Subir Ray

High VoltageEngineering

An Introduction to

Second Edition

SUBIR RAYProfessor

Department of Electrical and Electronics EngineeringMVJ College of Engineering

BangaloreFormerly, Professor

Electrical Engineering Department andAdvisor, ICT, College of Technology

Govind Ballabh Pant UniversityPantnagar

An Introduction toHigh Voltage Engineering

SECOND EDITION

Delhi-1100922013

AN INTRODUCTION TO HIGH VOLTAGE ENGINEERING, Second EditionSubir Ray

© 2013 by PHI Learning Private Limited, Delhi. All rights reserved. No part of this book may bereproduced in any form, by mimeograph or any other means, without permission in writing fromthe publisher.

ISBN-978-81-203-4740-3

The export rights of this book are vested solely with the publisher.

Seventh Printing (Second Edition) ººººº ººººº April, 2013

Published by Asoke K. Ghosh, PHI Learning Private Limited, Rimjhim House, 111, PatparganjIndustrial Estate, Delhi-110092 and Printed by Rajkamal Electric Press, Plot No. 2, Phase IV,HSIDC, Kundli-131028, Sonepat, Haryana.

ToMy Mother

For giving me the courage to fight against odds

My wife, SumitaFor showing me the virtue of sobriety

My daughter Suchandra and son SudiptoFor teaching me the essence of patience

v

Preface xi

Preface to the First Edition xiii

1. Breakdown Mechanisms in Gases under Static Uniform Field 1–17

1.1 Breakdown of Insulators—What is it? 11.2 Electric Field and Stress 11.3 Bohr’s Atomic Model Revisited 21.4 Ionization 31.5 Ionization Processes 4

1.5.1 Ionization by Collision 41.5.2 Photo-ionization 51.5.3 Ionization on the Surface of Electrodes 5

1.6 Townsend’s Mechanism 51.7 Time Lag for Breakdown 91.8 Streamer Theory of Breakdown of Gases 101.9 Comparison of Townsend’s and Streamer Mechanisms 111.10 Post-breakdown Current–Voltage Characteristic 121.11 Recovery of Electric Field Strength 131.12 Problems 13Review Questions 17

2. Breakdown Characteristics of Gases under Uniform Field 18–35

2.1 More About a And g 182.2 Paschen’s Law 192.3 Minimum Breakdown Voltage 202.4 Effect of Temperature 222.5 De-ionization 232.6 Desirable Properties of a Gaseous Insulator 242.7 Sulphur Hexafluoride (SF6) as an Insulator 242.8 Vacuum as a Dielectric 25

ContentsContentsContentsContentsContents

Contentsvi

2.9 Factors Affecting Time Lag for Breakdown 262.10 Breakdown in a Uniform AC Field 262.11 Breakdown under Impulse Voltage 27

2.11.1 Impulse Volt–Time Characteristics 282.12 Practical Breakdown Characteristics 292.13 Problems 33Review Questions 35

3. Breakdown of Gases in a Non-uniform Field 36–47

3.1 Breakdown in Uniform and Non-uniform Fields 363.2 Degree of Non-uniformity 363.3 Breakdown in a Non-uniform Field 373.4 Effect of Space Charge on the Breakdown Voltage 393.5 Effect of Pressure on Corona Inception and Breakdown Voltage 403.6 Corona Inception Stress in Air 413.7 Corona Current 423.8 Corona Loss in Air 42

3.8.1 Corona Loss on Conductors at dc Voltage 423.8.2 Corona Loss on Conductors at ac Voltage 44

3.9 Problems 46Review Questions 47

4. Lightning Phenomenon 48–57

4.1 What is Lightning? 484.2 Charge Formation in Cloud 48

4.2.1 Wilson’s Theory 494.2.2 Simpson’s Theory 50

4.3 Different Forms Taken by Lightning 514.3.1 Cloud Flashes 524.3.2 Air Discharges 524.3.3 Forked Lightning or Lightning Stroke to Ground 52

