direction that voltage sensing element of T 1 sees a higher voltage than T 2 . When the bus voltage goes still up, it will be T 1 which does a tapchange, reducing its voltage. The incremental circulating current flows in a direction to reduce the total circulating current, while the bus voltage reduces.

Fig. 11.10 Beckwith relay connections for circulating current method [15]

11.10 Staggered Switching of Tapchangers in Parallel

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11.10 Staggered Switching of Tapchangers in Parallel A finer control of output voltage can be obtained by operating one tapchanger at a time, in a group of parallel-connected transformers. There is a circulating current. This may be tolerated as a price for fine voltage control. A variation of the idea is often used in convertor transformers, if there is a tapchanger for each phase. The tapchangers can be operated one phase at a time, to give a smoother DC.

11.11 Protection Against Tapchanging Under Short Circuit It is well nigh impossible to protect a high-speed resistance tapchanger against tapchange during short circuit. In resistance transition tapchangers, the diverter switch moves without recourse to the motor drive. Therefore no external restraint can stop a tapchange after the diverter or selector switch has started moving. It is possible to provide for logic in the controls to stop the motor if a short circuit is detected beforehand. As the time for tapchange is of the order of 50 ms, the statistical coincidence of a short circuit with a tapchange, even in a fleet of 10,000 transformers is very small.

11.12 Tapchanging with Magnetizing Inrush Current The magnetizing inrush current of transformers can be very high. The waveform consists of high peak current for part of a half cycle, when the core saturates. The current in the other half cycle is low, with magnitude of normal magnetizing current. The peak current may well exceed the capacity of the tapchanger to interrupt. It is therefore a good practice not to operate the tapchanger too soon after charging the transformer. Magnetizing inrush may last for four to ten seconds.

11.13 Contact Coking in Pre-selectors In some applications the pre-selector may remain at the same position for a long time. The lack of movement enables non-conducting films to forms increasing resistance and heating. At worst a phenomenon characterized as “coking of contacts” may take place. This is a situation of film build up to very high thicknesses, reducing cooling and further aggravating the temperature. Contact coking also takes place in de-energized tapchangers. The solution is to provide for sufficient cooling, and operating the contacts once in a while, even if functionally not required.

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11.14 Application of DGA to LTCs Application of the well-established DGA procedure to tapchangers is mainly guided by References [17–21] are good practical guides to the Duval triangle method. DGA as a means of monitoring the insulation of transformers is well established. It is slightly more difficult to apply the well-developed DGA protocols to tapchangers because tapchangers produce similar gases even during normal operation. Normal gas formation resulting from normal operation of LTCs must be identified first as precisely as possible in service. Faulty or abnormal operation may then be detected by deviation from normal gas formation. Gases are formed in tapchangers due to arcing in oil, and by heating of the transition resistances. Arcing in oil mainly results in the formation of C2 H2 . Dissipation in transition resistances increases their temperature and produces thermal gases in oil (C2 H4 /CH4 /C2 H6 ). The gases produced depend on the specific type and design of tapchanger used, the parameters defining the behaviour being: 1. oil or vacuum tapchanger 2. compartment type or Intank type 3. operating conditions (at low or high powers or currents).

11.14.1 Approach to Analysis Two main methods are in vogue for the identification of faults in tapchangers: the IEEE gas ratios [22, 23] and the Duval triangle 2. The Duval triangle method is discussed more fully hereunder. The Duval gases are CH4 (Methane), C2 H4 (Ethylene), and C2 H2 (Acetylene). As in the case of the IEEE method, computed ratios of gases rather than absolute values are used. Readers may be familiar with Rogers and Doernenburg ratios [22–24]. The Duval approach using gas ratios is however more complex, involving the construction of the appropriate Duval triangle. The methodology is no doubt guided by the sound principle of gas thermodynamics, and energy balance concepts. But as electrical engineers are usually not well trained in these arts, the application with its routines takes on an almost mystic hue. We shall therefore discuss the details of how the triangle is constructed, but some preliminaries are required, as discussed in the next section.

