POLLUTED INSULATORS : A REVIEW OF CURRENT KNOWLEDGE

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POLLUTED INSULATORS : 
A REVIEW OF CURRENT KNOWLEDGE 
 
 
 
Task Force 33.04.01 
 
 
 
 
 
 
June 2000 
 
 
 
POLLUTED INSULATORS : 
 
A REVIEW OF CURRENT KNOWLEDGE 
 
 
 
 
 
 
 
 
 
 
 
PREPARED BY 
 
Task Force 33.13.01 (formerly 33.04.01) 
 
of Working Group 33.13 
(DIELECTRIC STRENGHT OF 
INTERNAL AND EXTERNAL INSULATION) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
MEMBERS OF TASK FORCE 01 OF WORKING GROUP 33.04 : 
 
D.A. SWIFT (Convenor, United Kingdom), J.P. REYNDERS (Secretary, South Africa), 
C.S. ENGELBRECHT (Compiler of documents, South Africa), J.L. FIERRO-CHAVEZ 
(Mexico), R. HOULGATE (United Kingdom), C. LUMB (France), R. MATSUOKA (Japan), 
G. MELIK (Australia), M. MORENO (Mexico), K. NAITO (Japan), W. PETRUSCH (Germany), 
A. PIGINI (Italy), G. RIQUEL (France), F.A.M. RIZK (Canada) 
1999-09-01 I
TABLE OF CONTENTS
1. INTRODUCTION.............................................................................................................................................................. 1
1.1 THE POLLUTION PROBLEM ............................................................................................................................................ 1
1.2 PREVIOUS REVIEW DOCUMENTS.................................................................................................................................... 1
1.3 RELEVANCE OF IEC 815 (1986) ................................................................................................................................... 2
1.4 INSULATOR TYPES AND DEFINITIONS OF SPECIFIC CREEPAGE LENGTH & SPECIFIC AXIAL LENGTH............................... 2
1.5 APPROACH FOR INSULATOR SELECTION AND DIMENSIONING ......................................................................................... 3
2. POLLUTION FLASHOVER PROCESS......................................................................................................................... 5
2.1 INTRODUCTION ............................................................................................................................................................. 5
2.2 MODELLING .................................................................................................................................................................. 6
2.2.1 Hydrophilic surface ............................................................................................................................................. 6
2.2.2 Hydrophobic surface.......................................................................................................................................... 10
2.3 ENVIRONMENTAL ASPECTS......................................................................................................................................... 10
2.3.1 Climates or atmospheric variables and typical environments ........................................................................... 10
2.3.2 Type of pollution ................................................................................................................................................ 13
2.3.3 Mechanisms of contamination accumulation on insulators ............................................................................... 21
2.3.4 Mechanisms of wetting....................................................................................................................................... 24
2.3.5 The natural cleaning processes.......................................................................................................................... 29
2.3.6 Critical wetting conditions................................................................................................................................. 29
2.3.7 Effect of various aspects of the insulator on its pollution accumulation ........................................................... 29
2.3.8 Physical and mathematical models of pollution deposit .................................................................................... 33
2.4 ICE AND SNOW ............................................................................................................................................................ 33
2.4.1 Flashover on insulators covered with ice. ......................................................................................................... 34
2.4.2 Flashover on insulators covered with snow....................................................................................................... 35
3. INSULATOR CHARACTERISTICS ............................................................................................................................ 37
3.1 INTRODUCTION ........................................................................................................................................................... 37
3.2 MATERIALS USED FOR OUTDOOR INSULATORS ............................................................................................................ 38
3.2.1 Porcelain and glass............................................................................................................................................ 38
3.2.2 Polymers ............................................................................................................................................................ 38
3.3 INSULATOR PERFORMANCE ......................................................................................................................................... 39
3.3.1 Ceramic insulators............................................................................................................................................. 40
3.3.2 Polymeric Insulators .......................................................................................................................................... 50
3.3.3 Effect of insulator orientation. ........................................................................................................................... 52
3.3.4 Influence of a non-uniform pollution deposit..................................................................................................... 56
3.3.5 Electric field at the surface of insulators ........................................................................................................... 57
3.3.6 Cold switch-on and thermal lag......................................................................................................................... 59
3.3.7 Contaminated insulators under transient overvoltages ..................................................................................... 59
3.3.8 Air density correction factors for polluted insulators ........................................................................................ 68
3.3.9 General trends for ice covered insulators.......................................................................................................... 69
3.3.10 General trends for snow covered insulators ...................................................................................................... 71