4.4 Mechanism of Forked Lightning 524.5 Multiple Strokes 544.6 Return Stroke Current 554.7 Energy in Lightning 57Review Questions 57

5. Breakdown in Liquids and Solids 58–74

5.1 Introduction 585.2 Breakdown in Liquids 58

5.2.1 Classification of Liquids 585.2.2 Liquid Breakdown Test Cells 595.2.3 Breakdown in Pure Liquids 595.2.4 Breakdown in Commercial Liquids 61

Contents vii

5.3 Breakdown in Solids 635.3.1 Intrinsic Breakdown 645.3.2 Electromechanical Breakdown 655.3.3 Thermal Breakdown 665.3.4 Mechanisms of Breakdown Occurring After Prolonged Operation 68

5.4 Breakdown of Composite Dielectrics 715.5 Problems 72Review Questions 74

6. Generation of High Voltages 75 –117

6.1 Introduction 756.2 Impulse Voltages 76

6.2.1 Characteristics of Impulse Voltage 766.2.2 Single-stage Impulse Generator Circuits 796.2.3 Multi-stage Impulse Generators 866.2.4 Constructional Features of Impulse Generators 896.2.5 Generation of Switching Impulse Voltages 91

6.3 Generation of High Alternating Voltages 926.3.1 Testing Transformers 926.3.2 Cascaded Transformers 946.3.3 Series Resonant Circuits 97

6.4 Generation of High Direct Voltages 986.4.1 Characteristic Parameters 996.4.2 Fundamentals of Rectifier Circuits 996.4.3 Voltage Doubler Circuit 1026.4.4 Cascade Circuit 1036.4.5 Van de Graaff Generator 107

6.5 Problems 109Review Questions 116

7. Measurement of High Voltages 118–154

7.1 Introduction 1187.2 Measurement of High Alternating Voltages 118

7.2.1 Peak Voltage Measurement with Sphere-gaps 1187.2.2 Peak Voltage Measurement Using Measuring Capacitors 1227.2.3 Peak Value Measurement with Capacitor Voltage Divider 1257.2.4 Measurement of rms Values by Electrostatic Voltmeters 1287.2.5 Capacitance Voltage Transformer (CVT) 1307.2.6 Digital Recording 132

7.3 Measurement of High Direct Voltages 1337.3.1 Electrostatic Voltmeters 1337.3.2 Generating Voltmeter 1337.3.3 Sphere-gap 1367.3.4 Measurement of Ripple Voltages 136

Contentsviii

7.4 Measurement of Impulse Voltages 1377.4.1 Peak Value Measurement Using a Sphere-gap 1377.4.2 Voltage Dividers 1407.4.3 Resistive Voltage Divider 1437.4.4 Capacitance Voltage Dividers 1457.4.5 Delay Cables 1487.4.6 Measurement of Peak Voltage by an Indicating Electronic Device 1507.4.7 Digital Recorders for Impulse Measurements 151

Review Questions 154

8. Overvoltage Transients in Power Systems 155–201

8.1 Basic Concepts of Transients 1558.2 Classification of Power System Transients 1568.3 Lightning Overvoltage 156

8.3.1 Interaction Between Lightning and the Power System 1578.3.2 Propagation of Lightning Current and Voltage Along Transmission

Lines 1598.3.3 Shape of Lightning Voltage Waves 1628.3.4 Reflection and Refraction of Rectangular Travelling Waves 1638.3.5 Successive Reflections 1688.3.6 Ground Wires 169

8.4 Switching Overvoltages 1718.4.1 Transient Initiated by the Clearing of a Fault 1728.4.2 Switching Overvoltages due to Disconnection of an Unloaded

Transformer 1748.4.3 Overvoltage due to Capacitance Switching 1778.4.4 Overvoltage due to Ferro-resonance 1798.4.5 Power Frequency Overvoltages 1828.4.6 Control of Overvoltages due to Switching 182

8.5 Transformer Winding Behaviour under Transient Conditions 1828.6 Protection of System Insulation Against Transient Overvoltages 1878.7 Insulation Coordination 1958.8 Problems 196Review Questions 201

