Electrical Equipment: A Field Guide [1 ed.] 1119768942, 9781119768944

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Electrical Equipment

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Electrical Equipment A Field Guide

B. Koti Reddy

This edition first published 2021 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2021 Scrivener Publishing LLC For more information about Scrivener publications please visit www.scrivenerpublishing.com. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. Wiley Global Headquarters 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no rep­ resentations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchant-­ ability or fitness for a particular purpose. No warranty may be created or extended by sales representa­ tives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further informa­ tion does not mean that the publisher and authors endorse the information or services the organiza­ tion, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Library of Congress Cataloging-in-Publication Data ISBN 9781119768944 Cover image: Pixabay.com and Wikimedia Commons Cover design by Russell Richardson Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines Printed in the USA 10 9 8 7 6 5 4 3 2 1

Contents Foreword xiii Preface xv 1 Introduction 1.1 Introduction 1.2 Electrical Power Supply 1.3 Classification of Voltages or Voltage Bands 1.4 Standards Agencies 1.5 Electrical Standards 1.6 Abbreviations 1.7 Constants 1.8 Types of Maintenance 1.9 Useful Life of Equipment

1 1 1 2 2 12 15 19 19 22

2 Transformers 2.1 Introduction 2.2 Types of Transformers 2.3 Transformer on No Load 2.4 Transformer on Load 2.5 Total Equivalent Circuit of Transformer (Referred to Primary Side) 2.6 Losses in a Transformer 2.7 Efficiency of Transformer 2.8 Parallel Operation of Transformers 2.9 Rating Plate of Transformer 2.10 Information to Be Given to Purchase a Transformer 2.11 Tests on Transformer 2.12 Maintenance of Transformers 2.13 Troubleshooting Chart for Transformers 2.14 Latest Trends Opportunities in Transformer Technology

25 25 27 27 28 29 29 30 31 32 49 52 54 65 65

v

vi  Contents 3 Generators 73 3.1 Introduction 73 3.2 Alternator 73 3.3 Field Poles 74 3.4 Construction of Field Poles 74 3.5 EMF Equation of Alternator 75 3.6 Capability Curve 76 3.7 Design of Alternator 80 3.8 Rating Plates 80 3.9 Voltage Regulation of Synchronous Generator 85 3.10 Excitation 86 3.11 Connections 87 3.12 Neutral Grounding 88 3.13 Cooling 88 3.14 Short-Circuit Ratio (SCR) 89 3.15 Pitch Factor (Kp) or Chording Factor (Kc) 90 3.16 Distribution Factor (Kd) 91 3.17 Leakage Reactance (Xl) 92 3.18 Armature Reaction 92 3.19 Operation of Generator When Connected to an Infinite Bus 93 3.20 Load Sharing of Grid-Connected Alternator 94 3.21 Typical Values of Various Reactances and Time-Constants 94 3.22 Load Characteristics of Alternators 94 3.23 Salient Pole Machine with Two Reaction Theory 97 3.24 Hunting 98 3.25 Stability and Swing Equation 98 3.26 Prime-Mover Rating Plates 99 3.27 Effect of Unbalanced Loads and External Faults 100 3.28 Voltage Regulators 101 3.29 Parallel Operation of Alternators Under Different Conditions 101 3.30 Induction Generator 103 3.31 Doubly Fed Induction Generator 105 3.32 Latest Trends in TG Technology 105 3.33 Maintenance 107 3.34 Fault Finding 107 3.35 Generator Failure Modes 107 3.36 Tests on a Turbo-Generator 107 3.37 Tests on Engine-Driven Generator 110 3.38 Gaps and Research Opportunities 111

Contents  vii 4 Induction Motors 113 4.1 Introduction 113 4.2 Comparison Between Various Types of Motors 113 4.3 Working Principle of 3-Phase Induction Motor 114 4.4 Construction of SCIM 115 4.5 Equivalent Circuit of SCIM 118 4.6 Torque-Speed Curve of SCIM 119 4.7 T-S Curve for SRIM 121 4.8 Torque-Speed Curve of Single-Phase Motor (Split Phase) 122 4.9 Name Plate or Rating Plate of SCIM 122 4.10 Power Stages of Induction Motor 140 4.11 Abnormal Conditions 140 4.12 Starting of Induction Motors 143 4.13 Speed Control of Induction Motors 143 4.14 Deep Cage Induction Motor 143 4.15 Double Cage SCIM 145 4.16 Selection of Motor Power for an Application 146 4.17 Design of Induction Motors 146 4.18 Characteristics of Loads 149 4.19 Circle Diagram 149 4.20 Alignment of Motor with Driven Equipment 152 4.21 Shaft and Bearing Currents in Large Motors 153 4.22 Special Motors for Hazardous/Explosive Areas 154 4.23 Identification of 3-Phase Winding Leads 155 4.24 Tests on Induction Motor 156 4.25 Maintenance 158 4.26 Trouble-Shooting 170 4.27 Heating and Cooling Curves of Induction Motor 170 4.28 Smart Motors 178 4.29 Single-Phase Induction Motors 179 4.30 Information to Be Given to Purchase a 3-Phase Induction Motor 181 4.31 Protection Against Faults 183 4.32 Motors for Electrical Vehicles 183 4.33 Future Scope 184 5 Circuit Breakers and Contactors 5.1 Introduction 5.2 Arcing Phenomenon 5.3 Types of Circuit Breakers

185 185 185 186

viii  Contents 5.4 AC and DC CBs 5.5 DCCB 5.6 CB Contacts 5.7 Selection of CB 5.8 Operation of CBs 5.9 Name Plate of CBs 5.10 Tests on CB 5.11 Information to Be Given to Purchase a CB 5.12 Maintenance of CB 5.13 Contactors 5.14 MCB, MCCB and RCCB

186 186 189 189 192 199 203 204 205 207 215

6 Protection and Measurement Systems 6.1 Introduction 6.2 Desirable Characteristics of Protective Device 6.3 Current Transformer 6.4 Voltage Transformer 6.5 Measuring Instruments 6.6 Multi-Function Meter 6.7 Desirable Characteristics of Meters 6.8 Meter Symbols and Codes 6.9 AVO or Multimeter 6.10 Meter Calibration Reports

225 225 225 226 242 249 253 253 254 254 255

7 Earthing and Lightning 7.1 Earthing 7.2 Earthing, Grounding and Bonding 7.3 System Neutral Grounding 7.4 LV Neutral Earthing 7.5 Types of Earth Electrodes 7.6 Measurement of Earth Resistance 7.7 General Guidelines on Earthing 7.8 Lightning Arrester 7.9 Protection Against Lightning 7.10 Definitions 7.11 Name Plate of LA 7.12 Protective Devices Against Lightning Surges 7.13 Surge Protective Device (SPD) 7.14 Lightning Conductor Size 7.15 Inspection and Maintenance of Lightning Protection System 7.16 Testing of LA

257 257 257 259 260 264 264 269 269 269 271 273 274 274 277 277 278

Contents  ix 8 Fuses 8.1 Introduction 8.2 Terms Used in the Fuse Field 8.3 Cut-Off Characteristic of Fuse 8.4 Fuse Law (Prece’s Law) 8.5 Types of Fuses 8.6 Application Categories and TCC of Fuses 8.7 Discrimination between an Over Current Relay and Fuse 8.8 Semi-Conductor Fuse 8.9 Examples of Selection of Fuse 8.10 Symbols of Fuse Letter Code 8.11 Desirable Characteristics of Fuse 8.12 Tests Recommended on Fuses 8.13 Market Models of Fuses

281 281 281 282 283 284 288

9 Protective Relays 9.1 Introduction 9.2 Terms Used in Relaying 9.3 Types of Protection 9.4 Types of Relays 9.5 Relay Block Diagrams of Three Generations 9.6 IDMT Relay Calculations 9.7 Inverse – Time Over-Current Relays 9.8 Comparison between Three Generation Relays 9.9 Thermal Overload Relays 9.10 Protections of Various Electrical Equipment 9.11 Relay Settings 9.12 Protection System Failure Modes 9.13 Maintenance of Relays 9.14 Field Testing 9.15 Relay Co-Ordination 9.16 Protective Device Numbers 9.17 Challenges and Opportunities

301 301 301 302 302 304 305 306 308 308 310 312 313 315 315 317 323 323

10 Cables and Overhead Conductors 10.1 Introduction 10.2 Conducting Materials 10.3 Cable Insulating Material

329 329 329 330

289 290 291 293 293 294 294

x  Contents 10.4 Construction of Cables 10.5 Overhead Conductor versus Cable 10.6 Comparison between PVC and XLPE Cables 10.7 De-Rating Factors 10.8 Special Cables 10.9 FRLS Cable Properties 10.10 Methods of Cable Laying 10.11 Identification Codes of Cables 10.12 Selection of Cable 10.13 Rule of Thumb for Industrial Work 10.14 Fault Location Methods 10.15 Maintenance on Cables 10.16 Cable Jointing 10.17 Tests on PVC Cables 10.18 Tests on XLPE Cables 10.19 Overhead Lines 10.20 FACTS

330 332 332 333 333 335 336 336 337 338 339 340 341 341 343 345 347

11 Solar Photovoltaics 11.1 Introduction 11.2 Solar Energy 11.3 Forms of Energy Resources 11.4 Solar Spectrum 11.5 Solar Energy Fundamentals 11.6 I-V and P-V Curves 11.7 Solar Photovoltaic Power Plants 11.8 Modelling of PV Modules 11.9 Performance Indicators 11.10 Maximum Power Point Tracking 11.11 Rating Plates of SPP 11.12 Opportunities and Future Scope

353 353 353 354 354 357 362 363 365 366 368 369 369

12 Storage Batteries 12.1 Introduction 12.2 Faraday’s Law of Electrolysis 12.3 Types of Batteries 12.4 Dry Cell 12.5 Technical Terms 12.6 Secondary Batteries 12.7 Lead – Acid Battery 12.8 Nickel-Cadmium Batteries

375 375 375 376 376 379 381 383 391

Contents  xi 12.9 Lithium Batteries 12.10 Latest Trends in Energy Storage Field 12.11 Maintenance 12.12 Some Other Notable Points on Battery 12.13 Batteries for Electric Vehicles 12.14 Smart Battery 12.15 Future Outlook

393 395 397 399 400 402 402

13 Computer Aided Studies in Power Systems (CASiPS) 13.1 Introduction 13.2 E-TAP 13.3 EDSA 13.4 PV Syst 13.5 Power Factory 13.6 Matlab-Simulink

405 405 406 408 409 410 412

14 Lighting 14.1 Introduction 14.2 Definitions 14.3 Type of Lighting Technologies 14.4 Estimation of Illumination 14.5 Recommended Illumination Levels 14.6 Lamps Rating Plate 14.7 Fluorescent Lamp Colour Temperatures

413 413 414 419 419 421 421 430

15 Electrical Safety 15.1 Introduction 15.2 Hazards and Effects of Electric Current 15.3 Electric Shock 15.4 Permit to Work System and Qualification and Training 15.5 Personnel Protective Equipment and Devices

439 439 439 440 442 446

Index 449

Foreword At the outset, I wish to congratulate Mr. B. Koti Reddy, the author of the Electrical Equipment: A Field Guide, for writing this high utility book, bringing out many practical aspects of all major equipments we find in power generation and distribution systems. He has put forth in this book all the practical experience he has acquired in the decades of his service at a government of India organization. The first four chapters of the book deal with the major constituents of a power supply system, generators, transformers and induction motors. These chapters supplement the fundamental theoretical aspects, like development of equivalent circuits, phasor diagrams, etc., of these equipments that are normally dealt with in regular textbooks of our undergraduate programs, with the vital aspects of international standards, testing and maintenance procedures and condition monitoring of these equipments and systems. In Chapters 4 to 9, the salient protective system components which are employed in power systems are dealt with, including circuit breakers, relay systems, current transformers, and potential transformers. Also covered are the philosophy of protection, schemes, operation and their coordination, three-phase winding connections, etc., including their testing and maintenance procedures. Chapter 10 deals with the cables and overhead lines, which are part and parcel of any industrial electricity supply system. Due emphasis has been placed on the selection, testing, fault location and type identification procedures. Nowadays, a lot of importance is being given to generating power from renewable energy sources to counter the challenges of global warming and climate change. In this context, the installation of solar photovoltaic power generation systems has assumed great importance. Also, because most of the renewable energy sources fluctuate and vary according to the time of day and the season, it has become essential to develop, design and install

xiii

xiv  Foreword energy storage systems like the battery energy storage systems to reliably operate the integrated systems. Chapters 11 and 12 deal with all the aspects of photovoltaic systems and battery energy storage systems, respectively. For installing any new industrial or plant electrical system, the design is first carried out in computer simulations using the software tools like ETAP, PSCAD or MATLAB simulink and only then is the hardware design is done. Chapter 13 deals with how these software tools are effectively used in the design of the system. For implementing energy conservation schemes, efficient systems with LED lighting of residential, commercial and industrial complexes have become essential and this is the subject matter of Chapter 14. Finally, the safety aspects of all electrical installations in the residential, commercial, industrial and distribution utility substations are of paramount importance and these are dealt with in Chapter 15. In conclusion, I wish to state that there are very few books available addressing all these practical aspects of electrical system design, installation, testing, maintenance and operation of modern electrical power systems. As a professor who has taught in various educational institutes like IITs and state university colleges, I feel a great need exists for introducing a core course like this at the final year undergraduate level for all electrical power engineering programs. This will facilitate the graduating students becoming readily employable in the field. This can also be a very suitable course for outgoing polytechnic students. Dr. R. Balasubramanian NTPC Chair Professor (Rtd.) Indian Institute of Technology, Delhi

Preface In the past, too much emphasis has been given to basic theory and analytical problems, both in academic texts and handbooks of electrical engineering. Even the in-depth study of these books cannot bridge the gap between the academic curriculum and the work at the application field. Hence there is a need for a suitable guide to work with various electrical appliances at field that will be useful for electrical engineering graduates. The author has been practicing electrical power systems for almost three decades, from selection of electrical equipment, to their operation and maintenance through their commissioning and testing with the advancement of technology and percolation of power electronics, digital controllers, computers and information technology. The face of electrical engineering is changing day by day, which necessitates regular updating, retrofits and up gradations by field practitioners. This requires thorough understanding of field equipment or appliances which thus enables the engineer to understand it practically from the operation and maintenance point of view. This is possible once the field engineer understands any equipment thoroughly by reading the technical specifications from the rating plate. This field book guides practitioners to deal with the above concept and the equipment to be dealt with, which are chosen according to present industry requirements. This book deals with almost all types of electrical appliances in domestic or industrial use. Each chapter deals with one electrical appliance and is arranged in the following order: basic theory, fundamental equations and graphs/curves, description of rating plate, relevant codes and standards, technical specifications and current trends in their technology. This book covers all the topics that are needed for domestic and industry practitioners to apply in their field activities. Tables from relevant International Standards like IEC, BIS, BS, ANSI, NEMA and IEEE are included at required sections of each chapter. In preparation of the manuscript, the author has received valuable help from several working colleagues, well wishers, and wishes to record sincere xv

xvi  Preface thanks to all of them. My special thanks goes to my wife Halini for typing in addition to sparing family time and son Dheeraj, daughter-in-law Sneha and daughter Sreebindu for sacrificing family time in order to bring out this book, and lastly to Prof. R. Balasubramanian for readily accepting and reviewing the book. The author wishes to record his appreciation of valuable guidance given by Mr. Phil Carmical to finalize the title and contents of this book and the excellent work done by Scrivener Publishing to bring out the book in record time. Suggestions are welcome from readers for consideration in future works. B. Koti Reddy

1 Introduction 1.1 Introduction Electrical Power System comprises so many components, appliances and equipment, which are interconnected in a manner that enables them to do an intended job. The major appliances are Generators, Transformers, Motors, Batteries, Conductors, Cables, Switchgears, Protective Relays, Instrument Transformers, Meters and Lamps. This chapter describes the general concept of electrical power systems, various applicable standards, commonly used abbreviations, generally used constants in electrical engineering and types of maintenance to be done on each and every piece of electrical equipment.

1.2 Electrical Power Supply As known to all, electricity is of two types viz., Alternating Current (AC) and Direct Current (DC). AC has both magnitude and periodically reversing direction whereas DC has only magnitude and unidirectional. Both AC and DC have their own advantages and disadvantages. AC is mostly used for distribution due to its advantage of step up and step down with the help of transformers. However DC is used at lower levels right from tiny electronic circuits to motors up to a certain extent and at higher levels for transmission purpose. DC has the upper hand where the service is essential since it is stored in batteries. In early days, DC motors were widely used where fine speed control was required, and when speed control of AC motors was not so simple. With the invention of power electronic devices, it is becoming very easy and simple to control the speed of AC motors. The supply frequency of AC varies from country to country, from 16.7 Hz to 60HZ. The rail systems in Austria, Sweden and some other European countries are using 16.7 Hz frequency, whereas other utilisation is at 50 Hz. The majority of Asian B. Koti Reddy. Electrical Equipment: A Field Guide, (1–24) © 2021 Scrivener Publishing LLC

1

2  Electrical Equipment countries including India are also using 50 Hz. In the United States, the supply frequency is mostly at 60 Hz level.

1.3 Classification of Voltages or Voltage Bands There are different nomenclatures of voltage levels like LV, MV, HV, EHV and UHV, etc., in both AC and DC side, which varies from region to region and country to country. A. Classification of AC and DC Voltages One of the Indian classifications based on AC and DC voltage bands is shown in Tables 1.1 and 1.2, respectively. This is almost equivalent to that of bands prescribed in IEC 60038-Standard voltages. However, there are different nomenclature and usual voltage notations of HV, EHV and UHV which varies from place to place and industry to industry. These classifications will also change from equipment to equipment and asset to asset like overhead lines, cables and converters. B. Different countries voltage and frequencies Various types of voltages (AC&DC) and frequencies used in different countries are shown in Table 1.3.

1.4 Standards Agencies There are different agencies worldwide that frame various standards and guidelines for electrical power systems with respect to generation, transmission, distribution, utilisation, switchgear and protection, measurements, instrumentation, machines, etc. Most of the countries have their own body of standards for almost all the above fields. Sometimes they adopt international standards like IEC, IEEE, and UL, etc. A. List of nations level standards formation agencies A list of some of the national level standards formation agencies are shown here. • Afghanistan – ANSA – Afghan National Standard Authority • Algeria – IANOR – Institut algérien de normalisation

66 KV, 132 KV, 220 KV

400 KV

Three

Three

IIIB

IIIC

The standard frequency shall be 50 HZ +3% in all the above case.

3.3 KV, 6.6 KV, 11 KV, 33 KV

Three

IIIA

Single

Three

240 V

Three

II

III

415 V

Not specified.

I

Preferred nominal AC system voltage

Phase(s)

AC voltage band

420 KV

72.5KV, 145KV, 245 KV Respectively

3.6 KV, 7.2 KV, 12 KV, 36 KV Respectively

264 V

457 V

Highest system voltage

Table 1.1  AC voltage bands for electrical installations including preferred voltages.

380 KV

60 KV, 120 KV, 200 KV Respectively.

3.0 KV, 6.0 KV, 10 KV, 30 KV Respectively

216 V

374 V

Lowest system voltage

+12.5%

+12.5%

+6 and -9%

+6%

+6%

Tolerance on declared voltage

Introduction  3

4  Electrical Equipment Table 1.2  DC voltage bands for electrical installations. DC voltage band

Pole to earth

Between poles

I

< 120 V

< 120 V

II

Above 120 V and up to 900 V.

Above 120 V and up to 1500 V.

