characteristics of creepage discharges along ester-pressboard interfaces under ac stress

213 Pages·2012·20.32 MB·English

CHARACTERISTICS OF CREEPAGE DISCHARGES ALONG
ESTER-PRESSBOARD INTERFACES UNDER AC STRESS

A thesis submitted to The University of Manchester for the degree of
PhD
in the Faculty of Engineering and Physical Sciences

2012

XIAO YI
School of Electrical and Electronic Engineering
2
Contents
Contents
Contents ........................................................................................................................................................ 3
List of Figures ................................................................................................................................................. 7
List of Tables .................................................................................................................................................13
Declaration ...................................................................................................................................................15
Copyright Statement .....................................................................................................................................17
Acknowledgement ........................................................................................................................................19
ABSTRACT .....................................................................................................................................................21
Chapter 1 Introduction ...........................................................................................................................23
1.1 Background ........................................................................................................................................... 23
1.1.1 Transformer insulation layouts .................................................................................................... 26
1.1.2 Transformer insulation design principle ...................................................................................... 28
1.1.3 Different insulating liquids for transformers ................................................................................ 33
1.2 Objectives of research ........................................................................................................................... 35
1.3 Outline of thesis..................................................................................................................................... 36
Chapter 2 Reviews on Discharge Phenomena in Liquid and Liquid-Solid Composite Insulation ..............39
2.1 Partial discharges in transformers and PD testing ................................................................................ 39
2.1.1 Partial discharges in transformers ............................................................................................... 39
2.1.2 PD detection and PD testing for transformers ............................................................................. 40
2.2 Streamer phenomena in liquid .............................................................................................................. 43
2.2.1 Mechanisms for streamers in insulating liquids .......................................................................... 45
2.2.2 Streamers under impulse stresses ............................................................................................... 46
2.2.3 Streamers under ac stress............................................................................................................ 54
2.3 Creepage discharges along liquid-solid interface .................................................................................. 62
2.3.1 Permittivity factor ........................................................................................................................ 63
2.3.2 Effects of solid orientations on streamer propagation ................................................................ 68
2.3.3 Effect of voltage polarity on creepage discharges ....................................................................... 72
2.3.4 Effect of solid spacer under dc stress........................................................................................... 76
3
Contents
2.3.5 Surface charging ........................................................................................................................... 78
2.3.6 Effect of oil ageing and reclamation ............................................................................................. 80
2.3.7 Effect of liquid type on creepage strength ................................................................................... 81
2.3.8 Creepage discharges under sustaining ac stress .......................................................................... 86
2.4 Summary of existing knowledge ............................................................................................................ 87
2.5 Forward looking remarks ....................................................................................................................... 89
Chapter 3 Experimental Description....................................................................................................... 91
3.1 Liquids under investigation .................................................................................................................... 91
3.2 Sample processing ................................................................................................................................. 91
3.3 Electrode configuration ......................................................................................................................... 93
3.4 Experiment description .......................................................................................................................... 94
3.4.1 Experiment setup ......................................................................................................................... 94
3.4.2 Measurement approaches ........................................................................................................... 96
Chapter 4 Creepage Discharge Characteristics on Pressboard Surfaces .................................................. 99
4.1 Voltage range for measurement ............................................................................................................ 99
4.2 The inception of creepage discharges ................................................................................................. 100
