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INTERPRETATION TECHNIQUE OF PARTIAL DISCHARGE PULSE COUNT
AND SURFACE DEFECT ANALYSIS FOR EVALUATION OF COMPOSITE
MATERIALS
AULIA
UNIVERSITI TEKNOLOGI MALAYSIA
INTERPRETATION TECHNIQUE OF PARTIAL DISCHARGE PULSE COUNT
AND SURFACE DEFECT ANALYSIS FOR EVALUATION OF COMPOSITE
MATERIALS
AULIA
A thesis submitted in partial fulfilment of the
requirements for the award of degree of
Doctor of Philosophy (Electrical Engineering)
Faculty of Electrical Engineering
Universiti Teknologi Malaysia
JUNE 2016
iii
DEDICATION
To my lovely parents who passed away, whose introduced me to Allah and Rasulullah and
Al-Quran and gave me the endless love, trust, constant encouragement over the years during
their lives, and for their prayers.
To my family members, my wife Yessi Anggraini, and my children Jundullah Ilhaq
Aulia, Mujahid Salafi Aulia, and Widad Elqudsi Aulia, for their patience, support,
love, and for enduring the ups and downs during the completion of this PhD thesis.
This thesis is dedicated to them.
iv
ACKNOWLEDGEMENT
I wish to express my deepest appreciation to all those who helped me, in one
way or another, to complete this PhD thesis. First and foremost I thank Allah who
provided me with strength, direction and purpose throughout my PhD thesis. Special
thanks to my supervisor, Professor Dr. Zulkurnain Abdul-Malek, and my co-
supervisor Dr. Yanuar Zulardiansyah Arief, for all their patience, guidance and support
throughout my PhD work duration. Through their expert guidance, I was able to
overcome all obstacles that I encountered in these enduring years. In fact, they always
gave me immense hope every time I consulted them over problems relating to my PhD
study.
v
ABSTRACT
High voltage insulation must be designed in such a way that it is very resistant
to ageing including that from partial discharge (PD). Many studies were previously
carried out on composites based on low density polyethylene (LDPE). However, the
use of natural rubber (NR) and nanosilica (SiO2 nanoparticle) in the LDPE-based
composites are relatively new. Furthermore, the PD resistant performance of the
composites is yet to be extensively researched. It is desired to know the weight
percentage of each component in the LDPE-NR-SiO2 composite, especially the
nanosilica filler weight percentage, for an optimum PD resistant performance. Due to
specific research requirements, a customized laboratory scale PD data acquisition
system (PD-DAQS) is desired to be developed. With the availability of several
parameters obtained from the PD experiment, there is another need to devise an
interpretation tool that can correctly correlate all parameters to the PD resistance. This
work aims to develop a new PD system for measuring, analysing and interpreting PD
signals, which can then be used to determine a new nanocomposite material with high
PD resistance. A new PD-DAQS comprising CIGRE Method II test cell, Picoscope
interfacing device, and LabVIEW based program was successfully developed.
Scanning electron as well as normal microscopes were used for composite sample’s
image capturing and morphological analyses. A new PD interpretation technique
based on PD pulse count and its surface image analysis was obtained and successfully
categorised the PD performance of a given dielectric sample on scores of 1 to 5
corresponding to very bad to very good. PD improvement index and a scoring system
were also introduced and utilised for the new dielectric work. Three groups of new
composites with varying compositions and nanosilica content were made and tested
by applying high voltage stress using the CIGRE Method II test cell for 60 minutes.
Results have shown that the addition of nanosilica filler into LDPE and LDPE-NR
base polymers have increased the PD resistance of the new composites. The highest
PD resistance score of 5 was achieved by the best nanocomposite sample of LDPE-
SiO2 with 4.5 weight percent of nanosilica. Even though the addition of natural rubber
to LDPE matrix has caused a decrease in PD resistance, the addition of 6 weight
percent of nanosilica as fillers in the LDPE-NR (80:20) composite has tremendously
improved its PD resistant performance. The LDPE as well as LDPE-NR based
nanocomposites can be potentially developed as a high voltage insulating material.
The developed PD measuring and interpretation system can also be utilised for future
PD and nanodielectric studies.
vi
ABSTRAK
Penebatan voltan tinggi mesti direkabentuk supaya mempunyai ketahanan
yang tinggi terhadap penuaan termasuk yang disebabkan oleh nyahcas separa. Banyak
kajian ke atas komposit berasaskan polyethylene berketumpatan rendah (LDPE) telah
dilakukan. Walau bagaimanapun, penggunaan getah asli (NR) dan nanosilika (SiO2)
di dalam komposit berasaskan LDPE masih baru. Tambahan pula, prestasi rintangan
PD komposit masih perlu kajian lanjut. Ingin diketahui adalah peratus berat setiap
komponen komposit LDPE-NR-SiO2, terutama peratus pengisi nanosilika bagi
prestasi PD optimum. Disebabkan oleh keperluan penyelidikan khusus, satu sistem
perolehan data (PD-DAQS) berskala makmal perlu dibangunkan. Dengan adanya
beberapa parameter yang diperolehi dari ujikaji PD, terdapat keperluan untuk
menyediakan satu alat penafsiran yang dapat mengaitkan semua parameter dengan
rintangan PD. Tujuan penyelidikan ini adalah untuk membangunkan satu sistem PD
baru untuk mengukur, menganalisis dan mentafsir isyarat PD yang kemudiannya boleh
digunakan untuk mendapatkan satu bahan nanokomposit baru dengan rintangan PD
yang tinggi. Sebuah PD-DAQS baru yang terdiri daripada sel ujian kaedah CIGRE II,
peranti antara muka Picoscope, dan program berasaskan LabVIEW berjaya
dibangunkan. Mikroskop pengimbas elektron dan mikroskop biasa digunakan untuk
mengambil gambar sampel komposit dan untuk analisis morfologi. Satu penafsiran
PD yang baru berdasarkan hitungan denyut PD dan analisis imej permukaan telah
diperolehi dan ianya berjaya mengklasifikasikan prestasi PD sampel dielektrik dengan
skor 1 hingga 5, atau sangat buruk hingga sangat baik. Indeks penambahbaikan PD
dan satu sistem penilaian juga diperkenal dan digunakan dalam penyelidikan dielektrik
baru. Tiga kumpulan komposit baru dengan pelbagai komposisi dan kandungan
nanosilika telah dibuat dan diuji menggunakan voltan tinggi dan sel ujian kaedah
CIGRE II selama 60 minit. Hasil menunjukkan bahawa penambahan pengisi
nanosilika ke dalam polimer asas LDPE dan LDPE-NR telah meningkatkan rintangan
PD komposit baru tersebut. Skor rintangan PD tertinggi 5 dicapai oleh sampel
nanokomposit terbaik iaitu LDPE-SiO2 yang mangandungi 4.5 peratus berat
nanosilika. Walaupun penambahan getah asli ke dalam LDPE telah menyebabkan
penurunan rintangan PD, namun penambahan 6 peratus berat nanosilika sebagai
pengisi dalam komposit LDPE-NR (80:20) telah menambahbaik rintangan PD secara
mendadak. Nanokomposit berasaskan LDPE serta LDPE-NR berpotensi dibangunkan
sebagai bahan penebat voltan tinggi. Sistem pengukuran PD dan sistem tafsiran yang
dibangunkan juga boleh dimanfaatkan untuk kajian PD dan nanodielektrik pada masa
depan.