• • • • • • • • • • • • • • • • • • • • • • •

Argentina – IRAM – Instituto Argentino de Normalización Australia – SA – Standards Australia Austria – ASI – Austrian Standards Institute Bahrain – BSMD Bangladesh – BSTI – Bangladesh Standards and Bangladesh Standards and Testing Institution Barbados – BNSI – Barbados National Standards Institution Belgium – NBN – Bureau voor Normalisatie/Bureau de Normalisation (formerly: IBN/BIN) Bolivia – IBNORCA – Instituto Boliviano de Normalización y Calidad Brazil – ABNT – Associação Brasileira de Normas Técnicas Brunei Darussalam – CPRU – Construction Planning and Research Unit, Ministry of Development Bulgaria – BDS – Bulgarian Institute for Standardization Canada – SCC – Standards Council of Canada and CSA – Canadian Standards Association Chile – INN – Instituto Nacional de Normalizacion China – SAC – Standardization Administration of China China – CSSN – China Standards Information Center Colombia – ICONTEC – Instituto Colombiano de Normas Tecnicas y Certificacion Costa Rica – INTECO – Instituto de Normas Técnicas de Costa Rica Croatia – DZNM – State Office for Standardization and Metrology Cuba – NC – Oficina Nacional de Normalización Czech Republic – CSNI – Czech Standards Institute Denmark – DS – Dansk Standard Egypt – EO – Egyptian Organization for Standardization and Quality Control El Salvador – CONACYT – Consejo Nacional de Ciencia y Tecnología

Introduction  5 Table 1.3  Voltage and frequencies used in different countries. Country

Single-phase voltage (Volts)

Three-phase voltage (Volts)

Frequency (Hz)

Abu Dhabi

230 V

400 V

50 Hz

Afghanistan

220 V

380 V

50 Hz

Algeria

230 V

400 V

50 Hz

Angola

220 V

380 V

50 Hz

Antigua

230 V

400 V

60 Hz

Argentina

220 V

380 V

50 Hz

Australia

230 V

400 V

50 Hz

Austria

230 V

400 V

50 Hz

Bahrain

230 V

400 V

50 Hz

Bangladesh

220 V

380 V

50 Hz

Barbados

115 V

200 V

50 Hz

Belgium

230 V

400 V

50 Hz

Bermuda

120 V

208 V

60 Hz

Bhutan

230 V

400 V

50 Hz

Bolivia

230 V

400 V

50 Hz

Botswana

230 V

400 V

50 Hz

Brazil

127 V/220 V

220 V/380 V

60 Hz

Brunei

240 V

415 V

50 Hz

Bulgaria

230 V

400 V

50 Hz

Cambodia

230 V

400 V

50 Hz

Cameroon

220 V

380 V

50 Hz

Canada

120 V

120/208 V/240 V/ 480 V/347/600 V

60 Hz

Chile

220 V

380 V

50 Hz (Continued)

6  Electrical Equipment Table 1.3  Voltage and frequencies used in different countries. (Continued) Country

Single-phase voltage (Volts)

Three-phase voltage (Volts)

Frequency (Hz)

China

220 V

380 V

50 Hz

Colombia

110 V

220 V/440 V

60 Hz

Croatia

230 V

400 V

50 Hz

Cuba

110 V/220 V

190 V

60 Hz

Cyprus

230 V

400 V

50 Hz

Czech Republic

230 V

400 V

50 Hz

Denmark

230 V

400 V

50 Hz

Dubai

230 V

400 V

50 Hz

Egypt

220 V

380 V

50 Hz

El Salvador

120 V

200 V

60 Hz

England

230 V

415 V

50 Hz

Fiji

240 V

415 V

50 Hz

Finland

230 V

400 V

50 Hz

France

230 V

400 V

50 Hz

Germany

230 V

400 V

50 Hz

Ghana

230 V

400 V

50 Hz

Great Britain

230 V

415 V

50 Hz

Greece

230 V

400 V

50 Hz

Guatemala

120 V

208 V

60 Hz

Guyana

120 V/240 V

190 V

60 Hz

Haiti

110 V

190 V

60 Hz

Hong Kong

220 V

380 V

50 Hz

Hungary

230 V

400 V

50 Hz

India

230 V

400 V

50 Hz (Continued)

Introduction  7 Table 1.3  Voltage and frequencies used in different countries. (Continued) Country

Single-phase voltage (Volts)

Three-phase voltage (Volts)

Frequency (Hz)

Indonesia

230 V

400 V

50 Hz

Iran

230 V

400 V

50 Hz

Iraq

230 V

400 V

50 Hz

Israel

230 V

400 V

50 Hz

Italy

230 V

400 V

50 Hz

Jamaica

110 V

190 V

50 Hz

Japan

100 V

200 V

50/60 Hz

Jordan

230 V

400 V

50 Hz

Kazakhstan

220 V

380 V

50 Hz

Kenya

240 V

415 V

50 Hz

Korea, North

220 V

380 V

50 Hz

Korea, South

220 V

380 V

60 Hz

Kosovo

230 V

230 V/400 V

50 Hz

Kuwait

240 V

415 V

50 Hz

Laos

230 V

400 V

50 Hz

Lebanon

230 V

400 V

50 Hz

Libya

230 V

400 V

50 Hz

Malaysia

240 V

415 V

50 Hz

Maldives

230 V

400 V

50 Hz

Mali

220 V

380 V

50 Hz

Myanmar

230 V

400 V

50 Hz

Mauritius

230 V

400 V

50 Hz

Mexico

127 V

220 V/480 V

60 Hz

Morocco

220 V

380 V

50 Hz (Continued)

8  Electrical Equipment Table 1.3  Voltage and frequencies used in different countries. (Continued) Country

Single-phase voltage (Volts)

Three-phase voltage (Volts)

Frequency (Hz)

Nepal

230 V

400 V

50 Hz

Netherlands

230 V

400 V

50 Hz

New Zealand

230 V

400 V

50 Hz

Nicaragua

120 V

208 V

60 Hz

Nigeria

230 V

415 V

50 Hz

Norway

230 V

230 V/400 V

50 Hz

Oman

240 V

415 V

50 Hz

Pakistan

230 V

400 V

50 Hz

Peru

220 V

220 V

60 Hz

Philippines

220 V

380 V

60 Hz

Poland

230 V

400 V

50 Hz

Portugal

230 V

400 V

50 Hz

Qatar

240 V

415 V

50 Hz

Romania

230 V

400 V

50 Hz

Russian Federation

220 V

380 V

50 Hz

Saudi Arabia

230 V

400 V

60 Hz

Singapore

230 V

400 V

50 Hz

South Africa

230 V

400 V

50 Hz

Spain

230 V

400 V

50 Hz

Sri Lanka

230 V

400 V

50 Hz

Sweden

230 V

400 V

50 Hz

Switzerland

230 V

400 V

50 Hz

Taiwan

110 V

220 V

60 Hz

Thailand

230 V

400 V

50 Hz (Continued)

Introduction  9 Table 1.3  Voltage and frequencies used in different countries. (Continued) Country

Single-phase voltage (Volts)

Three-phase voltage (Volts)

Frequency (Hz)

Turkey

230 V

400 V

50 Hz

Uganda

240 V

415 V

50 Hz

Ukraine

230 V

400 V

50 Hz

United Arab Emirates (UAE)

230 V

400 V

50 Hz

United Kingdom (UK)

230 V

415 V

50 Hz

United States of America (USA)

120 V

120/208 V/277/ 480 V/120/240 V/ 240 V/480 V

60 Hz

Venezuela

120 V

120 V

60 Hz

Vietnam

220 V

380 V

50 Hz

Yemen

230 V

400 V

50 Hz

Zimbabwe

240 V

415 V

50 Hz

• • • • • • • • • • • •

Ethiopia – QSAE – Quality and Standards Authority of Ethiopia Finland – SFS – Finnish Standards Association France – AFNOR – Association française de normalisation Germany – ◦◦ DIN – Deutsches Institut für Normung ◦◦ DKE – Deutsche Kommission für Elektrotechnik Elektronik Georgia – GEOSTM – Georgian National Agency for Standards, Technical Regulations and Metrology Ghana – GSA- Ghana Standards Authority Greece – ELOT – Hellenic Organization for Standardization Hong Kong – ITCHKSAR – Innovation and Technology Commission Hungary – MSZT – Magyar SzabványügyiTestület India – BIS – Bureau of Indian Standards Indonesia – BSN – BadanStandardisasi Nasional Iran – ISIRI – Institute of Standards and Industrial Research of Iran

10  Electrical Equipment • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Ireland – NSAI – National Standards Authority of Ireland Israel – SII – The Standards Institution of Israel Italy – UNI – EnteNazionaleItaliano di Unificazione Jamaica – BSJ – Bureau of Standards, Jamaica Japan – JISC – Japan Industrial Standards Committee Jordan – JISM – Jordan Institution for Standards and Metrology Kazakhstan – KAZMEMST – Committee for Standardization, Metrology and Certification Kenya – KEBS – Kenya Bureau of Standards Republic of Korea – KATS – Korean Agency for Technology and Standards Kuwait – KOWSMD – Public Authority for Industry, Standards and Industrial Services Affairs Kyrgyzstan – KYRGYZST – State Inspection for Standardization and Metrology Latvia – LVS – Latvian Standard Lebanon – LIBNOR – Lebanese Standards Institution Malaysia – DSM – Department of Standards Malaysia Malta – MSA – Malta Standards Authority Mauritius – MSB – Mauritius Standards Bureau Mexico – DGN – Dirección General de Normas Nepal – NTA – Nepal Telecommunication Authority Netherlands – NEN – Nederlandse Norm, maintained by the Nederlands Normalisatie Instituut (NNI) New Zealand – SNZ – Standards New Zealand Norway – SN – Standards Norway (Standard Norge) Oman – DGSM – Directorate General for Specifications and Measurements Pakistan – PSQCA – Pakistan Standards and Quality Control Authority Peru – INDECOPI – Instituto Nacional de Defensa de la Competencia y de la Protección de la Propiedad Intellectual Philippines – BPS – Bureau of Product Standards Poland – PKN – Polish Committee for Standardization Portugal – IPQ – Instituto Portugues da Qualidade Romania – ASRO – Asociatia de Standardizare din Romania Russian Federation – Rosstandart – Federal Technical Regulation and Metrology Agency Saudi Arabia – SASO – Saudi Arabian Standards Organization

Introduction  11 • Singapore – SPRING SG – Standards, Productivity and Innovation Board • South Africa – SABS – South African Bureau of Standards • Spain – UNE— Asociación Española de Normalizacion y Certificación (AENOR) • Sri Lanka – SLSI – Sri Lanka Standards Institution • Sweden – SIS – Swedish Standards Institute • Switzerland – SNV – Swiss Association for Standardization • Tanzania – TBS – Tanzania Bureau of Standards • Thailand – TISI – Thai Industrial Standards Institute • Turkey – TSE – TürkStandardlariEnstitüsü • United Arab Emirates – ESMA – Emirates Standardization and Metrology Association • United Kingdom – BSI – British Standards Institution • United States of America – ◦◦ ANSI – American National Standards Institute ◦◦ NISO – National Information Standards Organization ◦◦ NIST – National Institute of Standards and Technology • Uruguay – UNIT – Instituto Uruguayo de Normas Técnicas • Venezuela – FONDONORMA – Fondo para la Normalización y Certificacion de la Calidad • Vietnam – TCVN – Directorate for Standards and Quality B. List of International Standards Organizations A list of some of the international-level standards preparation agencies is provided here. These standards are widely used by different countries. • CENELEC – European Committee for Electrotechnical Standardization • AHRI – Air-conditioning, Heating, and Refrigeration Institute • ASME – American Society of Mechanical Engineers • IEC – International Electrotechnical Commission • IEEE – Institute of Electrical and Electronics Engineers • ITU – The International Telecommunication Union • ISO – International Organization for Standardization • CIE – International Commission on Illumination • NFPA – National Fire Protection Association (USA) • UL – Underwriters Laboratories • OSHA – Occupational Safety & Health Administration

12  Electrical Equipment • ANSI –American National Standards Institute • NEMA – National Electrical Manufacturers Association (USA) • CIGRE – The International Council on Large Electric Systems (originally this acronym meant the Conference Internationale des Grands ReseauxElectriques)

1.5 Electrical Standards The list of major standards used internationally in electrical power systems are shown here. There are standards mainly useful in standardisation of electrical equipment globally. • • • • • • • • • • • • • • • • • • • •

IEC 60027 Letter symbols to be used in electrical technology IEC 60028 International standard of resistance for copper IEC 60034 Rotating electrical machines IEC 60038 IEC Standard Voltages IEC 60044 Instrument transformers IEC 60050 International Electrotechnical Vocabulary IEC 60051 Direct acting indicating analogue electrical measuring instruments and their accessories IEC 60061 Lamp caps and holders together with gauges for the control of interchangeability and safety IEC 60071 Insulation co-ordination IEC 60072 Dimensions and output series for rotating electrical machines IEC 60073 Basic and safety principles for man-machine interface, marking and identification – Coding principles for indicators and actuators IEC 60076 Power transformers IEC 60079 Explosive atmospheres IEC 60095 Lead-acid starter batteries IEC 60099 Surge arresters IEC 60127 Miniature fuses IEC 60143 Series capacitors for power systems IEC 60183 Guidance for the selection of high-voltage A.C. cable systems IEC 60188 High-pressure mercury vapour lamps – Performance specifications IEC 60189 Low-frequency cables and wires with PVC insulation and PVC sheath

Introduction  13 • IEC 60192 Low-pressure sodium vapour lamps – Performance specifications • IEC 60204 Safety of machinery – Electrical equipment of machines • IEC 60205 Calculation of the effective parameters of magnetic piece parts • IEC 60211 Maximum demand indicators, Class 1.0 • IEC 60212 Standard conditions for use prior to and during the testing of solid electrical insulating materials • IEC 60214 Tap-changers • IEC 60216 Electrical insulating materials – Thermal endurance properties • IEC 60228 Conductors of insulated cables • IEC 60243 Electric strength of insulating materials • IEC 60255 Measuring relays and protection equipment • IEC 60269 Low-voltage fuses • IEC 60270 High-voltage test techniques – Partial discharge measurements • IEC 60282 High-voltage fuses • IEC 60335 Household and similar electrical appliances – Safety • IEC 60364 Low-voltage electrical installations • IEC 60376 Specification of technical grade sulfur hexafluoride (SF6) for use in electrical equipment • IEC 60377 Methods for the determination of the dielectric properties of insulating materials at frequencies above 300 MHz • IEC 60379 Methods for measuring the performance of electric storage water-heaters for household purposes • IEC 60417 Graphical symbols for use on equipment • IEC 60422 Mineral insulating oils in electrical equipment – Supervision and maintenance guidance • IEC 60502 Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um = 1.2 kV) up to 30 kV (Um = 36 kV) • IEC 60567 Oil-filled electrical equipment – Sampling of gases and analysis of free and dissolved gases – Guidance • IEC 60598 Luminaires • IEC 60599 Mineral oil-filled electrical equipment in service – Guidance on the interpretation of dissolved and free gases analysis

14  Electrical Equipment • • • • • • • • • • • • • • • • • • • • • • • • • •

IEC 60909 Short-circuit currents in three-phase a.c. systems EC61000 Electromagnetic compatibility (EMC) IEC61029 Safety of motor-operated transportable tools IEC61039 General classification of insulating liquids ISO3 Preferred numbers (voltages, currents, KVA, etc.) ISO281 Rolling bearings – dynamic load ratings and rating life ISO2372 Mechanical vibration of machines with operating speeds from 10 to 200 rev/sec. Basis for specifying evaluation standards ISO3046/IV Reciprocating internal combustion engines: Performance BS116 Oil circuit breakers (for alternating current systems above 1 kV) BS5311 High-voltage alternating current circuit breakers BS162 Electric power switchgear and associated apparatus BS170{BS5000: 2} Rotating electrical machines of particular types or for particular applications BS229 Flameproof enclosure of electrical apparatus BS4683: 3Electrical apparatus for explosive atmospheres BS5311 High-voltage alternating current circuit breakers IEEE32 Standard requirements, terminology and test procedures for neutral grounding devices IEEE80 IEEE guide for safety in AC substation grounding IEEE81 IEEE guide for measuring earth resistivity, ground impedance and earth surface potentials of a ground system IEEE112 IEEE standard test procedure for poly phase induction motors and generators IEEE344 IEEE recommended practice for seismic qualification of class 1Eequipment for nuclear power generating stations IEEE519 Recommended practices and requirements for harmonic control in electrical power systems IEEE979 IEEE guide for substation fire protection IEEE1100 Recommended practice for powering and grounding sensitive electronic equipment IS:732 - 1989 Code of practice for electrical wiring installations IS: 4648 - 1968 Guide for electrical layout in residential building IS:8061 - 1976 Code of practice for design, installation and maintenance of service lines up to and including 650V

Introduction  15 • IS:10118-1982 Code of practice for selection, installation and maintenance of switchgear and control gear • IS: 3043 -1987 Code of practice for earthing

1.6 Abbreviations The main abbreviations used frequently in electrical power systems are given here. A Amperes AC Alternating current or voltage ACB Air circuit breaker ACSR Aluminum conductor steel reinforced Ah Ampere-hour capacity of batteries AVR Automatic voltage regulator AWA Aluminum wire armor AWG American Wire Gauge B or b Electrical suceptance Bar or bar Pressure in atmospheres BEV Battery electric vehicle BIL Breakdown insulation level / Basic insulation level Btu British thermal unit C Electrical capacitance CACA Closed air circuit, air cooled CACW Closed air circuit, water cooled CB Circuit breaker CCR Central control room CFL Compact Fluorescent Light or Compact Fluorescent Lamp Cos φ Power factor CSI Current source inverter CT Current transformer db(A) Measurement unit of sound, Decibels, absolute DC Direct current or voltage DCS Distributed control system DOL Direct-on-line starter E Earth or ground ELCB Earth leakage circuit breaker EMA Ethylene methyl acrylate EMC Electro-magnetic compatibility EMI Electro-magnetic interference

16  Electrical Equipment EMF Electro motive force EOL End of life Ex Certification symbol for hazardous area equipment EV Electric vehicle F, or Hz Frequency Freq. Frequency FCEV Fuel Cell Electric Vehicle FAT Factory acceptance testing Figure Figure G Electrical conductance, or ground GIS Gas insulated switchgear GOR Gas operated relay GT Generator transformer GTO Gate turn off thyristor GFCI Ground Fault Circuit Interrupter circuit breaker H Solar insolation H or hr Hour HCL Hydrogen chloride acid HRC High rupturing capacity HV High voltage, above 600 volts HVAC Heating ventilation and air conditioning Hz Frequency in cycles per second, or hertz I Current in amperes ICE Internal combustion engine IDMT Inverse definite minimum time I/O Input or output signals or quantity inst. Instantaneous quantity IP Ingress protection code IR Insulation resistance J Energy in joules or Newton-meters δ Current density, amps/mm2 kA Kilo-amperes kg Kilogram km Length in kilometers kmph Kilometers per hour kV Kilo-volts kVA Kilo-volt-amperes kVAr Reactive kilo-volt-amperes kW Kilowatts L Electrical inductance in henries

Introduction  17 LED Light emitting diode LEL Lower explosive limit LF Low frequency Li-ion Lithium Ion (battery) LPG Liquified petroleum gas LV Low voltage L1-L3,N Line & neutral voltages/currents mA mille-amperes MB High-pressure mercury, without phosphor coating MBF High-pressure mercury, with phosphor coating MCB Miniature circuit breaker MCC Motor control centre MCCB Molded case circuit breaker MESG Maximum experimental safe gap Mho Unit of electrical admittance MIC Minimum ignition current mm Length or dimension in mille meters MMF Magneto-motive force MMI Man–machine interface MOCB Minimum oil circuit breaker MTBF Mean time between failures MTTR Mean time to repair MV Medium voltage mV mille volts MVA Mega-volt-amperes MVAr Reactive mega-volt-amperes MW Megawatts N Speed in RPM or RPS N Neutral NC Normally closed contact NDE Non-drive end NEMA The National Electrical Manufacturers Association (USA) NEC National Electric Code NGR Neutral grounding resistor NGT Neutral grounding transformer NFPA National Fire Protection Association (USA) Ni Nickel metal NO Normally open contact O/C Over current OCB Oil circuit breaker

18  Electrical Equipment OFAF Oil forced air forced OFAN Oil forced air natural O/L Overload ONAF Oil natural air forced ONAN Oil natural air natural OSHA Occupational Health and Safety Administration (USA) O/V Overvoltage P or W Active power, watts PCC Point of common connection Pb Lead metal PF Power factor Ph Phases of an electrical circuit PHEV Plug-in hybrid electric vehicle PLC Programmable logic controller PTFE Polytetrafluoro thylene pu Per unit PV Photo-voltaic PVC Polyvinylchloride PWM Pulse width modulation Q Reactive power, volt-amperes-reactive R Electrical resistance REF Restricted earth fault RLA Residual life assessment RMS Root mean square RMU Ring main unit RTD Resistance temperature detector RTU Remote terminal unit S Apparent power in volt-amperes or slip S/C Short circuit SCADA Supervisory control and data acquisition system SCR Silicon controlled rectifier SF6 Sulphur hexa fluoride SON-T Clear tubular outer bulb, single-ended SV lamp SPV Solar photo-voltaic ST Station transformer SWGR Switchgear Sync Synchronizing TEFC Totally enclosed fan cooled Temp. Temperature

Introduction  19 TG Turbo-generator TPN Three-phase and neutral power supply Tx Transformer U/C Undercurrent UAT Unit auxiliary transformer UEL Upper explosive limit UPS Uninterruptible power supply unit U/V Under voltage U Voltage or volts VA Volt-amperes VCB Vacuum circuit breaker VSI Voltage source inverter VSD Variable speed drive VT Voltage transformer w or ω Frequency in radians per second wdg. Winding of a machine or transformer X or x Electrical reactance XLPE Cross-linked polyethylene Y or y Electrical admittance Z or z Electrical impedance

1.7 Constants A list of most of the constants used in electrical engineering is shown in Table 1.4.

1.8 Types of Maintenance Any equipment needs to be maintained in order to get satisfactory performance from it during its lifetime. Maintenance is basically of two types, Preventive Maintenance (PM) and Breakdown Maintenance (BM). It is illustrated in Figure 1.1. Preventive maintenance is done to avoid breakdowns and to extend the life of equipment. On the other hand, breakdown maintenance is done to bring back failed equipment on faults. The latest concept is No– maintenance instead of preventive maintenance. For some of the equipment (e.g., a 5 HP motor), the cost of regular time-based maintenance is more than that of replacing a motor on failure.

