POLARIZATION AND DEPOLARIZATION CURRENT … · Tesis ini membentangkan hasil kajian eksperimen yang...

POLARIZATION AND DEPOLARIZATION CURRENT OF LINEAR LOW

DENSITY POLYETHYLENE-NATURAL RUBBER NANOCOMPOSITE

SUBJECTED TO ELECTRICAL TRACKING AND MOISTURE

NOR AKMAL BINTI MOHD JAMAIL

A thesis submitted in fulfilment of the

requirements for the award of the degree of

Doctor of Philosophy (Electrical Engineering)

Faculty of Electrical Engineering

Universiti Teknologi Malaysia

FEBRUARY 2015

v

ABSTRACT

Polymeric nanocomposites are widely used for high voltage outdoor

insulating application due to their efficient electrical performance. Recently, SiO2,

TiO2 and MMT nanofillers are used as fillers because there are listed as the main

nanofiller commonly used in electrical engineering due to the increases of effective

activation energy. Natural rubber (NR) is used because the nature of the interphase is

found to affect viscoelasticity and it develops several interphases with the Linear

Low-Density Polyethylene (LLDPE) matrix. This thesis presents the outcome of an

experimental study which has been carried out to determine the conductivity level

and dielectric response function of the LLDPE-NR compound, filled with different

amount of SiO2, TiO2 and MMT nanofillers using Polarization and Depolarization

Current (PDC) measurement technique. LLDPE and NR with the ratio composition

of 80:20 were selected as a base polymer. The PDC testing was done on samples at

various conditions: without any defect (normal condition), wet condition (moisture

absorption) and after electrical tracking effect condition based on BS EN

60587:2007 standards. One of the problems associated with outdoor polymeric

insulators is the tracking of the surface which can directly influence the reliability of

the insulator. The amount of water content can be used to monitor the dielectric

quality of insulation and as an indicator of possible deterioration for outdoor

degradation. Besides PDC, surface morphology analysis using FESEM was also

conducted to study the changes in physical structure which can explain the

conductivity and response function. This research found that an addition of certain

weight percentage of nanofiller and NR into the LLDPE improved conductivity level

of the insulator. LLDPE-NR/TiO2 at 5 wt% has become the best sample in terms of

the lowest conductivity in normal and under electrical tracking effect. Besides, this

sample has the fastest time response under electrical tracking and moisture

condition. This investigation has successfully identified the PDC pattern,

conductivity and dielectric response function of LLDPE-NR nanocomposite. Results

show that different compositions as well as the surface physical conditions affect the

PDC measurement results.

vi

ABSTRAK

Nanokomposit polimer telah digunakan secara meluas dalam aplikasi

penebatan luar voltan tinggi disebabkan prestasi elektrik yang bagus. Pada masa kini,

pengisi nano SiO2, TiO2 dan MMT digunakan sebagai pengisi kerana telah

disenaraikan sebagai pengisi nano utama yang digunakan dalam kejuruteraan

elektrik disebabkan oleh peningkatan tenaga pengaktifan yang berkesan. Getah asli

telah digunakan kerana sifat semulajadi interfasanya memberi kesan likat-kenyal dan

membentuk beberapa interfasa dengan matrik Linear Low-Density Polyethylene

(LLDPE). Tesis ini membentangkan hasil kajian eksperimen yang telah dijalankan

untuk menentukan tahap kekonduksian dan fungsi tindak balas dielektrik terhadap

komposit LLDPE-NR, diisi dengan pengisi SiO2, TiO2 dan MMT yang berlainan

jumlah menggunakan teknik Pengukuran Arus Polarisasi dan Depolarisasi (PDC).

LLDPE dan getah asli dengan komposisi nisbah 80:20 telah dipilih sebagai polimer

asas. Ujian PDC telah dilakukan ke atas sampel dalam pelbagai keadaan: tanpa

sebarang kecacatan (keadaan biasa), keadaan basah (penyerapan lembapan) dan

selepas keadaan kesan treking elektrik berdasarkan piawaian BS EN 60587:2007.

Salah satu masalah berkaitan dengan penebat polimer luar adalah treking permukaan

yang secara langsung boleh mempengaruhi kebolehpercayaan penebat. Jumlah

kandungan air boleh digunakan untuk memantau kualiti dielektrik penebat dan

sebagai petunjuk kemungkinan kemerosotan untuk degradasi luar. Selain PDC,

analisis morfologi permukaan menggunakan FESEM juga telah dijalankan untuk

mengkaji perubahan struktur fizikal yang berkaitan dengan fungsi kekonduksian dan

fungsi tindak balas. Kajian ini mendapati bahawa tambahan peratusan berat tertentu

pengisi nano dan getah asli ke atas LLDPE telah menambahbaik tahap kekonduksian

penebat. LLDPE-NR/TiO2 pada 5 % berat telah menjadi sampel yang terbaik dari

segi kekonduksian paling rendah dalam keadaan normal dan treking elektrik. Selain

itu, sampel ini mempunyai masa tindak balas yang paling pantas dalam keadaan

treking elektrik dan kelembapan. Kajian ini berjaya mengenal pasti corak PDC,

kekonduksian dan fungsi tindak balas dielektrik nanokomposit LLDPE-NR.

Keputusan menunjukkan bahawa komposisi yang berbeza serta keadaan fizikal

permukaan memberi kesan kepada keputusan pengukuran PDC.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