4.2.1 Definitions of PDIV ..................................................................................................................... 100
4.2.2 Procedures of PDIV measurement ............................................................................................. 101
4.2.3 PDIV results in open gap and on pressboard surface ................................................................. 102
4.3 Creepage discharges in MIDEL 7131 ................................................................................................... 104
4.3.1 Apparent charge measurement ................................................................................................. 104
4.3.2 Discharge current detection....................................................................................................... 107
4.3.3 Streamer channel visualization .................................................................................................. 110
4.3.4 Summary of discharge characteristics in MIDEL 7131 at 35.5 kV .............................................. 111
4.4 Creepage discharges in FR3 ................................................................................................................. 111
4.4.1 Apparent charge measurement ................................................................................................. 111
4.4.2 Discharge current detection....................................................................................................... 114
4.4.3 Streamer channel visualization .................................................................................................. 117
4.4.4 Summary of discharge characteristics in FR3 at 46.0 kV ............................................................ 117
4.5 Creepage discharges in Gemini X ........................................................................................................ 118
4.5.1 Apparent charge measurement ................................................................................................. 118
4.5.2 Discharge current detection....................................................................................................... 120
4.5.3 Streamer channel visualization .................................................................................................. 122
4
Contents
4.5.4 Summary of discharge characteristics in Gemini X at 50.0 kV ................................................... 123
4.6 Discussions .......................................................................................................................................... 123
4.7 Summary ............................................................................................................................................. 125
Chapter 5 Mechanism Investigations on Influence of Solid Surfaces on Creepage Discharges .............. 127
5.1 Creepage discharge tests on Perspex and glass .................................................................................. 128
5.1.2 Tests in Gemini X ........................................................................................................................ 134
5.2 Mechanisms for the influences of solid on discharges ........................................................................ 137
5.2.1 Introduction ............................................................................................................................... 137
5.2.2 Experimental evidence for the mechanisms for the influence of solid on discharges............... 139
5.3 Analysis on the correlations between consecutive discharges ............................................................ 144
5.3.1 Introduction of pulse sequence analysis.................................................................................... 146
5.3.2 Data analysis of test results using PSA ....................................................................................... 148
5.4 Summary ............................................................................................................................................. 156
Chapter 6 Surface Tracking on Pressboard Barriers .............................................................................. 159
6.1 Introduction ......................................................................................................................................... 159
6.2 Previous research on surface tracking of pressboard .......................................................................... 160
6.3 Experiment setup ................................................................................................................................. 161
6.3.1 Sample processing ..................................................................................................................... 161
6.3.2 Test configuration ...................................................................................................................... 162
6.4 Test results ........................................................................................................................................... 162
6.4.1 Initial conditions for surface tracking ........................................................................................ 162
6.4.2 Surface tracking process on pressboard in MIDEL 7131 ............................................................ 164
6.4.3 Surface tracking process on pressboard in FR3 ......................................................................... 172
6.4.4 Surface tracking process on wet pressboard in Gemini X .......................................................... 173
6.4.5 Extension of white mark at reduced voltage ............................................................................. 175
6.4.6 The gaseous nature of white marks ........................................................................................... 175
6.4.7 The carbonized tree-shaped marks on pressboard ................................................................... 177
6.4.8 The penetration of tree-shaped marks into pressboard ............................................................ 178
6.4.9 Surface tracking on composite pressboard structure ................................................................ 182
6.5 Discussions .......................................................................................................................................... 187
6.5.1 Two types of surface tree-shaped marks ................................................................................... 187