20  Electrical Equipment Table 1.4  List of constants. Name

Symbol

Value

1 Ton refrigeration

RT

3.5168525 kilowatts, 3023.95 kcal/h

Boltzmann

k

1.38 x 10-23 J/K

Electron Volt

eV

1.602 176 53 x 10-19 J

Electron Charge

e

1.602 176 53 x 10-19 C

Electron rest mass

me

9.109 x 10-31 kg

Faraday constant

F

9.649 x 104 C mol-1

Permeability of free space

μ0

4π x 10-7H/M

Permittivity of free space

ε0

8.854 x 10-12 F/M

Speed of light in vacuum

C

2.9979 x 108 m/s

Planck

h

6.626 069 3 x 10-34 J·s

Proton rest mass

mp

1.6726 x 10-27 kg

Density of Iron

ρ, D

7.8 g/cc

Density of Copper

ρ, D

8.94 g/cc

Density of Aluminum

ρ, D

2.73 g/cc

Density of PVC

ρ, D

1.03 g/cc

Resistivity of Copper

ρ

1.72 x 10-8 Ω -M

Resistivity of Aluminum

ρ

2.8 x 10-8 Ω -M

Resistivity of Silver

ρ

1.64 x 10-8 Ω -M

The following terms need to be remembered in the maintenance field: A. Availability It is the probability that the equipment is operating satisfactorily when it is required for use. It depends on reliability and maintainability, which are described in the next paragraph. It is function of RMS, i.e., Reliability, Maintainability and Supply effectiveness.

Introduction  21 Maintenance

Planned Maint. or Preventive Maint. Scheduled Maint.

No-Maint. (Replace faulty equipment)

Condition Based Maint.

Predictine Maint.

Opportunity Un-planned Maint. Maint. or Break down Maint or Corrective Maint.

Reliability Centered Maint

Figure 1.1  Types of maintenance.

B. Reliability It is the probability of any equipment or its components to satisfactorily perform the intended work for a desired period of time without failure, under the specified operating conditions. It is a probability-based concept of value lying between 0 and 1. For example, an equipment with 90% reliability indicates a 10% failure chance. It is measured as MTTF. C. Maintainability It is the average time taken to repair an equipment after a failure. It is the probability that maintenance of equipment will retain the equipment or restore it to a specified condition within a given time after a failure or maintenance work. It is the combined qualitative and quantitative characteristic of material design and installation. D. MTTF (Mean Time to Failure) It is used for equipment which fails and the equipment cannot be repaired but can be replaced. MTTF = 1/ʎ where ʎ = failure rate, e.g, 0.00005 per hour E. MTBF (Mean Time Between Failure) It is the average time between two consecutive failures, i.e., time from one failure to another failure. It is used for repairable equipment or parts. Also, MTBF = MTTF when the repair time is negligible. MTBF = Total operating hours in a time period, i.e., up time/Total number of failures happened in their period (i.e. breakdown).

22  Electrical Equipment

1.9 Useful Life of Equipment Every electrical equipment has its own shelf-life, based on its design and duty. The main part of the equipment that degrades with time is insulation. Also the operating conditions like ambient temperature, loading pattern, power supply quality and the preventive maintenance done on appliances decides their trouble-free operation. The approximate useful life of most of electrical equipment is shown in Table 1.5. Table 1.5  Useful life of equipment. Description of assets

Fair life (years)

Plant and machinery in generating stations including plant foundations:i) Hydro-electric

35

ii) Steam electric

25

iii) Diesel-electric and gas plant

15

Transformers (including foundations) having a rating of 100 KVA and over

25

Switchgear including cable connections

25

Synchronous condenser

35

Batteries

5

Underground Cable including joint boxes and disconnected boxes

25

Cable duct system

50

Lines on fabricated steel operating at nominal voltages higher than 66 KV.

35

Lines on steel supports operating at nominal voltages higher than 13.2 KV but not exceeding 66 KV

25

Lines on steel or reinforced concrete supports

25

Lines on treated wood supports

25

Meters

15 (Continued)

Introduction  23 Table 1.5  Useful life of equipment. (Continued) Description of assets

Fair life (years)

Self-propelled vehicles

5

Office furniture and fittings

15

Internal wiring including fittings and apparatus

15

Street light fittings

15

Motors

15

2 Transformers 2.1 Introduction A Transformer plays an important role in societal development. Electrical power generated at one place transforms a minimum of two to three times before reaching the end point of use. Global electricity demand is expected to be around 38,700 TWH by 2050, with the estimated population of 9.8 billion, which was 7.8 billion in 2017. It is the most efficient and reliable device in electrical power systems, particularly in the present era of Industrial Revolution 4.0 (RI 4.0). A transformer is basically a static device which transfers electrical power from one circuit (generally called primary winding) to another circuit (generally called secondary winding). In general, there are no rotating parts in a transformer. A transformer works on the principle of electromagnetic induction. When an alternating voltage is applied to the primary, an e.m.f, e2, is induced in secondary as per (2.1).



e2 =

d∅1M dt

(2.1)

Where Ø1M = Flux produced by primary winding that links with secondary winding. When the secondary circuit is closed, current will circulate in load thus causing transfer of energy from primary to secondary. The magnetic core in between the two windings will cause more flux linkages. The mutual flux is almost constant if the applied primary voltage is constant. The e.m.f induced in each turn of any winding, i.e., the average rate of change of flux,



Et =

∅m 1 4f

(2.2)

B. Koti Reddy. Electrical Equipment: A Field Guide, (25–72) © 2021 Scrivener Publishing LLC

25

26  Electrical Equipment Where, Øm= Maximum flux produced in core in Weber, as shown in Figure 2.1. = BmA where Bm= Maximum flux density in Wb/m2. A = Core area (m2) f = frequency in Hz



RMS value of induced emf per turn = 1.11 x 4 Ømf = 4.44Ømf

(2.3) where 1.11 = Form factor of sinusoidal waveform



E1 = EMF induced in Primary per phase = 4.44ØmfN1 (2.4)



E2 = EMF induced in Primary per phase = 4.44ØmfN2 (2.5)

where N1 = No. of turns on Primary winding N2 = No. of turns on Secondary winding Also:





i. Voltage transformation ratio: K =

N1 E1 = N 2 E2

(2.6)

Here, E1 and E2 are phase values in 3-phase system. If K >1, then it is a Step up Transformer and If K 12.5



Loading on the transformer-1 = Total Load KVA*(Z2/Z1+Z2)



Loading on the transformer-2 = Total Load KVA*(Z1/Z1+Z2)

(2.16) (2.17)

Where Z1, Z2 = % impedances of T1 and T2 respectively. Generally, transformers with lower %Z shares more load that that of higher %Z impedance rating. iv. Vector Group or Connection symbol: This is a connection symbol used to inform about type of connection of 3-phase windings and angle. The information is required in parallel operation. Since same vector grouped Transformers only to be connected in parallel. –– ––

The type of connection of high-voltage (HV) winding is denoted by a capital letter, like D = delta, Y = Star, Z = Zig-Zag connection and A = Auto Transformer. Similarly the type of connection of low voltage (LV) winding is denoted by a small letter, like d = delta, y = Star, z = Zig-Zag connection and a = Auto Transformer.

36  Electrical Equipment ––

The phase angle between HV and LV induced EMF’s is given by a clock number and to be specified in anti-clockwise direction like 11° clock means LV leads HV by 30°.

LV connection HV connection

VC A1

Dyn 11

B1 C1

Phase Neutral displacement connection

A2 B2 C2

a2

a1

b2 c2

b1 c1

Vc’

n

VA

Vb’

Va’

VB

Figure 2.11  Vector symbol Dyn11.

Windings and terminals

Symbol Dy1 –30º

n

A2 B2 C2

Yd1 –30º

D y 11 +30º

vector diagrams

A1 a1

a2

B1 b1

b2

C1 c1

c2

c1 n

C1. A2

B2 C2

C1 c 1

1

A2

n A1 a1

B2

B1 b1

C2

C1 c1

B2 C2

a1. c2

A1

A1 a 1 B1 b

Y d 11 +30º A2

b1

B1. C2

N A2

a2

A1. B2

N A1 a1

a2 b2 c2

b2. c1

C1

b1. a2

B1 a1

A1. C2

b1

a2

n

b2 c2

C1 B2

a2

B1 b 1

b2

C1 c1

c2

Figure 2.12  Main vector groups.

N

A2. B1

c1 a1. b2

A1

b1. c2

N C1

B1

a2. c1

Transformers  37 Table 2.3  Possible connections. Phase shift (o)

Connection

0

Yy0

Dd0

Dz0

180 lag

Yy6

Dd6

Dz6

30 lag

Yd1

Dy1

Yz1

30 lead

Yd11 Dy11

Yz11

60 lag

-

Dd2

Dz2

60 lead

-

Dd10 Dz10

120 lag

-

Dd4

Dz4

120 lead

-

Dd8

Dz8

150 lag

Yd5

Dy5

Yz5

150 lead

Yd7

Dy7

Yz7

Others like: 1 = 1° clock, i.e., LV lags HV by 30°, 0 = 12° clock, i.e., LV and HV are in phase, 6 = 6° clock, i.e., LV and HV opposite by 180°. Example: Dyn11 connection with all details is shown in Figure 2.11. Here the HV is delta connected and LV is star connected with neutral out and the LV e.m.f leads HV e.m.f by 300°. There are four main groups which are commonly used at field, as shown in Figure 2.12. The possible connections are shown in Table 2.3. v. Standard reference: The standard followed for design, construction and testing is mentioned. It can be either Indian Standard like IS 2026-1: 2011by Bureau of Indian Standards or IEC like IEC-60076-1. vi. Year of manufacturing: This is required to know the age of the transformer at any time and is also helpful in attending regular preventive maintenance and overhauling systematically. vii. Type (Core): It is required to know whether it is core type (of CRGO, i.e., Cold Rolled Grain Oriented or Amorphous) or shell type. CRGO core type is used for normal power and

38  Electrical Equipment distribution purpose whereas shell is the type used for larger power transformers. Amorphous material is latest in which core losses are very low and is presently used for distribution purpose. Core is generally made with CRGO silicon Steel (Cold Rolled Grain Oriented) sheets in the range of 0.27 to 0.35 thickness. To reduce the Iron loss, the thinnest (0.27 mm) plate is to be used. This will ensure lower core loss with conventional CRGO silicon steel. The main parts of a transformer with core formation are shown in Figures 2.13 and 2.14, respectively. CONSERVATOR EXPLOSION VALVE

BUCHHOLZ RELAY

DIAPHRAGM OIL LEVEL INDICATOR

OIL INLET VALVE

HT TERMINAL

L.T.PORCEL AIN BUSHES

BREATHER

TEMPERATURE GAUGE TAP CHANGER MANUALLY OPERATED

AIR OIL

COOLING TUBES TANK

EARTH

WHEEL OIL OUTLET DRAIN COCK EARTH

Figure 2.13  Parts of transformer.

Figure 2.14  Core formation.

Transformers  39 viii. Voltage at No-load: Here the line to line voltages of both HV (such as 11000 Volts) and LV (such as 433 Volts) windings are given at rated tap. Higher voltages cannot be given to primary side since it maybe leads to winding insulation failure. ix. Current: The rated current of both HV and LV windings are given. This is the maximum value to which the transformer can be loaded beyond which the temperature will go too high and damage windings, wires and insulations. x. Phases and frequency: The number of phases of HV and LV windings (1 or 3 in general) and supply frequency (50 or 60 Hz in general) are given. Transformers are to be charged with voltage of rated frequency only, otherwise it may lead to over fluxing or higher core losses. xi. Number of windings: Generally two windings, i.e., primary (to which supply is given) and secondary (from which the supply is taken out). There are some three-winding transformers having Primary, secondary and tertiary. The tertiary winding is either for other voltage or for harmonic compensating delta winding in case of both primary and secondary in star connected mode. Generally the LV winding is nearer to core and then HV winding which helps in better insulation, easy tapings, supports windings during short circuits. xii. Temperature (in °C): The value of ambient temperature (generally either 40°C or 50°C) and permissible temperature rise of oil (in case of immersed windings) and winding. These are the values for monitoring and setting of oil and winding temperature gauges for alarm and trip. xiii. Mass: The weight of total transformer oil (in case of oil-filled transformer), un-tanking and a core windings are given for foundation and transformation of tank purpose. xiv. Tap switch: It used for voltage regulation purpose by maintaining EMF per turn as constant. The tap changer is used to add or remove some turns from winding (generally HV) to maintain constant voltage at other side (generally LV). These are

40  Electrical Equipment Line

7 5 3

8 6 4 2

1 B

A

Neutral

Figure 2.15  OLTC.

A HV Winding

B

C

b

c

3 4 5 6 7 8

a LV Winding

Figure 2.16  OCTC.

either On-Load Tap Changer (OLTC) or Off-Circuit Tap Changer (OCTC). The taps and their switch position with voltages will be diagrammatically shown on name plate and a sample OLTC and OCTC are show in Figures 2.15 and 2.16, respectively. xv. Type of cooling: In case of liquid-filled transformers, the type of cooling is indicated by letters.

Transformers  41 ONAN = Oil Natural Air Natural ONAF = Oil Natural Air Forced OFAF = Oil Forced Air Forced It is known that each forced cooling will increase the capacity by 15 to 25% due to better heat dissipation. e.g., A 16 MVA transformer with ONAN cooling can cater the loads up to 20 MVA without exceeding its temperature limits if oil forced cooling (i.e., ONAF) is provided. Note: Other than the details mentioned above, it may contain additionally the name of the manufacturer, Current transformer (for winding temperature and neutral CT for earth fault protection) etc. xvi. Transformer oil It works as a coolant, i.e., to dissipate the heat in the transformer and also as insulator between windings, core and body. It also conserves the metal parts from corrosion by submerging active metal parts in it. Dry type transformers are available up to 33 KV rating. Most of the oil used is refined mineral oil. It can be either Paraffin based or Naphtha based. Naphtha oil is more easily oxidized, but the sludge, i.e., the product of oxidation, is easily soluble. Whereas in paraffin oil, the oxidation rate is less but the sludge cannot be soluble easily and settles at the bottom of the tank and obstructs cooling. The approximate quantity required for a transformer depends on the power rating and the amount of heat to be dissipated. As a rule of thumb, it is 1 liter per KVA up to 1600 KVA, 0.3 to 0.5 Liter/KVA up to 80 MVA and 0.6 liter per KVA for transformers of rating above 80 MVA. Also the amount of Silica gel required in breathers is 0.1 to 0.2% of transformer oil (approximately). A. The required technical specifications of new insulating oil, as per IEC 60296 are shown in Table 2.4. B. Tests on transformer oil in service The oil in service will be subject to different loading cycles and ambient conditions and loses its properties with time. The desired properties of oil in service, with action to be taken for degraded oil, are shown in Table 2.5.

42  Electrical Equipment Table 2.4  Characteristics of new oil. S. no.

Characteristic

Unit

Limiting value

1

Appearance

-

Clear and Bright.

2

Density at 20oC

kg/dm³

0.895 Max.

3

Kinematic Viscosity@ 40oC

CST

12 Maximum

4

Flash Point

o

C

135 Minimum

5

Pour Point

o

C

-40 Max

6

Electrical Strength

kV

30 Min(BDV)

7

Tan Delta@90oC

-

0.005 Max

8

Oxidation Stability 164 Hrs, @100oC

i

Neutralization Value

Mg. KOH/g

1.2 Max

ii

Sludge Content

Mass%

0.8 Max

9

Sulphur Content

%

No content

10

Oxidation Inhibitor Content, Max

%

Not detectable

11

Water Content

ppm

40 Max

12

Antioxidant additives

%wt

Not detectable

13

Interfacial Tension at 25°C

N/cm

40 Min

14

Gassing tendencies at 50HZ after 120 min

mm³/min

Not Required

15

Total furan

mg/kg

0.1 Max

16

Total PCB content

mg/kg

Not detectable

17

Polycylic aromatic

% Mass

0.3 Max

C. Maintenance of oil: The oil to be tested at regular intervals, preferably once in a year for different other properties is as follows: ■■ Dissolved Gas Analysis (DGA) ■■ Furan Analysis (FA)

Transformers  43 Table 2.5  Tests on transformer oil in service. Value as per IS:1866 Permissible limits

To be reconditioned

To be replaced

Electric Strength (Breakdown voltage) a. Below 72.5 kV b. 72.5 kV and less than 145 kV c. 145 kV and above

Min. 30kV 40kV 50kV

Less than the value specified in column 3.

-

2.

Specific resistance (Resistivity) Ohm/ cm at 27°C

Above 10 x 1012

Between 1 x 1012 to 10 x 1012

Below 1 x 1012

3.

Water content Below 145kV Above 145kV

Max. 35ppm 25ppm

Greater than the value Specified in column 3

-

4.

Dielectric dissipation factor, (tan delta) at 90°C

0.01 or less

Above 0.01 to 0.1

Above 0.1

5.

Neutralization value in mg KOH/g of oil

0.5 or less

Above 0.5

Above 1.0

6.

Interfacial tension N/m at 27°C

0.02 or more

0.015 and above but below 0.02

Below 0.015

7.

Flash point in °C

140 or more

125 and above but below 140

Below 125

8.

Sludge

Nondetectable

Sediment

Perceptible sludge

S. no.

Tests

1.

a. Dissolved gas analysis (DGA) of the oil of a transformer in operation is a specialized technique to assess the internal condition of the transformer. It is also one of the most useful techniques to detect incipient faults in liquid-filled transformers. DGA is performed by Gas Chromatography. The combinations of gas levels

44  Electrical Equipment Table 2.6  Range of gas levels (all values in PPM). Gas/Service age

0-4 years

4-10 years

10 years

Methane (CH4)

10 – 30

30 – 80

30 – 130

Ethane (C2H6)

10 – 30

30 – 50

30 – 110

Ethylene (C2H4)

10 – 30

30 – 50

50 – 150

Acetylene (C2H2)

10 – 16

10 – 30

10 – 40

Hydrogen (H2)

20–150

150 – 300

200 – 500

Carbon monoxide (CO)

00–300

300 – 500

500 – 700

Carbon dioxide (CO2)

3000–4000

4000 – 5000

4000 – 10,000

for different types of faults, based on age of transformers, are shown in Table 2.6. ■■ Gas Ratios There are different theories and methods which give approximate information about the condition of oil. Some of these methods are IEC ratio code, Roger’s method and Duval triangle method. One of the most frequently used DGA methods is Roger’s method, which is shown in Table 2.7. b. Furan Analysis (FA) When the cellulose molecule of paper is de-polymerized, furans are formed in oil. The extent of degradation of paper insulation can be identified with the help of furan analysis of oil. The content of 2-furaldehyde in oil relate to the average deterioration of the insulating paper. The amount wof furan compounds and condition of insulation is given in Table 2.8.

Transformers  45 Table 2.7  Roger’s method of hydrocarbon gas ratios. CH4H2

C2H6-CH4

C2H4C2H6

C2H2C2H4

0

0

0

0

Diagnosis If methane/hydrogen less than 0.1 - partial discharge. Normal deterioration

1

0

0

0

Slight overheating below 150°C

1

1

0

0

Slight overheating 150°C - 200°C

0

1

0

0

Slight overheating 200°C - 300°C

0

0

1

0

Normal conductor overheating

1

0

1

0

Circulating currents and/or overheated joints

0

0

0

1

Flashover without power follow through.

0

1

0

1

Tap changer selector breaking current.

0

0

1

1

Arc with power follow through or Persistent arcing.

Table 2.8  Range of furan values and condition. Furan level (PPM)

Status

0-0.1

Healthy transformer

0.1-1

Moderate deterioration

1-10

Excessive deterioration

>10

End of Life (EOL)

D. Degree of Polymerization (DOP) and Furan Analysis: Life assessment based on DOP and Furan Analysis is given in Table 2.9. Figure 2.17 shows the relationship between DOP and Remaining life or residual life of transformer.

Fural dehyde (2FAL) in PPB

58

100

150

200

300

400

500

600

700

800

S. no.

1

2

3

4

5

6

7

8

9

10

475

492

511

533

561

597

647

683

733

801

Degree of polymerisation (DP)

Table 2.9  LA based on DOP and FA.

62

64

67

70

74

78

84

88

93

99

Estimated percentage of remaining

25

26

27

28

30

31

34

35

37

40

Remaining life (Max=40 years)

(Continued)

Accelerated aging rate

Normal aging rate

Suggested interpretation

46  Electrical Equipment

Fural dehyde (2FAL) in PPB

900

1000

1500

2021

2500

3000

4000

5000

7237

S. no.

11

12

13

14

15

16

17

18

19

202

248

275

311

334

360

397

447

460

Degree of polymerisation (DP)

Table 2.9  LA based on DOP and FA. (Continued)

0

15

22

31

36

42

49

57

60

Estimated percentage of remaining

0

6

9

12

15

17

20

23

24

Remaining life (Max=40 years)

End of expected life

High risk of failure

Excessive aging. Danger zone.

