electrical characteristics of aged composite insulators...

Electrical Characteristics of Aged Composite Insulators

JianBin Zhou

September 2003

Electrical Characteristics of

Aged Composite Insulators

A thesis submitted for the degree of Masters Degree

by

JianBin Zhou, B. Eng

School of Electrical & Electronics Systems Engineering

Queensland University of Technology

September 2003

Declaration of originality The work contained in this thesis has not been previously submitted for a degree or diploma at

any other higher education institution. To the best of my knowledge and belief, the thesis

contains no material previously published or written by another person except where due

reference is made.

Signed:

Date:

Acknowledgements

The author wishes to express sincerely thanks to his principal supervisor Associate Professor

David Birtwhistle of the Electrical & Electronics Systems School of Queensland University of

Technology for his support, suggestions, and encouragement through this 3 years research.

Also the author wishes to thank his associate supervisor Dr. Greg Cash of the School of

Physical Sciences for his guidance, support, and help for this research. The author wishes to

express thanks to Professor R.S. Gorur from Arizona University, who provided valuable help

regarding new composite insulator assessment method described in Chapter 6.

Thanks are due to Mr. Ronald Penfold for his work in setting up lab for research test,

especially for the help on setting up the fog chamber in the lab. The author wishes to thank to

Dr. HePing Liu of School of Physical Sciences for performing part of the infrared emission

spectroscopy and Ms. Wen Hu of Faculty of Science for performing part of the scanning

electron microscopy analysis in the thesis.

The author gratefully acknowledges Powerlink of Queensland in funding the research and

financial support from the Electrical & Electronics School of Queensland University of

Technology for travel assistance.

Keywords:

composite insulator, electrical characteristic, aging, fog chamber, EPDM insulator, leakage

current, waveform, chemical analysis, ester/ketone ratio, oxidation index, scanning of electron

microscopy

Abstract:

Composite insulators are widely being used in power industry to alternate traditional

porcelain-based insulators for their advantages, including better pollution performance, low

maintenance cost, light weight, compact line design. However, due to the short application

history and experience, the degradation of composite insulators in natural environment is a

big concern for the power utilities. The knowledge on the degradation of composite insulators

is being studied world wide. The methods to assess the working conditions of composite

insulators are being studied and created. In Queensland University of Technology (QUT), the

approach based on chemical analysis methods was first developed. The work in this thesis

based on the previous research work is focused on correlating electrical characteristics with

chemical analysis results of the composite insulators and physical observations results. First,

the electrical characteristics of composite insulators were presented and analysed, including

leakage current, cumulative current, peaks of leakage current, the statistic results of the

leakage current. Among them, the characteristics of leakage current were mainly studied. The

shape of waveforms was found to relate to the degree of discharge activities of the composite

insulators. The waveforms analysed by FFT revealed that the odd harmonic components

became obvious during the discharge activities. The correlations between the electrical

characteristics of composite insulators and chemical analysis results showed that the

composition of composite insulators plays significant roles in terms of electrical performance.

The oxidation index (O.I.) and the ester/ketone ratio (E/K) differentiated the different

degradation reasons of the composite insulators in the test conditions. Finally, the thesis

presents one approach, which aims to assess the surface conditions of composite insulators in

an easy manner and in short time.

Publications Arising From This Thesis

J. Zhou and D. Birtwhistle, "Comparison of Electrical Performance of EPDM Composite Insulators with Chemical and Physical Indicators of Shed Material Condition", presented at Electricity Engineers' Association of NZ, Christchurch, NZ, 2002. J. Zhou, D. Birtwhistle, and G. Cash, "Chemical and Electrical Techniques for Condition Assessment of Composite Insulators", presented at The 7th International Conference on Properties and Applications of Dielectric Materials, Nagoya, Japan, 2003.

FIGURES

Figure 2-1 Section View of a Long Rod Composite Insulator 4 Figure 2-2 Section View of a Post-type Composite Insulator 4 Figure 2-3 Formation of Cross-linked Polymer 6 Figure 2-4 Chemical Structure of Polydimethyl Siloxane Polymer 7 Figure 2-5 Cause Failure Distribution 9 Figure 2-6 Degradation Process of EPDM due to Impurities 11 Figure 2-7 Scanning Electron Microscope Images of the Surface of an Aged EPDM Insulator 14 Figure 2-8 Live Line Tool for Sampling Composite Insulators 16 Figure 2-9 Diagram of the Infrared Emission Spectrometer 17 Figure 2-10 Infrared Spectrum of an Aged EPDM 19 Figure 2-11 Oxidation Index of a 275kV EPDM Aged Insulator 20 Figure 2-12 Chalking Index of an Aged EPDM Insulator 21 Figure 2-13 Expanded FTIR Spectra from the Carbonyl Region 22 Figure 2-14 Scatter Plot Relating the Oxidation Index to the Ester/Ketone Ratio of Insulators from Different Locations 25 Figure 3-1 Fog Chamber Test Systems 28 Figure 3-2 Fog Chamber 29 Figure 3-3 Plan View of the Fog Chamber 30 Figure 3-4 Control Panel Systems 31 Figure 3-5 Power Systems of the Fog Chamber Control Panel 32 Figure 3-6 Layout of the Fog Chamber and the Control Panel 32 Figure 3-7 Test System and Measurement & Protection Circuitry 34 Figure 4-1 IEC 1106 Accelerated Weather Aging Cycle under Operating Voltage 38 Figure 4-2 Classification of Leakage Current Measurement 50 Figure 5-1 Hydrophobicity Classification of EPDM Insulator in Fog Condition 56 Figure 5-2 Hydrophobicity Classification on Topside Shed of EPDM Insulator #3 57 Figure 5-3 Water Droplets on Core Surface of #3 Insulator 57 Figure 5-4 Water Droplets on Core Surface of #3 Insulator 58 Figure 5-5 Rectified Mean Values of LC (0-100hours) (#1 -#3) 59 Figure 5-6 Statistics of MVLC of the three Insulators 60 Figure 5-7 MVLC of the Three Insulators in Test 61 Figure 5-8 Waveforms of LC at Specific Time (#1-#3 Insulators) 62 Figure 5-9 SEM Images of Surface of the Insulators (#1-#3) before and after the Test 65

Figure 5-10 Mean Values of LC of Insulators in Test A 71 Figure 5-11 Cumulative Charge of LC of Insulators in Test A 71 Figure 5-12 LC Distribution of Two Composition Insulators in Test A 72 Figure 5-13 MVLC of Insulators #1- #4 in Test B 77 Figure 5-14 MVLC of Insulators #5- #8 in Test B 77 Figure 5-15 LC Distribution of the same material insulators in Test B 78 Figure 5-16 (a) LC Waveforms and FFT of #1 Insulator in Test A at Specific Time 82 Figure 5-16 (b) LC Waveforms and FFT of #3 Insulator in Test A at Specific Time 83 Figure 5-17 LC Waveforms and FFT Results of #1 Insulator in Test B at specific time 88 Figure 5-18 LC Waveforms and FFT Results of #5 Insulator in Test B at specific time 90 Figure 5-19 LC Waveforms and FFT Results of #3 Insulator in Test B at specific time 92 Figure 5-20 LC Waveforms and FFT Results of #7 Insulator in Test B at specific time 94 Figure 5-21 LC Waveforms and FFT Results of Flashovered Insulators before Flashover 99 Figure 5-22 Interval Time between Flashovers 101 Figure 5-23 SEM Images of the eight insulators after test C 103 Figure 5-24 Sequence of Discharges on Hydrophobic Silicone Rubber 109 Figure 5-25 Locations of Sampling for Chemical Analysis 110 Figure 5-26 Oxidation Index of Insulators #1 - #8 before and after the Tests 111 Figure 5-27 Ester/ketone Ratio of Insulators #1 - #8 before and after the Tests 112 Figure 6-1 Arrangement for Water Spray Test 118 Figure 6-2 Surface Conditions of sr1 before and after using Liquid Soup 120 Figure 6-3 LC Waveform of sr1 in Spray Water Test (conductivity=315µS/cm with liquid soup) 121 Figure 6-4 Surface Resistance of Insulators in Spray Water with Liquid Soup 121

Figure 6-5 Thermal effect on insulator surface caused by leakage current 123

Table

Table 2-1Number of Insulators that have Failed in Service 8

Table 2-2 Chemical Analysis Report of Aged Medium Voltage Insulators in Queensland 23

Table 4-1 Summary Test Methods on Composite Insulators 37

Table 4-2 Parameters of Standard 61109Test Conditions 40

Table 4-3 Overview of Discussible Parameters 40

Table 4-4 Main Characteristics of the Inert Material Used in Clean Fog Tests 41

Table 4-5 Kaolin Composition: Correspondence between the Reference Degrees of Pollution

on the Insulator and Volume Conductivity of the Slurry 42

Table 4-6 Relationship between θ and b 44 Table 4-7 Correspondence between the Value of Salinity, Volume Conductivity, and Density

of the Solution at a Temperature of 20ºC 47

Table 5-1 Parameters of Insulators (test voltage = 12kV) 53

Table 5-2 Parameters of Preliminary Test Conditions 54

Table 5-3 Distribution (number of times) of MVLC of the Three Insulators 60

Table 5-4 Chemical Analysis Results of the three Insulators before and after the Test 63

Table 5-5 Electrical Characteristics of the three Insulators 63

Table 5-6 HC and Contact Angle of Insulators before and after the Test 64

Table 5-7 Comparison of SEM Images of the three Insulators 67

Table 5-8 Test Parameters of Tests 69

Table 5-9 Parameters of Insulators of Batch I & II 70

Table 5-10 Insulators’ List after Test C 70

Table 5-11 LC Distribution of Two Composition Insulators in Test A 73

Table 6-1 Shape of Sample Insulators 119

Table 6-2 Configurations for Water Spray Test 119

Table 6-3 Thermal conductivity of four materials used in insulators 124

Table of Contents

CHAPTER 1 INTRODUCTION…………………………………………...1

CHAPTER 2 COMPOSITE INSULATORS……………………………...4

2.1 Construction & Material of Composite Insulators…………………………….........4

2.1.1 Construction……………………………………………………………………4

2.1.2 Housing and Weathershed Materials…………………………………………..5

2.1.2.1 Introduction……………………………………………………………….5

2.1.2.2 Ethylene Propylene Diene Monomer (EPDM)……………………………6

2.1.2.3 Silicone Rubber……………………………………………………………6

2.1.3 Service Report of Composite Insulators……………………………………….7

2.2 Aging of Field EPDM……………………………………………………………..10

2.3 New Condition Monitoring Techniques for Composite Insulators……………….13

2.3.1 Introduction…………………………………………………………………..13

2.3.2 New Method to Diagnose the EPDM Insulators……………………………..13

2.3.3 Oxidation Index Analysis Method……………..…………………………….15

2.3.3.1 Sampling…………………………………………………………….......15

2.3.3.2 Oxidation Index…………………………………………………………17

2.3.4 Chalking Index Analysis Method……………………………………………20

2.3.5 Ester / Ketone Ratio Index…………………………………………………..21

2.3.6 Investigation of Aging of Medium Voltage Insulators………………….......22

2.4 Summary…………………………………………………………………………25

CHAPTER 3 DEVELOPMENT OF THE TEST EQUIPMENT……..27

3.1 Literature Review………………………………………………………………..27

3.2 Fog Chamber System……………………………………………………………28

3.2.1 Fog Chamber…………………………………………………………………28

3.2.2 Fog Chamber Control System………………………………………………..30

3.2.3 High Voltage Supply Equipment…………………………………………….33

3.2.4 Data Acquisition System…………………………………………………….34

3.3 Summary………………………………………………………………………….35

CHAPTER 4 REVIEW OF ELECTRICAL TESTS ON COMPOSITE

INSULATORS…………………………………………………………….36

4.1 Introduction……………………………………………………………………….36

4.2 Electrical Test Standards………………………………………………………….36

4.2.1 Introduction…………………………………………………………………..36

4.2.2 IEC Standard……………………………………………………………........38

4.2.3 IEEE Standard………………………………………………………………..41

4.2.3.1 The Clean Fog Test………………………………………………………41

4.2.3.2 The Salt Fog Test………………………………………………………..46

4.3 Test Methodology…………………………………………………………………48

CHAPTER 5 AGING TESTS ON COMPOSITE INSULATORS…….52

5.1 Introduction……………………………………………………………………….52

5.2 Preliminary Tests on Composite Insulators………………………………………53

5.2.1 Test Introduction………………………………………………………..........53

5.2.2 Hydrophobicity Loss on Surface of Composite Insulators…………………..54

5.2.3 LC Measurement Results……………………………………………….........59

5.2.4 Comparison between Electrical Characteristics, Chemical Analysis and Physical

Analysis Results……………………………………………………………………63

5.2.5 Summary…………………………………………………………………….67

5.3 Electrical Tests on Composite Insulators………………………………………..69

5.3.1 Test Conditions and Insulators’ Parameters…………………………………69

5.3.2 Results of Test A…………………………………………………………….70

5.3.3 Results of Test B…………………………………………………………….74

5.3.4 Summary.………… …………………………………………………….......78

5.3.5 Waveforms of LC.. ………………………………………………………....80

5.3.5.1 Introduction …………………………………………………………..80

5.3.5.2 Waveforms and Analysis of LC in Test A - Clean Fog Test……….......80

5.3.5.3 Waveforms of LC in Test B – 2.5 kg/m3 Salt Fog………………….......86

5.3.5.4 Waveforms of LC in Test C – 5 kg/m3 Salinity Fog Test………………98

5.3.6 Summary……………………………………………………………….........101

5.4 Relationships between Physical, Chemical Analysis Results of Insulator………103

5.4.1 SEM (Scanning Electron Microscopy) Observations of Insulator Surface…103

5.4.2 Chemical Analysis Results and Comparison with Physical Observations….110

5.4.3 Discussion of the Relationships between Physical Characteristics and Chemical

Analysis Results of Composite Insulators………………………………………..115

CHAPTER 6 SURFACE RESISTANCE MEASUREMENT TO ASSESS

SURFACE CONDITIONS OF COMPOSITE INSULATORS11…….117

6.1 Introduction………………………………………………………………………117

6.2 Leakage Resistance Assessment Using a Water Spray…………………………..117

6.2.1 Test with Water Spray……………………………………………………….117

6.2.2 Test with Reduced Surface Tension Water………………………………….119

6.2.3 Surface Resistance of Artificially-wet Insulators……………………………120

6.3 Discussions……………………………………………………………………….122

6.4 Conclusions……………………………………………………………………….125

CHAPTER 7 SUMMARY……………………………………………...127

7. 1 Electrical Characteristics of Composite Insulators in Fog Tests………………….127

7. 2 Relationships between Electrical Characteristics and Surface Conditions of

Composite Insulators………………………………………………………………….129

7. 3 Future Work………………………………………………………………………130

Appendix

Appendix-1 - Control Circuitry of Fog Chamber

Appendix-2 - Equivalent Circuitry of Test Transformer and Variac

Appendix-3 - Power Supply in Fog Chamber Test System

Appendix-4 - Terminal Connection of Control Panel and Fog Box

Appendix-5 - LC Waveforms and FFT Analysis of #2 & #4 Insulators in Test A

Appendix-6 - LC Waveforms and FFT Analysis of #2, #4, #6 & #8 Insulators in Test B Appendix-7 - LC Waveforms and FFT results of the Eight Insulators before Flashover in Test C

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CHAPTER 1

INTRODUCTION

Insulators are important electrical equipment, which are installed on power transmission

towers or poles to suspend or support transmission power lines. The history of composite

insulators in application can be traced back in the late 1960s and 1970s [1]. In 1980s,

composite insulators were widely accepted as substitutes for traditional ceramic insulators [3]

[4]. The advantages of composite insulators over traditional insulators include lightweight,

better contamination performance, resistance to impact damage, particularly to gun-shot, and

the feasibility of compact line design [3].

· Lightweight: This great advantage of composite insulators over porcelain reduces the weight

of insulators dramatically, and this characteristic reduces the cost of design, construction, and

maintenance of power tower and poles. In inaccessible areas, the characteristic of lightweight

of composite insulators makes it possible for helicopters to set up high voltage power towers

[5].

· Resistance to damage: It is well known that porcelain insulators can be broken accidentally,

such as transportation, installation, and collision. Also vandalism is an important reason

responsible for the failure of porcelain insulators, especially by gunshots. Composite

insulators are resistant to deliberate damage and this is an important reason for power supply

companies to be fond of them.

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· Compact Line Design: The characteristics of light weight of composite insulators, which

means less volume than traditional insulators, simplify and optimise the traditional power

tower design, especially on high or extra-high voltage transmission towers. The compact line

towers have been applied in Queensland on 132 kV and 275 kV power lines. Report on

reasons of selection of composite insulators suggested lighter clarifies 30% [6]. In urban

areas, the compact line design reduces the visual impact and improves the urban landscape.

· Pollution Performance: The materials of sheds and sheath of composite insulators are

commonly silicone rubber (SIR), ethylene propylene rubber (EPR), or the combination of

these two materials and other materials, including fillers, anti-oxidants, colorants, UV

stabilisers. One important characteristic of SIR and EPR is hydrophobicity [7]. Compared

with porcelain insulators, which have high surface free energy, composite insulators have

lower surface free energy, which help them not easily form water films on surface. So they

provide good water resistant characteristic for insulators in moist environments. In polluted

environments, this characteristic gives composite insulators good resistance to form atom of

conductive electrolyte along insulator surfaces, which can initiate flashover process [8].

However the polymeric materials of composite insulators deteriorate with time under natural

environment and electric stress. Lifetime of composite insulators is therefore shorter in some

cases [9].

The deterioration of polymer materials of composite insulators over time is a multi-factor

aging process. The process happens in the conditions, which combine natural environmental

and electric stresses. The factors from the natural environment include: fog, ultraviolet (UV),

rain, moisture, temperature, and pollution. These factors combine electric stress in wet and

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pollution environments resulting in the acceleration of deterioration of polymeric materials.

Studies of the fundamental mechanisms of deterioration of composite insulators in different

environments and to find efficient methods of assessing rates of deterioration of composite

insulators become necessary and urgent with development of polymeric materials. Better

understanding of deterioration of composite insulators helps power supply companies to draw

up optimising policies to manage a large number of composite insulators. Efficient, fast, low

cost, and practical assessment methods on composite insulators help power supply companies

appraise the conditions of composite insulators and avoid insulator failures during service life.

Also the conditions of composite insulators provide useful feedback information to line

designers, which let them select composite insulators with knowledge and service experience.

The objective of this thesis is to investigate how the electrical characteristics of EPDM and

EPDM/silicone rubber insulators change in laboratory test conditions, and to correlate the

electrical characteristics with results of surface analysis of EPDM and EPDM/silicone rubber

insulators. Using procedures developed over the past two years at QUT, the knowledge of

relationships between electrical characteristics and chemical analysis results supplements the

understanding of the aging process and leads to improved aging assessment methods for

composite insulators. The thesis also investigates one new technique for diagnosis of aging

conditions of composite insulator surfaces based on surface resistance measurement. The

method explores a new direction for assessment of aged composite insulators in a fast,

efficient, and economical way.

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CHAPTER 2

COMPOSITE INSULATORS

2.1 Construction & material of Composite Insulators 2.1.1 Construction

There are two main types of composite insulators commonly used in the power industry. One

type is the suspension insulator. The other one is the post type insulator [10, 11]. A typical

suspension insulator is shown in Figure 2-1. Suspension insulators are mainly used in the

situations where insulators are in tension. Post type insulators (see Figure 2-2) are used in the

situations where there is compressive load and bending force.

Figure 2-1 Section View of a Long Rod Composite Insulator [10]

Figure 2-2 Section View of a Post-type Composite Insulator [10]

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Basically, composite insulators have four main components. They are core, protective

housing, weather sheds, and end fittings. The protective housing and weather sheds of

composite insulators are made of polymeric materials. In the centre of composite insulators, a

load-bearing core carries the mechanical load. The core is composed of glass fibre and

bonded with thermosetting resin. The protective housing and weather sheds envelope the

whole core and provide environmental protection and long surface electrical creepage length.