3.4 SPECIAL INSULATORS .................................................................................................................................................. 73
3.4.1 Hollow insulators............................................................................................................................................... 73
3.4.2 HVDC wall bushings.......................................................................................................................................... 75
3.4.3 Circuit breaker and isolator insulation.............................................................................................................. 75
3.4.4 Insulators in desert conditions........................................................................................................................... 76
3.4.5 Semiconducting Glaze insulators.......................................................................................................................76
3.5 CONCLUSIONS............................................................................................................................................................. 77
4. ENVIRONMENTAL IMPACT ...................................................................................................................................... 80
4.1 VISIBLE DISCHARGES .................................................................................................................................................. 80
4.2 AUDIBLE NOISE ........................................................................................................................................................... 80
4.3 RADIO INTERFERENCE................................................................................................................................................. 81
4.4 TELEVISION INTERFERENCE ........................................................................................................................................ 82
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4.5 CORROSION OF METAL HARDWARE - TELEVISION INTERFERENCE................................................................................ 82
4.6 CRITERIA FOR RADIO NOISE LIMITS OF INSULATORS..................................................................................................... 83
4.7 CORROSION OF METAL HARDWARE - MECHANICAL STRENGTH REDUCTION................................................................. 84
4.8 FIRES .......................................................................................................................................................................... 85
5. POLLUTION MONITORING ....................................................................................................................................... 87
5.1 INTRODUCTION ........................................................................................................................................................... 87
5.2 AIR POLLUTION MEASUREMENT .................................................................................................................................. 88
5.2.1 Directional dust deposit gauge .......................................................................................................................... 88
5.3 EQUIVALENT SALT DEPOSIT DENSITY (ESDD)............................................................................................................. 89
5.3.1 Advantages......................................................................................................................................................... 89
5.3.2 Disadvantages.................................................................................................................................................... 89
5.3.3 Further developments ........................................................................................................................................ 89
5.4 NON-SOLUBLE DEPOSIT DENSITY (NSDD) .................................................................................................................. 90
5.4.1 Optical measurement ......................................................................................................................................... 90
5.5 SURFACE CONDUCTANCE ............................................................................................................................................ 90
5.5.1 Advantages......................................................................................................................................................... 90
5.5.2 Disadvantages.................................................................................................................................................... 90
5.5.3 Further developments ........................................................................................................................................ 90
5.6 INSULATOR FLASHOVER STRESS.................................................................................................................................. 91
5.6.1 Advantages......................................................................................................................................................... 91
5.6.2 Disadvantages.................................................................................................................................................... 91
5.7 LEAKAGE CURRENT .................................................................................................................................................... 91
5.7.1 Surge counting ................................................................................................................................................... 92
5.7.2 I highest.................................................................................................................................................................. 92
5.8 CONCLUSIONS............................................................................................................................................................. 92
6. TESTING PROCEDURES FOR INSULATORS ......................................................................................................... 93
6.1 INTRODUCTION ........................................................................................................................................................... 93
6.2 CATEGORIES OF TEST METHODS .................................................................................................................................. 93
6.2.1 Testing under natural pollution conditions........................................................................................................ 93
6.2.2 Artificial pollution laboratory tests.................................................................................................................... 95
6.3 TEST PROCEDURES FOR PORCELAIN AND GLASS INSULATORS TO BE USED IN HIGH-VOLTAGE A.C. OR D.C. SYSTEMS ... 95
6.3.1 Standardised test procedures ............................................................................................................................. 95
