PRE-BREAKDOWN AND BREAKDOWN STUDY OF
TRANSFORMER OIL UNDER DC AND IMPULSE
VOLTAGES
A thesis submitted to The University of Manchester for the degree of
PhD
in the Faculty of Science & Engineering
2017
JING XIANG
School of Electrical and Electronic Engineering
3
Contents
CONTENTS
CONTENTS ......................................................................................................................... 3
LIST OF FIGURES ..................................................................................................................... 7
LIST OF TABLES ..................................................................................................................... 15
ABSTRACT ....................................................................................................................... 17
DECLARATION ....................................................................................................................... 19
COPYRIGHT STATEMENT ................................................................................................... 21
ACKNOWLEDGEMENT ......................................................................................................... 23
CHAPTER 1. INTRODUCTION ....................................................................................... 27
1.1 Background ................................................................................................... 27
1.2 Research Objectives ...................................................................................... 29
1.3 Major Contributions ...................................................................................... 30
1.4 Outline of Thesis ........................................................................................... 31
CHAPTER 2. LITERATURE REVIEW ........................................................................... 33
2.1 Introduction ................................................................................................... 33
2.2 Streamer and Breakdown in Liquids under DC Voltage ............................... 33
2.2.1 Methodologies .............................................................................. 33
2.2.2 Current and emitted light .............................................................. 35
2.2.3 Photography measurement ............................................................ 37
2.2.4 Breakdown properties in Liquids.................................................. 40
2.2.5 Space Charge ................................................................................ 41
2.3 Streamer and Breakdown in Liquids under Impulse Voltage ....................... 42
2.3.1 General Streamer Characteristics ................................................. 42
2.3.2 Streamer and Breakdown under Different Impulse Waveforms .. 48
2.4 Gas Generation in Liquids under Electrical Faults ....................................... 56
2.4.1 Background Knowledge of Fault Gas Analysis ............................ 56
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Contents
2.4.2 Fault Gas Studies under Electrical Faults ..................................... 59
2.5 Summary ....................................................................................................... 64
CHAPTER 3. EXPERIMENTAL DESCRIPTION ......................................................... 67
3.1 Liquids under Investigation ........................................................................... 67
3.2 Sample Preparation ....................................................................................... 67
3.3 Etched Tungsten Needles Based on Electrochemical Method ...................... 68
3.3.1 Basic Principle of Electrochemical Etching ................................. 68
3.3.2 Etching Procedure of Tungsten Needles ....................................... 69
3.4 DC Voltage Tests .......................................................................................... 71
3.4.1 Experimental Setup ....................................................................... 71
3.4.2 Experimental Procedures .............................................................. 73
3.5 Impulse Voltage Tests ................................................................................... 73
3.5.1 Experimental Setup ....................................................................... 73
3.5.2 Impulse Waveforms with Different Tail times ............................. 75
3.5.3 Experimental Procedures .............................................................. 76
3.6 Gassing Behaviour Tests ............................................................................... 77
3.6.1 Experimental Setup ....................................................................... 77
3.6.2 Fault Control and Data Acquisition System Design ..................... 79
3.6.3 Oil-loop System Design ................................................................ 81
3.6.4 Experimental Procedures .............................................................. 82
3.7 Summary ....................................................................................................... 84
CHAPTER 4. STREAMER AND BREAKDOWN PROPERTIES OF
TRANSFORMER LIQUIDS UNDER DC VOLTAGE ......................................................... 87
4.1 Introduction ................................................................................................... 87
4.2 Effect of Tip Radius on Streamer Initiation Voltage .................................... 87
4.3 Basic Characteristics of Streamers ................................................................ 91
4.3.1 Positive Streamer .......................................................................... 91
5
Contents
4.3.2 Negative Streamer ........................................................................ 95
4.4 Effect of Gap Distance on Breakdown Voltage ............................................ 99
4.4.1 Breakdown Phenomena ................................................................ 99
4.4.2 Breakdown Tests at Gap Distances from 2 mm to 30 mm ......... 100
4.5 Summary ..................................................................................................... 103
CHAPTER 5. STREAMER AND BREAKDOWN PHENOMENA OF
TRANSFORMER LIQUIDS UNDER DIFFERENT IMPULSE WAVEFORMS ............ 105
5.1 Introduction ................................................................................................. 105
5.2 Pre-breakdown Characteristics .................................................................... 105
5.2.1 Stopping Length .......................................................................... 105
5.2.2 Average Propagation Velocity .................................................... 106
5.3 Breakdown Voltage ..................................................................................... 108
5.3.1 Breakdown Tests in the Mineral Oil ........................................... 108
5.3.2 Prediction of Breakdown Voltage .............................................. 110
5.3.3 Verification in the Synthetic Ester Liquid .................................. 113
5.4 Effect of Impulse Waveform on Streamer Characteristics .......................... 115
5.5 Summary ..................................................................................................... 117
CHAPTER 6. CORRELATIONS BETWEEN GAS GENERATION AND SPARKING
FAULT IN TRANSFORMER LIQUIDS UNDER LIGHTNING IMPULSE VOLTAGE ....
..................................................................................................................... 119
6.1 Introduction ................................................................................................. 119
6.2 Data Processing ........................................................................................... 119
6.2.1 Calculation of Dissolved Gas Generation ................................... 119
6.2.2 Gas-in-total Calculation .............................................................. 121
6.2.3 Energy Calculation ..................................................................... 122
6.3 DGA Results and Analysis .......................................................................... 124
6.3.1 Comparison of Hydrogen Measurements ................................... 124
6.3.2 Effect of Spark Numbers ............................................................ 125
6
Contents
6.3.3 Effect of Gap Distance ................................................................ 128
6.3.4 Effect of Voltage Levels ............................................................. 131
6.3.5 Correlation between Fault Gas Generation and Fault Energy .... 133
6.4 Summary ..................................................................................................... 136
CHAPTER 7. CONCLUSIONS AND FUTURE WORK .............................................. 139
7.1 Conclusions ................................................................................................. 139
7.1.1 General ........................................................................................ 139
7.1.2 Summary of Results and Main Findings .................................... 140
7.2 Future Work ................................................................................................ 141
REFERENCES ..................................................................................................................... 143
APPENDIX I LIST OF PUBLICATIONS ................................................................... 149
7
List of Figures
LIST OF FIGURES
Figure 2-1 Typical discharge pulse in a needle to plane gap in a mineral oil under negative
polarity at room temperature, d = 5 mm, V = 14 kV [37]. ................................................... 35
Figure 2-2 Streamer current and emitted light signal in transformer oil under DC voltage,
positive polarity, d = 20 mm, V = 40 kV [27]. ...................................................................... 36
Figure 2-3 Streamer current waveforms in Hexane taken with a point cathode (Top –
Negative polarity) and with a point anode (Bottom – Positive polarity), point-sphere
electrode, d = 4.75 mm [29]. ................................................................................................. 37
Figure 2-4 Experimental setup of time delay system for streamer photograph capture under
DC voltage (re-produced) [24]. ............................................................................................. 38
Figure 2-5 The photograph of a streamer with the numbers indicating the sequence in which
the pictures were taken; time interval between each film 200 ns; (a) streamer photograph by
the camera; (b) streamer current signal; (c) streamer stopping length [29]. ......................... 39
Figure 2-6 Shadowgraph images of streamer expansion in transformer oil as a function of
time and pressure in a needle-to-plane geometry with a gap of 2.5 mm under negative
polarity [45]. ......................................................................................................................... 40
Figure 2-7 Breakdown voltage of oil under DC voltages of both polarities [48]. ................ 41
Figure 2-8 Typical images of positive streamers in a mineral transformer oil. (A) ‘1st mode’;
(B) ‘2nd mode’; (C) ‘2nd
mode’; (D) ‘3rd + 2nd modes’; (E) ‘4th mode’ [61]. ................... 43
Figure 2-9 Typical examples of positive streamer with emitted light (upper trace, arb. unit.),
transient currents (middle trace) and voltage (lower trace) in cyclohexane, d = 1.8 mm, rp =
1.2 μm, (A) 1st mode, V = 12 kV, (B) 2
nd mode V = 20 kV [71]. ......................................... 44
Figure 2-10 Propagation modes at high voltage in a natural ester liquid. d = 10 cm. Upper
oscilloscope trace: applied voltage, lower: streamer current. (A) 2nd
mode, V = 120 kV; (B)
3rd
+ 2nd
modes, V = 120 kV; (C) 3rd
mode, V = 142 kV; (D) 4th
mode, V = 152 kV [57]. .. 46
Figure 2-11 The propagation process of positive 3rd
+2nd
mode streamer at high voltage just
below Va; Diala S4 ZX-I, d = 50 mm; the ground electrode is at the bottom edge of the
streamer images [72]. ............................................................................................................ 46
Figure 2-12 The typical propagation process of 4th
+3rd
mode negative streamers in the GTL
oil, d=50 mm; the ground electrode is at the bottom edge of the streamer images from frame
3 to frame 6 [72]. .................................................................................................................. 47
8
List of Figures
Figure 2-13 Typical shapes of the positive 4th
mode streamers: (a) neat white oil, (b) neat
white oil at reduced pressure, and (c) DMA, all images taken just before breakdown, d = 80
mm [47]. ................................................................................................................................ 48
Figure 2-14 Typical shapes of the negative 4th
mode streamers: (a) neat white oil at reduced
pressure, (b) neat white oil, and (c) TCE, all images taken just before breakdown [47]. ..... 48
Figure 2-15 Characteristic voltage for oil breakdown on an impulse waveform, where L is
gap distance, ti and tb are the times corresponding to initiation and breakdown [73]. ......... 49
Figure 2-16 The effect of tail time on breakdown voltage in transformer oil at different gap
distance, positive point-plane electrode, gap distance: (o): 2.54 cm; (): 5.08 cm; (Δ):
10.16 cm; (V): 14.61 cm [73]. .............................................................................................. 49
Figure 2-17 Comparison of breakdown voltage and acceleration voltage in mineral oil
under lightning and step impulse, negative polarity [42]. .................................................... 50
Figure 2-18 Comparison of stopping length between step impulse and lightning impulse
under positive polarity, d = 50 [22]. ..................................................................................... 51
Figure 2-19 Comparison of average propagation velocity between lightning impulse and
step impulse under positive polarity, d = 50 mm [22]. ......................................................... 51
Figure 2-20 Comparison of (a) Breakdown voltage VbLI
and (b) time to breakdown tbLI
under lightning impulse versus gap distance in liquids of group IV (PMO, NMO, PB) [23,
42]. ........................................................................................................................................ 53
Figure 2-21 Average breakdown velocities vbLI
versus gap distance in insulating liquids, (a)
under lightning impulse; (b) under step impulse [23]. .......................................................... 54
Figure 2-22 Comparison of breakdown voltage in mineral oil PMO under step and lightning
voltages in point-sphere geometry [23]. ............................................................................... 55
Figure 2-23 Typical breakdown cases in mineral oil under lightning impulse voltage (full
lines) and step impulse voltage (dotted lines) [23]. .............................................................. 55
Figure 2-24 Comparison of breakdown voltage in natural ester NE under step and lightning
impulse voltage in point-plane geometry [23]. ..................................................................... 56
Figure 2-25 key gases generation versus number of breakdowns in mineral oil, d = 4 mm
[90] ........................................................................................................................................ 61
Figure 2-26 Comparison of fault gases generation in various liquids after 90 breakdowns
[90] ........................................................................................................................................ 61
Figure 2-27 Fault gases generation in mineral oil under different voltage levels, arc duration
= 15 mins [91]. ...................................................................................................................... 62
9
List of Figures
Figure 2-28 Fault gas generation in mineral oil under different arc durations, V = 20 kV
[91]. ....................................................................................................................................... 62
Figure 2-29 Fault gases generation in different insulation oils under sparking fault (re-
produced plot based on results in Table 2-7) [92]. ............................................................... 63
Figure 2-30 Fault gases generation per unit fault energy (µL/J) in Gemini X and FR3 under
sparking fault, averaged over a group of three tests. TCG = total combustible gases [92]. . 64
Figure 3-1 Principle of static etching (green A-C) and dynamic etching (orange A-D) [101].
............................................................................................................................................... 68
Figure 3-2 Measurement of the radius of curvature of needles [102]. .................................. 69
Figure 3-3 Failure results of etched tungsten needles without applying oscillating method.
............................................................................................................................................... 70
Figure 3-4 Ideal etched tungsten needle with the tip radius of 50 µm. ................................. 70
Figure 3-5 Sketch of the experimental setup used in streamer tests under DC voltage. ....... 71
Figure 3-6 Perspex made cubic test cell used for investigating streamer and breakdown
under DC voltage. ................................................................................................................. 72
Figure 3-7 Sketch of test setup of impulse tests with different tail-time. ............................. 74
Figure 3-8 The photo of the compact solid-state switch based impulse generator. .............. 75
Figure 3-9 The different impulse waveforms with tail time ranging from 8 µs to 3200 µs, V
= 24 kV. ................................................................................................................................ 76
Figure 3-10 The diagram of the experimental setup for gas generation tests under electrical
faults. ..................................................................................................................................... 78
Figure 3-11 The photos of the experimental setup for gas generation tests under electrical
faults. ..................................................................................................................................... 78
Figure 3-12 The photo of the cubic shaped stainless steel test cell used for gassing
behaviour test, (a) test cell; (b) electrode configuration with the gap distance of 10 mm. ... 79
Figure 3-13 The flow chart of the automatic control system with voltage output and data
recording. .............................................................................................................................. 80
Figure 3-14 Description of time domain in Labview setting. ............................................... 81
Figure 3-15 Sealing performance based on pressure reading in oil-loop system. ................ 81
Figure 3-16 The comparison of energy generation of individual breakdowns with different
time interval. ......................................................................................................................... 83
Figure 3-17 The flow chart of the experimental procedure for gassing behaviours tests ..... 84
10
List of Figures
Figure 4-1 The current and emitted light signals of streamer initiation under DC voltage (a).
Mineral oil – positive polarity (b). Mineral oil – negative polarity(c). Synthetic ester –
positive polarity (d). Synthetic ester – negative polarity; d = 10 mm, r = 10 µm. ............... 88
Figure 4-2 Weibull distribution plot of streamer initiation results with various tip radii (r =
5, 10, 20 and 50 µm) under DC voltage, d = 10 mm. ........................................................... 89
Figure 4-3 Effect of tip radius on streamer initiation voltage under DC voltage, d = 10 mm
(plot based on 50% initiation voltage). ................................................................................. 90
Figure 4-4 Initiation field versus tip radius in the mineral oil and the synthetic ester liquid
under positive and negative polarities. .................................................................................. 91
Figure 4-5 Typical positive streamer propagation in the synthetic ester liquid under DC
voltage, d = 10 mm, r = 10 µm, V = 28 kV; (a) voltage, current and monitor signals, (b)
streamer propagation, corresponding to the signals in (a). ................................................... 92
Figure 4-6 Stopping length of streamers in the mineral oil and the synthetic ester liquid
under positive polarity, d = 10 mm, r = 10 µm (error bars stand for one standard deviation).
............................................................................................................................................... 93
Figure 4-7 Average propagation velocity versus applied DC voltage in the mineral oil and
the synthetic ester liquid under positive polarity, d = 10 mm, r = 10 µm (error bars stand for
one standard deviation). ........................................................................................................ 94
Figure 4-8 Positive streamer stopping length as a function of maximum apparent charge in
the mineral oil and the synthetic ester liquid. ....................................................................... 95
Figure 4-9 Typical negative streamer propagation in the synthetic ester liquid under DC
voltage, d = 10 mm, r = 10 µm, V = -55 kV; (a) voltage, current and camera monitor signals,
(b) streamer propagation, corresponding to the signals in (a) .............................................. 96
Figure 4-10 Stopping length of streamers in the mineral oil and the synthetic ester liquid
under negative polarity, d = 10 mm, r = 10 µm (error bars stand for one standard deviation).
............................................................................................................................................... 97
Figure 4-11 Average propagation of streamers in the mineral oil and the synthetic ester
liquid under negative polarity, d = 10 mm, r = 10 µm (error bars stand for one standard
deviation). ............................................................................................................................. 98
Figure 4-12 Negative streamer stopping length as a function of maximum apparent charge
in the mineral oil and the synthetic ester liquid. ................................................................... 99
Figure 4-13 Breakdown in the synthetic ester liquid under DC voltage, positive polarity; d =
10 mm, r = 10 µm, exposure time 2 µs; (a) voltage, current and monitor signals, (b)
streamer propagation, corresponding to the signals in (a). ................................................. 100
11
List of Figures
Figure 4-14 Breakdown in the synthetic ester liquid under DC voltage, negative polarity; d
= 10 mm, r = 10 µm, exposure time 2 µs; (a) voltage, current and monitor signals, (b)
streamer propagation, corresponding to the signals in (a). ................................................. 100
Figure 4-15 Weibull distribution plot of breakdown results in the mineral oil and the
synthetic ester liquid with different gap distances under DC voltage, (a) Gemini X, (b)
MIDEL 7131, r = 10 µm ..................................................................................................... 101
Figure 4-16 Effect of gap distance on breakdown voltage in the mineral oil and the
synthetic ester liquid under DC voltage, r = 10 µm; based on 50% breakdown voltages .. 102
Figure 5-1 Stopping length of streamers in the mineral oil under positive polarity; d = 10
mm, r = 10 µm; error bars stand for one standard deviation. ............................................. 106
Figure 5-2 Stopping length of streamers in the synthetic ester liquid under positive polarity;
d = 10 mm, r = 10 µm; error bars stand for one standard deviation. .................................. 106
Figure 5-3 Average propagation velocity of streamers in the mineral oil under positive
polarity; d = 10 mm, r = 10 µm; error bars stand for one standard deviation. ................... 107
Figure 5-4 Average propagation velocity of streamers in the synthetic ester liquid under
positive polarity; d = 10 mm, r = 10 µm; error bars stand for one standard deviation. ...... 107
Figure 5-5 Weibull plot of breakdown voltages in the mineral oil under different impulse
waveforms, d = 10 mm, r = 10 µm, positive polarity. ........................................................ 108
Figure 5-6 Typical breakdowns in the mineral oil under different impulse waveforms, d =
10 mm, r = 10 µm, positive polarity. .................................................................................. 109
Figure 5-7 Time to breakdown in the mineral oil under different impulse waveforms, d = 10
mm, r = 10 µm, positive polarity. ....................................................................................... 109
Figure 5-8 Breakdown voltage and instantaneous breakdown voltages in the mineral oil
under different impulse waveforms; d = 10 mm, r = 10 µm, positive polarity. ................. 110
Figure 5-9 Flowchart of a mathematical model of breakdown voltage prediction. ............ 112
Figure 5-10 Simulated different impulse waveforms in the mineral oil based on the
mathematical model, positive polarity. ............................................................................... 113
Figure 5-11 Simulated different impulse waveforms in the synthetic ester liquid based on
the mathematical model, positive polarity. ......................................................................... 114
Figure 5-12 Typical breakdowns in the synthetic ester liquid under different impulse
waveforms, d = 10 mm, r = 10 µm, positive polarity. ........................................................ 114
Figure 5-13 Pre-breakdown in the mineral oil under different impulse waveforms, d = 10
mm, r = 10 µm, positive polarity, (a). 0.8/8 µs, V = 52 kV, lstoping = 7.62 mm (b). 0.8/14 µs,
12
List of Figures
V = 42 kV, lstoping = 7.71 mm (c). 0.8/30 µs, V = 34 kV, lstoping = 7.44 mm (d). 0.8/3200 µs,
V = 32 kV, lstoping = 7.85 mm............................................................................................... 115
Figure 5-14 Pre-breakdown in the synthetic ester liquid under different impulse waveforms,
d = 10 mm, r = 10 µm, positive polarity, (a). 0.8/8 µs, V = 40 kV, lstoping = 6.59 mm (b).
0.8/14 µs, V = 32 kV, lstoping = 6.42 mm (c). 0.8/30 µs, V = 30 kV, lstoping = 6.47 mm (d).
0.8/3200 µs, V = 26 kV, lstoping = 6.57 mm. ......................................................................... 115
Figure 5-15 Positive streamer area as a function of stopping length in the mineral oil under
different impulse waveforms, d = 10 mm, r = 10 µm. ........................................................ 116
Figure 5-16 Positive streamer area as a function of stopping length in the synthetic ester
liquid under different impulse waveforms, d = 10 mm, r = 10 µm. ................................... 117
Figure 6-1 Hydrogen concentration as gas in oil concentration from the TM1 hydrogen
monitor as a function of time during a 500 spark test, mineral oil, d = 10 mm, positive
polarity. ............................................................................................................................... 120
Figure 6-2 Hydrogen concentration as gas in oil concentration from the TM8 multi-gas
monitor as a function of time during a 500 spark test, mineral oil, d = 10 mm, positive
polarity. ............................................................................................................................... 120
Figure 6-3 The K factor based on Ostwald solubility coefficient under different temperature
in mineral oil ....................................................................................................................... 122
Figure 6-4 The voltage and current waveform in mineral oil on 99.9% breakdown voltage,
positive polarity, d = 10 mm, r = 50 µm. ............................................................................ 123
Figure 6-5 Typical spark in the mineral oil of positive polarity under lightning impulse, VB-
99.9%-positive = 31 kV, d = 5 mm, exposure time 0.5 µs. ......................................................... 125
Figure 6-6 Fault gas generation (GIT) in the mineral oil at different numbers of breakdowns,
positive polarity, d = 5 mm. ................................................................................................ 126
Figure 6-7 Fault gas generation (GIT) in the synthetic ester liquid at different numbers of
breakdowns, positive polarity, d = 5 mm. ........................................................................... 126
Figure 6-8 Comparison of hydrogen and acetylene generation (GIT) in the mineral oil and
the synthetic ester liquid under sparking fault at different numbers of sparks, positive
polarity, d = 5 mm. .............................................................................................................. 127
Figure 6-9 Individual fault gases as percentages of total fault gases in the mineral oil and
the synthetic ester liquid with a different number of sparks, d = 10 mm. .......................... 128
Figure 6-10 Fault gas generation (GIT) in the mineral oil as a function of the number of
breakdowns, Gemini X - Vb-99.9% = 39 kV, positive polarity, d = 10 mm. .......................... 129
13
List of Figures
Figure 6-11 Fault gas generation (GIT) in the synthetic ester liquid as a function of the
number of breakdowns, MIDEL 7131 - Vb-99.9% = 37 kV, positive polarity, d = 10 mm. .. 129
Figure 6-12 Comparison of hydrogen and acetylene generation (GIT) in the mineral oil and
the synthetic ester liquid at different gap distances under positive polarity. ...................... 130
Figure 6-13 Individual fault gases as percentages of total fault gases in the mineral oil and
the synthetic ester liquid with different spark numbers, Gemini X - Vb-99.9% = 39 kV,
MIDEL 7131 - Vb-99.9% = 37 kV, positive polarity, d = 10 mm. ......................................... 131
Figure 6-14 Fault gas generation in the mineral oil at different voltage levels after 200
sparks, Vb-99.9% = 39 kV, 1.5Vb-99.9% = 59 kV, d = 10 mm. ................................................. 132
Figure 6-15 Fault gas generation in the synthetic ester liquid at different voltage levels after
200 sparks, Vb-99.9% = 37 kV, 1.5Vb-99.9% = 56 kV, d = 10 mm. .......................................... 132
Figure 6-16 Individual fault gases as percentages of total fault gases in the mineral oil and
the synthetic ester liquid at different voltage levels with 200 sparks, d = 10 mm. ............. 133
Figure 6-17 Statistical analysis of fault energy for each spark in the mineral oil and the
synthetic ester liquid at the 10 mm gap distance, totally 1170 sparks. ............................... 134
Figure 6-18 Average energy per spark in the mineral oil and the synthetic ester liquid under
different test conditions. ...................................................................................................... 134
Figure 6-19 Fault gas volumes per unit fault energy (μL/J) of the mineral oil and the
synthetic ester liquid at the 5 and 10 mm gap distance with a different number of sparks.135
14
List of Figures
15
List of Tables
LIST OF TABLES
Table 2-1 Rate of rise corresponds to specific rising-voltage method [30]. ......................... 34
Table 2-2 Rate of rise corresponds to specific step-by-step method [30] ............................. 34
Table 2-3 Breakdown voltage and acceleration voltage obtained under step impulse and
lightning impulse [22]. .......................................................................................................... 52
Table 2-4 Bond Dissociation Energy [21]. .......................................................................... 57
Table 2-5 Indicator fault gases in mineral oil [76] ............................................................... 58
Table 2-6 Summary of experimental works of DGA analysis under electrical faults [90-99].
............................................................................................................................................... 60
Table 2-7 Fault gases volumes (µL) in Gemini X and FR3 for each of three groups of 15
breakdowns [92]. ................................................................................................................... 63
Table 3-1 Basic properties of testing liquids: MIDEL 7131 and Gemini X [42]. ................ 67
Table 3-2 The experimental conditions used to produce various tip radii of tungsten needles.