Mental end fittings consist of malleable iron or aluminium alloy, which are compressed

around the core.

The main differences between suspension insulators and post type insulators are the design of

end fittings and the size of the core. The diameter of the core for post insulators is larger than

that of the suspension insulators due to the fact that its load is dominated by bending force.

Some other forms of composite insulators for specific applications in electrical equipment,

e.g. circuit breakers and transformers, have hollow fibreglass-reinforced core [12].

2.1.2 Housing and Weathershed Materials

2.1.2.1 Introduction

A variety of polymeric insulating materials have been used in composite insulators for

overhead lines. They include, PTFE (Teflon), epoxy resins, polyethylene, instant set polymers

based on urethane chemistry, polymer concretes, various copolymers, ethylene-propylene

elastomers [11]. In this section, ethylene propylene diene monomer (EPDM) and silicone

rubber (SIR) while as by for the most common are described in detail.

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2.1.2.2 Ethylene Propylene Diene Monomer (EPDM)

EPDM is the copolymer of ethylene and propylene with the mixture of nonconjugated diene.

According to [13, 14], the chains of EPDM consist of randomly combined ethylene and

propylene units forming saturated polymer chains without double carbon bonds, which are

susceptible to attack by ozone and UV. A diene provides C=C double bonds capable of

forming cross-linking. The main chains without double carbon bonds possess the

characteristics of high resistance to weathering by UV and ozone [15]. The cross-linking

process of EPDM with the most common diene, ethylidene-norbornene, is shown in Figure 2-

3.

Figure 2-3 Formation of Cross-linked Polymer [13]

2.1.2.3 Silicone Rubber

Silicone rubber is based on polymers having a molecular backbone of alternate atoms of

silicon and oxygen. Some organic groups are attached to the main chains, which include

methyl, phenyl, or vinyl. The structure of silicone rubber provides properties of good

resistance to UV and ozone. One basic polymer used in silicone rubbers is shown in Figure 2-

4, which is predominantly based on linear chains of polydimethyl siloxane polymer [2].

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There are two basic types of silicone rubbers. One is high temperature vulcanised (HTV) or

room temperature vulcanised (RTV) and the other is liquid silicone rubber (LSR). Recently

the insulator industry has introduced new materials, which are blends of EPDM polymer and

silicone rubber. The purpose is obviously to combine the rigidity characteristic of EPDM with

the hydrophobic properties imparted by silicone rubber. Apart from the base polymers, the

shed materials also include: UV stabilisers, reinforcing filler (e.g. hydrated aluminium

Al2O3·3H2O), tracking and erosion fillers, processing aids and colorants. The amount of base

polymers range from 10% to 90% of the net weight of the insulator.

2.1.3 Service Report of Composite Insulators

The survey by CIGRE [4] revealed that the benefits of the improved technology and the

increased use of composite insulators have led to decrease in costs of composite insulators,

which has made them competitive with conventional porcelain insulators. According to the

CIGRE survey, the main reason for selecting composite insulators was that they possess good

performance under pollution environment. Silicone occupies over 90% of weathershed

material of insulators and 4.2% composite insulators are made of EPDM.

For power industry and insulator researchers, the life expectancy of composite insulators is

the main concern in terms of application. Table 2-1 abstracted from the report [8] shows the

Figure 2-4 Chemical Structure of Polydimethyl Siloxane Polymer [2]

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number of insulators, which have failed in service. The total number shows that rod failure is

the biggest failure reason followed by weathershed-related failures. The next reason is rod slip

out. End-fitting is the least reason to be responsible for composite insulator failures.

Table 2-1 Number of Insulators that have Failed in Service [8]

But for composite insulators under 200 kV power systems, the number of weathershed–

related failures is nearly three times of the number of rod failures. This reason also came the

first in the previous report [6]. That report combined two surveys carried out by the

Southeastern Electric Exchange (SEE), USA, in 1987 and the Electric Power Research

Institute (EPRI), USA, in 1988. The two surveys covered 45,817 composite suspension

insulators across over 58 utilities and a total of 26,967 composite post type insulators used by

51 utilities. Figure 2-5 shows the reasons for deterioration according to the survey. The entire

failure rate was 0.43%, which is higher than that of traditional ceramic insulators. In the

figure, deterioration of weathershed is the major reason responsible for the failures of

composite insulators (64%). In that case deterioration includes erosion, corona cutting,

chalking, and crazing of the surfaces.

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It is noted that the surveys [6] included the first generation design of composite insulators, so

the result of statistics may contain the failure modes of the first generation designs and these

modes may not be characteristic of later improved designs.

There were some reports about service failure accidents of composite insulators. In 1991

Christmas period in Florida, U.S.A, the Florida Power & Light Co. suffered record number of

contamination-related outages. According to the outage investigation report [16], FPL

experienced 172 outages on overhead lines in just 9 days that was far more than the normal

level. Bad weather conditions were blamed for the outages. Under serious fog conditions,

composite insulators lost dielectric properties quickly and resulted in a wide range of

flashovers. The investigation of the incident observed that foggy weather conditions and

industry pollution were responsible for flashovers on insulators of transmission lines and

substation power equipment. These cases show the fact that under specific environments

composite insulators could show poor insulation characteristic rather than reliable

performance. Aging of composite insulators was another important factor which was involved

Figure 2-5 Cause Failure Distribution [6]

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in the process of rapid-loss of insulation property under serious wether conditions. The

following section describes the mechanism of aging of EPDM insulator material.

2.2 Aging of Field EPDM

It is known that composite materials or polymer materials degrade with exposure in the

natural environment [15] and electrical environment [9]. Aging mechanism of composite

materials involves external and internal factors. The deterioration of composite materials

depends on how and to what extent it interacts with its surroundings. In terms of outdoor

environmental factors, the main components of the environment, which accompany

deterioration, are sunlight, temperature change, moisture, wind, dust, and pollutants. These

factors vary widely in duration, intensity and sequence. On the other hand electrical factors,

such as discharges, corona, and leakage current, etc. also play significant roles in the process

of aging [2]. The consequence of aging includes surface cracking or crazing, ablation of

surface material and exposure of insulator inner material. Aging may also cause breaching of

protective housing and even mechanical failure of the structural core. Among the factors, for

EPDM insulators, photo-oxidation is ascribed the main reason for material deterioration [17].

The following sections describe the photo-oxidation aging mechanism.

Theoretically, pure EPDM should not be affected by terrestrial ultraviolet (UV) light of

wavelength >290nm; however, experience showed that EPDM deteriorates under UV. The

research [18] revealed that polyethylene and polypropylene both are susceptible to light with

wavelength of 300nm, and terrestrial UV light with wavelength greater than 290nm was

identified as the main cause of deterioration. And the presence of impurities, such as

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hydroperoxides and carbonyl groups, which are left from the process of manufacturing, e.g. in

the process of high temperature extrusion and injection moulding processes, initiates the

process of aging. Because catalyst residues are able to absorb sunlight, they ultimately

produce a free radical, which initiates a chain photo-oxidation process. The process is

illustrated in Figure 2-6 [17].

Figure 2-6 Degradation Process of EPDM due to Impurities [17]

(i) The impurities, which are attached to the carbon backbone of the polymer chain, absorb

the sunlight (wavelength λ≥290nm), and produce free radicals, which are the triggers of a

chain reaction.

(ii) The presence of oxygen reacts with radical forming a peroxy radical.

(iii) The peroxy radical reacts with another polymer chain to produce another free radical and

a hydroperoxide (OOH).

(iv) The hydroperoxide absorbs UV and breaks down to produce an alkoxy radical (O•) and a

hydroxy radical (OH•).

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(v) Because the hydroxy radical is very active, it reacts with another polymer chain to produce

further free radical and produces water. Electrical-driven aging on composite insulators

depends on several factors, including environment, composition of insulators, electrical field

stress, and surface condition. In terms of aging, corona and leakage current play considerable

roles which are responsible for composite insulator deterioration [18]. There are three aspects

of aging mechanism caused by corona. (1) Electrical particles impact directly on the bonds of

insulator chains. It is estimated that the energy of one electron is about 3.2 eV, while one C-C

bond binding energy is 4 eV and ionisation energy is about 10-11 eV, so there is chance that

one high energy electron could cause scissoring C-C. When this process is repeated, it leads

to the gradual degradation of the polymer. (2) High temperature on composite insulators

caused by short-period discharge activities can reach as high as 1000 ºC, which would lead to

disintegration of the polymer producing caves, carbon deoxidation, melt, and dissolve the

structure of composite insulators. (3) The dynamic products left by discharge activities such

as O, O2*, and O2+, react with chemical molecular of composite insulators, strengthening the

process of aging. Leakage current brings about the formation of dry band, arcing [2], and this

induces tracking on insulator surface. Tracking is irregular and random between HV and

ground. Similar to the process of discharge activities, the effect of tracking results in the

decrease of hydrophobicity of the surface of insulators, decrease in the volume resistance of

insulators, and acceleration of aging of tracking sites.

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2.3 New Condition Monitoring Techniques for Composite Insulators

2.3.1 Introduction

According to a survey [19] conducted by electricity distribution organisations in Queensland,

Australia, over 7,000 EPDM insulators have been installed in the 66-132 kV range network.

EPDM insulators currently make up 91% of the composite insulator population. The survey

shows 6% of composite insulators are made from silicone rubber, and 3% of composite

insulators are made from the mixture of EDPM and silicone. More than 13,000 medium

voltage (11-33 kV) EPDM insulators are in service across Queensland [20]. The increase in

the use of EPDM insulators in power industry requires efficient, practical, and economical

method to indicate working condition of composite insulators.

2.3.2 New Method to Diagnose the EPDM Insulators

According to Vlastos [21] EPDM composite insulators show good hydrophobicity

characteristic during the first few service years, but deteriorate after they become hydrophilic.

Vlastos and Sherif [22] found that EPDM composite insulators from a +300 kV DC test line

in Sweden flashed over less often than porcelain and glass insulators installed in the same line

during the first three years of service. Unexpectedly, composite insulators flashed over more

often than ceramic insulators during the subsequent 5 years of operation. In 1991, Florida

Power and Light suffered the most serious outages in history [16]. Accident analysis revealed

that flashovers caused by fog environment and degradation of housing of composite

insulators. Two kinds of EPDM composite insulators had worse flashover records than

normal porcelain insulators with same creepage length. From these two cases, the reduced

performance of EPDM insulators was attributed to aging of the surface of the EPDM

insulator.

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Degradation of EPDM typically happens with the process of losing hydrophobicity on

insulator surface. Accompanied by losing hydrophobicity, surface of EPDM insulator would

appear surface cracks and produce surface layer known as chalking or flouring due to natural

and electrical aging factors [21].

(a) Top View (×1000) (b) Cleaved Section (×1000) Figure 2-7 Scanning Electron Microscope Images of the Surface of an Aged EPDM insulator Left: surface in plan view showing surface cracking and erosion. Right: section through surface indicating a layer of loose surface material (chalking) and bulk polymer

Figure 2-4 shows surface conditions of an aged EPDM insulator using SEM (Scanning

Electron Microscope). It is clear that on surface of the aged EPDM insulator, cracks and

chalking layer formed as the results of degradation. The loose powdery surface accumulated

chalking materials and deposits on insulator surface. The explanation for this is due to

degradation on insulator surface. The oxidation process on composite insulators results in

water repellence of polymer surface decreasing from hydrophobicity to hydrophilicity. The

predominant form of oxygen exists in oxidised hydrocarbon polymers in a single oxygen

atom is bound to a carbon atom in the polymer chain through a double bond, which is known

as carbonyl (C=O). Theoretically, carbonyl is the product of oxidation process, so monitoring

its development and existence provides evidence of the existence of surface oxidation. If

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using some technology to assess the composition of oxidation products in EPDM material, the

information on deterioration extent of EPDM insulators could be retrieved.

2.3.3 Oxidation Index Analysis Method

At QUT, researchers have developed a new method to quantify oxidation and chalking extent

for aged EPDM insulators [18, 24-27]. This method provides a simple, practical, and cost-

effective condition assessment technique for power companies to operate. The results provide

useful and reliable information related to assessing working condition of composite insulators.

It supplements the existing composite insulator assessment methods, which include visual

inspection (naked eyes and SEM), hydrophobicity classification [27], and on-line leakage

current measurement [29, 30]. The following section is the description of this method.

2.3.3.1 Sampling

The first step of this method is to take a material sample from the surface of composite

insulators. There are three sampling methods available:

Surface swabbing

For EPDM a cotton bud soaked in xylene is used to swab an area of 10 cm2 on the surface of

insulator to get suitable amount material for analysis. In this procedure traces of surface

polymer are dissolved by the xylene solvent. Some non-soluble material such as ATH fillers

and surface impurities are also removed by this method. The cotton bud is subsequently

rinsed in a bottle, which contains 0.5 cm3 of xylene. The solution that contains the dissolved

polymer material and other substance is then allowed to settle and polymer-solvent free of

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solid material is drawn off for chemical analysis. It is noted that the swabbing process does

not damage insulators.

Surface scraping

This technique is achieved by razor blade. The amount of the shed surface for analyse is about

1 mg. Infrared absorption spectroscopy is employed to analyse the surface and get chalking

degree information.

Surface planing

This sampling method is accomplished by use of a patented hot-stick device to cut thin slivers

of surface material from sheds. The slivers area is ~3 cm2 and they are ~0.25 mm thick. The

samples are analysed by XPS to determine the composition of surface layer of material. Also

scanning electron microscopy (SEM) is used to observe the surface condition of slivers. A

live line tool has been developed to make it possible to take samples without de-energising

lines [26]. Figure 2-8 shows the line tool to get samples from live power line insulators.

Figure 2-8 Live Line Tool for Sampling Composite Insulators

Swabbing tool

Planing tool

Attached live tool

Scraping tool

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2.3.3.2 Oxidation Index

From the surface swabbing method, the solvent contains minute traces of dissolved polymer,

which can be analysed using Fourier Transform Infrared (FTIR) spectroscopy. The oxidation

of insulator surface is determined by analysis of infrared spectrum. The theory of this method

is described as follows.

The frequency of vibration and the wavelength of absorbed or emitted radiation is

characteristic of the resonant frequency of the chemical bond, that means the vibrations of

carbonyl (C=O) bonds can be clearly different from those of sound polymer (C-H) bonds.

Researchers at QUT developed a method using infrared emission spectroscopy technique to

measure the wavelength of C=O and C–H bonds from the samples which can be retrieved

from the sampling method (A). A schematic diagram of the infrared emission spectrometer

[23] is shown in Figure 2–9.

Figure 2–9 Diagram of the Infrared Emission Spectrometer [23]

Using this apparatus, polymer solvent is dropped on a 6mm-diameter platinum hotplate by 5-

10 drops. The solvent is heated by an electrically heated platinum hotplate to 120ºC. This

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approach is to evaporate the xylene solvent just leaving a thin film of polymer residue on the

hotplate. As showed in the figure, a paraboloidal mirror collects the infrared light emitted by

the hot sample and the infrared light is directed into a Fourier Transform Infrared (FTIR)

spectrometer, which provides an intensity spectrum of the emitted light as a function of

wavenumber. The wavenumber is the reciprocal of wavelength and thus the unit for

wavenumber is cm–1. In the spectral diagram, the height of the spectral peaks is proportional

to the concentration of the molecular structure that produces the spectral peaks. Accordingly,

the degree of polymer oxidation can be determined by counting the ratio of the magnitude of

spectral peak heights combined with carbonyl (C=O) and sound polymer (C-H). A new

concept –Oxidation Index was introduced.

Oxidation Index = peak height of carbonyl (1735-1745 cm-1) / peak height of sound polymer

(1460 cm-1)

The characteristic wavenumber of carbonyl (C=O) is 1735 – 1745 cm-1, for sound polymer

(C-H), the characteristic wavenumber is 1460 cm-1. Figure 2–10 is an infrared spectrum

sample of an aged EPDM material insulator [23] and it shows the spectrum peaks used to

calculate the oxidation index.

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Figure 2–10 Infrared Spectrum of an Aged EPDM [23]

The oxidation index was used to diagnose a 275 kV EPDM insulator [25], which had been in

service at a polluted site close to a power station. In Figure 2–11 [25], the solid diamond

points how the oxidation indices along the length of the insulator. It is clear that the maximum

oxidation index occurred at the high voltage end of the insulator. It is consistent with the fact

that the electrical stress is the highest at the high voltage end of the insulator. It is noted that

an increase trend towards to the grounded end of the insulator. Another 275 kV EPDM

insulator without energised but was installed at the same site with the same environment also

has been investigated using oxidation index. The hollow diamond is the oxidation index along

the insulator. The oxidation indices of this insulator don’t change with position, and the value

of oxidation index is generally below the lowest value of oxidation index of energised

insulator. This result indicates that the increased oxidation indices are due to energization and

surface charge. Oxidation index provides a new quantitative analysis method for assessing

EPDM insulators.

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Figure 2–11 Oxidation Index of a 275 kV EPDM Aged Insulator [25]

2.3.4 Chalking Index Analysis Method

Another indicator – Chalking Index has also been developed for quantitatively evaluating the

amount of surface chalking. The following steps describe the approach of calculating chalking

index [24].

Scrap a small amount of the powdery surface material from degraded EPDM insulators. The

tool is suggested is a razor blade. This method is explained in 2.3.3.1(b).

Mix the sample material with 300 mg of potassium bromide powder, which is transparent to

infrared light.

Press the mixture into a 13mm diameter disk under a pressure of 10 tonnes. The result of this

step is to make a thin transparent disk with finely divided and evenly dispersed sample

material.

Using Fourier Transform Infrared (FTIR) spectrometer to get infrared absorption spectrum of

the sample.

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Chalking Index = peak height of alumina-tri-hydroxide (1020 cm-1) / peak of height of sound

polymer(2918 cm-1)

The characteristic wavenumber for alumina-tri-hydroxide (ATH) is 1020 cm-1, and the

characteristic wavenumber for sound polymer is 2918 cm-1. Figure 2-9 is a typical FTIR

absorption spectrum from a surface scraping and KBr disc sample [25].

Figure 2-9 Chalking index of an aged EPDM insulator [25]

2.3.5 Ester / Ketone Ratio Index

Ester/Ketone ratio is defined as the ratio of the peak heights associated with the ester carbonyl

(1735 cm-1) and the ketone carbonyl (1718 cm-1) [30].

Ester/Ketone Ratio = peak height of ester carbonyl (1735cm-1) / peak of height of ketone

carbonyl (1718 cm-1)

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Figure 2-10 [31] shows that an insulator from inland (Roma, 425 km from the sea) has a peak

at 1735 cm-1, which is the characteristic wavelength of ester carbonyl. The spectra for the

insulator from coastal (Beenleigh, 15 km from the sea; Ingham, 5 km from the sea) form a

peak at 1718 cm-1, which is the characteristic wavelength of ketone carbonyl. George [30]

explained the phenomena that for EPDM insulators, UV-induced degradation produces

carbonyl peaks that are centred on the ester group, with characteristic wavelength around

1734-1735 cm-1. While aging is mainly dominated by thermal oxidation, the carbonyl peaks

always focus on the ketone group with characteristic wavelength about 1717-1718 cm-1.The

significance of Ester/Ketone is that it explains the primary cause for EPDM aging. If the

Ester/Ketone is a high value, which indicates degradation of EPDM insulators is mainly

related to UV radiation. Whilst a low value of Ester/Ketone means the aging of EPDM

insulators is principally dominated by thermal degradation, which is strongly related to

discharge [25].

2.3.6 Investigation of Aging of Medium Voltage Insulators

A survey has been carried out using the chemical analysis methods to investigate the

condition of medium voltage insulators in Queensland [25]. Most sample insulators were

Figure 2–10 Expanded FTIR spectra from the carbonyl region [32]

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EPDM and a smaller proportion of EPDM/silicone rubber blends. Table 2–2 lists the analysis

results.