11.14.2 Types of Incipient Faults Identified by DGA Methods The inside area of the Duval triangle is divided into a normal area and several “fault zones”. The following types of faults are marked up within. The application involves locating the characteristic operating point of the test object, based on the measurements made. The operating point will fall in one of the areas marked up with the

11.14 Application of DGA to LTCs

(a)

447

(b)

Fig. 11.11 Duval triangle 2

following (See Fig. 11.11). Partial discharge (PD)—of the corona type occurs, which can result in deposits of “X-Wax” on paper insulation. PD of the sparking type may occur also, which can induce pinholes (carbonized punctures) in paper that may be difficult to find. 1. Discharges of Low Energy (D1)—occur in oil and/or paper, as indicated by large carbonized punctures in paper (pinholes), carbonization of the paper surface (tracking), or carbon particles in oil (as in an LTC). 2. Discharges of High Energy (D2)—occur in oil and/or paper, as indicated by extensive destruction and carbonization of paper or metal fusion at the discharge extremes, extensive carbonization in oil, and in some cases, tripping of the equipment confirming a large current follow-through. 3. Thermal Fault (T1)—occurs in oil and/or paper below 300 °C, turning the paper “brownish”. 4. Thermal Fault (T2)—occurs in oil and/or paper above 300 °C and below 700 °C, carbonizing the paper. 5. Thermal Fault (T3)—occurs in oil and/or paper above 700 °C with strong evidence of carbonization of the oil, metal colouration (at 800 °C), or metal fusion (below 1000 °C). 6. Normal (N).

11.14.3 Construction of the Duval Triangle 2 We shall confine our attention to the Duval triangle 2. Figure 11.11 shows a Duval triangle 2 according to Reference [20]. The three sides of the triangle are marked CH4 (Methane), C2 H4 (Ethylene), and C2 H2 (Acetylene), with arrows indicating the direction of increase. The limits of the “normal” zone are left to be decided for a

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specific tapchanger from observations. The fault zones are marked up, according to the list of the previous section. See also [21].

11.14.3.1

Method of Plotting Points on the Duval Triangle 2

Points are not directly plotted on raw measurements. The percentage of each gas of the total is determined, and each gas is plotted on its axis on the triangle. Suppose an oil sample has: 40 ppm methane, 60 ppm ethylene, and 100 ppm acetylene. Calculate T = 40 + 60 + 100 = 200. The percentage of each gas is then methane = 40/200 = 20%, ethylene = 60/200 = 30%, acetylene = 100/200 = 50%. Mark up a point on the methane side of the triangle at 20% (Fig. 11.9b) and draw a line parallel to the acetylene side of the triangle (horizontal). Drawing of such lines is facilitated by the short line segments marked upon the sides of the triangle as guide. Mark up 30% on the ethylene side and draw alike parallel to the methane side. Mark up 50% on the acetylene axis and draw a line parallel to the ethylene side. The lines of construction and the point representing the measurements are marked in red. The point lies in the X 3 zone. It has already emigrated from the normal N zone and seems to be travelling towards the T 2 and T 3 zones. By plotting these points from time to time, the nature of the final fault can be predicted, and remedial action is taken before a full-fledged fault develops. This is an advantage claimed for the Duval method. An example of the predictive nature of the method is given in Reference [20], however for transformer and not tapchanger fault.