vii
TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES xii
LIST OF FIGURES xiv
LIST OF ABBREVIATIONS xxiii
LIST OF SYMBOLS xxiv
1 INTRODUCTION 1
1.1 Preliminaries 1
1.2 Research Problem Statement 4
1.3 Research Objectives 5
1.4 Research Scopes 6
1.5 Research Contribution 6
1.6 Research Significance 9
1.7 Thesis Organization 9
2 LITERATURE REVIEW 11
2.1 Introduction 11
2.2 Partial Discharge Detection and Data Acquisition System 11
2.3 Partial Discharge Interpretation Techniques 16
2.3.1 Partial Discharge Pulse Count 16
viii
2.3.2 Surface Imaging 19
2.3.2.1 Image Histogram Method 20
2.3.2.2 Image Quantifying Method 21
2.3.2.3 Machine Vision 22
2.3.3 PD Improvement Index 23
2.3.4 Scoring System 23
2.4 Polymer Insulator 26
2.4.1 Role of Nanofiller 27
2.4.2 Polymeric Materials with Nanosilica 28
2.4.3 Partial Discharge Resistance of
Nanocomposite 31
2.5 Summary 32
3 RESEARCH METHODOLOGY 34
3.1 Introduction 34
3.2 Partial Discharge Measurement and Data Acquisition
System 35
3.2.1 Partial Discharge Measurement Procedure and
Data-Acquisition System Set-up 36
3.2.2 Partial Discharge Experimental Set-up 38
3.2.3 Hardware Specification of PD-DAQS 39
3.2.4 Schematic Diagram of the DAQS for PD
Measurement 40
3.2.5 PD Waveform 42
3.2.6 Noises and Power Supply Signals 43
3.2.7 Charge Calibration 44
3.2.8 Filtering the Partial Discharge Signals 44
3.2.9 PD Data Recording 45
3.2.10 Sub VI to Detect the PD Pulses 46
3.3 PD Interpretation Using PD Pulse Count and Surface
Image 48
3.3.1 Partial Discharge Pulse Count Analysis 48
3.3.2 Statistical Analysis 49
3.3.2.1 Mean 50
ix
3.3.2.2 Standard Deviation 50
3.3.2.3 Skewness 51
3.3.2.4 Kurtosis 51
3.3.3 Surface Defect Analysis 52
3.3.3.1 NI Vision Assistant 54
3.3.3.2 Image Histogram Analysis 56
3.3.3.3 Image Quantifying Analysis 58
3.3.4 PD Improvement Index 58
3.3.5 The Scoring System 59
3.4 Design of a New Composite 63
3.4.1 Sample Composition 63
3.4.2 Sample Blending Process 64
3.4.3 Sample Moulding 67
3.4.4 CIGRE Method II 68
3.4.5 New Dielectric Sample Analyses 69
3.5 Summary 71
4 RESULTS AND DISCUSSION 72
4.1 Introduction 72
4.2 Development of New Partial Discharge and Data
Acquisition System 73
4.2.1 Noises Elimination 74
4.2.2 Charge Calibration 76
4.2.3 Sampling Rate Effect 78
4.3 A New PD Data Interpretation Technique Using PD
Pulse Count and Surface Image 81
4.3.1 PD Pulse Count Analysis 81
4.3.2 Surface Defect Analysis 83
4.3.3 PD Improvement Index Analysis 84
4.3.4 Scoring Analysis 85
4.4 New Composite Material 85
4.4.1 PD Pulse Count Characteristics of LDPE-NR
Composites 86
4.4.1.1 Cumulative PD Pulse Count 86
x
4.4.1.2 Average Positive and Negative PD
Pulse Count 88
4.4.1.3 Positive and Negative PD Pulse
Count per Cycle 89
4.4.1.4 Total PD Pulse Count 90
4.4.2 PD Pulse Count Characteristics of LDPE-SiO2
Composites 92
4.4.2.1 Cumulative PD Pulse Count 93
4.4.2.2 Average Positive and Negative PD
Pulse Count 94
4.4.2.3 Positive and Negative PD Pulse Count
Pattern 95
4.4.2.4 Total PD Pulse Count 97
4.4.3 PD Pulse Count Characteristics of LDPE-NR-SiO2
Composites 97
4.4.3.1 Cumulative PD Pulse Count 98
4.4.3.2 Average Positive and Negative PD
Pulse Count 99
4.4.3.3 Positive and Negative PD Pulse Count
per Cycle 100
4.4.3.4 Total PD Pulse Count 102
4.4.3.5 Overall Pulse Count Analysis 102
4.4.4 Surface Image Analysis 105
4.4.4.1 Scanning Electron Macroscopic
Analyses 105
4.4.4.2 Surface Morphology Analysis Using
NI Vision Assistant 110
4.4.4.3 Surfaces Images Histogram Analysis 122
4.4.4.4 Percentage of Eroded Area Using the
Histogram Method 127
4.4.4.5 The Percentage of Eroded Area Using
Quantifying Method 130
4.4.5 PD Improvement Index 136
4.4.6 Scoring System for Composites Material
xi
Assessment 137
4.4.7 Comparison of Histogram and Quantifying Method 143
4.5 Summary 145
5 CONCLUSION AND FUTURE WORK 147
5.1 Conclusion 147
1. Partial Discharge Data Acquisition System 147
2. A New Partial Discharge Interpretation Tool 148
3. New Composite Material 149
5.2 Future Work 150
REFFERENCES 151
APPENDIX A 168
xii
LIST OF TABLES
TABLE NO. TITLE PAGE
2.1 Partial discharge testing related to the insulation condition 18
2.2 Various evaluation techniques and their respective
objectives 24
2.3 The evaluated criteria and its related result of different
composite 25
2.4 The summary of the base polymer and the filler
percentage of the nanosilica 29
3.1 PC and DAQS specification 39
3.2 The score rule for PD count analysis and PoEA analysis
using histogram and quantifying methods 61
3.3 The score rule of the item in each category 62
3.4 Samples prepared for three different categories 63
3.5 The picture of new dielectric samples prepared for three
different groups 70