Table 2–2 Chemical Analysis Report of Aged Medium Voltage insulators in Queensland [25]

(‘sea’ means the area close to sea (the Pacific Ocean) – no more than 200 meters and exposed to salt spray directly. ‘coastal’ refers to sites more than 200 meters but less than 100km from sea. The ‘inland’ locations are 100km from sea. )

It is noted that the oxidation indices for the seaside insulators of manufacturer A are higher

than those of coastal and inland insulators of the same manufacturer. The exception is the

insulator installed at Goodna–Coastal. This insulator was installed near the Brisbane River

where fog occurs frequently. It is supposed that fog weather condition resulted in more moist

environment; therefore, this moist condition brought about more dry band phenomena which

resulted in degradation of surface of EPDM insulators. Similarly, for insulators near seaside,

salt fog spray provided suitable condition for dry band formation. This could explain why the

oxidation indices for seaside insulators are higher than those of the insulators installed at the

other locations.

For insulators from manufacturer B, the oxidation indices are significantly smaller than those

of manufacturer A in spite of the fact that the insulators were installed on the same pole.

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According to SEM analysis, however, the insulators from manufacturer B showed more

extensive surface damage than those of manufacturer A. The thickness of degradation layer of

manufacturer A is around 10μm, whereas for manufacturer B, the thickness of degradation

layer is more than 20μm. For insulators from manufacturer C, the performance is the worst,

including the oxidation indices and the thickness of degradation layer. Another point is that

the insulator with blend material showed a higher chalking index than that of pure EPDM

insulators.

Figure 2-11 [31] uses ester/ketone ratios and oxidation indices to classify different insulators

into groups. This two-dimension scatter plot shows the ester/ketone ratio to the oxidation

index for insulators from manufacturer A. Several distinct groups can be identified according

to the indices. The insulators installed in the location of seaside of Miami and the insulators

from Goodna – Coastal (G) show high values of oxidation index. At the same time, the

coastal insulators indicate high values of ester/ketone ratio. This can be explained by the fact

that UV at Goodna is stronger than that of Miami. Another location with strong UV radiation

and sunny weather is Roma (R); it is clear that the insulators in this area indicate higher

values of ester/ketone ratio. The insulators from Beenleigh (B) and Ingham (I) show relative

lower values of oxidation index and ester/ketone ratio. Their values overlapped each other;

maybe it is expected that the weather conditions of these two places have some extent

similarity. Summarise the chemical analysis on EPDM insulators, the following lists the

suggested end of life criteria of EPDM insulators [25].

25

Figure 2–11 Scatter plot relating the oxidation index to the ester/ketone ratio for insulators from manufacturer A, B [31]

The ester / ketone ration determined from the FTIR spectrum is below 0.6.

The Oxidation Index FTIR is above 0.4.

Levels of surface aluminium from XPS are above 7%.

The degradation layer is thicker than 20 μm and the width of surface cracks exceeds 7 μm.

2.4 Summary

This chapter introduces the innovative chemical methods to assess surface conditions of

EPDM composite insulators developed in QUT. In other literature, electrical characteristics

are widely used to assess conditions of composite insulators [33, 34]. Leakage current and

flashover voltage are two important electrical characteristics. However, the relationships

between chemical surface conditions of composite insulators and electrical characteristics are

still blank at this stage. It requires research in this area. The following chapters in this thesis

aim to do this work. The strategy is that aged EPDM insulators can be acquired by

accelerating aging on composite insulators in the laboratory. The electrical characteristics of

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insulators can be recorded. Surface conditions of insulators can be acquired. There are

essential elements in my research. First is to design controlled test procedures. Secondly, it is

necessary to find and record proper electrical characteristics of composite insulators. Thirdly,

compare electrical characteristics of composite insulators with surface conditions to find

relationships between them.

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CHAPTER 3

DEVELOPMENT OF TEST EQUIPMENT

3.1 Literature Review

Papers dealing characteristics of insulators with aging show that universities and research

organisations in insulator area use fog chamber to age composite insulators and characteristics

of fog chambers can be found from the publications.

The fog chamber in Ohio State University, USA, is a 1.72 m (length) by 2.44 m (width) by

1.83 m (height) high chamber with a gable roof [34]. A fog chamber with a cubic size of 2.54

m has been built in University of Windsor, Canada [35]. At Dow Corning Corporation, the

first fog chamber is a cube with 1.52 m sides and a pyramidal roof. The details of the fog

chamber are described in [36]. A fog chamber at Arizona State University is in 3.65 m (l) by

3.65 m (w) by 2.44 m (h) [37]. In Japan, a fog chamber was set up at University of

Tokushima. The size is 2 m (l) by 2 m (w) by 3 m (h) [38].

A comprehensive review on different designs of fog chambers can be found in [39]. In this

paper, the authors classify fog chambers into ranges from large size, with dimension ≥ 5 m,

medium size with dimensions between 1 m and 5 m, to small size with dimensions smaller

than 1 m.

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3.2 Fog Chamber System

The whole fog chamber test system is composed of four parts. I - fog chamber. II – fog

chamber control system. III - high voltage supply equipment. IV – data acquisition system.

The following sections illustrate these parts individually. Figure 3-1 illustrates the system.

Figure 3-1 Fog Chamber Test Systems

3.2.1 Fog Chamber

Fog Chamber Test System

Fog Chamber

Fog Chamber Control System

High-voltage Test Power Equipment

Data Acquisition System

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Developing a fog chamber is one part of my research project. After surveying other fog

chambers, a fog chamber was developed in the High-Voltage Laboratory at QUT. Figure 3-2

illustrates the outline of the fog chamber.

Figure 3-2 Fog Chamber

The body of fog chamber is made of acrylic plastic. The outline of the fog chamber is a

square case. Its size is 2000 x 2000 mm at the base and 1500 mm in height. The fog chamber

body sits on a 20-mm thick wooden board, under which there are four universal wheels that

can move the fog chamber in any direction easily. Two doors are mounted on two opposite

sides that allow operator accessing the interior of the fog chamber. Eight brass hooks are

installed on ceiling of the fog chamber allowing eight insulators to be suspended vertically. A

35 kV class bushing is in the centre of the ceiling of the fog chamber. The end-fittings of

insulators are grounded through the measurement system. Two fog boxes are installed in

diagonal position. Two funnels with an area of 78 cm2 each are installed beside the fog boxes

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measuring precipitation rate of fog. The location of the bushing and the hooks is showed in

Figure 3–3.

Figure 3-3 Plan View of the Fog Chamber

3.2.2 Fog Chamber Control System

The function of fog chamber control system is to control the production of fog. Four

ultrasonic fog nebulizers are installed in two separate ancillary fog boxes attached to the main

chamber in a diagonal position. Each fog box has two ultrasonic fog nebulizers. Fog

generation speed is divided into three settings. At the maximum output rate, the fog nebulizer

consumes water 80ml per hour. Beside each fog box, there are two fans, which provide the

function of circulating fog in the fog chamber. One is installed beside of the fog box and the

other is on the floor. The locations of the fans enable circulating air avoiding to blow directly

on the insulators. One 60-litre barrel provides water for each fog box powered by a

submergible pump. In each fog box, two float switches control water level. If water level is

lower than the low-position float switch, the pump starts to work automatically,

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supplementing water until water level reaches the position of the high-position float switch.

The high-position float switch controls pump to stop supplementing water when water reaches

the optimum height. The optimum water height is about 3.4cm above nebulizers. The two

float switches maintain the water level in the optimum range for the ultrasonic fog nebulizers.

Water in the fog chamber is collected by a water tank installed under the base of fog chamber

through a slope drain hole (diameter = 10mm) on the floor of fog chamber. Water is not

recycled. In order to drain away water in the tank, another pump is mounted outside the tank

to pump tank water into sewer. A digital thermometer in the laboratory records the ambient

temperature and humidity. A removable control panel was built outside high-voltage test zone

that isolates the operating zone from the high-voltage zone. Inside the control panel, the

secondary control power source and the control circuit board are installed. Figure 3-4 shows

control panel systems.

Control Panel

Power Supply (Figure 4-5) Control Circuit

Fan Speed Controller and Switch

Pump Speed Controller and Switch

#1 Fan

#2 Fan

#3 Fan

#4 Fan

#1 Barrel Pump

#2 Barrel Pump

Tank Pump

Nebulizer Switch

#1 Nebulizer

#2 Nebulizer

#3 Nebulizer

#4 Nebulizer

Figure 3-4 Control Panel Systems

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Figure 3-5 Power Systems of the Fog Chamber Control Panel

Figure 3-6 Layout of the Fog Chamber and the Control Panel

Figure 3-5 illustrates power systems of the fog chamber control panel. The front-view of the

control panel is shown in Figure 3-6. The whole control circuitry is available in the Appendix-

1. Appendix-3 illustrates power supply of the fog chamber test system. Appendix-4 is the

terminal connection of the control panel and the fog box.

AC 24V

AC Power (240V)

AC 12V

DC Power

DC 12V

Fog Nebulizers

Fans

Pumps

Indicator Lights

Power Supply

Transformer (240V/24V/12V) Rectifier (240V AC/ DC )

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3.2.3 High Voltage Supply Equipment

A 250V/19100V, 5 kVA high-voltage test transformer is employed as the power supply. An

independent power transformer, which is separated from the laboratory’s lighting power

system, provides power to the high voltage test transformer. The input voltage of the testing

transformer is controlled by an autotransformer. The nameplate of the autotransformer

indicates that input/output voltage range is 240V/0-240V. A section of cable connects the

high-voltage testing transformer and the 35 kV bushing, providing power source for the test

samples in the fog chamber. The equivalent circuit of the testing high-voltage and

autotransformer (variac) is attached in the Appendix-2. Below are terms defined by IEC

60507 regarding the insulators test.

(a). Test voltage

The r.m.s value of the voltage with which the insulator is continuously energised throughout

the test.

(b). Specific creepage distance (ls) of an insulator

The overall creepage distance L of an insulator divided by the product of the test voltage and

3 , which is normally expressed in mm/ kV.

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3.2.4 Data Acquisition System

Through a literature review, it was found that there are two main methods to monitor leakage

current. One is the direct measurement method. And the other is the indirect measurement

method. The direct measurement method is described in [35], [36]. It uses resistance to

measure voltage caused by leakage current. The indirect measurement method uses

transducers to convert current signals into voltage signals, avoiding direct contact with the

HV source [40]. Comparing these two methods, they both have their own advantages; the

direct measurement is simple, practical, and economical; while the indirect method is safer,

but expensive. They both meet the required measurement accuracy. Considering the ratio of

price / function, in this project, the direct data acquisition system was chosen as the leakage

current monitoring method. Figure 3-7 shows the whole test system and the measurement &

protection board.

Figure 3–7 Test System and Measurement & Protection Circuit

In the figure, the signal into the LabVIEW board is a voltage signal, and the leakage current

flows a sampling resistor forming a voltage drop, which is proportional to the current. This

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signal is connected to the input terminals of an A/D card, which is controlled by LabVIEW

program. The input voltage range for the A/D board, 6023E, is –10V to +10V. In order to

avoid damage to the board, a back-to-back zener diode is used to limit the input voltage of the

A/D card in the range of –5V to +5V. The Data Acquisition System is based on a computer

with 233MHz CPU, 32M memory. A National Instrument Data Collection Board (6023E)

carries out the data acquisition task. It consists of a 12 bits, 16 channel Analog-Digital (A/D)

converter, which samples leakage current at the preset frequency. The data-recording program

was based on the LabVIEW program, which was designed to record leakage current of 8

insulators simultaneously. The waveforms of leakage currents can be displayed on screen.

Also waveforms of individual channel are recorded in a data file. The total sampling duration

and the sampling interval can be changed from the LabVIEW panel. The LabVIEW program

for recording the leakage current of one channel is shown in the Appendix-5.

3.3 Summary

This chapter describes the major test equipment, the fog chamber developed in QUT during

this thesis work. It comprises four parts, the body of fog chamber, the control system, the high

voltage supply system, and the data-acquisition system. The appendices include the details of

the fog chamber system.

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CHAPTER 4

REVIEW OF ELECTRICAL TESTS ON COMPOSITE INSULATORS

4.1 Introduction Chapter-2 describes the chemical analysis methods to assess surface conditions of EPDM and

EPDM/silicone rubber insulators at QUT. Chapter-3 describes the development of the

electrical test equipment, the fog chamber. This chapter reviews the test standards and

methodology, which describe test procedures, sample preparation, test parameters, and test

procedures on composite insulators. The objective of this chapter is to produce a set of

suitable test methods, based on reasonable standards and valuable experience from other

research results, for this research project. Chapter-5 will describe test details and test results

on EPDM composite insulators, using the test methods described in this chapter.

4.2 Electrical Test Standards

4.2.1 Introduction

There are two international standards relating to this research project. IEC standard 60507

[41], “Artificial pollution tests on high-voltage insulators to be used on a.c. systems” is used

for testing the power frequency withstand characteristics of ceramic and glass insulators for

applications. It is applicable to systems with rated voltages ranging from 1000V to 765 kV.

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IEC standard 61109 [42], “Composite insulators for a.c. overhead lines with a nominal

voltage greater than 1000V – Definitions, test methods and acceptance criteria”, defines the

terms used, prescribes test methods and prescribes acceptance criteria. These two standards

are fundamental guidelines for tests in my research, especially the definitions and terms

involving the electrical tests.

R. Barsch, H. Jahn, J. Lambrecht summarised electrical test methods for composite insulators

[43]. Namely, they are: inclined plane test, arc test, modified rotating wheel dip test, salt fog

test and hydrophobicity transfer evaluation. Table 4-1 lists the main test parameters of these

tests.

Table 4 - 1 Summary of Test Methods on Composite Insulators [43]

All the test methods, except hydrophobicity classification, involve high voltage electrical

tests. The criteria for assessing working conditions of composite insulators vary with

methods. Only the inclined plane test indicates quantitative criteria for assessing insulator

degradation. It is concluded that the other tests have subjective criteria, which may lead to

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different results from different assessors. In this thesis, the quantitative assessment method

and the other methods are combined together to assess the conditions of composite insulators.

4.2.2 IEC Standard

IEC 60507 [41] is a standard which describes aging test procedures for ceramic and glass

insulators. IEC 61109 (1992) [42] is a standard which defines aging test procedures for

composite insulators. It defines two aging test procedures, the 1000-hour salt fog test (clause

5.3) and the 5000-hour cyclic test (annex C) [44]. The 5000-hour cycle test is a multi-factor

aging test procedure, which aims to simulate the natural environment of the composite

insulators. Figure 4-1 lists the details of this multi-factor artificial aging test.

Figure 4-1 IEC 61109 Accelerated Weather Aging Cycle under the Operating Voltage [42]

In this IEC guide, insulators are subjected to repeating environment stress factors, which

include rain, fog, UV, and surrounding temperature. The duration of 5000 hours is

recommended by the standard. Perrot [45] carried out tests under this guideline, and found

good correlation between the multifactor accelerated aging test and degradation observed on

composite insulators recovered from the network in coastal areas. In another report related to

accelerating aging test carried out by Riquel [46], accelerating aging test was found effective

in producing similar degradation results to those in the real field application. Riquel induced a

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concept named as the acceleration ratio, which is defined as the ratio of time under test to

time in the field to produce a similar level of damage. It is clear that the acceleration ratio is

dependent on the environment factor because different environment has a wide range of

effects on insulators. According to Riquel, the ratio calculated by their test is about 15 for a

coastal location in France and about 7.5 for a highly polluted coastal industrial area of France.

No other publications have found using this ratio to conduct accelerated aging tests on

composite insulators. It is concluded that aging in the environment is a very complex multi-

factor process, although some ratios could be found in laboratory, different laboratories have

different test conditions, and that would result in different effects. So it is supposed that

different areas have different so-called acceleration ratios. The application of the ratio to other

areas needs more investigation. Although the 5000-hour multifactor aging test applies more

aging factors than the 1000-hour salt fog test, the 1000-hour salt fog test is still commonly

used and accepted by manufacturers and researchers to study characteristics of composite

insulators. IEC 61109 1000-hour salt fog test involves the following main test parameters,

which is listed in Table 4-2.

40

T

S

Table 4-2 Parameters of Standard 61109 Test Conditions [42]

In this test guide, the evaluation criterion is numbers of flashovers (“not more than three

overcurrent trip-outs for each specimen tested”) and the visual examination of degradation

(“no tracking, erosion does not reach the glass fibre core, sheds are not punctured, core is not

visible”). However, according to Gutman, some test parameters in this test and test criteria are

doubtfully specified, questionable, or not specified at all [47] [44]. Table 4-3 lists these

points.

Table 4-3 Overview of Discussible Parameters [47]

4.2.3 IEEE Standard

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Besides IEC standards, IEEE also provides a relevant test standard, “IEEE Standard

Techniques for High-Voltage Testing” [48]. It mainly applies to ceramic and glass insulators.

It defines some common parameters as in IEC 60507. In this standard, it also defines two

testing environments, the clean fog test and the salt fog test. The following section introduces

these two tests.

4.2.3.1 The Clean Fog Test

4.2.3.1.1 Preparation

In the clean fog test, a contamination layer is applied to the insulator surface using slurry

consisting of water, an inert material, such as kaolin, and an appropriate amount of sodium

chloride (NaCl). The amount of NaCl is determined by the required salt deposit density (Sdd)

or layer conductivity. The slurry composition consists of:

(i) 40 g kaolin

(ii) 1000 g tap water

(iii) suitable amount of NaCl of commercial purity

Table 4-4 lists main characteristics of the inert materials used for contamination purpose.

W

Table 4-4 Main characteristics of the inert material used in clean fog tests [48]

1 conductance (20 ºC)

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Before the insulators are subjected to slurry contamination, they are required to be processed

in the following steps: (a) To be cleaned by scrubbing the insulation surfaces with an inert

material, e.g. kaolin. (b) To be thoroughly rinsed with clean water. After the above process,

the samples are ready for contamination. Before applying high voltage on test samples, two

kinds of surface condition, dry and wet, are applied. In both cases, the standard recommends

that the test starts at the same time as the start of fog generation. The fog around the test

objects in chamber must be uniform and the temperature rise of fog chamber must not exceed

15ºC by the end of test. The desired volume conductivity of the contamination is reached by

adjusting the amount of salt in the slurry. Also, as a guide, the IEEE standard gives a

correspondence between the reference degree of pollution on the insulator and the volume

conductivity. (It is noted that the temperature of the slurry mentioned in the table is 20 ºC)

Table 4-5 Kaolin composition: correspondence between the reference degrees of Pollution on the insulator and volume conductivity of the slurry [48]

4.2.3.1.2 Application of the contamination layer

After cleaning the dry insulators following the steps described in the above section, the next

step is to use contamination slurry (described as the “Contamination”) to contaminate the

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insulators. Two methods are recommended for applying the contamination layer: using spray

nozzles or commercial-type spray guns. With the latter one, a distance of 20-40cm between

insulators and spray mouth is recommended by the standard. The purpose of this is to obtain a

reasonably uniform pollution layer. After spraying, the layer is left to dry prior to

commencement of the test.

To determine the conductivity of the contamination layer on insulators, this IEEE standard

describes one method, which defines the degree of contamination by salt deposit density or

layer conductivity. The following steps describe how to measure the salt deposit density – Sdd.

Remove carefully the deposit on the surface of a separate insulator. This insulator must be

identical to the tested insulator and have been subjected to the same contaminating process.

Dissolve the deposit in a known quantity of water, preferably demineralised water.

Stir the mixture of water and the deposit for at least 2 minutes.

Measure the volume conductivity Ơθ at the ambient temperature θ (ºC).

The value Ơ20(ambient temperature=20ºC) is calculated using the following formula:

Ơ20= Ơθ* [1 – b (θ – 20)] (1)

where

Ơ20 is the layer conductivity at a temperature of 20 ºC (in S/m)

Ơθ is the volume conductivity measured at the ambient temperature of θ ºC (in

S/m) θ is the temperature of the insulator surface (in ºC)

b is a factor depending on temperature, as given in Table 4-6:

44

Table 4-6 Relationship between θ and b [50]

The salinity, Sa ( in kg/m3 ), of the slurry is determined using the formula:

Sa = (5.7 Ơ20) 1.03 (0.004 ≤ Ơ20 ≤ 0.4 S/m) (2)

The Salt Deposit Density, Sdd, is then determined using the formula:

Sdd = A

SaV (3)

where V is the volume of the slurry (in cm3)

A is the area of the cleaned surface (in cm2)

The layer conductivity is determined by the following formula.