9. High Voltage Testing of Power System Equipment 202–224

9.1 Introduction 2029.2 Terms Used in Standards 2029.3 High Voltage Tests on Line Insulators 204

9.3.1 Arrangement of Insulators for Test 2049.3.2 Power Frequency Tests 2059.3.3 Impulse Tests 2059.3.4 Impulse Withstand Voltage Tests 2059.3.5 Pollution Testing 206

Contents ix

9.4 High Voltage Tests on Bushings 2069.4.1 Impulse Voltage Tests 206

9.5 High Voltage Tests on Transformers 2069.5.1 Induced and Applied Overvoltage Tests 2069.5.2 Impulse Testing of Transformers 2079.5.3 Lightning Impulse Testing 2079.5.4 Switching Impulse Tests 209

9.6 High Voltage Tests on Cables 2119.7 Non-destructive High Voltage Tests 211

9.7.1 Measurement of Capacitance and Dissipation Factor 2119.7.2 Partial-discharge Measurements (PD Measurements) 216

9.8 Tests on Lightning Arresters 2199.8.1 Impulse Current Test 220

9.9 Problems 222Review Questions 224

10. Electrostatic Field Estimation 225–242

10.1 Fundamentals of Electric Field 22510.1.1 Boundary Conditions 22710.1.2 Estimation of Electric Field by Direct Solution

of Laplace Equation 22910.2 Numerical Methods for Field Computations 234

10.2.1 Finite Element Method ( FEM) 23510.2.2 Charge Simulation Method (CSM) 23710.2.3 Surface Charge Simulation Method 240

10.3 Comparison of Numerical Techniques 24210.4 Electric Stress Control 242

Appendix Partial Discharges (PD) 243–244

References 245–248

Index 249–251

The first edition of this book was intended to provide a basic concept of high voltageengineering to the undergraduate students of Electrical Engineering. The topics dealt withfour major components — breakdown mechanism, generation and measurement of highvoltage, high voltage testing and high voltage phenomenon in transmission systems. Over thelast decade, electric field analyses using digital methods are extensively practiced forinsulation design. A new chapter on electric field estimation is being added in the secondedition. Instead of going into the details of field theory, the chapter has been devoted to itsapplication to high voltage systems. Techniques for measurement of partial discharges haveundergone a lot of changes. The fundamentals of partial discharges were adequately addressedto in the first edition. An appendix is being added to provide the readers with modernconcepts of partial discharge and its detection.

I am thankful to my daughter-in-law, Jayeetri for helping with the manuscript typing.Thanks are due to ex-Professor N. Chatterjee, Jadavpur University and Professor S. Sen ofIIT Kharagpur for their constructive suggestions. I must express my gratitude to the manage-ment of M.V.J. College of Engineering, Bangalore, especially Dr. K.S. Badrinarayan,Principal, for allowing me to bring out the second edition. I am also thankful to mypublishers PHI Learning for bringing out the second edition.

SUBIR RAY

PrPrPrPrPrefaceefaceefaceefaceeface

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Since the first public power station established in London more than 120 years ago, theelectric utility system has witnessed revolutionary changes. To meet the ever-increasingdemand of electric energy at locations spread over vast territories, the power system hasgrown into an integrated one interconnecting generating stations by gigantic transmissionnetworks. Tapping of hydel energy at places far remote from load centres in certain countrieshas necessitated long transmission lines intended to carry bulk power. Accordingly, thetransmission voltage levels have increased rapidly. High voltage dc links (sometimes hundredsof kilometres in length) are now being successfully used for providing a channel for powerflow.

Voltage levels of 1000 kV and above have been achieved over the last few decades insome countries. Such high transmission voltages necessitate a proper design of the insulationsystem. Not only should the insulation system be capable of withstanding such high normalsystem voltages, it should also be capable of withstanding transient overvoltages associatedwith external lightning discharges or internal switching operations without any outage. Toachieve this, the system engineers need to have an understanding of the properties ofinsulating materials to be used as well as knowledge of the characteristics of the overvoltagesappearing in the system.

