High Voltage Engineering

Transcription

1 Second Edition An Introduction to High Voltage Engineering Subir Ray

2 An Introduction to High Voltage Engineering SECOND EDITION SUBIR RAY Professor Department of Electrical and Electronics Engineering MVJ College of Engineering Bangalore Formerly, Professor Electrical Engineering Department and Advisor, ICT, College of Technology Govind Ballabh Pant University Pantnagar Delhi

3 AN INTRODUCTION TO HIGH VOLTAGE ENGINEERING, Second Edition Subir Ray 2013 by PHI Learning Private Limited, Delhi. All rights reserved. No part of this book may be reproduced in any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN The export rights of this book are vested solely with the publisher. Seventh Printing (Second Edition) º º April, 2013 Published by Asoke K. Ghosh, PHI Learning Private Limited, Rimjhim House, 111, Patparganj Industrial Estate, Delhi and Printed by Rajkamal Electric Press, Plot No. 2, Phase IV, HSIDC, Kundli , Sonepat, Haryana.

4 To My Mother For giving me the courage to fight against odds My wife, Sumita For showing me the virtue of sobriety My daughter Suchandra and son Sudipto For teaching me the essence of patience

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6 Contents Preface Preface to the First Edition xi xiii 1. Breakdown Mechanisms in Gases under Static Uniform Field Breakdown of Insulators What is it? Electric Field and Stress Bohr s Atomic Model Revisited Ionization Ionization Processes Ionization by Collision Photo-ionization Ionization on the Surface of Electrodes Townsend s Mechanism Time Lag for Breakdown Streamer Theory of Breakdown of Gases Comparison of Townsend s and Streamer Mechanisms Post-breakdown Current Voltage Characteristic Recovery of Electric Field Strength Problems 13 Review Questions Breakdown Characteristics of Gases under Uniform Field More About a And g Paschen s Law Minimum Breakdown Voltage Effect of Temperature De-ionization Desirable Properties of a Gaseous Insulator Sulphur Hexafluoride (SF 6 ) as an Insulator Vacuum as a Dielectric 25 v

7 vi Contents 2.9 Factors Affecting Time Lag for Breakdown Breakdown in a Uniform AC Field Breakdown under Impulse Voltage Impulse Volt Time Characteristics Practical Breakdown Characteristics Problems 33 Review Questions Breakdown of Gases in a Non-uniform Field Breakdown in Uniform and Non-uniform Fields Degree of Non-uniformity Breakdown in a Non-uniform Field Effect of Space Charge on the Breakdown Voltage Effect of Pressure on Corona Inception and Breakdown Voltage Corona Inception Stress in Air Corona Current Corona Loss in Air Corona Loss on Conductors at dc Voltage Corona Loss on Conductors at ac Voltage Problems 46 Review Questions Lightning Phenomenon What is Lightning? Charge Formation in Cloud Wilson s Theory Simpson s Theory Different Forms Taken by Lightning Cloud Flashes Air Discharges Forked Lightning or Lightning Stroke to Ground Mechanism of Forked Lightning Multiple Strokes Return Stroke Current Energy in Lightning 57 Review Questions Breakdown in Liquids and Solids Introduction Breakdown in Liquids Classification of Liquids Liquid Breakdown Test Cells Breakdown in Pure Liquids Breakdown in Commercial Liquids 61

8 Contents vii 5.3 Breakdown in Solids Intrinsic Breakdown Electromechanical Breakdown Thermal Breakdown Mechanisms of Breakdown Occurring After Prolonged Operation Breakdown of Composite Dielectrics Problems 72 Review Questions Generation of High Voltages Introduction Impulse Voltages Characteristics of Impulse Voltage Single-stage Impulse Generator Circuits Multi-stage Impulse Generators Constructional Features of Impulse Generators Generation of Switching Impulse Voltages Generation of High Alternating Voltages Testing Transformers Cascaded Transformers Series Resonant Circuits Generation of High Direct Voltages Characteristic Parameters Fundamentals of Rectifier Circuits Voltage Doubler Circuit Cascade Circuit Van de Graaff Generator Problems 109 Review Questions Measurement of High Voltages Introduction Measurement of High Alternating Voltages Peak Voltage Measurement with Sphere-gaps Peak Voltage Measurement Using Measuring Capacitors Peak Value Measurement with Capacitor Voltage Divider Measurement of rms Values by Electrostatic Voltmeters Capacitance Voltage Transformer (CVT) Digital Recording Measurement of High Direct Voltages Electrostatic Voltmeters Generating Voltmeter Sphere-gap Measurement of Ripple Voltages 136