The layer conductivity (K) = the layer conductance measured on the unenergized insulator ×F

F = ∫L

dllp0

)](/1[ (4)

where F is the form factor

p ( l ) is the circumference at partial creepage distance l along the surface

L is the total creepage distance

dl is the increment of integration

4.2.3.1.3 Test procedure

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In IEEE standard, two alternative test procedures are proposed. Basically the procedures

differ in the conditions of the pollution layer, which is dry or wet when the test voltage is

applied.

(a) Dry before energisation

After preparation described in above section, insulators are put into fog chamber with the

condition of dry contamination layer. The IEEE standard suggests that under the ambient

temperature, the steam input rate shall be within the range of 0.05± 0.01 kg/h per cubic meter

of the fog chamber volume. Fog is applied when the test voltage is applied to the insulators.

(b) Wet before and during energisation

This method requires that the prepared insulators are put into fog chamber, which is filled

with fog. The fog generation rate must be sufficient to ensure that the surface conductivity of

insulators reaches the maximum value in 20-40 minutes from the start of fog generation at the

ambient temperature. The maximum value of conductivity is assumed as the reference layer

conductivity. When the maximum conductivity is achieved, the test voltage is applied.

4.2.3.1.4 Test objective

The IEEE standard defines the withstand test and the acceptance criterion as follows [48].

“The objective of this test is to confirm the specified withstand degree of contamination at the

specified test voltage. The insulator complies with this specification if no flashover occurs

during three consecutive tests performed in accordance with Procedures.”

46

4.2.3.2 The Salt Fog Test

4.2.3.2.1 Preparation

In this test, insulators are subjected to salt fog, which is produced by salt water solution. The

contamination degree is defined by specified salinity of the salt water solution. Before the

test, the insulator surface is cleaned by the solution whish is mixed with water and neutral

detergent, such as trisodium phosphate (Na3PO3). And then the insulators are cleaned by tap

water. The last step allows the insulators to dry out in the natural environment. After finishing

these steps, the insulators are ready for the test. The test voltage is supplied when the fog

generation system starts.

4.2.3.2.2 Salt solution

The salt solution consists of sodium chloride (NaCl) of commercial purity and tap water.

IEEE standard lists the following salinity to be used in test: 2.5 kg/m3, 3.5 kg/m3, 5 kg/m3, 7

kg/m3, 10 kg/m3, 14 kg/m3, 20 kg/m3, 28 kg/m3, 40 kg/m3, 56 kg/m3, 80 kg/m3, 160 kg/m3, or

224 kg/m3. The salinity is decided by measuring the conductivity or by measuring the density

with a correction of temperature. The following Table 4-7 lists the correspondence between

the value of salinity, volume conductivity, and density of the solution at a temperature of 20

ºC.

47

Table 4-7 Correspondence between the Value of Salinity, Volume Conductivity, and Density of the Solution at the Temperature of 20ºC [48]

If the solution temperature is not 20ºC, the conductivity and density should be corrected by

the following formula,

δ20 = δθ [1 + (200 + 1.35 Sa) (θ – 20) × 10 –6] (5)

where

δ20 is the density at a temperature of 20 ºC (in kg/m3)

δθ is the density at a temperature of θ ºC (in kg/m3)

Sa is the salinity ( in kg/m3 ) - see formula (2)

θ is the solution temperature (in ºC)

4.2.3.2.3 Conditions before starting the test

Before the test, the insulators should be prepared according to 4.2.3.2.1. When the test voltage

is applied, the surface of the insulators should be wet. Additionally, the ambient temperature

should be in the range of 5ºC to 40ºC and the difference between the water solution and the

ambient temperature should be no more than 15ºC.

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4.2.3.2.4 Test procedure

The IEEE standard states that the objective of the salt fog test is to show whether samples will

withstand the application of test voltage, that is “to confirm the specified withstand salinity of

the insulator at the specified test voltage”. The acceptance criterion for the withstand test is

described as follows: “The insulator complies with this standard if no flashover occurs during

a series of three consecutive tests in accordance to the procedure in withstand test. If only one

flashover occurs, a fourth test shall be performed and the insulator then passes the test it no

flashover occurs.”

4.3 Test Methodology

After reviewing test guidelines, this section describes the test methodology for my research

project. The objective of this section is to determine the suitable electrical test parameters and

electrical characteristics based on the previous sections.

The significant factors influencing the electrical performance of composite insulators are the

decrease of hydrophobicity and material aging due to electrical and environmental stresses.

When a number of factors, such as UV, moist, corona, salty fog, etc., affect composite

insulators simultaneously, surface degradation on composite insulators could develop

considerably. From the electrical point of view, the electrical characteristics of composite

insulators may change dramatically during the aging process. In terms of electrical

characteristics, leakage current, partial discharge, corona, dry-band, and flashover, are typical

49

parameters to study the aging process of composite insulators [49] [29]. These electrical

characteristics can be measured or studied in the field [28] [51] or under laboratory test

conditions [51]. Among these electrical characteristics, leakage current is treated as one

typical parameter to represent surface conditions of composite insulators. Fernando and

Gubanski [49] summarised a review of leakage current measurement on composite insulators.

In the conclusion, the authors gave the following directions for future work on composite

insulators:

“However, knowledge of the correlation between the parameters of the leakage current

(current level, harmonic content and accumulated charge), and the state of the insulator

surface (contamination level, hydrophobicity and aging) on the other side, is not yet complete.

More work is needed to elucidate the above mentioned relations and, therefore, create a basis

for broader use of LC measurements in practical situations.”

Figure 4-2 shows the methods of measuring leakage current on composite insulators. They

contain the site measurement and the laboratory test measurement.

50

Figure 4-2 Classification of Leakage Current Measurement [49]

Gorur, etc. [52] presented leakage current measurements for EPDM and silicone composite

insulators in the clean fog environment and the salt fog environment. The composite

insulators were selected from different ranges: energized at field, field not energized, and kept

indoors with plastic bags. The ones kept indoors are treated as reference for comparing the

results. The leakage current measurement was carried out three times. The first time is

immediately after receiving insulators from the field; the second measurement was done

following the surface wetting of insulators, and the third time is carried out after the DC

flashover tests. For EPDM insulators, the results showed that the EPDM insulators kept

indoors without energized history showed low leakage current, whereas the EPDM insulators

which are subject to outdoor environment with high voltage stress showed higher leakage

current with fluctuation amplitude. Additionally, the authors used guideline from

“Hydrophobicity Classification Guide, STRI Guide 92/1”, to analyse the surface conditions of

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composite insulators in different test phrases. It is noted that the duration of measured leakage

current is roughly about 120 mins rather than flashover happened.

In this research project, a fog chamber has been built to carry out the aging tests. Combined

with test conditions, leakage current is selected as the electrical characteristic to assess surface

conditions of composite insulators. Flashover incidences will be recorded for reference. Due

to limitation of equipment, flashover voltage test could not be recorded in the laboratory. The

test parameters in LC tests will be introduced in the following chapter.

52

CHAPTER 5

AGING TESTS ON COMPOSITE INSULATORS

5.1 Introduction Chapter 4 reviewed test standards and test methodology that have been used to guide tests on

composite insulators in this chapter. They include terminology, test definition, sample

preparation, test condition parameters, and test procedures. This chapter describes details of

tests carried out on EPDM and EPDM/silicone rubber insulators based on the test

methodology. The test results are presented. Additionally, chemical analysis and physical

observation of aged composite insulators are compared with the electrical characteristics in

this chapter.

The objectives of this chapter have three aspects. The first objective is to find and understand

the electrical characteristics of EPDM insulators in test conditions. The second objective is to

analysis aged EPDM insulator surface conditions by chemical analysis methods. The third

objective is to compare these two assessment results and to find relationships between

electrical characteristics of aged EPDM insulators and chemical analysis results. In this

chapter, LC of composite insulators in test conditions is used to represent electrical

performance of composite insulators. SEM was carried out to investigate the physical change

on surfaces of composite insulators in the tests. Furthermore, hydrophobicity classification

and contact angle analysis supplement the assessment of physical changes of aged composite

insulators.

53

5.2 Preliminary Tests on Composite Insulators 5.2.1 Test Introduction

Preliminary electrical tests were carried out in clean fog environment. Three 11 kV class

composite long rod insulators were tested in the fog chamber. Insulator #1 was from

manufacturer A and the housing and sheath material was blend of EPDM and silicone rubber.

Insulators #2 and #3 were both from manufacturer B. The housing and sheath material was

EPDM material. #1 insulator was new, and #2 insulator had served in power distribution

network about 5 years. By visual observation, the surface of #2 insulator was in good

condition, and there was no obvious holes or cracks on the surface. #3 insulator was new.

This test was carried out in clean fog condition, which was based on IEC standard 60507 [41].

(Chapter 3.2.3.1.1) Table 5-1 lists the parameters of the insulators.

Insulator No. Housing Composition Creepage Length (mm) Core Diameter (mm)

#1 EPDM/silicone rubber 405 22 #2 EPDM 332 25 #3 EPDM 332 25

Insulator No.

Shed Height (mm)

Number of Sheds

Shed Spacing

(mm)

Manufacturer

Creepage Stress(mm/

kV) History

#1 35 4 26 A 33.75 new #2 38 4 31 B 25.5 field aged #3 38 4 31 B 25.5 new

Table 5-1 Parameters of Insulators (test voltage = 12 kV)

The preliminary electrical test recorded electrical characteristic - LC. The surface of the

insulators was not contaminated. The insulator surface was processed following the test

methodology (see Chapter-3.2.3.1). The surface of the insulators was cleaned by scrubbing

kaolin powder and rinsed with tap water. In the test, high voltage was applied to sample

insulators while the insulator surface was dry. Fog generation system in fog chamber started

54

to produce fog at the same time when high voltage was applied. Table 5-2 lists the parameters

of the test conditions.

Salinity (salt/ clean water)

Tap Water Conductivity (µS/cm)

Testing Temperature (ºC)

Fog Particle Size (µm)

Test Voltage (rms)

0 305 23-25 10-30 12 kV Table 5-2 Parameters of Preliminary Test Conditions

The whole test circuit is shown in Figure 3-7 (see page 34). The test lasted about 450 hours.

LabVIEW program recorded LC every 30 minutes. During each sampling period, voltage

waveforms were recorded at sampling frequencies of 2 kHz for 60 seconds.

5.2.2 Hydrophobicity Loss on Surface of Composite Insulators

Under moist conditions, wet areas could form on insulator surface. The wet and dry areas

affect the distribution of electric stress. The distribution of electric stress affects the intensity

of discharge and corona on insulator surface. In high electric field zones, the intensity of

discharge and corona is stronger than that of the less intensity electrical field zones. The

consequence of discharge and corona phenomena is the surface condition change on

composite insulator from hydrophobicity to hydrophilicity. Discharge and corona are active

factors to cause deterioration of composite weather shed and sheath materials [53]. The

degree of hydrophobicity is a useful index to assess surface conditions of composite insulators.

One method so-called Hydrophobicity Classification [54] is used to assess hydrophobicity

conditions of composite insulator surface, which was developed by Swedish Transmission

Research Institute. Observers compare test samples with the images of reference standard to

define the hydrophobicity class.

55

Following the instructions of assessing hydrophobicity degree, before the aging test, the

insulator surface under fog condition was investigated. Figure 5-1 shows images of surface

conditions of two EPDM insulators, #2 and #3 insulators, in fog conditions during the same

time. Both of them showed the similar change rate from hydrophobicity to hydrophilicity

(HC2-1 to HC-7).

(a) HC0 (b) HC1

(c) HC2 (d) HC3

2 HC – hydrophobicity classification

56

(e) HC4 (f) HC5

(g) HC6 (h) HC7

Figure 5-1 Hydrophobicity Classification of EPDM Insulator in Fog Condition

The results revealed that transition from HC1 (totally hydrophobic) to HC6 (totally

hydrophilic) of the two EPDM insulators (#2 and #3) took about 15 hours, whereas the new

EPDM/silicone rubber insulator (#1) took 12 hours to become hydrophilicity. Figure 5-2

shows the HC change with time on shed one (high-voltage side). The results showed that it

took after 15 hours for the shed-1 of #2 and #3 insulators (EPDM) to become totally wet in

fog environment.

57

Figure 5-2 Hydrophobicity Classification on Topside Shed of EPDM Insulator #3

The backside and core positions were also investigated. Take #3 insulator as an example, it

was found that these two positions took longer time to become totally hydrophilic compared

to the top sides of the sheds. By observation, it showed that back position on shed-1 (high-

voltage side) took 20 hours to accumulate tiny water droplets, which was classified as HC1.

Then it took another 2-3 hours to form small water droplets. And then about 14 hours later,

bigger water droplets formed on the backside of the shed. They were classified as HC1-HC2.

With time went on, water droplets accumulated into big water droplets at the rim of the shed

(HC3-HC5). Due to gravitation, big water droplets dropped. The inside of the rim was still

full of small water droplets. Figure 5-3 (a) shows the observations.

(a) HC1-3 (t=42hours) (b) HC3-5 (t=64hours)

Figure 5-3 Water Droplets on Back-shed Surface of #3 Insulator

01234567

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Time (hours)

Hydr

opho

bici

ty c

lass

ifica

tion

backside rim of the backside

inside of the rim of the backside

58

At t=32 hours, the water droplets on the inside rim changed into HC3-HC5. The number of

bigger water droplets on the inside surface of the shed increased while the droplets on the rim

still kept the same size. It was defined that the underside of the shed was HC=6. Figure 5-3

(b) shows the back surface status of this stage. It was not easy to form big water droplets on

the core surface because the core surface was vertical to the ground. As time progressed, if the

speed of accumulation of small water droplets exceeded the speed of splitting of water

droplets in saturated moist environment, some bigger water droplets formed. Figures 5-4 (a)-

(b) show HC at two occasions.

(a) HC=2 on Core (t= 42 hours) (b) HC=3-4 on Core (t=64 hours)

Figure 5-4 Water Droplets on Core Surface of #3 Insulator

core surface

core surface

59

5.2.3 LC Measurement Results

The LC data was analysed by MatLAB and EXCEL. The three insulators showed different

manner in terms of LC performance. Figure 5-5 shows the averaged MVLC (mean values of

LC) of the three insulators between 0-100 hours in the test.

The MVLC of #1 and #3 increased dramatically about 15 hours, then it became fluctuating,

and #1 insulator showed a faster increase than #3 insulator. Let us to analyse the results

combining with the HC investigation described in the previous section. Previous section

revealed that EPDM insulators (#2 and #3) took about 15 hours and EPDM/silicone rubber

insulator (#1) took about 12 hours to form water films on topside shed. The time coincides

with the increase periods of MVLC of #1 insulator (EPDM/silicone rubber) and #3 insulator

(EPDM). It is concluded that the formation of water films on insulator surface is the direct

reason of LC increase in the test.

Contrarily, the LC of #2 insulator remained at a very low-level for about 65 hours, then it

increased to the level as #1 and #3 insulators. This result could be explained by its surface

conditions. Because its five years service history, small erosion holes on its surface trapped

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 10 20 30 40 50 60 70 80 90 100

Time from start of test (hour)

Mea

n Va

lues

of L

eaka

ge C

urre

nts

(mA)

#1 insulator#3 insulator

#2 insulator

Figure 5-5 Rectified Mean Values of LC (0-100hours) (#1 -#3)

60

fog droplets. This prevented the formation of continuous water films on its surface. The

delayed formation of water film on the surface resulted in the slowly increased LC.

Figure 5-6 compares the number of times of VLC recorded in the specified ranges. Table 5-3

summarises the results.

(a) MVLC between 0-0.6 mA (b) MVLC greater than 0.6 mA

Figure 5-6 Statistics of MVLC of the three Insulators

0-0.2 mA 0.2-0.4 mA 0.4-0.6

mA 0.6-0.8

mA 0.8-1.0

mA 1.0-1.5

mA 1.5-2.5

mA >2.5 mA

#1 insulator 55 785 91 4 2 2 1 0 #2 insulator 166 611 157 5 0 1 0 0 #3 insulator 57 775 90 12 4 1 0 1

Table 5-3 Distribution (number of times) of MVLC of the Three Insulators

It is found 0.2-0.4 mA was the most common LC range in the test, which represented 77% of

all recorded number of times of MVLC values. The three insulators kept the same level

around 600-757 number of times, while #1 insulator was the most (785). #2 was remarkably

higher (157) than the other two (around 90) in the range of 0.4-0.6 mA, which was the second

common range. Due to the slow increase of LC in the first 65 hours, in the range of <0.2 mA,

the third range, #2 insulator also had higher distribution number of times than the other two.

In the range of 0.6-0.8 mA, #3 insulator recorded distribution number of MVLC twice higher

0

100

200

300

400

500

600

700

800#1 Blend(new)#2 EPDM(aged)#3 EPDM (new)

0-0.2mA 0.2-0.4mA 0.4-0.6mA

Dis

tribu

tion

Num

ber

0

100

200

300

400

500

600

700

800#1 Blend(new)#2 EPDM(aged)#3 EPDM (new)

0-0.2mA 0.2-0.4mA 0.4-0.6mA

Dis

tribu

tion

Num

ber

Num

ber (

times

)

0

2

4

6

8

10

12

#1 Blend(new)#2 EPDM(aged)#3 EPDM (new)

0.6-0.8mA 0.8-1.0mA 1.0-1.5mA 1.5-2.5mA >2.5mA

Dis

tribu

tion

Tim

e N

umbe

r (tim

es)

61

than the other two. In the range of >0.8 mA, the three insulators only had a few times. And

only #3 insulator had recorded values >2.5 mA. The range of 0.6 mA was just about 1% of all

number of times of MVLC values.

Figure 5-7 MVLC of the Three Insulators in Test

Figure 5-7 shows the whole MVLC of the three insulators in the test. The figure shows that

although there were high peak values, the amplitude level was around 0.5 mA. During the

time of 360 to 410 hours, #2 insulator showed that it was in a very unstable condition in

which its MVLC fluctuated 0.1 mA to 0.6 mA. #1 insulator had a period of instability of 5

hours started from 420 hours. #3 insulator remained relatively stable except there were 20

hours of unstable period starting from t=395hours. Between 420 and 440 hours, #1 insulator

had intense current activities. As a whole, LC of the three insulators stabilised around 0.3-0.4

mA level till the end of the test.

0 50 100 150 200 250 300 350 400 450 5000

0.5

1

1.5

2

2.5

3

Time (hours)

Leak

age

Cur

rent

(mA

)

#1 Blend(new)#2 EPDM(aged)#3 EPDM (new)

62

The above sections analyse the general situations of LC. Figure 5-8 lists LC waveforms,

which revealed momentary status of LC at 4 different moments. At t=55 hours, the LC

magnitude of #1 insulator fluctuated around 0.2 mA. The LC magnitude of #3 insulator was

0.1 mA. The magnitude of #2 insulator was a very low value of 0.02 mA. At t=70 hours, the

LC amplitude of #2 insulator reached 0.1 mA. The figure shows that the LC amplitudes of the

three insulators were similar, and #3 insulator now showed a higher peak value of 0.15 mA.