............................................................................................................................................... 71
Table 3-3 The parameters of the front resistor, tail resistor and charging capacitor used to
generate the four impulse waveforms. .................................................................................. 76
Table 3-4 The 99.9% breakdown voltages of the mineral oil and the synthetic ester liquid at
different gap distance under positive and negative lightning impulse. ................................. 82
Table 4-1 Weibull parameters of streamer initiation results with various tip radii at point-
plane electrode under negative and positive polarities ......................................................... 89
Table 4-2 Weibull parameters of breakdown results in the mineral oil and the synthetic
ester liquid with different gap distances at the point-plane electrode. ................................ 102
Table 5-1 Weibull parameters of breakdown results in the mineral oil obtained under
different impulse waveforms; d = 10 mm, r = 10 µm, positive polarity. ........................... 108
Table 5-2 Predicted breakdown voltage and experimental breakdown voltage in the mineral
oil obtained under different impulse waveforms; d = 10 mm, r = 10 µm, positive polarity.
............................................................................................................................................. 112
Table 5-3 Predicted breakdown voltage and experimental breakdown voltage in synthetic
ester liquid obtained under variable impulse voltages; d = 10 mm, r = 10 µm, positive
polarity ................................................................................................................................ 113
Table 5-4 Average charges injected into oil samples of the similar stopping length under
different impulse waveforms. ............................................................................................. 117
16
List of Tables
Table 6-1 Individual part energy as percentages of total energy generation based on 100
sparks in mineral oil, V = Vb-99.9%........................................................................................ 124
Table 6-2 Comparison of hydrogen measurements among the hydrogen monitor, the multi-
gas monitor and laboratory technique. ................................................................................ 125
Table 6-3 Fault gas volumes per unit fault energy (μL/J) of the mineral oil and the synthetic
ester liquid at different spark numbers, gap distances and voltage levels. ......................... 136
17
Abstract
ABSTRACT
Streamer characteristics, breakdown strengths and gassing behaviour of insulating liquids
under electric stresses are taken into account for a reliable design and safe operation of the
transformer. Ester liquids which are biodegradable and have high fire point have been
widely used in distribution transformers and some power transformers in recent years. It is
also interesting to introduce ester liquids into High Voltage Direct Current (HVDC)
converter transformers due to the fast development of HVDC transmission lines. Therefore,
this thesis aims to investigate the pre-breakdown, breakdown characteristics and gassing
behaviour of a synthetic ester liquid under DC and various impulse voltages where a
mineral oil is tested as the benchmark.
A comprehensive study of streamer characteristics and breakdown strength of the mineral
oil and the synthetic ester liquid under both positive and negative DC voltages was carried
out in the point-plane electric fields. Characteristics of streamer length, propagation
velocity and shape were analysed based on shadowgraph images obtained at a gap distance
of 10 mm, using a multi-channel ultra-high speed camera. Streamer inception voltages with
the tip radii of 5 µm, 10 µm, 20 µm and 50 µm and breakdown voltages at various gaps of 2
mm, 5 mm, 10 mm, 20 mm and 30 mm were also investigated. The results indicate that
there is no obvious streamer propagation (less than about 10% of the gap distance) under
negative polarity even when the applied voltage approaches breakdown voltage. At the
same applied voltage level, the streamer in the synthetic ester liquid propagates faster and
further than that in the mineral oil. As a result, the breakdown voltages of the synthetic
ester liquid are lower than those of the mineral oil at all the gap distances investigated
under both polarities.
Experimental and modelling studies of pre-breakdown and breakdown phenomena in the
mineral oil and the synthetic ester liquid under impulse waveforms with different tail-time
were carried out in the point-plane electric fields. A compact solid-state switch based
impulse generator was used to provide different impulse waveforms from short tail-time to
“step-like” tail-time: 0.8/8 μs, 0.8/14 μs, 0.8/30 μs and 0.8/3200 μs. A point-plane electrode
configuration with a small gap distance of 10 mm and a tip radius of 10 µm was used. The
results indicate that the shorter tail-time impulse waveform results in a shorter stopping
length and higher breakdown voltage; however it does not affect the instantaneous
breakdown voltage and time to breakdown. A mathematical model is therefore described to
predict the breakdown voltage under different impulse waveforms. In addition, with the
similar stopping length, higher energy injected from the short tail-time impulse caused the
streamers to have more branches than those under the long tail-time impulse.
The characteristics of fault gas generation in the mineral oil and the synthetic ester liquid
under various levels of electrical faults were studied. A test platform with functions of
automatic spark fault control and data acquisition was developed. The effects of spark
numbers (from 20 to 500), gap distance (5 mm and 10 mm) and voltage levels (Vb-99.9% and
1.5Vb-99.9%) on fault gas generation in liquids were studied. The key gases in the mineral oil
are H2 and C2H2, while the key gases in the synthetic ester liquid are H2, C2H2 and CO. The
amount of fault gas generation increases linearly with the number of sparks. However, the
number of sparks does not have an obvious effect on fault gas pattern and gas generation
per unit fault energy in µL/J. Spark at a larger gap distance or under a higher applied
breakdown voltage generates more fault gases due to higher injected fault energy.
18
19
Declaration
DECLARATION
I declare that no portion of the work referred to in the thesis has been submitted in support
of an application for another degree or qualification of this or any other university or other
institute of learning.
20
21
Copyright Statement
COPYRIGHT STATEMENT
(i). The author of this thesis (including any appendices and/or schedules to this thesis)
owns certain copyright or related rights in it (the “Copyright”) and s/he has given The
University of Manchester certain rights to use such Copyright, including for administrative
purposes.
(ii). Copies of this thesis, either in full or in extracts and whether in hard or electronic
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22
23
Acknowledgement
ACKNOWLEDGEMENT
I would like to express my very great appreciation to my supervisor Dr. Qiang Liu for his
great supervision, guidance and support during my PhD studies in the University of
Manchester. He helped me build up concrete confidence bit by bit afterwards; any tiny
steps of my growth in the past three and half years are owing to his kind encouragement
and inspirations.
I would also like to thank my co-supervisor, Prof. Zhongdong Wang, since she has given
me valuable suggestions and comments on my research work and keeps me in the right
direction of research.
Great thanks are also given to the Qualitrol (Serveron) for the financial and technical
contributions to the project. In particular, he is Dr. John Hinshaw of Serveron Company.
Also, I would also like to thank Dr. Pascal Mavrommatis from TJ|H2b Analytical Services
for the lab measurements and technical contributions to the project. Mr. Adrian Walker, the
manager of EPSRC (Engineering and Physical Sciences Research Council) Engineering
Instrument Pool, His technical support and guidance for properly operating the Specialized
Imaging SIM 16 Camera guaranteed the success of this study and are greatly appreciated
here. In addition, many thanks to Prof. Yunpeng Liu from North China Electric Power
University (NCEPU), for the help of the impulse system design.
I wish to extend my thanks to all my colleagues in the transformer research group and all
my friends in the School of Electrical and Electronic Engineering. Thank you for offering
me an enjoyable working condition. Special thanks to Dr. Wu Lu, Dr. Zhao Liu and Mr.
Xiongfei Wang for the great help of technical supports during my experiments. In addition,
many thanks to Jiawen Xi, Xinyu Zhou, Zhepeng Lv, Harisanka K, Buyang Qi and Jiaxin
Li who I supervised during my PhD study, for the contribution of experimental studies.
Lastly but not least, I wish to give my heartfelt thanks to my family, to my parents for their
continuous support and encouragement.
24
25
List of Figures
27
Chapter 1 Introduction
CHAPTER 1. INTRODUCTION
1.1 Background
With the rapid development of society and technology, power systems are required to have
high stability and reliability, since they ensure our basic standard of living and daily
operation of industries. Transformers were introduced into the high voltage AC network in
the late nineteenth century [1]. Nowadays, a reliable power supply largely depends on the
fault-free operation of the networks, where transformers play an important role. The great
majority of power transformers in the world are still oil-immersed transformers, due to their
long lifetime expectancy.
In practice, the safe operation of high-voltage power transformers largely depends on the
insulation system design. The reason is that transformers in service are often exposed to
various types of overvoltage. One is the switching impulse voltage, which occurs when
apparatus is connected or disconnected from the power system, and is represented by
standard switching impulse voltage (250/2500μs) [2]. Another is due to lightning strikes to
overhead lines that are connected to the power transformer, represented by the standard
lightning impulse (1.2/50μs) [2]. Switching impulses and lightning impulses have been
recognised as one of the main causes of apparatus failure [1]. To verify the working
performance of insulation design, both switching and lightning impulse tests are required
for a transformer with a voltage rating higher than 170 kV [3].
Insulating liquids have been widely employed in high-voltage transformers for many
decades. They have several functions including electrical insulation, cooling medium and
information carriers. There are also some other expected properties for insulating liquids,
such as high fire point, environmentally friendly and cellulose solid insulation compatibility
etc. Nowadays, the dominated insulating liquid for power transformers is still mineral oil,
due to its successful and widespread usage over many decades. In recent years, the
preference for renewable, sustainable energy sources on an international level has meant
that ester liquids are now being considered as alternatives to mineral oils, due to their
improved fire safety and outstanding environment performance [4-6]. Ester liquids are
classified into synthetic esters synthesised from a combination of chemicals and natural
esters derived from sustainable and biodegradable resources [7]. For the last few decades,
28
Chapter 1 Introduction
natural esters have been successfully used in more than 45,000 low and medium power
installations [8]. Moreover, there is an increasing preference for ester liquids in large power
transformers. The synthetic ester liquid used in a 238 kV installation for Vattenfall in
Sweden has successfully been in operation since 2004, a 400 kV installation for National
Grid in the UK has been delivery in summer 2015 [9], and a 420 kV natural ester
transformer (operated at 380 kV) planted in Germany [10]. However, differences between
ester liquids and mineral oils in chemical and physical characteristics require further
investigation before they can be widely used in other types of transforms, e.g. HVDC
converter transformers.
Breakdown strength is one of the essential properties of insulating liquids. To understand
the breakdown mechanism in insulating liquids, the pre-breakdown phenomenon is the
critical aspect to be analysed [11]. Pre-breakdown in insulating liquids is a cause of further
deterioration of its insulation ability, which may eventually lead to failure of the electrical
equipment [1]. The discharge phenomenon in liquids before breakdown is usually called a
“streamer” [12]. Streamers are generally low-density conductive channels initiated in high
local electrical fields [13]. Generally, streamer characteristic analysis under impulse
voltages is one of the most effective ways to identify the intrinsic dielectric property of
insulating liquids [11]. However, due to a lack of coherent theory to describe streamers in
liquid, further research into the study of streamer characteristics, including length, velocity,
shape, area, current and light signals of the streamer, is required.
Streamer characteristics under standard lightning impulse and step-like impulse voltage
have been carried out for many decades, but there is a lack of understanding of the effects
of different impulse waveforms. In practice, an in-service transformer might be struck by
various impulse voltages, e.g. repetitive impulse voltage and voltage waveform with
different front and tail times. In recent years, more and more researchers have been
focusing on streamer and breakdown studies under non-standard impulse voltage [14-16],
e.g. chopped impulse. Therefore, the analysis of streamer characteristics and breakdown
strength under non-standard impulse voltage is worth further investigation.
In recent years, there has been an increasing interest in applying High Voltage Direct
Current (HVDC) transmission lines in some countries, e.g. European countries, Brazil and
China, due to their long distance or submarine bulk energy transmission [17]. In HVDC
transmission systems, converter transformers are the essential components. The insulation
systems of HVDC converter transformers could suffer both AC voltage and DC biased AC
29
Chapter 1 Introduction
voltage [18]. The streamer phenomena under AC and impulse voltage have been reported
for many years. In contrast to impulse voltage which has an extremely short duration within
microseconds, DC voltage has considerable time duration to work on insulating liquids.
However, there is a lack of streamer studies regarding liquids under DC voltage. To
understand the electrical performance of insulating liquids in HVDC converter transformers,
it is worth investigating the streamer characteristics and breakdown strength under DC
voltage.
Besides electrical breakdown, gassing behaviour due to discharge and breakdown has to be
considered for a reliable insulation design and safe operation of liquid insulated equipment
[19]. The gassing behaviour of mineral oil and ester liquids has been studied under
electrical fault of AC voltage for many years. In addition to the electrical fault of AC
voltage, transformers during operation could suffer various electrical faults, e.g. impulse
voltage. [20]. Moreover, past study [21] indicated that gas generation is highly associated
with fault energy. It is thus interesting to investigate the gassing behaviour of mineral oil
and ester liquids subjected to electrical faults under repetitive impulse with accurate energy
control.
1.2 Research Objectives
The aim of this PhD thesis is to evaluate the streamer, breakdown and gassing
characteristics of insulating liquids under various voltage stresses. Two types of insulating
liquids including a mineral oil and a synthetic ester liquid will be studied. Streamer
phenomena and breakdown strength under DC voltage were analysed under the various tip
radius and gap distances in a non-uniform electric field. Streamer characteristics and
breakdown properties of the two liquids under different impulse waveforms (from the
short-tail impulse to “step” like impulse voltage) will be investigated in a non-uniform
electric field. Moreover, to understand the gassing behaviour in the liquids after breakdown,
the correlation between gas generation and energy injection under electrical faults with
controlled fault energy were carried out. The following topics will be covered in detail in
this thesis:
(i) Streamer phenomenon and breakdown strength of insulating liquids under DC
voltages
Due to the application of converter transformers in HVDC transmission systems, the
investigation of dielectric properties of mineral oil and ester liquids in HVDC converter
30
Chapter 1 Introduction
transformers is required. The first attempt is to investigate the streamer and breakdown
phenomena under DC voltage in a non-uniform field. The effects of both tip radius and gap
distance on streamer initiation and breakdown strength are considered in this study.
(ii) Streamer characteristics and breakdown properties of insulating liquids under
different impulse waveforms
Lightning impulse is widely used in the power industry as one of the standard waveforms
for testing insulation performance. Step impulse has the advantage of fast rise-time that
mitigates the effect of space charge injection on streamer initiation and of long tail-time
that provides a stable voltage stress for streamer propagation. Previous studies [22, 23]
indicated that breakdown voltage highly correlates to instantaneous breakdown voltage and
time to breakdown under lightning and step impulse voltage. More valuable results, such as
breakdown voltage, can be obtained based on mathematical programs without practical
experiments. Therefore, to elaborate on the previous findings, the objective is to study
streamer and breakdown phenomena of a mineral oil and a synthetic ester liquid under
variable impulse voltages. A needle-to-plane electrode configuration and four different
impulse waveforms under positive polarity are considered in this study.
(iii) Gassing behaviour of breakdown in liquids
In recent decades, the gassing behaviour of mineral oil and ester liquids has been
investigated under electrical faults of AC and impulse voltage including discharge, sparking
and arcing. However, most previous studies lacked auto-controlled electrical fault
generation especially under AC voltage, which resulted in the inaccurate measurement of
fault energy injection. Therefore, the objective is to build up a testing system with an auto-
controlled spark generator and investigate the correlation between gassing behaviour and
electrical faults in both the mineral oil and the synthetic ester liquid.
1.3 Major Contributions
The major contributions of this thesis are given as follows:
(i) The effects of tip radius on streamer initiation voltage and the effects of gap
distance on breakdown voltage were studied under DC voltage. Although
initiation voltages of the synthetic ester liquid are comparable to those of the mineral oil,
the breakdown voltages of the synthetic ester liquid are lower than those of the
mineral oil at all the gap distances investigated under both polarities.
31
Chapter 1 Introduction
(ii) (ii) Pre-breakdown and breakdown phenomena in the mineral oil and the
synthetic ester liquid under impulse waveforms with different tail-time were
carried out in the point-plane electric fields. A mathematical model is described
to predict the breakdown voltage in the mineral oil and the synthetic ester liquid
under impulse waveforms with different tail times.
(iii) A relationship between fault gas generation and fault energy was investigated in
both the mineral oil and the synthetic ester liquid under lightning impulse. The
amount of fault gas generation increases linearly with the number of sparks.
However, the number of sparks does not have an obvious effect on fault gas
pattern and gas generation per unit fault energy in µL/J.
1.4 Outline of Thesis
The following is a summary of the chapters presented in this thesis:
Chapter 1 Introduction
This chapter introduces the background and motivation of the PhD study and also gives an
overview of the thesis.
Chapter 2 Literature Review
This chapter first provides background knowledge of discharge phenomena and
measurement techniques in liquids under DC. Then, the streamer modes and the effects of
impulse waveforms on streamers and the breakdown characteristics of insulating liquids are
discussed. Finally, recent research work related to fault gas generation in oil under
electrical faults is summarised in this chapter.
Chapter 3 Experimental Description
This chapter first discusses the liquids (a mineral oil and an ester liquid) under investigation
and the preparation procedures for oil samples. Then, the methodology of etched tungsten
needles based on electrochemical technique is explained. Finally, the experimental setups
for streamer and breakdown tests under DC and variable impulse voltage, and the setup of
fault gas generation under electrical faults are described in detail.
32
Chapter 1 Introduction
Chapter 4 Streamer and Breakdown Phenomena of Transformer Liquids under DC
Voltage
This chapter describes the streamer characteristics and breakdown strength of the mineral
oil and the ester liquid in a strongly non-uniform point-plane field under DC voltage. The
effects of tip radius on streamer initiation voltage and the effects of gap distance on
breakdown voltage are studied. At a fixed gap distance, streamer characteristics including
length, average propagation velocity and shape are analysed under both positive and
negative polarities.
Chapter 5 Streamer and Breakdown Phenomena of Transformer Liquids under
Various Impulse Voltages
This chapter reports the streamer phenomena and breakdown properties of the mineral oil
and the ester liquid under variable impulse voltages. Streamer length, average propagation
velocity, area and shape at a small gap distance are investigated. The relationship between
breakdown voltage, instantaneous breakdown voltage and time to breakdown is analysed,
and a feasible mathematical model for breakdown voltage estimation is proposed and
verified.
Chapter 6 Correlation between Gas Generation and Sparking Faults of Transformer
Liquids under Impulse Voltage
This chapter investigates fault gas generation in the mineral oil and the ester liquid under
sparking faults. A multi-gas on-line dissolved gas monitor was used to detect eight fault
gases. The effects of breakdown numbers, voltage polarities and voltage levels on gas
generations were studied. The relationship between energy generation and gas generation
was found.
Chapter 7 Conclusions and Future Work
This chapter summarises the main conclusions of this thesis and provides the suggestions
for future work no pre-breakdown and breakdown studies of transformer liquids.
33
Chapter 2 Literature Review
CHAPTER 2. LITERATURE REVIEW
2.1 Introduction
This thesis focuses on streamer, breakdown and gassing behaviour of breakdowns in liquids
under variable electrical stresses. This chapter first discusses the fundamental knowledge
and recent findings of streamer and breakdown in liquids under DC voltage and then
summarises the most recent research into the streamer and breakdown in liquids under
impulse voltage. Finally, previous results of gassing behaviour of liquids under electrical
faults are summarised and discussed.
2.2 Streamer and Breakdown in Liquids under DC Voltage
Various measurement technologies including current measurement [24-27], acoustic
emission technique [18], ultra-high frequency technique [28] and image converter camera
technique [24, 29] were introduced to investigate streamer and breakdown phenomena
under DC voltage. A few studies on partial discharge (PD) of hexane under DC voltage
were published [24, 29], describing the correlation between current signal and streamer
shape.
2.2.1 Methodologies
It is important to define the method for applying DC stress, as it has an impact on the
results [30] e.g. streamer initiation, breakdown voltage etc. In the literature, there are three
main methods used: the rising-voltage method [18, 31-33], periodic step-by-step method
[34], which have been used for streamer and breakdown study under DC voltage over the
past decades. Each method has its own pros and cons, and also its own validity range.
When comparing the breakdown strength of oil samples, different testing methods might
lead to different results. Therefore, an applicable testing method is significant for
investigating breakdown strength under DC voltage.
Rising-voltage method
The rising-voltage method is normally used for breakdown tests under AC and DC voltage.
For a single test, the applied voltage should be initiated at 0 kV and increased at a constant
rate till breakdown occurs. This single test procedure is then repeated after a fixed time
interval, until a specific number of breakdowns is obtained [35]. In IEC 60243 [30], the
34
Chapter 2 Literature Review
rising-voltage method is classified into a short-time test (breakdown in 10 to 20 seconds), a
slow rate-of-rise test (breakdown in 120 to 240 seconds) and a very slow rate-of-rise test
(breakdown in 300 to 600 seconds). The rate of rise shall be initially selected as defined in
Table 2-1 [30].
Table 2-1 Rate of rise corresponds to specific rising-voltage method [30].
Rising-voltage methods Rate of rise
short-time test 100 V/s, 200 V/s, 500 V/s, 1 000 V/s, 2 000 V/s,
5 000 V/s, etc.
slow rate-of-rise test 2 V/s, 5 V/s, 10 V/s, 20 V/s, 50 V/s, 100 V/s,
200 V/s, 500 V/s, 1 000 V/s, etc.
very slow rate-of-rise test 1 V/s, 2 V/s, 5 V/s, 10 V/s, 20 V/s, 50 V/s,
100 V/s, 200 V/s, etc.
Step-by-step method
The step-by-step method is normally used to observe streamer initiation, propagation or
breakdown test under impulse voltage due to the application of discontinuous voltage.
Applied voltage starts at a level of expected percentage (normally 40% - 70%) of initiation
voltage or breakdown voltage, and increases step by step at a constant step interval (e.g. 1
kV per step) till initiation or breakdown occurs. Between each step, a fixed amount of time
should be allowed to prevent the accumulative effect of space charge, particles, bubbles etc.
In IEC 60243 [30], a voltage at 40 % of the probable short-time breakdown voltage shall be
applied to the oil sample based on a 20-second step-by-step test. If the oil sample
withstands this voltage for 20 seconds without breakdown, the voltage should be increased
in incremental steps as defined in Table 2-2. Each increased voltage should be immediately
and successively applied for 20 seconds until breakdown occurs. Unless otherwise specified,
the test should be carried out with a time intervals of 60 seconds.
Table 2-2 Rate of rise corresponds to specific step-by-step method [30]
Starting voltage (kV) Increment (kV)
1.0 or less 10 % of start voltage
Over 1.0 to 2.0 0.1
Over 2.0 to 5.0 0.2
Over 5.0 to 10.0 0.5
Over 10 to 20 1.0
Over 20 to 50 2.0
Over 50 to 100 5.0
Over 100 to 200 10.0
Over 200 20.0
35
Chapter 2 Literature Review
The step-by-step method under DC voltage was used in [34]. The experiment was carried
out using the periodic stress-grounding method, applying a DC voltage for 3 minutes and
then grounding for 2 seconds, and the same procedure was repeated 20 times. In addition,
different test methods were compared under impulse conditions, which indicates that 50%
breakdown voltage under 3 shots per step is lower than that under 1 shot per step [36].
2.2.2 Current and emitted light
The experiment of current measurement in a mineral oil under negative DC voltage was
carried out in [37]. The gap distance was between 3 and 25 mm with a needle-to-plane
geometry. A 50 Ω current shunt was placed at low voltage (LV) side of the test cell and was
connected to an oscilloscope through a 50 Ω coaxial cable. A coupling capacitor with 460
nF used for PD pulses detection was set in parallel with the test cell.
The negative streamer current in a mineral oil under DC voltage at a gap distance of 5 mm
in a needle-to-plane electrode is shown in Figure 2-1 [37]. When the applied voltage is
below 50% breakdown voltage, the oscillating discharge pulses might occur discreetly as
single pulses, in pairs, three pairs or more successive pulses with a stably increasing
magnitude [37]. For each single pulse, the rise time of about 1 ns indicates the PD
phenomena in mineral oil is as fast as that in air gap [37]. Based on the current
measurement, it is possible to understand the time domain from streamer initiation to
propagation.
(a) 100 ns/div (b) 5 ns/div
Figure 2-1 Typical discharge pulse in a needle to plane gap in a mineral oil under negative polarity at room
temperature, d = 5 mm, V = 14 kV [37].
The photomultiplier tube (PMT) is normally used to study streamers in optics, which
records the real-time light emission during streamer propagation. Previous studies of
streamers related to emitted light in oils were mostly under impulse voltage [38, 39],
36
Chapter 2 Literature Review
showing that emitted light signals correspond to current signals. A study of emitted light of
streamers in transformer oil under DC voltage was carried out in a needle-to-plane
electrode of 20 mm gap distance as shown in Figure 2-2 [27]. A Hamamatsu made PMT
H5783P with a spectral response from 300 nm to 650 nm was used to monitor the emitted
light of a streamer. Compared to the current signal, the unipolar light signal has less noise
and signal oscillation, which enables researchers to have a deeper understanding of
streamer studies.
Figure 2-2 Streamer current and emitted light signal in transformer oil under DC voltage, positive polarity, d
= 20 mm, V = 40 kV [27].
In addition, the results from [24, 29] indicated that the streamer current waveform is
significantly different between positive and negative polarities under DC voltage, which
confirms the previous finding under impulse voltage [40, 41]. Figure 2-3 shows the typical
current waveforms of positive (Anode point) and negative (Cathode point) polarities under
DC voltage [29]. The streamer current under negative polarity is characterised by a period
of growth where the pulse amplitudes increase systematically followed by a longer period
of large pulse. Under positive polarity, both continue and pulsed current waveforms were
observed.
37
Chapter 2 Literature Review
Figure 2-3 Streamer current waveforms in Hexane taken with a point cathode (Top – Negative polarity) and
with a point anode (Bottom – Positive polarity), point-sphere electrode, d = 4.75 mm [29].