6.5.2 Surface tracking on pressboard barriers in transformers .......................................................... 189
6.5.3 Comparisons between esters and mineral oil ............................................................................ 189
5
Contents
6.6 Summary ............................................................................................................................................. 191
Chapter 7 Conclusions and Future Work .............................................................................................. 193
7.1 Concluding remarks ............................................................................................................................. 193
7.1.1 General ....................................................................................................................................... 193
7.1.2 Summary of test results and key findings .................................................................................. 194
7.2 Future work ......................................................................................................................................... 196
References .................................................................................................................................................. 199
Appendix I Additional Approaches for PSA Analysis ................................................................................... 209
Appendix II List of Publications ................................................................................................................... 213
6
List of Figures
List of Figures
Figure 1-1 Chemical structure of the anhydrob bbb -D-glucopyranose unit [1, 2] ...........................................24
Figure 1-2 Manufacturing process for pressboard [9] ................................................................................25
Figure 1-3 Failure modes for transformers of 15-25 years old [14] ............................................................26
Figure 1-4 Structure of oil-pressboard composite insulation system in transformer .................................27
Figure 1-5 ac design curves for (a) bulk oil, (b) creepage path [24] ............................................................29
Figure 1-6 Experiment verifying effect of angle rings on creepage discharge elimination and flashover
prevention [24] ....................................................................................................................................31
Figure 1-7 An example of cumulative stress calculation [18] ......................................................................32
Figure 1-8 Basic molecular structures of mineral oil [30] ...........................................................................34
Figure 1-9 Typical molecule formulae of synthetic and natural ester [32] ..................................................34
Figure 2-1 Typical PD measurement circuit using pulse current method [48] ...........................................41
Figure 2-2 Test circuit for partial discharge calibration and measurement [48] ........................................42
Figure 2-3 Time sequence for application of test voltage with respect to phase to earth for transformer
PD test [9] ...........................................................................................................................................42
Figure 2-4 Luminous streamer channels in mineral oil captured by streak camera and corresponding
current signals ....................................................................................................................................45
Figure 2-5 Setup for streamer study under impulse [22] ............................................................................46
Figure 2-6 Typical shadowgraphs of streamers [68] ...................................................................................47
Figure 2-7 Average velocity and streamer modes in mineral oil (10 cm gap) [22, 55, 75] ...........................49
Figure 2-8 Typical images of positive streamers of different modes [55] ....................................................49
Figure 2-9 Propagation velocity of streamers versus voltage (10 cm gap) [23] ...........................................50
Figure 2-10 Current and light emission signal of streamer in cyclohexane [76] .........................................50
Figure 2-11 Influence of external hydrostatic pressure on streamer current (1.8 mm gap, 1.2 µm tip, 10
kV, in cyclohexane) [56] ......................................................................................................................51
Figure 2-12 Current signals for the 2nd (left: without front pulses) and the 3rd mode (right: with front
pulses) positive streamers in natural ester [21] ..................................................................................51
Figure 2-13 50% breakdown voltage V and acceleration voltage V for different gaps under 0.4/1400
b a
µs step impulse [23] ............................................................................................................................53
Figure 2-14 Breakdown voltage versus gap distance under impulse voltages in mineral oil ......................53
Figure 2-15 Positive and negative breakdown voltage versus gap distance for mineral oil under lightning
impulse [20] ........................................................................................................................................54
Figure 2-16 Breakdown probability for each polarity at different pressures [83] ......................................54
Figure 2-17 ac breakdown voltage versus gap distance in divergent field in mineral oil [84] ....................57
Figure 2-18 Schematic diagram for two ac positive streamer modes [85] ..................................................57
7
List of Figures
Figure 2-19 Effect of size of earthed plane electrode (left, high voltage at needle side, 3 µm tip, 5 cm gap)
and high voltage connection (right: 3 µm tip, 5 cm gap) on streamer length [87] ............................. 57
Figure 2-20 Breakdown from burst type to direct type with trigger wire shortened [73] .......................... 58
Figure 2-21 Average breakdown strength and streamer frequency at various oil conditions and
geometries [73] ................................................................................................................................... 59
Figure 2-22 Correlation between positive streamer length and ac instantaneous voltage or impulse crest