At t=396 hours, #3 insulator showed a series of high value current peaks (5 mA), which were

more than 10 times higher than the LC amplitude of the other two insulators. At t=422.5

hours, #1 insulator showed occasional high peaks (around 0.5 mA), which were about 4-5

times higher than its normal level (0.1-0.2 mA). #2 and #3 insulators kept the LC level at 0.1-

0.2 mA without extra peaks. The high-value peaks have been widely recognized as evidence

of discharge [55] [2]. The intensity of discharge is proportional to the amplitude of LC. The

Figure 5-8 Waveforms of LC at Specific Time Intervals (#1 - #3 Insulators)

(a) t=55 hours

-

-0.20

0.00

0.2

0.4

Time Interval (0.06s)

LC (m

A)

#3 insulator #2 insulator

#1 insulator

(C) t=396 hours

-10.00 -5.00 0.00 5.00

10.00


LC (m

A)

#3 insulator

(b) t=70 hours

-0.15-0.10-0.050.000.050.100.150.20

Time Interval (0.06s) #2 insulator#1 insulator

#3 insulator

LC (

mA

)

(d) t=422.5 hours

-0.50

0.00

0.50

1.00



#1 insulator LC (

mA

)

63

recorded LC waveforms revealed that #3 insulator had more active current activities than the

other two during the later part of the test.

5.2.4 Comparison between Electrical Characteristics, Chemical Analysis and Physical

Analysis Results

Oxidation indices of all insulators were measured before and after electrical testing. Table 5-4

compares chemical analysis results. Table 5-5 lists the features of LC of the three insulators.

Table 5-4 Chemical Analysis Results of the Three Insulators before and after the Test

Table 5-5 Electrical Characteristics of the three Insulators

It is noted that the oxidation indices (O.I.) of all three insulators increased. O.I. values of the

EPDM insulators #2 and #3 were significant different before the test due to their history. For

#2, which had been used in field service, the O.I. increased from 0.265 to 0.355, about 34%.

O.I. value of #3, a new one, changed from 0.224 to 0.405, about 81%. O.I. value of the

EPDM/silicone rubber insulator #1 changed from 0.726 to 0.946, about 30%. In terms of O.I.

change, #3 insulator showed the biggest change among the three. The LC analysis showed

that #3 was more active than #2 insulator because of its recorded high LC peaks. Another

index – Ester/ketone (E/K) ratio is also listed in the table. It was found that E/K of the three

insulators decreased in the test. The change rates were similar in all three insulators, which

Oxidation Index. Results Ester/ketone Ratio Insulator Type Field Aged History before after change (%) before after change

#1 Blend New 0.726 0.946 30% 1.849 1.299 29% #2 EPDM 5 years 0.265 0.355 34% 2.218 1.674 24% #3 EPDM New 0.224 0.405 81% 2.546 1.908 25%

#1 insulator #2 insulator #3 insulator

Features of LC characteristics

Mean values of LC increased the quickest, the maximum peak value of LC, LC under 0.5 mA level, more active in

the end of the test

Mean values of LC increased the slowest,

active between 300-450 hours, LC was 0.3 mA level

Mean values of LC increased slower and less

active than #1, LC was under 0.5 mA level

64

were 25% to 30%. So there exist potential relationships between electrical characteristics and

chemical analysis results that electrical discharge results in an increase of O.I. and a decrease

of E/K ratio. These conclusions need more tests to fully confirm their validity.

Additionally, physical characteristics of insulators were examined to assess the surface

change during the test. The physical characteristics include hydrophobicity classification

(HC), contact angle, and surface images by scanning electron microscopy (SEM). Table 5-6

compares the results of HC and contact angles of the insulators before and after the test.

Surface Contact Angle HC (shed 1) Before After Insulator before after Max ( o ) Min ( o ) Max ( o ) Min ( o )

#1 1 5 78 17 53 0 #2 1 5 92 0 70 0 #3 1 2 141 66 86 0

#1 and #2 insulators were nearly totally hydrophilic (HC=5) after the test. It is noted that #3

insulator showed sound water resistance (HC=2) after the test. #3 insulator showed the

maximum value of contact angle among the three before and after the test. #1 insulator

showed the minimum value of the maximum contact angle before the test. #2 insulator kept

the value between the two. It is noted that the minimum contact angles of the three insulators

were 0˚ after the test. The HC results accord with the contact angle results, especially on #3

insulator. The greater the angle, the more hydrophobic the surface becomes. However, the

leakage currents of #3 insulator were not less active than the other two insulators during the

test. It was assumed that in the period of the fog test, the characteristic of hydrophobicity did

not play significant role to delay or to prevent the conditions, which were helpful to form

leakage current and discharges.

Table 5-6 HC and Contact Angle of Insulators before and after the Test

65

(a) #1 Shed Surface before the Test (×1000) (b) #1 Cross Section before the Test (×1000)

(c) #2 Shed Surface before the Test (×1000) (d) Sheath Surface of #2 before the Test (×2000)

(e) #3 Shed Surface before the Test (×200) (f) #3 Cross Section before the Test (×2000)

Figure 5-9 SEM I mAges of Surface of the Insulators (#1-#3) before and after the Test (Continued)

66

(g) #1 Shed Surface after the Test (× 1000) (h) Sheath Surface of #1 after the Test (×200)

(i) #2 Shed Surface after the Test (×2000) (j) Sheath Surface of #2 after the Test (×200)

(k) #3 Shed Surface after the Test (×1000) (l) Sheath Surface of #3 after the Test (×200)

Figure 5-9 SEM Images of Surface of the Insulators (#1-#3) before and after the Test

67

#1 Insulator #2 Insulator #3 Insulator

before Fresh dark color, no erosion holes, smooth surface

A litter bit darker caesious color, dust and small white particles on surface

Fresh caesious color, no cracks or holes on surface, tiny dust on surface, end-fitting new

after

White dry band, erosion holes close to ground, some white chalking, tracking

White dry band, erosion holes close to ground, bulk exposed, tracking

White dry band, chalking, tracking, end fitting with white chalking coat

Figure 5-9 shows surface images of the three insulators before and after the test. Table 5-7

compares the SEM images of the three insulators before and after the test. It shows that the

deterioration conditions on #2 insulator, especially on sheath surface, were more serious than

those of the other two. For all three insulators, the sheath surface deteriorated (Figure 5-9 (h),

(j), (l)) than the shed surface. After the test, on shed surfaces of the three insulators, pits and

cracks were visible. On sheath surfaces of all insulators, loose chalking layer were formed. It

is noted that the minimum contact angles of the three insulators were all 0˚ after the test. But

#3 insulator kept the highest hydrophobicity value before the test and after the test in terms of

the maximum contact angle among the three. It agreed well with the surface image showed in

Figure 5-9 (k), which shows that #3 insulator deteriorated less compared with the other two

insulators (Figure 5- 9 (g), (j)).

5.2.5 Summary

This section describes preliminary test procedures and presents test results. The comparison

between electrical characteristics, chemical, and physical analysis results were illustrated. The

objectives of the preliminary test were to investigate the appropriate test procedures to

correlate the relations between different assessment methods. The electrical test results

revealed that 0.2-0.4 mA was the most active range of LC for the tested EPDM insulators.

The amplitude of mean LC values of insulators in test was under 0.8 mA. The results provide

Table 5-7 Comparison of SEM I mAges of the Three Insulators

68

fundamental test results to understand electrical characteristics of EPDM insulators in clean

fog test conditions. The intensity of leakage currents, especially characterized by waveforms,

changed with test time. Generally, the intensity of leakage current increased with test time;

however, the leakage currents of the insulators in the test showed the characteristic of

uncertainty, which was correlated with the surface conditions of the insulators. Other

methods, including chemical and physical assessment, supplemented methods of assessing

surface conditions of insulators. It was noted that the HC degree of the insulator #3 kept

sound hydrophobic state. It was assumed that the surface conditions of the insulator #3 did not

show significant deterioration after the test, although it showed active leakage current

performance in the test. Based on the preliminary test, a series of tests will be carried out on

EPDM and EPDM/silicone rubber insulators. The results will be presented in the next

sections. And the relationships between electrical characteristics and chemical analysis results

will be discussed.

69

5.3 Electrical Tests on Composite Insulators

5.3.1 Test Conditions and Insulators’ Parameters

In this section, a series of tests, including salt fog tests, on composite insulators in clean and

salt fog test conditions are presented. Three different fog conditions, which had three kinds of

different fog conductivity, were used to accelerate the aging process of composite insulators.

Table 5-8 lists the three test conditions.

Table 5-8 Test Parameters of Tests

In test A, B, and C, eight insulators numbered as #1 - #8 were tested. Table 5-9 lists the

parameters of insulators. Among them, #1, #2, #5, and #6 were EPDM/silicone rubber

insulators from the same batch I. #3, #4, #7, and #8 were EPDM insulators from the same

batch II. #1, #3, #5, and #7 insulators were contaminated with kaolin, with a conductivity

value of 408 μm/cm. The kaolin contamination was prepared according to 4.2.3.1. It was

intended to find any effects on aging process of insulators caused by kaolin. The ESDD

(equivalent salt deposit density) of contamination was measured at 3.4 mg/cm3.

3 Fog particle size was measured by a partial measurement instrument from the Science Department

at QUT.

Test A Test B Test C Test Period (hours) 450 240 240 Salinity 0 2.5kg/m3 5kg/m3 Water Conductivity (µS/cm) 305 1750 3100 Testing Temperature (ºC) 23-25 22-25 22-25 Fog Particle Size (µm)3 10-30 10-30 10-30

70

Batch. Housing Composition Creepage Length (mm) Core diameter (mm) I EPDM/silicone rubber 405 22.0 II EPDM 380 22.6

Batch Shed Height (mm)

Number of Sheds Shed Spacing(mm) Creepage Stress

(mm/ kV) I 27 4 26 19.5 II 35 5 23 18.3

Table 5-9 Parameters of Insulators of Batch I & II

In test A, #1, #2, #3 and #4 were tested. Then in test B, #5, #6, #7 and #8 were placed inside

the fog chamber. #1, #2, #3, and #4 insulators were still in the fog chamber and were

subjected to test B together with insulators #5 - #8. The tests, which the insulators were

subjected to, are summarised in Table 5-10.

Insulators Test A2 Test B Test C #1 blend (EPDM+ silicone rubber) #2 blend (EPDM+ silicone rubber) #3 EPDM #4 EPDM #5 blend (EPDM+ silicone rubber) #6 blend (EPDM+ silicone rubber) #7 EPDM #8 EPDM

Table 5-10 Insulators’ List after Test C

5.3.2 Results of Test A

Test A lasted about 450 hours. Figure 5-10 shows the mean values of LC of #1 -#4 insulators.

Figure 5-11 shows the cumulative charge4 of the four insulators. Figure 5-12 compares the LC

distribution of the two different composition insulators.

4 The cumulative charge was calculated by integrating the leakage current every 30 minutes.

71

Figure 5-10 Mean Values of LC of Insulators in Test A

Figure 5-11 Cumulative Charge of LC of Insulators in Test A

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

450.0

Cum

ulat

ive

Char

ge (m

C)



35050 100 150 200 250 300 400 470

Time (Hours)

#1 insulator


#4 insulator

72

(a) LC Distribution of EPDM/silicone rubber Insulators (b) LC Distribution of EPDM Insulators

Figure 5-12 LC Distribution of Two Composition Insulators in Test A

Figure 5-10 shows that EPDM/silicone rubber insulators (#1 and #2) had lower level of

MVLC than EPDM insulators (#3 and #4). It is interesting to see that for EPDM insulators,

kaolin-polluted insulator #3 had relatively higher leakage current level than non-polluted

insulator #4. However, for EPDM/silicone rubber insulators #1 and #2, the effects of kaolin

on LC were not obvious. Figure 5-11 also shows that EPDM/silicone rubber insulators (#1

and #2) had lower level of cumulative charge than EPDM insulators (#3 and #4), especially

after 200 hours. The cumulative charge was calculated by the intergradation of the recorded

leakage current waveforms, so the value could be positive and negative. In Figure 5-11, the

change of the cumulative charge accords with the mean value of the leakage current of the test

insulators. Figure 5-12 compares LC distributions of the four insulators. For EPDM/silicone

rubber insulators, 0.2-0.4 mA (65%) was the most common LC range, followed by the range

of 0-0.2 mA (20%) and 0.4-0.6 mA (12%). For EPDM insulators, 0.4-0.6 mA was the most

concentrated LC range (48%), followed by 0.2-0.4 mA (21%). Next range was 0.6-0.8 mA

(19%). EPDM/silicone rubber insulators showed lower MVLC level than EPDM insulators.

73

Table 5-11 lists the LC distribution of the four insulators with different surface pollution in

the test.

Table 5-11 LC Distribution of Insulators with different Surface Contamination in Test A

Overall, for the same composition insulators, the kaolin-contaminated insulators (#1 and #3)

and the non-contaminated insulators (#2 and #4) showed different LC distribution. 26% LC

distribution of #3 insulator was in the range of 0-0.2 mA, whereas #4 showed 14%. In the

range of 0.2-0.4 mA, the distribution of LC of #3 was 57% and #4 was 73%. In the range of

0.4-0.6 mA, #3 was 15%, which was 5% greater than #4 (10%). In the range of >0.6 mA, both

insulators showed very small distribution. The effects of kaolin on leakage current were not

obvious.

Kaolin-contaminated EPDM insulator #3 showed greater LC distribution (51%) in the range

of 0.4-0.6 mA than non-kaolin polluted #4 (44%). In the range of 0.6-0.8 mA, LC distribution

of #4 (20%) was greater than that of #3 (10%). In other ranges, the two kept the difference

under 5%. The kaolin-polluted insulator appeared to show more active leakage currents than

non-polluted insulator.

This section presents test results of test A performed on four EPDM and EPDM/silicone

rubber insulators. The results mainly focus on mean values of LC of insulators. EPDM and

EPDM/silicone rubber insulators are compared in terms of leakage current. Also this section

compares the electrical performance of kaolin-polluted insulators with non-polluted insulators

of the same composition. The test results revealed that kaolin did not produce significant pro-

0-0.2 mA 0.2-0.4 mA 0.4-0.6 mA 0.6-0.8 mA >0.8 mA #1 E/S (with kaolin) 26% 57% 15% <2% <1% #2 E/S (no kaolin) 14% 73% 10% 2% <1%

#3 EPDM (with kaolin) 10% 18% 51% 10% <2% #4 EPDM (no kaolin) 8% 23% 44% 20% <5%

74

aging or anti-aging effects on insulators. In this section, EPDM and EPDM/silicone rubber

insulators showed different leakage current characteristics in clean fog test conditions.

EPDM/silicone rubber insulators showed better resistance to leakage current than EPDM

insulators. LC distribution analysis classifies mean values of leakage currents into different

groups, which are helpful to differentiate the mean values of leakage currents.

5.3.3 Results of Test B

Figure 5-13 shows mean values of LC of insulators #1 - #4. Figure 5-14 presents mean values

of LC of #5-#8 insulators. The LC distribution of the eight insulators in test B was showed in

Figure 5-15.

According to Table 5-8, Test B was carried out in salt fog, which was produced from water

with 2.5 kg/m3 salinity. The conductivity of water was 1750 µS/cm. The test lasted about 250

hours. #1, #2, #3, #4 insulators have been subjected to the Test A. #5 and #6 insulators were

newly introduced EPDM/silicone rubber insulators; #7 and #8 insulators were newly

introduced EPDM insulators. #5 and #7 insulators were polluted by kaolin, which was

prepared according to. 4.2.3.1. During the test no flashovers occurred except that power was

turned off once.

Figure 5-13 clearly shows that EPDM insulators, #3 and #4 had higher MVLC level (0.5-0.6

mA) than EPDM/silicone rubber insulators #1 and #2 (0.2-0.3 mA). The results agreed with

the results of Test A. In the figure, for EPDM/silicone rubber insulators (#1 and #2), the

difference between kaolin-polluted #1 and non-polluted #2 insulator was not obvious, and

both of them kept the same level of MVLC. For EPDM insulators (#3 and #4), the kaolin-

75

polluted #3 and non-polluted #4 did not show significant difference in leakage current level in

the test.

Figure 5-14 shows that the differences of MVLC between EPDM insulators, #7 and #8, and

EPDM/silicone rubber insulators, #5 and #6, were not as significant as the results in Figure 5-

13. #7 and #8 insulators showed little higher levels (0.4-0.5 mA) of MVLC than #5 and #6

insulators (0.2-0.3 mA). #7 and #8 insulators showed more current peaks than #5 and #6. It is

noted that the MVLC level of EPDM insulators #3 and #4 (0.5-0.6 mA), which have been

subjected to test A, was higher than that of EPDM insulators #7 and #8 (0.4-0.5 mA), which

have not been subjected to Test A. However, for EPDM/silicone rubber insulators, #1 and #2,

which have been subjected to Test A, and #5 and #6, which had the same composition but

have not been subjected to Test A, the difference was not clear. Their MVLC level stayed at

around 0.2 mA. In the figure, for EPDM insulators (#7 and #8), the kaolin-polluted #7

insulator showed a bit higher MVLC level than the non-polluted #8 insulator. There was no

clear difference on MVLC level between kaolin-polluted EPDM insulator #5 and non-

polluted #6 insulator.

Figure 5-15 compares LC distribution of the insulators, which were of the same composition

but with different test history. Figure 5-15 (a) shows that EPDM/silicone rubber insulators, #1

and #5, showed similar LC distribution. The most common LC distribution was 0-0.4 mA.

Figure 5-15 (b) shows that the LC distribution of #2 insulator was mush higher than that of #6

insulator in the range of 0.2-0.4 mA. But the LC distribution of #6 was nearly twice as that of

#2 insulator in the range of 0-0.2 mA. #2 insulator still had higher LC distribution than #6

insulator in the range of >0.4 mA but both of them kept low level of LC distribution.

76

Figure 5-15 (c) compares #3 and #7 EPDM insulators. In the range of 0-0.2 mA, the LC

distribution of #7 insulator was slightly higher than that of #3 insulator. In the range of 0.2-

0.4 mA, the LC distribution of #7 insulator was twice as many as that of #3 insulator. The LC

distribution of #3 was twice as many as that of #7 insulator in the range of 0.4-0.6 mA. In the

range of 0.6-0.8 mA, the LC distribution of #3 was nearly ten times as many as that of #7

insulator. Both insulators were at a very low level of LC distribution in the range greater than

0.8 mA. Figure 5-15 (d) shows that in the range of 0-0.2 mA, the LC distribution of #8 was

slighter higher than that of #4 insulator, and both of them kept similar LC distribution in the

range of 0.2-0.6 mA.

77

Figure 5-14 MVLC of Insulators #5- #8 in Test B

Figure 5-13 MVLC of Insulators #1- #4 in Test B

1esc - #1 insulator (EPDM/silicone) contaminated with kaolin 2es - #2 insulator (EPDM/silicone) without contamination 3epc - #3 insulator (EPDM) contaminated with kaolin 4ep - #4 insulator (EPDM) without contamination

5esc - #5 insulator (EPDM/silicone) contaminated with kaolin 6es - #6 insulator (EPDM/silicone) without contamination 7epc - #7 insulator (EPDM) contaminated with kaolin 8ep - #8 insulator (EPDM) without contamination

78

Test C was carried out under the fog condition with a fog conductivity value of 3100 μS/cm.

The fog was produced from water with 5kg/m3 salinity. During the test, total seven

flashovers occurred. The data of recorded leakage current was broken seven times. The results

are presented in the section of 5.3.5.3.

5.3.4 Summary

The test results reveal that the composition of insulators, the test history, and the test

conditions are factors that could affect insulators’ performance during the test. In this section,

Figure 5-15 LC Distribution of the Same Material Insulators with Different Test History (#1, #3, #5 and #7 with kaolin; #2, #4, #6, and #8 no kaolin)

0%10%20%30%40%50%60%70%

0-0.2 0.2-0.4 0.4-0.6 0.6-0.8 >0.8 (mA) 0%

10%

20%

30%

40%

50%

60%

0-0.2 0.2-0.4 0.4-0.6 0.6-0.8 >0.8 (mA)

#4 insulator

#8 insulator

0%

10%

20%

30%

40%

50%

60%

0.2-0.4 0.4-0.6 >0.6 (mA)0-0.20%

10%20%30%40%50%60%70%80%

0-0.2 0.2-0.4 0.4-0.6 >0.8 (mA)

#4 insulator #8 insulatr


(a) #1 & #5 insulators (b) #2 & #6 insulators


(c) #3 & #7 insulators (d) #4 & #8 insulators


79

the leakage currents of composite insulators were analysed. Mean values of leakage current,

and LC distribution are presented to assess performance of composite insulators. The results

revealed that EPDM/silicone rubber insulators showed better resistance to leakage current

than EPDM insulators. The test results seemed to show that the effect caused by kaolin

coating on EPDM insulators was greater than that on EPDM/silicone rubber insulators. But

the kaolin contamination on insulator surface did not play significant effects on the mean

values of leakage current. The results show that the test history significantly took effect on the

electrical characteristics of leakage currents.