11.15 Acoustic Signals as a Means of Estimating Tapchanger Operation Tapchangers produce characteristic acoustic signals from mechanical sources such as contact movements, and application of brakes (when fitted), as well as arc phenomena. References [25–27] describe the capture and analysis of acoustic signals generated by tapchanger operation for the purpose of condition monitoring. The method is applicable both with the transformer energized and de-energized. The acoustic signals are captured by typically three accelerometers mounted on the transformer tank. It is claimed that it is possible to relate features of the acoustic signals to mechanical events such as contact make and break, duration of dwell, as well as current interruption and commutation process. Dedicated instrumentation designated as T 4 by the manufacturer has been developed by Zensol Automation Inc. of Canada (http:// www.zensol.com/en/) for the purpose. The method still seems to need some skill, and is subject to interpretation. Reference [25] warns of incorrectly placed sensors generating false information. The acoustic signals are compared to samples taken from new tapchangers. During such calibration, high-speed photography is used to relate signals to actual physical events. The raw acoustic signal is analysed by a

11.15 Acoustic Signals as a Means of Estimating Tapchanger Operation

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patented algorithm developed by Hydro-Quebec, which extracts a low-frequency and a high-frequency envelope from the signal [28]. These envelops normalize the captured raw information, so that comparisons are enabled. The low-frequency envelope yields information on the contact movement, while the high frequency gives information of the arcing process. Acoustic measurements are sometimes supported by simultaneous capture of the motor current.

11.16 Operation Monitoring by Extended Voltage Regulating Relays The basic function of voltage regulating relays was discussed in Chapter Sect. 10.22. Once the relays adopted microprocessor technology, it was a short step to expand the functions of the automatic voltage regulating relays to make them function as transformer monitors. Several such monitors and controllers are available on the market. The following section summarizes the various capabilities of such controller monitors. The summary is based on the features offered by the more familiar monitors such as MR Tapcon [29], MR TapGuard [30], A.Eberle RegSys [31], and ABB RAYA [32]. Not all the features described in the summary below may be available on each monitoring system mentioned above. The reader is advised to consult individual manufacturer’s information for details.

11.16.1 Features Offered by Monitoring and Data Acquisition Systems The basic function remains the independent, but user-programmable monitoring and control voltage and to supervise signals from the relay to the tapchanger. Thus parameters such as the desired voltage band, time delay, and other feature of tapchanger control are programmable in the field by the user. Provision is also made for takeover by manual operation, either from remote or local stations. Monitoring is used to perform both simple and complex measurements, control and regulation tasks on tapchanging transformers. To achieve these tasks, the basic measurement and operation control unit are equipped with optional add-on components. The add-on functions could include supervisory modules, remote I/O modules, and an assortment of communication cards. The system usually displays all measured variables of the network, such as the various switching operations of the tapchanger, voltage and current, tap number, cooler operation status, and more on a screen. A paralleling module provides for parallel control of several tapchangers using the master/follower, circulating current of other control stratagems. Line drop compensation is an additional feature

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Fig. 11.12 Typical communication architecture for grid integration

which is usually incorporated in the paralleling module. Some devices display transformer interconnection through a single line diagram on a screen. A transformer control and monitoring module could be added enabling continuous monitoring of various conditions of the transformer and tapchanger. Information such as hotspot temperature (IEC 60 354 or IEC 60 076) and transformer loss-of-life are calculated, and the result could be called on the screen by an user control programme. This module can also perform functions of cooler control and cooler supervision. A most useful feature supported by most monitors is data logging. Thus number of tapchanges, the number of these by manual modes, number of voltage excursions beyond the set value, and other parameters can be data logged for analysis. Data logging enables the system to display advance information on forthcoming maintenance schedules. Some monitors record motor current, torque, or acoustic data generated during a tapchanger, so that any abnormality can be detected in time, and potentially damaging operations blocked off. Most control and monitoring systems can communicate between themselves and to the external remote network (Fig. 11.12) through IEC 61 850, IEC 60 870, MODBUS RT-U and other popular communication protocols.

11.17 Tapchanger Failures Contact interruption duties, short-circuit currents and tapchanger voltage stresses are so well studied and so well published that there is a high degree of knowledge among the transformer fraternity. It is rare that a properly chosen tapchanger will fail on account of these. Tapchanger failures more often occur due to more mundane issues like mechanical obstruction, component and fastener failures, and lack of oil maintenance. These constitute the majority of failures. Apart from these, troubles rather than serious failures may occur more frequently with the drive mechanism.