TABLE OF CONTENT

LIST OF TABLES

LIST OF FIGURES

LIST OF ABBREVIATIONS

LIST OF SYMBOLS

LIST OF APPENDICES

ii

iii

iv

v

vi

vii

xi

xii

xix

xxi

xxii

1 INTRODUCTION 1

1.1 Background of the Research

1.2 Problem Statement

1.3 Aim and Objectives of the Research

1.4 Scope of the Research

1.5 Significance of the Study

1.6 Thesis Organisation

1

3

5

6

6

8

2 LITERATURE REVIEW 9

2.1 Introduction

2.2 Polymer Nanocomposite

2.3 DC Conductivity of Polymer Nanocomposite

9

9

13

viii

2.3.1 Conduction Current Measurement System

2.3.2 Current and Conduction Analysis

2.4 Moisture Absorption in Polymer Nanocomposite

2.5 Polarization and Depolarization Current (PDC)

2.5.1 Basic Principle of PDC

2.5.2 PDC Measurement for Solid Insulation

2.6 Conductivity Variation using PDC Measurement

2.7 Conductivity and Moisture Content Analysis

using PDC Measurement

2.7.1 Effect of Moisture Content and

Temperature on Transformer Insulations

2.7.2 Conductivity of Transformer Insulation

2.7.3 Machine Insulation Results

2.7.4 Effect of Moisture Content and

Temperature on Cable Insulation

2.7.5 Conductivity of Cable Insulation

2.8 Dielectric Response of Polymer

2.9 Electrical Tracking

2.9.1 Analysis of Carbon Track

2.9.2 Leakage Current and Dry-band Arcing

2.10 Summary

14

16

19

21

24

28

29

30

31

33

35

37

38

40

41

41

43

45

3 RESEARCH METHODOLOGY 46

3.1 Introduction

3.2 Development of the New Polymer

Nanocomposite Sample for HV Insulating

Material

3.2.1 Material Preparation

3.2.2 Compounding and Mixing of Polymer

Nanocomposite

3.2.2.1 Compounding setup and

equipment

3.2.3 Cutting of Mixed Material

46

47

48

51

51

53

ix

3.2.4 Drying Process

3.2.5 Compression Moulding

3.2.5.1 Estimation of Mass

3.2.5.2 Compression Moulding Setup and

3.3 Morphological Studies

3.3.1 Platinum Coating Procedure

3.3.2 FESEM Setup and Procedure

3.4 Energy Dispersion X-Ray (EDX)

3.5 Capacitance Measurement

3.5.1 Test cell

3.5.2 Capacitance Test Procedure


Measurement

3.6.1 PDC Test Setup and Equipment

3.6.2 PDC Test Procedure

3.7 Electrical Tracking Test

3.7.1 Electrical Tracking Test Setup and

Equipment

3.7.2 Electrical Tracking Test Procedure

3.8 Adding Moisture to Solid Insulation

3.8.1 Adding Moisture Procedure

3.9 Summary

55

55

57

58

59

60

61

67

67

68

69

70

70

73

74

74

77

79

80

4 RESULTS AND DISCUSSIONS 81

4.1 Identification of Material Composition Using

Energy Dispersive X-ray (EDX)

4.2 Physical Characteristic

4.2.1 Characteristic of New Sample

4.2.1.1 Colour

4.2.1.2 Morphology

4.2.2 Sample with Electrical Tracking Effects

4.2.2.1 Erosion

81

88

88

88

90

94

94

x

4.2.2.2 Morphology

4.3 Moisture Absorption Characteristic

4.4 Polarization and Depolarization Current

Measurement Test Result

4.4.1 Normal Condition Result


4.4.3 After Immersed in Water Condition

4.5 Conductivity Variation



4.5.3 After Immersed in Water Condition

4.6 Dielectric Response Function



4.6.3 After Immersion in Water Condition

4.7 Maximum Conductivity Analysis

4.8 Response Function Analysis

4.9 Summary

98

102

108

109

114

119

126

128

131

134

139

139

142

145

149

156

161

5 CONCLUSION AND RECOMMENDATION 164

5.1 Conclusions

5.2 Summary of the Original Contributions

5.3 Recommendations

164

166

166

REFERENCES

168

Appendices A-B 180-188

xi

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Machine Insulation Current for Different Condition 37

2.2 Cable Conductivity for Different Condition [56, 57] 39

3.1 Details properties of materials 50

3.2 Technical data for mixer 52

3.3 Compound formulations and designations 57

4.1 Colour characteristic for all samples 89

4.2 Water absorption percentage of saturation 108

4.3 Minimum and maximum value of polarization current(Ipol) 125

4.4 Minimum and maximum value of depolarization current

(Idepol) 126

4.5 Minimum and maximum conductivity value 138

4.6 The initial time of response function to reach steady state

condition 148

4.7 Lowest conductivity for samples A, B and C 155

4.8 Initial time response for samples A, B and C 161

xii

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Conduction current measurement system [9, 43, 46] 15

2.2 Current for TiO2 based polyimide nanocomposite at

different % amount added [44] 17

2.3 The influence of the MgO nano-filler content on the

volume resistivity [9, 43] 18

2.4 I - V characteristics in alumina nanoparticles filled LLDPE

samples at 20°C [42] 19

2.5 Moisture absorption of three materials as aging time [50] 20

2.6 Characteristic of Polarization and Depolarization Currents

[13, 73, 74, 85, 88]

26

2.7 PDC curve in dB plot [89] 27

2.8 Principle of test arrangement for PDC measurement with

isolated shield [54, 60]

28

2.9 Principle of test arrangement for PDC measurement with

grounded shield [54, 60]

29

2.10 Effect of paper moisture content on polarization current

measured at 15C and 10C [65]

32

2.11 Effect of paper moisture content on depolarization current

measured at 15C and 10C [65]

33

2.12 Variation of polarization currents with paper conductivity.

(a) σpaper = 1.5pS/m; σpaper = 0.15 pS/m; σpaper =

0.015pS/m, (b) σpaper = 4.2fS/m; σpaper = 42fS/m;

σpaper = 420fS/m [13, 86]

34

2.13 Variation of polarization currents with different oil

conductivity [12, 92]

35

2.14 Polarization currents for epoxy-mica bars and a polyester-

mica coil before (dry) and after (humid) one week under

xiii

humid atmosphere [52] 36

2.15 Depolarization currents for epoxy-mica bars and a

polyester-mica coil before (dry) and after (humid) one

week under humid atmosphere [52]

36

2.16 Polarization and depolarization current of 2 km XLPE

cable after 4 years in service (T=21C) and 5 years later

after supplying fault current (T=30C )[54]

38

2.17 Polarization and depolarization current of aged XLPE

cable before and after repair due to free water [54]

38

2.18 Schematic Diagram of IPT setup [106] 43

2.19 Surface tracking development process [113] 44

3.1 Flow chart of research methodology 47

3.2 Linear Low Density Polyethylene (LLDPE) 49

3.3 Natural Rubber grade SMR CV 60 49

3.4 Nanofiller (a) Silicon Oxide (SiO2), (b) Titanium Oxide

(TiO2) and (c) Monmorillonite clay (MMT-organoclay) 50

3.5 Compound mixer setup (a) temperature controller, (b)

mixer bowl, (c) two- hand control device, (d) drive unit (e)

blades and (f) loading weight

52

3.6 Compounding sample (a) LLDPE-NR/SiO2, (b) LLDPE-

NR/TiO2 and (c) LLDPE-NR/MMT 53

3.7 Plastic granulation (a) feeding hopper, (b) lock and (c)

START and STOP button 54

3.8 LLDPE/NR nanocomposite in small particle 54

3.9 Oven for drying process 55

3.10 Genesis compression moulding equipment (a) heat power

ON, (b) heat power OFF, (c) clamp CLOSE, (d) master

power ON, (e) master power OFF , (f) clamp OPEN, (g)