6.3.2 Non-standardised test procedures...................................................................................................................... 96
6.3.3 Non-standardised test procedures for laboratory tests on naturally polluted insulators .................................. 98
6.4 TEST PROCEDURES FOR POLYMERIC INSULATORS TO BE USED IN HIGH-VOLTAGE A.C. OR D.C. SYSTEMS..................... 98
6.5 TEST PROCEDURES FOR INSULATORS COVERED WITH ICE OR SNOW............................................................................. 98
6.5.1 Laboratory test methods with ice ....................................................................................................................... 98
6.5.2 Laboratory test methods with snow.................................................................................................................. 100
6.6 ADDITIONAL INFORMATION ON PARTICULAR POINTS OF POLLUTION TESTING ............................................................ 100
6.6.1 Ambient conditions during testing ................................................................................................................... 100
6.6.2 Leakage current measurement ......................................................................................................................... 103
6.6.3 Testing of insulators for the UHV range up to 1100 kV................................................................................... 104
6.6.4 Comparison of test results obtained with different pollution test methods ...................................................... 104
6.6.5 Comparison of test results obtained from test stations ....................................................................................104
7. INSULATOR SELECTION AND DIMENSIONING ................................................................................................ 106
7.1 INTRODUCTION ......................................................................................................................................................... 106
7.2 SELECTION OF INSULATOR CHARACTERISTICS .......................................................................................................... 106
7.2.1 Selection of profile ........................................................................................................................................... 107
7.2.2 Selection of insulator dimensions..................................................................................................................... 107
7.2.3 Deterministic method ....................................................................................................................................... 108
7.2.4 Probabilistic method. ....................................................................................................................................... 108
7.2.5 Static and dynamic methods in the probabilistic approach. ............................................................................ 109
7.2.6 Present status of the probabilistic approach.................................................................................................... 110
7.2.7 Dynamic method .............................................................................................................................................. 113
7.2.8 Truncation of the distribution .......................................................................................................................... 114
7.2.9 Conclusions...................................................................................................................................................... 115
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7.3 SELECTION OF INSULATORS FOR APPLICATION UNDER ICE AND SNOW ....................................................................... 115
7.4 SELECTION OF INSULATORS FOR D.C. ENERGISATION................................................................................................. 116
7.4.1 Introduction ..................................................................................................................................................... 116
7.4.2 Selection of a site severity correction factor.................................................................................................... 116
7.5 INSULATOR POLLUTION DESIGN OF PHASE-TO-PHASE SPACERS ................................................................................ 117
7.5.1 Introduction ..................................................................................................................................................... 117
7.5.2 Design Practice................................................................................................................................................ 117
8. PALLIATIVES AND OTHER MITIGATION MEASURES .................................................................................... 118
8.1 INTRODUCTION ......................................................................................................................................................... 118
8.2 MAINTENANCE PROCEDURES .................................................................................................................................... 118
8.2.1 Live-insulator washing of ceramic insulators .................................................................................................. 118
8.2.2 Live-insulator washing of polymeric insulators............................................................................................... 128
8.3 USE OF GREASES AND RTV COATINGS...................................................................................................................... 129
8.3.1 Introduction ..................................................................................................................................................... 129
8.3.2 Hydrocarbon and silicone greases .................................................................................................................. 129
8.3.3 RTV rubber coatings ........................................................................................................................................ 130
8.3.4 Summary .......................................................................................................................................................... 130
8.4 BOOSTER SHEDS ....................................................................................................................................................... 131
8.5 METHODS FOR INCREASING INSULATOR RELIABILITY UNDER ICE AND SNOW CONDITIONS......................................... 132
8.5.1 Some measures to prevent flashovers during ice conditions............................................................................ 132
8.5.2 Some measures to prevent flashovers during snow conditions ........................................................................ 133
9. THERMAL EFFECTS OF CONTAMINATION ON METAL OXIDE ARRESTERS (MOA) ............................ 134
9.1 INTRODUCTION ......................................................................................................................................................... 134