voltage [88, 89] ................................................................................................................................... 60
Figure 2-23 Streamer inception field under ac and impulse stresses with different electrode sizes [55].... 61
Figure 2-24 Comparison for breakdown voltage between ac stress and impulse stress in mineral oil
(10mm sphere-plane, 1mm gap) [83] ................................................................................................. 61
Figure 2-25 Equivalent model of solid-liquid interface [93] ....................................................................... 64
Figure 2-26 Flashover strength along solid in uniform field as function of permittivity of solid materials
[93] ...................................................................................................................................................... 64
Figure 2-27 Electrodes for ac flashover test on different solid spacers [94] ............................................... 65
Figure 2-28 Flashover voltage with different spacer materials [94] ........................................................... 65
Figure 2-29 Comparison of dielectric performance between traditional pressboard and PMP conditioned
pressboard [95] ................................................................................................................................... 66
Figure 2-30 Winding model for dielectric tests for traditional pressboard and PMP conditioned
pressboard [95] ................................................................................................................................... 66
Figure 2-31 Effect of permittivity match between solid and liquid on flashover [96] ................................ 68
Figure 2-32 Layouts of solid barriers in impulse creepage discharge tests ................................................ 68
Figure 2-33 Comparison between different layouts and polarities, tests on hard glass at ± 50 kV [97, 98]
............................................................................................................................................................ 69
Figure 2-34 50% breakdown voltage versus needle tip-barrier distance (total gap is 10 cm, the radius of
pressboard barrier disk is 7.5 cm) [99] .............................................................................................. 70
Figure 2-35 Streamer velocity versus voltage for tests with vertical pressboard barrier (gap 10 cm) [99] 70
Figure 2-36 Velocity of positive impulse streamer with and without pressboard [99] ................................ 71
Figure 2-37 Conceptual sketch indicating half of sphere enclosing streamer is removed by pressboard [64]
............................................................................................................................................................ 71
Figure 2-38 Positive streamer velocity in open gap and on pressboard surface [100] ................................ 72
Figure 2-39 Electrodes for Nakao’ s impulse creepage tests (10 µm tip) [29] ............................................. 72
Figure 2-40 Setup to study creepage streamer on solid rod under screening effect [101] .......................... 73
Figure 2-41 Relationships between streamer length and electric potential of solid surface [101] .............. 73
Figure 2-42 Setup for study of surface groove effect on creepage streamer [102] ...................................... 74
Figure 2-43 Effect of groove on creepage streamer [102] ........................................................................... 74
Figure 2-44 Electrodes to study influence of solid surfaces on streamers .................................................. 75
Figure 2-45 Influence of solid spacers on time to breakdown at different gap lengths [103] ..................... 75
Figure 2-46 Influence of paper surface on time to breakdown [103] .......................................................... 75
Figure 2-47 Electric field measured by Kerr effect ..................................................................................... 76
8
List of Figures
Figure 2-48 Study on dc conduction phenomenon along HDPE-oil interface [108] ...................................77
Figure 2-49 Effect of surface charging on positive streamer propagation [101] .........................................78
Figure 2-50 Setup to study effect of surface charging on flashover [109] ...................................................79
Figure 2-51 Decay of surface charges on pressboard [109] .........................................................................79
Figure 2-52 Effect of surface charging on flashover strength [109] ............................................................79
Figure 2-53 Time constant to achieve steady state in two liquids [110] ......................................................80
Figure 2-54 Setup and oil condition to study effect of oil quality on flashover [112] ..................................81
Figure 2-55 Flashover voltages along pressboard impregnated with types of liquids [112] .......................81
Figure 2-56 Electrode used for creepage test by Weidmann and Cooper Power Systems [113] .................82
Figure 2-57 Creepage breakdown voltage for mineral oil and FR3 with gaps of 10 mm, 20 mm and 35
mm [113] .............................................................................................................................................83
Figure 2-58 Creepage breakdown stress with 1% probability versus gap distance [113] ...........................83
Figure 2-59 Impulse creepage dielectric tests on coil to coil electrodes with pressboard spacers [114] .....84
Figure 2-60 Impulse creepage dielectric tests on phenolic tap selector [114] ..............................................84
Figure 2-61 Needle vertical to rod electrode with pressboard barrier [115, 116] ........................................85
Figure 2-62 Creepage streamer length with applied voltage in three liquids [115, 116] .............................85
Figure 2-63 Breakdown voltage of natural ester and mineral oil in needle-plane geometry under positive
lightning voltages [117] .......................................................................................................................86
Figure 2-64 Streamer velocity versus voltage (needle to plane, 40 µm tip, 100 mm gap) [117] ..................86
Figure 2-65 Electrodes for partial discharge tests [24, 104] .......................................................................87
Figure 3-1 Test vessel used for the experiments ..........................................................................................94