4.1 Memory used and numbers of files could be saved every
minute for different sampling rates 80
4.2 Pulse waveform in different sampling rates 80
4.3 Template for image histogram analysis of sample N before
and after the high voltage stress 83
4.4 Percentage of eroded area and corresponding histogram
for sample N after the high voltage stress 84
4.5 The blank score distribution of LDPE-NR-SiO2 composite
samples 85
4.6 LDPE-NR composites intensity histogram of sample A1
(100:0) and sample A3 (80:20) before and after 60
minutes high voltage stress 123
xiii
4.7 LDPE-SiO2 composites histogram of samples B1 (100:0),
B2, B3 and B4 (80:20) before and after 60 minutes of high
voltage stress 124
4.8 Histogram of samples C1, C2, C3 and C4 before and after
60 minutes of high voltage stress 126
4.9 Percentage of eroded area and related histogram of LDPE-
NR composites of sample A1 (100:0) and A3 (80:20) after
60 minutes high voltage stress 131
4.10 Percentage of defect area and histogram for samples B1
(100:1.5), B2 (100:3), B3 (100:4.5) and B4 (100:6) after
60 minutes of high voltage stress 132
4.11 Percentage of defect area and histogram of samples C1,
C2, C3 and C4 after 60 minutes of high voltage stress 134
4.12 The score distribution of LDPE-NR-SiO2 composite
samples after 60 minutes of high voltage stress; A1
(100:0:0), A3 (80:20:0), B1 (100:0:1.5), B2 (100:0:3), B3
(100:0:4.5), B4 (100:0:6), C1 (80:20:1.5), C2 (80:20:3),
C3 (80:20:4.5) and C4 (80:20:6) 139
xiv
LIST OF FIGURES
FIGURE NO. TITLE PAGE
3.1 PD experimental procedure using PD-DAQS 37
3.2 Test set-up for the partial discharge test 38
3.3 Picoscope 5203 series [163] 40
3.4 (a) Schematic of PD-DAQS, (b) graphical user interface
(GUI) unit of PD-DAQS using LabVIEW 41
3.5 Typical parameters describing the shape of an idealized
PD pulse [164] 42
3.6 High pass filter Sub VI used in the program to digitally
filter the input signals to extract the PD pulses 45
3.7 The file formats and conditions to create a new file 46
3.8 (a) Peak detector SubVI and (b) Statistics SubVI 47
3.9 The PD pulse count analysis flow chart 49
3.10 Positive and negative skewness 51
3.11 An illustration of kurtosis; (a) negative kurtosis and (b)
positive kurtosis 52
3.12 The flow chart for the image analysis 53
3.13 (a) The general steps in designing an NI Vision
application, (b) Inspection steps for building a Vision
application 56
3.14 The steps to calculate PoEA using QM 58
3.15 The flow chart for the scoring system 60
3.16 Schematic flow diagram of the mixing process in
preparing the LDPE-NR composite, nanosilica LDPE
composite, and nanosilica LDPE-NR composite 65
3.17 (a) Conceptual view of rubber mill rolls, (b) The actual
picture of two roll mill 66
xv
3.18 Hot press machine for molding process 67
3.19 The mould for the ClGRE Method II electrode cell. (a) A
detailed dimension of the mould, and (b) the actual mould. 68
3.20 Structure of the ClGRE method II test cell 69
4.1 (a) PD and 50 Hz power supply signals, and (b) zoom out
view of corresponding PD signal waveform. 75
4.2 (a) PD signal [176] and input voltage (kV), (b) PD signal
waveform 76
4.3 Charge calibration for Picoscope reading. (a) Positive
charge, and (b) negative charge 77
4.4 Effect of sampling rate to PD pulse waveform; (a) 1 MS/s,
(b) 42 MS/s, (c) 125 MS/s, (d) 250 MS/s and (e) 500 MS/s 79
4.5 Four templates for PD pulse count analysis; (a) cumulative
PD pulse count, (b) average PD pulse count, (c) PD pulse
count pattern, and (d) total PD pulse count. 82
4.6 PD improvement index (A-PD Index) 84
4.7 Cumulative positive and negative PD pulse counts for
different LDPE-NR compositions over 60 minutes stress
duration; (a) sample A1 (100:0), (b) sample A2 (90:10),
(c) sample A3 (80:20), and (d) sample A4 (70:30) 87
4.8 Average positive and negative PD pulse count over 10
minutes stress duration for six consecutive cycles (60
minutes total stress duration) for different LDPE-NR
composites; (a) sample A1 (100:0), (b) sample A2 (90:10),
(c) sample A3 (80:20), (d) sample A4 (70:30) 88
4.9 Comparison of average PD pulse count over 10 minutes
stress duration for six consecutive cycles (60 minutes total
stress duration) of different LDPE-NR composites; sample
A1 (100:0), sample A2 (90:10), sample A3 (80:20),
sample A4 (70:30). (a) positive PD and (b) negative PD 89
4.10 The comparison of average PD pulse count for every 10
minutes stress duration for six consecutive cycles (60
minutes total stress duration) of different LDPE-NR
composites; sample A1 (100:0), sample A2 (90:10),
xvi
sample A3 (80:20), sample A4 (70:30). (a) positive PD
and (b) negative PD 90
4.11 Positive and negative PD pulse counts of different LDPE-