Suggested interpretation

Transformers  47

48  Electrical Equipment 120 100

90

80

79 66 50

60 46

42

40 20

800

700

600

500

400 DP

380

360

% Remnant life

100

0 340

Figure 2.17  DP versus RLA of transformer.

E. Alternate Oils: Mineral oils have proven reliable as an insulating medium but they have certain disadvantages: –– Flammability can cause heavy damage to adjacent equipment and buildings. –– Environmentally unfriendly and non-biodegradable – spilled oil is like a toxic waste and if that escapes into water, it is highly harmful to the environment. –– Shortened insulating paper life – water, which is soluble in oil, trapped in the paper shortens the life of the paper and the transformer. –– Later on Synthetic oils like Askarels were used due to higher flash point but they are harmful to the environment due to which they are almost phased out. –– Esters are a broad class of organic compounds available from agricultural products (natural esters) or chemically synthesized from organic precursors (synthetic esters). Due to their high cost compared to other less flammable fluids, synthetic fluids are generally limited to use in traction and mobile transformers, and other specialty applications. –– Vegetable oils: Now lots of research is going on vegetable oil suitability for use in transformers. Being renewable and environmental friendly, they are gaining momentum for their better use in transformers. Some important properties of vegetable oils are shown in Table 2.10.

Transformers  49 Table 2.10  Vegetable oils vs. mineral oil. Property

Mineral oil

Silicone oil

Synthetic ester

Soya bean oil

Coconut oil

Palm oil

BDV (KV)

50

40

43

39

60

25

Moisture 0.15 Content(%)

0.1

0.1

0.2

0.1

0.19

Pour point(oC)

-40

-55

-50

-1

23

15

Flash point(oC)

140

295

270

234

225

242

Density (g/cc)

0.89

0.87

0.97

0.92

0.92

1.5

Viscosity (CST) at 40oC

9.2

37

29

35

29

30

2.10 Information to Be Given to Purchase a Transformer The following minimum information shall be given by the user while purchasing a new transformer. i.

Rating and general data: The following normal information shall be given in all cases: a. Particulars of the specifications to which the transformer shall comply like IEC; b. Kind of transformer, for example, separate winding transformer, auto-transformer or booster transformer; c. Single or three-phase unit; d. Number of phases in system; e. Frequency; f. Dry-type or oil-immersed type. If oil immersed type, whether mineral oil or synthetic insulating liquid. If dry-type, degree of protection; g. Indoor or outdoor type; h. Type of cooling;

50  Electrical Equipment i. Rated power for each winding and, for tapping range exceeding +5%, the specified maximum current tapping, if applicable. If the transformer is specified with alternative methods of cooling, the respective lower power values are to be stated together with the rated power (which refers to the most efficient cooling); j. Rated voltage for each winding; k. For a transformer with tappings: a. which winding is tapped, the number of tappings, and the tapping range or tapping step; b. whether “off-circuit” or “on-load” tap changing is required; c. if the tapping range is more than +5%, the type of voltage variation, and the location of the maximum current tapping, if applicable. l. Highest voltage for equipment (Um) for each winding [with respect to insulation]; m. Method of system earthing (for each winding); n. Insulation level, for each winding; o. Connection symbol and neutral terminals, if required for any winding; p. Any peculiarities of installation, assembly, transport and handling. Restrictions on dimensions and mass; q. Details of auxiliary supply voltage (for fans and pumps, tap-changer, alarms, etc); r. Fittings required and an indication of the side from which meters, rating plates, oil-level indicators, etc, shall be legible; s. Type of oil preservation system; and t. For multi-winding transformers, required power-­ loading combinations, stating, when necessary, the active and reactive outputs separately, especially in the case of multi-winding auto-transformers. ii. Special Information The following additional information may need to be given:

Transformers  51 a. If a lightning impulse voltage test is required, whether or not the test is to include chopped waves; b. Whether a stabilizing winding is required and, if so, the method of earthing; c. Short-circuit impedance, or impedance range. For multi-winding transformers, any impedances that are specified for particular pairs of windings (together with relevant reference ratings, if percentage values are given); d. Tolerances on voltage ratios and short-circuit impedances as left to agreement, or deviating from values given in respective standards; e. Whether a generator transformer is to be connected to the generator directly or through switchgear, and whether it will be subjected to load rejection conditions; f. Whether a transformer is to be connected directly or by a short length of overhead line to gas-insulated switchgear (GIS); g. Altitude above sea-level, if in excess of 1000 m (3300 ft); h. Special ambient temperature conditions or restrictions to circulation of cooling air; i. Expected seismic activity at the installation site which requires special consideration; j. Special installation space restrictions which may influence the insulation clearances and terminal locations on the transformer; k. Whether load current wave shape will be heavily distorted. Whether unbalanced three-phase loading is anticipated. In both cases, details to be given; l. Whether transformers will be subjected to frequent over currents, for example, furnace transformers and traction feeding transformers; m. Details of intended regular cyclic overloading n. Any other exceptional service conditions; o. If a transformer has alternative winding connections, how they should be changed, and which connection is required ex works; p. Short-circuit characteristics of the connected systems (expressed as short-circuit power or current, or system impedance data) and possible limitations affecting the transformer design;

52  Electrical Equipment q. Whether sound-level measurement is to be carried out; r. Vacuum withstand of the transformer tank and, possibly, the conservator, if a specific value is required; and s. Any special tests not referred to above which may be required. iii. Parallel operation If parallel operation with existing transformers is required, this shall be stated and the following information on the existing transformers given: a. Rated power; b. Rated voltage ratio; c. Voltage ratios corresponding to tappings other than the principal tapping; d. Load loss at rated current on the principal tapping, corrected to the appropriate Reference temperature; e. Short-circuit impedance on the principal tapping and at least on the extreme tappings, if the tapping range of the tapped winding exceeds ±5%; and f. Diagram of connections, or connection symbol, or both. Note: On multi-winding transformers, supplementary information will generally be required.

2.11 Tests on Transformer Transformers shall be subjected to tests as specified below. A. Routine tests ¾¾ Below the list of routine tests to be performed on liquid oil immersed transformers: • • • •

Measurement of winding resistance Measurement of voltage ratio Checking of phase displacement Measurement of short-circuit impedance and load loss • Measurement of no-load loss and current • Dielectric routine tests (please check below according to the Um (i.e.. highest system voltage) • Insulation test and functional verification of accessories

Transformers  53 • • • •

Tests on on-load tap changers, where appropriate Auxiliary wiring insulation test Leak testing with pressure (tightness test) Tightness tests and pressure tests for tanks for gasfilled transformers • Check of the ratio and polarity of built-in current transformers • Check of core and frame insulation for liquid immersed transformers with core or frame insulation. ​ ¾¾ Additional routine tests for transformers with Um >75 kV • Determination of capacitances windings-to-earth and between windings • Measurement of DC insulation resistance between each winding to earth and between windings • Measurement of dissipation factor (tan δ) of the insulation system capacitances • Measurement of dissolved gases in dielectric liquid from each separate oil compartment except diverter switch compartment • Measurement of no-load loss and current at 90% and 110% of rated voltage B. Type tests Below is the list of the type of tests to be performed on liquid immersed transformers: • Temperature rise test • Dielectric tests • Determination of sound level for each method of cooling for which a guaranteed sound level is specified • Measurement of the power taken by the fan and liquid pump motors • Measurement of no-load loss and current at 90% and 110% of rated voltage C. Special tests • Dielectric special tests • Winding hot-spot temperature-rise measurements

54  Electrical Equipment • Determination of capacitances windings-to-earth and between windings • Measurement of dissipation factor (tan δ) of the insulation system capacitances • Determination of transient voltage transfer characteristics • Measurement of zero-sequence impedance(s) on three phase transformers • Short-circuit withstand test • Measurement of DC insulation resistance between each winding to earth and between windings • Vacuum deflection test on liquid immersed transformers • Pressure deflection test on liquid immersed transformers • Vacuum tightness test on site on liquid immersed transformers • Measurement of frequency response (Frequency Response Analysis or FRA); the test procedure shall be agreed between manufacturer and user. • Check of external coating (ISO 2178 and ISO 2409 or as specified) • Measurement of dissolved gasses in dielectric liquid • Mechanical test or assessment of tank for suitability for transport • Determination of weight with transformer arranged for transport. For transformers up to 1.6 MVA by measurement and for larger transformers by measurement or calculation as agreed between manufacturer and • Detection of harmonic content in no load current • Thermographic detection of hot-spots • Measurement of winding inductance • Recurrent voltage surges test • In-rush current test • Measurement of the transmitted vibrations • Test on vibration floor (seismic).

2.12 Maintenance of Transformers The widely used preventive maintenance schedule of transformers is shown in Tables 2.11 and 2.12.

Dehydrating breather

i. Oil level in transformer ii. Connection iii. Dehydrating breather

Bushing

Monthly

Quarterly

Examine for cracks & dirt deposits.

Check transformer oil level. Check tightness. Check that air passages are clear. Check color of active agent.

Check that air passages are clear. Check color of active agent.

Check against rated figure.

iii. Voltage

Daily (if manned)

Check against rated figure. Check oil temperature & ambient temp.

i. Load (amp.) ii. Temperature

Hourly (if manned)

Inspection notes

Items to be inspected

Inspection

Table 2.11  Maintenance schedule for transformer of capacities less than 1000 KVA.

Clean or replace. (Continued)

If low, top up with dry oil. Examine transformer for leaks. If silica gel is pink, change it. The gel may be reactivated for use again.

If silica gel is pink, change it. The gel may be reactivated for use again.

Reduce load if higher. Switch off, if the oil temperature is high. Take corrective action.

Action required if inspection shows unsatisfactory condition

Transformers  55

i. Transformer oil

Yearly

5 Yearly

i. Non-conservator transformer ii. Cable boxes, gasketed joints, gauges and general paintwork.

Half Yearly

Core and windings

iii. Relay, alarms and their circuits etc.

ii. Earth resistance

Items to be inspected

Inspection

Overall inspection including lifting of core and coils.

Examine relay and alarm contacts, their operation, fuses, etc. Check relay accuracy, etc.

Wash with clean dry oil.

Take suitable action to restore quality of oil. Take suitable action if earth resistance is high. Clean components and replace contacts and fuses if necessary, change the setting, if necessary.

Attend defects, if any.

Inspect for leaks, cracks etc.

Check di-electric strength and water content. Check for acidity and sludge. Check values of earth resistance

Improve ventilation and check oil.

Action required if inspection shows unsatisfactory condition

Check for moisture under cover.

Inspection notes

Table 2.11  Maintenance schedule for transformer of capacities less than 1000 KVA. (Continued)

56  Electrical Equipment

3. Dehydrating breather

1. Bushing 2. Transformer oil

Check for any crack or damage. Check that air passages are free. Check color of active agent.

2. Explosion vent 3. Dehydrating breather

Quarterly

Check oil level.

1. Oil level in transformer

Daily

Examine for cracks & dirt deposits. Check for dielectric strength & water content Check oil level in oil cup and ensure air passages are free.

-Check that temperature rise is within limit. It should be reasonable. Check against rated figures. Check & record.

1. Ambient 2. Temperature 3. Winding temperature 4. Oil temperature 5. Load (amps.) 6. Voltage

Hourly

Inspection notes

Items to be inspected

Inspection

Table 2.12  Maintenance schedule for transformer of capacities of 1000 KVA & above.

(Continued)

Clean or replace. Take suitable action to restore quality of oil. Make up oil if required.

If low, top up oil, examine transformer for leaks. Replace if cracked or broken. If silica gel is pink, change it. The gel may be reactivated for use.

-Shut down the transformer and investigate if it is persistently higher than normal. An improper condition can cause excessive core loss. Switch off, if excessively high.

Action required if inspection shows unsatisfactory condition

Transformers  57

Check for sealing arrangements. Examine compound for cracks. Examine for dirt deposits. Examine for cracks & dirt deposits. Examine relay alarm contacts their operation, fuses, etc. Check relay accuracy etc. Pockets holding thermometers should be checked.

5. Cable boxes

9. Temperature indicator

6. Arcing horns 7. Surge diverter and gaps 8. Relays, alarm, their circuits, etc.

3. Oil-filled bushings 4. Gasket joints

Check for acidity & sludge. Compare with value at the time of commissioning Test oil. Check for leakage or cracks.

1. Transformer oil 2. Insulation resistance

Yearly

Inspection notes

Items to be inspected

Inspection

Change setting if necessary. Oil to be replenished if required. (Continued)

Clean. Clean or replace. Clean components. Replace contacts and fuses, if necessary.

Filter or replace. Tighten the bolts evenly to avoid un-even pressure. Replace gaskets if leaking.

Filter or replace if not in order. Process if required.

Action required if inspection shows unsatisfactory condition

Table 2.12  Maintenance schedule for transformer of capacities of 1000 KVA & above. (Continued)

58  Electrical Equipment

Items to be inspected Overall inspection including lifting of core & coils. Overall inspection including lifting of core & coils.

1000 to 3000 KVA Capacity Transformer

Above 3000 KVA Capacity Transformer

b) 7-10 yearly

Inspection notes

a) 5 yearly

Complete overhauling

Inspection

Wash with clean oil.

Wash with clean oil.

Action required if inspection shows unsatisfactory condition

Table 2.12  Maintenance schedule for transformer of capacities of 1000 KVA & above. (Continued)

Transformers  59

60  Electrical Equipment Table 2.13  Transformer yearly maintenance report. Transformer yearly maintenance report WP No: Date: TR No: Area: Yes / No

S. no

Description

1.

Check transformer connection/tightness and cleaned with CTC if required.

2.

Check all bushings for tightness clean with CTC if required.

3.

Check oil level in conservator (min.35%).

4.

Check operation of all gauges.

5.

Check that all cooling fan motors are operational in manual and auto mode.

6.

a) Check oil sample from bottom of the tank and test for BDV, moisture and acidity. b) Fill the oil if required.

7.

Check condition of silicagel breather if required reactivate silicagel and fill Sump cup with oil.

8.

Check transformer thoroughly for any oil leak.

9.

Check buchholz relay for alarm & trip by draining the oil.

10.

Check OTI and WTI that their contacts are actuated for alarm and trips.

11.

Check whether the transformer HV and LV breakers are maintained.

12.

Check and record IR Values of transformers and cables. (Limits Primary 35 MΩ & Secondary 7.5 MΩ)

Manpower utilized: Spares used: 1. Signature

Outage: 2.

day

3.

Value, if required

Remarks

Transformers  61 Table 2.14  Transformer overhauling report. Transformer overhauling report WP No: Date: TR No: Area: 1. Name Plate details: S. No.: Make: 2. Whether work-permit taken 3. Note down leakage points (a) (b) (c) (d) (e) (f) 4. Shutdown on HV side arranged (including Control Supply) 5. Shutdown on LV side arranged 6. Note down tap switch position 7. Disconnect H.V. cable (Phase sequence) (Left to Right, by facing Transformer from cable box) 8. Disconnection of neutral at two places 9. Disconnect HV& LV Bushings 10. Disconnect neutral CTs (Note polarity of two cables) 11. Disconnect control wires in Marshalling box(note down the wiring details) (Left to Right, viewing Tank) 1. 4. 7. 10. 13. 2. 5. 8. 11. 14. 3. 6. 9. 12. 15. 12. Take oil sample for testing and record IR Values    (a) BDV = ___ KV (b) Water content = _____ ppm    (c) TAN = ____ mg of KOH/gm of oil    (d) IR value (temp = oC) : HV to E = MΩ (e) LV to E = MΩ    (f) HV to LV=M Ω 13. Draining of 1 to 2 drums of oil from main tank 14. Close Radiator valves (top & bottom) 15. Draining complete oil from Radiators. 16. Check the operation of Radiator valves 17. Disconnection of assemblies and closing with blanking plates.    (a) Conservator (Disconnect control wires of MOG)    (b) Oil temp& Wdg. sensors (c) Radiators (d) Silica gel breather    (e) Earth connections (f) Buchholz Relay (Wires : Alarm : ----- , Trip ------)    (g) Explosion vent 18. Complete draining of oil from Tank into Vessel/drums & do filtration in Vessel/drums (minimum of 6 Circulations) 19. Removal of top cover bolts (Continued)

62  Electrical Equipment Table 2.14  Transformer overhauling report. (Continued) Transformer overhauling report 20. Lifting of core assembly along with top cover 21. Cleaning of core & windings through hot oil jet (use oil as filtered in 18 above) 22. Visual inspection of core assembly for healthiness and adjustments/repair of   a.) Shrinkage   b.) Spacers/insulating cylinders    c.)  Bushings end connections   d.) Tap switch    e.)  Tie - rod/Fish plates   f.) Core bolts    g.)  Core adjusting screws    h.)  Wooden supports of core    i.) Other active parts 23. Visual inspection of tank & removal of sludge from tank 24. Replacement of top cover gasket 25. Placing core assembly in tank & fixing top cove 26. Replacement of entire gaskets 27. Fixing of blanking plates for    (a) Radiator valves    (b) Explosion vent    (c) conservator pipe line 28. Connecting vacuum pump to transformer and Creation of vacuum in tank (for few hours) vacuum = ___ mm of Hg 29. Filling of tank with filtered oil (as in 18 above) & circulation of oil in tank with filter Machine until 65o C temp. is achieved. 30. Drain complete oil into separate vessel/drum & Filter this oil separately 31. Record I R Values : HV - E = __ M Ω, LV-E = _____ M Ω, HV-LV= ____ M Ω 32. Create vacuum in Main tank ( ________ mm of Hg) and hold for few Hours. Started at _____ hrs. on ______ & stopped at ____ hrs on --33. Fill the oil in tank by holding vacuum & circulation of oil in tank through filter Machine until 65o C temp. is achieved.    Started at ___hrs. on _______& Stopped on _____hrs. on _________ 34. Servicing of Accessories Conservator : (a) Remove sludge & Clean it and fix to transformer (b) Connect MOG wires (as in 11 above) Radiators : Clean by flushing with oil & install on transformer Radiators Fans : Check Bearings, IR Values (Continued)

Transformers  63 Table 2.14  Transformer overhauling report. (Continued) Transformer overhauling report Explosion vent : Check the diaphragm & replaces if required & fix to Transformer (Diaphragm replaced/Not replaced.) Breather : Clean and fill with fresh/activated silica gel and connect to conservator and fill sump cup with fresh oil Buchholz relay : Clean and check operation, after connecting wires as in 17f. Bushings: Clean and check, replace if required OTI Sensor: Calibrate & fix to transformer (OTI = o C, Std. Meter = o C) WTI Sensor: Calibrate & fix to transformer (WTI = o C, Std. Meter = o C) Control cables: Connect them in Marshalling box Neutral CTs: Connect them (as in 10 above) 35. Painting of Transformer accessories if required (Use Epoxy gel in Bus-bar Chambers) 36. (a) Connecting HV cable   (b) Connecting LV Bus-duct   (c) Connecting Neutral points   (d) Connecting earth 37. Topping up if required & continue filtration at temp. of 65° C 38. Stop filtration after attaining Required values of BDV, IR Values & Water content.     Record IR Values 7 other readings:     I R Values: HV - E =____ M Ω, LV-E =_________ M Ω,         HV-LV= _____ M Ω     BDV = _______ kV, Water content:______ ppm: ______ 39. Take Radiator & conservator into Line & circulate oil for few hours 40. Check the position of all Valves for their correctness and keep all in open condition   Conservator   (a) Radiators    (b) Bottom sample valve    (c) Top sample valve    (d) Tank drain valve    (e) Tank air-vent 41. Release air from    (a) Transformer tank    (b) Radiators    (c) Buchholz Relay   (d) Conservator (Continued)

64  Electrical Equipment Table 2.14  Transformer overhauling report. (Continued) Transformer overhauling report 42. Ratio test (at different taps)    a. Test : Volts applied Measured volts on secondary on primary Tap (3 phase) 2U-2V 2V-2W 2W-2V 2U-2N 2V-2N 2W-2N Result 1 2 3 4 5

   b. Set Tap Switch to original set figure, Set at 43. Core balance test with Tap switch at

: ____

Volts applied across

Measured volts across

Result

2U-2N=

v

2V-2N=

v

2W-2N=

v

2V-2N=

v

2W-2N=

v

2U-2N=

v

2W-2N=

v

2U-2N=

v

2V-2N=

v

44. Transformer Charged on ------------ at ---------- hrs. Primary voltage (in kV)

Secondary voltage in volts

1U-1V=

2U-2V=

1V-1W=

2V-2W=

1W-1V=

2W-2V=

Result

45. Materials consumed:    (a) Transformer Oil = liters (b) HV Bushings = Nos. (c) LV    Bushings = Nos.    (d) Oil seals(LV) = Nos.   (e) Oil seals(HV)= Nos. (e)   (e)     (f)    (f) Signature

Transformers  65 Note: a. The silica gel may be reactivated by heating to 150-200°C. b. Every time the oil is changed, oil seal should also be changed. c. No work should be done on any transformer unless it is disconnected from all external circuits and the tank and all windings have been solidly earthed. d. In case of anything abnormal occurring during service, maker’s advice should be obtained giving complete particulars regarding the nature and extent of occurrence, together with the name plate particulars. e. The detailed sample checklists for yearly and overhaul of transformer are shown in Tables 2.13 and 2.14, respectively.

