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Transcript of JianBin Zhou Thesis
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Electrical Characteristics of Aged Composite Insulators
JianBin Zhou
September 2003
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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
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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:
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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.
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Keywords:
composite insulator, electrical characteristic, aging, fog chamber, EPDM insulator, leakage
current, waveform, chemical analysis, ester/ketone ratio, oxidation index, scanning of electron
microscopy
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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.
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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.
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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
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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=315S/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
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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 and b 44 Table 4-7 Correspondence between the Value of Salinity, Volume Conductivity, and Density
of the Solution at a Temperature of 20C 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
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Table of Contents
CHAPTER 1 INTRODUCTION...1
CHAPTER 2 COMPOSITE INSULATORS...4
2.1 Construction & Material of Composite Insulators.........4
2.1.1 Construction4
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 Rubber6
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 Index17
2.3.4 Chalking Index Analysis Method20
2.3.5 Ester / Ketone Ratio Index..21
2.3.6 Investigation of Aging of Medium Voltage Insulators.......22
2.4 Summary25
CHAPTER 3 DEVELOPMENT OF THE TEST EQUIPMENT..27
3.1 Literature Review..27
3.2 Fog Chamber System28
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3.2.1 Fog Chamber28
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 Test41
4.2.3.2 The Salt Fog Test..46
4.3 Test Methodology48
CHAPTER 5 AGING TESTS ON COMPOSITE INSULATORS.52
5.1 Introduction.52
5.2 Preliminary Tests on Composite Insulators53
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 Results63
5.2.5 Summary.67
5.3 Electrical Tests on Composite Insulators..69
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5.3.1 Test Conditions and Insulators Parameters69
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 Test98
5.3.6 Summary.........101
5.4 Relationships between Physical, Chemical Analysis Results of Insulator103
5.4.1 SEM (Scanning Electron Microscopy) Observations of Insulator Surface103
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 Introduction117
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 Insulators120
6.3 Discussions.122
6.4 Conclusions.125
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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 Work130
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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
Al2O33H2O), 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 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 C, 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 CH 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 29.
Figure 29 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 120C. 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 cm1. 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 210 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 210 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 211 [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 dont 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 211 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 210 Expanded FTIR spectra from the carbonyl region [32]
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EPDM and a smaller proportion of EPDM/silicone rubber blends. Table 22 lists the analysis
results.
Table 22 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 GoodnaCoastal. 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 10m, whereas for manufacturer B, the thickness of degradation
layer is more than 20m. 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].
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Figure 211 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 m and the width of surface cracks exceeds 7 m.
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 33.
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 laboratorys 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 37 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.
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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 C)
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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
15C 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 C)
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 at the ambient temperature (C).
The value 20(ambient temperature=20C) is calculated using the following formula:
20= * [1 b ( 20)] (1)
where
20 is the layer conductivity at a temperature of 20 C (in S/m)
is the volume conductivity measured at the ambient temperature of C (in
S/m) is the temperature of the insulator surface (in C)
b is a factor depending on temperature, as given in Table 4-6:
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Table 4-6 Relationship between and b [50]
The salinity, Sa ( in kg/m3 ), of the slurry is determined using the formula:
Sa = (5.7 20) 1.03 (0.004 20 0.4 S/m) (2)
The Salt Deposit Density, Sdd, is then determined using the formula:
Sdd = ASaV (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.
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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
C.
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Table 4-7 Correspondence between the Value of Salinity, Volume Conductivity, and Density of the Solution at the Temperature of 20C [48]
If the solution temperature is not 20C, the conductivity and density should be corrected by
the following formula,
20 = [1 + (200 + 1.35 Sa) ( 20) 10 6] (5) where
20 is the density at a temperature of 20 C (in kg/m3)
is the density at a temperature of C (in kg/m3)
Sa is the salinity ( in kg/m3 ) - see formula (2)
is the solution temperature (in C)
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 5C to 40C and the difference between the water solution and the
ambient temperature should be no more than 15C.
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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
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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.
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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