NR compositions over 60 minutes stress duration; sample
A1 (100:0), sample A2 (90:10), sample A3 (80:20),
sample A4 (70:30) 91
4.12: The difference of positve and negative PD pulse counts of
different LDPE-NR composition over 60 minutes stress
duration; sample A1 (100:0), sample A2 (90:10), sample
A3 (80:20), sample A4 (70:30) 91
4.13 Total PD pulse counts of different LDPE-NR composition
over 60 minutes stress duration; sample A1 (100:0),
sample A2 (90:10), sample A3 (80:20), sample A4 (70:30) 92
4.14 Cumulative PD pulse count for various samples of LDPE-
SiO2 composites over 60 minutes stress duration; (a)
sample B1 (100:1.5), (b) sample B2 (100:3), (c) for
sample B3 (100:4.5), and (d) for sample B4 (100:6) 93
4.1 Average positive and negative PD pulse count over 10
minutes stress duration for six consecutive cycles for
various samples of LDPE-SiO2 composites; (a) sample B1
(100:1.5), (b) sample B2 (100:3), (c) for sample B3
(100:4.5) and (d) for sample B4 (100:6) 94
4.16 The comparison of average PD pulse count for every
sample over 10 minutes stress duration for six consecutive
cycles of various samples of LDPE-SiO2; sample B1
(100:1.5), sample B2 (100:3), sample B3 (100:4.5) and
sample B4 (100:6). (a) Positive PD and (b) Negative PD 95
4.17 Comparison of PD pulse count over 10 minutes stress
duration for six consecutive cycles for various samples of
LDPE-SiO2; sample B1 (100:1.5), sample B2 (100:3),
sample B3 (100:4.5) and sample B4 (100:6), (a) positive
PD, and (b) negative PD 96
4.18: The difference between positive PD and negative PD pulse
counts of various samples of LDPE-SiO2 composite,
xvii
sample B1 (100:1.5), sample B2 (100:3), sample B3
(100:4.5) and sample B4 (100:6) 96
4.19 The total PD pulse counts for various samples of LDPE-
SiO2 composite, sample B1 (100:1.5), sample B2 (100:3),
sample B3 (100:4.5) and sample B4 (100:6) 97
4.20 The cumulative PD pulse count for various samples of
LDPE-NR-SiO2 composite; (a) sample C1 (80:20:1.5), (b)
sample C2 (80:20:3), (c) sample C3 (80:20:4.5) and (d)
sample C4 (80:20:6) 98
4.21 Average positive and negative PD pulse count over 10
minutes stress duration for six consecutive cycles of
various samples of LDPE-NR-SiO2; (a) sample C1
(80:20:1.5), (b) sample C2 (80:20:3), (c) sample C3
(80:20:4.5) and (d) sample C4 (80:20:6) 99
4.22 Comparison of average PD pulse count for every sample
over 10 minutes high voltage stress duration for six
consecutive cycles of various samples of LDPE-NR-SiO2;
sample C1 (80:20:1.5), sample C2 (80:20:3), sample C3
(80:20:4.5) and sample C4 (80:20:6). (a) positive PD and
(b) negative PD 100
4.23 The average PD pulse count for every 10 minutes high
voltage stress duration for six consecutive cycles of
various samples of LDPE-NR-SiO2; sample C1,
(80:20:1.5), sample C2 (80:20:3), sample C3 (80:20:4.5)
and sample C4 (80:20:6). (a) positive PD and (b) negative
PD 101
4.24 The difference of positive PD pulse count and negative PD
pulse count of different samples of LDPE-NR-SiO2
composite; sample C1 (80:20:1.5) ,sample C2 (80:20:3),
sample C3 (80:20:4.5) and (e) sample C4 (80:20:6) 101
4.25 The total PD pulse count of different samples of LDPE-
NR-SiO2 composites; sample C1 (80:20:1.5), sample C2
(80:20:3), sample C3 (80:20:4.5) and (e) sample C4
(80:20:6) 102
xviii
4.26 The summary of the positive and negative PD pulse counts
of LDPE-NR-SiO2 composites for all samples after 60
minutes high voltage stress applied to them; A1 (100:0:0),
A2 (90:10:0), A3 (80:20:0), A4 (70:30:), B1 (100:0:1.5),
B2 (100:0:3), B3 (100:0:4.5), B4 (100:0:6), C1
(80:20:1.5), C2 (80:20:3), C3 (80:20:4.5) and C4
(80:20:6). (a) positive PD pulse count and, (b) negative
PD pulse count 103
4.27 Total PD pulse counts of LDPE-NR-SiO2 composites for
all samples after 60 minutes high voltage stress; A1
(100:0:0), A2 (90:10:0), A3 (80:20:0), A4 (70:30:), B1
(100:0:1.5), B2 (100:0:3), B3 (100:0:4.5), B4 (100:0:6),
C1 (80:20:1.5), C2 (80:20:3), C3 (80:20:4.5) and C4
(80:20:6) 104
4.28 SEM images of LDPE-SiO2 in different compositions
prior to being stressed with the 500 and 20,000
magnification respectively, (a1) and (a2) for sample B1
(100:1.5), (b1) and (b2) for sample B2 (100:3), (c1) and
(c2) for sample B3 (100:4.5) and (d1) and (d2) for sample
B4 (100:6) 106
4.29 SEM cross sectional images of LDPE-SiO2 in different
compositions prior to being stressed; (a) sample B1
(100:1.5), (b) sample B2 (100:3), (c) for sample B3 107
4.30 SEM images of LDPE-NR-SiO2 in different compositions
prior to being stressed with the precision of 500 Mag and
20,000 Mag respectively, (a1) and (a2) for sample C1
(80:20:1.5) , (b1) and (b2) for sample C2 (80:20:3), (c1)
