High Voltage Engineering Fundamentals

Contents
Preface to second edition xi
Preface to first edition xv
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 recorders176
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
Contents ix
7.3.8 Digital PD instruments and measurements 453

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 results472
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 P V ) 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


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