HIGH VOLTAGE ENGINEERING Second Edition M S Naidu Department of High Voltage Engineering Indian Institute of Science Bangalore V Kamaraju Department of Electrical Engineering College of Engineering Jawaharlal Nehru Technological University Kakinada McGraw-Hill New York San Francisco Washington, D.C. Auckland BogotA Caracas Lisbon London Madrid Mexico City Milan Montreal New Delhi San Juan Singapore Sydney Tokyo Toronto McGraw-Hill $7 iX$ A Division of The McGraw-Hill Companies First published © 1995, Tata McGraw-Hill Publishing Company Limited Copyright © 1996 by The McGraw-Hill Companies, Inc. All rights reserved. Printed in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publica- tion may be reproduced or distributed in any form or by any means, or stored in a data base or retrieval system, without the prior written per- mission of the publisher. 1234567890 BKP/BKP 90098765 ISBN 0-07-462286-2 Printed and bound by Quebecor/Book Press. Information contained in this work has been obtained by McGraw-Hill, Inc., from sources believed to be reliable. However, neither McGraw-Hill nor its authors guarantee the accuracy or completeness of any information published herein, and neither McGraw-Hill nor its authors shall be responsible for any errors, omissions, or damages arising out of use of this information. This work is published with the understanding that McGraw-Hill and its authors are supplying information but are not attempting to render engineering or other professional ser- vices. If such services are required, the assistance of an appropri- ate professional should be sought. About the Authors M S NAIDU is Professor in the Department of High Voltage Engineering, Indian Institute of Science, Bangalore. A Ph D from the University of Liverpool, he served as a visiting scientist at the High Voltage Laboratory of the Eindhoven University of Technology, Netherlands. He has also lectured at many high voltage laboratories in West Germany, Switzerland and France. Prof. Naidu is a Chartered Engineer and a Fellow of the Institution of Engineers (India) and also a Fellow of the National Academy of Engineering. His research interests include gaseous insulation, circuit breaker arcs, pollution under HVDC etc. He has published many research papers and has authored Advances in High Voltage Breakdown and Arc Interruption in SF6 and Vacuum (Pergamon Press, 1981). V KAMARAJU obtained his Ph D in High Voltage Engineering from the Indian Institute of Science, Bangalore and is currently a Professor of Electrical Engineering at the Engineering College, Kakinada, Andhra Pradesh. He has done extensive research in the area of liquid and solid dielectrics, composite insulation and partial discharge. He is a Chartered Engineer and a Fellow of the Institution of Engineers (India). He has published many research papers and has been a consultant to various industries and to the Andhra Pradesh State Electricity Board. JT C/u* (onidcfaen, May their world be filled with understanding, love and peace Preface The demand for the generation and transmission of large amounts of electric power today, necessitates its transmission at extra-high voltages. In the developed countries like USA, power transmission voltages have reached 765 kV or 1100 kV, and 1500 kV systems are also being built. In our country, 400 kV a.c. power systems have already come into operation, and in another 10 years time every state is expected to be linked by a National Power Grid operating at 400 kV or at 800 kV. At this juncture, a practising electrical engineer or a student of electrical engineering is expected to possess a knowledge of high voltage techniques and should have sufficient background in high voltage engineering. Unfortunately, at present only very few textbooks in high voltage engineering are available, compared to those in other areas of electrical engineering; even among these, no single book has covered broadly the entire range of topics in high voltage engineering and presented the material in a lucid manner. Therefore, an attempt has been made in this book, to bring together different topics in high voltage engineering to serve as a single semester course for final year undergraduate students or postgraduate students studying this subject This book is also intended to serve power engineers in industry who are involved in the design and development of electrical equipment and also engineers in the electricity supply and utility establishments. It provides all the latest information on insulating materials, breakdown phenomena, overvoltages, and testing techniques. The material in this book has been organized into five sections, namely, (i) insulating materials and their applications in electrical and electronic engineering, (h) breakdown phenomena in insulating materials—solids, liquids, and gases, (iii) generation and measurement of high d.c., ax., and impulse voltages and currents, (iv) overvoltage phenomena in electrical power transmission systems and insulation coordination, and (v) high voltage testing techniques, testing of apparatus and equipment, and planning of high voltage laboratories. Much of the information on these topics has been drawn from standard textbooks and reference books, which is simplified and reorganized to suit the needs of the students and graduate engineers. Many research publications have also been referred to, and relevant standard specifications have been quoted to help the reader to gain an easy access to the original references. We have been associated with the subject of High Voltage Engineering for the last 30 years, both as teachers and researchers. This book is useful for undergraduate students of Electrical Engineering, and postgraduate students of Electrical Engineering, Electronics and Applied Physics. It is also useful for self study by engineers in the field of electricity utilities and in the design, development and testing of electrical apparatus, transmission line hardware, particle accelerators, etc. Major changes incorporated in the second edition are: * Chapter 2 has been expanded to include vacuum insulation, including vacuum breakdown and practical applications of vacuum insulation. * Chapter 4 includes various aspects of breakdown of composite insulation/ insulation systems. * Chapter 8 incorporates many new aspects of high voltage and extra high voltage AC power transmission. * In Chapters 6 and 7, certain aspects of production and measurement of high voltages have been deleted; instead, the recent developments have been incorporated. Many smaller changes have been made throughout the book to update the material and improve the clarity of presentation. The authors acknowledge with thanks the permission given by the Bureau of Indian Standards, New Delhi for permitting them to refer to their various specifications and to include the following figures and table in this book. (i) Fig. 6.14: Impulse waveform and its definitions, from IS: 2071 Part II-1973. (ii) Fig. 10.1: Computation of absolute humidity, and Fig. 10.2: Humidity correction factor from IS: 731-1971. (iii) Table 7.6: Relationship between correction factor K and air density factor d, from IS: 2071 Part 1-1973. We also wish to express our thanks to the persons who helped us during the preparation of this second edition. Mr. Mohamed Saleem and Mrs Meena helped with the typing work, while Mr. Dinesh Bhat and Mr. S.T. Paramesh helped with the technical preparation of the manuscript. Technical information derived from various research publications is gratefully acknowledged. We owe our special gratitude to the Director, Indian Institute of Science, Bangalore and to the Vice- Chancellor, Jawaharlal Nehru Technological University, Hyderabad for their encouragement. M S NAIDU V KAMARAJU Contents Preface .................................................................................. vii 1. Introduction ................................................................... 1 1.1 Electric Field Stresses ....................................................... 1 1.2 Gas/Vacuum as Insulator .................................................. 2 1.3 Liquid Breakdown ............................................................... 3 1.4 Solid Breakdown ................................................................ 3 1.5 Estimation and Control of Electric Stress .......................... 4 1.6 Surge Voltages, their Distribution and Control .................. 10 References .................................................................................. 11 2. Conduction and Breakdown in Gases ......................... 12 2.1 Gases as Insulating Media ................................................ 12 2.2 Ionization Processes .......................................................... 12 2.3 Townsend's Current Growth Equation ............................... 16 2.4 Current Growth in the Presence of Secondary Processes ........................................................................... 16 2.5 Townsend's Criterion for Breakdown ................................. 17 2.6 Experimental Determination of Coefficients α and γ ......... 18 2.7 Breakdown in Electronegative Gases ................................ 20 2.8 Time Lags for Breakdown .................................................. 23 2.9 Streamer Theory of Breakdown in Gases ......................... 24 2.10 Paschen's Law ................................................................... 26 2.11 Breakdown in Non-Uniform Fields and Corona Discharges