2.2.3 Photography measurement
Besides the methods of electrical and optical measurements, photographing setup remains
the most widely used technique to study streamers. Recent studies of streamer images are
mainly obtained under impulse voltage [12, 38, 42, 43]. Research into streamer shapes was
normally limited to impulse studies due to difficulty in correlating the shuttering of the
camera with the initiation stage of streamer under DC voltage [29]. Under continuous
stressing of DC voltage, the randomness and uncertainty of a streamer initiation make
streamer image capture more difficult.
Figure 2-4 shows the experimental setup of streamer photograph investigation in Hexane
under DC voltage [24, 29]. A needle-to-plane electrode made of stainless steel at a gap
distance of 3.2 mm was set as electrode geometry. A 10 MΩ current limit resistor was
38
Chapter 2 Literature Review
connected following high-voltage DC power supply to prevent a high current when the
breakdown occurs. A laser used for illumination when the streamer occurs was placed at the
left of the test cell. An image converter camera was placed at the right side of the test cell to
capture the streamer image. Lenses were placed between the test cell and camera to provide
high magnification. An image preserving optical delay was developed to preserve the image
long enough to shutter the camera. A Pockels cell shutter prevents the continuous
illumination generated by the laser from overexposure in image converter camera. At LV
side of the test cell, a current sensitive amplifier was directly connected to the needle
electrode to measure the current signal. The Pockels cell shutter and the camera are both
triggered simultaneously by a streamer current signal via the synchronising circuit and
current amplifier. The shutter opens after a streamer initiates, and after the camera is used
to photograph the growth of the discharge, a second pulse closes the shutter [29].
Figure 2-4 Experimental setup of time delay system for streamer photograph capture under DC voltage (re-
produced) [24].
The synchronised imaging technique, allows the features of streamer shape under DC
voltage to be investigated. Previous findings of streamers under impulse conditions
indicated that the intensity and occurrence time of streamers in photograph normally
correlate to those of current and light signals [12, 44]. A similar streamer feature
phenomenon was found under DC voltage with the applied voltage level of 15.5 kV [24,
29]. Figure 2-5 shows the streamer shape, current and stopping length in Hexane at a
needle-to-plane geometry of 3.2 mm gap distance under DC voltage [29]. The frames were
captured 200 ns apart and the exposure time was 40 ns. It was found that the isolated
current pulses were generated when the streamer propagated and the occurrence of a small
current pulse between 3 and 4 µs appears to be correlated with some slow development of
CW Argon
Laser 10 X
Image – Preserving
Optical Delay Pockels Cell
Shutter
Image
Converter
Camera
Trigger
Amp. Synchronizer
Digital Storage
Oscilloscope
DS
PS
10 M
250 pF
Mon.
100 K
39
Chapter 2 Literature Review
the streamer. Once these current pulses ended, the streamer stopped growing and dissipated
slowly. Although the streamer can be captured by the synchronised imaging technique,
there is still an obvious time delay of the photograph at the initiation stage of the streamer
due to the difficulty of triggering a small current at an early stage.
Figure 2-5 The photograph of a streamer with the numbers indicating the sequence in which the pictures were
taken; time interval between each film 200 ns; (a) streamer photograph by the camera; (b) streamer current
signal; (c) streamer stopping length [29].
Figure 2-6 shows a comparison of streamer shapes leading to breakdown in transformer oil
under different external pressures in a needle-to-plane geometry with a gap distance of 2.5
mm under negative DC voltage [45]. A voltage amplifier was used to trigger the pulse
generator to initiate intensified charge coupled device (ICCD) camera. All pulse generators
were set to a minimum internal delay of 80 ns. This indicates that the structures of the
streamer formations shown for 300 torr (about 40 kPa) are identical to streamer formations
observed at atmosphere (about 101 kPa), presented as a uniform, luminous channel after
breakdown and no expanded regions [45]. When the environment pressure is at 30 torr
(about 4 kPa) or below, the streamer shapes begin to show some expansion and once
breakdown occurs, luminosity is seen throughout the expanded region. This finding
assumes that the breakdown mechanism in liquids of negative polarity under DC voltage is
based on bubble formation with subsequent carrier amplification in the gas phase, which
indicates pressure dependence, i.e. the expansion velocity decreases with increasing
pressure. These results support the bubble theory streamer formation and breakdown under
negative polarity. However, a similar experiment was investigated under positive polarity
(a)
(a)
(b)
(c)
Time
delay
40
Chapter 2 Literature Review
and indicated no pressure dependence [46]. This finding confirms the previous results under
lightning impulse tests, which indicated that positive streamers rely more on electronic
processes, while negative streamers depend more on gaseous processes [47].
Figure 2-6 Shadowgraph images of streamer expansion in transformer oil as a function of time and pressure
in a needle-to-plane geometry with a gap of 2.5 mm under negative polarity [45].
2.2.4 Breakdown properties in Liquids
Breakdown strength is one of the parameters used to assess the performance of insulating
liquids. The breakdown studies under DC voltage, which are enhanced by adding semi-
conductive nanofluids (SNFs) in transformer oil are presented in [33]. The high voltage DC
source is capable of generating up to 200 kV. The point-sphere geometry with a gap
distance of 10 mm and a tip radius of 3 μm was chosen in the tests. The rising-voltage
method is applied to stress the oil sample with a ramping rate of 1 kV/s. The initial standing
time was 5 minutes, and the time interval between two successive shots was fixed at 1
minute. All experiments were performed at room temperature. After one breakdown
occurred, the oil sample and electrodes were changed; 10 samples in total were carried out
for each type of oil. Similar to the breakdown tests under impulse voltage [40], DC
breakdown voltages of positive polarity are almost half of those under negative polarity.
41
Chapter 2 Literature Review
Figure 2-7 shows the breakdown tests in oil with two different gap distances in a needle-
plate electrode system under DC voltage of both polarities [48]. Similar polarity effects to
the results shown in [40] were observed; negative DC breakdown voltages are much higher
than positive DC breakdown voltage. Moreover, the polarity effect under oil alone
condition becomes significant with an increase of gap distance, and this is similar to what
was observed in past studies [38].
Figure 2-7 Breakdown voltage of oil under DC voltages of both polarities [48].
2.2.5 Space Charge
A lot of publications related to DC voltage focused on investigating space charge, as
insulating paper and oil are widely used as major insulation materials in HVDC equipment,
such as converter transformers, DC bushings and DC cables [49-52]. Most of the space
charge studies focused on oil paper insulation system, since the formation and dynamics of
space charge under high electric fields will change the distribution of electric fields [49].
In liquid only, the theory of space charge induced field distortion can be also used to
explain the polarity effect on the breakdown in liquids [11]. Under positive polarity, after
ionization occurs in the region near the needle electrode, electrons move quickly towards
the positive needle electrode, while positive ions move away slowly to the negative plane
electrode. The accumulated positive ions act as an extension of the positive needle electrode
and thus enhance the boundary electric field at the head of the positive streamer. This
promotes streamer propagation and consequently decreases breakdown voltage. Under
negative polarity, after ionization occurs in the region near the needle electrode, positive
ions move back slowly to the negative needle electrode, while electrons dissipate quickly
into the liquids. Therefore the diluted negative charges act as a shielding of the negative
42
Chapter 2 Literature Review
needle electrode and reduce the electric field at the head of the negative streamer, which
slows down streamer propagation and consequently increases negative breakdown voltage.
2.3 Streamer and Breakdown in Liquids under Impulse Voltage
The transformer in service could be exposed to voltages in excess of the normal operating
voltage, such as transient overvoltage due to lightning strikes to earth near overhead lines
connected to the transformer (represented by standard lightning impulse 1.2/50 μs). In order
to understand the breakdown mechanism in liquids, pre-breakdown phenomena, also
known as streamer phenomena, shall be thoroughly investigated. Previous streamer and
breakdown measurements of insulating liquids under lightning impulse were studied in [12,
38, 42, 43, 53-55], and those under step impulse were published in [56-60]. The correlation
between breakdown voltage, instantaneous breakdown voltage and time to breakdown
under lightning impulse and step impulse at large gap distance was discussed in [22, 23],
and it was shown that more comprehensive data can be obtained without complex
experiments. In the following sections, general knowledge of streamer and breakdown
phenomena under various impulse voltages is discussed.
2.3.1 General Streamer Characteristics
The breakdown in liquids generally consists of the initiation and propagation of a streamer.
If the streamer stops propagating before bridging the electrodes, the phenomenon is treated
as pre-breakdown or streamer. Pre-breakdown phenomena in liquids normally behave as
luminous filaments propagating at a velocity from about 100 m/s up to more than 100 km/s
[61]. From early publications [62-65] of discharges in liquids, the terms ‘corona’, ‘leader’
and ‘streamer’ were used to define the phenomenon. As time progressed, the term ‘streamer’
with defined modes was widely accepted to describe all types of discharge phenomena in
liquids.
So far there are two main theories used to describe streamer mechanisms: ‘electronic
ionisation model’ and ‘gaseous bubble theory’. The electronic ionisation model is borrowed
from early studies of electrical breakdown in gases, which indicated breakdown occurs on
account of electron avalanches caused by continuous collision ionisation [66]. In the
gaseous bubble theory, a streamer initiates from local micro bubbles in liquids, which is
possibly formed by the vaporisation of liquid due to the charge injection induced heating
process or the natural existence due to the impurity of the liquid [67]. The bubble theory is
supported by a few previous studies [68, 69] under various pressure tests, such as the
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Chapter 2 Literature Review
streamer image shown in Figure 2-6 [45], in which both streamer initiation and propagation
are affected by the applied pressure. With more and more findings on the streamer, these
two theories are both feasible and reasonable to describe breakdown phenomena, probably
in the different stages of streamer, named as propagation ‘modes’ [11].
Streamer modes
The idea of streamer ‘modes’ was first proposed by Hebner [70] in 1988, who put forward a
classification of modes based mainly on streamer propagation velocities. The development
and the usage of the high-speed camera by Lesaint and others have led to the creation of
experimental phenomena (e.g. streamer shapes) and, as a result, the concept of ‘modes’ can
then be decided by such experimental phenomena. [12, 61]. In an identical nature of oil
sample, when the test conditions are changed (e.g. higher applied voltage), a rapid
transition in the streamer propagation velocity is observed. Not only does the streamer
velocity dramatically increase, but the streamer shape, current and emitted light signal are
also influenced. This phenomenon indicated that different physical processes do exist in
these streamer modes. The images of Figure 2-8 show the four different positive streamer
modes (1st, 2
nd, 3
rd and 4
th modes) induced in a mineral oil with a wide range of applied
impulse voltage (from 6 to 424 kV) and a needle-to-plane geometry of gap distances from 6
mm to 10 cm [61]. It was found that the streamer propagates towards the plane electrode
with an average propagation velocity from 100 m/s at small gap distance (A), to over 100
km/s at large gap distance under much higher voltage (E).
Figure 2-8 Typical images of positive streamers in a mineral transformer oil. (A) ‘1st mode’; (B) ‘2nd mode’;
(C) ‘2nd
mode’; (D) ‘3rd + 2nd modes’; (E) ‘4th mode’ [61].
Streamer images (A) and (B) presented as shadowgraphs indicate the streamer is
constituted by the low-density gas phase. All streamer modes are able to emit light but with
different intensity. The light intensity of 1st mode streamer in liquids is weakest, where the
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Chapter 2 Literature Review
light intensity is almost close to detection capability of PMT. It is difficult to capture these
kinds of streamers even by a sensitive high-speed camera due to the weak illumination of
the streamer. According to different literature, this kind of streamer is known by different
names such as ‘primary’, ‘bush-like’, ‘subsonic’ or ‘slow’ streamer. The 1st mode streamer
is normally observed only at the low voltage level and preferably in the pre-breakdown
stage. The typical streamer current signal in liquids is composed of a continuous current at
the early stage and a train of rapid pulses with nanosecond duration afterwards as shown in
Figure 2-9 (A) [71]. The emitted light signal has an extremely low intensity, which
indicates the irregular nature of the recorded signal. Therefore, the observation of 1st mode
streamer can only be achieved by a shadowgraph or the Schlieren technique.
When the applied voltage is increased, the shape of the streamer is suddenly transformed
into to the 2nd
mode streamer with the appearance of fine filaments (µm in diameter) as
shown in Figure 2-8 (B) [61]. Since the 2nd
mode streamer is faster, streamer currents in
Figure 2-9 (B) are made up of continuous components with significant intensity [71].
Moreover, the intensity of emitted light by 2nd
mode streamer is much stronger than that of
the 1st mode streamer. Therefore, it then becomes possible to record the 2
nd mode streamers
more clearly owing to more luminous branches and offshoots [61].
Figure 2-9 Typical examples of positive streamer with emitted light (upper trace, arb. unit.), transient currents
(middle trace) and voltage (lower trace) in cyclohexane, d = 1.8 mm, rp = 1.2 μm, (A) 1st mode, V = 12 kV, (B)
2nd
mode V = 20 kV [71].
Streamer pictures (C), (D) and (E) were obtained with a gated image intensifier showing the
emitted light integrated during propagation [61]. Streamer photographs (C), (D) and (E)
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Chapter 2 Literature Review
mainly show the most luminous filaments, while weakly luminous or dark filaments cannot
be seen in these images. The light emission of fast streamers at high voltage in 4th
mode (E)
is considerably greater than in (C). Due to strong light intensity in the image (E), the
aperture of the high-speed camera must be relatively reduced to avoid overexposure. By
contrast, weakly luminous filaments surrounding the main luminous channel cannot be
observed, due to the low light intensity of side streamer branches.
Figure 2-10 shows the typical transient currents of 2nd
, 3rd
and 4th
mode streamers in a
natural ester liquid at a gap distance of 10 cm [57]. As shown in Figure 2-10 (A), the 2nd
mode streamer was observed with a uniform velocity about 2 km/s and a stopping length ls
of 3 cm without breakdown. The associated current at the early stage is in the form of a
continuous current and afterwards is transformed into a train of large single pulses with
increasing amplitude, and has similar characteristics to streamers observed in Figure 2-9 but
different liquid nature. In mineral oil, the presence of aromatic molecules helps to provide a
stronger light emission, while the emitted light in ester liquids is weaker compared to
mineral oil [44].
When the applied voltage is increased to a certain level, a very different 3rd
mode streamer
can be observed at the early stage of streamer propagation in Figure 2-10 (B). Several
intense current pulses up to 1 A correspond to the bright streamer channel in picture (B).
The picture (B) also indicates that the 3rd
mode streamer does not propagate in continuous
form. The corresponding average propagation velocity of the 3rd
mode streamer is larger
than 10 km/s. Once the initial 3rd
mode streamer has ended, the streamer then continues to
propagate with the 2nd
mode at relatively lower velocity. In addition, when a voltage level is
repeated, either 2nd
mode or 3rd
mode streamer usually appears. For example in Figure 2-10
(A) and (B), both streamers were obtained at the same voltage – 120 kV, and both 2nd
and
3rd
mode streamers randomly showed up [61]. For the pure 3rd
mode streamer in Figure
2-10 (C), the time duration of current pulses (about 200 ns) is much longer than those of the
2nd
mode streamer, and the instantaneous velocity per step reaches above 100 km/s [61].
At a much higher voltage, streamer propagation from the needle-to-plane electrode is
completed in one step at an extremely fast velocity of about 120 km/s, which is called the
4th
mode streamer. The streamer current in the 4th
mode is completely in the form of
continuous components as shown in Figure 2-10 (D).
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Chapter 2 Literature Review
Figure 2-10 Propagation modes at high voltage in a natural ester liquid. d = 10 cm. Upper oscilloscope trace:
applied voltage, lower: streamer current. (A) 2nd
mode, V = 120 kV; (B) 3rd
+ 2nd
modes, V = 120 kV; (C) 3rd
mode, V = 142 kV; (D) 4th
mode, V = 152 kV [57].
Investigations of streamers in the 3rd
and 4th
modes were also carried out in [72]. Figure
2-11 shows the propagation process of positive 3rd
+2nd
mode streamer in Diala S4 at high
voltage just below acceleration voltage Va, which indicates more structure information of
the 3rd
mode streamer [72]. At the voltage just below Va, in the first stage during streamer
propagation, the 3rd
mode positive streamers appear, with speeds varying between 6 km/s to
11 km/s. These 3rd
mode streamers consist of one or two main branches surrounded by
numerous side branches. However, the 3rd mode streamers turn into 2nd mode streamers
afterwards with a speed of around 3 km/s during propagation.
Figure 2-11 The propagation process of positive 3rd
+2nd
mode streamer at high voltage just below Va; Diala
S4 ZX-I, d = 50 mm; the ground electrode is at the bottom edge of the streamer images [72].
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Chapter 2 Literature Review
Figure 2-12 shows the propagation process of negative 4th
and 3rd
mode streamers in Diala
S4 at high voltage above Va. When applied voltage is above Va, the propagation of negative
streamers starts with 4th
mode over a very short time (0.5 μs). The 4th
mode streamer has a
bright filamentary shape and a velocity of approximately 20 km/s. However, this bright
channel quickly dies out and streamer propagation falls to the 3rd
mode with a much lower
velocity of 5-6 km/s. The shape of the 3rd
mode streamer is similar to that of the 2nd
mode
streamer at lower voltage but has more branches. The 3rd
mode streamer then propagates for
about 3 μs before the streamer switches back to the 4th
mode again at the end of streamer
propagation, with a velocity of over 10 km/s.
Figure 2-12 The typical propagation process of 4th
+3rd
mode negative streamers in the GTL oil, d=50 mm; the
ground electrode is at the bottom edge of the streamer images from frame 3 to frame 6 [72].
Figure 2-13 and Figure 2-14 show the typical shapes of the positive and negative 4th
mode
streamers in neat white oil, neat white oil with reduced pressure (RP) and neat white oil
with dimethylaniline (DMA) under lightning impulse [47]. When applied voltage is above
Va, the 4th
mode streamers are observed. Under positive polarity, the streamers consist of
only a main bright channel with some lateral branches. Compared to the 2nd
and 3rd
mode
streamers, the 4th mode streamers have a much lower number of branches. This mode
streamer is as fast as 100 ~ 200 km/s [47]. Under negative polarity, streamers are composed
of some main channels surrounded by numerous lateral branches. The streamers are as fast
as 10 ~ 40 km/s, which are slower than those under positive polarity [47].
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Chapter 2 Literature Review
Figure 2-13 Typical shapes of the positive 4th
mode streamers: (a) neat white oil, (b) neat white oil at reduced
pressure, and (c) DMA, all images taken just before breakdown, d = 80 mm [47].
Figure 2-14 Typical shapes of the negative 4th
mode streamers: (a) neat white oil at reduced pressure, (b) neat
white oil, and (c) TCE, all images taken just before breakdown [47].
As well as investigating of streamer modes, paper [47] also indicated that both electronic
and gaseous processes exist during streamer propagation of both low and fast mode
streamers with a long gap. Fast positive streamers rely more on electronic processes, while
fast negative streamers depend on both electronic and gaseous processes [47].
Overall, it is feasible to observe transitions among streamer shapes corresponding to the
different streamer modes. The 1st mode streamer shows a ‘bubble-like’ shape, and the 2
nd
mode streamer shows a ‘tree-like’ shape composed of branched fine filaments. The 3rd
mode streamers consist of a bright filamentary channel and tree-like dark channels. The 4th
modes streamers are normally formed of filamentary channels with high light intensity [61,
72].
2.3.2 Streamer and Breakdown under Different Impulse Waveforms
Breakdown measurements in liquids under lightning impulse voltage are more
representative of conditions used to test real apparatus such as transformers. Step impulse
has the advantage of fast rise time and long tail time and is favourably used to interpret
breakdown phenomena since the applied voltage remains constant while the streamer
propagates [23]. An early study [73] in 1979 talked about the interactions between voltage
shape, streamer propagation, and breakdown. Figure 2-15 shows the characteristic voltage
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Chapter 2 Literature Review
for oil breakdown on an impulse waveform. In order for breakdown to occur a streamer
must initiate (Vi) and propagate across the gap before the voltage drops to zero. In fact,
breakdown occurs on the tail at a voltage which is considerably larger than zero, which may
confirm that a minimum voltage for breakdown Vm exists [73].
Figure 2-15 Characteristic voltage for oil breakdown on an impulse waveform, where L is gap distance, ti and
tb are the times corresponding to initiation and breakdown [73].
Figure 2-16 shows the effect of tail time of impulse on breakdown voltage at different gap
distances carried out with point-plane geometry in transformer oil [73]. It is clear that the
breakdown voltages increase with the decrease of tail time. When time constant is large
enough (step-like impulse), V0 is comparable to the minimum voltage for breakdown Vm.
This means the effects of parameters such as wave shape on impulse behaviour can be
predicted [73].
Figure 2-16 The effect of tail time on breakdown voltage in transformer oil at different gap distance, positive
point-plane electrode, gap distance: (o): 2.54 cm; (): 5.08 cm; (Δ): 10.16 cm; (V): 14.61 cm [73].
An interesting phenomenon related to breakdown voltage and acceleration voltage in
mineral oil under lightning and step impulse was also observed in [42] as shown in Figure
ti
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Chapter 2 Literature Review
2-17. At very small gaps, lightning breakdown voltage is identical to breakdown voltage
obtained under step voltage. With increasing gap distances, lightning breakdown voltage
becomes higher than the breakdown voltage under step voltage, probably due to the short
tail of lightning waveform, but is still lower than the acceleration voltage under step voltage.
When the gap distance reaches 55 mm, lightning breakdown voltage is almost the same as
the acceleration voltage obtained under step voltage [42].
Figure 2-17 Comparison of breakdown voltage and acceleration voltage in mineral oil under lightning and
step impulse, negative polarity [42].
Afterwards, two recent publications [22, 23] indicated that different impulse waveforms
have an influence on breakdown mechanism in both polarities, and came up with a theory
of breakdown voltage prediction. Experimental results of streamer under lightning (1.2/50
µs) and step impulse (0.4/1400 µs) were shared and compared [22]. Figure 2-18 shows the
comparison of the stopping lengths between lightning and step impulse voltage under
positive polarity [22]. Under positive polarity, the solid line indicates the growing trend of
stopping length with increasing voltage applied. It was found that the stopping length
increases linearly with applied voltage under both lightning and step impulse voltage.
Nevertheless, at the same applied voltage level, streamers under the step impulse propagate
further than those under the lightning impulse voltage. The measured stopping length under
lightning impulse is about 60% of that measured under step impulse. The shorter stopping
length under lightning impulse voltage is as a result of voltage decaying during streamer
propagation [22].
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Chapter 2 Literature Review
Figure 2-18 Comparison of stopping length between step impulse and lightning impulse under positive
polarity, d = 50 [22].
Average propagation velocity va is determined by the ratio of stopping length to
propagation time measured, based on the streamer current or the light signal. In the case of
a breakdown event, va is calculated by using gap distance d divided by time to breakdown tb.
Figure 2-19 shows the comparison of average propagation velocity between lightning and
step impulse voltage under positive polarity. Under positive polarity, the va under lightning
impulse follows exactly the same trend as that under step impulse, at about 2 km/s for the
2nd
mode streamer (responsible for Vb) [22].
Figure 2-19 Comparison of average propagation velocity between lightning impulse and step impulse under
positive polarity, d = 50 mm [22].
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Chapter 2 Literature Review
Based on the results in Figure 2-18 and Figure 2-19, different impulse voltage waveforms
have an influence on streamer and breakdown properties under positive polarity. Table 2-3
gives the comparison of 50% breakdown voltage and acceleration voltage under both
lightning and step impulse [22]. When the streamer propagates in the 2nd
mode (about 2
km/s), the time that the streamer takes to bridge the whole gap (50 mm) is 25 µs. During
this time, lightning impulse voltage drops from 126 kV to about 94 kV. The moment of
voltage drop (94 kV) presents the instantaneous breakdown voltage Vi. An interesting
phenomenon was observed when the instantaneous breakdown voltage under lightning
impulse is very close to the breakdown voltage (93 kV) measured under step impulse. In
terms of acceleration voltage, fast streamers bridge the whole gap in a short time, in which
the voltage drops under lightning impulse could be ignored [22].
Table 2-3 Breakdown voltage and acceleration voltage obtained under step impulse and lightning impulse
[22].
` Breakdown voltage (kV) Acceleration voltage (kV)
Positive Negative Positive Negative
Step impulse 93 174 260 240
Lightning impulse 126 228 260 250
At small gap distance (typically d < 1 cm) under positive polarity, the 50% breakdown
voltage under lightning and step impulse are almost the same due to the short time to
breakdown. At considerably large gap distances (typically d < 12 cm), the short tail time of
lightning impulse is not able to cause the streamer to bridge the whole gap in the 2nd
mode,
and so breakdown can only occur when the streamer is transformed into the fast streamer
[22, 42]. In addition, this phenomenon may not apply to the breakdown under negative
polarity, due to large variation and uncertainty [22].