9.2 OPERATIONAL EXPERIENCE AND FIELD TESTS.......................................................................................................... 134
9.3 ARTIFICIAL POLLUTION TESTS OF LIGHTNING ARRESTERS........................................................................................ 135
9.3.1 Test Techniques................................................................................................................................................ 135
9.3.2 Laboratory Test Results ................................................................................................................................... 135
9.4 STANDARDISATION OF A LABORATORY TEST............................................................................................................ 139
10. ADITIONAL INFORMATION AND RESULTS ................................................................................................... 142
10.1 INSULATOR PROFILES AND DIMENSIONS .................................................................................................................... 142
10.2 RANKING OF INSULATORS ......................................................................................................................................... 158
10.2.1 Ceramic Insulators........................................................................................................................................... 158
10.2.2 Polymeric insulators ........................................................................................................................................ 162
10.3 INSULATOR PERFORMANCE AS A FUNCTION OF POLLUTION SEVERITY ....................................................................... 164
10.4 AGEING OF INSULATORS ........................................................................................................................................... 165
11. REFERENCES........................................................................................................................................................... 1 67
1999-09-01 1
1. INTRODUCTION
1.1 The Pollution problem
The performance of insulators used on overhead transmission lines and overhead distribution lines, and in outdoor substations
is a key factor in determining the reliability of power delivery systems. The insulators not only must withstand normal
operating voltage, but also must withstand overvoltages that may cause disturbances, flashovers andline outages. The
reduction in the performance of outdoor insulators occurs mainly by the pollution of the insulating surfaces from air-borne
deposits that may form a conducting or partially conducting surface layer when wet.
The presence of a conducting or partially conducting layer of pollution on the insulator surface will dictate flashover
performance. It is impractical in many situations to prevent the formation of such a layer and consequently insulators must be
designed so that the flashover performance remains high enough to withstand all types of anticipated voltage stresses despite
the presence of the pollution layer. In certain situations where pollution is extremely severe, further preventative or curative
measures - such as periodic washing or greasing - may be necessary.
It is clear that the environment, in which the insulator must operate, together with the insulator itself, will determine the
severity of the pollution layer on the insulator. Translating the environment into parameters that can be used to design the
insulation, therefore, presents one of the fundamental problems in designing external insulation with respect to polluted
conditions. This is due to the vast range of possible conditions such as those found in coastal, industrial, agricultural and
desert areas; also in areas with ice and snow or at high altitude. Combinations of these conditions may also occur. A further
complicating factor is that environments have an inherent statistical behaviour that is to a large extent unpredictable.
Furthermore, the increase of available electrical energy in an area, through the construction of a new substation, may trigger
industrial growth that can contribute to the pollution and affect thus the behaviour of the insulation. It is, therefore, difficult to
quantify the effect of the environment on insulator performance.
This document attempts to address these problems by serving as a review of current knowledge on insulator pollution with the
intention of providing information for the selection and maintenance of insulators in polluted environments. A very extensive
list of references is provided.
It is recognised that ageing may influence the performance of insulators, particularly in the case of polymer insulators.
However, this report is restricted to discussing the pollution performance of insulators, since Cigré Study Committees 15 and
22 are mandated to deal with material and insulator ageing.
1.2 Previous review documents
The first large-scale review of pollution effects on insulators was published in 19711. That document describes theories of the
flashover process as well as artificial and natural test methods for assessing insulator performance in pollution conditions.
Various parameters that influence insulator performance, such as surface conductance and insulator dimensions, are also
discussed. Furthermore, several methods for measuring pollution severity are described and preventative procedures such as
greasing are reviewed.
In 1979, a major review on insulator pollution was published as two separate reports: one on the measurement of pollution
severity2 and the other as a critical comparison of artificial pollution test methods3.
The report on pollution severity measurement analysed the main methods in use in terms of the pollution flashover process.
The conclusion was that there is no single best method but rather that the best results are obtained when several methods are
used in parallel. Factors pertaining to the equipment - i.e. cost, availability, etc. - and the power delivery system - i.e. extent,
voltage level and type, etc. - were identified as being important for selecting a pollution site severity measurement method. It
was noted that the cost of optimisation also should be weighed against the cost of a detailed site severity assessment before
such measurements are undertaken.
The report on artificial pollution test methods gave an analysis of available test methods with the intent of indicating which
methods are best suited for international standardisation. This report also recommended the natural conditions best
represented by each test method.
Another report4 combined the experience of electric utilities, manufacturers, and research laboratories in a comprehensive
summary on the design and maintenance of outdoor insulators in polluted environments. In addition to providing a
description of the flashover process, this report also contains discussions on pollution severity measurement, test procedures,
design practice, and maintenance procedures.
1999-09-01 2
1.3 Relevance of IEC 815 (1986)
The present edition of IEC Publication 815 (1986)5 is based on knowledge obtained mainly from experience with