water cooling switch and (h) MANUAL or AUTO cycle

59

3.11 Platinum coating equipment and setup (a) coating

chamber, (b) auto fine coater and (c) START button 60

3.12 FESEM test arrangement (a) FESEM JEOL, (b) personal

computer and (c) exchange rod 61

3.13 FESEM main switch (a) VAC switch and (b) Main switch 62

3.14 Microscope OS switch 62

xiv

3.15 Liquid Nitrogen Cold-trap 63

3.16 Exchange chamber switch (a) EVAC switch, (b) HLDR

switch, (c) VENT switch and (d) EXCH POSN switch 64

3.17 Sample holder 64

3.18 Exchange rod 65

3.19 FESEM Trackball 66

3.20 FESEM control panel 66

3.21 Capacitance test setup (a) test cell and (b) LCR meter 68

3.22 PDC test cell (a) positive electrode (unguarded electrode),

(b) negative electrode (guarded electrode), (c) positive

terminal and (d) negative terminal 69

3.23 Principal of test arrangement for PDC measurement 70

3.24 PDC test workstation 71

3.25 User panel for PDC measurement 72

3.26 Switching panel 72

3.27 Schematic Diagram of IPT setup [10, 106] 75

3.28 Schematic Diagram of IPT setup [10, 106] 76

3.29 High voltage transformer 76

3.30 Online analysis system 77

3.31 Test specimen arrangement 78

3.32 Weight scale 78

3.33 Sample immersed in distilled water 79

4.1 EDX spectrum of LLDPE-NR (Sample R) 82

4.2 EDX spectrum of LLDPE-NR/SiO2 (Sample A1) 82




4.6 EDX spectrum of LLDPE-NR/TiO2 (Sample B1) 84



xv


4.10 EDX spectrum of LLDPE-NR/MMT (Sample C3) 86



4.13 FESEM images of sample R in normal condition 90

4.14 FESEM images of samples for group A in normal

condition. (a) Sample A1, (b) Sample A3, (c) Sample A5,

(d) Sample A7 91

4.15 FESEM images of samples for group B in normal

condition. (a) Sample B1, (b) Sample B3, (c) Sample B5,

(d) Sample B7 92

4.16 FESEM images of samples for group C in normal

condition. (a) Sample C3, (b) Sample C5 (c) Sample C7 93

4.17 Test samples R effect of electrical tracking test 95

4.18 Test samples for group A effect of electrical tracking test.

(a) Sample A1, (b) Sample A3, (c) Sample A5, (d) Sample

A7 95

4.19 Test samples for group B effect of electrical tracking test.

(a) Sample B1, (b) Sample B3, (c) Sample B5, (d) Sample

B7 96

4.20 Test samples for group C effect of electrical tracking test.

(a) Sample C3, (b) Sample C5, (c) Sample C7

97

4.21 FESEM images of sample R effect of electrical tracking

test

99

4.22 FESEM images of sample for group A effect of electrical

tracking test. (a) Sample A1, (b) Sample A3, (c) Sample

A5, (d) Sample A7

100

4.23 FESEM images of sample for group B effect of electrical

tracking test. (a) Sample B1, (b) Sample B3, (c) Sample

B5, (d) Sample B7

101

4.24 FESEM images of sample for group C effect of electrical

tracking test. (a) Sample C3, (b) Sample C5, (c) Sample C7

102

4.25 Weight of water absorption for sample R and samples in

Group A

104

4.26 Percentage of water absorption for sample R and samples

in Group A

104

xvi


Group B

105


in Group B

105


Group C

107


in Group C

107

4.31 Polarization current pattern for samples R, A1, A3, A5 and

A7 at normal condition

110

4.32 Depolarization current pattern for samples R, A1, A3, A5

and A7 at normal condition

111

4.33 Polarization current pattern for samples R, B1, B3, B5 and

B7 at normal condition

112

4.34 Depolarization current pattern for samples R, B1, B3, B5

and B7 at normal condition

112

4.35 Polarization current pattern for samples R, C3, C5 and C7

at normal condition

113

4.36 Depolarization current pattern for samples R, C3, C5 and

C7 at normal condition

114


A7 after electrical tracking test

115


and A7 after electrical tracking test

115


B7 after electrical tracking test

116


and B7 after electrical tracking test

117

4.41 Polarization current pattern for sample R, C3, C5 and C7

after electrical tracking test

118

4.42 Depolarization current pattern for sample R, C3, C5 and

C7 after electrical tracking test

118


A7 after water absorption

119


and A7 after water absorption

120

xvii


B7 after water absorption

121


and B7 after water absorption

121

4.47 Polarization current pattern for samples R, C3, C5 and C7

after water absorption

122

4.48 Depolarization current pattern for samples R, C3, C5 and

C7 after water absorption

123

4.49 Conductivity variations for sample LLDPE-NR and sample

LLDPE-NR/SiO2 at different amount of nanofiller at

normal condition

129


LLDPE-NR/TiO2 at different amount of nanofiller at

normal condition

130


LLDPE-NR/MMT at different amount of nanofiller at

normal condition

131


LLDPE-NR/SiO2 at different amount of nanofiller after

electrical tracking test

132


LLDPE-NR/TiO2 at different amount of nanofiller after


133


LLDPE-NR/MMT at different amount of nanofiller after


133



water absorption

134



water absorption

135



water absorption

136

4.58 Response functions for sample LLDPE-NR and sample

LLDPE-NR/SiO2 at different amount of nanofiller at

normal condition

140


LLDPE-NR/TiO2 at different amount of nanofiller at

xviii

normal condition 141


LLDPE-NR/MMT at different amount of nanofiller at

normal condition 142

4.61 Response functions for sample LLDPE-NR/SiO2 at

different amount of nanofiller after electrical tracking test 143

4.62 Response functions for sample LLDPE-NR/TiO2 at


4.63 Response functions for sample LLDPE-NR/MMT at




water absorption 146



water absorption 147



water absorption

148

4.67 Actual conductivity to weight percentage of nanofiller (wt

%) for all samples in normal condition

150


%) for all samples in with effect of electrical tracking. 150


%) for all samples in with effect of moisture absorption 151

4.70 Conductivity to weight percentage of nanofiller (wt %) for

all samples in normal condition after fitting 153


all samples with effect of electrical tracking 153


all samples with effect of moisture absorption 154

4.73 Actual response function to weight percentage of nanofiller

(wt %) for all samples in normal condition 156


(wt %) for all samples with effect of electrical tracking 157


(wt %) for all samples with effect of moisture absorption 157

xix

4.76 Response function to weight percentage of nanofiller (wt

%) for all samples in normal condition 159

4.77 Response Function to weight percentage of nanofiller (wt

%) for all samples with effect of electrical tracking 159

4.78 Response function to weight percentage of nanofiller (wt

%) for all samples with effect of moisture absorption 160

xx

LIST OF ABBREVIATION

Al - Aluminium

ATH - Alumina trihydrate

ASTM - American Society for Testing and Materials

BS EN - British Standard European Norm

C - Carbon

DONL - Degree of Nonlinearity

DC - Direct Current

EMAA - Ethylene/Methacrylic Acid copolymer

EDX - Energy Dispersion X-Ray

FESEM - Field Emission Scanning Electron Microscopy

HDPE - High Density Polyethylene

HV - High Voltage

IEEE - Institute of Electrical and Electronics Engineers

IPT - Inclined Plane Tracking

LabVIEW - Laboratory Virtual Instrumentation Engineering

Workbench

LC - Leakage Current

LDH - Layered Double Hydroxide

LLDPE - Linear Low- Density Polyethylene

LS - Layer Silicates

MV - Medium Voltage

MDH - Magnesium Dihydroxide

MgO - Magnesium Oxide

MMT - Montmorillonite

NR - Natural Rubber

O - Oxygen

xxi

OLTC - On-Load Tap Changer

OMMT - Organically Modified Montmorillonite

PI - Polyimide

PDC - Polarization and Depolarization Current

Si - Silicone

SEM - Scanning Electron Microscope

SMR - Standard Malaysian Rubber

Ti - Titanium

TEM - Transmission Electron Microscope

XLPE - Extruded Cross-Linked Polyethylene

XRD - X-Ray Diffraction

ZnO - Zinc Oxide

xxii

LIST OF SYMBOLS

Ip(t) - Polarization current

Id (t) - Depolarization current

U0 - Potential difference

C0 - Geometric capacitance

Cm - Measured capacitance

f(t) - Dielectric response function

σ - DC conductivity

Ɛ0 - Dielectric constant

Ɛr - Relative static permittivity

ρ - Density

wt - Weightage

xxiii

LIST OF APPENDICES

APPENDIX TITLE PAGE

A List of Publications 180

B Dielectric Response Function 184

CHAPTER 1

INTRODUCTION

1.1 Background of the Research

Nanocomposite material application in the field of dielectric and electrical

insulation has attracted many researchers’ attention recently. Polymer

nanocomposite materials were applied as insulation material since the 21st century