This phenomenon was confirmed again based on a large number of experimental results in
[23]. The breakdown tests were carried out in both mineral oil and ester liquids under point-
plane and point-sphere electrode geometries with a large range of gap distance up to 35 cm
[23, 42]. Two different impulse waveforms were used, including the standard lightning
impulse (1.2/50 µs) and a specific step impulse. Figure 2-20 shows the breakdown voltage
and time to breakdown versus gap distance in point-sphere geometry in various insulating
liquids [23]. It was found that with the increase of gap distance, the increase of breakdown
voltages does not follow the exact same trend in terms of liquid nature. The different
transition points marked as a dashed line exist in various liquids as shown in Figure 2-20
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Chapter 2 Literature Review
(a). In addition, the transition points in breakdown voltage correspond to the reduction of
time to breakdown. The reduction of time to breakdown in a large gap distance indicates
the transformation from slow streamer to fast streamer, which leads to the conclusion in
[22].
Figure 2-20 Comparison of (a) Breakdown voltage VbLI
and (b) time to breakdown tbLI
under lightning
impulse versus gap distance in liquids of group IV (PMO, NMO, PB) [23, 42].
Figure 2-21 shows breakdown velocity versus gap distance in different liquids under
lightning impulse (a) and step impulse (b) [23]. Under lightning impulse, a uniform
streamer propagation velocity at about 2 km/s in a mineral oil was observed at small gap
distance (3 cm ≤ d ≤ 12 cm), which corresponds to the propagation of the 2nd
mode
streamer [44]. Under step impulse, the streamer propagation velocity at gap distance from 2
cm to 20 cm remains constant at 2 km/s. It is reasonable that under large gap distances, a
higher crest voltage of lightning impulse must be induced to allow the streamer to
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Chapter 2 Literature Review
accelerate (vp > 10 km/s) and achieve breakdown such behaviour strongly depends on the
liquid nature [23].
Figure 2-21 Average breakdown velocities vbLI
versus gap distance in insulating liquids, (a) under lightning
impulse; (b) under step impulse [23].
Figure 2-22 shows the comparison of breakdown voltage and instantaneous breakdown
voltage in mineral oil under lightning and step impulse. Firstly, breakdown voltage under
lightning VbLI
and step impulse VbST
are extremely close in small gaps (d ≤ 2 cm). Then, VbLI
becomes larger than VbST
with the increase of d (d ≥ 2cm). However, the instantaneous
breakdown voltage ViLI
(at the moment of breakdown) is identical to VbST
with the gap
distance d less than 12 cm. Finally, the difference between ViLI
and VbST
becomes larger
with d above 12 cm. Two cases (A and B) corresponding to different breakdown
mechanisms are able to explain the behaviour in mineral oil as shown in Figure 2-23. In
case A, to compensate for the voltage decay under lightning impulse, a crest voltage VbLI
higher than the minimum propagation voltage Vmin (similar to VbST
) must be applied to
induce breakdown. At this stage, the average propagation velocity in mineral oil still
remains constant, and VbLI
should not exceed the acceleration voltage Va. A past study [74],
indicated that breakdown can be induced only if Vi does not drop below Vmin before the
streamer propagates to the opposite electrode. Therefore, the breakdown criterion is able to
be written as [23]:
𝑽(𝒕 = 𝒕𝒃) = 𝑽𝒎𝒊𝒏 (2-1)
When time to breakdown corresponds to 2nd
mode streamers, an interesting phenomenon
was observed in which ViLI
highly corresponds to the criterion VbST
≈ ViLI
≈ Vmin. Moreover,
the average breakdown velocity under lightning impulse voltage should remain constant
(a) (b)
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Chapter 2 Literature Review
and similar to step voltage at about 2 km/s as shown in Figure 2-21. At a short gap distance
of less than 3 cm, the time to breakdown is relatively shorter, e.g. tb = 10 µs at d = 2 cm.
The lightning impulse voltage has less time duration for the voltage decay, about a 10%
drop. As a result, VbLI
is quite close to Vmin (VbST
≈ VbLI
≈ Vmin). In case B where the gap
distance is larger than 12 cm in mineral oil, VbLI
is potentially close to Va, which results in
the occurrence of a fast streamer as shown in Figure 2-20 and Figure 2-21.
Figure 2-22 Comparison of breakdown voltage in mineral oil PMO under step and lightning voltages in
point-sphere geometry [23].
Figure 2-23 Typical breakdown cases in mineral oil under lightning impulse voltage (full lines) and step
impulse voltage (dotted lines) [23].
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Chapter 2 Literature Review
Figure 2-24 compares the breakdown voltages Vb in a natural ester under lightning and step
impulse in point-plane geometry with the gap range from 2 cm to 20 cm [23]. The results
indicate that breakdown voltages under step voltage VbST
are nearly equal to breakdown
voltage under lightning impulse VbLI
, which is reasonable since breakdown is a result of the
fast streamer. Due to the high velocity of fast streamers, the decay of the lightning impulse
voltage is nearly negligible at this moment, which explains why VbST
≈ VbLI
in a natural
ester.
Figure 2-24 Comparison of breakdown voltage in natural ester NE under step and lightning impulse voltage
in point-plane geometry [23].
Overall, the breakdown characteristics of various liquids at various gap distances were
studied under lightning and step impulses. However, there is still a lack of breakdown
studies regarding other waveforms to develop the theory further. Therefore, a highly
flexible voltage generator will be built to investigate the breakdown characteristics in
liquids under impulse waveforms with different tail-time in this study.
2.4 Gas Generation in Liquids under Electrical Faults
2.4.1 Background Knowledge of Fault Gas Analysis
2.4.1.1 Theory
Discharges and breakdown cause chemical degradation of insulating liquids and generate
fault gases [75]. The insulating liquids are composed of different hydrocarbon atomic
groups, e.g. CH2, CH3 and CH. The molecular bonds used to link the molecular group
together (e.g. C-C and C-H bonds), will be broken when electrical or thermal energy is
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Chapter 2 Literature Review
stressed on the liquids. Newly formed unstable radical or ionic fragments will recombine
swiftly into gas molecules in the forms of ethylene (CH2=CH2), ethane (CH3-CH3),
methane (CH3-H), hydrogen (H-H), acetylene (CH≡CH), CO (C≡O) and CO2 (O=C=O).
Different energy levels are required to break different kinds of molecular bonds. Therefore,
different types and concentration of fault gases will be associated with the severity and
category of the transformer faults. The energy which is compulsory to crack the typical
molecular bond inside the insulating liquids is shown in Table 2-4 [21].
Table 2-4 Bond Dissociation Energy [21].
Bond C-C
(CH3-CH3) C-H (average)
C=C
(H2C=CH2)
C≡C
(HC≡CH)
Dissociation energy
(kJ/mol) 356 410 632 837
The IEC standard 60599 [76] classifies the electrical fault into three types: partial discharge
(PD), D1 (discharge of low energy) and D2 (discharge of high energy). Partial discharge
stands for the kind of streamer that only partially bridges the insulation gap between two
electrodes. Breakdown occurs after the streamer fully propagates through the gap of the
electrode, and it will act as small arcs known as sparking faults, called D1 and D2.
Compared to the PD fault, the sparking fault generates more amount of fault gases under
the same fault time and could have a critical effect on transformer operation.
Partial discharge, low energy sparking and arcing are some of the common faults that could
occur in oil-immersed transformers. When any of these faults occur, the insulating liquids
will be decomposed, and then a certain amount of combustible and non-combustible fault
gases will be generated [77-79].
2.4.1.2 Measurement
Dissolved gas analysis (DGA) is one of the most widely used diagnostic tools of oil-filled
transformers. It is known as the non-interrupt test method which has already functioned for
many decades. The function of DGA, similar to a human blood test, is to detect the health
problems of the transformer by analysing these fault gases dissolved in insulating liquids
[80]. These fault gases could be detected by DGA in a series of processes: collect oil
sample, extract dissolved gas, gas chromatograph and data interpretation.
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Chapter 2 Literature Review
According to IEC 60475 [81], the procedure of oil sampling involves carefully transferring
the oil sample from oil-filled equipment into a gas-sealed syringe. The international
standard IEC 60567 [79] describes four main methods for extracting gas, including Toepler
pump extraction, partial degassing extraction, stripping extraction and Headspace method.
After gases have been extracted from the dissolved oil, every single gas component can be
measured by means of various detection methods. IEC 60567 [79] describes the method of
gas chromatography (GC), which has been used to analyse dissolved gas in transformer oils
over 60 years. Furthermore, there is another method, known as photoacoustic spectroscopy
(PAS), which could also be used to measure gas components [82].
2.4.1.3 Data Interpretation
After the measurement of fault gas concentration, data interpretation is used to analyse the
type of fault based on the gas concentration and patterns in transformer oil. Based on the
international standard IEC 60599 [76], the main methods of data interpretation are the key
gas method, various gas ratio methods and the Duval triangle method.
The key gas method is a simple and acceptable method to monitor and determine the fault
types in transformer oil. The total dissolved combustible gases (TDCG) is calculated by the
percentage (%) of each combustible gas component in the total volume of evolved gases,
and normally used to determine the fault severity and its nature [83]. The key gases and
secondary gases that correspond to fault types are given in Table 2-5 [76].
Table 2-5 Indicator fault gases in mineral oil [76]
Fault types Key gases Secondary gases
PD in oil H2 CH4, C2H4 and C2H6
Breakdown in oil C2H2 H2, CH4 and C2H4
CO, CO2 (if involved cellulose)
Overheated oil C2H4 CH4, C2H2 and C2H6
Overheated cellulose CO CO2
There are several gas ratio methods, including the Dornenburg ratio method, Rogers ratio
method and IEC ratio method [83]. The most popular one is IEC 60599 ratio method which
calculates the three different ratios of the concentration of two specific fault gases,
including C2H2/C2H4, CH4/H2 and C2H4/C2H6. The Duval triangle method has been widely
used to detect the fault type in transformer oil since 1860 [84]. So far, there are seven
different Duval triangles used for different conditions, such as load tap changer, non-
mineral oil, specific temperature and thermal fault etc [85].
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Chapter 2 Literature Review
The on-line DGA monitor is a fully sealed system that can reflect the real-time operating
condition of the transformer. Compared with traditional DGA measurement methods, it
saves time transporting oil sample to a laboratory and avoids gas leakage in transit. An on-
line DGA monitor can be divided into single-gas monitor and multi-gas monitor.
The hydrogen monitor is a low-cost device mainly used in distribution transformers. It can
be directly installed into the valve of the transformer position between the cooling tank and
main tank for better oil circulation [86]. Unlike traditional DGA measurements, a hydrogen
monitor directly measures hydrogen in ppm level in the insulating oil. The hydrogen
monitor utilises a patented, solid-state hydrogen sensor that is immersed directly into the oil,
eliminating membranes and the potential for rupture [87]. It has potential benefit since all
types of electrical faults and some types of thermal faults involve a certain amount of
hydrogen generation.
The multi-gas monitor is a costly device and is mainly used in large power transformers.
Generally, it can measure multiple gases, including H2, CH4, C2H2, C2H4, C2H6, CO, CO2
and O2. Compared with the hydrogen monitor, a multi-gas monitor performs more accurate
measurements (the accuracy can be ± 5% of reading) [75]. Some multi-gas monitors are
similar to traditional lab based DGA measurements (e.g. Serveron TM8), since they have a
complete GC system of DGA measurements. Others might use photoacoustic spectroscopy
technique [82].
2.4.2 Fault Gas Studies under Electrical Faults
Most of the previous studies focused on electrical faults under AC voltage and very few
past studies carried out DGA analysis of electrical faults under impulse voltage. Statistics
show that the failure rate of HVDC converter transformers is approximately twice that of
AC transformers [88]. The valve windings ensure AC, DC, and repetitive impulse
combined voltages caused by inverters for an extended period of time [89]. The summary
of previous studies on fault gas generation under electrical faults is shown in Table 2-6 [90-
99]. In these studies, the applied number of breakdowns is up to 200. The point-plane
geometry with the non-uniform electric field could be successfully applied. The gap
distance is formed in a wide range from 1 to 45 mm. Both mineral oil and ester liquids were
considered.
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Chapter 2 Literature Review
Table 2-6 Summary of experimental works of DGA analysis under electrical faults [90-99].
Author Voltage
shape
Number of
breakdown
s
Electrode Gap
(mm) Oil type
Oil sample
(Liters)
Jovalekic
[90] Impulse 90 Point-Point 4
Mineral oil, Nature
Esters, Synthetic
Ester
1.62
Suwarno
[91] AC N/A Point-plane 1, 2, 3
Mineral oil, Nature
Esters N/A
Z. D. Wang
[92] AC 15 Point-plane 35
Mineral oil,
Synthetic Ester 2.57
Perrier
[93] AC 100
Spherical
electrodes 2.5
Mineral oil, Nature
Esters, Synthetic
Ester
0.4, not
sealed
Muhamad
[94] AC 200 Point-plane 45
Mineral oil,
Synthetic Ester N/A
Bandara
[95] AC
50, 75,
100
Three
electrodes 2
Mineral oil, Nature
Esters N/A
Gómez [96] AC 12 Plane-plane 2.5 Mineral oil, Nature
Esters N/A
Ghani [97] AC 6 Spherical
electrodes 2.5
Mineral oil,
Synthetic Ester
0.35 -
0.5
Eberhardt
[98] AC 10
U shaped
bow-plane 10, 20
Mineral oil, Nature
Esters, Synthetic
Ester
17
Khan [99] AC 3 Point-plane 10
Mineral oil, Nature
Esters, Synthetic
Ester
1.5
2.4.2.1 Electrical Fault under Impulse Voltage
A DGA test in a mineral oil and three ester liquids (two natural ester and one synthetic ester
liquids) under the sparking fault stressed by lightning impulse was carried out in [90]. A 4-
stage impulse generator is used as the voltage supply. A test cell with a volume of 1.618 L
contains a point-to-needle electrode of 4 mm gap distance. The applied impulse voltage
(1.2/50 µs) was set at 134 kV. Figure 2-25 shows the growing trend of key gases in mineral
oil with an increasing number of breakdowns [90]. The two dominated fault gases − H2 and
C2H2, increase linearly with the number of breakdowns in mineral oil. When the mineral oil
suffers fewer than 30 breakdowns, the generation of C2H2 is comparable to H2. And then,
the concentration of C2H2 exceeds the H2 afterwards and the difference becomes greater
with the increasing number of breakdowns. Figure 2-26 shows the concentration of fault
gases in four different liquids after 90 breakdowns [90]. It was found that both H2 and C2H2
are key gases in four liquids. Compared with ester liquids, the concentrations of H2 and
C2H2 in mineral oil are almost twice those in ester liquids, which is different from the
results under AC sparking fault in [92]. Therefore, it is worth investigating the
characteristics of gas generation under impulse voltage. In addition to H2 and C2H2, small
61
Chapter 2 Literature Review
amounts of C2H4, CH4 in mineral oil and C2H4, CH4, CO in ester liquids were measured
after breakdown.
Figure 2-25 key gases generation versus number of breakdowns in mineral oil, d = 4 mm [90]
Figure 2-26 Comparison of fault gases generation in various liquids after 90 breakdowns [90]
2.4.2.2 Electrical Fault under AC Voltage
The fault gas generation under arcing fault in different voltage levels and arc durations was
carried out in [91]. The arcing fault was generated in a needle-to-plane electrode geometry
made of steel under AC voltage. Through the adjustment of the gap distance between the
point and plant electrode in 1, 2 and 3 mm, three types of arc energy under different applied
voltage (12 kV, 20 kV and 24 kV) were achieved. The duration of arc application was
chosen as 5, 10 and 15 minutes, respectively.
62
Chapter 2 Literature Review
Figure 2-27 and Figure 2-28 show the comparison of fault gas generation under different
voltage levels and arc durations in mineral oil [91]. The results indicate that either under
different voltage levels or arc durations, C2H2 dominates in the combustible gases generated
by the arc. The concentration of all the combustible gases increases almost exponentially
with the applied AC voltage. In contrast to voltage levels, fault gas generation under
different arc duration increases almost linearly with the duration of arc application. Based
on the key gas method, oil samples under 12 and 20 kV indicate C2H2 as key gas and
correspond to arcing fault. However, when applied voltage increased to 24 kV, key gas of
C2H4 was also found indicating overheating. This phenomenon probably occurs because
high-voltage arcing releases a large amount of energy that heats the oil sample.
Figure 2-27 Fault gases generation in mineral oil under different voltage levels, arc duration = 15 mins [91].
Figure 2-28 Fault gas generation in mineral oil under different arc durations, V = 20 kV [91].
63
Chapter 2 Literature Review
Table 2-7 and Figure 2-29 show fault gas generation in Gemini X and FR3 under sparking
fault [92]. In the experimental setup, a test cell with a volume of 2.57 L was used and a
needle-to-plane electrode geometry with 35 mm gap distance was installed in the test cell.
The results show the volumes of fault gases generated for each of the three groups of 15
breakdowns. It was found that the key gases were H2 and C2H2 in both liquids, with the
amount of H2 nearly twice that of C2H2, which is opposite to the results obtained in [91].
The higher concentration of H2 in this study is owing to the calculation of the total fault gas
generation, rather than dissolved gas only in the oil. Normally, due to the low solubility of
H2, a large amount of H2 generated under electrical fault easily escapes to the air or
headspace in a sealing system, which explains why a lesser amount of H2 was measured in
past studies [78, 91, 93]. In addition, compared with Gemini X, FR3 generates larger
amounts of CO, which leads to smaller percentages of hydrocarbon gases in FR3.
Table 2-7 Fault gases volumes (µL) in Gemini X and FR3 for each of three groups of 15 breakdowns [92].
Gas Gemini X FR3
Group 1 Group 2 Group 3 Group 4 Group 1 Group 2 Group 3 Group 4
H2 553.5 544.5 586.5 555.0 499.5 499.5 546.0 514.5
CH4 37.5 37.5 39.0 37.5 16.5 16.5 16.5 16.5
C2H6 4.5 4.5 0.0 3.0 1.5 0.0 1.5 1.5
C2H4 45.5 46.5 45.0 46.5 37.5 30.0 36.0 34.5
C2H2 258.0 265.5 261.0 261.0 237.0 217.5 259.5 238.5
CO 3.0 4.5 1.5 3.0 108.0 97.5 127.5 111.0
TCG1 880.5 903.0 933.0 906.0 900.0 861.0 987.0 916.5
1TCG = total combustible gases
Figure 2-29 Fault gases generation in different insulation oils under sparking fault (re-produced plot based on
results in Table 2-7) [92].
64
Chapter 2 Literature Review
Figure 2-30 shows the fault gas generation per unit fault energy in Gemini X and FR3 [92].
It was found that the respective generation of H2 and hydrocarbons per unit fault energy
produced in FR3 were comparable to those produced in Gemini X. Nevertheless, a higher
amount of carbon monoxide was generated in FR3. Therefore, the total combustible gas
volume per unit fault energy (μL/J) in FR3 is slightly higher (< 20%) than that in Gemini X.
This finding suggests that the conventional gas methods for diagnosing sparking faults in
mineral oil are also applicable to natural ester liquids (FR3) [92].
Figure 2-30 Fault gases generation per unit fault energy (µL/J) in Gemini X and FR3 under sparking fault,
averaged over a group of three tests. TCG = total combustible gases [92].
In addition, some other papers [93-99] carried out similar electrical breakdown tests under
AC voltage. In summary, hydrogen and acetylene are key gases in a mineral oil, a synthetic
ester and five vegetable oils under low energy discharge [93]. Most of the fault gases
increase proportionally with moisture level [94]. The dissolved gas concentration increases
linearly with the number of sparks or arc duration and increases exponentially with applied
voltage in the mineral oil under electrical breakdown fault [95-98]. Compared to mineral oil,
FR3 generates significantly higher amounts of fault gases but less readily absorbs these
gases into the fluid as dissolved gases, and MIDEL 7131 generates the lowest amount of
fault gases [99].
2.5 Summary
An overview of streamer and breakdown in liquids under DC voltage was firstly given in
this chapter, mainly based on experimental methodologies and previous publications related
to streamers and breakdowns. It indicates that the basic feature of streamers in mineral oil
65
Chapter 2 Literature Review
including current, emitted light and shape under DC voltage are comparable to previous
studies under impulse voltage. Moreover, the effect of background pressure on streamer
shapes and the effect of voltage polarity on breakdown voltage under DC voltage were
observed. However, to date there is a lack of study of dielectric performance in ester liquids
under DC voltage.
Then, the fundamental knowledge of streamer features and breakdown properties under
lightning and step impulse was reviewed. The publications have shown that, at a certain
range of gap distance, the relationship between breakdown voltage, instantaneous
breakdown voltage and time to breakdown follows a mathematical rule under different
impulse waveforms in liquids. Based on this theory, more comprehensive data without
complicated experiments can be obtained by simply measuring tb and Vi. And then, a
programme can be used to search the breakdown voltage from a large range of breakdown
values, which meets the criteria. However, the supporting publications only involved two
impulse waveforms. To support and promote this theory, there is a strong need to
investigate this phenomenon in different liquids under various impulse waveforms
Furthermore, the general knowledge of gassing behaviour in liquids and the past studies of
fault gas generation under electrical faults was explained. The publications have shown that
under spark fault, acetylene and hydrogen are the main gases, and the amount of hydrogen
and acetylene increases with the number of sparks and breakdown voltage levels. Most
studies have focused on electrical faults generated under AC voltage, which has a poor
record of breakdown number and fault energy control. It is worth creating a test system
with an auto-controlled spark generator in the attempt to find out the correlation between
gas generation and energy injection under electrical faults.
66
Chapter 2 Literature Review
67
Chapter 3 Experimental Description
CHAPTER 3. EXPERIMENTAL DESCRIPTION
3.1 Liquids under Investigation
In this thesis, a mineral oil Gemini X and a synthetic ester liquid MIDEL 7131 were
investigated. The mineral oil with 3% aromatic content was tested in all the experiments as
the benchmark for cross comparison between the ester liquid and the mineral oil. The
synthetic ester liquid is a type of pentaerythritol ester, formulated synthetically by a
combination of chemicals. The basic properties of the investigated liquids are listed in
Table 3-1.
Table 3-1 Basic properties of testing liquids: MIDEL 7131 and Gemini X [42].
Property Unit Gemini X MIDEL 7131
Density g/cm3 0.882 0.97
Viscosity mm2/s 8.7 28
Pour point °C -60 -60
Flash point °C 144 275
Acidity mgKOH/g <0.01 <0.03
Aromatic content % 3
Water content mg/kg <20 50
Furan content mg/kg <0.1
Antioxidant, phenols Wt% 0.38
Dissipation factor, 90°C - <0.03 <0.03
Breakdown voltage kV
Before treatment - 40-60 (2.5 mm)
After treatment - > 70 (2.5 mm) > 75 (2.5 mm)
Permittivity - 2.2 3.2
3.2 Sample Preparation
For streamer studies under DC and impulse voltages, a pre-processing procedure including
filtering, degassing and dehydrating was performed on all liquid samples, in order to
minimise the effect of moisture and impurity on the test results. For gassing behaviour tests,
the oil samples were only dehydrated and degassed. The detailed procedure is described in
the following paragraphs.
First of all, the experimental oil samples were filtered by a Nalgene® MF 75 nylon
membrane filter with a pore size of 0.2 μm. The particle numbers in unfiltered and filtered
liquids were measured by using HIAC/ROYCO 8000A 8-channel particle counter with an
HIAC/ROYCO ABS2 automatic, which can detect particles in the diameter ranges of 1, 2,
5, 15, 25, 50, 100 and 200 μm. Based on previous results [11], the cumulative particle
68
Chapter 3 Experimental Description
number larger than 5 µm of filtered oil samples could reduce to about 500 per 100 mL oil
sample, which is close to the clean oil condition according to CIGRE Brochure [100].
Secondly, filtered oil samples were dehydrated and degassed in a vacuum oven at 85 ˚C and
500 Pa for over 48 hours in Gemini X and 72 hours in MIDEL 7131. After that, the oil
samples were given another 24 hours to cool down to ambient temperature under vacuum
conditions. The water content of the liquid samples after this processing was less than 10%
relative humidity.
3.3 Etched Tungsten Needles Based on Electrochemical Method
3.3.1 Basic Principle of Electrochemical Etching
Due to the demand of a large number of tungsten needles in the experiment, a feasible
method of producing the tungsten needle is required. Electrochemical etching is widely
accepted as an efficient and reliable method of producing sharp needles [101]. Figure 3-1
shows the principle of electrochemical etching in a static way and dynamic way [101]. In
the case of static etching, the meniscus shifts over time and the final probe becomes
irregular in shape. The dynamic electrochemical etching technique is optimised to produce
tungsten needles with controllable shape and radius of curvature, where the probe was
slowly lifted off from the liquid using a stepper motor [101]. The standard procedure is to
dip a tungsten wire into an electrolyte, and then bias DC voltage in the electrolyte as a
cathode and tungsten wire as an anode to start etching. The choice of DC voltage level and
electrolyte normally rely on the wire material. For the tungsten wire, the DC voltage level is
normally set at a lower voltage level, which ensures a much smoother tip. Generally,
potassium hydroxide (KOH) is used for tungsten wires.
Figure 3-1 Principle of static etching (green A-C) and dynamic etching (orange A-D) [101].
69
Chapter 3 Experimental Description
In IEC 60897 [102], it described a method for determining the radius of curvature of the
needle electrode by a metallographic microscope as shown in Figure 3-2. The radius R is
the distance from the tip circle of the needle to the centre of the circle along which the
needle curves. The simple way of measuring R without parameter a and b, is to draw a
circle overlapping the margin of the tip area as shown by the red dashed circle in Figure 3-2.
The radius of the circle is then identical to the radius of the tip.