9 viii Contents 7.4 Measurement of Impulse Voltages Peak Value Measurement Using a Sphere-gap Voltage Dividers Resistive Voltage Divider Capacitance Voltage Dividers Delay Cables Measurement of Peak Voltage by an Indicating Electronic Device Digital Recorders for Impulse Measurements 151 Review Questions Overvoltage Transients in Power Systems Basic Concepts of Transients Classification of Power System Transients Lightning Overvoltage Interaction Between Lightning and the Power System Propagation of Lightning Current and Voltage Along Transmission Lines Shape of Lightning Voltage Waves Reflection and Refraction of Rectangular Travelling Waves Successive Reflections Ground Wires Switching Overvoltages Transient Initiated by the Clearing of a Fault Switching Overvoltages due to Disconnection of an Unloaded Transformer Overvoltage due to Capacitance Switching Overvoltage due to Ferro-resonance Power Frequency Overvoltages Control of Overvoltages due to Switching Transformer Winding Behaviour under Transient Conditions Protection of System Insulation Against Transient Overvoltages Insulation Coordination Problems 196 Review Questions High Voltage Testing of Power System Equipment Introduction Terms Used in Standards High Voltage Tests on Line Insulators Arrangement of Insulators for Test Power Frequency Tests Impulse Tests Impulse Withstand Voltage Tests Pollution Testing 206

10 Contents ix 9.4 High Voltage Tests on Bushings Impulse Voltage Tests High Voltage Tests on Transformers Induced and Applied Overvoltage Tests Impulse Testing of Transformers Lightning Impulse Testing Switching Impulse Tests High Voltage Tests on Cables Non-destructive High Voltage Tests Measurement of Capacitance and Dissipation Factor Partial-discharge Measurements (PD Measurements) Tests on Lightning Arresters Impulse Current Test Problems 222 Review Questions Electrostatic Field Estimation Fundamentals of Electric Field Boundary Conditions Estimation of Electric Field by Direct Solution of Laplace Equation Numerical Methods for Field Computations Finite Element Method ( FEM) Charge Simulation Method (CSM) Surface Charge Simulation Method Comparison of Numerical Techniques Electric Stress Control 242 Appendix Partial Discharges (PD) References Index

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12 Preface The first edition of this book was intended to provide a basic concept of high voltage engineering to the undergraduate students of Electrical Engineering. The topics dealt with four major components breakdown mechanism, generation and measurement of high voltage, high voltage testing and high voltage phenomenon in transmission systems. Over the last decade, electric field analyses using digital methods are extensively practiced for insulation design. A new chapter on electric field estimation is being added in the second edition. Instead of going into the details of field theory, the chapter has been devoted to its application to high voltage systems. Techniques for measurement of partial discharges have undergone a lot of changes. The fundamentals of partial discharges were adequately addressed to in the first edition. An appendix is being added to provide the readers with modern concepts 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 of IIT Kharagpur for their constructive suggestions. I must express my gratitude to the management 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 my publishers PHI Learning for bringing out the second edition. SUBIR RAY xi