Figure 3-2 Schematic diagram of the experiment circuit ............................................................................95
Figure 3-3 Set up for shadowgraph visualization ........................................................................................97
Figure 4-1 Four PDIV tests conducted on one pressboard sample ........................................................... 102
Figure 4-2 Q-φ PD patterns in open gap in MIDEL 7131 at various voltages .......................................... 104
Figure 4-3 Q-φ PD patterns on pressboard in MIDEL 7131 at various voltages ...................................... 105
Figure 4-4 Maximum PD amplitudes in open gap and on pressboard in MIDEL 7131 ........................... 106
Figure 4-5 Minimum phase angles for PD occurrences in open gap and on pressboard in MIDEL 7131 106
Figure 4-6 Current signals of discharges in MIDEL 7131 at 35.5 kV ....................................................... 107
Figure 4-7 Current signals of negative discharges with small continuous dc components in MIDEL 7131
.......................................................................................................................................................... 108
Figure 4-8 Current signals of positive discharges with large front pulse in MIDEL 7131 ....................... 109
Figure 4-9 Maximum pulse peaks of discharge current signals in open gap and on pressboard in MIDEL
7131 ................................................................................................................................................... 109
Figure 4-10 Maximum sustaining time of discharge current signals in open gap and on pressboard in
MIDEL 7131 ..................................................................................................................................... 110
Figure 4-11 Streamer images in open gap and on pressboard in MIDEL 7131 at 35.5 kV ....................... 111
Figure 4-12 Q-φ PD patterns in open gap in FR3 at various voltages ...................................................... 112
9
List of Figures
Figure 4-13 Q-φ PD patterns on pressboard in FR3 at various voltages .................................................. 113
Figure 4-14 Maximum PD amplitudes in open gap and on pressboard in FR3 ....................................... 113
Figure 4-15 Minimum phase angles for PD occurrence in open gap and on pressboard in FR3 ............ 114
Figure 4-16 Current signals of discharges in FR3 at 46.0 kV ................................................................... 115
Figure 4-17 Current signals of negative discharges with small continuous dc components in FR3 ......... 115
Figure 4-18 Current signals of positive discharges with large front pulse in FR3 ................................... 116
Figure 4-19 Maximum pulse peaks of discharge current signals in open gap and on pressboard in FR3116
Figure 4-20 Maximum sustaining time of discharge current signals in open gap and on pressboard in
FR3 ................................................................................................................................................... 116
Figure 4-21 Streamer images in open gap and on pressboard in FR3 at 46.0 kV .................................... 117
Figure 4-22 Q-φ PD patterns in open gap and on pressboard in Gemini X ............................................. 119
Figure 4-23 Maximum PD amplitudes in open gap and on pressboard in Gemini X .............................. 119
Figure 4-24 Minimum phase angles for PD occurrence in open gap and on pressboard in Gemini X .... 120
Figure 4-25 Current signals of discharges in Gemini X at 50.0 kV .......................................................... 121
Figure 4-26 Negative discharge current with continuous dc component in Gemini X ............................. 121
Figure 4-27 Positive discharge currents with large front pulse in Gemini X ........................................... 122
Figure 4-28 Streamer images in open gap and on pressboard in Gemini X ............................................. 122
Figure 5-1 PD patterns for one minute in open gap and on solid surfaces at 39.0 kV ............................. 129
Figure 5-2 Maximum PD amplitudes in open gap and on solid surfaces ................................................. 130
Figure 5-3 Minimum PD occurrence phase angles in open gap and on solid surfaces ............................. 131
Figure 5-4 Discharge current signals in open gap and on solid surfaces in FR3 at 39.0 kV .................... 132
Figure 5-5 The longest streamer channels in both half cycles in open gap for FR3 at 43.0 kV ................ 133
Figure 5-6 The longest streamer channels in both half cycles on Perspex in FR3 at 43.0 kV .................. 133
Figure 5-7 Negative streamers on Perspex and on glass in Gemini X at 50.0 kV ..................................... 135
Figure 5-8 Negative discharge current on Perspex and glass in Gemini X at 50.0 kV ............................. 136
Figure 5-9 Discharge initiated from small oil wedge between needle and glass in Gemini X at 50.0 kV . 137
Figure 5-10 Discharges at zero-crossing instant of applied ac voltage on Perspex in mineral oil at 50.0 kV
.......................................................................................................................................................... 140
Figure 5-11 Sketch showing influence of space charges on streamer at zero-crossing instant ................. 141
Figure 5-12 Streamer channel and residual low density channels of five consecutive discharges on
Perspex surface in FR3 at 43.0 kV ................................................................................................... 142
Figure 5-13 Creepage discharge signals in FR3 at 46.0 kV ...................................................................... 145
Figure 5-14 Streamer shapes corresponding to each discharge current signals in Figure 5-13 ............... 145
Figure 5-15 Basic principles of the PSA method [151] .............................................................................. 146
Figure 5-16 PSA results for PDs before and after treeing initiation in polyethylene [149] ...................... 148
Figure 5-17 Traditional PSA on PDs in open gap and on pressboard in FR3 at 46.0 kV ......................... 149
Figure 5-18 An example of discharges occurring in 24 consecutive ac cycles on pressboard in FR3 at 46.0
kV ..................................................................................................................................................... 150
10