Figure 3-2 Measurement of the radius of curvature of needles [102].
3.3.2 Etching Procedure of Tungsten Needles
In this thesis, tungsten needles with tip radii of 5 μm, 10 μm, 20 μm and 50 μm were
produced under well-controlled procedures by using an original 200 μm tungsten wire for
studying the effects of tip radius on streamer initiation. Before the process of etching
tungsten needles, a room temperature of 24 and humidity of 33.1% were measured. A 5
mol/L KOH was used as an electrolyte. A piece of tungsten wire was connected with the
cathode electrode in the KOH solutions. Another smooth and straight tungsten wire to be
etched was connected with the anode electrode. The selection of the radius of a tungsten
wire relies on the expected tip radius of etched tungsten needles. The radius of a tungsten
wire has to be twice as large as the expected tip radius of etched tungsten needles. When the
15 V DC was biased on the sample, it is necessary to apply a dynamic method to etch the
tungsten wire as shown in Figure 3-1 [101]. After electrochemical etching, all etched
tungsten needles were checked using a microscope to ensure that the tip surface is smooth.
70
Chapter 3 Experimental Description
The acceptable tip radius range is set as ± 10 % of the expected value. Without the
automatic system, the dynamic method can be achieved by manual operation at a certain
frequency, e.g. 1 second per shot. The time duration of tungsten wire immersed in a
solution should be as short as possible, in order to improve the accuracy of electrochemical
etching. Otherwise, the etched tungsten needles are likely to be ineffective as shown in
Figure 3-3.
Figure 3-3 Failure results of etched tungsten needles without applying oscillating method.
Figure 3-4 Ideal etched tungsten needle with the tip radius of 50 µm.
Table 3-2 summarises the experimental conditions used to etch tungsten needles with the
four tip radii in this thesis. The time durations per shot for 20 and 50 µm are both 1 second,
and the number of shots is 51 and 37 times respectively. However, for 5 and 10 µm, a pre-
processed procedure of 5 seconds per shot was applied to greatly reduce the tip radius.
Then, a well-controlled procedure of 1 second per shot was applied to etching accurately.
Figure 3-4 shows the ideal etched tungsten needle with a tip radius of 50 µm. A red circle
shows the exact tip radius based on the measurement of software in the microscope.
71
Chapter 3 Experimental Description
Table 3-2 The experimental conditions used to produce various tip radii of tungsten needles.
Tip Radius (µm) DC Voltage (V) Time Duration/Shot (second) Shots
5 15 5 38
10 15 5 27
20 15 1 51
50 15 1 37
3.4 DC Voltage Tests
3.4.1 Experimental Setup
The experimental setup used to investigate the streamer propagation and breakdown
strength of insulating liquids under DC voltage is shown in Figure 3-5. A bipolar high-
voltage DC source with a maximum voltage of 100 kV was used to deliver the continuous
DC voltage. A 2 MΩ resistor RL was placed in series with the DC source and the test cell to
limit the breakdown current as well as to protect the high-voltage DC source. A
compensated RC voltage divider (VD-100) was used to measure the DC voltage applied in
the test cell. A current shunt (10 Ω) was placed at the low voltage potential side of the test
cell to record the current signals. All the experiments were carried out at room temperature
and ambient pressure.
Figure 3-5 Sketch of the experimental setup used in streamer tests under DC voltage.
A 10 ×10 ×10 cm Perspex made test cell with a volume of 1 litre was used to hold the
electrodes and liquid sample as shown in Figure 3-6. The test cell contained a needle-to-
plane electrode system with an adjustable gap distance ranging from 0 to 50 mm. The
Oscilloscope
HV Divider
DC
RL
Current
Shunt
Test cell
Power Supply
Flood
Light SIM 16 Camera
72
Chapter 3 Experimental Description
needle electrode is held by a brass cylinder tube on the top of the test cell, and the length of
the needle that is exposed from the brass tube is fixed at 2.5 cm. The tungsten needles with
the tip radii of 5 μm, 10 μm, 20 μm and 50 μm were produced based on electrochemical
technique. The brass made plane electrode has a diameter of 70 mm.
Figure 3-6 Perspex made cubic test cell used for investigating streamer and breakdown under DC voltage.
A 16-channel ultra-high speed camera Specialized Imaging SIM16 with high resolution
intensified CCD sensors was used to study streamer characteristics. The high resolution of
1360 × 1024 pixels ensures streamer structure analysis, e.g. the stopping length resolution
is down to 0.02 mm. In total 16 frames of photos can be captured. With tuning of time
delay, explosion time aperture value and the distance to the flood light, the shadowgraph
image quality was optimised.
An oscilloscope with a bandwidth of 1GHz is not only used for recording voltage and
current signal but also used to synchronously trigger the high-speed camera. Once there is a
streamer current triggered in the oscilloscope, a 5 V TTL signal will be sent out from the
oscilloscope to fire the camera. However, due to the time delay of transferring TTL signal
from the oscilloscope to the camera (about 75 ns) and camera trigger time (75 ns), there is
an approximate 150 ns delay of the streamer image compared with the current signal.
73
Chapter 3 Experimental Description
3.4.2 Experimental Procedures
The probability of the streamer initiation was determined by using a rising-voltage method.
Firstly, the initial voltage applied was set at 60% of expected streamer initiation voltage.
Then, the applied voltage was increased in 1 kV steps. For each step of applied DC voltage,
60-second duration was applied to allow the capturing of streamer initiation, and a
minimum 60-second interval was used between each step. Finally, twenty initiation
voltages per sample were obtained to determine the 50% streamer initiation voltage with tip
radii of 5 μm, 10 μm, 20 μm and 50 μm.
Then, the streamer characteristics were studied using a rising-voltage method from
initiation to near breakdown level. The DC voltage was increased step by step with an
increment of 5 kV under both positive and negative polarities. At each step of voltage level,
a series of ten streamer phenomena were investigated. To minimise the cumulative effect of
DC voltage stress on the oil sample, a minimum 60-second interval was allowed between
each streamer investigation. The oil sample and needle electrode were renewed after one set
of tests under each polarity.
Finally, the breakdown voltage was measured by also using a rising-voltage method. The
initial voltage applied was set at 70% of expected breakdown voltage and increased in 2 kV
steps. For each step of applied DC voltage, 60-second duration was applied to allow the
capturing of breakdown, and a minimum of 120-second interval was used between each
step. After breakdown occurs, a minimum of 5 minutes was allowed before starting the next
breakdown test. Twenty breakdowns per sample were obtained to determine the 50%
breakdown voltage under gap distances of 2 mm, 5 mm, 10 mm, 20 mm and 30 mm. The
oil sample and needle electrode were renewed after the test of each tip radius or gap
distance.
3.5 Impulse Voltage Tests
3.5.1 Experimental Setup
The test setup used to study streamer and breakdown of insulating liquids under impulse
waveforms with different tail times is shown in Figure 3-7. A compact impulse generator
was used to generate different impulse waveforms. A compensated RC voltage divider
(VD-100) was used to measure the output voltage. A similar test cell used for DC voltage
tests was also used to study the streamer under impulse waveforms with different tail-time.
74
Chapter 3 Experimental Description
The needle electrodes of tungsten needles with a fixed tip radius of 10 µm were produced
based on an electrochemical technique. The etched tungsten needle was regularly changed
after each set of breakdown tests. The gap distance of the electrode was fixed at 10 mm.
The current was recorded by a 10 Ω non-inductive current shunt placed at the low voltage
potential side of the test cell. The same setup of ultra-high speed camera used in DC voltage
tests was also applied to investigate streamer phenomena under impulse voltage.
Figure 3-7 Sketch of test setup of impulse tests with different tail-time.
To simply generate the impulse waveform with the different tail times, a flexible and
compact solid-state switch (BEHLKE HTS-901-10-GSM) based impulse generator was
developed as shown in Figure 3-8. This compact impulse generator can be divided into two
parts: high voltage part (a) and control part (b). For the high voltage part, A bipolar high-
voltage DC source with a maximum voltage of 100 kV was used as a DC charging source.
A 500 MHz oscilloscope with the sampling rate of 1GS/s was used to monitor the impulse
voltage and current signals. A 2 MΩ resistor was placed in series with the DC source and
the impulse generator to limit the breakdown current as well as to protect the high-voltage
DC source as shown in Figure 3-7. Three parallel capacitors with a total capacitance of 1.8
µF were used as a charging capacitor C. Rs1 and Rs2 are 600 Ω protective resistors (grey
cylinder) used to prevent overcurrent damage to the solid-state switch. A set of the
replaceable front resistor Rf and tail resistor Rt (red cylinder), allows the generation of
positive impulse waveforms including 0.8/8 µs, 0.8/14 µs, 0.8/30 µs and 0.8/3200 µs. The
control part includes a shielding box and a control unit. The shielding box is used to
Oscilloscope
HV Divider
DC
Rf
Current
Shunt
SIM 16
Camera
Test cell
Power Supply
Flood
Light
A
B
Rt
Rs1
Rs2
RL
C
Control Unit
Impulse Generator
75
Chapter 3 Experimental Description
mitigate the effect of electromagnetic noise on the control signal. The separated control unit
is not only used to control the solid-state switch by a function generator, but also to monitor
the operation mode of the solid-state switch with LED indicators.
Figure 3-8 The photo of the compact solid-state switch based impulse generator.
3.5.2 Impulse Waveforms with Different Tail times
In this thesis, four impulse waveforms with different tail times (0.8/8 μs, 0.8/14 μs, 0.8/30
μs and 0.8/3200 μs) were used to investigate the streamer characteristics and breakdown
properties in liquids. Table 3-3 summarises the parameters of the front resistor, tail resistor
and charging capacitor used to generate the four impulse waveforms. The front time of four
impulse waveforms was fixed at 0.8 µs (Rf = 6.4 kΩ). The tail times of impulse waveforms
are determined by both tail resistor and charging capacitor. With the same charging
(a) High voltage unit
(b) Control unit
Solid-State
Switch
Charging
Capacitor
RS1
Protective
Resistors
Tail Resistor
Front
Resistor
76
Chapter 3 Experimental Description
capacitor, higher resistance of the tail resistor gives the longer tail-time impulse waveform.
With the same tail resistor, higher capacitance of the charging capacitor also gives the
longer tail-time impulse waveform.
Table 3-3 The parameters of the front resistor, tail resistor and charging capacitor used to generate the four
impulse waveforms.
Impulse
Waveforms
Front Resistor Rf
(kΩ)
Tail Resistor Rt
(kΩ)
Charging Capacitor C
(nF)
0.8/8 µs 6.4 3.2 1.8
0.8/14 µs 6.4 6.4 1.8
0.8/30 µs 6.4 40 1.2
0.8/3200 µs 6.4 54 1.8
Figure 3-9 shows the different impulse waveforms with tail time ranging from 8 µs to 3200
µs used in further streamer and breakdown studies. The voltage of the 0.8/3200 µs impulse
waveform does not decay for 10 µs, which is more than the maximum streamer propagation
time at the 10 mm gap distance. The 0.8/3200 µs impulse waveform is thus used as a ‘step-
like’ impulse.
Figure 3-9 The different impulse waveforms with tail time ranging from 8 µs to 3200 µs, V = 24 kV.
3.5.3 Experimental Procedures
The streamer characteristics were studied by using a rising-voltage method from initiation
to near breakdown level. The impulse voltage was increased step by step with an increment
of 2 kV under positive polarity. At each step of voltage level, a series of ten streamer
-2 0 2 4 6 8 10 12 14 16 18 20
0
5
10
15
20
25
30
Vo
ltag
e (k
V)
Time (us)
-2 0 2 4 6 8 10 12 14 160
5
10
15
20
25
Vo
ltag
e (k
V)
Time (us)
0.8/8 us
0.8/14 us
0.8/30 us
0.8/3200 us
77
Chapter 3 Experimental Description
phenomena were investigated. To minimise the cumulative effect of impulse voltage
stressed on the oil sample, a minimum 60-second interval was allowed between each
impulse shot. The oil sample and needle electrode were renewed after the test of each
impulse waveform.
The breakdown voltages under different impulse waveforms were also measured by using a
rising-voltage method. The initial voltage applied was set at 70% of the expected
breakdown voltage. Voltage level was increased step by step with an increment of 1 kV.
Ten breakdowns per impulse waveform were obtained to determine the 50% breakdown
voltage.
3.6 Gassing Behaviour Tests
3.6.1 Experimental Setup
Figure 3-10 shows the sketch of the experimental setup used to investigate the gassing
behaviour of insulating liquids under electrical faults. Figure 3-11 shows the photos of the
testing area and the control area in the laboratory. In the testing area (Figure 3-11a), a
compact impulse generator used for previous impulse tests was also used to generate the
electrical sparks in gassing behaviour study. A 100 kV voltage divider was used to measure
the fault voltage level. A special designed gas-tight test cell was used to hold the electrodes
and liquid sample. A 10 Ω non-inductive current shunt used to measure the fault current
was placed at the low voltage potential side of the test cell. The on-line DGA monitors,
including a palladium sensor based Serveron TM1 hydrogen monitor and a gas
chromatography (GC) based Serveron TM8 multi-gas monitor, were employed to measure
fault gas generation. A stainless steel buffer chamber was built and connected to the TM1
hydrogen monitor to the oil circulation loop.
78
Chapter 3 Experimental Description
Figure 3-10 The diagram of the experimental setup for gas generation tests under electrical faults.
Figure 3-11 The photos of the experimental setup for gas generation tests under electrical faults.
In the testing area (Figure 3-11b), a national instrument (NI) data acquisition (DAQ) system
and a NI high-speed digitizer with the sampling rate of 5 GS/s were used to control and
acquire data. A high voltage DC source was used to charge the charging capacitor of the
compact impulse generator. A 5 V DC power supply supplies electric energy to the solid-
state switch of the impulse generator.
A cubic shaped stainless steel test cell with a volume of 1 litre used to hold a needle-to-
plane electrode system is shown in Figure 3-12. Two white bushings, capable of
withstanding up to 100 kV AC voltage (more than 100 kV for impulse voltage) are
connected at the each side of the cubic test cell. Two Perspex made windows are mounted
(a) Testing area (b) Control area
High-speed
digitizer
High voltage
DC source
500 MHz
Oscilloscope
5 V DC
source
Spark
generation
Voltage
divider
DGA test
system
79
Chapter 3 Experimental Description
on the front and back of the test cell, which allows the streamer/breakdown observation.
Two optic fibre connectors and one oil-in connector are fixed at the lid on the top of the test
cell. An oil-out connector is fixed at the bottom of the test cell. The gaskets were used to
fill the space between all the mating surfaces, which prevents leakage of oil sample from
the joined objects. Figure 3-12b shows the electrode configuration of the test cell. The
needle electrode with a tip radius of 50 µm was produced based on an electrochemical
technique. The brass plane electrode had a diameter of 20 mm. The gap distance of the
needle-to-plane electrode was fixed at 5 or 10 mm.
Figure 3-12 The photo of the cubic shaped stainless steel test cell used for gassing behaviour test, (a) test cell;
(b) electrode configuration with the gap distance of 10 mm.
3.6.2 Fault Control and Data Acquisition System Design
To generate a controlled number of breakdowns, an automatic control system of voltage
output and data recording was built up. The flow chart in Figure 3-13 shows the working
(a)
(b)
80
Chapter 3 Experimental Description
principle of the auto-controlled system. Since the spark generator is fully controlled by the
electronic trigger signal, a national instrument (NI) data acquisition (DAQ) system was
used to accurately control the breakdown numbers and the time interval of each breakdown.
In addition to the voltage output control, an NI high-speed digitizer was used to
automatically record the voltage and current signals. Once there is an electrical fault
generated in the liquid sample, the simultaneous fault voltage and current signal are stored
in the PC drive for further data analysis.
Figure 3-13 The flow chart of the automatic control system with voltage output and data recording.
The way of generating a certain number of breakdowns is achieved by using the time
domain in Labview setting as shown in Figure 3-14. The initial delay is starting time, which
is set as 0 second in this study. The high time is the time duration of triggering the impulse
generation, which should be larger than the time duration of impulse waveform. Insufficient
high time can chop the tail of impulse waveform. Low time is the time interval between
each triggering. The equation (3-1) shows the calculation of total time based on a certain
number of breakdowns.
81
Chapter 3 Experimental Description
Figure 3-14 Description of time domain in Labview setting.
𝐓 = 𝒕𝟏 × 𝒏 + 𝒕𝟐 × 𝒏 + 𝒕𝟎 (3-1)
Where t1 means high time; t2 means low time; t0 means initial delay; T means time out; n
means number of breakdowns.
3.6.3 Oil-loop System Design
With a volume of 1 litre in the test cell, a volume of 1.2 litres in the buffer chamber and an
extra volume of 0.5 litres in the TM8 multi-gas monitor, the total volume of the oil-loop
system is about 2.7 litres. To avoid leakage of dissolved gases during the experimental tests,
the sealing performance of the oil-loop system was tested before filling the oil sample. A
pressure gauge was connected within the oil-loop system. A syringe was connected at the
highest position of the oil loop and this was used to inject air into the oil-loop system. Once
the pressure of the oil-loop system reaches 100 mbar, all the valves should be switched off
and the pressure gauge monitored for 24 hours. Figure 3-15 shows the inner pressure
readings of the oil-loop system for 24 hours. The results indicate that there is only a 12%
drop of inner pressure after 24 hours. Considering the longest time duration of the
experiments is within 9 hours, the sealing performance of the test setup is acceptable.
Figure 3-15 Sealing performance based on pressure reading in oil-loop system.
82
Chapter 3 Experimental Description
Before filling the oil sample, the test cell and buffer chamber were washed and dried in an
oven at about 80˚C. Then, the processed oil sample was injected into the system by an oil
pump of the TM8 multi-gas monitor. At the end of the oil filling process, a syringe was
connected at the highest position of the oil loop to bleed the air. The test system is able to
carry out the experiment once the headspace existed in the test cell and the buffer chamber
is minimised. A fixed headspace of about 75 mL exists in the on-line DGA monitor and
another headspace of about 5 mL exists in the test cell. The oil sample is circulated by an
oil pump of the TM8 multi-gas monitor with an oil flow rate of 250 mL/min.
3.6.4 Experimental Procedures
Before carrying out the gassing behaviour tests, the breakdown voltages in both liquids
were determined based on a rising-voltage method as described before. Table 3-4
summarises the 99.9% breakdown voltages of both liquids under lightning impulse with the
gap distances of 5 and 10 mm derived through the normal cumulative distribution.
Table 3-4 The 99.9% breakdown voltages of the mineral oil and the synthetic ester liquid at different gap
distance under positive and negative lightning impulse.
Oil
Types
Voltage
Polarities
Gap Distances
5 mm 10 mm
99.9% VB (kV) 99.9% VB (kV) 1.5 times 99.9% VB (kV)
Gemini
X Positive 31.3 39.1 58.6
MIDEL
7131 Positive 26.8 36.8 55.2
Moreover, to improve experimental efficiency and also to avoid the accumulative effect of
each breakdown on fault energy, an experiment of energy measurement with different time
intervals between each breakdown was carried out. Figure 3-16 shows the comparison of
fault generation with different time intervals of 1 minute, 2 minutes and 5 minutes. The data
is plotted for every 5 breakdowns with a total number of breakdowns of 200. The results
indicate that the energy generation remained stable in all tested time intervals. It shows
well-controlled output energy and no accumulative effect is observed even with the time
interval of 1 minute, probably due to the continuous circulation of the oil in the test system.
Therefore, 1-minute interval was used in the following study.
83
Chapter 3 Experimental Description
Figure 3-16 The comparison of energy generation of individual breakdowns with different time interval.
Figure 3-17 shows the flow chart of the gassing behaviour tests in both liquids from 20 to
500 breakdowns at either a 5 mm or 10 mm gap distance. Firstly, a 10-hour background
measurement by TM8 multi-gas monitor was allowed to measure the dissolved gases in the
oil sample. Then, after a certain number of breakdowns applied, another 8-hour
measurement by TM8 multi-gas monitor was allowed to measure the concentration of the
dissolved gases after the electrical fault. Finally, the tested oil of 50 mL was sampled from
the oil-out connector at the bottom of the chamber by using a full-sealed syringe. After
sampling, the buffer chamber is replenished by a new 50 mL oil sample. The difference
between the post-test reading and background reading is used to indicate gas generation due
to the breakdown.
84
Chapter 3 Experimental Description
Figure 3-17 The flow chart of the experimental procedure for gassing behaviours tests
At the 99.9% breakdown voltage level, the effect of voltage polarity was firstly investigated
with a different number of breakdowns (sparking) including 20, 50, 100, 200, 300 and 500
breakdowns under positive polarity at a gap distance of 5 mm. Then, the effect of gap
distance was investigated under positive polarity at a gap distance of 10 mm. Finally, an
additional set of 200 breakdowns test was investigated at the 10 mm gap distance but with a
higher voltage level i.e. 1.5 times of the 99.9% breakdown voltage. Each set of tests was
repeated twice under the same conditions to confirm the credibility of results. The time
interval between each breakdown was set at 1 minute, and the oil was continuously
circulated to help the dispersion of breakdown by-products e.g. gas bubbles, if there were
any.
3.7 Summary
Two types of liquid, a mineral oil – Gemini X and a synthetic ester liquid – MIDEL 7131,
are used in this study. The liquid samples were pre-processed through filtering, dehydrating
and degassing
An electrochemical method used to produce the etched tungsten wire was presented. The
etched tungsten wire with various tip radii of 5 μm, 10 μm, 20 μm and 50 μm were
produced under well-controlled procedures in this study.
85
Chapter 3 Experimental Description
For DC voltage tests, needle-to-plane electrodes with the tip radii of 5 μm, 10 μm, 20 μm
and 50 μm and adjustable gap distances up to 50 mm were contained in a cubic test cell. A
high-speed camera and a current shunt were used to observe the streamer characteristics.
An advanced oscilloscope was used to record the current signal and trigger the camera
synchronously.
For impulse voltage tests, a compact solid-state switch based impulse generator was built
up to deliver impulse waveforms with different tail times (0.8/8 µs, 0.8/15 µs, 0.8/30 µs and
0.8/3200 µs). Needle-to-plane electrodes with a fixed gap distance of 10 mm and a tip
radius of 10 µm are contained in a cubic test cell. A high-speed camera and a current shunt
were used to observe the streamer characteristics.
For gas generation tests, a DGA test platform based on a sealed circulating oil loop with
functions of automatic spark fault control and data acquisition was developed. Needle-to-
plane electrodes with a tip radius of 10 µm and a fixed gap distance of either 5 or 10 mm
were contained in a cubic shaped stainless steel test cell. A TM1 hydrogen monitor and a
TM8 multi-gas monitor were used to measure the gas generation throughout the experiment.
86
Chapter 3 Experimental Description
87
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
CHAPTER 4. STREAMER AND BREAKDOWN PROPERTIES
OF TRANSFORMER LIQUIDS UNDER DC VOLTAGE
4.1 Introduction
This chapter presents experimental studies on streamer characteristics and breakdown
strengths of a mineral oil and a synthetic ester liquid under DC voltage. The effects of tip
radius and gap distance on streamer initiation and breakdown voltage are analysed under
both positive and negative polarities. Streamer shape, stopping length, average propagation
velocity and area are compared between the mineral oil and the synthetic ester liquid.
Finally, the dielectric strengths of the mineral oil and the synthetic ester liquid are defined.
4.2 Effect of Tip Radius on Streamer Initiation Voltage
The typical current and emitted light signals of initiated streamer in the mineral oil and the
synthetic ester liquid observed under both positive and negative polarities are shown in
Figure 4-1. Under positive polarity, the current and emitted light signals are more intensive
and consist of continuous current components with several discrete large single pulses.
Under negative polarity, only one single pulse is detected for both the current and light
signals. The time duration of negative pulses is extremely short (less than 100 ns). The
maximum pulse peak of current signals remains at 2 mA at the initiation stages for the
mineral oil and the synthetic ester liquid under both positive and negative polarities. Higher
magnitudes of emitted light signals were recorded under positive polarity, which explains
why streamer initiation under positive polarity is brighter than under negative polarity.
88
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
Figure 4-1 The current and emitted light signals of streamer initiation under DC voltage (a). Mineral oil –
positive polarity (b). Mineral oil – negative polarity(c). Synthetic ester – positive polarity (d). Synthetic ester
– negative polarity; d = 10 mm, r = 10 µm.
To statistically analyse the streamer initiation voltage, Weibull distribution was used to fit
the initiation results of 20 tests and calculate 50% streamer initiation voltage. Its cumulative
distribution function is given as shown in equation (4-1). Knowing the specific shape and
scale parameters of Weibull function, the initiation voltage at a specific initiation
probability can be easily deduced. The Weibull distribution plots of the streamer initiation
voltages obtained under various tip radii of the mineral oil and the synthetic ester liquid are
shown in Figure 4-2, and the parameters of shape and scale are summarised in Table 4-1.
𝐹𝑊𝑒𝑖𝑏𝑢𝑙𝑙(𝑥) = 1 − 𝑒−(
𝑥
𝛽)𝑎
(4-1)
where, α is shape parameter and β is scale parameter.