Insulations may be solid, liquid, or gaseous. In electrical systems a combination of theseinsulating media is used. The properties of gases, as well as those of liquid and solidinsulating materials, are of fundamental significance to high voltage engineering—a field ofelectrical engineering which is concerned with the physical phenomena and technicalproblems associated with high voltages. While we have now a reasonably satisfactory idea ofbreakdown characteristics of several gaseous media supported by theoretical treatment, thesame cannot be said of the other two media. So, experiment constitutes the backbone ofresearch in this area. Our idea of overvoltages appearing in power systems and their effectson the system is also not supported by accurate theoretical treatment. This is because of thestatistical nature of overvoltages and the complications involved in quantization. One has todepend on national or international standards which are essentially based on experiments andstatistical inferences, for equipment insulation design. Accordingly, it is necessary to test highvoltage equipment during its development stage and prior to commissioning, to verify if theinsulation can withstand the overvoltage as recommended in the standards.

PrPrPrPrPreface to the Feface to the Feface to the Feface to the Feface to the First Editionirst Editionirst Editionirst Editionirst Edition

xiii

From what has been said in the preceding paragraph it is clear that research anddevelopment in the field of high voltage technology cannot be carried out without expensivehigh voltage laboratories. For the purpose of effective teaching in this area, students need tobe exposed to such laboratory practices. Such laboratory facilities are not available at all thetechnical institutes, and only the institutes having such facilities offer specialized courses onhigh voltage technology.

A qualitative change is taking place as regards manpower requirement in power systemssince 1990. Flexible ac transmission systems (FACTS) technology employing powerelectronic devices is gaining popularity and the process of unbundling of generation andtransmission areas (to be managed by separate companies) has started all over the world. So,more and more technical personnel drawn from specialized areas other than high voltageengineering are being exposed to high voltage systems. It is, therefore, being felt that anintroductory course on high voltage engineering should be included in the curriculum at theundergraduate level in electrical engineering and may also form a part of the package ofpower system courses.

The present book is intended to address the situation by providing a basic concept ofhigh voltage engineering qualitatively, and wherever possible quantitatively. The book will beuseful not only to students studying the first course on high voltage engineering, but also topracticing engineers who are not exposed to formal training on high voltage technology.

The book deals with the following broad topics: (i) breakdown mechanisms andcharacteristics of insulating media, (ii) generation and measurement of high voltages,(iii) high voltage phenomenon in electric power systems, and (iv) high voltage testing ofequipment.

Chapters 1 to 4 are devoted to the study of breakdown of gaseous insulation. WhileChapters 1 and 2 deal with the breakdown mechanisms and characteristics in uniform field,Chapter 3 discusses the same topic under a non-uniform field. In Chapter 4, lightningdischarges have been dealt with as a special case of breakdown of a long non-uniform air-gap. Breakdown mechanisms of liquid and solid dielectric materials have been covered inChapter 5. Chapters 6 and 7 describe the principles of generation of high voltages in thelaboratory for testing purposes and the measurement of such high voltages, respectively. InChapter 8, the transient overvoltage phenomenon in power systems is discussed and thenecessity of high voltage testing has been emphasized. Finally, Chapter 9 describes someimportant high voltage tests performed on power system equipment.

It is expected that the reader will have the following pre-requisites to appreciate thecontents of the book: (i) elementary concepts of atomic models and electric field,(ii) concepts of circuit theory including transient analysis, (iii) knowledge of electric powertransmission and electrical measurements.

I would whole-heartedly welcome constructive criticism and appraisal of this book fromstudents and faculty alike. I sincerely invite their comments and suggestions for improvingthis book.

I am deeply indebted to Dr. P.L. Gautam, Vice-Chancellor of Govind Ballabh PantUniversity of Agriculture and Technology and Dr. C.S. Jaiswal, former Dean, Faculty ofTechnology, for their constant encouragement, constructive suggestions, and valuablecomments during the writing of this book.

Preface to the First Editionxiv

An Introduction To High VoltageEngineering

Publisher : PHI Learning ISBN : 9788120347403 Author : Subir Ray

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