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14 Preface to the First Edition Since the first public power station established in London more than 120 years ago, the electric utility system has witnessed revolutionary changes. To meet the ever-increasing demand of electric energy at locations spread over vast territories, the power system has grown into an integrated one interconnecting generating stations by gigantic transmission networks. Tapping of hydel energy at places far remote from load centres in certain countries has necessitated long transmission lines intended to carry bulk power. Accordingly, the transmission voltage levels have increased rapidly. High voltage dc links (sometimes hundreds of kilometres in length) are now being successfully used for providing a channel for power flow. Voltage levels of 1000 kv and above have been achieved over the last few decades in some countries. Such high transmission voltages necessitate a proper design of the insulation system. Not only should the insulation system be capable of withstanding such high normal system voltages, it should also be capable of withstanding transient overvoltages associated with external lightning discharges or internal switching operations without any outage. To achieve this, the system engineers need to have an understanding of the properties of insulating materials to be used as well as knowledge of the characteristics of the overvoltages appearing in the system. Insulations may be solid, liquid, or gaseous. In electrical systems a combination of these insulating media is used. The properties of gases, as well as those of liquid and solid insulating materials, are of fundamental significance to high voltage engineering a field of electrical engineering which is concerned with the physical phenomena and technical problems associated with high voltages. While we have now a reasonably satisfactory idea of breakdown characteristics of several gaseous media supported by theoretical treatment, the same cannot be said of the other two media. So, experiment constitutes the backbone of research in this area. Our idea of overvoltages appearing in power systems and their effects on the system is also not supported by accurate theoretical treatment. This is because of the statistical nature of overvoltages and the complications involved in quantization. One has to depend on national or international standards which are essentially based on experiments and statistical inferences, for equipment insulation design. Accordingly, it is necessary to test high voltage equipment during its development stage and prior to commissioning, to verify if the insulation can withstand the overvoltage as recommended in the standards. xiii

15 xiv Preface to the First Edition From what has been said in the preceding paragraph it is clear that research and development in the field of high voltage technology cannot be carried out without expensive high voltage laboratories. For the purpose of effective teaching in this area, students need to be exposed to such laboratory practices. Such laboratory facilities are not available at all the technical institutes, and only the institutes having such facilities offer specialized courses on high voltage technology. A qualitative change is taking place as regards manpower requirement in power systems since Flexible ac transmission systems (FACTS) technology employing power electronic devices is gaining popularity and the process of unbundling of generation and transmission 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 voltage engineering are being exposed to high voltage systems. It is, therefore, being felt that an introductory course on high voltage engineering should be included in the curriculum at the undergraduate level in electrical engineering and may also form a part of the package of power system courses. The present book is intended to address the situation by providing a basic concept of high voltage engineering qualitatively, and wherever possible quantitatively. The book will be useful not only to students studying the first course on high voltage engineering, but also to practicing engineers who are not exposed to formal training on high voltage technology. The book deals with the following broad topics: (i) breakdown mechanisms and characteristics of insulating media, (ii) generation and measurement of high voltages, (iii) high voltage phenomenon in electric power systems, and (iv) high voltage testing of equipment. Chapters 1 to 4 are devoted to the study of breakdown of gaseous insulation. While Chapters 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, lightning discharges have been dealt with as a special case of breakdown of a long non-uniform airgap. Breakdown mechanisms of liquid and solid dielectric materials have been covered in Chapter 5. Chapters 6 and 7 describe the principles of generation of high voltages in the laboratory for testing purposes and the measurement of such high voltages, respectively. In Chapter 8, the transient overvoltage phenomenon in power systems is discussed and the necessity of high voltage testing has been emphasized. Finally, Chapter 9 describes some important high voltage tests performed on power system equipment. It is expected that the reader will have the following pre-requisites to appreciate the contents of the book: (i) elementary concepts of atomic models and electric field, (ii) concepts of circuit theory including transient analysis, (iii) knowledge of electric power transmission and electrical measurements. I would whole-heartedly welcome constructive criticism and appraisal of this book from students and faculty alike. I sincerely invite their comments and suggestions for improving this book. I am deeply indebted to Dr. P.L. Gautam, Vice-Chancellor of Govind Ballabh Pant University of Agriculture and Technology and Dr. C.S. Jaiswal, former Dean, Faculty of Technology, for their constant encouragement, constructive suggestions, and valuable comments during the writing of this book.

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