0 0.5 1 1.5 2-2
0
2
4
Time [us]
Str
eam
er C
urr
ent
[mA
]
0 0.5 1 1.5 2
-0.1
-0.05
0
0.05
Em
itte
d L
igh
t [A
rb]
Streamer current
Emitted light
0 0.5 1 1.5 2
-2
0
2
Time [us]
Str
eam
er C
urr
ent
[mA
]
0 0.5 1 1.5 2
-0.02
-0.01
0
0.01
Em
itte
d L
igh
t [A
rb]
Streamer current
Emitted light
0 0.5 1 1.5 2
0
5
10
Time [us]
Str
eam
er C
urr
ent
[mA
]
0 0.5 1 1.5 2
-0.1
-0.05
0
Em
itte
d L
igh
t [A
rb]
Streamer current
Emitted light
0 0.5 1 1.5 2
-2
0
2
Time [us]
Str
eam
er C
urr
ent
[mA
]
0 0.5 1 1.5 2
-0.02
-0.01
0
0.01
Em
itte
d L
igh
t [A
rb]
Streamer current
Emitted light
(a) (b)
(c) (d)
89
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
Figure 4-2 Weibull distribution plot of streamer initiation results with various tip radii (r = 5, 10, 20 and 50
µm) under DC voltage, d = 10 mm.
Table 4-1 Weibull parameters of streamer initiation results with various tip radii at point-plane electrode
under negative and positive polarities
Tip Radius
(µm)
Polarity Shape Scale
50 %
Breakdown
Voltage
5
Gemini X (+) 8.8 6.8 6.5
(-) 6.5 5.8 5.5
MIDEL 7131 (+) 8.6 6.8 6.5
(-) 6.7 6.1 5.7
10
Gemini X (+) 9.1 7.6 7.4
(-) 6.1 5.5 5.3
MIDEL 7131 (+) 8.7 7.1 6.9
(-) 7.7 6.2 6.0
20
Gemini X (+) 11.1 8.9 8.6
(-) 8.1 7.4 7.1
MIDEL 7131 (+) 7.8 8.3 7.9
(-) 6.9 7.2 6.9
50
Gemini X (+) 7.6 11.5 11.9
(-) 8.5 8.4 8.0
MIDEL 7131 (+) 10.1 9.9 9.5
(-) 11.0 8.3 8.0
r = 5 µm r = 10 µm
r = 20 µm r = 50 µm
90
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
Figure 4-3 shows the effect of tip radius on streamer initiation voltage of the mineral oil
and the synthetic ester liquid under DC voltage. With the increase of tip radius, streamer
initiation voltages of both positive and negative streamers increase with tip radius. The
polarity effect is observed which means that that initiation voltage under negative polarity
is slightly lower than that of positive polarity. This is because positive space charges that
accumulate ahead of the positive point weaken the near tip field and suppress streamer
initiation under positive polarity, while positive space charges that accumulate ahead of the
negative tip enhance the near tip field and then decrease the initiation voltage under
negative polarity. Under positive polarity, when tip radius r = 5 μm, the initiation voltages
of the mineral oil are the same as that of the synthetic ester liquid; when r > 5 μm, initiation
voltage of the mineral oil becomes noticeably higher than that of the synthetic ester liquid.
However, under negative polarity, the initiation voltages of the mineral oil are almost the
same as those of the synthetic ester liquid for all tested tip radii.
Figure 4-3 Effect of tip radius on streamer initiation voltage under DC voltage, d = 10 mm (plot based on 50%
initiation voltage).
From the initiation voltage presented in Figure 4-3, initiation fields Ei at the local needle
tips were calculated according to equation 𝑬𝒊 =𝟐𝑽𝒊
𝒓𝒑 𝐥𝐧(𝟒𝒅
𝒓𝒑)
(4-1). With the increase of tip radius, the initiation voltage is increasing, but the initiation
field, calculated based on the inception voltage and electrode geometry, actually is reducing
as shown in Figure 4-4 under both positive and negative polarities. This probably can be
91
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
explained by the area effect that large surface area of electrode increases the probability of
defects on the electrode and the stressed oil samples near the tip electrode.
𝑬𝒊 =𝟐𝑽𝒊
𝒓𝒑 𝐥𝐧(𝟒𝒅
𝒓𝒑) (4-1)
where Ei means initiation field, Vi means initiation voltage, rp means tip radius and d means
gap distance.
Figure 4-4 Initiation field versus tip radius in the mineral oil and the synthetic ester liquid under positive and
negative polarities.
4.3 Basic Characteristics of Streamers
4.3.1 Positive Streamer
An example of the current signal and shadowgraph of positive streamer propagation in the
synthetic ester liquid at a gap distance of 10 mm is shown in Figure 4-5. The current signal
is intensive and consists of continuous current components superimposed with large
discrete pulses. A camera monitor signal was used to correlate the streamer shadowgraph
and the current signal. Streamer propagation based on shadowgraphs is shown in Figure 4-5
(b), where only one main branch with small offshoots was observed. A clearly growing
path of the main branch was observed from Frame 1 to Frame 3. The streamer tip continued
propagation for a short distance even after the current pulses ended, as shown in Frame 4.
At this stage, it was observed that the initial channel, root, of the streamer, has dissipated
92
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
into the liquid where the streamer tip propagation continues due to either local ionisation or
to simply gaseous channel expansion. The final stopping length of streamer in Figure 4-5(b)
is 7.34 mm with average propagation velocity of 1.56 km/s.
Figure 4-5 Typical positive streamer propagation in the synthetic ester liquid under DC voltage, d = 10 mm, r
= 10 µm, V = 28 kV; (a) voltage, current and monitor signals, (b) streamer propagation, corresponding to the
signals in (a).
Stopping Length
Below the breakdown voltage, streamers can initiate and propagate, but finally stop at a
certain distance that is less than the full gap distance. The final length is usually called
stopping length ls, which is one of the most important parameters to characterise a streamer.
Stopping length means the straight-line distance from the farthest tip point of a streamer to
the point electrode, which is measured based on the streamer images.
The results of streamer stopping length in the mineral oil and the synthetic ester liquid at a
10 mm gap under positive polarity are shown in Figure 4-6, where 50% breakdown
voltages are also stated as reference (VB-Gemini X and VB-MIDEL 7131 stand for 50% breakdown
voltage of the mineral oil and the synthetic ester liquid respectively). At each voltage level,
the mean and standard deviation are given based on 10 measurements. For the shadowgraph,
each frame has a resolution of 1360×1024 pixels, and the length (mm) per pixel is 0.0212
mm/pixel.
Under positive polarity, the synthetic ester liquid behaves similarly to the mineral oil at
inception voltage level V = 15 kV; the stopping length increases with an increase of applied
voltage. When applied voltage is higher than 15 kV, the stopping length of the synthetic
0 2 4 6 8 10 12
0
5
10
Curr
ent (
mA
)
0 2 4 6 8 100
10
20
30
40M
onito
r Sig
nal (
V)
Time (us)
0 2 4 6 8 10
0
5
10
Curr
ent (
mA
)
0 2 4 6 8 10-5
0
5
10
Mon
itor S
igna
l (V
)
Time (us)
0 2 4 6 8 10
0
5
10
Curre
nt (mA
)
0 2 4 6 8 100
20
40
Monito
r Signa
l (V)
Time (us)
0 2 4 6 8 10
0
2
4
Monito
r Signa
l (V)
Time (us)
30 kV
2µs
5 mA
Frame 1 Frame 2 Frame 3 Frame 4
(a)
(b)
Frame 1 Frame 2 Frame 3 Frame 4
93
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
ester liquid becomes longer than that of the mineral oil. The higher increasing rate, mm/kV,
of the synthetic ester liquid, results in a lower breakdown voltage compared with the
mineral oil. Figure 4-6(b) shows the fully propagated streamers at the different voltage
levels in both liquids. It is clear that higher voltage level indeed promotes streamer growth
in terms of length, whereas it does not have much effect on branching in the investigated
range.
(a) Streamer stopping length versus applied DC voltage
(b) Corresponding streamer shape at different voltage levels
Figure 4-6 Stopping length of streamers in the mineral oil and the synthetic ester liquid under positive
polarity, d = 10 mm, r = 10 µm (error bars stand for one standard deviation).
Average Propagation Velocity
Average streamer propagation velocity va, is calculated by the ratio of stopping length l to
propagating time t, with the assumption that streamers in the 2nd
mode propagate at a
constant velocity. Once breakdown occurs, va is determined by using the gap distance d
A1 B1 C1 D1
A2 B2 C2
94
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
divided by time to breakdown tb.
Figure 4-7 shows the comparison of average streamer propagation velocity in mineral oil
and synthetic ester liquid at a 10 mm gap and 10 µm tip radius under positive polarity. It
was found that, at the same applied voltage level, average propagation velocity in synthetic
ester liquid is slightly higher than that in mineral oil. The velocity of both liquids remains in
the range from 1.5 km/s to 2 km/s, from initiated voltage to near 50% breakdown voltage.
Figure 4-7 Average propagation velocity versus applied DC voltage in the mineral oil and the synthetic ester
liquid under positive polarity, d = 10 mm, r = 10 µm (error bars stand for one standard deviation).
Streamer Charge
The apparent charge from PD measurement serves as a key indicator of insulation defect.
This raises the question of whether the apparent charge threshold adopted can suitably
reflect the presence of streamers in liquids under DC voltage.
Synchronised measurements of streamer shape and current allow the correlations between
stopping length (shown in Figure 4-6) and maximum apparent charge (calculated based on
the integration of current signals) to be established. Figure 4-8 shows the positive streamer
stopping length as a function of maximum apparent charge in both the mineral oil and the
synthetic ester liquid under positive DC voltage. The results indicate that the correlations
between streamer stopping lengths and apparent charges increase linearly in both the
mineral oil and the synthetic ester liquid. This is probably because no obvious side branches
were developed during the growth of streamer and the charge density within the streamer
channels seems uniform and constant. It also indicates that injected charge is driving the
95
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
streamer growth. The rate is about 0.31 mm per 100 pC in the mineral oil and 0.26 mm per
100 pC in the synthetic ester liquid.
Figure 4-8 Positive streamer stopping length as a function of maximum apparent charge in the mineral oil and
the synthetic ester liquid.
4.3.2 Negative Streamer
A similar experimental procedure was carried out under negative polarity. Figure 4-9 shows
an example of the current signal and shadowgraph of negative streamer propagation in the
synthetic ester liquid at a gap distance of 10 mm. A train of clearly recognised discrete
pulses with increasing amplitude is observed in the current signal, which is similar to the
previous finding of the negative streamer observed under ac and impulse voltages [38, 103].
A camera monitor signal was used to correlate the streamer shadowgraph and the current
signal. Streamer propagation based on shadowgraphs is shown in Figure 4-9 (b). Unlike the
streamer observed under positive polarity, the negative streamer slowly propagates in the
form of a cloud like from Frame 1 to Frame 5, and it dissipates slower than the positive
streamer after the end of the current signal. The final stopping length of the streamer in
Figure 4-9 (b) is 1.13 mm, with an average propagation velocity of 0.12 km/s.
96
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
Figure 4-9 Typical negative streamer propagation in the synthetic ester liquid under DC voltage, d = 10 mm,
r = 10 µm, V = -55 kV; (a) voltage, current and camera monitor signals, (b) streamer propagation,
corresponding to the signals in (a)
Stopping Length
Figure 4-10 shows the streamer stopping length in the mineral oil and the synthetic ester
liquid at a 10 mm gap under negative polarity, where 50% breakdown voltages are also
stated as a reference (VB-Gemini X and VB-MIDEL 7131 stand for 50% breakdown voltage of the
mineral oil and the synthetic ester liquid respectively). Under negative polarity, a
significant difference in the streamer stopping length characteristic exists in both liquids.
The streamers in both liquids increase extremely slowly from inception voltage to the
voltage before breakdown. A stopping length of only about 1 mm can be observed in the
synthetic ester liquid at the applied voltage of 50 kV (VB-MIDEL 7131 = 59.8 kV), and only
about 0.7 mm for the mineral oil at the applied voltage of 80 kV (VB-Gemini X = 92.9 kV).
Similar to the results of positive streamers, the stopping length of negative streamers of the
synthetic ester liquid is longer than that of the mineral oil. The higher increasing rate,
mm/kV, of the synthetic ester liquid, results in a lower breakdown voltage compared with
the mineral oil.
Figure 4-10(b) shows the fully propagated streamers at the different voltage levels in both
the liquids. At the inception voltage level (A1 and A2), only a tiny streamer tip around the
point electrode is observed in both the liquids. At the voltage level before breakdown (D1
and D2), the negative streamers barely propagate (stopping length < ~0.1d) and are in the
form of a cloud like with no clear branches and offshoots. This is obviously different from
0 5 10 15 20-20
0
20
Cu
rren
t (m
A)
0 5 10 15 20
-50
0
Mo
nit
or
Sig
nal
(V)
Time (us)
0 5 10 15 20
0
2
4
Mo
nit
or
Sig
nal
(V)
Time (us)
0 5 10 15 20-20
-10
0
10C
urre
nt (m
A)
0 5 10 15 20-5
0
5
10
Mon
itor S
igna
l (V
)
Time (us)
0 5 10 15 20-20
0
20
Curre
nt (m
A)
0 5 10 15 20
-50
0
Mon
itor S
ignal
(V)
Time (us)
0 5 10 15 20
0
2
4
Mon
itor S
ignal
(V)
Time (us)
10 mA
(a)
5 µs
- 55 kV
F 1 F 2 F 3 F 4 F 5 F 6 F 7
(b)
Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6
97
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
the negative streamer characteristics observed under impulse voltages, where the stopping
length increases exponentially with an increase in applied voltage [40, 53]. After ionisation
occurs near the streamer tip region, the dissipated negative space charges seem to form a
strong shielding effect and hence weaken the boundary field, which prevents further
propagation of the streamer.
(a) Streamer stopping length versus applied DC voltage
(b) Streamer area versus applied DC voltage
Figure 4-10 Stopping length of streamers in the mineral oil and the synthetic ester liquid under negative
polarity, d = 10 mm, r = 10 µm (error bars stand for one standard deviation).
Average Propagation Velocity
Figure 4-11 shows the comparison of average streamer propagation velocity in the mineral
oil and the synthetic ester liquid at a 10 mm gap under negative polarity. At the inception
voltage levels, there is only one single pulse of the current signal that was captured for the
negative streamer. The propagation time based on the current signal is thus extremely short,
A1 B1 C1 D1
A2 B2 C2 D2
98
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
from about 50 ns to 200 ns. In addition, the streamer length at this stage is extremely short,
so there is uncertainty about calculating the streamer velocity. With the increase of applied
voltage, the propagation time is measured based on multiple current pulses the average
propagation velocity can be reasonably calculated based on the ratio of stopping length and
propagation time. It is found that the negative streamer velocity in synthetic ester liquid is
comparable to that in mineral oil, almost remaining constant from 0.1 km/s to 0.3 km/s,
which corresponds to the 1st mode of streamer propagation [61].
Figure 4-11 Average propagation of streamers in the mineral oil and the synthetic ester liquid under negative
polarity, d = 10 mm, r = 10 µm (error bars stand for one standard deviation).
Streamer Charge
Figure 4-12 shows the negative streamer stopping length as a function of maximum
apparent charge in both the mineral oil and the synthetic ester liquid. Unlike the linear
results under positive polarity, the apparent charge firstly increases with the increase of
stopping length, and then the increase rate slow down when the apparent charge above 50
pC. This result indicates that although the streamer does not propagate much further as
shown in stopping length, the discharge becomes more intensive and the charge density
within the streamer channels become higher.
99
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
Figure 4-12 Negative streamer stopping length as a function of maximum apparent charge in the mineral oil
and the synthetic ester liquid.
4.4 Effect of Gap Distance on Breakdown Voltage
4.4.1 Breakdown Phenomena
Figure 4-13 shows an example of the streamer leading to the breakdown in the synthetic
ester liquid under positive polarity. A dramatic voltage drop from 30 kV to 0 kV was
observed when the breakdown occurred. A short train of consecutive pulses with increasing
amplitude is commonly observed for the positive streamer. Compared with the current
signal of pre-breakdown phenomena, the current signal becomes gradually stronger with
larger amplitude. At about 8 µs, the streamer propagated to the opposite plane electrode
with one main branch with strong luminance, indicating the breakdown event, as shown in
Figure 4-13 (b).
Figure 4-14 shows the streamer leading to the breakdown in the synthetic ester liquid under
negative polarity. Similar to positive polarity, a significant voltage drop from -60 kV to 0
kV was observed when the breakdown occurred. A long current train with discrete pulses is
observed before breakdown. A few small discrete current pulses with relatively lower
amplitude existed before Frame 1 as shown in the black dashed circle A in Figure 4-14 (a).
The amplitude of these small discrete pulses is nearly equal to the current amplitude (≈ 10
mA) as shown in Figure 4-9 (a). The average propagation velocity of the breakdown event
is about 1.01 km/s, which belongs to the 2nd
mode streamer. This result indicates that the
negative streamer changes from the 1st mode streamer in Figure 4-11 to the 2
nd mode
100
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
streamer in Figure 4-14 at the breakdown level. Comparing Figure 4-13 (b) and Figure 4-14
(b), the negative streamer is clearer and much thicker, and it has more branches with many
small offshoots than the positive streamer.
Figure 4-13 Breakdown in the synthetic ester liquid under DC voltage, positive polarity; d = 10 mm, r = 10
µm, exposure time 2 µs; (a) voltage, current and monitor signals, (b) streamer propagation, corresponding to
the signals in (a).
Figure 4-14 Breakdown in the synthetic ester liquid under DC voltage, negative polarity; d = 10 mm, r = 10
µm, exposure time 2 µs; (a) voltage, current and monitor signals, (b) streamer propagation, corresponding to
the signals in (a).
4.4.2 Breakdown Tests at Gap Distances from 2 mm to 30 mm
To statistically analyse the breakdown voltage, Weibull distribution was used to fit the
breakdown results and to calculate 50% streamer inception voltage. Figure 4-15 shows the
0 1 2 3 4 5 6 7 8 9 10-10000
-5000
0
5000
Cu
rren
t (m
A)
0 1.4 2.8 4.2 5.6-2
0
2
4M
on
ito
r S
ign
al
(V)
Time (us)
0 1 2 3 4 5 6 7 8 9 10-1
0
1
Mo
nit
or
Sig
nal
(V)
Time (us)
0 2 4 6 8 10-10
0
10
20
Cur
rent
(mA
)
0 2 4 6 8 10-5
0
5
10
Mon
itor S
igna
l (V
)
Time (us)
0 2 4 6 8 10-50
0
50
Curre
nt (m
A)
0 2 4 6 8 10-10
0
10
Mon
itor S
igna
l (V)
Time (us)
0 2 4 6 8 10
0
2
4
Mon
itor S
igna
l (V)
Time (us)
0 5 10 15 20 25 30-1
0
1x 10
4
Cu
rren
t (m
A)
0 8 16 32
-4
-2
0
2
Mo
nit
or
Sig
nal
(V)
Time (us)
0 5 10 15 20 25 30-1
0
1
Mo
nit
or
Sig
nal
(V)
Time (us)
20 25 30 35 40-4
-2
0
2
Cur
rent
(A
)
20 25 30 35 40-5
0
5
10
Mon
itor
Sig
nal (
V)
Time (us)
20 25 30 35 40-2000
0
2000
Curre
nt (m
A)
0 10 20 30 40 50 60-10
0
10
Monit
or Sig
nal (V
)
Time (us)
20 25 30 35 40
0
2
4
Monit
or Sig
nal (V
)
Time (us)
2 µs
10 mA
30 kV
Frame 1 Frame 2 Frame 3 Frame 4
Frame 1 Frame 2 Frame 3 Frame 4
(a)
(b)
5 µs 2 A
F 1 F 2 F 3 F 4 F 5 F 6 F 7
-60 kV
(b)
Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6
(a)
101
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
breakdown results of the mineral oil and the synthetic ester liquid under both positive and
negative polarities, and parameters are summarised in Table 4-2. Strong polarity effect was
observed and breakdown voltages of negative polarity are nearly 2.5 times those of positive
polarity. This conclusion is valid for both the mineral oil and the synthetic ester. The reason
is, under positive polarity, the concentration of positive charges enhances the boundary
field at the head of the charge cloud, which promotes streamer propagation and then
weakens breakdown voltages. However, the diluted negative space charges, like a shielding
of the negative tip, weaken the boundary field, which slows down the streamer propagation
and then increases the negative breakdown voltage.
Figure 4-15 Weibull distribution plot of breakdown results in the mineral oil and the synthetic ester liquid
with different gap distances under DC voltage, (a) Gemini X, (b) MIDEL 7131, r = 10 µm
(a)
(b)
102
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
Table 4-2 Weibull parameters of breakdown results in the mineral oil and the synthetic ester liquid with
different gap distances at the point-plane electrode.
Oil Type Polarity Gap distance
(mm) Shape Scale
50 % Breakdown
Voltage
Gemini X
Positive
(+)
5 22 18.9 21.5
10 33.9 37.0 36.9
20 39.1 59.5 58.9
30 44.7 92.1 91.4
Negative
(-)
2 32.3 33.9 33.5
5 61.4 55.9 61.0
10 49.3 93.5 92.8
MIDEL
7131
Positive
(+)
5 17.4 34.0 17.2
10 33.7 35.6 25.4
20 37.9 39.0 38.6
30 44.0 61.3 60.8
Negative
(-)
2 31.6 22.2 21.9
5 37.9 59.9 37.7
10 32.1 60.7 59.8
20 33.8 87.7 86.8
The effect of gap distance on breakdown voltages under both positive and negative DC
voltages are shown in Figure 4-16. It is clear that breakdown voltages of both positive and
negative polarities almost increase linearly with the gap distance in the investigated range.
In the mineral oil, the breakdown voltages of negative polarity are about 2.5 times higher
than those under positive polarity. In the synthetic ester liquid, the breakdown voltages of
negative polarity are about 2.2 times higher than those under positive polarity.
Figure 4-16 Effect of gap distance on breakdown voltage in the mineral oil and the synthetic ester liquid
under DC voltage, r = 10 µm; based on 50% breakdown voltages
Under both positive and negative polarities, the breakdown voltages of the mineral oil are
about 1.5 times higher than those of the synthetic ester, while the initiation voltages of both
103
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
the liquids are similar. It is because breakdown in the non-uniform electrical field is
controlled by propagation rather than initiation.
4.5 Summary
Electrical strengths of a mineral oil and a synthetic ester liquid in a non-uniform field under
DC voltage in both positive and negative polarities were examined in this chapter. Point-
plane electrodes with the various tip radii and gap distances were used throughout the
investigation.
It can be summarised that streamer initiation voltage in the synthetic ester liquid is
comparable to that of the mineral oil (Figure 4-3). Once a streamer is initiated, streamer
propagation is easier in the synthetic ester liquid than that in the mineral oil (Figure 4-6 and
Figure 4-10). Although initiation voltages of synthetic ester liquid are comparable to those
of mineral oil, breakdown voltages of the synthetic ester liquid are nearly 40% lower than
those of the mineral oil for the investigated divergent point-plane gaps. Only slow streamer
mode was observed in both the mineral oil and the synthetic ester liquid in this study.
In addition, an extremely strong polarity effect of streamer propagation in both the liquids
is observed. At pre-breakdown stage, under negative polarity, the streamer barely
propagates (lstopping < 0.5 mm in the mineral oil and lstopping < 1 mm in the synthetic ester
liquid) even with the applied voltage close to the breakdown voltage. The negative streamer
only changes propagation mode from 1st mode to 2
nd mode at the breakdown voltage level.
104
Chapter 4 Streamer and Breakdown Properties of Insulating Liquids under DC Voltage
105
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse Voltages
CHAPTER 5. STREAMER AND BREAKDOWN PHENOMENA
OF TRANSFORMER LIQUIDS UNDER DIFFERENT IMPULSE
WAVEFORMS
5.1 Introduction
This chapter reports the pre-breakdown and breakdown properties of a mineral oil and a
synthetic ester liquid at a small gap distance of 10 mm under different positive impulse
waveforms. The different impulse waveforms from the short tail-time waveform with 8 µs
to “step-like” waveform with 3200 µs were stressed on oil samples. A 16-channel high-
speed camera was used to characterise the streamer stopping length, average propagation
velocity and shape. A comparison of breakdown voltages and instantaneous breakdown
voltages between the different impulse waveforms is given. A mathematical model of
breakdown voltage prediction is described in detail and verified by tests in another liquid.
5.2 Pre-breakdown Characteristics
5.2.1 Stopping Length
Figure 5-1 shows the comparisons of streamer stopping length in the mineral oil, where 50%
breakdown voltages are stated as the reference (Vb-8 µs, Vb-14 µs, Vb-30 µs and Vb-3200 µs stand
for 50% breakdown voltage with different tail times respectively). At each voltage level,
the average and standard deviation are given based on ten measurements. The results
indicate that, streamer stopping length increases gradually with voltage applied for all the
impulse waveforms investigated. At the same voltage level, streamers under longer-tail
impulses propagate much further than those under shorter tail-time impulses.
Figure 5-2 shows the comparisons of streamer stopping lengths in the synthetic ester liquid,
which follows similar trends as presented in the mineral oil. At lower voltage levels, a very
small difference in the stopping length can be observed. However, with the increase of the
applied voltage, the streamers propagate further under a longer tail-time impulse waveform
than those under a shorter tail-time impulse waveform at the same applied voltage level.