Also the test history showed that the clean fog test and 2.5 kg/m3 salt fog test could not result

in flashover. Test A and Test B showed that mean values of LC of EPDM insulators mainly

concentrated in the range of 0.2-0.6 mA. For EPDM/silicone rubber insulators, the mean

values of LC mainly concentrated in the range of 0-0.4 mA. In the following sections, the

characteristics of momentary of leakage currents - waveforms of LC, will be presented and

analysed.

80

5.3.5 Waveforms of LC

5.3.5.1 Introduction

In the previous sections, mean values of LC of insulators in different test conditions were

analysed. The mean values provided general characteristics of LC. In this section, waveforms

of LC were studied.

Waveforms contain momentary state of LC of insulators, so it is helpful to study the effects of

LC on the insulator surface. In paper [56], the author studied waveforms of LC of suspension

insulators in wet contaminant and clean fog test conditions, and found that there were six

stages in the LC development from the start of test till flashover. The test found the threshold

values of LC for insulators before flashovers. Butler, etc. studied incipient behaviours of LC

waveforms to detect incipient of discharges of insulators. The results showed that LC

waveforms had erratic behaviour with bursts lasting from half a cycle to several seconds. The

objectives in this section focus on the shape of waveforms of LC, frequency-domain analysis

of waveforms, amplitude of waveforms and development stage of waveforms. The LC

waveforms are analysed to study surface activities of the insulator.

5.3.5.2 Waveforms and Analysis of LC in Test A - Clean Fog Test

In this section, LC waveforms were analysed with fast fourier transform (FFT) using

MATLab. The amplitude and shape of waveforms are presented. This section presents the

characteristics of waveforms of two insulators, #1 and #3, in test A. Appendix 6 lists the LC

waveforms and frequency-domain analysis of the other two insulators, #2 and #4. The reason

81

to raise #1 and #3 insulators to analyse is that #1 was EPDM/silicone rubber insulator and #3

was EPDM insulator, both of them contaminated with kaolin. The previous hydrophobicity

classification revealed that EPDM insulators are apt to become hydrophilic than

EPDM/silicone rubber insulators in moist environment. Figure 5-16 shows LC waveforms

and the FFT analysis of #1 and #3 insulators.

82

0 500 1000 1500 2000 2500 3000-0.5

0

0.5

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

600

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-0.1

-0.05

0

0.05

0.1

Sig

nal

Time

0 100 200 300 400 500 600 700 8000

20

40

60

80

100

120

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-4

-2

0

2

4

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-3

-2

-1

0

1

2

3

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

500

1000

1500

2000

Mag

nitu

de

Frequency [hertz]

(i) t = 2 hours (ii) t = 26 hours

(iii) t = 98 hours (iv) t = 122 hours

(v) t = 218 hours (vi) t = 338 hours

Figure 5-16 (a) LC Waveforms and FFT of #1 Insulator in Test A at Specific Time (cont.)

83

0 500 1000 1500 2000 2500 3000-2

0

2

4

6

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

(vii) t = 362 hours (viii) t = 410 hours

0 500 1000 1500 2000 2500 3000-2

-1

0

1

2

3

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

Mag

nitu

deFrequency [hertz]

Figure 5-16 (a) LC Waveforms and FFT Results of #1 Insulator in Test A at Specific Time (continued)

0 500 1000 1500 2000 2500 3000-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-4

-2

0

2

4

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

500

1000

1500

2000

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-2

-1

0

1

2

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

1000

1200

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-2

-1

0

1

2

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

500

1000

1500

Mag

nitu

de

Frequency [hertz]

(i) t =2 hours (ii) t =26 hours

(iii) t =98 hours (iv) t =122 hours

Figure 5-16 (b) LC Waveforms and FFT Results of #3 Insulator in Test A at Specific Time (Continued)

84

Analysis:

The figure shows that the LC of #1 and #3 can be divided into several stages.

#1 insulator:

Stage 1: LC waveforms were sine signals (Figure 5-16(a)-(i)). It revealed voltage and current

had linear relationship. So the insulator surface was resistive. No discharge was observed

during this stage.

0 500 1000 1500 2000 2500 3000-10

-5

0

5

10

15

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

1000

1200

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-4

-2

0

2

4

6

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

1000

1200

Mag

nitu

de

Frequency [hertz]

(v) t =218 hours (vi) t =338 hours

0 500 1000 1500 2000 2500 3000-6

-4

-2

0

2

4

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

500

1000

1500

Mag

nitu

de

Frequency [hertz]

(vii) t =362 hours (viii) t =410 hours

Figure 5-16 (b) LC Waveforms and FFT Results of #3 Insulator in test A at Specific Time (continued)

0 500 1000 1500 2000 2500 3000-3

-2

-1

0

1

2

3

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

Mag

nitu

de

Frequency [hertz]

85

Stage 2: The shape of waveforms became thinner while discharge emerged (Figure 5-16(a) –

(ii) (iii) (iv). The magnitude of LC increased from 0.1 mA to 0.6 mA, which was about triple

times as those of stage one. Audible discharges emerged. Odd harmonic components arose in

the FFT analysis results.

Stage 3: Discharges on insulator surface were visible and audible in this stage. The shape of

waveforms became sharper and that was explained by the fact that 3rd and 5th harmonic

components appeared in the waveforms. Peaks due to discharges were recorded in the

waveforms (Figure 5-16 (a) – (v) (vi) (vii) (viii)). The peaks of LC reached to 4 mA. The

general amplitude of waveforms remained at the level of 0.7 mA.

#3 insulator:

Stage 1: This stage lasted about 40 hours. The waveforms of LC of #3 insulator in Figure 5-

16 (i) were of a sine shape, which indicated that the relationships between voltage and current

were linear. The amplitude of LC waveforms was at the level of 0.2-0.4 mA. The FFT

analysis results showed that there was no obvious odd frequency component in the

waveforms. Stage 2: This stage lasted about 150hurs, and it was the longest period in the test. Figure 5-16

(b)-(ii) (iii) (iv) show that the amplitude of LC waveforms increased dramatically, reaching to

1.2-2.0 mA. The shape of waveforms became stretched. The FFT analysis results revealed

that 3rd and 5th harmonics became significant. Slightly visible and audible discharges were

observed.

Stage 3: This stage lasted about 60 hours. Peaks of waveforms were obvious in this stage.

The general LC amplitude was at the level of 0.5-0.7 mA. Discharges became more severe.

The discharges had big current peaks as high as 12 mA showed in Figure 5-16 (b)-(v). The

86

FFT results showed many harmonic components in the waveforms. These were indication of

intense discharges on the insulator surface.

Stage 4: Stage 4 lasted about 100 hours. Current peaks became more and more frequent. The

amplitude of peaks increased to 2.0-4.0 mA. The shape of LC was distorted and it became

lengthened. The FFT analysis results showed that odd number harmonic components became

considerable (Figure 5-16 (b) - (vi) (vii)), especially 3rd and 5th harmonic components.

Stage 5: The stage lasted about 100 hours. The LC waveforms performed in a relatively

stable manner. The waveforms were of a similar shape to those in stage 2, but the amplitude

was in the level of 2 mA. Figure 5-16 (b) - (viii) shows that no obvious big peaks were

recorded. The FFT analysis results revealed that there were 50Hz and considerable 3rd and 5th

of harmonic components in the waveforms.

The analysis of mean values of LC showed #3 EPDM insulator was more active than #1

EPDM/silicone rubber insulator in all tests. In this section, the LC waveforms of #1 and #3

insulators showed evidence that discharges on #3 insulator was more active than that on #1

insulator. First, the general amplitude of LC waveforms (no peaks) of #1 was at the level

under 1.5 mA, which was obviously less than that of #3 insulator, which ranged from 2 to 4

mA. Secondly, the frequency and intensity of discharge peaks of #1 insulator were less and

weaker than those of #3. Thirdly, the discharge waveforms of #1 appeared later than that of

#3. Finally, Figures 5-17 (a) – (v) (vii) (viii)) show that the number of harmonic components

of LC waveforms of #1 insulator was less than those of #3 insulator.

The waveforms of #2 insulator were similar to those of #1 insulator. The waveforms of #4

insulator were similar to those of #3 insulator. From the waveform analysis point of view,

87

there was no obvious difference between the kaolin-contaminated insulators and the non-

contaminated insulators.

5.3.5.3 Waveforms of LC in Test B with 2.5 kg/m3 Salt Fog

5.3.5.3.1 Results

Eight insulators (#1 - #8) were subjected to the test. The parameters of insulators and test

conditions were described in 5.3.1. This section studies the LC waveforms and the FFT

results of #1, #5, #3, and #7 insulators. Their LC waveforms of these insulators are listed in

Figure 5-17 to 5-20. The LC waveforms of the other four insulators are listed in the

Appendix-7.

88

0 100 200 300 400 500 600 700-0.05

0

0.05

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

5

10

15

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1.5

-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1.5

-1

-0.5

0

0.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]

(i) t = 1 hour (ii) t = 12 hours



0 100 200 300 400 500 600 700-8

-6

-4

-2

0

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

120

Mag

nitu

de

Frequency [hertz]

Figure 5-17 LC Waveforms and FFT Results of #1 Insulator in Test B at Specific Time

89

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.5

0

0.5

1

1.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

120

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

0.6

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]


(ix) t = 168 hours (x) t = 196 hours

(xi) t = 216 hours (xii) t = 238 hours

Figure 5-17 LC Waveforms and FFT Results of #1 Insulator in Test B at Specific Time (continued)

90

0 100 200 300 400 500 600 700-0.1

-0.05

0

0.05

0.1

0.15

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

1.5

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

0.6

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

0.6

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]





91

0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-40

-20

0

20

40

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

200

400

600

800

1000

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

0

1

2

3

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-3

-2

-1

0

1LC

(mA

)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.2

0

0.2

0.4

0.6

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-2

-1.5

-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]



(xi) t = 216 hours (x) t = 237 hours


92

0 100 200 300 400 500 600 700-0.1

-0.05

0

0.05

0.1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

5

10

15

20

25

30

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1.5

-1

-0.5

0

0.5

1

1.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

1.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]





93

0 100 200 300 400 500 600 700-8

-6

-4

-2

0

2

4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-3

-2

-1

0

1

2

3

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-2

-1

0

1

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

1.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1.5

-1

-0.5

0

0.5

1

1.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1.5

-1

-0.5

0

0.5

1

1.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]

(xii) t = 120 hours (xiii) t = 144 hours


(xi) t = 216 hours (xiii) t = 239 hours


94

0 100 200 300 400 500 600 700-0.1

-0.05

0

0.05

0.1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

5

10

15

20

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-2

-1

0

1

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-2

-1

0

1

2LC

(mA

)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-10

0

10

20

30

40

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-6

-4

-2

0

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

120

Mag

nitu

de

Frequency [hertz]




Figure 5-20 LC Waveforms and FFT Results of #7 Insulator in Test B at specific time

95

0 100 200 300 400 500 600 700-6

-4

-2

0

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

120

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-2

-1

0

1

2

3

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1LC

(mA

)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1.5

-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1.5

-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]





96

Analysis:

#1 insulator

In Test B, the general amplitude of LC waveforms of #1 insulator kept around 1.0 mA, which

was greater than that in Test A (>0.5 mA). In Figure 5-17, the peaks ranged from 1.0 mA to

7.0 mA, and discharge peaks emerged earlier (t=12 hours) than those in Test A. FFT results

revealed that harmonic components in waveforms appeared earlier in Test B than in Test A.

Comparison between #1 insulator and #5 insulator

• Sawtooth shape waveforms of LC of #5 insulator emerged later (t=48h) than those of #1

insulator (t=24h).

• The LC waveforms of #5 insulator showed clear stages, which were similar to the

pattern of #1 insulator in Test A.

• The peaks of #5 insulator and FFT results revealed there were more intense discharges

in the later phases in Test B (t=144 hours). They were more active than #5 insulator.

The peak values reached as high as 20 mA (t=144hours).

#3 insulator

• The sawtooth shape waveforms of LC appeared earlier (t=48hours) than those in test A2

(t=338 hours).

• The amplitude of waveforms of LC in Test B stabilized at the level of 1.0 mA. At t=120

hours, the amplitude of LC waveforms sharply increased to 4.0 mA. At t=144 hours, the

amplitude remained at the level of 1.0 mA. For #3 insulator, the amplitude of

waveforms of LC was in the range of 1.0-2.0 mA.

97

• The sawtooth shape of LC waveforms of #3 in Test B emerged much earlier than those

in Test A. It revealed that discharge appeared earlier in Test B, but the peaks in

waveforms showed that the intensity of discharges was weaker than that in Test A.

Comparison between #3 and #7 insulators

• In test B, the sawtooth waveforms of discharges of #7 insulator appeared earlier (t=12

hours) than those of #3 insulator (t=48 hours).

• The current peaks in LC waveforms of #7 insulator appeared earlier (t=12 hours) than

those of #3 insulator (t=48 hours).

• The current peaks in Test B of #7 (I max=5 mA-31 mA) were higher than those of #3

insulator (I max=6 mA) in the test.

• In test B, the general amplitude of LC waveforms of #3 and #7 insulators was at around

1.0 mA.

5.3.5.3.2 Conclusions

• Under the 2.5 kg/m3 salinity fog test condition, no flashover occurred on all insulators.

• The leakage current of #1 - #4 insulators, which have been subjected to clean fog Test

A, showed more active waveforms than those of the insulators #5 - #8, which had no

clean fog test history.

• EPDM/silicone rubber insulators showed lower level of mean values of LC than EPDM

insulators. In Figure 5-10, EPDM showed obviously higher mean values of LC after 300

hours than EPDM/silicone rubber insulators in test A2. In Figure 5-13, the similar

results were found after around 100 hours in test B.

• Three characteristics of LC waveforms described the electrical performance of

insulators in tests. The first one is the mean value of LC. The second one is the

98

waveform shape. The lengthened shape of waveforms and distorted triangle waveforms

was explained by the phenomena of forming dry band, which resulted in discharges.

Sawtooth shape waveforms revealed the intensity increase of discharge activities. The

third one is the current peak, which represented intensity of discharges or arcs.

• The signatures of transitional phase in the waveforms were the lengthened shape and the

distorted waveforms. #1 - #4 insulators, which had been subjected to Test A, did not

show clear transitional phase in Test B. The sawtooth shape of waveforms of #1 - #4

insulators appeared in the early stage compared with those of #5 - #8 insulators.

5.3.5.4 Waveforms of LC in Test C with 5kg/m3 Salinity Fog Test

In Test C, 5 kg/m3 salt density was applied. The conductivity of salt water was 3100 µS/cm.

The test conditions with higher conductivity were more serious than the previous two, Test A

and Test B. The test was carried out 7 days after Test B. Insulators under Test C were the

same insulators #1- #8. In this test, flashover occurred seven times. Figure 5-21 lists LC

waveforms and FFT results of insulators, which had incidence of flashover in the test. All LC

waveforms and FFT results of the eight insulators before flashover were presented in

Appendix-8.

99

0 500 1000 1500 2000 2500 3000-2

0

2

4

6

8

10LC

(mA

)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

2000

2500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-10

-8

-6

-4

-2

0

2

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-30

-20

-10

0

10

20

30

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-5

0

5

10

15

20

25

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-60

-40

-20

0

20

40

60

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

2000

2500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-40

-30

-20

-10

0

10

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

2000

Mag

nitu

de

Frequency [hertz]

(i) 1st flashover - #1 insulator (ii) 2nd flashover - #2 insulator

(iii) 3rd flashover - #5 insulator (iv) 4th flashover - #5 insulator

(v) 5th flashover - #5 insulator (vi) 6th flashover - #1 insulator

Figure 5-21 LC Waveforms and FFT Results of Flashovered Insulators before Flashover

100

The characteristics of the LC waveforms of insulators before flashover are described as

follows.

• Big discharge peaks, which ranged up to 45 mA, emerged in the LC waveform before

flashover.

• The odd-frequency components in the waveforms of the peaks, especially the 5th and 7th

harmonic components were significant before flashover.

• FFT results showed different harmonic components in different waveforms of insulators

before each flashover. #1 insulator (1st flashover) and #2 (2nd flashover) showed

dominant 50Hz components in waveforms, whereas for other insulators, odd harmonic

components were considerable.

• The flashover of insulators happened randomly. In this test, #5 insulator showed active

manner compared with other insulators. Test history of insulators did not increase the

chance of flashover.

0 500 1000 1500 2000 2500 3000-50

0

50

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

2000

Mag

nitu

de

Frequency [hertz]

(vii) 7th flashover - #7 insulator

Figure 5-21 LC Waveforms and FFT Results of Flashovered Insulators before Flashover (continued)

101

• The waveforms of insulators with no flashovers. They included all forms of waveforms,

sine shape, lengthened waveforms, distorted shape of waveforms, and sawtooth shape

of waveforms.

• The interval time5 between two flashovers is shown in Figure 5-22. The figure shows

that the interval time was around 9-10 hours except for the fourth flashover, which was

just 1 hour.

Figure 5-22 Interval Time between Flashovers

5.3.6 Summary of Waveform Results

Section 5.3.5 presented test results on eight insulators under three different test conditions.

The analysis of test results focused on mean values of leakage currents, the distribution of

leakage current, waveforms of leakage current, FFT analysis of waveforms, and the

characteristics of waveforms before flashover.

The LC waveforms of the testing insulators experienced different phases. It is generally

known that leakage current is divided into two parts; one is current by electrolytic

5 The interval time means the time between the start of energising to flashover.

10 109

1

9 98

0

2

4

6

8

10

12

1 2 3 4 5 6 7

Flashover

Tim

e (h

ours

)

102

conductivity, and the other is by dry band arcing. Dry band arcing is the main reason resulting

in the degradation on the composite insulators [57]. The waveforms of leakage current from

each part show obviously different features. In the beginning of the tests, the waveforms

appeared in sinusoidal shape, which represent that the voltage and current on the surface of

the insulators were resistant relationship. The energy carried by the current dissipated into the

surface of the insulators. At this stage, no discharge activities were observed. Consequently,

the currents did not significantly affect the structure of the surface of the insulators. However,

the distorted waveforms, which appeared after the sinusoidal leakage currents, showed

observable discharge activities, including arcs, which significant affected the surface

conditions of the insulators. The sawtooth shape of the waveforms represented faint discharge

appeared on the surface of the insulators. The distorted sharper and thinner waveforms of the

leakage currents represented the discharge activities became more serious and arcs appeared

on the surface of the insulators. By observation, the discharge activities and arcs were more

serious in the top end of the insulators and the bottom end of the insulators. The large peaks in

the waveforms showed that strong arcs appeared on the insulator surface. The energy in the

current was strong enough to cause deformation on the insulator surface, which including pits,

scissions. The peaks in the waveforms were the indication of flashover. The FFT analysis

revealed that the 3rd harmonic was the main part of the odd harmonic components in the

leakage currents during the discharge activities. Also 5th and 7th harmonic were obvious in the

leakage currents during the discharge activities.

The characteristics of waveforms of the leakage current are explained to understand the

activities on insulator surface during the tests. Next section investigates the physical change

on insulator surface using scanning electron microscopy (SEM) techniques.

103

5.4 Relationships between Physical, Chemical Properties of Insulator and

Electrical Characteristics

5.4.1 Scanning Electron Microscopy (SEM) Observations of the Insulator Surface

SEM was used to observe the physical changes on the surface of eight insulators following

Test C. They are showed in Figure 5-23.