11.17 Tapchanger Failures

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11.17.1 Failure of a Diverter, or Selector Switch to Complete a Tapchange Failure to complete a tapchange is easily the most frequent cause of tapchanger failure. This results in the transition resistance carrying current for a long duration. The transition resistance or the connections to it then open out interrupting the current, with disastrous consequences. Tapchange getting stuck may happen due to 1. Mechanical obstruction. Mechanical obstruction could be due to foreign material getting between moving parts. Unfortunately this type of failure is most frequent following a maintenance work, when sundry extraneous materials like wrenches, spare fasteners, and so on are unintentionally left behind. In worst cases there could be a mistake in the assembly of parts repaired or replaced. 2. Sometimes fasteners which work loose end up among the machinery, causing a block up. Tapchangers work with a great deal of vibration. Accelerations achieved in transit from tap to tap of diverter and selector switches involve several “G”s. It is very necessary to ensure integrity of fastener tightness and locking. Lock sheets, lock nuts, castle nuts, and thread adhesives must be used. Spring washers as a locking device are of practically no effect in a tapchanger environment. Thread inserts (“Nylock” for instance) also have a mixed record of service. Nuts manufactured as part of design with a few distorted threads in a controlled manner have vindicated themselves well for sizes larger than M10. 3. Slowing down of the switching element. Such slowing down may not result in stoppage with incomplete tapchange, but may expose the transition resistances to current for too long. This mode of failure is more common in selector switches. This may happen due to interference of the fixed and moving contacts due to extreme wear. In some cases of worn contacts, there may be a minor contact weld resulting in slowing down. A prelude to this failure is that in most cases the Buchholz causes an alarm. Such alarms should be thoroughly investigated. 4. In selector switches, there is an over-travel of the moving contact system at the end of contact movement. As the Geneva drive locks, the shaft is torsionally strained by the inertia of the moving parts, and the moving contacts are driven beyond their operating point. The return to the correct position is uncontrolled and uncertain. In due course this causes an elongation of the holes of the drive pins, and the contact system becomes slack. The main contact may end up not making with the fixed contact properly. In this case the transition resistance may end up carrying current. 5. Broken main drive springs, or when they are “set” due to fatigue, is also a cause for failure of complete drive. However this cause of failure may be expected only after a very large number of operations, e.g. 200,000 or more.

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11.17.2 Component Failure Tapchangers may fail due to the gradual worsening of minor problems with time. 1. The case of elongation of holes in shafts with usage has been mentioned. This is a slow and insidious development, which is often not noticed. 2. Contact springs are usually lightly insulated from the contacts so as not to form a parallel path for the current. Damage of the insulation or more frequently displacement may create a passage for the current through the springs. The springs may heat up, and in course of time the material is annealed. This cause loss of spring action and contact force. 3. Barring design and manufacturing defects, incorrect installation, and accidental breakage, all springs should easily outlast a tapchanger. We may compare with the valve springs of a four-stroke automobile, which do about 750 operations a minute. In about 10 h, when a car would be regarded still new, these springs would have completed more operations than a tapchanger of “reasonably long life”.

11.18 Oil Maintenance Surprise is often expresses by users that while small compartment tapchangers require more frequent oil maintenance than high-voltage line end units. These small compartment types are mostly applied at the middle of delta-connected windings of 33 kV transformers. The working voltage between phases is about 16 kV. This is applied to an insulation distance which stands 70 kV during ACSD. The working voltage to ground of a 132 kV line end tapchanger is 38 kV. This is applied to an insulation distance which corresponds to 230 kV during ACSD. It is seen that the 33 kV insulation is relatively more highly stressed in service.