106
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse
Voltages
Figure 5-1 Stopping length of streamers in the mineral oil under positive polarity; d = 10 mm, r = 10 µm;
error bars stand for one standard deviation.
Figure 5-2 Stopping length of streamers in the synthetic ester liquid under positive polarity; d = 10 mm, r =
10 µm; error bars stand for one standard deviation.
5.2.2 Average Propagation Velocity
In order to characterise the streamer behaviours in the mineral oil and the synthetic ester
liquid with more details, the average streamer propagation velocity of the two testing
liquids under different impulse waveforms was also calculated at a wide range of voltage
levels, i.e. from the voltage where streamer propagates in the 2nd
mode to the voltage before
the 50% breakdown voltage.
107
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse Voltages
Figure 5-3 shows the comparisons of streamer average propagation velocities in the mineral
oil under different positive impulse waveforms. The results indicate that the average
propagation velocities remain almost constant in the range from 1.5 km/s to 1.7 km/s with
the increase of applied voltage for all the impulse waveforms investigated. At the same
applied voltage level, there is no difference in the average propagation velocities among
different tail-time impulse waveforms. In addition, it confirms that all the streamers
observed during the tests were corresponding to the 2nd
mode streamer. The similar
phenomenon was observed in the synthetic ester liquid as shown in Figure 5-4.
Figure 5-3 Average propagation velocity of streamers in the mineral oil under positive polarity; d = 10 mm, r
= 10 µm; error bars stand for one standard deviation.
Figure 5-4 Average propagation velocity of streamers in the synthetic ester liquid under positive polarity; d =
10 mm, r = 10 µm; error bars stand for one standard deviation.
108
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse
Voltages
5.3 Breakdown Voltage
5.3.1 Breakdown Tests in the Mineral Oil
To statistically analyse the breakdown voltages, Weibull distribution was employed to fit
the breakdown results and to estimate the breakdown voltages at 50% breakdown
probability. Figure 5-5 shows Weibull distribution plots of the breakdown results obtained
in the mineral oil under different impulse waveforms.
The 50% breakdown voltages of the mineral oil under different impulse waveforms are
summarised in Table 5-1. In the following discussions of this chapter, 50% breakdown
voltage is defined as the breakdown voltage Vb of the testing liquids. It is observed that the
50% breakdown voltage decreases with the increase of impulse tail-time.
Table 5-1 Weibull parameters of breakdown results in the mineral oil obtained under different impulse
waveforms; d = 10 mm, r = 10 µm, positive polarity.
Different Impulse
Waveforms Shape Scale
50% Breakdown
Voltage (kV)
0.8/8 µs 45.48 54.76 54.1
0.8/14 µs 39.17 43.46 42.9
0.8/30 µs 41.4 34.84 34.7
0.8/3200 µs 56.85 32.22 32
Figure 5-5 Weibull plot of breakdown voltages in the mineral oil under different impulse waveforms, d = 10
mm, r = 10 µm, positive polarity.
109
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse Voltages
Figure 5-6 shows an example of typical breakdown voltage waveforms under each type of
impulse waveform. The breakdown voltage is normally defined as the highest voltage value
of impulse waveform as shown in the grey-dashed lines A1 ~ A4. There is another term
called instantaneous breakdown voltage Vi, which is the point before voltage drop down as
shown in the black-dash circle. The time to breakdown tb is measured based on the voltage
waveform from the voltage rising point to the point before the voltage drops.
Figure 5-6 Typical breakdowns in the mineral oil under different impulse waveforms, d = 10 mm, r = 10 µm,
positive polarity.
Figure 5-7 Time to breakdown in the mineral oil under different impulse waveforms, d = 10 mm, r = 10 µm,
positive polarity.
-2 0 2 4 6 8 10 120
10
20
30
40
50
60
Time (us)
Vo
ltag
e (k
V)
0.8/8 us
0.8/14 us
0.8/30 us
0.8/3200 us
A1
Time to breakdown
A2
A3
A4
Breakdown voltage
Instantaneous
breakdown voltage
110
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse
Voltages
Figure 5-8 Breakdown voltage and instantaneous breakdown voltages in the mineral oil under different
impulse waveforms; d = 10 mm, r = 10 µm, positive polarity.
The time to breakdown is measured based on the start point of impulse waveform and the
end of the instantaneous breakdown point, and it was found to be almost the same for
different impulse waveforms. Further detailed results are summarised in Figure 5-7. This
can be simply explained by the similar streamer average propagation velocity shown in
Figure 5-3. The peak voltage of the impulse waveform is commonly referred as the
breakdown voltage whereas instantaneous breakdown voltage Vi, represents the instant
voltage when the breakdown occurs. 50% breakdown voltages and instantaneous
breakdown voltages obtained under different impulse waveforms are shown in Figure 5-8.
Although the 50% breakdown voltage decreases with the increase of tail-time, the
instantaneous breakdown voltage remains almost the same under different impulse
waveforms. In addition, the breakdown voltage and instantaneous breakdown voltage under
the long tail impulse waveform, i.e. 0.8/3200 µs are the same due to the negligible
reduction of instant voltage during the streamer propagation.
5.3.2 Prediction of Breakdown Voltage
This behaviour in the mineral oil indicates a correlation between breakdown voltage,
instantaneous breakdown voltage and time to breakdown. In this case, a mathematical
model of predicting the breakdown voltage under desired impulse waveforms can be
established. Equation (5-1) is used to simulate the desired impulse voltage waveform [104]:
111
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse Voltages
𝒗(𝒕) = 𝑽𝟎(𝒆−𝜶𝒕 − 𝒆−𝜷𝒕) (5-1)
where a and β are used to define impulse voltage waveform; V0 means the voltage level of
impulse waveform.
Based on the results shown in Figure 5-6, Figure 5-7 and Figure 5-8, it is assumed that Vi
and tb remain stable under impulse waveforms with different tail times. In this case, Vi and
tb can be obtained by conducting breakdown tests under one impulse waveform to predict
the breakdown voltage of another impulse waveform.
Figure 5-9 shows the flowchart of the breakdown voltage prediction model. In this model,
four parameters α, β, Vi and tb are used as the input values to process breakdown voltage
prediction. Firstly, parameter α and β are used to determine the shape of the desired impulse
waveform. For the standard lightning impulse voltage (1.2/50 µs), normally α is 0.0143 and
β is 4.87. Then, the parameters Vi and tb are determined by one set of breakdown tests under
one type of impulse waveform. The prediction is controlled by tb. The applied voltage V is
initialised at the voltage level of Vi and increased by 1% of Vi per step. Finally, when the
time to breakdown of simulated impulse waveform meets the criterion tb, the breakdown
voltage prediction program stops and outputs the breakdown voltage Vb.
In the process of breakdown voltage prediction in the mineral oil, the experimental data of
0.8/8 µs are selected as the test impulse waveform. Another three impulse waveforms
(0.8/14 µs, 0.8/30 µs and 0.8/3200 µs) are treated as the desired impulse waveforms. Table
5-2 summarises the experimental results of 50% breakdown voltages and predicted
breakdown voltages under four impulse waveforms. The results indicate that the predicted
breakdown voltages are very close to the experimental results, and the relative difference is
within 6%. The three simulated breakdown voltage waveforms are shown in Figure 5-10,
where the shapes of the simulated waveforms are identical to the impulse waveforms
obtained by experiments shown in Figure 5-6.
112
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse
Voltages
Figure 5-9 Flowchart of a mathematical model of breakdown voltage prediction.
Table 5-2 Predicted breakdown voltage and experimental breakdown voltage in the mineral oil obtained
under different impulse waveforms; d = 10 mm, r = 10 µm, positive polarity.
Impulse Voltage
Waveforms α β
50% Breakdown
Voltage (kV)
Predicted Breakdown
Voltage (kV)
Input 0.8/8 µs 0.1100 4.87 54.1 -
Output
0.8/14 µs 0.0543 4.87 42.9 40.4
0.8/30 µs 0.0243 4.87 34.7 35.8
0.8/3200 µs 0.0013 4.87 32 31.9
Desired impulse
waveform
Determine
α and β
Experimental
breakdown test
under test impulse
waveform
Determine
Vi and tb
Estimated time to
breakdown t ≥ tb
Increasing simulated
impulse voltage V by 1%
of Vi
Outputting
breakdown voltage V
≈ Vb
YES
NO
Calculating time to
breakdown t based on Vi
113
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse Voltages
Figure 5-10 Simulated different impulse waveforms in the mineral oil based on the mathematical model,
positive polarity.
5.3.3 Verification in the Synthetic Ester Liquid
To verify the prediction model of breakdown voltage, other experimental breakdown tests
have been conducted in the synthetic ester liquid. The test conditions including gap distance,
tip radius, electrode configuration and voltage polarity are the same. First of all, the
experiment of breakdown measurements in the synthetic ester liquid has been investigated
under test impulse waveform 0.8/3200 µs. The instantaneous breakdown voltage Vi is
calculated as 26.6 kV and the time to breakdown tb is calculated as 5.1 µs. Then, parameters
α and β are obtained for the desired impulse waveforms of 0.8/8 µs, 0.8/14 µs and 0.8/30,
as given in Table 5-2. Finally, with the input values of α, β, Vi and tb, the breakdown
voltages can be predicted as given in Table 5-3. Based on these predicted breakdown
voltages, the simulated impulse voltage waveforms under breakdown conditions are plotted
in Figure 5-11.
Table 5-3 Predicted breakdown voltage and experimental breakdown voltage in synthetic ester liquid
obtained under variable impulse voltages; d = 10 mm, r = 10 µm, positive polarity
Impulse Voltage
Waveforms
Predicted Breakdown
Voltage (kV)
50% Breakdown
Voltage (kV)
Input 0.8/3200 µs - 26.6
Output
0.8/8 µs 42.9 42.0
0.8/14 µs 32.6 34.0
0.8/30 µs 29.4 29.9
-2 0 2 4 6 8 10 120
10
20
30
40
50
60
Time (us)
Vo
ltag
e (k
V)
0.8/8 us
0.8/14 us
0.8/30 us
0.8/3200 us
O
InstantaneousBreakdownPoint
Time to breakdown
114
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse
Voltages
Figure 5-11 Simulated different impulse waveforms in the synthetic ester liquid based on the mathematical
model, positive polarity.
Figure 5-12 Typical breakdowns in the synthetic ester liquid under different impulse waveforms, d = 10 mm,
r = 10 µm, positive polarity.
To verify the accuracy of breakdown voltage prediction, experimental breakdown
measurements have been conducted under different impulse waveforms with 0.8/8 µs,
0.8/14 µs and 0.8/30 µs. The typical breakdown waveforms in the synthetic ester liquid
under the different impulse waveforms are shown in Figure 5-12. Both the waveform
shapes and voltage levels including ‘instantaneous breakdown voltage’ and ‘time to
breakdown’ are comparable to the simulated breakdown voltage waveforms as shown in
Figure 5-11. The 50% breakdown voltages are also given in Table 5-3, which are very close
-2 0 2 4 6 8 10 120
5
10
15
20
25
30
35
40
45
Time (us)
Vo
ltag
e (k
V)
0.8/8 us
0.8/14 us
0.8/30 us
0.8/3200 us
O
InstantaneousBreakdownPoint
-2 0 2 4 6 8 10 12-5
0
5
10
15
20
25
30
35
40
45
Time (us)
Voltage (
kV
)
0.8/8 us
0.8/14 us
0.8/30 us
0.8/3200 us
Time to breakdown
Time to breakdown
115
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse Voltages
to the predicted values and the relative difference is within 5%. Therefore, it is confirmed
that this breakdown voltage prediction model is also applicable to the synthetic ester liquid.
5.4 Effect of Impulse Waveform on Streamer Characteristics
A series of streamer images have been captured using a high-speed camera to analyse
streamer propagation and eventual breakdown. During the process of streamer propagation,
with a similar stopping length the short tail-time impulse waveform needs a higher voltage
level to force streamers to propagate as shown in Figure 5-1 and Figure 5-2. This raises the
question of the impact of impulse waveform on steamer characteristics.
Figure 5-13 and Figure 5-14 show the typical images of positive streamers with a similar
stopping length obtained in the mineral oil and synthetic ester liquid, respectively, under
different impulse waveforms. The stopping lengths of the selected streamers in the mineral
oil are about 7.44 ~ 7.85 mm and in the synthetic ester liquid, they are about 6.42 ~ 6.59
mm. In both liquids, it is found that streamers under a short tail-time impulse waveform
have more branches with more small offshoots as shown in Figure 5-13 (a) and Figure 5-14
(a). However, only one or two main branches with a few small offshoots are observed under
a long tail-time impulse waveform as shown in Figure 5-13 (d) and Figure 5-14 (d).
Figure 5-13 Pre-breakdown in the mineral oil under different impulse waveforms, d = 10 mm, r = 10 µm,
positive polarity, (a). 0.8/8 µs, V = 52 kV, lstoping = 7.62 mm (b). 0.8/14 µs, V = 42 kV, lstoping = 7.71 mm (c).
0.8/30 µs, V = 34 kV, lstoping = 7.44 mm (d). 0.8/3200 µs, V = 32 kV, lstoping = 7.85 mm.
Figure 5-14 Pre-breakdown in the synthetic ester liquid under different impulse waveforms, d = 10 mm, r =
10 µm, positive polarity, (a). 0.8/8 µs, V = 40 kV, lstoping = 6.59 mm (b). 0.8/14 µs, V = 32 kV, lstoping = 6.42
mm (c). 0.8/30 µs, V = 30 kV, lstoping = 6.47 mm (d). 0.8/3200 µs, V = 26 kV, lstoping = 6.57 mm.
(a) (b) (c) (d)
(a) (b) (c) (d)
116
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse
Voltages
Figure 5-15 and Figure 5-16 present the streamer areas corresponding to the stopping
lengths in the mineral oil and the synthetic ester liquid under different impulse waveforms.
The streamer area is defined as the apparent area occupied by all streamer branches in the
2D image, which is estimated using a programme [105]. Similar to the photographs
observed in Figure 5-13 and Figure 5-14, positive streamers under a short tail-time impulse
waveform generally have larger areas due to more branches when compared with those
under a long tail-time impulse waveform of a similar stopping length.
Due to the rapidly decaying voltage of short tail-time impulse waveforms, streamers need
minimum voltage (e.g. Vi) and time to breakdown from higher applied voltage to maintain
the streamer propagation and achieve breakdown. However, Figure 5-3 and Figure 5-4
indicate that all the positive streamers captured in the mineral oil and the synthetic ester
liquid are the 2nd
mode streamers [44, 61]. The average propagation velocities almost
remain constant at the pre-breakdown stage. Meanwhile, the statistical analyses of the
apparent charge and energy at the similar stopping length based on ten measurements are
given in Table 5-4. This indicates that the apparent energy acting on oil samples is higher
under a short tail-time impulse waveform with a similar stopping length. In this case, the
redundant energy injected into oil samples does not compel streamers to propagate further
but helps to create more branches.
Figure 5-15 Positive streamer area as a function of stopping length in the mineral oil under different impulse
waveforms, d = 10 mm, r = 10 µm.
117
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse Voltages
Figure 5-16 Positive streamer area as a function of stopping length in the synthetic ester liquid under different
impulse waveforms, d = 10 mm, r = 10 µm.
Table 5-4 Average charges injected into oil samples of the similar stopping length under different impulse
waveforms.
Variable Impulse
Waveforms
Charge in
Gemini X (pC)
Charge in MIDEL
7131 (pC)
0.8/8 µs 6.52 13.97
0.8/14 µs 4.43 12.07
0.8/30 µs 4.16 6.97
0.8/3200 µs 2.46 3.94
5.5 Summary
Pre-breakdown and breakdown characteristics of stopping length, average propagation
velocity, streamer shape and breakdown voltage under positive impulse waveforms with
different tail times in a mineral oil and a synthetic ester liquid with a small gap distance of
10 mm were investigated in this chapter.
At the pre-breakdown stage, it was found that with similar stopping lengths, streamers
under a short tail-time impulse waveform have denser branches with more small offshoots,
while only one or two main branches with a few small offshoots were observed under a
long tail-time impulse waveform. This is due to higher energy injection into an oil sample
under a short tail-time impulse waveform, which encourages a streamer to grow densely.
Compared to the impulse voltage with a longer tail-time, the shorter tail-time impulse
waveform results in higher breakdown voltage, but does not have an obvious effect on the
118
Chapter 5 Streamer and Breakdown Phenomena of Insulating Liquids under Various Impulse
Voltages
instantaneous breakdown voltage and the time to breakdown. In both tested liquids, the
breakdown voltage waveforms of different impulse waveforms almost intersect one point at
the moment of voltage drop, which results in the same instantaneous breakdown voltage.
Therefore, a mathematical model for breakdown voltage prediction under impulse voltage
with different tail time has been described in detail. At the same testing environment and
liquid nature, the breakdown voltage of insulating liquids under desired impulse waveform
characterised by α, β can be estimated based on Vi and tb from one set of breakdown tests
under a known impulse waveform.
119
Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
CHAPTER 6. CORRELATIONS BETWEEN GAS
GENERATION AND SPARKING FAULT IN TRANSFORMER
LIQUIDS UNDER LIGHTNING IMPULSE VOLTAGE
6.1 Introduction
In this chapter, experimental studies that investigate the correlation between fault gas
generation and sparking faults in the mineral oil and the synthetic ester liquid were carried out
by using a hydrogen monitor and a multi-gas monitor. The well-controlled electrical fault with
different test conditions of gap distances, spark numbers and voltage levels were achieved using
a compact impulse spark generator. The sparking fault energy was derived from the recording
of voltage and current waveforms, hence the correlations between fault gas generation and
sparking fault energy were obtained.
6.2 Data Processing
6.2.1 Calculation of Dissolved Gas Generation
For the experiment, a single gas on-line DGA monitor – TM1 (hydrogen monitor) was used
to continuously monitor hydrogen generation and a multi gas on-line DGA monitor – TM8
was used to measure all eight fault gases and the oil temperature.
An example of dynamic hydrogen production measured by the TM1 hydrogen monitor
during the 500-spark test is shown in Figure 6-1. The reference cycle is an internal sensor
calibration that is typically performed every twelve hours. The TM1 hydrogen monitor
makes one measurement every 30 minutes [87]. The electrical fault stress in the oil sample
started at 0 minutes and ended at 500 minutes. The dissolved hydrogen concentration in the
oil increased gradually during the process of spark fault generation. It continued to increase
for a short period after the end of fault generation, and then reached a steady state upon
attaining gas–liquid equilibrium. In order to calculate the net hydrogen generation, the
dynamic hydrogen readings were divided into three stages: background detection, spark
fault period detection and post-test detection. Background detection measured the average
hydrogen concentration before the fault tests. Post-test detection measured the average
hydrogen concentration after cessation of spark fault generation. Therefore, hydrogen
120
Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
generation due to the spark faults is the difference between the post-test detection and the
background detection, which is 142.5 ppm in the case shown in Figure 6-1.
Figure 6-1 Hydrogen concentration as gas in oil concentration from the TM1 hydrogen monitor as a function
of time during a 500 spark test, mineral oil, d = 10 mm, positive polarity.
Figure 6-2 Hydrogen concentration as gas in oil concentration from the TM8 multi-gas monitor as a function
of time during a 500 spark test, mineral oil, d = 10 mm, positive polarity.
Figure 6-2 shows an example of hydrogen measurement before and after 500 sparks
measured by the TM8 multi-gas monitor. The TM8 multi-gas monitor was not allowed to
measure fault gas concentration during the test period of sparking fault, but allowed the
pump to continuously circulate the oil sample. At the stage of background measurement,
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
the average hydrogen concentration in the oil sample maintains at about 85.8 ppm. After
500 sparks, hydrogen concentration increases to about 219.4 ppm. Thus, the hydrogen
generation due to the 500 sparks is 133.6 ppm, which is comparable to the hydrogen
generation (142.5 ppm) measured based on the TM1 hydrogen monitor as shown in Figure
6-1. The different background measurement of hydrogen between hydrogen monitor and
multi-gas monitor is probably due to the different hydrogen sensors in these two DGA
monitors.
6.2.2 Gas-in-total Calculation
Both the TM1 hydrogen monitor and the TM8 multi-gas monitor can only measure the fault
gas concentration in oil. However, when the oil system has a headspace volume, part of the
available fault gas migrates into the headspace and affects the measured fault gas
concentration in the total oil volume. Therefore, it is necessary to calculate the gas-in-total
(GIT) concentration using the gas-in-oil (GIO) concentration. The equations for calculating
GIT are shown in equation (6-1) to (6-4) [19, 92]. The equations are based on the
conversation of mass law and the relationship between GIO concentration and gas-in-gas
(GIG) concentration [19, 92].
𝑪𝑶 = 𝑪𝑮 ∗ 𝑲 (6-1)
𝑴𝑻 = 𝑴𝑶 + 𝑴𝑮 (6-2)
𝑪𝑻 ∗ 𝑽𝑶 = 𝑪𝑶 ∗ 𝑽𝑶 + 𝑪𝑮 ∗ 𝑽𝑮 (6-3)
𝑪𝑻 = 𝑪𝑶(𝟏 +𝟏
𝑲∗
𝑽𝑮
𝑽𝑶) (6-4)
where CO, CG and CT mean the gas concentration of oil, gas and total respectively; MO, MG
and MT mean the mass of gas dissolved in the oil, in the gas and the total mass respectively;
VO and VG mean the volume of oil and gas respectively; K means the solubility coefficient
of the gas.
The ineradicable headspace volume in the entire test system including the online DGA
monitors was estimated as 80 mL. The value of the K factor was based on the Ostwald
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
solubility coefficient, which is affected by oil temperature. For example, the Ostwald
solubility coefficient under different oil temperatures in mineral oil is shown in Figure 6-3.
The oil temperature measured by the TM8 multi-gas monitors was 21±2 . In addition, the
Ostwald solubility coefficients used in the synthetic ester liquid are provided by the oil
supplier, which have not been pulished.
Figure 6-3 The K factor based on Ostwald solubility coefficient under different temperature in mineral oil
The fault gas generation in ppm represents the ratio between total gas volume and oil
volume. It is more reasonable to correlate total volume of fault gas generation with fault
energy. The equation for converting fault gas concentration in ppm to generated fault gas
volume in μL is shown in equation (6-5).
𝑻𝒐𝒕𝒂𝒍 𝑮𝒂𝒔 𝑽𝒐𝒍𝒖𝒎𝒆 = 𝑪𝑻 × 𝑽𝑶 (6-5)
where CT means the gas concentration in total; VO means the oil volume.
6.2.3 Energy Calculation
The lightning impulse voltage is measured by a voltage divider and the fault current is
measured by a current shunt. Equations (6-6) and (6-7) show the product of the
instantaneous voltage V and current I resulting in the instantaneous fault energy. To obtain
a more quantitative comparison of fault gas generation in the mineral oil and the synthetic
ester liquid, the concentrations of fault gases were correlated with the sparking fault energy
E (joule) by integrating power (VI) with time.
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
𝑬 = ∫ 𝑽𝑰 𝒅𝒕 (6-6)
𝑬 = 𝑽𝟏𝑰𝟏∆𝒕 + 𝑽𝟐𝑰𝟐∆𝒕 + ⋯ ⋯ + 𝑽𝒏𝑰𝒏∆𝒕 (6-7)
Figure 6-4 shows an example of the voltage and the current waveform of a spark in the
mineral oil at 39 kV (99.9% breakdown voltage). The voltage and current waveform can be
divided into three parts. Part 1 is pre-breakdown stage, which interprets a process of the
streamer initiating from needle electrode and propagating to plane electrode with time
duration of 7 μs. Part 2 is a sparking fault stage. The voltage drops sharply and current
initially rushes to a peak and then decay gradually. The sparking arc lasts for about 25 μs
and then distinguished. Afterwards, the current becomes near zero and voltage recovers
slightly and slowly decays to zero, which is called Part 3. The reason is probably that the
breakdown conductive channel disappeared at about 42 μs, and the dielectric strength of the
oil sample was recovered, which leads to drop of the current.
Figure 6-4 The voltage and current waveform in mineral oil on 99.9% breakdown voltage, positive polarity, d
= 10 mm, r = 50 µm.
0 10 20 30 40 50 60 70 80 90 1000
10
20
30
40
Time (us)
Volt
age
(kV
)
0 10 20 30 40 50 60 70 80 90 100
0
2
4
6
8
Time (us)
Cu
rren
t (A
)
Part 1 Part 2 Part 3
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
Table 6-1 shows the average energy generation as a percentage for each part of voltage and
current waveform based on 100 sparks in the mineral oil. The pre-breakdown energy in part
1 only accounts for 1.45% at 99.9% breakdown voltage and 1.56% at 150% breakdown
voltage. Most of the energy is accumulated in part 2, since the sparking arc occurred at this
stage. The energy in part 3 occupied 7.98% on 99.9% breakdown voltage and 7.97% on 150%
breakdown voltage, which might be due to residual discharge or current signal distortion.
Overall the calculation is representative for sparking fault energy.
Table 6-1 Individual part energy as percentages of total energy generation based on 100 sparks in mineral oil,
V = Vb-99.9%.