and were given more attention because of the material properties that can be

drastically improved by adding a few percent of nano-sized filler. This

nanocomposite can be used as nanodielectric. This material had widely been used as

power apparatuses insulator and cables insulator. Polymers nanocomposites are

made by adding nanometer sized fillers and homogenously dispersed into the matrix

of polymers composite. The filler is dispersed homogenously in the composite

matrix by a certain weight percentages (wt%). The fillers that are added to the matrix

are done in just a small quantity, typically less than about 10 wt%. Polyethylene or

epoxy resin are widely used as insulators in industrial applications as the base

material of many recent studies, while SiO2, Al2O3, MgO, TiO2 or layer silicates

(LS) and nano clay(MMT) function as typical nanofillers [1-4].

Lately, significant attention has been given into the monitoring of the

condition of electrical apparatus such as transformer, cable and rotating machine

insulation system. These equipments are important units in power system. Hence,

these equipment insulation systems should be monitored frequently to prolong the

2

equipment’s life time. For this reason, several new techniques for monitoring high

voltage (HV) equipment through its insulation have been developed in recent years.

One of these techniques is Polarization and Depolarization Current (PDC)

measurement. PDC is the time domain based techniques that are widely accepted by

many utilities due to the advancement in hardware and software interpretation

schemes [5]. Results from these techniques are severely influenced by several

environmental factors, predominantly the temperature and aging process of the

electrical insulation. Polarization and conduction phenomena occur in every

electrical insulating material. Each problem in the dielectric is produced by the

mechanism of polarization and conduction. Polarization takes place in all molecules

of a dielectric and causes chemical change or deterioration in the material.

This dielectric technique has been considered by many researchers for power

system insulations condition assessment due to its ability to provide information on

the conductivity and moisture contents. Conductivity can be estimated within the

initial periods (seconds) and the moisture content for long periods after a DC step

voltage application.

Polymer nanocomposites are the 21st century engineering materials with wide

range of markets such as transportation, electrical and electronics, food packaging

and building industries. They are particularly attractive as advanced dielectrics and

electrical insulation in the viewpoint of their inherent excellent properties.

Polymeric nanocomposites have gained importance due to their material properties

in providing enhanced performances, namely through their electrical and mechanical

properties. But until now, there are no investigations done on the conductivity and

response function in time domain of linear low-density polyethylene when it blends

with natural rubber (LLDPE-NR) nanocomposite as a new insulation.

One of the objectives of this research in the field of LLDPE-NR

nanocomposite dielectrics is to obtain new materials using improved dielectric

properties of HV insulation by adding SiO2, MMT and TiO2 nanofiller. This research

3

presents results of an experimental activity which is aimed to find the conductivity

variation and dielectric response functions of LLDPE-NR nanocomposite that was

focused on normal condition, the effect of electrical tracking and the effect of

moisture absorption. This research is focused on formulations of natural rubber

blends nanocomposites obtained from polyethylene filled with nanoparticles of SiO2,

MMT and TiO2. The conductivity and dielectric response function was done by

using Polarization and Depolarization Current (PDC) measurement technique. Study

on these parameters is required as an increasing number of power utilities nowadays

choose polymer nanocomposite as a new insulation due to its unique properties. The

study on conductivity is important in discovering how well the material can conduct

the movement of charges when there was voltage supplied to the electrode. The

lower the resistance, the greater is its conductivity because conductivity is the

inverse of electrical resistance. Response function in time domain is important to

determine how fast or slow the dipole would take to relax and return to the original

state when there was no voltage supply.

1.2 Problem Statement

Polyethylene such as linear low-density polyethylene (LLDPE) is obtained

via the copolymerization of ethylene with various alpha olefins. They have grown in

importance because of the specific properties that can be obtained by varying co-

monomer content and polymerization condition [6]. Several studies have been

conducted on the LLDPE-NR and found that the composition of 80:20 ratio has

shown to be the best composition due to the lowest carbon track development as

compared with virgin LLDPE (Piah et al., 2003). Natural rubber was used because

the nature of the interphase is found to affect viscoelasticity. It develops several

interphases with the LLDPE matrix. The current project used LLDPE-NR as base

material because it has shown significant improvement in terms of degradation due

surface tracking and moisture effect. SiO2, TiO2 and MMT nanofillers are being used

as filler because they are listed as the main nanofiller commonly used in electrical

engineering due to the increase in effective activation energy.

4

Most of the high voltage apparatus’ polymer insulation such as transformer

insulation and cable insulation face degradation due to surface tracking and water

trees aging. Faults usually occur in the bodies of oil-paper cables, high voltage

circuit breakers, surge arresters, power capacitors and transformers. One of the

factors that affect the life cycle of a electrical equipment is the operating state of its

insulation system.There are many insulation defects in power system equipment such

as the presence of partial discharging (corona) in insulator, external flashover of

bushings due to salt pollution and degradation of insulation due to ingress of

moisture and surface tracking. This project was focused on degradation condition

subjected to electrical tracking and moisture of LLDPE-NR nanocomposite because

these conditions leave the most effect on insulation properties and they can represent

outdoor aging. Carbon track is a process that takes place in the surface of insulator

due to the surface tracking phenomena. The presence of carbon in most of polymers

causes possible carbon track to occur.

Most of previous researches performed morphology studies, breakdown

strength test, space charge formation and dielectric loss test to determine the

electrical properties of polymer nanocomposite [7-9]. These studies showed physical

structure in static condition, breakdown strength and electrical strength. However,

these dielectric measurements are not adequate because they do not reveal

information on the polarization current, depolarization current and the variation

conductivity of nanocomposite under the defect of electrical tracking and under

moisture condition. Nonetheless, without the accurate information of these

parameters, it is difficult to determine the cause for the degradation of the insulator.

Condition monitoring techniques such as PDC measurement technique can be

used to predict the remaining life of the electrical apparatus and also reduce

unscheduled outages, improve maintenance planning and increase system reliability.

Analysis of the fundamental dielectric processes has shown that the polarization

phenomena are strongly influenced by the morphology and degradation of the

polymeric insulation. Therefore, research into the dielectric properties of polymer

nanocomposites based on PDC measurement would provide invaluable information

to further understand the electrical insulating performance of polymer

5

nanocomposites used in high voltage applications. PDC value is very sensitive to

moisture and electrical tracking which happen under wet contamination.