(a) Shed-1 of #1 insulator (× 2000) (b) Shed-1 of #3 insulator (× 2000)

cracks chalking

Figure 5-23 SEM I mAges of the Eight Insulators after Test C

Loose Area

(c) Core 1-2 of #1 insulator (×2100) (d) Core 1-2 of #3 insulator (×1000)

Loose Area

Loose area

chalking

104

(e) Shed-1 of #2 insulator (×1250) (f) Shed-1 of #4 insulator (×3000)

(g) Core 1-2 of #2 insulator (×100) (h) Core 1-2 of #4 insulator (×150)

discharge pits

Figure 5-23 SEM Images of the Eight Insulators after Test C (continued)

105

(i) Shed-1 of #5 insulator (×100) (j) Shed-1 of #7 insulator (×40)


(k) Core 1-2 of #5 insulator (×100) (l) Core 1-2 of #7 insulator (×200)

106

(m) Shed-1 of #6 insulator (×1000) (n) Shed-1 of #8 insulator (×80)

(o) Core 1-2 of #6 insulator (×1500) (p) Core 1-2 of #8 insulator (×200)


dented trace

107

Observation Results:

I. Figures 5-23 (a) and (b) clearly show that the surface of #3 EPDM insulator had

deteriorated more than that of #1 EPDM/silicone rubber insulator. The cracking and

chalking on #3 insulator surface were obvious. In contrast, #1 insulator did not have any

obvious cracking or chalking, and its surface remained smooth.

II. Figure 5-23 (c) shows marked roughness of the surface of core 1-2 of #1 insulators. This

can be compared to the smooth surface of shed in (a). The surface in (c) appears to have

polynmer leaving ATH behind. Figure 5-23 (d) shows that severe chalking areas and

cracks have formed on core 1-2 surface of insualtor #3.

III. Figures 5-23 (e) and (f) compare the shed surface conditions of #2 and #4 insulators. It

shows that there were some surface deposits on #2 insulator, and there was no obviously

deteriorated cracking and holes on it. Whereas, #4 insulator shed surface showed more

deposits on surface, and there were apparent chalking and cracks on surface.

IV. Figures 5-23 (g) and (h) show that at core 1-2 of #2 insulator, direct discharge evidence

was present, with white color discharge pits scattering on the core surface. On #4

insulator core 1-2, the scrap-like loose flakes were present on the surface. It deteriorated

more than the core 1-2 of #2 insulator.

V. Figures 5-23 (i) shows that on #5 insulator shed surface there was no obvious discharge

pits but discharge traces were present. On #7 insulator shed surface, the evidence of

surface discharges, white discharge pits, was visible. The deterioration on #7 insulator

surface was somewhat more sereve than that of #5 insulator.

VI. Figures 5-23 (k) and (l) show the surface conditions on core 1-2 of #5 and #7 insulators.

#5 insulator core surface did not present holes and chalking material but show obvious

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discharge traces. #7 insulator presented discharge pits, which changed color to white in

SEM. The centre of pits was more loose than around areas.

VII. Figures 5-23 (m) and (n) show the shed-1 surface of #6 and #8 insulators. There were

what apparent to be loose materials on #6 insulator surface, but no obvious cracks or

chalking. On #8 insulator surface, the whole shed surface became roughened and no flat

areas can be found.

VIII. Figures 5-23 (o) and (p) show the images of core 1-2 surface of #6 and #8 insulators.

For #6 insulator, the core 1-2 surface showed slight discharge dented traces and the

whole areas were relatively smooth. On #8 insulator core 1-2, it had similar situations as

its shed surface that some small loose grains scattered on surface and the core surface

roughened.

Summary:

(a) #1-#4 insulators deteriorated more severe than #5 - #8 insulators. Insulators #1-#4 have

been subjected to Test A, B, and C, and #5-#8 have been subjected to Test B and C.

(b) EPDM insulators, #3, #4, #7, and #8, suffered more serious surface deterioration than

EPDM/silicone rubber insulators, #1, #2, #5, and #6 insulators.

(c) The photos demonstrated that core surface of insulators was more degraded than shed

surface.

(d) #1, #3, #5, and #7 insulators were contaminated with kaolin before the tests. Comparing

with #2, #4, #6, and #8 insulators, which were not contaminated, kaolin-contaminated

insulators did not show more deteriorated surface conditions than non-contaminated

ones.

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Blackmore [2] investigated discharges on hydrophobic material surface. The results found

that the discharge occurred between water drops in series forming a chain of discharge spots

between electrodes. Figure 5-24 reproduces four pictures from Blackmore, which clearly

demonstrate the discharge development between electrodes from initial discharge to intense

glow discharges.

It is noted that discharges started at the edges of water droplets on surface. Initially, the

discharges presented in blue and purple colour. Then the size of discharge spots increased,

and discharge colour changed to orange red (Figure 5-24 (c)). In Figure 5-24 (d), the

Figure 5-24 Sequence of Discharges on Hydrophobic Silicone Rubber – transition from Initial discharges to intense glow discharges

(σ=16000μS cm-1, Tapart=0.04s) [2]

(a) (b)

(c) (d)

electrodes

halla


110

discharges were very intense and discharge spots began to joint together forming discharge

band areas. The whole discharge pattern was composed of round-shape discharge spots and

band-shape discharge areas. The discharge glowed in bright colour. After the test, the author

found that there were small pits visible to the naked eyes forming erosion parts on rubber

surface. In our fog tests, Figure 5-23 (g) (j) (k) show that there were similar discharge traces

on insulator surface. The results provide evidence of deterioration of the insulator surface due

to discharge.

5.4.2 Chemical Analysis Results and Comparison with Physical Observations

The eight insulators were analysed with chemical assessment methods described in Chapter 2.

Samples were taken from three locations on each insulator. Figure 5-25 shows the three

sampling locations.

Figure 5-25 Locations of Sampling for Chemical Analysis

S1

S4

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(a) O.I. of Insulators #1 and #2 (b) O.I. of Insulators #3 and #4

(c) O.I. of Insulators #5 and #6 (d) O.I. of Insulators #7 and #8

Figure 5-26 Oxidation Index of Insulators #1 - #8 before and after the Tests

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(a) E/K ratios of Insulators #1 and #2 (b) E/K ratios of Insulators #3 and #4

(c) E/K ratios of Insulators #5 and #6 (d) E/K ratios of Insulators #7 and #8

Figure 5-27 Ester/ketone Ratio of Insulators #1 - #8 before and after the Tests

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Figure 5-26 shows the values of oxidation index (O.I.) of the insulators before and after the

tests. The marked points are the average values of three oxidation indices. The error bars

show the standard deviations of the mean values. The results show that the O.I. of insulators

increased after each test. Figure 5-26 (a) shows that the O.I. of EPDM/silicone rubber

insulators #1 and #2 increased 15% and 39% respectively after all the tests. Figure 5-26 (b)

shows that the O.I. of EPDM insulators #3 and #4 increased 84% and 76% respectively after

all the tests. The increases were obviously higher than those of #1 and #2 insulators. It is

noted that while EPDM insulators #7 and #8 were not subjected to the clean fog Test A, their

O.I. increased 150% and 108% respectively (Figure 5-26 (d)). The O.I. of EPDM/silicone

rubber insulators #5 and #6, which had the same test history as #7 and #8, increased about

14% (Figure 5-26 (c)).

Figure 5-27 shows that the average values of the ester/ketone (E/K) ratio for each insulator. In

the figure, the marked points are the average values of E/K ratios from the three locations.

The error bars show standard deviations of the mean values. It is obvious that E/K ratios

decreased with the tests. This indicated that there was an increase in ketone with the tests. The

E/K ratios of EPDM/silicone rubber insulators #1 and #2 insulators decreased 49% and 42%

respectively after all the tests (Figure 5-27 (a)). The E/K ratios of EPDM insulators #3 and #4

decreased 48% and 39% respectively after all the tests (Figure 5-27 (b)). And the E/K ratios

of EPDM/silicone rubber insulators #5 and #6 decreased 46% and 54% respectively after the

tests (Figure 5-27 (c)). The E/K ratios of EPDM insulators #7 and #8 decreased about 28%

and 39% respectively after the tests (Figure 5-27 (d)).

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Among the insulators #1- #4, it is noted that the change rate of E/K ratios of the insulators in

Test A was much higher than that in Test B and Test C. The E/K ratios of insulators #1, #2,

#3 and #4 decreased dramatically after Test A. The E/K ratios of #1 and #2 insulators

decreased slightly after Test B and Test C. The E/K ratios of #3 also decreased after Test B,

but after Test C, its value seems to have recovered. This could be due to a measurement error.

The E/K ratio of #4 insulator stayed stable after Test B and Test C. For #5-#8 insulators, the

E/K ratios decreased dramatically after Test B, but did not change significantly after Test C.

The results can be summarised as follows. Firstly, EPDM insulators in general showed higher

O.I. change rate than EPDM/silicone rubber insulators, which indicate EPDM insulators are

more susceptible to oxidation conditions on insulator surface than EPDM /silicone rubber

insulators. It is noted that #2 and #4 insulators showed decreased O.I. after Test C, which

contradicts the other O.I. measurement results. Several factors may have affected the

measurement results. (a) The sampling areas after Test C were different from the sampling

areas after Test A and Test B. (b) the deteriorated materials on sampling surface were partly

washed off by water formed by fog moist on insulator surface.

Secondly, the decreased E/K ratio indicated that the thermal effects caused by discharge

activities and leakage currents dominated the degradation process on the insulators in the

tests. This conclusion is based on the fact that ester degradation products are associated with

photo oxidation, and ketone product is associated with thermal oxidation [30]. UV from

sunlight was not a factor in the tests, but some UV would be present during the tests. It is

believed that discharges are apt to happen in moist environment. The oxidation on the surface

of insulators was mainly caused by leakage currents. Discharge is a far more effective

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initiator of degradation than UV. It is noted that both EPDM and EPDM/silicone rubber

material insulators showed similar E/K ratio change during the tests.

5.4.3 Discussion of the Relationships between Physical Characteristics and Chemical Analysis Results of Composite Insulators

The first conclusion to be drawn from the results of physical and chemical analysis is that

EPDM and EPDM/silicone rubber insulators showed different surface deterioration under the

same test conditions. EPDM insulators showed greater surface damages than EPDM/silicone

rubber insulators. Correspondingly, EPDM insulators showed more active electrical

characteristics, including a higher level of leakage currents and more active discharge

activities than EPDM/silicone rubber insulators under the test conditions. Secondly, the

oxidation index revealed that the extent of oxidation on EPDM insulator surface was greater

than that of EPDM/silicone rubber insulators. Thus, the physical observations of the insulators

in the test conditions were consistent with the chemical analysis results. Thirdly, E/K ratios

verified that thermal effect caused by leakage currents and discharges was the dominant

degradation mechanism. Fourthly, the insulators, which had been contaminated with kaolin,

did not show significant differences from the non-contaminated insulators regarding leakage

currents, oxidation index, and E/K ratio.

In Chapter 2, it was noted that silicone rubber can recover hydrophobicity characteristic on its

surface due to the migration of silicone molecular from bulk to surface [21]. It is believed that

the reason for using a combination of EPDM and silicone rubber is to introduce this

characteristic to improve the hydrophobicity of the composition. The results in the fog tests

116

showed that EPDM/silicone rubber insulators did perform better than EPDM insulators in

terms of physical observations, electrical characteristics and chemical analysis results.

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CHAPTER 6

SURFACE RESISTANCE MEASUREMENT TO ASSESS SURFACE

CONDITIONS OF COMPOSITE INSULATORS

6.1 Introduction

Under some circumstances, there is a need for a simple reliable technique for assessing the

surface conditions of composite insulators prior to installation. For example, when insulators

have been stored for a long period in a dusty environment the appearance of the insulators

may change and power supply company may be reluctant to use the insulators. New

techniques for determining the insulator condition from chemical analysis of housing material

samples have been introduced in the previous chapter. Recently, low voltage tests for

measurement of insulation resistance of insulators has been introduced by an IEEE Working

Group [58]. This following examines a new procedure for measurement of surface leakage

resistance for field applications and the results are compared with chemical analysis results.

6.2 Leakage Resistance Assessment Using a Water Spray

Five high-voltage (275 kV and 330 kV) composite insulators were used as test samples. Three

silicone rubber insulators were numbered as sr1, sr2, and sr3. Insulators sr1 and sr2 had been

stored in a dusty environment for over one year after being removed from service. The

original grey color of sr1 had changed to deep grey color. sr3 was a new silicone rubber

insulator. Two EPDM insulators, ep1 and ep2, were removed from service because of concern

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about color change. ep2 had once flashed over but no obvious surface deterioration could be

seen. One 11 kV porcelain insulator was tested as a reference.

6. 2. 1 Test with Water Spray

In the test, it was initially proposed to use the leakage impedance method of assessing

composite insulator condition but with the exception that a hand-held water spray was to be

tried as a replacement for a fog generated by a traditional fog chamber. The arrangement of

the initial tests is shown in Figure 6-1.

Figure 6-1 Arrangement for Water Spray Test

Aluminium foil was wrapped around the shanks of insulators mid-way between the sheds.

Copper wire was bound on the foil as electrodes to connect the AC power supply (50Hz) and

the data-recording system. As shown in Figure 6-1, one pair of electrodes were separated by

several sheds. The creepage distance of the test sections of all insulators was determined to be

close to 450mm. Table 6-1 shows the shape of the insulators. At first, the insulators were

subjected to spraying with tap water (conductivity=315 µS/cm) and test voltage at 2 kV (rms).

The test conditions are listed in Table 6-2.

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Insulators sr1 sr2, sr3, ep1 ep2 porcelain

Shape

Table 6-1 Shape of Sample Insulators

The AC power supply was a 0-19 kV test transformer and the current-measurement used a

shunt and a waveform recorder. Mean values of leakage current (LC) and LC waveforms

were measured. Surface resistance was calculated from Ohm’s Law.

Leakage Distance Between Electrodes 45 cm

Voltage Applied 2 kV (rms) Method of Wetting Hand-held spray Time Duration 5 minutes

315 µS/cm Water Conductivity 2000 µS/cm

Table 6-2 Configurations for Water Spray Test

Results revealed that tests with spraying tap water (conductivity=315µS/cm) did not yield any

measurable LC on silicone, EPDM, and porcelain insulators. Test with salt water spray

(conductivity =2000 µS/cm) also showed no LC on any insulator surface. It was concluded

that under the test conditions, it is difficult to differentiate surface conditions of composite

insulators.

6.2.2 Tests with Water with Reduced Surface Tension

The question arose as to whether it might be possible to increase leakage current to a

measurable value by spraying with water with artificially reduced surface tension in order to

give a uniform coating of moisture on the insulator surface. Professor Ravi Gorur, who was a

visiting Professor of QUT suggested that liquid soup be added to 450 ml water in the spray

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bottle and well shaken. The purpose of the soap was to reduce the surface tension of water

deposited on the surface of composite insulators. The spray water was used on all the test

samples simultaneously. Figure 6-2 (a) shows surface of insulator sr1 after being sprayed with

tap water. Figure 6-2 (b) shows the surface of sr1 after being sprayed with tap water

containing liquid soup. It is obvious that the surface showed a uniform coating of moisture on

the insulator surface.

Figure 6-2 Surface Conditions of sr1 before and after Using Liquid Soup

6.2.3 Leakage Currents of Artificially-wet Insulators

During test of spraying with 315 µS/cm water with liquid soup, in every minute, six

waveforms (0.02 s×6=0.12 s) of LC were recorded. During the test, no flashover happened.

Figure 6-3 combines the six waveforms of LC to demonstrate the change of LC in the test. It

shows that the amplitude of waveform decreased with test duration, indicating that there was

an increase in surface leakage resistance with time. The surface leakage resistance was

calculated by dividing the peak supply voltage by the peak leakage current. Surface resistance

results measured in two different test conditions are shown in Figure 6-4 (a) and (b). As

(a) Spay-tap Water (b) Spay-tap Water with Addition of

Drops of Liquid Soup

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insulator sr3 had no measurable leakage current under all test conditions, no data is shown in

Figure 6-4. Insulator sr3 was regarded as a sound insulator.

Figure 6-4 (a) shows in 315 μS/cm water spray test, sr1 had the fastest increase of surface

resistance value, whereas the porcelain insulator increased more slowly and kept the lowest

-2.5-2.0-1.5-1.0-0.50.00.51.01.52.0

Leak

age

Cur

rent

(mA

) Envelope of LC

2nd min 3rd min 4th min 5th 0.12s 0.12s 0.12s 0.12s 0.12s

1st min

Figure 6-3 LC Waveform of sr1 in Spray Water Test (conductivity=315 µS/cm with liquid soup)

Ω

Ω

Ω

(a) 315 µS/cm water spray (b) 2000 µS/cm water spray

Figure 6-4 Surface Resistance of Insulators in Spray Water with Liquid Soup

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level of surface resistance value. The surface resistance value of insulators sr2, ep1, and ep2

increased steadily during test period. Figure 6-4 (b) indicates that in 2000 μS/cm spray water

test, silicone rubber and EPDM insulators had obviously different performance regarding

surface resistance value. The surface resistance of sr1 increased sharply to infinity value after

1 minute from the start of test. sr2 also showed a significantly quick increase in surface

resistance. For EPDM insulators, ep1 retains very low level of surface resistance (around 0.5

MΩ) during the test period while ep2 had a significant increase after 4 minutes from 1.2 MΩ

to infinity level. The porcelain insulator had the lowest surface resistance and maintained this

with little change during the test period.

6.3 Discussion of Results

The different performance of aged composite insulators in terms of surface resistance in water

spray tests is discussed as follows. Firstly, it was noted that the water with higher conductivity

resulted in higher level of leakage current and thereby produced more energy in water film

layer, accelerating the speed of evaporation of water film. Consequently, the surface

resistance value of composite insulators recovered faster in the 2000 µS/cm water spray test

than the 315 µS/cm water spray test. Figure 6-4 shows the different recovery speed of surface

resistance of the test insulators in the two tests.

Secondly, water film on insulator surface determined the impedance for current to flow across

insulator surface. Leakage current flowing across water film was decided by test voltage and

the impedance of the water film. Consequently, leakage current causes heating. In this test,

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leakage current on the insulator surface heated water film and resulted in water molecules

receiving heat energy to escape from water layer to air. As a result, the decrease of the

volume of water film on insulator surface brought about the increase of surface resistance. On

the other hand, the heat produced by leakage current on water film not only conducted to

water film but also conducted to the underlying polymer layer. Figure 6-5 demonstrates the

situations. The equation 1 quantifies the heat conducted along Z direction.

q = -dzdT⋅κ (1)

where q is the heat flux in J/(m2·sec), -dzdT represents the change in the temperature gradient

in Z direction (degrees Kelvin/m) and κ is the thermal conductivity.

If considering different composite materials have the same surface area A with water film, the

energy transferred from the surface contacting layer along Z direction is determined by

Figure 6-5 Ther mAl effect on insulator surface caused by leakage current

leakage current Water

film

Insulator surface

q

Z

A

Heat loss by convection and evaporation

Heat loss to insulation by conduction

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thermal conductivity. The thermal conductivity (at room temperature) of four materials used

in insulators is listed in Table 6-3 [59] [60].

material Polypropylene Polyethylene (medium density)

Silicone Elastomer Porcelain

Thermal conductivity (W/m·k) 0.1-0.3 0.35 0.63 1.7

Table 6-3 Thermal conductivity of four materials used in insulators [59][60]

Porcelain has the highest thermal conductivity among the four materials and is 2 to 17 times

higher than that of silicone elastomer, polyethylene and polypropylene. It was mentioned that

heating caused by leakage current not only led to the evaporation of surface water but also

conducted to the underlying layer of polymer. According to the equation 1, the rate of

conducting heat to the underlying layer of polymer was dependent upon the thermal

conductivity of the insulator material. Compared with other materials, porcelain transferred

more energy from the water film to the insulator interior parts, thus reducing the energy

conducting to surface water film. Therefore the existence of water film layer on insulator

surface kept the surface resistance value low. The thermal conductivity of EPDM and silicone

materials was lower, which resulted in less energy transferring to the insulator interior parts.