2.13 Troubleshooting Chart for Transformers Guidelines, as in Table 2.15, can be followed in troubleshooting of transformers.

2.14 Latest Trends Opportunities in Transformer Technology a. Higher losses in Core requires improvement which is being done with the help of (a) Amorphous core, as discussed elsewhere in this chapter. (b) High-frequency operation like Solid State Transformer (SST), as shown in Figure 2.18. b. Go for copper loss reduction with the help of Super Conducting Transformer (SCT), as shown in Figure 2.19. c. Problems with Mineral oil of flammability, sludge formation, exhaust of natural resources, etc., leads to use of vegetable oils which are renewable. d. Opportunities: Lots of opportunities are existing for further research transformer technology: • Adequate cooling and action plan for operation of SCT during fault conditions, if cooling fails.

66  Electrical Equipment Table 2.15  Troubleshooting of transformers. Trouble

Cause

Remedy/Action to be taken

Rise in Temperature High temperature

Over voltage

Change the circuit voltage or transformer connections to avoid over excitation.

Over current

If possible, reduce load. Heating can often be reduced by improving power factor of load. Check parallel circuits for circulating currents which may be caused by improper ratios or impedances.

High ambient temperature

Either improve ventilation or relocate transformer in lower ambient temperature.

Insufficient cooling

If unit is artificially cooled, make sure cooling is adequate.

Lower oil level or sludge in oil

Fill to proper level. Use filter press to wash off core and coils. Filter oil to remove sludge.

Short-circuited core

Test for exciting current and no load loss. If high, inspect core and repair.

Lightning. Short circuit overload Oil of low dielectric strength Foreign material Core insulation breakdown (Core, bolts, clamps, or between laminations)

Usually, when a transformer winding fails, it is automatically disconnected from the power source by opening of the circuit breaker or fuse. Smoke or cooling liquid may be expelled from the core, accompanied by noise. When there is any such evidence of a winding failure, the transformer should not be re-energized at full rated voltage because this might result in additional internal damage. Also it would introduce a fire hazard in transformers.

Electrical Troubles winding failure

(Continued)

Transformers  67 Table 2.15  Troubleshooting of transformers. (Continued) Trouble

Cause

Remedy/Action to be taken After disconnection from both source and load, the following observations and tests are recommended: a. External mechanical or electrical damage to bushing, leads, both heads, disconnection switches, or other accessories. b. Level of insulating liquid in all compartments. c. Temperature of insulating liquid wherever it can be measured. d. Evidence of leakage of insulating liquid or sealing compound.

High exciting current

Short-circuited core Open core joints

Test core loss. If high, it is probably due to a short-circuited core. Test core insulation. Repair if damaged. If laminations are welded together, refer to manufacturer. Core-loss test will show no appreciable increase. Pound joints together and retightenclamping structure.

Incorrect voltage

Improper ratio Supply voltage abnormal

Change terminal board connection or ratio-adjuster position to give correct voltage. Change tap connections or readjust supply voltage.

Audible internal arcing and ratio interference

Isolated metallic part Loose connections

The source should be immediately determined. Make certain that all normally grounded parts are grounded, such as the clamps and core. Same as above. Tighten all connections. Maintain proper liquid level.

Low liquid level, exposing live parts. Bushing flashover

Lightning Dirty bushings

Provide adequate lightning protection Clean bushing porcelains, frequency depending on dirt accumulation. (Continued)

68  Electrical Equipment Table 2.15  Troubleshooting of transformers. (Continued) Trouble

Cause

Remedy/Action to be taken

Mechanical Troubles Leakage through screw joints

Foreign material in threads Oval nipples Poor threads Improper filler Improper assembly

Make tight screw joints or gasket joints

Leakage at gasket

Poor scarfed joints Insufficient or uneven compression Improper preparation of gaskets and gasket surfaces

Make tight screw joints or gasket joints.

Leakage in welds

Shipping strains, imperfect weld

Repair leaks in welds.

Pressure relief diaphragm cracked

Improper assembly. Mechanical damage

Replace diaphragm. Inspect inside of pipe for evidence of rust or moisture. Be sure to dry out transformer if there is a chance that drops of water may have settled directly on windings or other vulnerable locations, as oil test may not always reveal presence of free water.

Pressure relief diaphragm ruptured

In conservator transformer obstructed oil flow or breathing In gas seal transformer obstructed pressure relief valve. In sealed transformer liquid level too high.

Check to see that valve between conservator and tank is open and that ventilator on conservator is not blocked. Make certain that relief valve functions and that valves in discharge line are open. Liquid level should be adjusted to that corresponding with liquid temperature to allow ample space for expansion of liquid. (Continued)

Transformers  69 Table 2.15  Troubleshooting of transformers. (Continued) Trouble

Cause

Remedy/Action to be taken

Moisture condensation in open-type transformers and air-filled compartments

Improper or insufficient ventilation

Make sure that all ventilator openings are free.

Moisture condensation in sealed transformers.

Cracked diaphragm Moisture in oil

Filter oil.

Audio noise

Leaky gaskets and joints. Accessories and external transformer parts are set into resonant vibration giving off loud noise.

Make certain all joints are tight. Tighten loose parts. In some cases parts may be stressed into resonant state. Releasing pressure and shimming will remedy this condition.

Rusting and deterioration of paint finish.

Abraded surfaces and weathering

Bare metal of mechanical parts should be covered with grease.

Fractured metal or porcelain parts of bushings

Unusual strains placed on terminal connections

Cables and bus bars attached to transformer terminals should be adequately supported. In the case of heavy leads, flexible connections should be provided to remove strain on the terminal and bushing porcelain.

Condensation in open type transformer from improper ventilation Broken relief diaphragm Leaks around cover accessories Leaky cooling coil

Make certain that ventilation openings are unobstructed.

Oil Troubles Low dielectric strength

Replace diaphragm Re-gasket, if necessary. Test cooling coil and repair. (Continued)

70  Electrical Equipment Table 2.15  Troubleshooting of transformers. (Continued) Trouble

Cause

Remedy/Action to be taken

Badly discoloured oil

Contaminated by varnishes Carbonized oil due to switching Winding or core failure

Retain oil if dielectric strength is satisfactory.

Oxidation (sludge or acidity)

Exposure to air High operating temperature

Wash down core and coils and tank. Filter and reclaim or replace oil. Same as above. Either reduce load or improve cooling.

HVDC

HFAC

LV

LVDC

LVAC LVAC

HVAC HVDC

HV

HFAC

LVDC

Figure 2.18  SST.

Power source

Current lead

SC transformers Cryogenic dewar

cryogenic system

Superconduct -ing joints

Samples for testing

central control system

data monitoring system

Figure 2.19  Super conducting transformer.

Transformers  71 • Reduction of harmonics due to the presence of power electronic converters and present capacity limitation of 3 MVA due to iron losses in SST. • Use of Industrial Internet of Things (IIoT), Artificial intelligence (AI) and drones in maintenance works. e. Continuous monitoring of operating parameters with manual intervention to be automated with Smart Transformer as shown in Figure 2.20.

Online Transformer Monitoring Cooling protection

• Low oil level • Ambient temperature

1 Communication (wired, wireless or cellular)

Transformer monitoring application suite

Overload protection 2 OLTC (optional) • Tap position and counter • Time since last through tap position • Low oil level

• Voltage, Current, Frequency • Active Power, Apparent Power, Power Factor • Active Power Angle, Tan Phi, Cos Phi • Remote operation LBS or MCCB

5 3 RTU

Core and coil protection

Thermal protection • Oil temperature • Winding temperature

4

• Dissolved gas (BuchholzRelay) • Moisture in oil

Figure 2.20  Smart Transformer. (Source : http://www.rtccuae.com/transformer-monitoringsolution)

3 Generators 3.1 Introduction An Electrical Generator, which is driven by a prime-mover, converts mechanical energy into electrical energy. There are various types of conventional generators, as shown in Figure 3.1. At present, most of the generators are of AC, which will be explained in subsequent sections.

3.2 Alternator Unlike DC generators, alternators have a stationary armature. The main advantages of stationary armature are: i.

The output can be taken easily from armature which avoids brush gear thus avoids sparking and wear & tear problems. ii. Ease in insulation and can built generators up to 30 KV or more. iii. No slip rings and less maintenance. iv. Low voltage is supplied to rotating field, thus avoids brush gear and other problems. Generators AC 1-ph

DC 3-ph

Self excited

seperately excited

Asynchronous Synchronous Compound shunt Series or Induction short Long Salient Cylindrical shunt shunt pole

Figure 3.1  Types of generators. B. Koti Reddy. Electrical Equipment: A Field Guide, (73–112) © 2021 Scrivener Publishing LLC

73

74  Electrical Equipment v. The armature can be braced which prevents the deformation of windings due to mechanical forces on occurrence of severe faults such as short circuits.

3.3 Field Poles There are two type of poles, which are explained in Table 3.1.

3.4 Construction of Field Poles The basic construction model of salient pole and cylindrical pole type generators are shown in Figures 3.2 and 3.3, respectively. Table 3.1  Types of field poles of AC generators. S. no.

Salient pole

Cylindrical pole

1

Projected or protruding poles.

Wound rotor type poles.

2

Used for low speeds like diesel driven Generators.

Used for high speeds and large capacities like turbo Generators.

3

Needs damper winding.

No need of damper winding since rotor body itself acts as a damper.

4

Has larger diameter of order of 2 to 6 meters to accommodate several poles and short axial length of around 1M.

Has a small diameter of order of 1 meter and long axial length of order of 2 to 5 meters.

5

Little vibrating and noisy operation.

Better balance of rotor and quite operation.

6

Have different axis reactances (xd and xq) which are to be analysed based on two reaction theory. Here, x s = x d + jx q

Have only one direct axis reactance. Here, xs = xd

7

Have several poles

Generally have 2 poles only.

Generators  75 S

Salient pole Field winding

N

N

Slip rings

Field Supply (DC)

S

Figure 3.2  Salient pole construction.

Cylindrical pole N

+ +

+ +

+ + +

S

+

Field Supply (DC) + +

+

S

Field winding

+ +

+

+ +

N

Figure 3.3  Cylindrical pole construction.

3.5 EMF Equation of Alternator The flux produced by north and south poles of the magnetic field is as shown in Figure 3.4. Average e.m.f = dØ/dt

76  Electrical Equipment Ф Фm 0

π

t

one revolution of rotor

Figure 3.4  Flux waveform.

Where dØ = Øp and dt = 60/N in which; Ø = flux p = number of poles and N = speed in RPM. Average e.m.f. per conductor, dØ/dt=ØNP/60 where, N=120f/P = 2 Øf = 4 Øf per turn per phase



RMS value of e.m.f per phase = 4.44Øf T

(3.1)

where T = number of turns. If winding factors are considered;



e.m.f per phase = 4.44Øf T KpKd

(3.2)

where Kp = Pitch factor = Cos(α/2) in which α= Chording angle Kd = Distribution factor = Sin(mß/2)/mSin(ß/2) where m = Slots/phase/pole and ß = Slot pitch in electrical degrees =180/(slots/pole)

3.6 Capability Curve The capability curve or operating chart of a Synchronous generator gives information about the boundary within which the generator can operate

Generators  77 safely. It is based on the phasor diagram of the generator. A sample capability curve is shown in Figure 3.5. The safe operating region can be seen as follows: a. The MVA load shall not exceed the rated MVA of generator. It is the limit of armature heating due to armature current. b. The MW load shall not exceed the MW rating of prime mover of generator, so as to avoid its overloading. c. The field current shall not exceed the specified value (as given in rating plate) to avoid overheating of field winding (Rotor). d. The load angle shall be less than 90o for a stable operations of machine (transient stability). One example of a 30 MW TG Capability curve is shown in Figure 3.6, the turbine technical details of which are shown in Table 3.2. The rating plate of a 570 MW Steam Turbine Generator unit is shown in Figure 3.7 The rating plate of a 4000 KW Water Turbine is shown in Figure 3.8.

Field limit δ (max)

Steady state stability limit KW

stator limit P (max)

S (max)

Stable zone M

Theoritical stability limit

Ef (max) (maximum excitation)

3 Vt Ef / Xs

Practical stability limit Ef = 0

P

Rotor overheating

ϕ O’

δ 3 Vt2 Xs Leading p.f (Reactive power out of Generator)

Figure 3.5  Generator capability curve.

O

N KVAR Lagging power factor (Reactive power in to Generator)

Q

THEORETICAL STABILITY LIMIT.

ACTIVE POWER (%)

78  Electrical Equipment

100

LE AD

P.F .’S

0.9 0.8

C

0.7

AB-ROTOR HEATING ELEMENT. BC-TURBINE OUTPUT LIMIT. CD-PRACTICAL STABILITY LIMIT WITH AVR., 9

0.

90 80

B

8 LAG

0.

P.F 7 .’S

0.

0.6

6

0.

70

0.5

0.5

0.4

0.4

40

20

100

80

60

D 40 LEAD

A 20

0 % MVAR

20

40

60 LA6.

LOADING DIAGRAM OF 30 MW, 11KV, 0.85 P.F. TG.

Figure 3.6  Capability curve of a 30 MW TG.

Table 3.2  Turbine details. Normal output

27 MWe

Maximum output

30 MWe

Normal speed

3000 RPM

Inlet steam pressure

100 kg/cm2 (g)

Permissible deviation

105 kg/cm2(g)

Pressure in HP Wheel chamber

69.7 kg/cm2 (g) (Max.)

Steam temperature

480oC

Permissible deviation

488.3oC (Average)

Permissible deviation

480oC for 15 minutes

Extraction Pressure

32.5 kg/cm2 (g)

Pressure variation

33.5 to 31.5 kg/cm2 (g)

Cooling water temperature

33oC

Condenser vacuum

0.1 kg/cm2 (a)

80

100

Generators  79

Figure 3.7  570 MW steam turbine generator unit.

Figure 3.8  4000 KW water turbine.

80  Electrical Equipment

3.7 Design of Alternator Following are the important formulae, with Figure 3.9 as reference.



Output in KVA = 11 BavacD2Ln *10-3

(3.3)

where Bav= Average flux density i.e. magnetic loading = 0.54 to 0.65 Wb/M2 for Cylindrical pole machine = 0.32 to 0.62 Wb/M2 for Salient pole machine ac = Ampere conductors per meter, i.e., electrical loading = 50,000 to 70,000 for Cylindrical pole machine = 20,000 for Salient pole machine D = Diameter Ln = Length of machine Kw = Window space factor ≈ 0.95

3.8 Rating Plates The sample rating plates of cylindrical pole type generator (i.e., Turbo generator) and salient pole type generator (i.e., Diesel generator) are shown in Figures 3.10 and 3.11, respectively. The prime mover or the machine which runs them generally will be either a Steam turbine or Diesel (or Gasoline) Engine. The prime mover is generally rated in terms of Horsepower (HP) or Mega-Watt (MW).

D L

Figure 3.9  Machine design.

Generators  81 All the parameters of their rating plate will be explained in subsequent sections. a. Power rating: It will be mentioned generally either in MVA or KVA. Since it is a source whose power factor is not known in exact terms, it is generally given as apparent power only. However, the power factor will be assumed as 0.8 and all the required calculations are done for sizing of other equipment, protection and capability curve. b. Power factor: For excitation and other purposes, it will be considered as 0.8. The effects of operating at lower power factor are as follows: i. The generator will not be released fully for loads. ii. For the same active power loading, the current will be high which causes more current, more heat, more voltage drop and reduced life of generator. c. Insulation class: It is indicated by a classification letter like B, F, H, etc. Generally all the rotating electrical machines will have primary insulation belonging to class-F category, whereas certain old generators have class-B insulation. The classification of insulating materials with their maximum

Prime Mover

Steam Turbine

Power

30,000 KW= 30 MW(35.29 MVA)

Voltage

11 KV

PF

0.85 Lag

Current

1852 Amperes

No. of phases

3

Frequency

50 Hz

Speed

3000 RPM

Excitation (DC) 172 V, 550 A Connection

Star (Y) (NGT Grounding)

Cooling

Air

Class of Insulation Ref.

Figure 3.10  Turbo generator rating plate.

B IS 4722/ 5422

82  Electrical Equipment Prime Mover Diesel Engine Continuous power 1250 KVA (1000 KW)

Over load capacity

Power factor Rated voltage

0.8 3 ph, 415 / 240 V

Rated current

1739 Amps

Frequency

50 Hz

Insulation class

F (stator & rotor) B (Exciter)

Type of enclosure

IP 23 for Alternator, IP 44 for Terminal box

Speed

1500 RPM

Connection

Star (Neutral Solid Grounding)

Excitation

23.6 V DC at 3 Amps

Cooling Make

Air AVK (Germany)

(Brushless, Self-excited) rotating f ield

110% for 1 hour in every 12 hours.

TD’

0.02 Sec

TD’

0.376 Sec

TDO’

3.962

Ef f iciency

94.3% at 100% load & 94% at 50% load

Resistance: Stator winding Field winding Pilot exciter stator Pilot exciter rotor

1.448 (ohms at 20ºC) 0.45 7.2 0.067

Xd”

0.132 pu

Xd’

0.236 pu

Xd

2.487 pu

X2

0.149 pu

SCR

0.463

Field build up time

0.4 sec

Total voltage build up time 14 sec

Figure 3.11  Diesel generator rating plate.

Generators  83 Table 3.3  Classification of insulating materials. Class

Temperature (°C)

Materials

Y

90

Cotton, paper, silk, and similar organic materials.

A

105

Impregnated paper, silk, polyamide, cotton, and resins.

E

120

Enameled wire insulation on the base of powdered plastics, polyvinyl epoxy resins.

B

130

Inorganic materials impregnated with varnish.

F

155

Mica, polyester epoxide varnished in the high heat resistance.

H

180

Composite materials on mica, glass, fiber.

C

>180

Glass, mica, quartz, ceramics, Teflon.

temperature withstand capacity along with material examples is shown in Table 3.3. d. Type of enclosure/degree of protection: It is indicated by two letters followed by two numbers for the protection against liquids and solids. The first two letters “IP” indicate Ingress Protection and the first number indicates the type of protection against solid particles and the second letter indicates the protection against entry of liquids. The detailed classification is shown in Table 3.4. e. Electrical parameters like voltage and current: i.

The rated voltage of alternator is indicated in volts of either Line to Line or Phase to Neutral. e.g. 6600 V, 415V, 240 V, 11 KV. ii. The rated current is derived from the VA rating of alternator. e.g., I = 35.29*103/ (√3*11) = 1852 Amps. iii. The frequency is either 50Hz or 60Hz based on the system frequency of region/country. iv. The speed is indicated in RPM which is also the speed of the prime-mover. f. Phasor diagram of Synchronous generator: The equivalent circuit and phasor diagrams at lag and lead power factors are shown in Figures 3.12 and 3.13.

84  Electrical Equipment Table 3.4  Degree of protection. First digit

Protection type

0

No protection

1

Protected against solid particles>50 mm

2

Protected against solid particles>12.5 mm

3

Protected against solid particles>2.5 mm

4

Protected against solid particles>1 mm

5

Dust protected

6

Dust tight

Source: IEC 60528-2001.

IA

If

+

jXs

Rf

RA

+

Vf

–

+

VBA*

EA

Lf –

Field supply

–

* shown for 1 phase and same for other 2 phases

Figure 3.12  Equivalent circuit.

EA

δ Ф

IA

VA

jIAXS

a. Lagging pf

Figure 3.13  Phasor diagram with different load.

IS

EA VA b. Leading pf

jIAXS IARA

Generators  85



Induced e.m.f. per phase =VA+IARACosØ ± jIAX



here (+ ve for lag and –ve for lead p.f)

(3.4)

where VA = Terminal voltage per phase = Load current per phase IA = Armature resistance per phase RA X = Synchronous reactance per phase δ = Load angle Ø = Power factor angle



% Voltage regulation = 100*(EA-VA)/VA

(3.5)

g. Power angle or load angle (δ): It is the angle between stator and rotor magnetic fields, where both are rotating at synchronous speed. It is also the phase angle between terminal voltage vector and induced e.m.f vector. Power developed is maximum where it is generally between 45 to 55o.

3.9 Voltage Regulation of Synchronous Generator For generators, the terminal voltage will reduce with the increase in load of lagging power factor with constant speed and field current. Voltage regulation is the change in voltage with a change in load and corresponding power factor. The basic idea is shown in Figure 3.14.



Voltage regulation = 100*(Eg-Vt)/Vt

(3.6)

Eg IZ

δ Vt

Ø I

Figure 3.14  Generator voltage regulation.

IX IR

86  Electrical Equipment where Eg = Generated voltage per phase Vt = Terminal voltage per phase There are different methods to find out voltage regulation of alternators such as Synchronous impedance method, Ampere-Turn method and Potier-Triangle method. It shall be noted that the terminal voltage will increase with the increase in load of lower leading power factors. Here the voltage regulation is considered as negative.