Conductivity and dielectric response of the insulating material can be determined by

using the relevant PDC technique. Response function in time domain is important to

determine the response for the dipole when it is relaxed and returned to the original

state when there is no voltage supply.

1.3 Aim and Objectives of the Research

The aim of this research is to identify the best LLDPE-NR nanocomposite

uses for HV insulation by determining the conductivity and dielectric response

function in normal condition and after degradation factors: electrical tracking and

moisture effect. In order to achieve this aim, the study was divided into three

objectives. The objectives of this research are:

i. To study the characteristics of electrical tracking test and moisture

absorption effect of the newly developed LLDPE-NR nanocomposite

materials. The effect of different weight percentages of nanofillers on

material was also investigated.

ii. To analyse the Polarization and Depolarization Current (PDC) and

conductivity trend of the newly developed LLDPE-NR nanocomposite

materials, LLDPE-NR nanocomposite effect of moisture absorption and

LLDPE-NR nanocomposite effect of electrical tracking condition.

iii. To determine the dielectric responsive function of the newly developed

LLDPE-NR nanocomposite materials, LLDPE-NR nanocomposite

effect of moisture absorption and LLDPE-NR nanocomposite effect of

electrical tracking condition.

6

1.4 Scope of the Research

This research focused on the improvement of LLDPE-NR nanocomposite

material. The scopes and limitation of this research are listed as below:

i. Insulating material that is used focused on polymer nanocomposite

material. Linear Low Density Polyethylene (LLDPE) is selected as a

base polymer and Natural Rubber (NR) grade SMR CV 60 with the

ratio composition of 80:20. LLDPE-NR is filled with a small weight

percentage of TiO2, SiO2 and MMT nanofillers.

ii. The wt % of nanofiller used are 1 wt %, 3 wt %, 5 wt % and 7 wt %.

iii. PDC testing is done on sample of the new material at the condition of:

without any defect (normal condition), wet condition (moisture

absorption) and after applied an electrical tracking condition based on

BS EN 60587:2007 standard [10].

1.5 Significance of the Study

Polymer nanocomposites possess promising high performance characteristic

as engineering materials if they are prepared and fabricated properly. Some work has

been recently done on such polymer nanocomposites as dielectrics and electrical

insulation [11]. One of the targets in this field is to obtain new materials with

improved dielectric properties for high voltage (HV) insulation application. In

general, a good insulator must have extremely low conductivity level. All the

samples have undergone degradation process of surface tracking and moisture

absorption. These conditions can represent major outdoor degradation. By

considering this condition, the conductivity performance in normal and degraded

conditions of all the samples studied can be determined.

PDC patterns of mineral oil and paper as transformers’ insulation have been

studied by many researchers [12, 13]. PDC analysis is a non-destructive dielectric

7

testing method for determining the conductivity and dielectric response function.

PDC analysis is normally used to determine the water content in the oil-paper

insulation barrier and also its oil conductivity. However, there are no investigations

on PDC done for LLDPE-NR nanocomposite. Study on the PDC pattern of LLDPE-

NR nanocomposite is required as it is a newly introduced insulation material for

electrical equipment. Others dielectric measurements are not focused on important

parameters such as information on the polarization current, depolarization current,

variation conductivity and dielectric response function of nanocomposite.

Nonetheless, without the accurate information of these parameters, it is difficult to

determine the cause for the degradation of the insulator. PDC is also very sensitive to

moisture condition. For this reason, PDC was selected as an accurate testing method

to determine the conductivity and response function in time domain.

Therefore in this work, PDC tests were performed on insulation of LLDPE-

NR nanocomposite samples at various conditions, namely the normal condition, the

moisture absorption condition and the electrical tracking effect condition. These

conditions are chosen because electrical tracking is one of the major problems in

outdoor insulation systems and it similarly occurs due to degradation of aging factor.

Effect of moisture absorption is considered as one of the major insulation

degradation cause. The amount of water content can be used to monitor the dielectric

quality of insulation and as an indicator of possible deterioration. Deterioration will

adversely affect its electrical properties and this will eventually cause dielectric

breakdown. The higher the moisture contents in the insulation, the higher the

conductivity will be.

The test data are used to find the dielectric responsive function and maximum

conductivities of LLDPE-NR nanocomposite. PDC is carried out to find the

dielectric responsive function and trend of this material conductivities level based on

the test conditions. In order to estimate the dielectric response function f(t) from a

depolarization current measurement, it is assumed that the dielectric response

function is a continuously decreasing function in time, then if the polarization period

is sufficiently long, as in ( )cf t t ≅0, the dielectric response function f(t) is

proportional to the depolarization current.

8

1.6 Thesis Organisation

The present thesis consists of six chapters. Chapter One provides an

introduction to the problem statement, objectives, scope of the research and

significance of the research. Chapter Two outlines the literature review which covers

topic of polymer nanocomposite, DC conductivity and moisture content of polymer

nanocomposite, dielectric response, Polarization and Depolarization Current (PDC)

and electrical tracking. Chapter Three outlines some of the test methods used to

investigate the performance of insulation materials. Detailed explanation of

experimental setup, procedure and technique employed in this research include

material preparation, morphological test, capacitance test, polarization and

depolarization current test and electrical tracking test. In Chapter Four, all

experimental results are presented and discussed. Chapter Five consists of

maximum conductivity and response function analysis. Finally, Chapter Six draws

conclusions and makes recommendations for future research.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

In this chapter, key subjects that are related to this research are summarised,

firstly by providing an outline of the history of polymer nanocomposite, then

focusing on the electrical properties and literature on polymer nanocomposite for

high voltage insulation. This chapter also reviews the Polarization and

Depolarization Current (PDC) measurement technique and electrical tracking effect

on insulation, moisture effect on insulation and polymer nanocomposite, theories,

technologies and research done which are briefly explained and discussed thereafter.

This part presents comprehensive review and analysis on conductivity value based

on the experimental results from previous researchers to find patterns and trends of

the test material. The review and analysis from previous researches and studies on

current and conductivity using different methods and samples are also compared.

2.2 Polymer Nanocomposite

In the 1990s, the commercialisation of Polyamide 6/clay nanocomposite that

was manufactured as engineering plastics by applying polymer nanocomposite for

engineering level has encouraged researchers to study the possibility of combining

different polymers with different nano inorganic fillers. Since then, much effort has

been made to develop and apply polymer nanocomposites in electrical and

10

electronics engineering, transportation, food package and industries. Polymer

nanocomposites which are newly developed advanced materials have wide

application potential [11].

Nanodielectrics for the electrical insulation field have commenced in year

1994 when John Lewis published a paper entitled “Nanometric Dielectrics” in a

publication titled IEEE Transactions on Dielectrics and Electrical Insulation [14].