conventional porcelain and glass insulators on a.c. systems. It applies only to these insulators, and only when they are used in
a.c. applications.
Minimum specific creepage distances are specified in this document for different pollution severity levels. These pollution
severity levels do not consider all aspects of the environment that can affect the performance of various insulator profiles.
Apart from some restrictions on insulator profile and corrections for diameter, IEC 815 thereby implies that no other factors
need to be considered when designing insulators for use in polluted conditions.
It is now recognised that a broader approach for insulator design and selection is required to address the optimised design of
porcelain and glass insulators as well as polymeric insulators for a.c. and d.c. systems world-wide. Other areas where IEC
815 lacks information have been identified.
This review document is based on the following list of areas where IEC 815 is perceived to be weak, and where input is
needed for its revision:
• Performance of polymeric insulators
• Insulator orientation
• Extension of applicability to voltages above 525 kV a.c.
• Design for d.c. application
• Insulators with semiconducting glaze
• Surge arrester housing performance, particularly with reference to polymeric materials
• Longitudinal breaks in interrupter equipment
• Radio interference, television interference, and audible noise of polluted insulators
• Effect of altitude
• Effect of heavy wetting
The revision of IEC 815 was started in 1998 and it is expected that the work will be completed by the end of the year 2005.
The revision will appear as five parts under the number ‘IEC 60815’.
1.4 Insulator types and definitions of Specific Creepage Length & Specific Axial
Length
For the purpose of this document, insulators are divided into the following four broad categories:
1. Ceramic insulators for a.c. systems
2. Polymeric insulators for a.c. systems
3. Ceramic insulators for d.c. systems
4. Polymeric insulators for d.c. systems.
Ceramic insulators have an insulating part manufactured either of glass or porcelain, whereas polymeric insulators have a
composite insulating part consisting of a polymer housing such as Silicone Rubber (SR), Ethylene Propylene Diene Monomer
(EPDM) and others, fitted onto a glass fibre core.
In Section 10, details are given of some of the available types of insulators. The tables presented therein are used throughout
this document to identify insulators and provide data for analysis.
For the purpose of this review, the electrical stress over an insulator is considered in two ways; one is related to the leakage
path length and the other to the axial length of the insulator.
In IEC 815, the leakage path of an insulator is specified by the ‘Specific Creepage Distance’ defined as the leakage distance
of the insulator in mm divided by the maximum system phase-to-phase voltage in kV. The Leakage Distance is defined as the
shortest distance, from on end of the insulator to the other, along the surface of the insulating parts. In this document, the
Specific Creepage Length (SCL) defined as the Leakage Distance of the insulator divided by the actual voltage across the
insulator - i.e. the phase-to-ground voltagein most instances.
The corresponding Specific Axial Length (SAL) of an insulator is defined as the axial length of the insulator divided by the
actual voltage across the insulator. The axial length refers to the shortest distance between fixing points of the live and
1999-09-01 3
earthed metalware, ignoring the presence of any stress control rings, but including intermediate metal parts along the length of
the insulator - as is shown in Figure 1-1.
Axial Length
Figure 1-1: Definition of axial length of an insulator as is used in this review.
1.5 Approach for insulator selection and dimensioning
The process of insulator selection and axial dimensioning together with its influencing parameters is shown in Figure 1-2.
The flow chart in this figure forms the basis of this review document for which an overview is given below.
The process of insulator selection starts with the collection of the basic data consisting of information on:
1. Insulator application
2. Insulator characteristics
3. Power system parameters
4. The environment
5. Constraints.
6. Field performance
1. The application of the insulator is an important aspect from the pollution performance viewpoint as it determines both
the radial dimension and the orientation of the insulator. Section 3 addresses the application of insulators under a variety of
headings.
2. An integral part of the basic data is the characteristics of the available insulators. These are discussed throughout this
report, but especially in Section 3. Information may also be obtained from manufacturers.
3. Power system parameters that form part of the basic data consist of:
• The electrical environment in which the insulator is applied, i.e. a.c. or d.c. voltage; maximum system voltage; and
lightning, switching and temporary overvoltages and their effects on insulator performance. These aspects are
comprehensively addressed in Section 2.2 and Section 3.
• The performance required from the insulator. This is determined mainly by power quality criteria such as the power
system’s sensitivity to outages.
4. Each environment where the insulators are to be installed has a different set of conditions under which the insulator must
operate reliably. An insulator that has a good performance under one set of conditions might have a bad performance in a
different set of conditions. It is therefore necessary to characterise the environment in terms specific to insulator performance.
In Section 2.3, the environmental aspects and how they affect the pollution flashover process are discussed. Methods to
monitor the environment are described in Section 5.
5. Constraints may also influence the selection of insulators. For example, limitations on the width of the right of way may
dictate the use of structures for which special insulator designs are required. In such cases, the range of available insulators
may be restricted. Cost and the need to minimise the visual impact may also be important factors that have to be built into the
selection process.
6. Field performance of insulators in service is an invaluable source of data for future applications. Unfortunately, these
data are not always available, and, as noted earlier, their applicability to different environments must always be questioned.
Nevertheless, service experience is usually a very important component of the basic data since it forms the basis for
determining whether the selection of a particular insulator leads to acceptable performance. Service experience also may
indicate whether certain artificial pollution tests are appropriate for a specific environment, and it may also contribute
information on insulator characteristics.
1999-09-01 4
Methods to assess insulator field performance are given in Section 5. References to service experience are given throughout
the document, but especially in Sections 2 and 3.
5) Constraints4) Environment3) Power System
parameters
2) Insulator
Characteristics
Basic data
Alternative solutions
Field tests
necessary?
Field test station
Test program
Test results
Lab tests
necessary
?
Representative
