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Banery wysuwane

Okładka w niebieskim kolorze z rysunkiem przedstawiającym przebieg impulsów elektrycznych
Impact of Fast Transient Phenomena on Electrical Insulation System

ISBN
978-83-7464-480-8
Publication type
monografia
Format
B5
Binding
twarda
Number of pages
326
Publication date
2012
Edition
1
Description

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.

Contents

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

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