3.10 Excitation The DC excitation voltage and current are indicated in volts and amperes respectively. Some important points to be known when excitation fails are as follows: i.

ii.

iii. iv.

v.

In case of stand-alone generator: Failure of excitation will cause the generated output to be zero and loads will not get any voltage. There will not be any problem to the generator but it runs idle which wastes fuel to the prime mover and power interruption to loads. In case of grid-connected generator (where the grid has other generators in parallel): The field winding gets supply from stator (i.e., grid) and runs as induction generator at an over speed, with reduced load. Even though there is no immediate damage to generator, the higher leading field current causes excessive heating of field winding. It shall be rectified at the earliest or be stopped with a field failure protection relay. Field forcing: During fault conditions, turbo generators try to fall out of step. This can be managed by giving more field current which in turn maintains the synchronism. Field flashing: For turbo generators, initially field supply is to be given up to building up of 30% of rated voltage from external source. After which, the AVR will take supplying input from generator terminals and gives supply to field winding. But Field flashing will continue along with main excitation up to 70% of voltage build up, where it will cut off from excitation system. A sample field excitation circuit for Turbo generator is shown in Figure 3.15.

Generators  87 Excitation transformer

Control

Field f lashing circuit

PT AVR

CT

Field breaker

File winding G

Brushgear

Figure 3.15  Sample field excitation circuit of turbo generator.

Exciter stator

AVR Rotating Bridge Rectif ier Auxiliary winding

Exciter Field

Main Stator Main Field

Figure 3.16  Sample brushless field excitation circuit of diesel generator.

vi. A sample brushless field excitation circuit for Diesel Generator is shown in Figure 3.16.

3.11 Connections Three phase alternators can be connected either in Delta or Star. But almost all the generators are connected in Star mode which reduces the requirement of more insulation for higher voltages and permits easiness in conducting connections. In case of Star connected system, the neutral

88  Electrical Equipment (i.e., Star) point to be connected to ground either directly (Salient pole in general) and through a Neutral Grounding Transformer (NGT for cylindrical pole machine in general).

3.12 Neutral Grounding The neutral point of a Star connected generator is required to protect generator from damage due to internal ground fault currents. It also ensures the safety of people working near the generator and to facilitate the operation of protective systems. The following points to be noted for neutral grounding work: i. For low-voltage generators, solid grounding is prepared. ii. For high-voltage generators, either resistance grounding or Neutral Grounding Transformer (NGT) are preferred, as shown in Figure 3.17. iii. Typical values of NGT: % x = 4% KVA = 10 to 67 KVA

3.13 Cooling All the generators produce heat while dissipating their power losses. This heat is to be removed from generators to protect them from excessive heat. There are many types of cooling systems: Generator

NGT

Figure 3.17  Neutral grounding transformer.

Generators  89 i.

For small generators (25 KW): Natural air cooling with suitable exhaust system is adequate. It takes fresh and cool air from the atmosphere and sends out the hot air to the surrounding atmosphere. ii. For medium-size generators: Water-cooled system is used to circulate cool water around machine and take away the heat. iii. For large generators: Hydrogen cooling is used. Having higher thermal conductivity, hydrogen-cooled system pulls out the heat at a faster rate. This is the most reliable and effective cooling system, but the cost is higher when compared with other cooling methods. It increases the rating, life span and efficiency of generator. Ensuring the purity of hydrogen and avoiding the explore mixture formation are the key factors in hydrogen cooling. The pressure shall be between 4 to 5 bar. Light weighted Hydrogen gives better cooling. • Cooling • Pressure between 4 to 5 bar. • Light weighted Hydrogen gives better cooling (Density of H2=1/14 of air) • Higher heat transfer capability (2 times of air) • No oxidation and no degradation with H2 cooling. • Hermetic sealing is required. • Flammability of H2 (4 to 76% by volume in air) requires special protection and a comprehensive system is needed.

3.14 Short-Circuit Ratio (SCR) It is the ratio between field current required to get open circuit voltage (OCC-open circuit characteristic) to the field current required to circulate rated short-circuit current (SCC-short circuit characteristic). SCR = If for rated OC voltage/If for rated S.C. current, as shown in Figure 3.18.

SCR = If for rated OC voltage/If for rated short circuit current (3.7)

90  Electrical Equipment

OCC / SCC

OC Voltage

SC Current

Field current (If )

Figure 3.18  OCC and SCC of alternator.

SCR = 0.5 to 0.6 for TGs and 1 to 1.5 for low-speed Hydro-generators and for rapid- loading. SCR α 1/xd α1/lg α Cos Ø where xd= Direct axis synchronous reactance Lg=air gap and cosØ = Power factor It gives a measure of relative strengths of field ampere-turns and stator ampere-turns. Higher SCR means higher short-circuit currents, better voltage regulation and higher sensibility limits, but large air-gap which leads to bulky machine. Low SCR means poor voltage regulation and problems in parallel operation.

3.15 Pitch Factor (Kp) or Chording Factor (Kc) It is the ratio between the e.m.f induced in fractional pitched winding to the e.m.f. induced with full pitched winding. Normally the two sides of a coil (a and b) in armature are differ by 180 electrical degrees, which is called full pitched winding. But in some cases, the winding pitch is made shorter than 180 electrical degrees by an angle α, i.e., short pitch, as shown in Figure 3.19, to eliminate the voltage harmonics. Es = Ea + Eb = 2Ea Resultant e.m.f. in coil a b = Ea + Eb = 2Ea Cos(α/2) ∴ Pitch factor (Kp) = Vector sum of induced emf per coil/arithmetic sum of induced emf per coil



= 2Ea Cos(α/2)/2Ea = Cos(α/2)

(3.8)

Generators  91 Ea

Eb

Es=2 Ea (a) Full pitch

α 2 Eb

Es α 2

α Ea (b) Short Pitch

Figure 3.19  Winding pitch.

To avoid nth order of harmonic; Kpn = Cos(nα/2) Advantages of short pitched winding: i.

When the coil pitch is shortened by th of the pole-pitch, nth harmonic will be eliminated, i.e., the induced e.m.f., thus possible to get a sinusoidal waveform. ii. Saves copper of end connections. iii. With the elimination of higher frequency harmonics, the magnetic losses (eddy current and hysteresis) are reduced, thus improves the efficiency of generator.

3.16 Distribution Factor (Kd) It is also called breadth factor, winding factor or spread factor. The conductor of a phase or different phases are not bunched or concentrated in one slot but are distributed in different slots to form polar groups under each pole. Hence the net e.m.f. is the vector sum of e.m.f.s induced in all conductors of that phase, which are distributed in different slots.

Kd = emf with distributed winding/emf with concentrated winding

= Vector sum of coil emf/Arithmetic sum of coil emf



= Sin(m/ß2)/mSin(ß/2)

where m = slots/ole/phase ß = Slot pitch in electrical degrees=180/(slots per pole)

(3.9)

92  Electrical Equipment Advantages: Even though this distribution reduces the net induced e.m.f., it has the following advantages: i. Reduces harmonic effect. ii. Diminishes armature reaction. iii. Helps in better cooling to the machine.

3.17 Leakage Reactance (Xl) It results from self-flux linking the armature slot and overhang conductors, producing a corresponding voltage. It is actually an ampere-turn balance between the armature current and part of the field current. Leakage produces true reactance (Xl) and armature reaction reactance (Xa). The voltage vector with different reactances is shown in Figure 3.20. Eo = No load e.m.f. Ea = Induced e.m.f. V = Terminal voltage.

3.18 Armature Reaction It is the effect of that lines of force produced by the stator current which passes through the magnetic circuit comprising the rotor (field MMF) and stator (armature MMF) iron. It develops upon the magnitude of current and its power factor. The MMF (Magneto Motive force) due to armature reaction combines with the MMF produced by the rotor field ampere-turns

R

XI

Xa Eo

Eo

IXa

V Load

δ V

Ø I

Figure 3.20  Leakage reactance diagram.

Ea IR

IXI

Generators  93 (AT) to produce a resultant flux. The effect of armature reaction on main flux at different power factors is as follows: i.

Unity P.F.: Cross- magnetizing effect tending to distort the main flux. ii. Lagging of P.F.: It weakens (demagnetize) the main flux, thus reduces the total flux available, so less e.m.f. is generated which requires more excitation. iii. Leading P.F.: Armature flux has a direct magnetizing effect on rotor thus assisting the field ATs which results in higher e.m.f.

3.19 Operation of Generator When Connected to an Infinite Bus An infinite bus is a large system whose voltage and frequency remains constant which is independent of the power exchange between the synchronous machine and bus and is independent of synchronous machine excitation. In general, a change in excitation changes the induced e.m.f. and reactive power

V-Ref. vector Generator (Unity p.f) Under excited

Over excited Producing Power

Motor

II

I

(Zero pf lag)

III

IV

Under excited

Consuming Power

Motor (Zero pf lead) Over excited

Motor Unity pf

Figure 3.21  Behavior of synchronous machine on infinite bus.

94  Electrical Equipment Grid 6.6 KV

2 MW 6.6 KV

X

G X

6.6 KV Bus Load

Figure 3.22  Alternator connected to an infinite.

shared by the machine, but when connected to grid, voltage remains constant and p.f. of machine changes. If a synchronous machine is connected to bus bars infinite capacity, it can run either as generator or as motor based on excitation, either over-excited or under-excited. The four ways of a synchronous machine operation in four quadrants is shown in Figure 3.21.

3.20 Load Sharing of Grid-Connected Alternator The load showing between either two parallel connected alternators or between a grid and alternator is explained with the help of an example, as shown in Figure 3.22. Various load conditions and the results are shown in Table 3.5. Equal load sharing example:

3.21 Typical Values of Various Reactances and Time-Constants The typical values of alternator reactances (p.u) and time constants (Seconds) used for different applications are shown in Table 3.6.

3.22 Load Characteristics of Alternators In the case of D.C generators, the terminal voltage depends on load, i.e., armature current only, whereas in the case of A.C. generators, the terminal voltage depends not only on load current but also on power factor of load. Important load characteristics of alternators like terminal voltage versus field currents, terminal voltage versus load current and load current versus field current are shown in Figure 3.23, 3.24, and 3.25, respectively.

Condition

Equal load sharing

Increased load (Grid absorbs)

Reduced grid frequency (More share by Generator)

Increased grid voltage

Capacitor of 0.85 MVAR added

S. no.

1

2

3

4

5

Table 3.5  Load sharing details.

4

4

4

5

4

MW

Load

0.88

0.88

0.88

0.85

0.80

p.f-lag

2.15

2.15

2.15

3.09

3

MVAR

2

2

1.8

3

2

MW

0.95

0.91

0.94

0.89

0.8

p.f-lag

From grid

0.65

0.9

0.65

1.85

1.5

MVAR

2

2

2.2

2

2

MW

0.8

0.85

0.82

0.8

0.8

p.f-lag

By generator

1.5

1.25

1.5

1.5

1.5

MVAR

Generators  95

96  Electrical Equipment Table 3.6  Typical reactances & time constants. Parameter

TG

DG

Hydro-generator

Syn. reactance-Xd

1.1

2.5

1.15

Transient-Xd’

0.23

0.24

0.37

Sub-transient reactance-Xd”

0.12

0.13

0.24

Negative sequence reactance-X2

0.16

0.15

-

Aero seq. reactance-X0

0.28

0.033

-

Transient time constant-Td’

1.1

0.376

1.8

Sub-transient time constant-Td”

0.035

0.02

0.035

% phase voltage

pf lead OCC Unity pf 0.8 lag zero lag pf

f ield current

Figure 3.23  Field current vs. phase voltage.

% Normal Voltage

Lead pf

0

Figure 3.24  Voltage versus current.

Unity pf Lag pf IL

Generators  97 0.8 pf lead

% Load current

150

Unity pf 0.8 pf lag

100

zero pf lag

0

100 150 Field current

200

Figure 3.25  Field current versus load current.

3.23 Salient Pole Machine with Two Reaction Theory In cylindrical rotor machines, the air gap between stator and rotor is uniform and has only one Synchronous reactance X (also can be termed direct axis reactance), whereas in the case of salient pole machine, the effect of saliency of rotor poles is non-uniform air gap flux. Here the armature reaction will be considered w.r.t. two axes, viz., detect axis (Xd) and quadrature pole axis (Xq). The phasor diagram of same is shown in Figure 3.26.



Here Id = (Ep-VpCosδ)/xd

(3.10)



and Iq = (Vp-VpSinδ)/xq

(3.11)

Ia(Xd-Xq) +d

IaXd

o

Iq

IqXq

IaXq

δ

V1 IaRa

θ

E1

Ia Id

Figure 3.26  Phasor diagram of salient pole machine.

N

Stator -d

IdXd

+q

-q

S

S

+d

direct axis

N

quardant axis

98  Electrical Equipment

3.24 Hunting The oscillation of load angle (δ) about the mean position, is called hunting of machine. Whenever the load on machine changes, load angle also changes for stable operation. But the load angle tends to overshoot due to inertia, which leads to power reversal and adjusts after some time. To overcome this problem, damper winding is used in salient pole machines. This is not required for cylindrical pole machines because the cylindrical poles themselves acts as dampers. They also help in maintaining balanced three-phase voltages during unbalanced load condition. They are mounted on rotor surface of salient pole machine. Under normal operations, they do not carry any current since rotor runs at synchronous speed.

3.25 Stability and Swing Equation The tendency of Synchronous machine to remain in synchronism in the power network is called stability. The machine operation is said to be stable if the machine does not fall out of step during system disturbances like sudden increase in load and faults. The stability is of two types, namely, steady state stability and transient stability. Steady State Stability is nothing but the maximum power transfer possible with minor disturbing forces, which occurs at a load angle of 90°.

Steady State Stability maximum Power, Pss = VpEp(Sin90o)/ (3.12) xs=VpEp/xs Transient Stability is nothing but the maximum power transfer without losing synchronism under sudden load changes in the network. By assuming a safe load angle of 30°.



Transient Stability Power, Pts =VpEp(Sin30o)/xs= 0.5 VpEp/xs

(3.13)

i.e., Pts = 50% of Pss The dynamics of a generator is determined by the swing equation;



M (d2δ/dt2)= Pa-Pi-Pe

(3.14)

Generators  99 where M = Inertia constant δ = load angle t = time = accelerating power Pa = mechanical power input Pi = electrical power output Pe Following are the essential factors in stability related issues: i. ii. iii. iv. v.

Inertia of prime mover and generator Input torque of prime mover Shaft load output torque Internal voltage of generator Reactance of generator and system

3.26 Prime-Mover Rating Plates i.

Diesel Engine: a. Cummins make: Model: VTA 1710 C – 800 –G Where V = Type of cylinders ’arrangement (V – shape) T = Exhaust Turbo – charger A = After cooled 1710=Displacement in Cubic machines G = Application of engine (Generator) C = Internal charge air to water cooling 800 = H.P. Rating b. MTU make: Model: 12-V-396-TC-33-4 Where 12 = Number of cylinders V = V-shaped cylinders 396 = Engine series of 100 times displacement = 3.96 liters TC = Turbo Charged

ii. Steam Turbine: The rating plate of a 12.5 HP Marine Steam turbine is shown in Figure 3.27 as a sample.

100  Electrical Equipment

Figure 3.27  Marine steam turbine.

3.27 Effect of Unbalanced Loads and External Faults During external faults, i.e., at grid side, the generator voltage becomes unbalanced causing negative phase sequence current component in stator current. The flux due to this negative phase sequence (NPS) current rotates in the direction opposite to that of main rotor direction. This causes overheating of rotor winding. The duration of such non-symmetrical NPS current (I2) shall not be be continued for more time and is to be protected with the help of NPS protection relay. The permissible time duration of NPS current (I2) based unit’s magnitude is given in Table 3.7.



% S = 100 ×(ratio of NPS current/rated current)



= 100×(I2/Ir)

(3.15)

In case of a L-L fault, I2 = Isc/√3 where Isc = Rated short-circuit current The governing equation of a generator’s ability to withstand unbalanced currents is:

I2 2 t = K



(3.16)

Table 3.7  Permissible NPS current and time. I2(p.u)

0.1

0.2

0.3

0.4

0.5

0.7

Time (milliseconds)

1000

200

60

30

20

10

Generators  101 where K = a constant which depends upon type of machine and cooling. = 7 to 60 As per standards, t < 30 for cylindrical machine < 40 for salient pole machine < 70 for Salient Pole Machine with air-cooled stator The maximum value shall not go beyond 70. An example of a 30 MW TG is: K = 8% continuous = 13% for 30 minutes

3.28 Voltage Regulators The common working principle of voltage regulation is use of a voltage sensitive element acting against a standard reference setting to change the effective resistance of exciter field. It works based on the error between the reference setting and real voltage which is independent of speed. They will manage reactive power during parallel operation of alternators. They work on PID (Proportionate, Integrate and Derivative) principle. There are various types of Automatic Voltage Regulators (AVR) like Static and Digital Voltage Regulators (DVR). The main characteristic requirement of AVR is Excitation Response Ratio (ERR). It is the rate of change of excitation voltage and shall do the job of output voltage from 100% to 200% in less than 0.3 seconds.

ERR = Rate of voltage build up(volts/sec)/normal exciter voltage on full load (3.17)

= 4 to 5.

High initial response systems will attain 95% of ceiling voltage (maximum voltage of exciter) in 0.1 second or less.

3.29 Parallel Operation of Alternators Under Different Conditions An example of two alternators connected to a grid are considered and their operation is illustrated in the vector diagram, as shown in Figure 3.28.

102  Electrical Equipment Et2

Et

Et1 jIaXs

δ1 ф2 Ia1

δ δ2

фL

jIa2Xs

jIa1Xs

VL=VT IL

Figure 3.28  Vector diagram for two alternators working in parallel.

Here ET, ET1 and ET2 = Terminal voltage of load, alternator 1 and alternator 2 respectively I2 = Load current δ, δ1 and δ2 = Load angle of total, alternator 1 and 2, respectively. In parallel operation, a. The active power supplied to the common load by each generator is controlled by their prime mover throttling. b. The reactive power sharing between two generators depends on their droop characteristics. The conditions to be satisfied for parallel operation of two generators are: a. Same voltage b. Same frequency c. Zero phase angle between two systems. In case of mismatch of above conditions, the operation will be as follows: a. Voltage difference: Here an unbalance of 5% is allowed and effects of major difference is illustrated in Figure 3.29. Here, V1 = Voltage of existing generator or bus V2 = Voltage of incoming generator V12 = Resultant Voltage The current is lagging w.r.t. V1 which causes a demagnetizing effect on field of running machine. Whereas the current is loading w.r.t. V2 and will cause strengthening of machine field.

Generators  103

V12

V2

O Motoring action

.. I

Generator action

V1

Figure 3.29  Voltage difference operation.

b. Phase difference: A phase angle difference up to 3° is acceptable. If this difference is high, mechanical reactions of an oscillating nature will be transmitted to the machine foundation. c. Frequency difference: It is a common practice, to synchronize the incoming generator with which they will adjust to a common frequency. If the difference is too high, oscillations will appear in the system, but will be damped by rotor losses.

3.30 Induction Generator Induction Generator (IG) is nothing but an Induction Motor running above its synchronous speed (Ns) as shown in Figure 3.30. It is driven by mechanical (wind turbine or mini hydro-turbine) or other means at above its Ns and always operates with leading power factor. Induction generator draws its excitation current capacitively from the source, unlike inductively by Motor. Here the slip is negative, since it runs at super-synchronous speed. It is mostly used in wind power applications and is popularly known as WEG (Wind Energy Generator).



Motor: P = VI Cos Ø and S = -VI Sin (-Ø)



Generator: P = -VI Cos Ø and S = VI Sin Ø



Reactive power drawn from source, Q = √3*Vr*Im*10-3 (3.18)

where Vr = Rated Voltage of machine Im = Magnetizing current of machine

104  Electrical Equipment + Torque Tmax.

N

0.7

- Torque

0.8

0.9

Motoring action

Ns X

X

Speed

Generating action

Figure 3.30  Induction generator principle.

The excitation requirement: It is to be met either from grid or initially charged capacitors, as shown in a schematic diagram in Figure 3.31. Later on it can take from its own generation. The major drawback of IG is its high consumption level of magnetizing current of order of 20 to 35%. Excitation Capacitor requirement (Delta connected): The capacitor estimation (Delta connected) is explained with an example: Let us consider an Induction Motor of 20HP (15KW, 1480RPM, 3ph, 415V, 50Hz, 0.8 p.f, efficiency of 85% rating to be used as Induction Generator. Motor: I = 15000/√3*415*0.8*0.85 = 30 A S = √3*415*30*10-3 = 21.6 KVA P = 21.6*0.8 = 17 KW Q = √21.62-172 = 13 KVAR Reactive Power per phase = 13000/3 = 4,333 VA Capacitive Current (Ic) = 4333/415 = 10.44 A Capacitive Reactance (xc) = 415/10.44 = 40 Ω Capacitance per phase = 1/(2*Π50*40) = 80 uF.

P T

IG Q

Figure 3.31  Schematic diagram of IG.

