High Voltage Engineering Fundamentals

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High Voltage Engineering Fundamentals

Electronics and Electrical Design Engineering

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Preface

The first edition as well as its forerunner of Kuffel and Abdullah published in 1970 and their translations into Japanese and Chinese languages have enjoyed wide international acceptance as basic textbooks in teaching senior undergraduate and postgraduate courses in High-Voltage Engineering. Both texts have also been extensively used by practising engineers engaged in the design and operation of high-voltage equipment. Over the years the authors have received numerous comments from the text’s users with helpful suggestions for improvements. These have been incorporated in the present edition. Major revisions and expansion of several chapters have been made to update the continued progress and developments in high-voltage engineering over the past two decades.

As in the previous edition, the principal objective of the current text is to cover the fundamentals of high-voltage laboratory techniques, to provide an understanding of high-voltage phenomena, and to present the basics of highvoltage insulation design together with the analytical and modern numerical tools available to high-voltage equipment designers.

Chapter 1 presents an introduction to high-voltage engineering including the concepts of power transmission, voltage stress, and testing with various types of voltage. Chapter 2 provides a description of the apparatus used in the generation of a.c., d.c., and impulse voltages. These first two introductory chapters have been reincorporated into the current revision with minor changes.

Chapter 3 deals with the topic of high-voltage measurements. It has undergone major revisions in content to reflect the replacement of analogue instrumentation with digitally based instruments. Fundamental operating principles of digital recorders used in high-voltage measurements are described, and the characteristics of digital instrumentation appropriate for use in impulse testing are explained.

Chapter 4 covers the application of numerical methods in electrical stress calculations. It incorporates much of the contents of the previous text, but the section on analogue methods has been replaced by a description of the more current boundary element method.

Chapter 5 of the previous edition dealt with the breakdown of gaseous, liquid, and solid insulation. In the new edition these topics are described in two chapters. The new Chapter 5 covers the electrical breakdown of gases. The breakdown of liquid and solid dielectrics is presented in Chapter 6 of the current edition.

Chapter 7 of the new text represents an expansion of Chapter 6 of the previous book. The additional areas covered comprise a short but fundamental introduction to dielectric properties of materials, diagnostic test methods, and non-destructive tests applicable also to on-site monitoring of power equipment. The expanded scope is a reflection of the growing interest in and development of on-site diagnostic testing techniques within the electrical power industry. This area represents what is perhaps the most quickly evolving aspect of highvoltage testing. The current drive towards deregulation of the power industry, combined with the fact that much of the apparatus making up the world’s electrical generation and delivery systems is ageing, has resulted in a pressing need for the development of in-service or at least on-site test methods which can be applied to define the state of various types of system assets. Assessment of the remaining life of major assets and development of maintenance practices optimized both from the technical and economic viewpoints have become critical factors in the operation of today’s electric power systems. Chapter 7 gives an introduction and overview of the fundamental aspects of on-site test methods with some practical examples illustrating current practices.

Chapter 8 is an expansion of Chapter 7 from the previous edition. However, in addition to the topics of lightning phenomena, switching overvoltages and insulation coordination, it covers statistically based laboratory impulse test methods and gives an overview of metal oxide surge arresters. The statistical impulse test methods described are basic tools used in the application of insulation coordination concepts. As such, an understanding of these methods leads to clearer understanding of the basis of insulation coordination. Similarly, an understanding of the operation and application of metal oxide arresters is an integral part of today’s insulation coordination techniques.

Chapter 9 describes the design, performance, application and testing of outdoor insulators. Both ceramic and composite insulators are included. Outdoor insulators represent one of the most critical components of transmission and distribution systems. While there is significant experience in the use of ceramic insulators, composite insulators represent a relatively new and quickly evolving technology that offers a number of performance advantages over the conventional ceramic alternative. Their use and importance will continue to increase and therefore merits particular attention. The authors are aware of the fact that many topics also relevant to the fundamentals of high-voltage engineering have again not been treated. But every textbook about this field will be a compromise between the limited space available for the book and the depth of treatment for the selected topics. The inclusion of more topics would reduce its depth of treatment, which should be good enough for fundamental understanding and should stimulate further reading.

