The book “Impact of fast transient phenomena on electrical insulation systems” addresses problems related to electronic converters, which affect electric insulation systems with high slew rate and repetition frequency of switching pulses. Exploitation stresses are a cause of degradation of high voltage insulation systems. The assessment of intensity and dynamics of these processes, being a consequence of local, working electric field strength has been considered mainly in view of sinusoidal voltage. However, in power electronics converters applications, the voltage stress has usually a form of fast switching pulses making up repetitive sequences. Such pulse trains usually have a modulated width and very short rise- and fall-times. Such conditions have essential influence on the inception and development of partial discharges in insulating systems subjected to nonsinusoidal stimuli.
The detection of partial discharge forms can be a basis for the assessment of structural transformations in polymeric insulations. Phase resolved partial discharge patterns are perceived as an effective tool for diagnosing insulation systems of electrical equipment, which can provide visualization of partial discharge trends and dynamics at various stages of insulation degradation. The insulation degradation mechanism is especially important for cables and electrical machines subjected to non-sinusoidal waveforms.
The fast transient phenomena in electrical networks are stressing the insulation systems of power and distribution transformers. The impulse transients having fast wavefront rise time or high frequency oscillatory components may lead to internal resonance overvoltages, and stress transformer insulation systems, in some cases even despite applied overvoltage protection.
The book presents both theoretical analyses and experimental results of application of non-sinusoidal waveforms to machine, cable and transformer insulation.
Monografia dotyczy zjawisk przejściowych powstających podczas eksploatacji urządzeń elektrycznych oraz ich oddziaływania na układy izolacyjne tych urządzeń. Jest to zespół narażeń elektrycznych przepięciowych, zarówno losowych, jak i zdeterminowanych parametrami znamionowymi napięć zasilających. Narażenia tego typu w warunkach pracy urządzeń elektrycznych ograniczają ich niezawodność i planowany czas pracy, są też przyczyną degradacji materiałów w układach izolacyjnych, przy czym intensywność i dynamika tych procesów zależą od poziomu natężenia pola elektrycznego, jego przebiegu czasowego i częstotliwości.
Treścią monografii są analizy teoretyczne i wyniki badań laboratoryjnych wykonanych przez zespół Autorów w ramach realizowanego w latach 2008-2011 projektu rozwojowego i umowy z Narodowym Centrum Badań i Rozwoju.
Intencją autorów prac badawczych przedstawionych w monografii było, aby otrzymane wyniki poszerzyły wiedzę o mechanizmach narażeń szybkozmiennych, przyczyniły się do poprawy konstrukcji urządzeń elektroenergetycznych, a także pozwoliły na ulepszenie standardów w tej dziedzinie i zaleceń w zakresie technologii układów izolacyjnych.
- Spis treści
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Summary 8
Streszczenie 9
Symbols and abbreviation 10
Introduction 13
I. IN-SERVICE STRESSES OF INSULATION SYSTEMS OF ELECTRICAL EQUIPMENT
1. Characteristics of in-service voltage stresses of electrical insulation systems 19
1.1. Electrical exploitation stresses 19
1.2. Characteristics of overvoltages 20
1.2.1. External overvoltages 21
1.2.2. Internal overvoltages 22
1.3. Characteristics of shapes and duration of overvoltages 23
II. VERY FAST STRESSES OF MOTOR INSULATION SYSTEMS FED BY PULSE WIDTH MODULATION INVERTERS
2. Characteristics of Pulse Width Modulation (PWM) supply voltage of electrical motors 29
2.1. Introduction 29
2.2. Inverter topology and switching sequence 31
2.3. Basics of PWM sequence 34
2.3.1. Sinusoidal PWM 34
2.3.2. Space-Vector PWM 37
3. Stresses of insulation systems of cables and motors fed by frequency inverters 41
3.1. Introduction 41
3.2. Fundamental principle of generation of inverter surge voltage 42
3.3. The stresses and deterioration processes for random and form-wound stator windings insulation system 44
3.3.1. The types of exploitation stresses 44