Grid or load Excitation capacitor

Generators  105

W

Wind Turbine Gear Box

T

Stator Field (Rotor)

Grid or Load 3 Phase

DC C

Power Converters

Figure 3.32  DFIG for WECS.

3.31 Doubly Fed Induction Generator The latest trend is to use a Doubly Fed Induction Generator (DFIG) for power conversion, particularly for wind turbine application. The schematic diagram of DFIG used to Wind Energy Conversion System (WECS) is shown in Figure 3.32. In DFIG, both field winding (rotor) and armature winding (stator) are separately connected to the external equipment as shown in the figure. The type of convertors are of bidirectional power flow for supply of DC excitation from grid or to give supply to grid when DFIGS are used for wind power application, possible by adjusting the field winding supply. Lots of research is going on regarding the topology of converts and protection issues.

3.32 Latest Trends in TG Technology The TG market is expected to see a CAGR of above 3.81% between 2020 and 2025 with 2019 as the base year. Now research is going on for cryogenic cooling of TG. From the present established practice of liquid or hydrogen cooling practices. In cryogenic cooled Turbojet Generators, rotor will have a superconducting winding. A prototype model of superconducting Turbo-Generator is shown in Figure 3.33. The main advantages of these generators are reduced resistive loss, improved efficiency, and reduced size which gives higher power density. But the associated problems are the cost of cryogenic cooling and its maintenance.

106  Electrical Equipment

Bearing

Super conducting coil Shield Damper core Stator coil

Vaccuming Coil cover

cooling chamber

Bearing slip rings

Seal

Figure 3.33  Superconducting Generator. (Source: http://engineering.electrical-equipment.org) PREVENTIVE MAINTENANCE OF SALIENT POLE ALTERNATOR Tag No./ Name of the equipment:

KVA DG Set

Supply: 3 phase, 415 Volts, 50 Hz. S1.No. of Alternator: S. No 01. 02. 03.

04. 05. 06. 07. 08. 09.

10.

Description Complete cleaning / dust blowing of surface if alternator. Visual check of terminal box, terminal insulators and studs. Check of Insulation Resistance(IR) values (with 500V IR Tester for main winding and 250 V Megger for other wdgs.) (Minimum 1.5 MΩ) Visual check of the Stator and field windings. Check the wedges and other internals for looseness Air blowing of internals and cleaning with cloth Check the rotating bridge and diode assembly for loose contacts and tighten them. Greasing of bearings Checking the Automatic Voltage Regulator and Power Regulator and their cleaning Checking the earth connections.

Figure 3.34  PM checklist of generator.

Make: Result

Remarks

Main winding= Aux. winding= Field winding=

DE:

MΩ MΩ MΩ

NDE:

Generators  107

3.33 Maintenance The Preventive Maintenance checklist of a Salient pole generator is shown in Figure 3.34.

3.34 Fault Finding Following are the brief details of troubleshooting of generator (Table 3.8).

3.35 Generator Failure Modes In general, failure of a generator took place due to any one of the following reasons. i. Electrical ii. Thermal and iii. Mechanical i.

Electrical failures are due to ageing & degradation of insulation, internal & external short circuits, loose connections, inter turn faults, rotor bars damage, end ring problem and earth faults. ii. Thermal failures are due to insulation breakdown, aging, overloads, loose connections in adequate cooling, higher, ambient temperature and rotor bars & end ring failures. iii. Mechanical failures are due to wrong alignment, poor lubrication, weak foundation and uneven air gap.

3.36 Tests on a Turbo-Generator There are different types of tests to be done on each and every machine as per respective regions’ applicable standards. The main tests are shown in Figure 3.35. i.

Type tests: Following are the type tests: a. Measurement of dc resistance of stator and rotor windings;

108  Electrical Equipment Table 3.8  Troubleshooting of generator. Fail to start or accelerate

1. loose connection of wires and cables 2. overloading 3. broken or shorted rotor bars 4. open end rings

Severe vibrations

1. foundation weakening 2. wrong alignment 3. faulty coupling 4. broken bars or faulty end-rings

More noise

1. wrong alignment 2. broken rotor bars 3. unequal air-gap between stator and rotor 4. air-gap contamination/choking with foreign material/dust particles

Winding overheating

1. overloads 2. loose connections 3. low voltage or unbalanced voltage in Poly phasor system 4. problem in cooling system 5. internal short circuits

Bearings overheating

1. lack of lubrication 2. leak of oil, in case of oil lubricated bearings 3. higher ambient temperature 4. wrong alignment 5. ageing 6. bearing 7. foundation platforms

Sparks from generator or terminal box

1. insulation failure 2. loose connections 3. overloads 4. carbon brushes failure

Rotor earth fault

1. ingress of oil or dust into the generator 2. sparking at brush gear of excitation

b. Insulation of stator and rotor winding (before and after high voltage tests), bearings and embedded temperature detectors; c. High voltage of stator and rotor windings;

Generators  109 Tests Type tests

Routine tests

Acceptance tests

To be done on f irst machine of each design

To be done on each and every machine

To be done after erection at location

Figure 3.35  Types of tests.

d. e. f. g. h. i. j. k. l. m. n. o. p. q.

Phase sequence; Determination of open-circuit characteristics; Determination of short-circuit characteristics; Over speed; Determination of efficiency by separation of losses method; Vibration; Air leakage for hydrogen cooled generators: Impedance of the rotor windings; Temperature rise; Instantaneous short-circuit test and determination of transient and sub-transient reactances and time constants; Determination of THF (Total harmonic Factor); Determination of zero and negative phase sequence reactances; Heat run test with one cooler out of operation; Retardation test.

ii. Routine tests: a. Measurement of dc resistance of stator and rotor windings; b. Insulation resistance of stator and rotor winding (before and after high-voltage tests), bearings and embedded temperature detectors. c. High voltage; d. Phase sequence; e. Determination of open-circuit characteristics: f. Determination of short-circuit characteristics; g. Over speed; h. Pressure test on coolers for close-circuit cooling;

110  Electrical Equipment i. Determination of efficiency by separation of losses method; j. Over pressure hydraulic test on stator frame for hydrogen-cooled machines; k. Vibration; l. Air leakage for hydrogen cooled generators m. Impedance of the rotor windings. iii. Acceptance tests: a. Measurement of the insulation resistance of stator and rotor winding against the frame and between phases after drying of the machine, b. Measurement of dc resistance of all windings, if required. In case of generators provided with brushless excitation resistance of stator windings only to be measured. c. High voltage test at 80% of the test voltage. The test made on the windings on acceptance shall, as far as possible, not be repeated. d. Temperature rise test on load; e. Check up of insulation resistance of bearings only in case the bearing pedestals are insulated in two horizontal planes separated by a metallic sheet. f. Vibration test. g. Hydrogen leakage test (if applicable). h. Pressure tests in the coolers (if applicable).

3.37 Tests on Engine-Driven Generator Following are the various types of tests: i.

Type tests: a. Measurement of resistance; b. Phase sequence test (for 3-phase generators only); c. Regulation test; d. Measurement of open circuit characteristic only; e. Measurement of short circuit characteristic only; f. Efficiency test; g. Temperature rise test; h. Over speed test (120% of rated speed);

Generators  111 i. Insulation resistance test (before and after high voltage tests); j. High voltage test; k. Determination of deviation of voltage wave-form from sinusoidal; l. Momentary over current test; m. Test for vibration severity. ii. Routine Tests a. Measurement of resistance, b. Insulation resistance test, c. Phase sequence test (for 3-phase generators only), d. Regulation test, e. Measurement of open-circuit characteristic, f. Measurement of short circuit characteristic, g. High-voltage test.

3.38 Gaps and Research Opportunities The following gaps are visible in conventional generators: • Cost of cryogenic cooling and its maintenance for super conducting generators • Slow responding speed governors, which shall be as fast as a wind gale • Low excitation Response Ratio, which shall be as fast as clouds • Lack of skilled people • Low efficiency of Coal-based power plants of around 35 to 40% • TG capacity now 1300 MVA Opportunities for research and business: • • • • •

Alternate cooling systems Digital speed governors Digital Voltage Regulators for better than now AI in TGs Business of RLA Study, Condition Monitoring and retrofits with new technologies.

4 Induction Motors 4.1 Introduction Electrical motors are the prime movers of any work whether in domestic, commercial or industrial applications. They have been working continuously with highest reliability since their invention by Tesla and Ferraris in 1889. There are various types of electrical motors based on their input supply, working principle, speed and construction. The broad classification of electrical motors is shown in Figure 4.1. Even though there are various types of motors, this chapter mostly deals with squirrel cage induction motors, which are widely used in industries globally.

4.2 Comparison Between Various Types of Motors i. AC and DC motors: AC motors work on the inductive principle, i.e., rotor gets flux through induction only. In contrast, DC motor works on conduction principle, i.e., both stator and rotor gets supply separately. For the same rating, the size of DC motor is more than that of AC motor with lesser efficiency. The wear and tear maintenance work of a DC motor is more than that of an AC motor due to the presence of rotor winding and brushes. ii. 1-phase and 3-phase AC motors: Even though the working principle of both are the same, a single-phase motor will not produce a rotating synchronous magnetic field but a pulsating field. Hence some starting arrangement is required. For the same power rating, single-phase motors are bulky and less efficient than poly phase motors. However, they are generally used for small H.P. ratings, where only single phase is available. iii. Synchronous and Asynchronous (Induction) motors: Syn­ chronous motor always runs at synchronous speed only, whereas an induction B. Koti Reddy. Electrical Equipment: A Field Guide, (113–184) © 2021 Scrivener Publishing LLC

113

114  Electrical Equipment Motors AC 1 phase

DC 3 phase

Self-excited

Synchronous Asynchronous or Induction AC Series Squirrel Cage

Special Unexcited Synchronous Universal Repulsion

Separatelyexcited

Shunt Compound Series

Slip Ring

Long Shunt

Short Shunt

Reluctance Hysteresis Concentrated pole Distributed

Repulsion induction Repulsion start-induction run Capacitor start and run

Induction

Split phase

Capacitor start & induction run

Shaded

Figure 4.1  Classification of motors.

motor runs at a slip of 1 to 5%. A synchronous motor requires a starting arrangement along with field supply. T α V2 → Induction motor T α V→ Synchronous motor iv. Squirrel Cage and Slip Ring Induction Motors (SCIM and SRIM) are compared as shown in Table 4.1.

4.3 Working Principle of 3-Phase Induction Motor When a three-phase supply is given to a stator of 1200 displayed, in space, windings, they produce a resultant magnetic flux (Ø) of constant magnitude (=1.5 Øm), as shown in Figure 4.2 at Synchronous speed (Ns = 120f/p). Here the magnetic field is rotating, since the winding is distributed and the resultant vector of three phases is changing its position at every position as if it rotates at synchronous speed. The resultant flux passes through the air gap, sweeps past the rotor surface and cuts the rotor conductors. An e.m.f. is induced in the stationary

Induction Motors  115 Table 4.1  SCIM and SRIM. S. no.

SCIM

SRIM

1

For starting: Either DOL (Direct On Line) or other starting arrangement is required.

Slip rings, brush gear, shortcircuiting device and starting resistance are required.

2

Starting Torque is poor

It can be increased with external resistance in rotor circuit.

3

Lesser cost than SRIM

Costlier

4

High efficiency

Low efficiency

5

Less maintenance

More maintenance

6

Copper losses are comparatively less

Higher copper losses

7

Less overhang portion of internals

More overhang portion

8

Used in almost all applications

Used for cranes, lifts and generators

9

Simple and rugged construction

Complicated construction

rotor conductors, due to the relative speed between stator and rotor. Now the current starts circulating and has a frequency of s x f (slip times supply frequency). Now the rotor starts rotating to catch up the rotating flux and causes torque development. Slip-Ring Induction Motor: In SRIM, the wound rotor is provided with distributed winding, the number of poles rotor are equal to that of rotor. The rotor winding is starred internally. The rotor winding is connected with external resistors with the help of slip rings as shown in Figure 4.3. At start-up, the resistance is in winding to get more torque and taken out after attaining full speed.

4.4 Construction of SCIM The exploded view of a squirrel cage Induction Motor, containing major parts, is shown in Figure 4.4.

116  Electrical Equipment I(R)

Ø

II(Y)

III(B)

Øm 1

0

60º

4

3

2 120º

θ

180º

(a) Three phase waveform II Y R Ø1

I

Ø2

Ø2

120 B

III

Ør = 1.5 Øm θ=0º

(b) Normal

Ø

3

θ = 120º

Ø1

θ = 60º (d) Φ=60

60 60 (f )

09

Ør = 1.5 Øm

Ø2

09

Ø2 Ør = 1.5 Øm

(c) Φ=0

60

60

Ør

Ø3

120 120 Ø3

Ø1

60 60

Ør = 1.5 Øm θ = 180º

(e) Φ=120

Figure 4.2  Three-phase induction motor working principle.

(f) Φ=180

Ø3

Induction Motors  117

Starting resistance

Stator supply

Brushes SRIM Slip rings

Rotor control

Figure 4.3  SRIM Schematic diagram.

Stator Fan Stator winding Rotor

End cover Bearing

Terminal Connections

Figure 4.4  Exploded view of SCIM.

Stator: It is made up of a large number of stampings, i.e., slots, to accommodate windings. The laminations are made up of high permeability silicon steel. Its outer frame is made up of mild steel (MS) or Cast Iron (CI) and it generally has fins for cooling purpose. The winding is made up of copper, either wires for lower H.P ratings or bars for higher H.P. rating. Rotor: It is made up of silicon steel laminations and slots for either aluminum (lower rating) or copper conductors (for higher rating). The conductors are generally bars and are brazed or electrically welded in slots. The end windings are short-circuited with end-rings. More bars in a rotor gives almost sinusoidal current. Other accessories: Some other accessories like cooling fans, terminal boxes, etc., are used in motors based on their HP and voltage rating, type of cooling, type of loading and installation location. The exploded view of a DC motor, Single-phase AC motor and Slip Ring Induction Motor are shown in Figures 4.5, 4.6 and 4.7, respectively.

118  Electrical Equipment Fan Commutator

Brushes

Rotor

Stator End cover

Figure 4.5  Exploded view of DC motor. Stator

Neutral Earth

Phase

Rotor

End cover

Body frame Shaft

Figure 4.6  Exploded view of 1-phase AC motor.

Winding

Shaft

Brishes

Slip Rings

Figure 4.7  Exploded view of SRIM.

4.5 Equivalent Circuit of SCIM It is shown in Figure 4.8. Here, K = E2/E1 is constant



RL′ =

R L R  2  1  =  − 1 with load resistance referred to rotor K2 K2  s  (4.1)

Induction Motors  119 I

I2

I1 R1

X1

I0

Z1

X2

RL‘ =

‘

Z2 R0

X0

V1

Stator

‘

‘

R2

‘

=

‘

RL K2 R2

K2

1 s-1

E2 = E1

Magnetisation circuit

Rotor

Figure 4.8  Exact equivalent circuit of SCIM.

I1 X01

R01

RL’

Z01 V1

Figure 4.9  Approximate equivalent circuit.

R2 K2 X X 2′ = 22 K

Where,  R2′ =



I2 =

sE2

R22 + (sx 2 )2



(4.2)

For simplicity, the above circuit can be drawn, as shown in Figure 4.9, which is popularly known as approximate equivalent circuit. Here the excitation circuit is neglected.

4.6 Torque-Speed Curve of SCIM The Torque-speed curve of a SCIM is shown in Figure 4.10, which is based on the following equation:

120  Electrical Equipment

Torque

Starting torque

Pull out or maximum torque = 2 - 3.5 TFL

Tmotor

= 1.2 - 1.5 TFL

Pull up torque

Accelaration torque

Tload 0

Speed

Full load torque N Ns

Slip

Figure 4.10  T-N Curve of SCIM.



sE22 R2∅ T= 2 R2 + (sx 2 )2

(4.3)

Where, T = Torque developed by motor Ø = Flux



s = slip = (Ns-N)/Ns

(4.4)

in which Ns = Synchronous Speed = 120f/p where f = frequency p = number of poles E2 = Rotor induced e.m.f. R2 = Rotor resistance X2 = Rotor reactance N = Normal Speed i. When s = 0, T = 0 i.e., Torque is zero at synchronous speed. ii. At start, where s = 1 the Torque is starting torque. It takes a dip, due to inertia, and goes to maximum and then settles at normal speed level, where the load torque and motor torque are equal. iii. At normal speeds (i.e., N): (sx2)2 is small and hence neglected T α s/R2 i.e., T α s if R2 is constant, so the curve is a straight line.

Induction Motors  121 iv. At higher slip values, the Torque is also higher and reaches a maximum point (Tmax) further increase in slip, R2 is negligible, when compared to sx2; T α 1/s, the curve is hyperbola. Ts = Starting Torque 1.2 to 1.5 TL Ts is maximum when R2 = X2

TL = 3



sE2 R2 R + (sx 2 )2

(4.5)

2 2

TL is maximum when R2 = sx2 Ts/Tmax = 2a(1+a2), where a = R2/X2

4.7 T-S Curve for SRIM The Torque–slip curve of SRIM is shown in Figure 4.11. The starting torque can be increased by inserting more resistance in rotor circuit, as shown in the figure. This addition will also help in the following ways: R5

R4

R3

R2

R1

Increasing Resistance R1

Torque

R6

R1100

T6

85

>85

A1

B1

C1 L1

A2

B2

L2

230 V AC

C2

Figure 4.28  Winding polarity check.

glows brighter than L1; then similar ends (i.e., B1 & C1) are joined. The test can be repeated for other phases. In place of Lamp, a voltmeter can be used in place of lamps and if V2 (i.e., L2) reads more than V1 (i.e., L1), then similar ends (B1 & C1) are joined.

4.24 Tests on Induction Motor The tests specified in any standard shall normally be carried out at the manufacturer’s works. The following types of tests are to be done on induction motors: A. Type tests: a. Dimensions b. Measurement of resistance of windings of stator and wound rotor c. No load test at rated voltage to determine input current power and speed

Induction Motors  157 d. Open circuit voltage ratio of wound rotor motors (Slip ring motors) e. Reduced voltage running up test at no load (for squirrel cage motors up to 37 kW only) f. Locked rotor readings of voltage, current and power input at a suitable reduced voltage g. Full load test to determine efficiency power factor and slip h. Temperature rise test i. Momentary overload test j. Insulation resistance test k. High voltage test *l. Test for vibration severity of motor *m. Test for noise levels of motor *n. Test for degree of protection by enclosure *o. Temperature rise test at limiting values of voltage and frequency variation; *p. Over speed test and *q. Test on insulation system * These are optional tests subject to mutual agreement between purchaser and the manufacturer. B. Routine Tests The following shall constitute the routine tests: a. Insulation resistance test b. Measurement of resistance of windings of stator and wound rotor c. No load test d. Locked rotor readings of voltage, current and power input at a suitable reduced voltage e. Reduced voltage running up test (see 23.2) (for squirrel cage motors) f. Open circuit voltage ratio of stator and rotor windings (for slip ring motors); rotor; g. High-voltage test C. Performance Tests i.

No Load Test The motor shall be run at rated voltage and frequency given on the rating plate. The motor shall run to its normal speed and shall not show abnormal electrical or mechanical

158  Electrical Equipment

ii.

iii.

iv. v.

noise. The input power, current and speed shall be measured and used in the determination of no load losses and efficiency at full load. Reduced Voltage Running Up Test The test is applied to squirrel cage motors. The test is made to check the ability of motor to run up to its rated speed at no load. The motor up to 37 KW shall be supplied with reduced voltage l/√3 of rated value for each direction of rotation. For motors above 37 kW, the voltage shall be 1/√3 of rated value or less but motor shall be run only in the specified direction of rotation. Open Circuit Voltage Ratio Test for Wound Rotor (Slip Ring) Motors The stator of the motor is supplied with rated voltage and open circuit voltage at the slip ring shall be determined (by lifting the slip ring brushes). The voltage shall comply with the declared values of the manufacturer. Locked Rotor Test The test may be carried out at reduced voltage. The readings of the input current, power and breakaway torque shall be determined. Full Load Test The motor shall be supplied with rated voltage and load on the shaft shall be adjusted such that it delivers the rated output. The value of voltage, power input, current and speed shall be measured. The efficiency determined for full load shall not be less than the declared values.

4.25 Maintenance Preventive maintenance (PM) of IM will have the following objectives: • • • • •

To reduce the motor failure and increase the availability To optimally use Men and Material (Spares) To maximize the performance of motor To extend the useful life beyond the designed shelf life To be SDRAM • Sustainable – able to be maintained at a certain rate/level • Dependable – faithful trusty • Reliable – Consistently good in quality

Induction Motors  159 • Available – the quality of being able to use • Maintainable – capable of being repaired • To decrease the maintenance cost, as shown in Figure 4.29. A. PM can have different models based on location and experience. Some of the models are shown in Figure 4.30. B. Recommended Maintenance Schedule is as follows: I.

Daily Maintenance –– Examine visually earth connections and motor leads. –– Check motor windings for overheating (the permissible maximum temperature is above which can be comfortably felt by hand). –– Examine control equipment. –– In the case of oil ring lubricated motors: –– Examine bearings to see that oil rings are working; –– Note temperature of bearings; –– Add oil, if necessary; and –– Check and play.