The role of nanometric interfacial processes in initiating electrical breakdown in

insulating materials has gained much attention since. Studies showed that the

attainment of equilibrium or a steady state in the interface of dielectric and

conductive is a requirement. In general that charge is transported towards or away

from it through the bulk phases on either side that depend on their electrical

conductivities. If one or both possess a highly insulating characteristic, the time to

achieve this attainment can be exceedingly long [14].

In 2002, first experimental data were produced on nanometric dielectrics by a

researcher [15]. It was proved that nanometric fillers mitigate the interfacial

polarization characteristic of conventional materials and hence, the internal field

accumulation was reduced. Large interaction zone in reduced mobility (free volume)

nanofilled polymers should lead to a significant change in electrical properties.

Studies of electrical behaviour thus have provided an opportunity for both a

fundamental study of this interaction zone, as well as an opportunity for optimising

performance for specific and critical applications. Nanotechnology was overviewed

with a focus on material potentialities, implications and development which

concluded that a new class of materials, nanodielectrics, is emerging [16].

Since 2002, some works that have been done in IEEE focusing particularly

on nanocomposite. Several reviews were made on polymer nanocomposites for

electrical insulation and high voltage dielectric application. A multi-core model

which has a far-distance effect and is closely related to an “interaction zones” was

proposed by considering electrical and chemical structures mesoscopic analysis of an

existing interface with finite thickness [11]. To have a deeper understanding of

11

nanodielectrics in general, further studies were done toward future research and

development of the material [17].

The second generation resins that are used in insulation engineering are

called polymer nanocomposites. These materials consist of polymers that are filled

with many micron-sized inorganic fillers in the order of 50 wt%. The recently

produced polymer composites can be defined as a small amount of nanofillers that

fill in a substance of polymers. The size of the nanofillers varies from 1 to 100 nm,

while the content varies from 1 to 10 wt%. Nanofillers should be dispersed in the

polymer matrix homogeneously. Although nanofillers have a small content

percentage (low wt %), they possess very large surface areas if compared to those of

microfillers [11]. These newly developed polymer nanocomposite advanced

materials have a wide application in food packaging, building, electrical and

electronics engineering, transportation and other fields of industries. The materials

are used in application of coating materials, foamed materials, barrier-functional

materials, and flame-retardant materials. The most used product in the market is

polyolefin.

When polymer nanocomposite can change one or several of its dielectric

properties, it can be used as nanodielectric [18]. Nanodielectric is widely used in

rotating machine insulations, HV cables, solid insulation in switchgear, and outdoor

insulation for power electronic devices packaging. Besides that, it is also used in

rotating machine insulations [19]. It has been proved that nano-sized filler improved

the material dielectric properties for power system insulation application [20].

A lot of research was conducted to study the polymer nanocomposite

dielectric properties [21-27]. Most of these studies focused on epoxy nanocomposite

which has different nanofiller influencing the dielectric properties. Most of these

researchers measured electrical properties of the material such as resistivity of dc

volume, strength of ac dielectric, permittivity of dielectric and tan delta (400 Hz - 1

MHz). Results from these studies showed the dielectric behaviours of

nanocomposites. It was also found that some electrical properties of these materials

are unique and very useful to be used in existing and potential electrical systems at

12

certain loadings of nanoparticle. Ar [28] has performed dielectric strength, volume

resistivity and surface resistivity testing on HDPE clay nanocomposite based on

ASTM D149 and ASTM D257 standards. Observation showed that there was

significant improvement in nanoclay when organoclay was treated with titanate and

silane compounds. Nanofiller mixture composite can greatly improve the discharge

surface resistance and breakdown strength of insulation [29, 30]. According to [31],

from various nanoparticulates, clay minerals and carbon nanotubes are more

preferable and often used to enhance physical, mechanical and thermal properties of

polymers. In terms of material, the four main groups that were significantly used in

nanocomposite for electrical engineering are silicone oxides (silica), metallic oxides

and hydroxides (Al2O3, TiO2, MgO, ZnO etc), nanoclays (Montmorillonite, hectorite

etc) and carbon nanotubes [4, 32]. Pettarin et al. [33] in their testing prepared their

base material with high-density polyethylene (PE), an organically modified

montmorillonite (OMMT), while ethylene/methacrylic acid copolymer (EMAA) was

used to melt compounding. The composites morphology was examined by XRD,

SEM and TEM and the mechanical properties were evaluated under static

conditions.

Another research [34] used morphology and thermal stabilisation mechanism

to study LLDPE/MMT and LLDPE/LDH nanocomposites. The research found that

all nanocomposites show significantly enhanced thermal stability compared to virgin

LLDPE due to the increase of effective activation energy during the degradation

process. According to [35], the dynamic mechanical properties of LLDPE/nano-SiO2

composites were improved compared with the corresponding value for

LLDPE/untreated SiO2 composites. Some researchers conducted studies on silica

nanopartices as insulators [3, 30, 35-38]. Researcher [39] performed morphology,

water treeing and electrical behaviour test on a linear low density polyethylene

(LLDPE) with silica nanoparticles surface treatment. This research found that

samples with silane-treated nanoparticles have improved the dielectric properties

compared to the unsurface-treated sample. Researchers [40] developed

nanocomposites formed from isotactic polypropylene (iPP) filled with MMT. Thus,

this material is regarded as a candidate for power cable insulation material with

13

enhanced properties. The research found that, the material has increased loss factor

and permittivity compared to unfilled iPP.

2.3 DC Conductivity of Polymer Nanocomposite

Extensive studies on polymer nanocomposites for HV insulation were

conducted in these recent years because it has unique properties and able to enhance

electrical performance. Polymer nanocomposites electrical conduction is used as one

of the components to monitor its dielectric behaviour. Many studies were done to

investigate the electrical properties of this material, such as strength of electric

breakdown, resistance of partial discharge, space charge and conduction current

measurement [9, 41-43].

The result shows that DC conductivity exhibits significant reduction with the

addition of any type of nanofiller in a certain amount. However exact percentage of

the nanofiller addition is not the same for different types of base materials. This was

due to the decreasing of initiation probability of short circuit treeing in each material.

The conductivity analysis done found that the addition of nanofiller into the polymer

at certain percentage has improved its insulation properties.

Ciuprina and Plesa conducted a research on DC conductivity and the

variation of the real part of complex conductivity with the frequency for three

formulations of nanocomposites obtained from polyethylene filled with

nanoparticles of Al2O3, SiO2 and TiO2 [41]. Three types of polyethylene

nanocomposites were prepared by researchers [41] with nanofillers of SiO2, TiO2

and Al2O3. Nanoparticles content of the tested formulations were recorded at 2, 5

and 10 wt%. The average diameter of the nanoparticle for Al2O3 was 40 nm while for

SiO2 and TiO2, the average diameter was 15 nm.

Similar research were also conducted by [42] on electrical conduction at

various applied electric fields. It was found that the conduction current shows a

14

minimum reading at a 1% b.w. concentration nano alumina particle. Researchers [9,

43] studied DC conduction of MgO/LDPE nanocomposite. Through the DC

conduction pattern, they found that the addition of MgO nanofiller leads to the

improvement of DC electrical insulating properties of LDPE.