Test technique
Lab testing
Test results
Design Procedure
Deterministic
?
Preliminary
Design
Acceptable
Failure rate
?
Preliminary
Solution
Cost optimisation
Preventative
Measures
?Identify
measures
Insulator selection
Insulator
monitoring
NoYes
NoYes
Yes No
Yes No
YesNo
6) Insulator
field performance
1) Insulator
application
Figure 1-2: An overview of the process of insulator selection, as based on a published 6 diagram.
Once these basic data are collected, the various options for insulator selection can be identified for further study. Depending
on whether or not information is available on service experience, insulator characteristics and the environment, the need for
further field tests should be determined. However, it should be noted that these tests normally take 2-4 years. An overview of
the available methods for site severity measurement and field tests is given in Section 5.
Since the time required for field tests is very long, such tests are usually augmented with laboratory tests. A brief overview of
laboratory test methods and some examples of field test stations are given in Section 6.
When the basic data and field and laboratory test results have been compiled, the actual design procedure - as described in
Section 7 - can begin. The choice between a deterministic or statistical approach will depend on the criticality of the design.
Economic and time constraints may dictate a shortened selection procedure with the possible concomitant reduction in
confidence in the design.
In the event that a reliable insulator design is not achieved, mitigation methods may be necessary. Examples of such methods
are given in Section 8.
Improvement in the design procedure requires verification of performance that also will provide further service experience for
future designs.
1999-09-01 5
2. POLLUTION FLASHOVER PROCESS
2.1 Introduction
The pollution flashover process of insulators is greatly affected by the insulator’s surface properties. Two surface conditions
are recognised: either hydrophilic or hydrophobic. A hydrophilic surface is generally associated with ceramic insulators
whereas a hydrophobic surface is generally associated with polymeric insulators, especially silicone rubber. Under wetting
conditions - such as rain, mist etc. - hydrophilic surfaces will wet out completely so that an electrolyte film covers the
insulator. In contrast, water beads into distinct droplets on a hydrophobic surface under such wetting conditions.
In the Electra No. 64 publication2, the pollution flashover process for ceramic insulators - that is, insulators with a hydrophilic
surface - is described essentially as follows:
a) The insulator becomes coated with a layer of pollution containing soluble salts or dilute acids or alkalis. If the pollution
is deposited as a layer of liquid electrolyte - e.g. salt spray, stages (c) to (f) may proceed immediately. If the pollution is
non-conducting when dry, some wetting process (stage (b)) is necessary.
b) The surface of the polluted insulator is wetted either completely or partially by fog, mist, light rain, sleet or melting snow
or ice and the pollution layer becomes conductive. Heavy rain is a complicating factor: it may wash away the electrolytic
components off part or all of the pollution layer without initiating the other stages in the breakdown process, or it may -
by bridging the gaps between sheds - promote flashover.
c) Once an energised insulator is covered with a conducting pollution layer, a surface leakage current flows and its heating
effect starts to dry out parts of the pollution layer.
d) The drying of the pollution layer is always non-uniform and, in places, the conducting pollution layer becomes broken by
dry bands that interrupt the flow of leakage current.
e) The line-to-earth voltage is then applied across these dry bands, which may only be a few centimetreswide. It causes air
breakdown to occur and the dry bands are bridged by arcs, which are electrically in series with the resistance of the
undried portion of the pollution layer. A surge of leakage current occurs each time the dry bands on an insulator
sparkover.
f) If the resistance of the undried part of the pollution layer is low enough, the arcs bridging the dry bands are able to burn
continuously and so may extend along the insulator; thereby spanning more and more of its surface. This in turn
decreases the resistance in series with the arcs, increases the current and permits the arcs to bridge even more of the
insulator surface. Ultimately the insulator is completely spanned and a line-to-earth fault is established.
Figure 2-1: Schematic representation of the pollution flashover process across a hydrophilic surface.
The key processes involved in the flashover process are shown in Figure 2-1. The environment, in which the insulator must
operate in, influences the first two processes - pollution deposit and wetting - whereas electrical aspects govern the last two
processes. This Section, therefore, discusses the flashover process from these two viewpoints.
1999-09-01 6
To date, no clear description exists of the complete insulator flashover process for insulators with a hydrophobic surface - but
the key aspects, as defined, will still be present to a greater or lesser extent.
The aforementioned points do not include the effects of ice and snow on the electrical strength of insulators. Such additional
points are discussed in provided in Section 2.4.
2.2 Modelling
2.2.1 Hydrophilic surface
It is assumed that, in general, the flashover process across ceramic insulators applies to hydrophilic surfaces - i.e. where this
surface is covered with a film of electrolyte. The models are, therefore, based on the study of an arc in series with a resistance
- representing a dry band arc and a wet polluted surface respectively.
2.2.1.1 d.c. Model
Mathematical modelling of the pollution flashover of ceramic insulators has already been the subject of an extensive review
published in Electra 7. Therefore, only a brief summary of the results will be given here.
For modelling of pollution flashover under d.c. energisation, the basic approach8 involves the determination of the minimum
voltage needed to sustain a dry band arc of a given length in series with the corresponding pollution surface resistance. The
arc length is then varied in order to obtain the critical position that corresponds to the highest value of the supply voltage. The
latter is taken as the insulator withstand voltage for the pollution severity concerned9. An alternative approach10, still for the
d.c. case, is to consider that the dry band arc will continue to elongate as long as:
E Ea p< (2-1)
where Ea is the arc voltage gradient and E p is the mean voltage gradient of the pollution layer.
The static arc characteristic for a current ‘i’ is of the form:
E i Na
n = 0 (2-2)
where N o and ‘n’ are constants.
Assuming a constant surface resistance rp per unit leakage path, the critical arcing distance xc was found to be:
x
L
nc
=
+ 1
(2-3)
were L is the leakage path length. The corresponding critical voltage ‘Uc’ was determined as:
U N r Lc n p
n
n= • •+ +0
1
1 1 (2-4)
The critical d.c. current ic - i.e. the maximum leakage current not leading to flashover - can be obtained from :
i
N
rc
o
p
n
=