TOC

Chapter 1 Introduction 1
1.1 Generation and transmission of electric energy 1
1.2 Voltage stresses 3
1.3 Testing voltages 5
1.3.1 Testing with power frequency voltages 5
1.3.2 Testing with lightning impulse voltages 5
1.3.3 Testing with switching impulses 6
1.3.4 D.C. voltages 6
1.3.5 Testing with very low frequency voltage 7
References 7
Chapter 2 Generation of high voltages 8
2.1 Direct voltages 9
2.1.1 A.C. to D.C. conversion 10
2.1.2 Electrostatic generators 24
2.2 Alternating voltages 29
2.2.1 Testing transformers 32
2.2.2 Series resonant circuits 40
2.3 Impulse voltages 48
2.3.1 Impulse voltage generator circuits 52
2.3.2 Operation, design and construction of impulse generators 66
2.4 Control systems 74
References 75
Chapter 3 Measurement of high voltages 77
3.1 Peak voltage measurements by spark gaps 78
3.1.1 Sphere gaps 79
3.1.2 Reference measuring systems 91
3.1.3 Uniform field gaps 92
3.1.4 Rod gaps 93
3.2 Electrostatic voltmeters 94
3.3 Ammeter in series with high ohmic resistors and high ohmic resistor voltage
dividers 96
3.4 Generating voltmeters and field sensors 107
3.5 The measurement of peak voltages 109
3.5.1 The Chubb–Fortescue method 110
3.5.2 Voltage dividers and passive rectifier circuits 113
3.5.3 Active peak-reading circuits 117
3.5.4 High-voltage capacitors for measuring circuits 118
3.6 Voltage dividing systems and impulse voltage measurements 129
3.6.1 Generalized voltage generation and measuring circuit 129
3.6.2 Demands upon transfer characteristics of the measuring system 132
3.6.3 Fundamentals for the computation of the measuring system 139
3.6.4 Voltage dividers 147
3.6.5 Interaction between voltage divider and its lead 163
3.6.6 The divider’s low-voltage arm 171
3.7 Fast digital transient recorders for impulse measurements 175
3.7.1 Principles and historical development of transient digital recorders
176
3.7.2 Errors inherent in digital recorders 179
3.7.3 Specification of ideal A/D recorder and parameters required for h.v.
impulse testing 183
3.7.4 Future trends 195
References 196
Chapter 4 Electrostatic fields and field stress control 201
4.1 Electrical field distribution and breakdown strength of insulating materials
201
4.2 Fields in homogeneous, isotropic materials 205
4.2.1 The uniform field electrode arrangement 206
4.2.2 Coaxial cylindrical and spherical fields 209
4.2.3 Sphere-to-sphere or sphere-to-plane 214
4.2.4 Two cylindrical conductors in parallel 218
4.2.5 Field distortions by conducting particles 221
4.3 Fields in multidielectric, isotropic materials 225
4.3.1 Simple configurations 227
4.3.2 Dielectric refraction 232
4.3.3 Stress control by floating screens 235
4.4 Numerical methods 241
4.4.1 Finite difference method (FDM) 242
4.4.2 Finite element method (FEM) 246
4.4.3 Charge simulation method (CSM) 254
4.4.4 Boundary element method 270
References 278
Chapter 5 Electrical breakdown in gases 281
5.1 Classical gas laws 281
5.1.1 Velocity distribution of a swarm of molecules 284
5.1.2 The free path of molecules and electrons 287
5.1.3 Distribution of free paths 290
5.1.4 Collision-energy transfer 291
5.2 Ionization and decay processes 294
5.2.1 Townsend first ionization coefficient 295
5.2.2 Photoionization 301
5.2.3 Ionization by interaction of metastables with atoms 301
5.2.4 Thermal ionization 302
5.2.5 Deionization by recombination 302
5.2.6 Deionization by attachment–negative ion formation 304
5.2.7 Mobility of gaseous ions and deionization by diffusion 308
5.2.8 Relation between diffusion and mobility 314
5.3 Cathode processes – secondary effects 316
5.3.1 Photoelectric emission 317
5.3.2 Electron emission by positive ion and excited atom impact 317
5.3.3 Thermionic emission 318
5.3.4 Field emission 319
5.3.5 Townsend second ionization coefficient 321
5.3.6 Secondary electron emission by photon impact 323
5.4 Transition from non-self-sustained discharges to breakdown 324
5.4.1 The Townsend mechanism 324
5.5 The streamer or ‘Kanal’ mechanism of spark 326
5.6 The sparking voltage–Paschen’s law 333
5.7 Penning effect 339
5.8 The breakdown field strength (Eb) 340
5.9 Breakdown in non-uniform fields 342