Researchers [42] have studied the electrical conduction processes in linear

low density polyethylene (LLDPE) filled with nano alumina particles. LLDPE used

in this study is a commercial linear low density polyethylene from Atofina. It has a

density of 0.934 g/cm3 at a melt index of 0.87 g/10 min. Nanoparticles of aluminium

oxide from Degussa with a particle size of about 13 nm was used as filler.

Researchers [9, 43] studied DC conduction of MgO/LDPE nanocomposite.

The employed polymer was LDPE mixed with MgO nanofiller. MgO nanofiller has

an average diameter of tens of nm. MgO nanofiller went through surface treatment

with a silane coupling agent, and then treated with the jet grinding treatment before

kneading is done. High concentration of MgO nanofiller master batch was prepared

first by the twin-screw extruder.

Researchers [44] studied surface potential decay and dc conductivity of TiO2-

based Polyimide Nanocomposite Films. In-situ dispersion polymerization process

with a thickness of 70 μm was used to prepare Polyimide/TiO2 (PI/TiO2)

nanocomposite films containing surface modified nano-TiO2 particles by employing

silane coupling agent. Researcher [38] found that, the conduction via electron

tunnelling will occur between polymer-covered particles under the influence of an

electric field only within the range of 1 - 4 nm. Due to the large surface area of the

nanofiller, conductive pathways throughout the composite can be lead [45].

2.3.1 Conduction Current Measurement System

Figure 2.1 shows the electrode configuration and the experimental setup for

the conduction current measurement used by [9, 43]. A gold electrode of 40 mm in

15

diameter was formed by vacuum evaporation on one side of the film. On the other

side, a gold electrode of 26 mm in diameter was formed as the main electrode and a

gold electrode consisting of 32 mm in inner diameter and 40 mm in outer diameter

was formed as the guard electrode. The conduction current measurement was

performed at 303 Kelvin at room temperature. The conduction current at 10 min

after the DC voltage application was employed to determine the volume resistivity.

Figure 2.1: Conduction current measurement system [9, 43, 46]

Researchers [44] have done the DC conductivity test in the oven, with 20 mm

diameter gold electrodes on both sides at 6.5 kV for 1 hour. The testing temperature

was fixed at 30 °C to avoid the influence of small temperature fluctuation in the

room on measured current [44].

Both conduction current measurements set up by researchers [9, 43] and

researcher [44] showed different DC measurement setup with different electrode

configuration. The injected voltage and measurement period also differed from both

experiments. Even though there are some differences, results from both experiments

can be used to determine the DC conductivity of the samples.

16

2.3.2 Current and Conduction Analysis

Current and conductivity had been applied by many researchers to study the

best concentration percentage of nanofiller. Researcher [44] measured current for the

TiO2 based polyimide nanocomposite at different added % amounts and these values

are shown in Figure 2.2. From the results, 3% is seen to possess the best insulation

property, 1% is slightly better than 5%, while 7% is noted to be the worst. It can be

concluded that adding nanoparticles into dielectrics can improve their electrical

properties. However, different amount of nanofillers will yield different results.

Larger amount of nanofiller can create agglomeration inside a material which can

contribute to larger overlapping interaction zone. This will cause the charge moving

increase when the content of nanofiller is increased. As 1% and 3% samples reduce

the conductivity to the lowest point, it can be known that a small amount of

nanofillers is separated inside the dielectric with a certain wide distance, which can

be known as extra traps for the dielectric, and therefore improve the insulation

property [44]. These nanofillers will fill the empty zones inside the material which

can contribute to function as the extra trapping mechanism for the dielectric.

Nanofillers may align together and help charge moving inside the material.

However, if more nanoparticles are added into the dielectrics, for example, a

5% sample has better insulation property than a pure sample, but its conductivity is

higher than 1% and 3% samples. This is because some of the nanofillers are too

close to each other, and each nanoparticle has an interaction zone around it, which

results to some overlap within the interaction zones. For the 7% sample or higher

amount nano dielectrics, the probability of interaction zones overlap is higher, and

therefore, the nanoparticles may align with each other, which help the charges to

move across the dielectric [44].

17

Figure 2.2: Current for TiO2 based polyimide nanocomposite at different % amount added

[44]

Figure 2.3 shows the influence of the MgO nanofiller content on the volume

resistivity under the field application of 40 or 80 kV/mm. The volume resistivity was

calculated by multiplying the applied field and the main electrode area to the

reciprocal of conduction current. The open sign is each value of three measurements

is in each condition. The volume resistivity of the film with MgO nanofiller under

each applied field was higher than that without MgO nanofiller. Above the MgO

nanofiller content of 1 or 2 phr, the volume resistivity of the film with MgO

nanofiller showed a saturated value [9, 43].

18

Figure 2.3: The influence of the MgO nano-filler content on the volume resistivity [9, 43]

Figure 2.4 shows I-V characteristic in alumina nanoparticles filled LLDPE

samples. The conduction current in PE-1 sample is lower than the sample without

any alumina nanoparticles. This means that the addition of a small amount of

alumina hinters the movement of charge in the bulk of the material. As the amount

of alumina increases the conduction current in PE-5 increases significantly compared

to PE-0. The conduction current in PE-10 sample shows significant reduction at

lower voltages but becomes very high once the applied voltage exceeds 5 kV [42].

19

Figure 2.4: I - V characteristics in alumina nanoparticles filled LLDPE samples at 20°C

[42]

Based on the research output highlighted earlier, it can be concluded that DC

conductivity measurement can be used to compare nano dielectrics properties.

Different types of nanofiller and the percentage of concentration will result to

different values of current and conductivity. The conduction current increases when

the filler content increase up to certain wt%. The presence of nanofillers influences

the conduction, especially for high electric fields [46]. Besides, the volume

resistivity of LDPE can be increased by addition of MgO nanofiller.

2.4 Moisture Absorption in Polymer Nanocomposite

Some research was done to study the moisture absorption of nanocomposite

polymer at different level of water content [38, 47-49]. They discussed how water

absorption by nanofillers can significantly affect the material’s electrical properties.

Researchers [50, 51] conducted tests for thermal properties, moisture absorption and

dielectric properties of pure epoxy and epoxy composites with micron-sized and

nano-sized filler. The test results found that nanocomposite studied has a higher

moisture absorption compared with microcomposite. These results were based on the

20

filler’s density, shape, as well as its surface treatment. This indicated that the

effectiveness of the dielectric constant of the composite was dependent on the

dielectric constant ratio between the filler and polymer and also the degree of

interaction between them. Different amount of nanofiller can effect the interaction

between filler and polymer and dielectric constant from the dissolving the nanofiller

uniformly inside the material. With the experimental data and molecular dipole

polarization calculations, it was shown that the addition of silane coupling agent

could form chemical bonding and improve the interphase interaction in the

composite so that the interphase dielectric constant can be increased [50].