Mean Value Standard Deviation
Vb-99.9%
Part 1 1.45 % 0.3114
Part 2 90.56 % 0.3631
Part 3 7.98 % 0.2063
Vb-150%
Part 1 1.56 % 0.3493
Part 2 90.48 % 0.6094
Part 3 7.97 % 0.2730
6.3 DGA Results and Analysis
6.3.1 Comparison of Hydrogen Measurements
Hydrogen is the lightest fault gas and has the highest mass transfer coefficient, which
means it is easier to escape from the oil compared to other fault gases. Therefore
comparison of hydrogen measurements between the TM8 multi-gas monitor, TM1
hydrogen monitor and laboratory gas chromatography (GC) system was carried out first.
When the TM1 hydrogen and TM8 multi-gas monitors had finished hydrogen
measurements after each group of spark tests, the oil was sampled with gas-tight syringes
for laboratory DGA measurement, where the fault gases were extracted by the headspace
method and analysed by GC system. Table 6-2 compares the hydrogen measurements by
the TM1 hydrogen monitor, the TM8 multi-gas monitor and the laboratory technique.
The results indicate that the hydrogen measured by the multi-gas monitor is comparable to
that measured by the hydrogen monitor and laboratory GC systems. The high relative
differences in sample 1 and sample 2 are due to the low concentration of hydrogen
measured. However the absolute differences were within a few parts-per-million, which are
negligible in terms of practical implication. When the hydrogen concentration is greater
than about 15 ppm, the reading from hydrogen monitor TM1 generally has higher values,
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
and the relative difference was within 20%. When the hydrogen is less than 15 ppm, the
relative difference was within a few parts-per-million. In the following, measurements from
the TM8 multi-gas monitor are used in the analyses.
Table 6-2 Comparison of hydrogen measurements among the hydrogen monitor, the multi-gas monitor and
laboratory technique.
Oil Sample
Hydrogen Concentration (ppm)
100%(TM1 -
TM8) /TM1
100%(TM1 - DGA)
/TM1 TM 8
Multi-gas
Monitor
TM1
Hydrogen
Monitor
Laboratory
DGA
Sample 1 3 3 5 0% - 67%
Sample 2 8 8 11 0% - 37%
Sample 3 15 18 16 17% 11%
Sample 4 52 57 48 9% 16%
Sample 5 108 135 109 20% 19%
Sample 6 211 227 192 7% 15%
6.3.2 Effect of Spark Numbers
To observe the occurrence of the spark in the liquids, a high-speed camera was used to
capture the image of the electrodes. Figure 6-5 shows a typical positive spark in the mineral
oil at the 5 mm gap distance under standard lightning impulse. It takes about 3 µs from the
streamer initiation to breakdown.
Figure 6-5 Typical spark in the mineral oil of positive polarity under lightning impulse, VB-99.9%-positive = 31 kV,
d = 5 mm, exposure time 0.5 µs.
Based on the equations (6-1) to (6-4), all the DGA results measured in GIO by TM8 on-line
DGA monitors were converted to GIT. The fault gas generation (GIT) based on the average
of two-group results in the mineral oil under positive sparking fault with the gap distance of
5 mm is shown in Figure 6-6. It is clear that fault gas generation increased gradually with
the number of sparks. When spark numbers were smaller than 50, only C2H2 was measured
with a small amount of 1.2 ppm. This phenomenon can be explained that a few sparks
generated very few other fault gases, which do not reach the measurement threshold of the
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
multi-gas monitor. When the spark number was increased to 100, a small amount of H2 was
measured with a value of 2.5 ppm. When the spark numbers were increased up to 500, H2
was generated with a value of 7.1 ppm and C2H2 of 26 ppm. It is also important to note that
very low amount of C2H4 normally created at the high energy or temperature was measured
[80]. The small amount of C2H4 was generated probably due to local high temperature
within the sparking arc. In addition, CH4, CO, and C2H6 were not measured during the tests.
Figure 6-6 Fault gas generation (GIT) in the mineral oil at different numbers of breakdowns, positive polarity,
d = 5 mm.
Figure 6-7 Fault gas generation (GIT) in the synthetic ester liquid at different numbers of breakdowns,
positive polarity, d = 5 mm.
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
Figure 6-7 shows the fault gas generation (GIT) in the synthetic ester liquid under positive
sparking fault with the gap distance of 5 mm. It is clear that fault gas generation increased
gradually with the number of sparks. When the spark numbers were smaller than 100, there
was no fault gas measured, due to the measurement threshold. When the spark number was
increased to 500, H2 with a value of 3.3 ppm, CO with a value of 8.8 ppm and C2H2 with a
value of 4.8 ppm were measured.
Figure 6-8 shows the comparison of H2 and C2H2 generation (GIT) in the mineral oil and
the synthetic ester liquid under sparking fault with different numbers of sparks. It is clear
that H2 and C2H2 generations increase linearly with the number of sparks. C2H2 generation
is higher than H2 generation, which implies the sparking fault at this small gap distance
tends to have high energy intensity. In addition, a number of fault gases in the mineral oil is
higher than that in the synthetic ester liquid, in particular for C2H2.
Figure 6-8 Comparison of hydrogen and acetylene generation (GIT) in the mineral oil and the synthetic ester
liquid under sparking fault at different numbers of sparks, positive polarity, d = 5 mm.
Figure 6-9 shows the individual fault gases as percentages of total fault gases in the mineral
oil and the synthetic ester liquid under positive sparking fault. Clearly key gases in the
mineral oil under sparking fault are C2H2 and H2 which accounted for more than 95% of the
overall gas concentration as shown in Figure 6-9 (a). However, H2, CO and C2H2 which
accounted for 20%, 30% and 50% respectively, are key gases in the synthetic ester liquid
under positive sparking fault as shown in Figure 6-9 (b). With the increase of breakdown
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
numbers, the fault gas patterns remain almost the same, which means there is no
accumulative effect induced secondary reactions during the sequential tests
200 300 500
(a). Gemini X – Positive
200 300 500
(b). MIDEL 7131 – Positive
Figure 6-9 Individual fault gases as percentages of total fault gases in the mineral oil and the synthetic ester
liquid with a different number of sparks, d = 10 mm.
6.3.3 Effect of Gap Distance
Normally, the larger gap distance results in higher breakdown voltage and longer
discharge/sparking channel. To investigate the effects of these factors on fault gas
generation, similar experiments were carried out at the gap distance of 10 mm.
Figure 6-10 shows the fault gas generation (GIT) in the mineral oil with the number of
sparks under positive sparking fault at the 10 mm gap distance. The fault gas generation
increased gradually with the number of sparks. After 20 sparks applied, a small amount of
H2 and C2H2 were measured with the values of 5.7 ppm and 5.2 ppm, respectively. When
the spark number was increased to 500, a large amount of H2 and C2H2 were measured with
the values of 191.4 ppm and 143.7 ppm, and a small amount of CH4, CO and C2H4 were
measured with the value less than 10 ppm.
Figure 6-11 shows the fault gas generation (GIT) in the synthetic ester liquid with the
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
number of sparks under positive sparking fault at the 10 mm gap distance. After 20
breakdowns applied, a small amount of H2, CO and C2H2 were measured with the values of
5.1 ppm, 3.2 ppm and 5.4 ppm, respectively. When the breakdown number was increased to
500, a large amount of H2, CO and C2H2 were measured with the values of 146.9 ppm, 65.2
ppm and 99.4 ppm, and a small amount of CH4 and C2H4 were generated with the values
less than 9 ppm.
Figure 6-10 Fault gas generation (GIT) in the mineral oil as a function of the number of breakdowns, Gemini
X - Vb-99.9% = 39 kV, positive polarity, d = 10 mm.
Figure 6-11 Fault gas generation (GIT) in the synthetic ester liquid as a function of the number of
breakdowns, MIDEL 7131 - Vb-99.9% = 37 kV, positive polarity, d = 10 mm.
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
Figure 6-12 shows the comparison of H2 and C2H2 in the mineral oil and the synthetic ester
liquid at 5 and 10 mm gap distance under positive sparking fault. It is clear that fault gas
generation in both liquids at the 10 mm gap distance is much higher than that at the 5 mm
gap distance. This can be explained that the longer discharge/sparking channels with high
total fault energy at the 10 mm gap distance induce much more fault gas generation
compared to the case of 5 mm gap distance. The amount of H2 generation is found to be
slightly higher than C2H2 generation, which implies the sparking fault at this large gap
distance tends to have low energy intensity. Similar to the results at the 5 mm gap distance,
fault gas generation in the mineral oil at the 10 mm gap distance is relatively higher than
that in the synthetic ester liquid.
Figure 6-12 Comparison of hydrogen and acetylene generation (GIT) in the mineral oil and the synthetic ester
liquid at different gap distances under positive polarity.
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
200 300 500
(a). Gemini X - Positive
200 300 500
(a). MIDEL 7131 - Positive
Figure 6-13 Individual fault gases as percentages of total fault gases in the mineral oil and the synthetic ester
liquid with different spark numbers, Gemini X - Vb-99.9% = 39 kV, MIDEL 7131 - Vb-99.9% = 37 kV, positive
polarity, d = 10 mm.
Figure 6-13 shows the individual fault gases as percentages of total fault gases in the
mineral oil and the synthetic ester liquid with the spark numbers of 200, 300 and 500. It is
clear that H2 and C2H2 are both key gases in the mineral oil and the synthetic ester liquid. In
addition, it should be noted that CO is an additional key gas in the synthetic ester liquid
which accounts for around 20%. With the increase of spark numbers, the percentage
patterns of fault gas generation almost remain constant.
In previous studies which investigated the dissolved fault gases in the mineral oil and the
ester liquid, the volume of C2H2 is usually larger than that of H2, which is possibly due to
the lower solubility of H2 in the mineral oil and the ester liquid than that of C2H2. The
amounts of CH4, C2H4, and C2H6 were almost negligible: the energy dissipated from the
sparking fault was insufficient to generate those gases.
6.3.4 Effect of Voltage Levels
To investigate the effect of the higher voltage level on the fault gas generation, a similar
experiment was carried out at 1.5 times 99% breakdown voltage at the 10 mm gap distance.
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
Figure 6-15 shows fault gas generation (GIT) in the mineral oil at two different voltage
levels with 200 sparks.
Figure 6-14 Fault gas generation in the mineral oil at different voltage levels after 200 sparks, Vb-99.9% = 39
kV, 1.5Vb-99.9% = 59 kV, d = 10 mm.
Figure 6-15 Fault gas generation in the synthetic ester liquid at different voltage levels after 200 sparks, Vb-99.9%
= 37 kV, 1.5Vb-99.9% = 56 kV, d = 10 mm.
At the same number of sparks, higher breakdown voltage leads to more fault gas generation
including H2, C2H2 and C2H4. The concentration of H2 under 1.5Vb-99.9% is about 1.70 times
as large as that under Vb-99.9% and C2H2 is about 1.29 times as large as that under Vb-99.9%.
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
Figure 6-15 shows fault gas generation (GIT) in the synthetic ester liquid at different
voltage levels with 200 sparks. Similar to the phenomena in the mineral oil, higher
breakdown voltage results in more fault gas generation in the synthetic ester liquid
including H2, C2H2, C2H4 and CO. The concentration of H2 under 1.5Vb-99.9% is about 1.74
times as large as that under Vb-99.9%, 1.21 times for C2H2, 1.29 times for CO, and 1.19 times
for C2H4.
Figure 6-16 shows the individual fault gases as percentages of total fault gases in the
mineral oil and the synthetic ester liquid at different voltage levels with 200 sparks. It is
clear that H2 and C2H2 are both key gases in the mineral oil and the synthetic ester liquid at
both the tested voltage levels. CO is also an additional key gas in the synthetic ester liquid
which accounts for around 20%. With the increase of voltage levels, the percentage patterns
of fault gas generation almost remain constant.
99.9% VB 1.5 times 99.9% VB
(a). Gemini X – 200 sparks
99.9% VB 1.5 times 99.9% VB
(b). MIDEL 7131 – 200 sparks
Figure 6-16 Individual fault gases as percentages of total fault gases in the mineral oil and the synthetic ester
liquid at different voltage levels with 200 sparks, d = 10 mm.
6.3.5 Correlation between Fault Gas Generation and Fault Energy
The fault gas generation is variable with different number of sparks, gap distances, voltage
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
levels and liquid natures. This raises the need to understand the correlation of fault energy
and fault gas generation.
The previous finding [106] indicated that energy is needed for the breaking of the chemical
bonds which comes from the energy of sparking faults. Figure 6-17 shows the statistical
analysis of fault energy for each spark of one group test with the total spark number of 1170.
It is clear that the fault energy for each spark almost remains constant in both the liquids.
The average energy per spark is about 0.121 J in the mineral oil and 0.111 J in the synthetic
ester liquid.
Figure 6-17 Statistical analysis of fault energy for each spark in the mineral oil and the synthetic ester liquid
at the 10 mm gap distance, totally 1170 sparks.
Figure 6-18 Average energy per spark in the mineral oil and the synthetic ester liquid under different test
conditions.
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
Figure 6-18 shows the average energy per spark in the mineral oil and the synthetic ester
liquid under different test conditions of gap distance, voltage level, and liquid nature. In
terms of different gap distances, larger gap distance results in higher fault energy, which
thus leads to more fault gas generation at the 10 mm gap distance than that at the 5 mm gap
distance. In terms of different voltage levels, higher voltage level also results in higher fault
energy, which thus leads to more fault gas generation at 1.5 times 99.9% VB than that at
99.9% VB. For the liquid nature, higher fault energy in the mineral oil than that in the
synthetic ester liquid was observed at the same test condition, which explains why sparking
fault in the mineral oil generated more fault gases than that in the synthetic ester liquid.
With the known fault energy of each breakdown, the fault gas volumes per unit fault energy
were calculated. Figure 6-19 shows the fault gas volumes per unit fault energy (μL/J) of the
mineral oil and the synthetic ester liquid at the 5 and 10 mm gap distance with different
number of sparks and voltage levels. When the applied spark numbers are between 20 and
100, the results are unstable due to the statistic uncertainty of measurement of low
concentration fault gases. When the applied spark numbers are equal to or above 200, the
gas generation rates almost remain stable. The results indicate that although more sparks
generate more fault gases, the fault gas volumes per unit fault energy almost remain
constant in both liquids. Therefore, in the investigated range of spark numbers, the different
number of sparks does not have an obvious effect on fault gas volumes per unit fault
energy.
Figure 6-19 Fault gas volumes per unit fault energy (μL/J) of the mineral oil and the synthetic ester liquid at
the 5 and 10 mm gap distance with a different number of sparks.
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
Based on the results from 200 sparks onward, fault gas volumes per unit fault energy in
μL/J of the mineral oil and the synthetic ester liquid at different spark numbers, gap
distances and voltage levels are summarised in Table 6-3. At the 10 mm gap distance, fault
gas generation rates at the high voltage level are almost identical to those at the low voltage
level. Fault gas generation rate of H2 and C2H2 in the mineral oil are also similar to those of
the synthetic ester liquid. In addition, it is clear that fault gas generation rates at the 10 mm
gap distance are higher than those at the 5 mm gap distance. It is possibly due to the
different discharge/sparking channel that results in different contact area of the spark in the
oil sample. Also, the limitation of fault energy calculation gives the difficulty of result
analysis, which needs further study of fault energy analysis.
Table 6-3 Fault gas volumes per unit fault energy (μL/J) of the mineral oil and the synthetic ester liquid at
different spark numbers, gap distances and voltage levels.
Gap Distance Voltage Level Gas Average Fault Gas Volumes per Unit Fault Energy (µL/J)
Gemini X MIDEL 7131
10 mm
99.9%
Breakdown
Voltage
H2 7.6812 6.2842
C2H2 6.3843 5.0476
CO - 3.2026
1.5 times
99.9%
Breakdown
Voltage
H2 7.7613 7.8992
C2H2 5.1288 4.7324
CO - 3.2106
5 mm
99.9%
Breakdown
Voltage
H2 0.6251 3.3651
C2H2 3.2435 1.0805
CO - 2.0967
6.4 Summary
This chapter discusses the gassing behaviour of sparking fault in the mineral oil and the
synthetic ester liquid. Two on-line DGA monitors: TM1 hydrogen monitor and TM8 multi-
gas monitor were used to measure dissolved gases in liquids after electrical faults. The three
main topics are a) effect of spark number (from 20 to 500); b) effect of gap distance (5 mm
and 10 mm); c) effect of voltage level (99.9% VB and 1.5 times 99.9% VB).
At the investigated range of sparking faults, the key gases in the mineral oil are H2 and
C2H2, while the key gases in the synthetic ester liquid are H2, C2H2 and CO. The amount of
fault gas generation increases linearly with the number of sparks. However, the number of
sparks does not have an obvious effect on fault gas generation per unit fault energy in µL/J.
At the same gap distance of 10 mm but with a higher applied breakdown voltage, more
fault gases were generated due to the higher injected fault energy, whereas the fault gas
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
generation per unit fault energy in µL/J remained stable. Fault gas generation rates at the 10
mm gap distance are higher than those at the 5 mm gap distance, which is possible due to
difficult underlying mechanisms. At the same test condition, sparking fault in the mineral
oil has more fault gas generation than that in the synthetic ester liquid.
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
139
Chapter 7 Conclusions and Future Work
CHAPTER 7. CONCLUSIONS AND FUTURE WORK
7.1 Conclusions
7.1.1 General
This thesis focuses on pre-breakdown and breakdown performance and gassing behaviour
of the mineral oil and the synthetic ester liquid under DC and impulse voltages by
considering the effects of tip radius, gap distance, voltage waveform, voltage polarity,
liquid nature and different electrical fault levels. Through experimental investigations and
data analyses, the research objectives have been achieved and thus some useful conclusions
and findings have been made.
Research topics covered in this thesis are:
Streamer phenomenon and breakdown properties of transformer liquids under DC
voltages
Effect of tip radius on streamer initiation voltage
Streamer characteristics including current, stopping length, velocity, charge and
shape
Effect of gap distance on breakdown voltage
Streamer characteristics and breakdown phenomena of transformer liquids under
different impulse waveforms
Effect of different impulse waveforms on streamer characteristics
Prediction of breakdown voltage using a mathematical model
Correlations between gas generation and sparking fault in transformer liquids under
lightning impulse voltage
Effect of spark number on fault gas generation
Effect of gap distance on fault gas generation
Effect of voltage level on fault gas generation
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
7.1.2 Summary of Results and Main Findings
In this thesis, the streamer characteristics and breakdown strength of the mineral oil and the
synthetic ester liquid under both positive and negative DC voltages under a divergent field
were studied. Streamer inception voltages with the tip radii of 5 µm, 10 µm, 20 µm and 50
µm and breakdown voltages at various gaps of 2 mm, 5 mm, 10 mm, 20 mm and 30 mm
were tested. Streamer initiation voltages increase with tip radius under both positive and
negative polarities. Streamer initiation voltage of the synthetic ester liquids is slightly lower
than that of the mineral oil. The stopping length of positive streamers gradually increases
with the applied voltage. However, the negative streamer remains in the 1st mode and
barely propagates with the increase of applied voltage, which is different from that was
observed in previous studies under impulse voltage [42, 107]. The apparent charge is
correlated to the streamer length. The correlations between streamer stopping lengths and
apparent charges increase linearly in both the liquids under positive polarity, while a 3-
stage power-law relationship is shown under negative polarity. In the investigated range,
there is no obvious difference in streamer shape between the mineral oil and the synthetic
ester liquid and hence the correlations between streamer stopping lengths and apparent
charges are identical in both liquids. At the same applied voltage level, the streamer in the
synthetic ester liquid propagates faster and further than that in the mineral oil. As a result,
the breakdown voltages of the synthetic ester liquid are lower than those of the mineral oil
at all the gap distances investigated under both polarities.
Pre-breakdown and breakdown characteristics of stopping length, average propagation
velocity, streamer shape and breakdown voltage under positive impulse waveforms with
different tail times (0.8/8 μs, 0.8/14 μs, 0.8/30 μs and 0.8/3200 μs) were investigated in the
mineral oil and the synthetic ester liquid at a gap distance of 10 mm. Compared to the
impulse waveform with longer tail times, the shorter tail-time impulse waveform results in
higher breakdown voltage, but does not have an obvious effect on instantaneous breakdown
voltage and time to breakdown. A mathematical model for breakdown voltage prediction
under impulse waveforms with different tail time has been described. At the same testing
environment and liquid nature, the breakdown voltage of both the mineral oil and the
synthetic ester liquid under any desired impulse waveform characterised by α, β can be
predicted based on Vi and tb from one set of breakdown tests under a known impulse
waveform. At the pre-breakdown stage, it was found that with the similar stopping length,
streamers under a short tail-time impulse waveform have denser branches, while only one
or two main branches are observed under a long tail-time impulse waveform. This is due to
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Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
the higher energy injected into oil sample under a short tail-time impulse waveform, which
encourages the streamer to grow densely.
Fault gas generation in the mineral oil and the synthetic ester liquid was investigated under
various levels of electrical stresses: different spark numbers (from 20 to 500), gap distances
(5 mm and 10 mm) and voltage levels (99.9% VB and 1.5 times 99.9% VB). The key gases in
the mineral oil are H2 and C2H2, while the key gases in the synthetic ester liquid are H2,
C2H2 and CO. The amount of fault gas generation increases linearly with the number of
sparks. However, the number of sparks does not have an obvious effect on fault gas
generation per unit fault energy in µL/J. Spark at a large gap distance has higher fault
energy, which results in more fault gas generation. At the same gap distance but with a
higher applied breakdown voltage, more fault gases were generated due to the higher
injected fault energy, whereas the fault gas generation per unit fault energy in µL/J
remained stable. At the same test condition, sparking fault in the mineral oil has more fault
gas generation than that in the synthetic ester liquid.
7.2 Future Work
In this thesis, some useful conclusions can be made about the dielectric performance and
gassing behaviour of the mineral oil and the synthetic ester liquid under DC and impulse
voltage. However, new questions have arisen and more research work could be done in the
future.
For investigations of streamer phenomena under DC voltage:
i. The work in this thesis focused mainly on streamer propagation and breakdown under
DC voltage. It might also be worth studying the streamers characteristics in mineral oil
and ester liquids under DC bias AC voltages.
ii. The work in this thesis focused mainly on streamer and breakdown study at the room
temperature. However, the oil temperature in transformers is practically higher than
room temperature. It is worth carrying similar experimental study at different oil
temperature.
iii. The results showed that the positive streamer gradually propagates with the applied
voltage, whereas negative streamers barely propagate with applied voltage in both
liquids. This kind of strong polarity effect was not observed in previous streamer
142
Chapter 6 Correlations between Gas Generation and Breakdown in Liquids
studies under AC and impulse voltage. Therefore, it might be worth carrying out
further experiments (e.g. gap distances, hydrostatic pressure) to investigate the
mechanism of negative streamers under DC voltage.
For investigations of streamer and breakdown phenomena under impulse voltage:
i. A mathematical model was developed to predict the breakdown voltage in the mineral
oil and the synthetic ester liquid based on limited experiments. It might be worth
applying this model to larger gap distance and other insulating liquids, e.g. natural ester
and Gas-to-Liquid (GTL).
ii. The work in this thesis focused mainly on positive polarity under impulse voltage. It
might also be worth investigating the relationship between breakdown voltage,
instantaneous breakdown voltage and time to breakdown under negative polarity.
For investigations of gassing behaviour under electrical faults:
i. This part of the study mainly focused on gassing behaviour in liquids under sparking
fault. Due to the insufficient energy of discharge fault generated by the current test
platform, an auto-controlled fault generator with higher voltage level (> 100 kV) is
expected to build up. It is worth investigating the gassing behaviour in liquids under
discharge faults.
ii. The work in this thesis focused mainly on fault gas generation under positive sparking
fault. As the previous finding indicated that the positive discharge relies on electronic
processes and the negative discharge depends more on gaseous processes. It is
interesting to find out the polarity effect on fault gas generation.
143
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Appendix I List of Publications
APPENDIX I LIST OF PUBLICATIONS
Journal Papers:
[1]. J. Xiang, X.Y. Zhou, Q. Liu, Z.D. Wang, J. Hinshaw and P. Mavrommatis,
“Correlation between Hydrogen Generation and Electrical Faults in a Mineral
Transformer Oil”, Electrical Insulation Magazine, IEEE, 2017. (Under 2nd
Review)
[2]. J. Xiang, Q. Liu and Z.D. Wang, “Streamer Characteristic and Breakdown in a
Mineral Oil and a Synthetic Ester Liquid under DC Voltage”, IEEE Transaction on
Dielectric and Electrical Insulation. (Submitted)
[3]. Q. Liu, J. Xiang, Z.D. Wang and O. Lesaint, “Prediction of Breakdown Voltage of
Insulating Liquids under Different Impulse Waveforms”, IEEE Transaction on
Dielectric and Electrical Insulation. (To be Submitted)
International Conference Papers:
[4]. J. Xiang, Q. Liu and Z.D. Wang, “Current and emitted light characteristics of
streamers in insulating liquids under ac voltages”, The 19th International Symposium
on High Voltage Engineering (ISH), Pilsen, Czech Republic, pp. 295, 23-28 August,
2015.
[5]. J. Xiang, Q. Liu and Z.D. Wang, “Inception and breakdown voltages of insulating
liquids under DC stress”. High Voltage Engineering and Application (ICHVE),
Chengdu, China, pp. 1-4, September, 2016.