3.3.2. Random-wound insulation system 45
3.3.3. Form-wound insulation system 48
3.4. Partial discharges in motor insulation 49
3.4.1. Mechanism of PD initiation 50
3.5. The technical foundation for the evaluation of insulation systems of electrical machines fed by voltage inverters 52
3.5.1. The types of insulation systems 52
3.5.2. Qualification tests of insulation systems Type I and Type II 52
3.5.3. Partial discharge test for Type I insulation systems 53
4. Parameters identification of an equivalent circuit of induction motor 55
4.1. Introduction 55
4.2. High frequency equivalent circuit of induction motor 55
4.3. Parameters of equivalent scheme of the 3kW motor 59
5. Characteristics of impulse waveforms used for modelling of fast stressesimpact 62
5.1. Introduction 62
5.2. Spectra of characteristic testing voltages 62
5.2.1. Lightning impulse (LI) 62
5.2.2. Very Fast Transient Overvoltages (VFTO) 65
5.2.3. Pulse Width Modulated voltages (PWM) 66
6. Theoretical analysis of overvoltages generated in insulation systems of induction motors fed by PWM inverters 71
6.1. Introduction 71
6.2. Model of supplying of induction motors 72
6.3. Wave phenomena in cables at impulse supplying 74
6.3.1. Development in frequency domain 74
6.3.2. Development in time domain – lossless cable 76
6.4. Simulation results of overvoltages on insulation system of the induction motor 78
7. Modelling of fast stresses in insulation systems of induction motors fed by PWM inverters 80
7.1. Introduction 80
7.2. High frequency modelling of induction motors supplied by use of PWM 81
8. Measurements of overvoltages in motor windings 85
8.1. Introduction 85
8.2. Laboratory measurements of motor terminal overvoltages 87
8.3. Overvoltage distribution in windings of induction motors at surge stresses 91
8.3.1. Characteristic of the experimental motor 91
8.3.2. Results of investigations for overvoltages in windings 92
9. Mechanism of partial discharges in insulation systems 96
9.1. Classification of partial discharges 96
9.2. Physical mechanism of partial discharges 97
9.3. Structural changes of dielectric materials exposed to partial discharges 98
9.4. Partial discharge pulses 99
9.5. Partial discharge model at sinusoidal voltage 102
9.6. Partial discharge model at trapezoidal voltage 105
9.7. Partial discharge model at overvoltages generated by semi-square voltage 107
9.8. Partial discharge model at impulse voltage 107
9.9. Partial discharges in insulation system of motor fed by frequency inverter 108
10. Detection and measurements of partial discharges in insulation systems 111
10.1. Introduction 111
10.2. Partial discharges detection and acquisition 113
10.3. Partial discharge detection in VHF/UHF range 121
10.4. Application of correlation techniques during PD pulses digital acquisition 124
10.5. Selected non-electrical PD detection methods 127
11. Generation of testing voltage waveforms for laboratory research 130
11.1. Introduction 130
11.2. High voltage arbitrary waveform generator 131
11.3. Rectangular pulse testing voltage systems 132
11.3.1. Bipolar IPM-based pulse voltage generator 132
11.3.2. Bipolar pulse voltage generator with LC resonant circuit 133
11.3.3. Unipolar/bipolar IGBT-based PWM system 134
11.4. Parameters of testing voltage waveforms generated by systems used in experiments 137
12. Breakdown characteristics of models representing motor winding insulation systems at PWM-like and sinusoidal voltages 139
12.1. Modelling of random–wound windings 139
12.2. Generation of semi-square and PWM-like voltage with different rise time 142
12.3. Breakdown voltage in twisted-pair samples with different number of twists 143
12.4. The influence of semi-square voltage rise time on breakdown voltage of twisted-pair samples 145
12.5. Time to breakdown of twisted-pair samples at PWM-like voltage at two frequencies 148
12.6. Time to breakdown of twisted-pair samples at PWM-like voltage and sinusoidal voltage at 50 Hz frequency 151
12.7. Summary 152
13. Partial discharges of models representing motor winding insulation systems at PWM-like and sinusoidal voltage 153
13.1. Partial discharge measurements in twisted-pair samples 153
13.2. Corona discharges in twisted-pair samples with parallel wires 155