and (c2) for sample C3 (80:20:4.5) and (d1) and (d2) for
sample C4 (80:20:6) 109
4.31 SEM Cross section images of LDPE-NR-SiO2 in different
compositions prior to being stressed, (a) sample C1
(80:20:1.5) , (b) sample C2 (80:20:3), (c) sample C3
(80:20:4.5) and (d) sample C4 (80:20:6) 110
xix
4.32 The surface images of LDPE-NR composite sample A1
(100:0) before and after 60 minutes high voltage stress; (a)
and (b) are the original image, (c) and (d) are a texture
image two-dimensional (2D), (e) and (f) are a texture
image three-dimension (3D) 112
4.33 The surface images of LDPE-NR composite sample A3
(80:20) before and after 60 minutes high voltage stress; (a)
and (b) are the original image, (c) and (d) are a texture
image two-dimensional (2D), (e) and (f) are a texture
image three-dimension (3D) 113
4.34 The surface images of LDPE-SiO2 composite sample B1
(100:1.5) before and after 60 minutes high voltage stress;
(a) and (b) are the original image, (c) and (d) are a texture
image two-dimensional (2D), (e) and (f) are a texture
image three-dimension (3D) 114
4.35 The surface images of LDPE-SiO2 composite sample B2
(100:3) before and after 60 minutes high voltage stress; (a)
and (b) are the original image, (c) and (d) are a texture
image two-dimensional (2D), (e) and (f) are a texture
image three-dimension (3D) 115
4.36 The surface images of LDPE-SiO2 composites sample B3
(100:4.5) before and after 60 minutes high voltage stress;
(a) and (b) are the original image, (c) and (d) are a texture
image two-dimensional (2D), (e) and (f) are a texture
image three-dimension (3D) 116
4.37 The surface images of LDPE-SiO2 composites sample B4
(100:6) before and after 60 minutes high voltage stress; (a)
and (b) are the original image, (c) and (d) are a texture
image two-dimensional (2D), (e) and (f) are a texture
image three-dimension (3D) 117
4.38 The surface images of LDPE-NR-SiO2 composite sample
C1 (80:20:1.5) before and after 60 minutes high voltage
stress; (a) and (b) are the original image, (c) and (d) are a
xx
texture image two-dimensional (2D), (e) and (f) are a
texture image three-dimension (3D) 119
4.39 The surface images of LDPE-NR-SiO2 composites
sample C2 (80:20:3) before and after 60 minutes high
voltage stress; (a) and (b) are the original image, (c) and
(d) are a two-dimensional texture image (2D), (e) and (f)
are a three-dimension of texture image (3D) 120
4.40 The surface images of LDPE-NR-SiO2 composites sample
C3 (80:20:4.5) before and after 60 minutes high voltage
stress; (a) and (b) are the original image, (c) and (d) are a
texture image two-dimensional (2D), (e) and (f) are a
texture image three-dimension (3D) 121
4.41 The surface images of LDPE-NR-SiO2 composites sample
C4 (80:20:6) before and after 60 minutes of high voltage
stress; (a) and (b) are the original image, (c) and (d) are
the texture image two-dimensional (2D), (e) and (f) are the
texture image three-dimension (3D) 122
4.42 Percentage change of key parameters for LDPE-NR-SiO2
samples after 60 minutes of high voltage stress; A1
(100:0:0), A3 (80:20:0), B1 (100:0:1.5), B2 (100:0:3), B3
(100:0:4.5), B4 (100:0:6), C1 (80:20:1.5), C2 (80:20:3),
C3 (80:20:4.5) and C4 (80:20:6) 128
4.43 The percentage of eroded area using histogram approach
of LDPE-NR-SiO2 composite after 60 minutes high
voltage stress; A1 (100:0:0), A3 (80:20:0), B1 (100:0:1.5),
B2 (100:0:3), B3 (100:0:4.5), B4 (100:0:6), C1
(80:20:1.5), C2 (80:20:3), C3 (80:20:4.5) and C4 (80:20:6) 129
4.44 (a) Changes of eroded area of samples B1 (100:0:1.5), B2
(100:0:3), B3 (100:0:4.5), B4 (100:0:6) with respect to the
reference sample A1 (100:0:0), and (b) Changes of eroded
area of samples C1 (80:20:1.5), C2 (80:20:3), C3
(80:20:4.5) and C4 (80:20:6) with respect to the reference
sample A3 (80:20). The histogram method was used for
the computation. 130
xxi
4.45 The percentage of eroded area using quantifying method
of LDPE-NR-SiO2 composite after 60 minutes high
voltage stress; A1 (100:0:0), A3 (80:20:0), B1 (100:0:1.5),
B2 (100:0:3), B3 (100:0:4.5), B4 (100:0:6), C1
(80:20:1.5), C2 (80:20:3), C3 (80:20:4.5) and C4 (80:20:6) 135
4.46 (a) Changes of eroded area for samples B1 (100:0:1.5), B2
(100:0:3), B3 (100:0:4.5), B4 (100:0:6) with respect to the
reference sample A1 (100:0:0), and (b) Changes of eroded
area for samples C1 (80:20:1.5), C2 (80:20:3), C3
(80:20:4.5) and C4 (80:20:6) with respect to the reference
sample A3 (80:20). The quantifying method was used for
the computation. 136
4.47 PD improvement index of LDPE-SiO2 composites samples
refer to the sample A1 as reference and PD improvement
index of LDPE-NR-SiO2 refer to sample A3 after 60