+
1
1
(2-5)
Several refinements have been introduced to the d.c. model. In another paper11, an insulator model was introduced with two
different surface resistances per unit length rp1 and rp2 - corresponding to the stem and the shed of a longrod insulator. A
circular insulator disc model was also investigated 12. The contribution of arc current concentration at the roots to the
pollution layer surface resistance was included 13 14. Other refinements include the consideration of the arc electrode voltage
drops 13, effect of temperature on the pollution layer resistance14 and the influence of multiple parallel arcing that takes place
on many insulators - especially on those of large diameter15.
The d.c. model has been used to study the polluted insulator : test source interaction 16. This contributed to the interpretation
of the experimental results and to the determination of the minimum requirements for d.c. sources in polluted insulator tests17.
1999-09-01 7
Unfortunately, the d.c. model has been frequently used to account for polluted insulator performance under a.c. energisation11
13 14, despite there being important basic differences - as is shown below.
2.2.1.2 a.c. Model
At the instant of voltage and current peak, the circuit equation of an a.c. arc burning in series with the insulator pollution
surface resistance is identical to that of the d.c. circuit equation. However, it has been amply demonstrated experimentally
that, for the same pollution severity, the peak a.c. withstand voltage far exceeds the corresponding value under d.c. conditions.
It has also been observed experimentally that arc-propagation across the insulator surface can take several cycles and,
therefore, the arc is subject to an extinction and re-ignition process at around current zero 18 19. This means that the d.c.
criterion for arc propagation, i.e. Ea < E p , referred to previously will not be sufficient to predict insulator flashover under
a.c. energisation. An arc can start propagation when this criterion becomes fulfilled, but if the voltage is not sufficient to
cause re-ignition after current zero, the arc will extinguish without leading to flashover.
It has been demonstrated, both theoretically18 and experimentally20, that for the current ‘i’ in a resistive circuit the re-ignition
voltage ‘U’ can be expressed as:
U
A x
im
=
•
(2-6)
where ‘x’ is the residual arc length and ‘A’ and ‘m’ are constants
Inserting this relationship in the circuit equation results in:
A x
i
N x
i
R im
o
n px
•
=
•
+ (2-7)
Where R px is the pollution surface resistance corresponding to an arc length ‘x’.
Since the voltage drop of a burning arc is much smaller than the re-ignition voltage, an acceptable - although not accurate -
approximation would be to put n ≅ m. This simplifies the analysis and yields a critical arc length x c :
x
L
mc
=
+1
(2-8)
For constant r p , the corresponding critical voltage ‘Uc’ is:
U B r Lc p
m
m= • •+1 (2-9)
where ‘B’ is a constant.
Expression 2-9 is similar to that of equation 2-4 for the d.c. case, although instead of n ≅ 0.8 - valid for the d.c. static
characteristic of a free-burning arc - m ≅ 0.5 in the a.c. arc re-ignition expression 2-6. Also, the constants in equations 2-9
and 2-4 are quite different. In fact, numerical evaluation of these expressions shows that for a high pollution severity - i.e.
relatively low values of rp - the critical a.c. voltage (rms) is much higher than the critical d.c. voltage. This difference
diminishes, however, at lower pollution severity and ultimately - with no pollution at all - the a.c. sparkover voltage peak
value is nearly equal to the corresponding d.c. voltage.
The a.c. model 21 has been used to investigate the source: polluted insulator interaction and has revealed the effect of the
parallel capacitance on insulator performance. It proved, therefore, to be quite useful in determining the minimum source
requirement 22. Recently, the model has been further used to investigate the effect of altitude on the performance of a.c.
insulators under pollution conditions23; see also the discussion in Section 3.3.8.
2.2.1.3 Evaluation of the pollution flashover mechanism under transient overvoltages
Consider an impulse voltage with a time to crest (tcr ) much smaller than the time to half value (th ). The main influence on the
leakage current flashover stress is given by th 
24 25 (see Figure 2-2). At very short times to half value (th less than 200 µs), no
pre-arc will occur and mainly streamer discharges develop.Then the flashover voltage is determined by the requirement for a