Figure 2.5: Moisture absorption of three materials as aging time [50]

Figure 2.5 shows the moisture absorption of composites with 20 wt% filler as

a function of time after the moisture uptake was normalised to the polymer weight

percentage. From the figure above, the moisture absorption behaviour of the polymer

matrix is not affected by the micron-sized filler. On the other hand, a rapid increase

in moisture uptake was shown by the nanocomposite. It is believed that the moisture

was attracted by the hydrophilic nature of the nano-sized silica surface. The

nanocomposite has a larger interface area between the resin and the filler compared

to the microcomposite. Thus, the larger numbers of sites attract moisture. Since the

surface becomes more hydrophilic, the interfacial tension between the surface and

21

water is reduced [4]. And finally, the increased polymer dynamics in the

nanocomposite also helps the moisture diffusion. The particles with spherical shape

have a surface constituted of hydroxyl [32]. This can contribute to higher moisture

absorption.


Polarization and Depolarization Current (PDC) test were used to monitor

condition of many electrical apparatus such as machine stator, transformer and

power cables. PDC measurement with time domain polarization based technique is

widely accepted by many utilities due to the advancement in hardware and software

interpretation schemes [5]. Also this technique is very useful for estimating the

conductivity and moisture contents of the insulations. PDC test was used for the

maintenance and diagnostic tests periodically conducted on machine stator insulation

systems to continuously monitor both charge and discharge current during a step

voltage test [52, 53].

Researcher [54] has applied the PDC analyser for the insulation assessment

of power cables since 2002. He found that the most advantageous aspect of this

technique is its easy identification between “conduction” and “polarization”

phenomena in a dielectric. Researches [55-57] applied PDC technique for XLPE

insulation that was subjected to wet aging. These testing were done using AVO

Megger S1-5010 at the medium voltage (MV) underground power cables with

extruded cross-linked polyethylene (XLPE) insulation. Furthermore, there was a

research done to describe the polarization and depolarization current responses from

XLPE cable insulation with and without water trees [55]. This researcher used

apparent conductivity which was based on difference between polarization and

depolarization currents and degree of nonlinearity factor which is the ratio of

apparent conductivity at different voltage as the parameter to determine the condition

of the cables. From the results obtained, it showed that the difference of polarization

and depolarization current is proportional to the conductivity of the insulation and

22

can therefore be as a first indicator of insulation aging. Conductivity is the ability of

the material to conduct an electric current. Equation (2.4) shows that PDC value is

proportional to conductivity. The higher is the difference between these currents or

the higher the conductivity, the greater is the degradation of the cable insulation [56].

Researchers [58, 59] too assessed the insulation condition of an underground cable

by measuring its polarization and depolarization currents. PDC measurements can be

used to determine the severity of insulation degradation. The level of the

conductivity can represent the severity of degradation in term of moisture

absorption. A higher content of moisture absorption can yield to a higher level of

conductivity. This is because PDC is very sensitive to moisture content and outdoor

degradation. Polarization and conduction phenomena occur in every electrical

insulating material.

Research also has been conducted by [60] on PDC analysis for power

transformer. The scope of the analysis was not only limited to insulation between

windings, but also on insulation systems of winding-to-ground. Sometimes this can

reveal trouble in the transformer accessories, such as on-load tap changer (OLTC)

which focuses on water and contaminant in a new OLTC, moisture and surface

humidity and free water in a refurbished transformer. This technique had also been

already applied as a quality assurance tool for the assessment of refurbishment

efficiency of power transformers by researcher [61] and moisture assessment of

transformer bushings by researchers [62]. Estimation of water content and

conductivity in a power transformer that is focused on paper insulation also were

done using PDC measurement by researcher [12, 63-72]. Moisture and aging

strongly influence the dielectric properties of oil-paper insulation system of power

transformer. Moisture accelerates the aging process in the oil-paper insulation of the

transformer. PDC analysis was also done by researcher [73 and 74] on the influence

of thermal transients and thermal runaway of oil-paper insulation. These researches

were done to identify and locate overheating/thermal fault in the transformer solid

insulation using PDC analysis [75]. There was also research done on paper oil

insulation under conditions of copper corrosion using PDC technique [76].

23

The PDC patterns of mineral oil, biodegradable oil and paper as transformer

insulations were studied by many researchers such as [12, 13, 63]. Researches [13,

68, 73, 74, 77, 78] were done to investigate the moisture content and conductivity of

the oil insulation that is focused on transformer oil. There was also research carried

out to find the dielectric responsive function and maximum conductivities of

biodegradable and mineral transformer oils with different moisture levels (dry,

normal, or wet) for comparative analysis by researcher [12]. This research examined

the condition of the insulation oils at different moisture levels. Comparison on oils

conductivity level (σ) and measured capacitance value (Cm) in each condition with

the PDC fingerprint of the oils during the initial installation was done. The trends of

the PDC response were found to be different for those two insulation oils. They

extended the research for faults identification of biodegradable oil-filled

transformers by comparing them with the PDC pattern [79]. The PDC technique

provides significant information that can be used in the evaluation of the transformer

condition. Each fresh insulation oil has its own unique PDC fingerprint response

[12].

Polarization and depolarization currents analysis were done in biodegradable

polymers that can be used in electrical apparatus and cable [80]. For instance, one

research presented and discussed the data on polarization and depolarization

characteristics of thermally degraded epoxy mica composites [81]. Besides,

researchers [82] obtained conduction currents results using PDC for epoxy resin

nanocomposites. A DER 332 epoxy resin was chosen as matrix and nanosilica and/or

Boron Nitride were chosen as fillers. They obtained the dielectric response in the

frequency domain.

Other researches were done to monitor insulation condition on transformer

oil, XLPE cable insulation, rotating machines insulation and biodegradable polymers

using PDC test. Similarly, a lot of research were done on application of PDC

technique in assessing the conductivity and moisture content of the insulation of the

electrical equipment. These studies examined the effects of moisture level, aging and

water trees of the insulation.

24

In conclusion, the PDC method is a simple and reliable technique based on

time domain analysis to monitor HV insulation systems. From the PDC results,

dielectric responsive function of each sample was determined to examine the

dielectric response based on time domain. This is a natural response of the material

itself due to the depolarization current that was measured from the sample.

In this research, PDC measurement was performed on polymer

nanocomposite as an insulating material at various conditions. The viability and

reliability of the PDC method in providing information on the conductivity and

moisture content of polymer nanocomposite has not been explored and done before,

and thus it was investigated in this research.

2.5.1 Basic Principle of PDC

The measurement principle of polarization and depolarization current is

based on applications of DC voltage across the test object for a long time. According

to [41], the test voltage at 500 V has led to satisfactory results on measurement

performed on LDPE nanocomposite for DC conductivity. Most of the researchers in

PDC testing used 1000 V in the same test with duration of 10,000 seconds. During

the test time, the current arising from the activation of the polarization process with

different time constant that corresponded to different insulation materials due to the

conductivity of the object was measured. Then the voltage was removed and the test

object was short circuited. The activated polarization process then gave rise to the

discharging current in the opposite direction, where no contribution of the

conductivity was present. The charging and discharging currents were influenced by

the properties of the insulating materials as well as by the geometric structure of the

insulating system.

According to [12, 13, 52, 83, 84], polarization and depolarization currents

measurement can be used to investigate the polarization process of a dielectric

material in the time domain analysis. The test object can be a single dielectric

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