minutes high voltage stress; A1 (100:0:0), A3 (80:20:0),
B1 (100:0:1.5), B2 (100:0:3), B3 (100:0:4.5), B4
(100:0:6), C1 (80:20:1.5), C2 (80:20:3), C3 (80:20:4.5)
and C4 (80:20:6) 137
4.48 The score distribution for 20 items from 6 categories of
LDPE-NR-SiO2 composite sample after 60 minutes high
voltage stress; A1 (100:0:0), A3 (80:20:0), B1 (100:0:1.5),
B2 (100:0:3), B3 (100:0:4.5), B4 (100:0:6), C1
(80:20:1.5), C2 (80:20:3), C3 (80:20:4.5) and C4 (80:20:6) 140
4.49 The number of score for each sample of LDPE-NR-SiO2
composite after 60 minutes high voltage stress; A1
(100:0:0), A3 (80:20:0), B1 (100:0:1.5), B2 (100:0:3), B3
(100:0:4.5), B4 (100:0:6), C1 (80:20:1.5), C2 (80:20:3),
C3 (80:20:4.5) and C4 (80:20:6) 141
4.50 The total score of LDPE-NR-SiO2 composite after 60
minutes high voltage stress; A1 (100:0:0), A3 (80:20:0),
B1 (100:0:1.5), B2 (100:0:3), B3 (100:0:4.5), B4
(100:0:6), C1 (80:20:1.5), C2 (80:20:3), C3 (80:20:4.5)
and C4 (80:20:6) 141
xxii
4.51 The qualitative score of LDPE-NR-SiO2 composite after
60 minutes high voltage stress; A1 (100:0:0), A3
(80:20:0), B1 (100:0:1.5), B2 (100:0:3), B3 (100:0:4.5),
B4 (100:0:6), C1 (80:20:1.5), C2 (80:20:3), C3 (80:20:4.5)
and C4 (80:20:6) 142
4.52 The comparison between the results of histogram method
and quantifying method calculating the percentage of
eroded area (PoEA) of LDPE-NR-SiO2 composite after 60
minutes high voltage stress; A1 (100:0:0), A3 (80:20:0),
B1 (100:0:1.5), B2 (100:0:3), B3 (100:0:4.5), B4
(100:0:6), C1 (80:20:1.5), C2 (80:20:3), C3 (80:20:4.5)
and C4 (80:20:6) 143
4.53 The difference between the results of histogram method
and quantifying method in calculating the percentage of
eroded area of LDPE-NR-SiO2 composite after 60 minutes
high voltage stress; A1 (100:0:0), A3 (80:20:0), B1
(100:0:1.5), B2 (100:0:3), B3 (100:0:4.5), B4 (100:0:6),
C1 (80:20:1.5), C2 (80:20:3), C3 (80:20:4.5) and C4
(80:20:6) 144
xxiii
LIST OF ABBREVIATIONS
ADC - analog-digital converter
CAD - computer aided design
CNR - contrast-to-noise ratio
DAQS - data acquisition system
LDPE - low density polyethylene
GS/s - giga sampling per second
IEC - International Electrotechnical Commission
IHM - image histogram method
IQM - image quantifying method
NR - natural rubber
PC - personal computer
PD - partial discharge
PD-DQAS - partial discharge and data acquisition system
PE - polyethylene
PoEA - percentage of eroded area
PP - polypropylene
RGB - red, green and blue
SEM - scanning electron microscope
SiO2 - nanosilica
XLPE - cross-linked polyethylene
xxiv
LIST OF SYMBOLS
S - standard deviation
V - voltage
q - charge
i - current
I - average discharge current
n - repetition rate
P - discharge power
y - filtered sequence
h - FIR filter coefficient
CHAPTER 1
INTRODUCTION
1.1 Preliminaries
A partial discharge (PD) is a dielectric phenomenon that occurs in solid or
liquid insulating materials. It is said to have occurred when an electrical pulse or
discharge partially bridges a gas void (whether they are adjacent or not). A PD is
usually initiated inside voids, cracks, or inclusions within a solid dielectric or at
conductor-dielectric interfaces within solid or liquid dielectrics or in bubbles within
liquid dielectrics [1]. PD can also occur along the surface of solid insulating materials
if the surface tangential electric field is high enough to cause a breakdown along the
insulator surface. This dielectric phenomenon only partially bridges the gap between
phase insulation and ground or between phase-to-phase insulation. Thus the PD
phenomenon could be used as an effective assessment technique in maintenance and
monitoring of electrical distribution equipment conditions [2-5].
However, despite possessing good electrical properties, polymer insulation
materials have been observed to have low PD resistance. Furthermore, the voids and
defects in the structure of polymer, which are usually because of imperfections in the
production process, contribute to accelerating the ageing of the material. The
dimension of voids could be in nanometer or micrometer size in which the gases are
trapped. It is in these voids in which ionizations are most likely to start under any high
electric field stress.
2
Partial discharge measurement is a well-known and promising method for
condition assessment of insulation systems in high voltage equipment. It is used for
preventive maintenance for assessing the medium voltage motors and cable joints [6-
8]. The partial discharge measurements represent a universally accepted technique
which gives the indication or status of the insulation. Partial discharge analysis is a
noninvasive or nondestructive technique which monitors the integrity of the insulation.