More energy was conducted away from water film in the case of EPDM than was the case for

silicone rubber. This causes the water film to be hotter for SIR than for EPDM leading to

ending evaporation and subsequently only pushed up of film resistance for SIR and can be

seen in Figure 6-4.

However, EPDM showed different change in terms of surface resistance with silicone

insulators in Figure 6-4 (b). To better understand this phenomenon, surface conditions of

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insulators were considered. In this test, ep1 and ep2 were analysed by chemical assessment.

Their oxidation indices (ep1-1.18, ep2- 0.855) were higher than normally acceptable

oxidation index value (0.4) [19]. The degraded surface had cracking and pits, which provided

space for water to reside. The water in extra space eventually prolonged the evaporation time.

One factor may be considered to explain the different results between EPDM and silicone

rubber insulators is the fact that silicone rubber has ability to recover its surface

hydrophobicity due to migration of low molecular weight silicone from the bulk to the

surface.

6.4 Conclusion

Spray water tests on different insulators were used to verify the efficiency of using simple and

practical methods to assess the surface condition of composite insulators. Silicone rubber

insulators showed better recovery of surface resistance than EPDM insulators under water

spray test conditions with added “liquid soup”. Hydrophobicity and thermal effects caused by

leakage current are hypothesised to be responsible for the recovery of surface resistance.

The question arises as to whether water spray with soap added provides useful information

about the surface of the insulation tested. Certainly water spray with no added soap provides a

direct indication of the surface hydrophobicity. However, adding soap causes immediate lost

of hydrophobicity as normally observed from water droplets on the surface of the material. In

the case with soap there is interesting behaviour of the leakage current as the surface dries

out. The leakage current gradually falls in magnitude and eventually is extinguished. In what

we have done in the lab and simple analysis, it was showed that the rate of which the leakage

126

current affected primarily by the thermal parameter of the shed material. This is not

considerately related to the surface conditions. It is concluded that there is no directly useful

information about the condition of the insulator that can be inferred from measurement with

soap-add to spray water.

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CHAPTER 7

SUMMARY

This thesis is aimed at improving the understanding the electrical characteristics of composite

insulators during their aging in the fog tests. The surface conditions of composite insulators

assessed by chemical and physical methods were correlated with the electrical performance.

The summary of the thesis is presented as follows.

7.1 Electrical Characteristics of Composite Insulators in Fog Tests

The thesis presented the electrical characteristics of composite insulators in fog tests. The test

methodology was based on IEC 60507 – “Artificially pollution tests on high-voltage

insulators to be used on AC systems”, IEC 61109 – “Composite insulators for AC overhead

lines with a nominal voltage greater than 1000V – Definitions, test methods and acceptance

criteria”, and IEEE Standard Techniques for High-Voltage Testing. The tests adopted three

different salinity fog conditions to conduct the tests on composite insulators with different

contamination conditions. Mean values of leakage currents, cumulative charges of leakage

currents, and waveforms of leakage currents were recorded as electrical characteristics of

composite insulators. The recorded waveforms of leakage currents showed the following

features.

1) In clean fog and salty fog test, EPDM and EPDM/silicone rubber insulators showed

different leakage current distributions. The mean values of leakage currents of EPDM

insulators mainly concentrated in the range of 0.2-0.6mA. For EPDM/silicone rubber

insulators, the range of was 0-0.4mA. EPDM insulators clearly showed a more active

electrical performance than EPDM/silicone rubber insulators. The recorded data showed

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that the general mean values of leakage currents of all testing insulator samples were

under 1 mA.

2) The shape of waveforms of leakage currents indicated the surface conditions of the

composite insulators in the tests. The sine shape of waveforms represented there were no

discharges on the surface of the insulators, and the voltage and the current were linear

relationship. The deformed waveforms of leakage currents indicated the occurrence of

discharges. The sawtooth shape of waveforms indicated the strengthened discharge

activities. The intense discharges had peak values of current of 30-40 mA. From the

development of discharge activities, audible discharge sound and visible discharges were

observable.

3) The FFT analysis results revealed that the 3rd (150 Hz) and the 5th (250Hz) frequency

were the main harmonics of the leakage currents during the stable discharge. The

waveforms of big current peaks (10 to 40 mA) showed the harmonics ranged from 50 Hz

to 400 Hz.

4) The chemical analysis on composite insulators revealed that thermal effects caused by

leakage current were the main aging factors for the test insulators under the test

conditions. UV produced by discharges played minor roles in helping aging of the test

insulators.

5) Under Test A (σ = 305 μS/cm) and Test B (σ = 1750 μS/cm), no flashover occurred. The

fog with conductivity σ = 3100 μS/cm was suitable for the occurrence of flashover.

6) Surface contamination on the test insulators in fog tests did not show obvious effects on

the physical and chemical analysis results.

7) The physical observations showed that the deteriorated surface conditions on the aged

test insulators were similar to a single aged insulator material, which was subjected to the

129

fog test condition. It was believed that the same discharge mechanism resulted in the fact

that the same phenomena were observed on the different test samples under the fog test

conditions.

8) The results of the preliminary test of the surface resistance measurement on different

material insulators showed that the surface resistance increased with test time. Different

insulators of different compositions showed different change speed due to the different

values of thermal conductivity of the materials.

7.2 Relationships between Electrical Characteristics and Surface Conditions of

Composite Insulators

The EPDM insulators clearly showed more active electrical characteristics than

EPDM/silicone rubber insulators in the fog tests. The EPDM insulators showed more

deteriorated surface conditions than EPDM/silicone rubber insulators after the tests. The

oxidation index (O.I.) change rate of EPDM insulators was higher than that of EPDM/silicone

rubber insulators after the fog tests. The ester/ketone (E/K) change rate of the EPDM

insulators was similar to that of the EPDM/silicone rubber insulators after the test.

Ester/ketone ratio was found to be effective in differentiating the aging factors of the

composite insulators in the fog tests. The ester product was strongly correlated with UV, and

ketone product was strongly correlated with thermal effects. The EPDM insulators were more

susceptible to oxidation conditions on the surface than the EPDM/silicone rubber insulators.

The EPDM and EPDM/silicone rubber insulators were found to be of the same sensibility to

thermal effects. Discharges were found to be directly responsible for the surface damaging of

the test insulators in the fog tests.

130

7.3 Future Work

The future research work is described as follows.

1) The discharge intensity was different at different locations on the test insulators. More

work is needed to study the factor of shape parameters of the composite insulators, which

affects the electrical characteristics, in the energised state.

2) Different discharge intensity resulted in the different deterioration level on the surface of

the insulators. The physical observation results showed that the core locations of the test

insulators were more deteriorated than the shed locations of the test insulators. The

difference of the chemical analysis results on the different locations of the test insulators

needs more tests to compare the results.

3) Discharges were noticeable during the fog tests. However, due to the limitation of

equipment, catching the images of the discharges on the insulators in the fog tests were

not achieved. The continuous and detailed discharge images on the test insulators are

useful to describe the process of discharges on the test insulators, especially the intensity

of the discharges on the test insulators.

4) The preliminary test of surface resistance measurement was a testing process. More test

in different testing conditions are needed to find the results to analyse. Such as the

different test voltage, the creepage length of the test samples, and the tests on the

standardized insulators or the standardized insulator materials.

5) The test conditions need more aging factors to simulate the true application environment

of the composite insulators, such as UV, rain, and dust.

6) Silicone rubber now is widely used in power industry. Chemical analysis assessment is

being developed. The electrical characteristics of silicone rubber insulators in the test

131

conditions and field are also being studied. However, the relationships between the

chemical analysis results and electrical characteristics of silicone rubber insulators have

not been investigated yet. Based on the work in this research, it is necessary to add this

part into the future research work.

132

Reference:

[1] CIGRE, Working Group 22.03, "Worldwide Service Experience with HV Composite

Insulators", No. 130, May 1990.

[2] P. D. Blackmore, "Degradation of polymeric outdoor high voltage insulation: surface

discharge phenomena and condition assessment techniques". Brisbane, Australia:

Queensland University of Technology, 1997, PhD Thesis, pp. 177.

[3] J. F. Hall, "History and Bibliography of Polymeric Insulators for Outdoor

Applications", IEEE Transactions on Power Delivery, vol. 8, pp. 376-385, 1993.

[4] CIGRE, Working Group 22.03, "Worldwide Service Experience with HV Composite

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[5] T. Gillespie, "Australian Experience with Composite Insulators", vol. 1, presented at

Symposium on Outdoor Insulators, Brisbane, Australia, 2002.

[6] H. M. Schneider, J. F. Hall, G. Kardady, and J. D. Renowden, "Nonceramic Insulators

for Transmission Lines", IEEE Transactions on Power Delivery, vol. 4, pp. 2214-

2221, 1989.

[7] T. Sorqvist and A. E. Vlastos, "Performance and Aging of Polymeric Insulators",

IEEE Transactions on Power Delivery, vol. 12, pp. 1657-1665, 1997.

[8] J. Mackevich and M. Shah, "Polymer Outdoor Insulating materials Part I: Comparison

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Appendix 1 Control Circuitry of Fog Chamber

Appendix 2 Equivalent Circuitry of Test Transformer and Variac

Appendix 3 Power Supply in Fog Chamber Test System

Appendix 4 Terminal Connection of Control Panel and Fog Box

Appendix - 5 LC Waveforms of #2 & #4 Insulators in Test A

0 500 1000 1500 2000 2500 3000-0.3

-0.2

-0.1

0

0.1

0.2

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

Mag

nitu

de

Frequency [hertz]


0 500 1000 1500 2000 2500 3000-2

-1

0

1

2

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

600

Mag

nitu

de

Frequency [hertz]


Appendix - 5 LC Waveforms and FFT Results of #2 Insulator in Test A at Specific Time

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

600

Mag

nitu

de

Frequency [hertz]


0 500 1000 1500 2000 2500 3000-5

0

5

10

15

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

1000

1200

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1.5

-1

-0.5

0

0.5

1

1.5

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800M

agni

tude

Frequency [hertz]


Appendix - 5 LC Waveforms and FFT Results of #2 Insulator in Test A at Specific Time (Continued)

0 500 1000 1500 2000 2500 3000-4

-2

0

2

4

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

Mag

nitu

de

Frequency [hertz]


0 500 1000 1500 2000 2500 3000-2

0

2

4

6

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-2

-1

0

1

2

3

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

600

Mag

nitu

de

Frequency [hertz]

(ix) t = 386 hours (x) t =410 hours


0 500 1000 1500 2000 2500 3000-1

0

1

2

3

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-3

-2

-1

0

1

2

3

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

(iii) t= 98 hours (iv) t= 122 hours

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-6

-4

-2

0

2

4

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

Mag

nitu

de

Frequency [hertz]

(i) t= 2 hours (ii) t= 26 hours


0 500 1000 1500 2000 2500 3000-3

-2

-1

0

1

2

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-4

-2

0

2

4

6

8

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

500

1000

1500

Mag

nitu

de

Frequency [hertz]

(vii) t= 256 hours (viii) t= 290 hours

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

1.5

2

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

600

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

600

Mag

nitu

de

Frequency [hertz]

(v) t= 218 hours (vi) t= 242 hours


0 500 1000 1500 2000 2500 3000-3

-2

-1

0

1

2

3

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

1000

1200

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1.5

-1

-0.5

0

0.5

1

1.5

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

200

400

600

800

1000

Mag

nitu

de

Frequency [hertz]

(xi) t= 434 hours (xii) t= 458 hours

0 500 1000 1500 2000 2500 3000-8

-6

-4

-2

0

2

4

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

500

1000

1500

2000

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-4

-2

0

2

4

LC (m

A)

Time (1/8000 s)

0 100 200 300 400 500 600 700 8000

500

1000

1500

Mag

nitu

de

Frequency [hertz]

(ix) t= 338 hours (x) t= 386 hours


0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.5

0

0.5

1

1.5

2

2.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]


0 100 200 300 400 500 600 700-30

-20

-10

0

10

20

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]

(iii) t = 48 hours

0 100 200 300 400 500 600 700-3

-2

-1

0

1

2

3

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

Mag

nitu

de

Frequency [hertz]

(iv) t = 72 hous

Appendix - 6 LC Waveforms of #2, #4, #6 & #8 Insulators in Test B

Appendix – 6 (a) LC Waveforms and FFT Results of #2 Insulator in Test B at Specific Time

0 100 200 300 400 500 600 700-3

-2

-1

0

1

2

3

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-4

-2

0

2

4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

120

Mag

nitu

de

Frequency [hertz]


0 100 200 300 400 500 600 700-0.2

-0.1

0

0.1

0.2

0.3

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

Mag

nitu

de

Frequency [hertz]


Appendix - 6 (a) LC Waveforms and FFT Results of #2 Insulator in Test B at Specific Time (continued)

0 100 200 300 400 500 600 700-0.2

-0.1

0

0.1

0.2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-60

-40

-20

0

20

40

60

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

Mag

nitu

de

Frequency [hertz]


Appendix – 6 (a) LC Waveforms and FFT Results of #2 Insulator in Test B at Specific Time (continued)

0 100 200 300 400 500 600 700-0.2

-0.1

0

0.1

0.2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1.5

-1

-0.5

0

0.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-3

-2

-1

0

1

2

3

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-20

-15

-10

-5

0

5

10

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

Appendix – 6 (b) LC Waveforms and FFT Results of #4 Insulator in Test B at Specific Time



0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

1.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-6

-4

-2

0

2

4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

1.5

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]

Appendix - 6 (b) LC Waveforms and FFT Results of #4 Insulator in Test B at Specific Time (continued)



0 100 200 300 400 500 600 700-40

-20

0

20

40

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

200

400

600

800

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-60

-40

-20

0

20

40

60

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

2000

4000

6000

8000

10000

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-15

-10

-5

0

5

10

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

120

Mag

nitu

de

Frequency [hertz]

Appendix – 6 (b) LC Waveforms and FFT Results of #4 Insulator in Test B at Specific Time (continued)



0 100 200 300 400 500 600 700-40

-30

-20

-10

0

10

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

deFrequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

Mag

nitu

de

Frequency [hertz]

Appendix – 6 (b) LC Waveforms and FFT Results of #4 Insulator in Test B at Specific Time (continued)

(xiiii) t = 240 hours (xiv) t = 248 hours

(xv) t = 268 hours

0 100 200 300 400 500 600 700-0.05

0

0.05

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

5

10

15

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-8

-6

-4

-2

0

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-2

-1

0

1

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-2

-1

0

1

2

3

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

Mag

nitu

de

Frequency [hertz]



Appendix – 6 (c) LC Waveforms and FFT Results of #6 Insulator in Test B at Specific Time

0 100 200 300 400 500 600 700-1.5

-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-6

-4

-2

0

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1.5

-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

1.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]



Appendix – 6 (c) LC Waveforms and FFT Results of #6 Insulator in Test B at Specific Time (continued)

0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

1.5

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-2

0

2

4

6

8

10

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]



Appendix – 6 (c) LC Waveforms and FFT Results of #6 Insulator in Test B at Specific Time (continued)

0 100 200 300 400 500 600 700-10

0

10

20

30

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-0.5

0

0.5

1

1.5

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

Mag

nitu

de

Frequency [hertz]

(xiii) t =226 hours (xiv) t = 239 hours

Appendix - 6 (c) LC Waveforms and FFT Results of #6 Insulator in Test B at Specific Time (continued)

0 100 200 300 400 500 600 700-5

0

5

10

15

20

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-4

-2

0

2

4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

120

Mag

nitu

de

Frequency [hertz]

(iii) t = 24 hours (iv) t = 48 hour

Appendix – 6 (d) LC Waveforms and FFT Results of #8 Insulator in Test B at Specific Time

0 100 200 300 400 500 600 700-0.05

0

0.05

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

1

2

3

4

5

6

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-30

-20

-10

0

10

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

deFrequency [hertz]


0 100 200 300 400 500 600 700-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-3

-2

-1

0

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]


0 100 200 300 400 500 600 700-15

-10

-5

0

5

10

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-2

-1

0

1

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]


Appendix – 6 (d) LC Waveforms and FFT Results of #8 Insulator in Test B at Specific Time (continued)

0 100 200 300 400 500 600 700-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-20

-10

0

10

20

30

40

LC (m

A)

Time (1/2000 s)

0 100 200 300 400 500 600 700 800 900 10000

100

200

300

400

Mag

nitu

de

Frequency [hertz]


0 100 200 300 400 500 600 700-4

-2

0

2

4

6

8

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-1

-0.5

0

0.5

1

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]



0 100 200 300 400 500 600 700-5

0

5

10

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]

0 100 200 300 400 500 600 700-2

-1

0

1

2

LC (m

A)

Time (1/2000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

120

Mag

nitu

de

Frequency [hertz]

(xiii) t = 224 hours (xiv) t = 239 hours


0 500 1000 1500 2000 2500 3000-5

0

5

10

15

20

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

2000

2500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-0.1

0

0.1

0.2

0.3

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1.5

-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]

Appendix - 7 LC Waveforms Insulators #1 - #8 before Flashover in Test C

(i) before 1st Flashover (ii) before 2nd Flashover

(iii) before 3rd Flashover (iv) before 4th Flashover

Appendix – 7 LC Waveforms and FFT Results of Insulator #1 before Flashovers

0 500 1000 1500 2000 2500 3000-2

-1

0

1

2

3

4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-40

-30

-20

-10

0

10

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

2000

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

Appendix – 7 LC Waveforms and FFT Results of Insulator #1 before Flashover (continued)

(v) before 5th Flashover (vi) before 6th Flashover

(vii) before 7th Flashover

0 500 1000 1500 2000 2500 3000-2

0

2

4

6

8

10

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

2000

2500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-10

-8

-6

-4

-2

0

2

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-0.6

-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-8

-6

-4

-2

0

2

4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

600

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

0

1

2

3

4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-6

-4

-2

0

2

4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

600

Mag

nitu

de

Frequency [hertz]



Appendix – 7 LC Waveforms and FFT Results of Insulator #2 before Flashovers (continued)

0 500 1000 1500 2000 2500 3000-10

-5

0

5

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

200

400

600

800

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1.5

-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-4

-2

0

2

4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-8

-6

-4

-2

0

2

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-30

-20

-10

0

10

20

30

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-5

0

5

10

15

20

25

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-60

-40

-20

0

20

40

60

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

2000

2500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-2

-1

0

1

2

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

600

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1.5

-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-8

-6

-4

-2

0

2

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-30

-20

-10

0

10

20

30

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-5

0

5

10

15

20

25

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-60

-40

-20

0

20

40

60

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

2000

2500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-2

-1

0

1

2

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

600

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1.5

-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-8

-6

-4

-2

0

2

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-30

-20

-10

0

10

20

30

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-5

0

5

10

15

20

25

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-60

-40

-20

0

20

40

60

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

2000

2500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-2

-1

0

1

2

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

600

Mag

nitu

deFrequency [hertz]

0 500 1000 1500 2000 2500 3000-1.5

-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-0.05

0

0.05

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

5

10

15

20

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-3

-2

-1

0

1

2

3

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

1.5

2

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-0.5

0

0.5

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-0.6

-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]


0 500 1000 1500 2000 2500 3000-1.5

-1

-0.5

0

0.5

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

Mag

nitu

de

Frequency [hertz]



0 500 1000 1500 2000 2500 3000-20

-15

-10

-5

0

5

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-50

0

50

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

500

1000

1500

2000

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-4

-2

0

2

4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-4

-3

-2

-1

0

1

2

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

500

600

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

1.5

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-0.6

-0.4

-0.2

0

0.2

0.4

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

120

Mag

nitu

de

Frequency [hertz]




0 500 1000 1500 2000 2500 3000-2

-1.5

-1

-0.5

0

0.5

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

Mag

nitu

de

Frequency [hertz]

0 500 1000 1500 2000 2500 3000-0.2

-0.1

0

0.1

0.2

0.3

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

50

100

150

Mag

nitu

deFrequency [hertz]

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

1.5

2

LC (m

A)

Time (1/8000 s)

0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Mag

nitu

de

Frequency [hertz]

(vii) before 7th Flashover (iv) before 4th Flashover



electrical characteristics of aged composite insulators...

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