5.10 Effect of electron attachment on the breakdown criteria 345
5.11 Partial breakdown, corona discharges 348
5.11.1 Positive or anode coronas 349
5.11.2 Negative or cathode corona 352
5.12 Polarity effect – influence of space charge 354
5.13 Surge breakdown voltage–time lag 359
5.13.1 Breakdown under impulse voltages 360
5.13.2 Volt–time characteristics 361
5.13.3 Experimental studies of time lags 362
References 365
Chapter 6 Breakdown in solid and liquid dielectrics 367
6.1 Breakdown in solids 367
6.1.1 Intrinsic breakdown 368
6.1.2 Streamer breakdown 373
6.1.3 Electromechanical breakdown 373
6.1.4 Edge breakdown and treeing 374
6.1.5 Thermal breakdown 375
6.1.6 Erosion breakdown 381
6.1.7 Tracking 385
6.2 Breakdown in liquids 385
6.2.1 Electronic breakdown 386
6.2.2 Suspended solid particle mechanism 387
6.2.3 Cavity breakdown 390
6.2.4 Electroconvection and electrohydrodynamic model of dielectric breakdown 391
6.3 Static electrification in power transformers 393
References 394
Chapter 7 Non-destructive insulation test techniques 395
7.1 Dynamic properties of dielectrics 395
7.1.1 Dynamic properties in the time domain 398
7.1.2 Dynamic properties in the frequency domain 404
7.1.3 Modelling of dielectric properties 407
7.1.4 Applications to insulation ageing 409
7.2 Dielectric loss and capacitance measurements 411
7.2.1 The Schering bridge 412
7.2.2 Current comparator bridges 417
7.2.3 Loss measurement on complete equipment 420
7.2.4 Null detectors 421
7.3 Partial-discharge measurements 421
7.3.1 The basic PD test circuit 423
7.3.2 PD currents 427
7.3.3 PD measuring systems within the PD test circuit 429
7.3.4 Measuring systems for apparent charge 433
7.3.5 Sources and reduction of disturbances 448
7.3.6 Other PD quantities 450
7.3.7 Calibration of PD detectors in a complete test circuit 452
7.3.8 Digital PD instruments and measurements 453
References 456
Chapter 8 Overvoltages, testing procedures and insulation coordination 460
8.1 The lightning mechanism 460
8.1.1 Energy in lightning 464
8.1.2 Nature of danger 465
8.2 Simulated lightning surges for testing 466
8.3 Switching surge test voltage characteristics 468
8.4 Laboratory high-voltage testing procedures and statistical treatment of results 472
8.4.1 Dielectric stress–voltage stress 472
8.4.2 Insulation characteristics 473
8.4.3 Randomness of the appearance of discharge 473
8.4.4 Types of insulation 473
8.4.5 Types of stress used in high-voltage testing 473
8.4.6 Errors and confidence in results 479
8.4.7 Laboratory test procedures 479
8.4.8 Standard test procedures 484
8.4.9 Testing with power frequency voltage 484
8.4.10 Distribution of measured breakdown probabilities (confidence in measured PV) 485
8.4.11 Confidence intervals in breakdown probability (in measured values)
487
8.5 Weighting of the measured breakdown probabilities 489
8.5.1 Fitting of the best fit normal distribution 489
8.6 Insulation coordination 492
8.6.1 Insulation level 492
8.6.2 Statistical approach to insulation coordination 495
8.6.3 Correlation between insulation and protection levels 498
8.7 Modern power systems protection devices 500
8.7.1 MOA – metal oxide arresters 500
References 507
Chapter 9 Design and testing of external insulation 509
9.1 Operation in a contaminated environment 509
9.2 Flashover mechanism of polluted insulators under a.c. and d.c. 510
9.2.1 Model for flashover of polluted insulators 511
9.3 Measurements and tests 512
9.3.1 Measurement of insulator dimensions 513
9.3.2 Measurement of pollution severity 514
9.3.3 Contamination testing 517
9.3.4 Contamination procedure for clean fog testing 518
9.3.5 Clean fog test procedure 519
9.3.6 Fog characteristics 520
9.4 Mitigation of contamination flashover 520
9.4.1 Use of insulators with optimized shapes 520
9.4.2 Periodic cleaning 520
9.4.3 Grease coating 521
9.4.4 RTV coating 521
9.4.5 Resistive glaze insulators 521
9.4.6 Use of non-ceramic insulators 522
9.5 Design of insulators 522
9.5.1 Ceramic insulators 523
9.5.2 Polymeric insulators (NCI) 526
9.6 Testing and specifications 530
9.6.1 In-service inspection and failure modes 531
References 531
Index 533