The discharge analysis is an efficient means used by stake holders to avoid insulation
failures in electrical equipment and to reduce their maintenance costs. The analysis is
used to identify deterioration mechanisms in insulation systems such as slot discharge,
internal voids, wedge and winding looseness. These problems need to be monitored
and corrected because their individual failures can lead to catastrophic failures of the
system.
Under high voltage stress, the insulation material suffers from the electron
bombardment that causes defect to the insulation surface gradually. The level of the
defect depends on internal and external factors. The internal factors are such as the
insulation quality and the behavior of the insulation to the high voltage stress. The
external factors are like the level of the high voltage stress and the electrode shape.
The size of surface damage varies in size from micrometer, centimeter, or the complete
damage of the surface. By looking at the picture of the defected area, the affected area
can be analyzed with image histogram analysis.
Surface defect is a common problem facing the industrial process. The defect
is identified by the abnormality of texture and colour. The surface defect detection is
a common process in an industrial engineering to detect the inferior product. The
defect detection process can be done automatically using a high-resolution camera
supported by a suitable software, but the basic defect detection is done by human visual
inspection supported by image-processing software [9]. This technique was used to
detect the texture defect of the surface like metal, wood, steel, wafer and even for non-
flat object like fruits and aircraft surfaces [10].
Nowadays the nanofillers are gaining interest from researchers to get a better
polymeric insulator due to its promising properties [11-13]. The improvement of
3
nanofiller filled polymeric insulator is due to very small size of the filler and hence
very large interfacial surface between the polymer and the filler [14-16], and the
behavior of the nanocomposite molecule inside the polymer [17, 18].
Undeniably, the addition of nanofillers in polymer matrices has resulted in
enhanced partial discharge resistance of the polymeric nanocomposite. It was found
that the addition of silica nanofiller to the epoxy resin improved the PD resistance of
the composite material [19]. It is also highlighted that epoxy/clay composite is
superior in partial discharge resistance compared to base epoxy resins. The conclusion
was drawn based on the evaluation on the depth of erosion. At the same time, the
research on the surface discharge degradation of metal oxide nano-filled epoxy showed
that nanoalumina and nanotitania filled epoxy had considerably improved its
resistance to surface discharges [15]. These facts show that there are potential
applications of nanocomposites in electrical insulation [20].
Although there are many experimental techniques that were used to determine
partial discharge characteristics of composites, a comprehensive method to evaluate
the PD resistance is still a big challenge. It is true that the PD phenomenon in
composites was extensively studied and different PD models were also introduced by
many researchers to understand the PD phenomenon. However, in order to choose the
best performance of the composite samples, there is no evaluation technique yet
proposed to rank the composite samples based on the PD data. Due to the complicated
nature of composite materials, there is no single characterization method that can
reveal the complete characteristics of composite materials. It is true that the
breakdown test can be taken as one characteristic of the composite materials, but the
information from the test is very general and cannot explain the physical
characteristics or the internal structures of materials [21].
A complete and expensive partial discharge data acquisition system (PD-
DAQS) is mostly used by the electrical power companies [6-8]. In contrary, only a
limited number of PD-DAQS are owned by other institutions, for example, a testing
laboratory. The limited market for PD-DAQS makes the price of the PD-DAQS to be
relatively on the high side for laboratories that intend to purchase. A comprehensive
4
PD-DAQS is a basic necessity for any study on partial discharge characteristics in a
laboratory environment.
Equally important for PD studies, especially when trying to determine the
performance of a new composite or dielectric is the interpretation technique of the
massive collected PD data. The interpretation should not be based on just PD
magnitude but rather on other PD characteristics and PD related changes on the studied
materials such as the morphological profiles of the materials.
1.2 Research Problem Statement
Experimental work is vital in PD studies, and hence the availability of complete
experimental setups is a must for a successful research. One key setup for PD studies
is the partial discharge data acquisition system (PD-DAQS). Because of specific
research requirements, a customized laboratory scale PD-DAQS may incur a high
procurement cost, especially when the facility itself is usually sold in a limited market.
It is therefore desired to develop a custom-made PD-DAQS that suits the objectives of
this research.
The partial discharge pulse count is one of PD characteristics normally
measured in PD experiments and then used to reflect how resistant a material is to a
PD activity, based on the correlation between the PD pulse count and the PD resistance
of a given material. However, the use of just the PD pulse count to decide on the PD
resistant property of a material is not sufficient. There is a need to also consider other
PD characteristics apart from the PD pulse count, such as images of material surface
defects caused by PD. In fact, because of the massive amount of data captured in
typical PD experiments, the PD pulse count itself need to be statistically analysed to
give several representative parameters before it can be used to determine the PD
resistance. With the availability of several parameters obtained from the PD
experiment, there is another need to devise an interpretation tool that can correctly
correlate all parameters to the PD resistance. Furthermore, the tool also need to be
5
validated. The interpretation tool is vital when designing a new dielectric material with
an improved PD resistance.
Studies had been carried out to obtain new materials with high PD resistance.
These materials include low density polyethylene (LDPE), natural rubber (NR), and
their composites. Attempts had also been made to also include micro and nano fillers,
such as nanosilica. Even though some other studies were previously carried out using
LDPE-NR-SiO2 composites, the PD resistant performance of the composites is yet to
be extensively researched. In particular, it is desired to know the weight percentage
of each component in the LDPE-NR-SiO2 composite, especially the nanosilica filler
weight percentage, for an optimum PD resistant performance.
1.3 Research Objectives
This research work was undertaken to address the above problem statements.
It aims to provide a comprehensive PD measuring and assessment system so that a
new and improved LDPE based composite material can be found. To achieve this aim
the following objectives of the research are listed.