II. Weekly maintenance –– Check belt tension. In cases where this is excessive, it

Maintenance cost curve In adequate maint.

Excessive Optimum maint. Maint. Optimum level of preventive maintenance Cost of preventive maintenance work Total cost

Cost

Cost of equipmentrelated production losses Amount of preventive maintenance work done

Figure 4.29  PM cost.

160  Electrical Equipment Preventive Maintenance

Time scheduled maint.

CBM (Condition Based Maint.)

Scheduled replacement maint.

• Time bound • PM-1 (Half-yearly) • PM-2 (Overhaul-2 - 4 Years) > Less expensive > Widely used

Replacement of • Parts like bearings • Motors > Simple > Expensive

On-line tests

Off-line Shut down maint.

> Measuring motor paraments with Sensors/Instruments & Prediction • Visual • Vibrations • V.I. PF • Sound • Temperature > Leads to results based maint

Digital methods

Classical Methods > Electro-mechanical devices > Less ef f icient > Slow response > Low reliability

Use of ICs, uPs, uCs

(a) General PM of motor Tiered type Maint. Model (Based on Population, predictive tools, criticality, Safety & economics)

Moderate maint.

Minor maint. - For non-critical eqpt. - Normal motors • Failures are acceptable • Replace on fail

- Elec parts can fail - Care of Mech. parts like bearings

Comprehensive maint.

Trend able maint.

- For large size motors

- Extensive maint. - For critical eqpt. - Safety oriented - Economics

(b) Tiered PM of motor IEEE-Preventive / Predictive Techniques

Trendable tests V, I, N Temperature IR, PI, CSA Hi-Pot, Vibration

Inspection techniques External oil: Oil level air passage Smell, sound

Other diagnostic tests PD, Tan delta Surge comparison tests

(c) IEEE Recommended PM of motor

Figure 4.30  PM models of IM.

Induction Motors  161 –– Should immediately be reduced and in the case of sleeve bearing machines the air gap between rotor and stator should be checked. –– Blow out windings of protected type motors situated in dusty locations. –– Examine starting equipment for burnt contacts where motor is started and stopped frequently. –– Examine oil in the case of oil ring lubricated bearings for contamination by dust, grit, etc. (This can be roughly judged from the color of the oil.) III. Monthly maintenance –– Overhaul Controllers. –– Inspect and clean oil circuit breakers. –– Renew oil in high-speed bearings in damp and dusty locations. –– Wipe brush holders and check bedding of brushes of slip-ring motors. IV. Half Yearly maintenance –– Clean windings of motors subjected to corrosive or other elements; also bake and varnish, if necessary. –– In the case of slip-ring motors, check slip-rings for grooving or unusual wear. –– Check grease in ball and roller bearings and make it up where necessary taking care to avoid overfilling. –– Drain oil bearings, wash with petrol to which a few drops of new oil are added; flush with lubricating oil and refill with clean oil. V. Annual Maintenance –– Check all high-speed bearings and renew, if necessary. –– Blow out all motor winding thoroughly with clean dry air. Make sure that the pressure is not so high as to damage the insulation. –– Clean and varnish dirty and oily windings. –– Overhaul motors which have been subjected to severe operating conditions.

162  Electrical Equipment –– Renew switch and fuse contacts, if damaged. –– Check oil. –– Renew oil in starters subjected to damp or corrosive elements. –– Check insulation resistance to earth and between phases of motor winding, control gear and wiring. –– Check resistance of earth connections. –– Check air gaps. –– Test the motor overload relays and breakers. VI. Records: Maintain a register giving one or more pages for each motor and record therein all important inspection and maintenance works carried out from time to time. These records should show past performance, normal insulation level, air gap measurements, nature of repairs and time between previous repairs and other important information which would be of help for good performance and maintenance. C. Condition Monitoring Techniques and Tests Even though SCIM are the most reliable electrical equipment, there are chances of failures due to various reasons mentioned above. These faults, before they lead to major damage, are to be identified along with a sound Condition Monitoring and Corrective Action System (CMCAS) to avoid future faults. This will avoid unwanted shutdowns and also enhance the life of the motor. Also the present trend is to go for condition monitoring (CM) instead of traditional scheduled maintenance to cope with the current time and financial constraints. A simple flow chart of CM is shown in Figure 4.31. D. Tests to be conducted on L.T. Motor: The following tests, to the extent possible at field, can be done to assess the condition of motor: • • • • • • • • •

Visual inspections. Measurement of Stator winding resistance. Measurement of IR, PI and DAR. Leakage Current Measurement. Flux loop test. Surge comparison test. No-load test. Load test. Alignment and Coupling.

Induction Motors  163 M

Start CM

NO

Example:

Measure required signal

Vibration Fault Yes Diagnosis

Prognosis Action

Poor lubrication Likely course of action is Greasing Apply grease

Figure 4.31  CM flow chart.

• Shock pulse test and Vibration analysis. • IR Test (Insulation resistance): It is a function of type and condition of insulating material used to evaluate the insulation. It is to be done by injecting 500V DC supply between a Conductor and body/ground, to know the healthiness of insulating material. The minimum value shall be KV+1 in Mega Ohms. This value shall be taken after 60 seconds of injecting voltage and the readings will be as shown in Figure 4.32. • DAR test (Dielectric Absorption ratio): For some of the equipment, where the absorption current (to be explained in next section) decreases, IR measurements after 30 seconds and 60 seconds will be taken to find out the quality of insulation. • DAR= R60/IR30< 1.25 (Questionable, not good & requires re-insulation); < 1.6 OK (Adequate); >1.6 Good but DAR>1 is dangerous. • PI test (Polarization Index): When D.C. supply given to insulation, the total leakage current (It) is divided into IL, Ic, Ia and IG as shown in Figure 4.33. • Capacitive current (Ic): If a DC voltage is applied to an insulation, electrons will rush into the negative plate and be

164  Electrical Equipment

to record this reading

IR

0

Time

60 Sec

Figure 4.32  A typical IR vs. time curve (Spot reading).

Relative current

100

Leakage (IL)

10

Total (IT)

Absorption (Ia) Conductance (IG) 1

Capacitance (IC) 0

1 5 10 Time Of Voltage Application (minutes)

Figure 4.33  PI test on insulation.

drawn from the positive plate. Initially, this current will be very large but it will die fast and reaches zero. • Absorption current (Ia): The charges that form on the plates of the capacitor attract charges of the opposite polarity causing the charge movement and, thus current. Initially it is high but goes to lowest minimum in a few seconds. • Conduction current (Ic): This is due to aging and degradation of insulation.

Induction Motors  165 • Leakage current (Il): No insulator is perfect dielectric even as new and will have some leakages. • PI Value= IR after 10 minutes/IR after 1 minute. • This value is used to assess the quality of insulation. • PI= 2 (minimum); 2 Excellent insulations. • So for values less than 2, a decision for re-insulation, if drying does not improve the PI value, is to be taken. However, if the IR value is more than 5000 Mega ohms, there is no need to concentrate on. • Flux-loop Test: With this test, the damages to thin stator laminations due to rubbing of stator and Rotor or other problems which reduces stator failures and core losses will be identified. This also helps in detecting hot spots of stator core and necessary corrective actions like re-staggering. This test is done by circulating the rated current over the core as shown in Figure 4.34. • Surge comparison Test: It is used to find shorts and coil insulation weaknesses. A set of fast rising pulses are passed through the windings; the uniformity of all three waves indicates healthy winding. • No-load and load Tests: These tests are done to ensure that the current, speed, losses and temperature rise are within the prescribed limits. • PD Test: It is a small local insulation breakdown (Voids) ◦◦ Cause high-frequency destructive currents and overheat LV AC Supply Wire AC f lux

Slots Stator core

Figure 4.34  Flux-loop test arrangement.

166  Electrical Equipment ◦◦ Measuring tan delta and taking minor corrective actions at incipient and thus eliminates major breakdowns. ◦◦ API 541 recommends a partial discharge (PD) level of 100 pC (pico-Coulombs). • Shock Pulse Test: This test is used to identify the condition of bearings, lubrication and remaining life with replacement. This is used by measuring the shock pulses of bearing with the help of Shock Pulse Tester T2000 of SPM make, as shown in Figure 4.35. Any bearing generates shocks whose magnitude varies based on the age and their condition. It is done by inputting the shaft diameter and RPM of Motor form which its initial shock value (dBi) is known, as shown in Figure 4.36. • By measuring the carpet value (dBc of 15 to 35 is OK, which is due to aging) and maximum shock pulse value (dBm of above 35 is not acceptable). Based on the results, the condition is known as Green (OK), Yellow (caution) and Red (damaged and replacement is necessary). Pictorial view of dBi, dBc and dBm is shown in Figure 4.37. A sample checklist is shown in Table 4.17. • Vibration: Vibration measurement is an extended CM technique which is used to detect mechanical faults like bearings or mechanical imbalances. Can be used to record vibrations. For normal motors, a maximum of 3 mm/Sec is acceptable as per ISO-2372.

Discription of keys 2

1

3

4

5 6 13

7 8 12

9 10 11

Figure 4.35  SP tester.

1. Display 2. Condition Scale 3. Peak Indicator 4. Measuring Key (M) 5. Select Key (SPM/VIB) 6. Set (SET) 7. Arrow Key (UP) 8. Arrow key (DOWN) 9. Connector for shock pulse transducers (SPM) 10. Connector for tachometer probe and earphone (TAC) 11. Connector for vibration transducer (VIB) 12. Master reset (unmarked) 13. Program version display (unmarked)

Induction Motors  167 5600

dBi

3200

40

1800

35

1000

30

560

25

320 180

20

100

15

56

10

32

5

18 5

10 5,6

10

3,2

15 560

1000

320

180

100

56

32

18

10

Shaft dia in mm

0

20

Figure 4.36  dBi Values chart.

70

60

60

50

50

40

40

30

30

20

20 10

dBi

10

dBN

dBm dBc

0

0 –9

Figure 4.37  SP readings.

dBi = Initial value of a bearing dBc = Carpet value (weak pulses) dBm = Maximum value (strong pulses) dBN - Unit for normalized shock level

Shaft dia in mm

DBm> 20db (Green range).

IM No.

Date:

Speed in RPM

Condition Monitoring of Motors

Table 4.17  Checklist for CM of IM.

dBi DE

dBc NDE

SPM Reading DE

dBm NDE

DE

NDE

Condition Remarks

168  Electrical Equipment

Induction Motors  169 E. Motor Current Signature Analysis (MCSA) MCSA is a condition monitoring technique used to diagnose problems in induction motors like: a. Static and/or dynamic air-gap irregularities b. Broken rotor bar or cracked rotor end-rings, rotor eccentricity c. Stator faults (opening or shorting of one coil d. Changing frictional forces e. Bearing degradation f. Current unbalance A FFT (Fast Fourier Transformation) creates the spectral display and the algorithm perform a special analysis on V & I waveforms to detect faults. F. Thermal Scanning Thermal imagers capture heat-based images, using color to correlate every pixel of the electronic image to know hot spots. The image makes immediately clear what part of the motor is overheating and to what extent for necessary corrective before a failure. G. Life Extension As explained in preceding paragraphs, a sound CM policy with necessary follow up preventive measures will definitely increase the life of motors. An example of extending life with timely taken corrective action is shown in Figure 4.38. H. PM Checklists PM Checklists: Sample checklists for different maintenance techniques are shown in Tables 4.18 and 4.19. I. Latest Trends • Maintenance personnel can predict when a motor can fail based on historical events and experience. • In the modern age, Machine learning (AI) software can now assist them in predictive maintenance. • Smart motors: IoT-based sensors attached to body/frame of motor can monitor the performance wirelessly and convey status to control room/Smart phones. This technique is simple to install and all the data can be stored in the cloud with suitable communication protocol. • AI can be used for CM and fault diagnosis (GA/Fuzzy logic/ ANN).

170  Electrical Equipment After repair

New motor 1000 100

Effect of aging & Contamination

IR

Insulation failure

10 1 1

2

Year

3

4

5

Figure 4.38  Life extension example.

• Since CBM is based on data obtained from CM (Condition Monitoring), AI is best suitable for CBM. J. Software Tools: • All-Test Pro Device: Handheld device for motor monitoring purpose. • Useful in knowing the imbalances in voltage & current, • Harmonic measurement, Power factor, health of Mechanical systems like bearings. • Areva’s EMPATH (Elec. Motor Performance Analysis & Trending Hardware): • Measure and analyze motor V & I to check Rotor bar deterioration, eccentricity, bearing condition, etc.

4.26 Trouble-Shooting The details of commonly occurring faults and causes with necessary corrective action are shown in Table 4.20.

4.27 Heating and Cooling Curves of Induction Motor The motor losses are dissipated in the form of heat. The increase in temperature depends upon the amount of heat developed and dissipated. After start up, the motor attains a steady-state temperature where the heat produced is equal to the heat dissipated.

Induction Motors  171 Table 4.18 PM-1 (Half yearly). PM-1

PM MOTORS

Permit No.:

Date: From

To

Tag No./Name of the equipment: Sl. No. of motor:

Make:

S. No.

Description

Yes

No

Remarks

01.

Check the terminals, bushings insulators and studs in terminal box.

02.

Check the IR values (with 500V IR Tester) (Min. = 1.5 M Ω)

Phase to Earth = M Ω Continuity = OK/Not OK

03.

Check space heater healthiness (if available)

IR value = M Ω Resistance = Ω

04.

Lubrication of Bearings, if required at both DE & NDE sides.

05.

Visual observation of the motor for air flow at the cooling vents

06.

Check the earth connections of motor & Push-Button Station (PBS).

07.

Check & ensure the motor terminal box is sealed with Flash-Strip Tape (FST).

08.

Check terminations at Panel

09.

Other checks, if any

10.

Spares & Consumables used:

Outage: days

172  Electrical Equipment Table 4.19  PM-2 (Overhaul). PM-2 Overhauling of LT Motor Permit: Area: Date: from---- to----------KW rating: Frame; Bearing: DE/NDE Name of equipment: S. No. of Motor: Make: S. No.

Description

Yes

No

Remarks

01.

Visual check of terminal box, insulators and studs.

02.

Check of IR values (with 500V IR Tester) before & after overhauling (Min 1.5 MΩ)

03.

Visual check of the rotor and stator

04.

Check the wedges and short circuit ring looseness

05.

Air blowing of rotor cooling vents and cleaning with cloth

06.

Shaft condition at bearing seating area

07.

Bearing clearance shall be seen & Visual inspection of races, for any sittings/cracks.

DE: NDE:

08.

Whether bearing is replaced or not

DE: NDE:

09.

Preheat rotor winding after cleaning

10.

Application of bectol red on rotor

11.

Visual inspection of core for hot spots and looseness of stampings

12.

Checking of wedges for looseness by knocking and replacement if required

13.

Tightening of overhang position bracers

14.

Preheating of the stator after cleaning with petrol

Before= MΩ After= MΩ

(Continued)

Induction Motors  173 Table 4.19  PM-2 (Overhaul). (Continued) PM-2 Overhauling of LT Motor 15.

Checking of space heater IR & Resistance (500V IR tester) (min.1.5MΩ)

16.

Varnishing of the stator winding and baking Started at ______hrs. on Stopped at_____hrs. on

17.

Application of bectol red in the overhang portion and in the end Covers

18.

Painting of cooling fans and covers Terminal Box

19.

Assembly of the Motor

20.

Check stator winding resistance (R) and inductance (L) measurement with motor checker (RYB coils).

21.

Painting and shifting to the location

22.

Check alignment of the motor: ↑ ↑  ←

→

←

IR = MΩ Resistance = Ω

→

↓ ↓

23.

IR values of cable and cable termination including Incoming cables (min. 1.5 MΩ)

24.

Check no load run and direction and measure 3 Phase currents (I in Amps)

25.

Visual observation of the motor for air flow at the cooling vents

26.

Spares & Consumables used:

IR = MΩ

Outage: days

– Check shaft for warping or bearing wear. – Dismantle motor and remove dirt or dust with jet of dry air.

4. Uneven air gap

5. Dirt in the air gap

(Continued)

– Check motor alignment with machine running.

3. Incorrect alignment

– Brushes may be worn, dirty or incorrectly fitted.

6. Brushes

– Shaft can be bent; check rotor balance and eccentricity.

– Try to start motor under no-load conditions. If it starts there may be an overload condition or a blocking of the starting mechanism. Reduce load to rated load level and increase torque.

5. Overload

2. Distorted shaft

– Tighten all connections.

4. Loose connection at terminals

– Vibrations can be eliminated by balancing rotor. If load is coupled directly to motor shaft, the load can be unbalanced.

– Compare connections with the wiring diagram on the motor nameplate.

3. Wrong control connections

1. Unbalance

– Check voltage supply and ascertain that voltage remains within 10% of the rated voltage shown on the motor nameplate.

2. Low voltage supply

High Noise Level

– Check feed connections to control system and from control to motor.

1. No voltage supply

Motor fails to start

Corrective measures

Probable cause

Failure

Table 4.20  Troubleshooting chart.

174  Electrical Equipment

Slip Ring Motor Operating at Low Speed with External Resistance Disconnected

Failure

– Test circuit with a magneto, or other means, and undertake necessary repairs. – Clean slip rings and insulation assembly. – Select brushes of correct size. – Check pressure on each brush and adjust it accordingly. – File, sand and polish. – Machine on lathe or with portable tool without removing from machine.

3. Open circuit on rotor circuits (including connections with control apparatus)

4. Dirt between brush and slip ring

5. Brushes gripe on brush holders

6. Incorrect pressure on brushes

7. Rough surfaces on slip rings

8. Eccentric rings

(Continued)

– Bring control closer to motor.

– Check lubrication. Replace bearing if noise is excessive and continuous.

8. Worn bearings

2. Control too far from motor

– Tighten all foundation studs. If necessary, realign motor.

7. Loose motor foundation

– Reduce load or replace brushes.

– Dismantle motor and clean. Remove trash or debris from motor vicinity.

6. Extraneous matter stuck between fan and motor casing.

1. High current density on brushes

Corrective measures

Probable cause

Table 4.20  Troubleshooting chart. (Continued)

Induction Motors  175

Intense Bearing Vibration

Overheating of bearings

Failure

– Replace bearings. – Flush out housings and relubricate.

7. Hardened grease causes locking of balls

8. Foreign material in grease

(Continued)

– Before altering shaft or housing dimensions, it is advisable to ascertain that bearing dimensions correspond to manufacturer’s specifications.

– Add grease to bearing.

6. Lack of grease

3. Bearing rings too tight on shaft and/or bearing housing

– Check end shields for close fit around circumference and tightness.

5. Loose or poorly fitted motor end shields

– If bearing rings are in perfect condition, clean re-lubricate/replace the bearing

– Replace bearings before they damage shaft.

4. Rough bearing surface

2. Dirty or worn bearing

– Have shaft straightened and check rotor balance.

3. Deformed shaft

– Balance rotor statically and dynamically.

– Reduce belt tension.

2. Excessive strain on belt

1. Unbalanced rotor

– Remove grease bleeder plug and run motor until excess grease is expelled.

– Reset brushes correctly.

9. Poorly set brushes

1. Excessive grease

Corrective measures

Probable cause

Table 4.20  Troubleshooting chart. (Continued)

176  Electrical Equipment

Overheating of Motor

Brush Sparking

Failure

– Compare values on motor nameplate with those of mains supply. Also check voltage at motor terminals under full load. – Exchange motor for another that meets needs. – Check bearing wear and shaft curvature. – Check for unbalanced voltages or operation under single-phase condition.

3. Incorrect voltages and frequencies

4. Frequent inversions

5. Rotor dragging on stator

6. Unbalanced electrical load (burnt fuse, incorrect control)

– Balance the rotor, check the brushes for free movement within holders.

5. Excess of Vibration

– Check application, measuring voltage and current under normal running conditions.

– Polish the slip rings with an emery and machine the same on lathe.

4. Oval slip rings. Rough surfaces and scored rings.

2. Overload

– Clean rings and reset brushes.

3. Slip rings in poor condition

– Clean and dry motor; inspect air vents and windings periodically.

– Reduce load or install motor with higher capacity.

2. Overload

1. Obstructed cooling system

– Check brush setting; adjust for correct pressure.

– Take bearing apart and clean. Reassemble only if rotating and support surfaces are unharmed.

4. Extraneous solid particles in bearing

1. Poorly set brushes with insufficient pressure

Corrective measures

Probable cause

Table 4.20  Troubleshooting chart. (Continued)

Induction Motors  177

178  Electrical Equipment Rise in temperature above cooling medium in 0°c.



θ = θf (1-e-t/T)

(4.24)

where θf = final steady state temperature in °C W = where W = power in watts A∝ A = Area of cooling media α = Co–efficient of cooling in J/sec/m2 t = time in seconds where θ is required T = Heating time constant (i.e., thermal constant of motor) = MS/Aα where M = mass of motor active parts in kg S = Specific heat of material in J/kg/°c 40 and t = 3 Seconds, if Ib/Ith