13.3. The comparison of partial discharge patterns at sinusoidal and semi-square wave shape 160
13.4. Partial discharge pulse acquisition at semi-square voltage 165
13.5. Acquisition and analysis of partial discharge signals in time domain 167
13.6. Partial discharge pulse registration at semi-square voltage with different frequency 170
14. Impact of fast transient stresses on insulation systems of electrical machines based on twisted-pair samples in long-term tests 171
14.1. Characteristic of long-term tests 171
14.1.1. Researches methodology 172
14.1.2. Measured PD attributes 172
14.1.3. Derived PD dependences 173
14.2. The long-term test at PWM-like voltage 173
14.2.1. Measurement results 173
14.2.2. Analysis of breakdown mechanism in TP samples 182
14.2.3. PD patterns and charge distributions 185
14.3. The long-term test at AC voltage 189
14.3.1. Research methodology 189
14.3.2. Measurements results 189
14.4. Final remarks 193
15. Analysis of electrical field distributions in elements of motor windings for assessment of partial discharge conditions 194
15.1. The exposition of twisted pair samples to electric field 194
15.2. Simulations of electric field in parallel configuration of wires in twisted pair sample 196
15.3. Electric field distribution in twist configuration of twisted pair samples 200
15.4 Impact of wire insulation conductivity on electric field distribution in twisted-pair samples 203
16. Mechanisms of deterioration processes in polymer insulation of electric motors caused by partial discharges 205
16.1. Basic physical phenomena of partial discharges in polymers 205
16.2. The stages in deterioration test of partial discharge action in elements of motor windings 207
16.3. The initial stage in partial discharge mechanism 208
16.3.1. PD inception mechanism in gaseous contents 208
16.3.2. The role of space charge in PD mechanism 213
16.3.3. Electron injection from electrode 214
16.4. The aging stage 215
16.4.1. Erosion of polymer surface 215
16.4.2. Changes of insulation surface conductivity 216
16.5. The model of partial discharge mechanism in elements of motor windings 218
17. Deterioration processes of polymer cable insulation by partial discharges 221
17.1. Characteristic of power cable insulation 221
17.2. Electric field distribution in polymer cable insulation with local defects 223
17.3. Laboratory investigations of partial discharges action on XLPE and EPR insulation 227
17.3.1. Methods and aging conditions 227
17.3.2. Partial discharge inception voltage 228
17.3.3. Partial discharge phase-resolved patterns 230
17.3.4. Amplitude charge distributions 237
17.3.5. Partial discharge phase range 237
17.3.6. Transformation of PD impulse sets in long-term test 241
17.3.7. Mechanism of surface partial discharges 246
17.3.8. Microscopic assessment of erosion processes of polymeric materials 249
18. Very fast voltage stresses of transformers insulating systems 253
18.1. Transient overvoltages in transformer 253
18.2. Stress of insulation systems of windings from overvoltages 258
19. Analysis of transient voltage distributions in transformer windings at different voltage stimuli 264
19.1. Introduction 264
19.2. Initial voltage distributions in transformer windings at ultra fast stresses 265
19.3. Impact of oil and temperature on initial voltage distributions in transformer windings at ultra fast stresses 271
19.4. Influence of oil temperature on frequency characteristics of disc and layer transformer windings 276
20. Impact of resonance overvoltages in transformers on internal insulation systems 281
20.1. Introduction 281
20.2. Test object and stimuli description 282
20.3. Experimental waveforms of internal resonance overvoltages in the winding 284
21. Application of transfer function to recognition of resonance overvoltages in transformer winding 288
21.1. Introduction 288
21.2. Investigation results of resonance overvoltages in windings 290
21.3. Transfer function based recognition of resonance overvoltage prone zones 293
IV. ASSESSMENT OF INFLUENCE OF FAST STRESSES ON DEGRADATION PROCESSES IN INSULATION SYSTEMS
22. Assessment of fast stresses influence on degradation processes in electrical insulation systems 299
Literature 305
Index 323