1. To develop a partial discharge data acquisition system (PD-DAQS) to
acquire, store and analyze the PD data.
2. To develop and validate new partial discharge interpretation techniques
based on PD pulse count and percentage of eroded area.
3. To determine a new low density polyethylene (LDPE) and natural rubber
(NR) based nanocomposites with superior PD resistance performance.
6
1.4 Research Scopes
The study carried out in this work was limited to several scopes described
below.
The partial discharge measurements were carried out using CIGRE Method II
set-up which was suitable for small samples of dielectrics. Therefore all measured PD
data such as PD pulse counts were based on this set-up with specified PD inception
voltage levels. All other requirements of the set-up including noise elimination and
equipment calibration were observed.
The morphological analyses carried out in this work are limited by the capacity
of available microscopes including the SEM. The materials used for the determination
of the new dielectric are limited to LDPE and natural rubber. Only nanosilica was
used as the filler material. Sample preparation and finishing techniques are as
described in this thesis.
1.5 Research Contribution
Major contributions of this study are as listed below.
1. Development of a partial discharge data acquisition system (PD-
DAQS)
Partial discharge data acquisition (PD-DAQ) systems are available in
different scales and for different purposes. However, most of them suffer
from many disadvantages such as the lack of memory capacity for long
data storage. In this study, a mobile, economical and customizable partial
discharge data acquisition system (PD-DAQS) suitable for laboratory scale
work had been successfully developed. The developed PD-DAQS
successfully handled huge PD data in a much shorter time compared to the
7
alternative MS Office Excel based system and with no memory crash
problem.
2. PD data interpreting techniques by using different methods as listed
below.
a. An improved PD pulse count analysis
PD pulse count is one important property of PD characteristics and was
used for a long time to justify the PD resistance of an insulation. PD pulse
count represents the frequency of PD occurrence during a period of time.
In the usual statistical method, the PD pulse count was mostly presented
together with PD phase and PD amplitude to form the well-known PD
pattern graph. In this research work, the PD pulse count was successfully
evaluated using a statistical approach by taking the advantage of histogram
parameters, namely, mean, standard deviation, kurtosis, and skewness. It
was clearly seen that the PD pulse count for each sample was different in
each interval of time reflecting the random behavior of the sample response
to the electric field stress.
b. The surface defect analysis
The surface image analysis using two main processing techniques called
the image histogram and image quantifying methods to calculate the
percentage of eroded area (PoEA) was successfully evaluated. Both
methods were successfully applied to identify the percentage of the eroded
area (PoEA) of the samples.
c. Correlation between PD pulse count and surface defects
PD phenomenon causes defect on the insulation material including its
surface. Until now, the correlation between PD events and related surface
defects are still not widely discussed in depth. In this study, a correlation
analysis between the PD pulse count and the corresponding surface defect
had been presented. It was found that there is a strong correlation between
8
the frequencies of the pulse count or frequency of PD, which reflects the
higher value of PoEA.
d. An Improved PD resistance index (APD index)
The PD improvement (APD Index) of the composite can help the
researchers to identify the contribution of nanofillers whether positive or
negative compared to the base of the polymer. In this study, an improved
PD resistance index (APD index) to analyze the PD resistance had been
developed and proposed.
e. A Scoring System
To evaluate the performance of groups of composite samples, a
comprehensive technique is needed. A scoring system was developed by
considering the result of PD pulse count analysis, and combining with the
results of PD pulse count and image analyses. The PD pulse count, the
image histogram, and PoEA were considered to give the score for each
parameter, which ranges from 1 to 5 representing qualitative values from
VERY BAD to VERY GOOD. The evaluation system identified that
addition of nanosilica to LDPE improved the quality of LDPE from GOOD
to VERY GOOD. Also, the addition of nanosilica to LDPE-NR (80:20)
composite material improved the composite quality from BAD to GOOD.
3. New composite material compositions with improved PD resistance
Additions of nanosilica filler to LDPE and to LDPE-NR composites had
improved the PD resistance performance. Using the developed evaluation
system, it was successfully identified that LDPE-SiO2 composite with 4.5
wt% of nanosilica filler loading has the highest score, followed by LDPE-
NR-SiO2 composite with 6 wt% nanosilica filler.
9
1.6 Research Significance
The surface defect of the composite sample has a strong correlation with the
PD pulse count. Smaller PD pulse count is in line with the smaller PoEA. This
correlation can make the PoEA analysis as an alternative to PD pulse count analysis,
or as a complement to PD pulse count analysis. In the absence of PD-DAQS, the
surface defect analysis can be a good technique to classify the PD performance based
on the PoEA of samples.
The PD improvement (APD Index) of the composite help the researchers to
understand the contribution of nanofillers whether positive or negative compared to
the base polymer. By adopting the PD resistance index to analyze the PD resistance,
a new set of dielectric materials can be better understood not only for experts in this
research area but also for the management team and those in common engineering.
The new compositions of composite materials have a higher PD resistance
compared to their base polymers. The current proposed compositions can be a starting
point for further research by doing different aspects of study in order to get a more
comprehensive knowledge of these composites while looking at the opportunity to be
applied as a high voltage insulating material.
1.7 Thesis Organization
Chapter 2 presents a literature review with respect to the background of
composite materials, partial discharge measurements, PD data acquisitions, PD data
interpretations, image processing methods to quantify the percentage of eroded area
(PoEA), the evaluation system in general, and techniques used in the composite
diagnosis. Chapter 3 presents the research methodology that explains the sample
preparations, the experimental procedure of PD and data acquisition system, image
processing method to compute the PoEA, and the evaluation procedure to select the
best composite sample.
10
Chapter 4 presents the results and discussions of the SEM observations, the
experimental results of PD pulse count, and the image processing used to count the
PoEA of tested composites. The morphological characteristics of each sample are also
described. The evaluation of result is also presented. Chapter 5 concludes the thesis
and also presents recommendations for future work.
151
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