References 1. ABB Technical guide (2009) Sudden pressure relay. 1ZUA 5663-210-Rev 2-08.09.2009 2. Edvard (2012) Sudden pressure relay in oil-filled power transformers. EEP Electrical Engineering Portal, March 2012. http://electrical-engineering-portal.com/sudden-pressure-relay-in-oilfilled-power-transformer 3. Slade PG. Interpretation of low dc current measurements of a transformer’s winding resistance and the effect of the tapchanger’s contacts, SFIEEE 4. Lewand L. The role of corrosive sulfur in transformers and transformer oil. In: Proceedings of th 69th Annual Int’l Doble Client conference 5. Slade PG. Electrical contacts, 2nd edn. Book. CRC Press, 20171221. VitalBook file 6. Holm R (2000) Electric contacts, theory and applications. Springer, Berlin 7. On-Load Tapchanger Testing Methods, Nada Cincar, Goran Milojevi´c. amforum.org/site/journal/On-Load%20Tap%20Changer%20Testing%20Methods.pdf

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8. Asset management solutions for transformers: Evaluation of the condition of on-load tapchangers by means of performing the TAPSCAN® DRM—dynamic resistance measurement. Werner Fleischmann 9. Erbrink JJ et al (2011) On-load tap changer’s dynamic resistance measurement: settings and interpretation. In: IEEE conference electrical insulation 10. Megger technical brochure. A guide to diagnostic insulation testing above 1 kV. https://www. instrumart.com/assets/Megger-insulationtester.pdf 11. Kuffel, Zaengel, Kuffel (1984) High voltage engineering: book. Pub. Elsevier 12. IEC 60 076 International standard power transformers (To complete) 13. IEEE C57.152 (To complete) 14. Krämer A (2000) On-load tap-changers for power transformers: book. MR Publication. Sect 5.5.2 15. Beckwith Electric Co Inc. (2003) Tech data brochure, digital tapchanger control M-2001C 16. Beckwith Electric Co Inc. Tapchanger controls. Application Note #11 17. Paralleling of LTC transformers by the circulating current method 18. IEEE C 57.139:2015. IEEE guide for dissolved gas analysis in transformer load tap changers, 2015 19. Duval M et al (2006) JTF D1.01/A2.11, Recent developments in DGA interpretation. CIGRE Brochure # 296, June 2006 (available from [email protected]) 20. Duval M. New frontiers of DGA interpretation for power transformers and their accessories. www.seeei.org.il/prdFiles/2922_desc2.pdf 21. Dukarm J (2010) Introduction to Duval’s diagnostic triangles. Presented at the Chemlab Conference Calgary AB – Sep 9–10 2010. Delta-X Research Inc. Victoria BC Canada, engineering.richmondcc.edu/Courses/EUS%20210/Notes/7-Duval-triangles.pdf 22. Arora RK. Different DGA techniques for monitoring of transformers: Global R&D Centre, Crompton Greaves Ltd, Mumbai, India. Email: [email protected] 23. Understanding Dissolved Gas Analysis (DGA) Techniques and Interpretations Jeff Golarz. www.electricenergyonline.com/show_article.php?article=91 24. Golarz J. IEE Online. Guest Editorial Understanding Dissolved Gas Analysis (DGA) Techniques and Interpretations 25. Frotscher R (2008) DGA for MR tap-changers. MR Academy Convention Report, Orlando, pp 119–135 26. Brikci F. Vibro-acoustic testing applied on tapchangers and circuit breakers. Ph.D. http://www. zensol.com/pdf/Article-TechCon-2010_.PDF 27. McPhail D. Condition analysis and assessment of on load tap changer acoustic monitoring principles and techniques. Graduate Electrical Engineer—Ergon Energy, Queensland [email protected] 28. Quebec Hydro TAP-4 On-load tap changer analyzer. www.hydroquebec.com/innovation/en/ innovations.html 29. MR Technical brochure Tapcon 250 30. MR technical brochure Tapguard 31. Eberle Technical brochure System for OLTC Control & Transformer Monitoring REGSys™ (REG-D, PAN-D). https://www.a-eberle.de/en/ 32. ABB Technical brochure RAYA https://library.e.abb.com/…/1MRK504006-BEN_en_RAYA_ _Voltage_regulating_relay/