streamer discharge to occur and may attain a value close to that for dry conditions.
1999-09-01 8
For very long times to the half value - i.e. longer than 3000 µs, a long pre-arc could be formed. In this case, the leakage
current flashover stress will be determined by the pre-arc only and reaches a value of approximately 0.7 kV/cm.
With a virtual impulse duration longer than 100 ms, a further decrease of the flashover voltage will be observed. This is not
caused by a new flashover mechanism. It is due to the fact that the pollution layer will be heated for a longer time duration by
the current flowing and so the surface conductivity will be increased.
In the range between 200 and 3000 µs of th - i.e. SI range, the performance is more complicated; as is analysed below.
Figure 2-2: Flashover strength vs. the voltage-time duration for a cylindrical model insulator under pollution conditions 25.
2.2.1.4 Evaluation of the discharge process under switching overvoltages
Discharge without dry bands (application of SI only).
Based on the analysis of experimental data as well as on simplifying assumptions for the very complex flashover mechanism
for a leader discharge 26 27, a flashover model has been developed 28. Whereas the a.c./d.c. flashover is governed by the pre-
arc 29 10 7, this is not the case with the SI stress - now the leader discharge becomes more important. Because of its
comparatively short lifetime and low energy dissipation, the leader can not produce any dry bands - which is contrary to the
case of an existing pre-arc. Furthermore, the leader gradient is much higher than the gradient in the pre-arc; i.e. the current
flowing in the bridged layer can not be neglected, as in the case of a.c./d.c. stresses. From this consideration it follows that,
instead of the usual voltage (U) - current (I) characteristic for a.c. and d.c. cases, only the strength (E) - current (I)
characteristic is applicable for SI28, for the instantaneous discharge parameters (Figure 2-3).
Analogous to the a.c./d.c. flashover criterion, a critical condition for the SI flashover arises. For this condition, the dotted
straight line in Figure 2-3b - which represents the negative slope of the layer resistivity per unit length - becomes a tangent of
the E-I leader characteristic - as given by the full curve in Figure 2-3b.
1999-09-01 9
Figure 2-3: Flashover models for a.c./d.c. (a) and SI (b) 28.
Discharge with dry bands (application of SI with pre-stress).
As reported by Garbagnati et al 152, the SI strength can be reduced due to the presence of dry bands. If the flashover strength
is drawn versus the dry band length, typical U-curves are obtained.
Figure 2-4: Approach for the evaluation of the minimum flashover strength in the presence of dry bands 28.
In the presence of a short dry band, having a length ‘ar’ (Figure 2-4), the flashover under positive SI first occurs from this dry
band in a very short time (air breakdown in the µs range). This is followed by the flashover along the contaminated layer of
the length ‘ag’ during a much longer time period (leakage - current flashover in the ms range).
1999-09-01 10
For dry band lengths smaller than 1 m, the strength of the air gap corresponds to the positive steamer gradient, i.e. 450 kV/m.
For longer dry band lengths, the mean breakdown strength corresponds to the minimum possible breakdown voltage per unit
length of a long air gap under positive SI.
To check if the proposed approach works, even for insulators of practical interest, the results of calculation are compared with
available experimental data. Because non-uniform contamination is to be regarded as the worst case, only the presence of dry
bands of critical lengths shall be considered in the following case.
As an example, Figure 3-28 shows the results obtained for a post insulator, where the experimental data of Garbagnati et al 152
are used. As is evident from the broken-line curve, the calculated values meet the measured ones quite well up to the longest
investigated insulator length of 12 m.
Another example is reported in Figure 3-33. Here, the calculated minimum curve agrees satisfactorily with the experimental
one presented by Garbagnati et al 152 for practical insulators up to 12 m length.
2.2.2 Hydrophobic surface
The superior performance of new polymeric insulators under pollution conditions is generally attributed to its water repellent -
i.e. hydrophobic - properties. Because the surface does not wet, water forms as isolated drops rather than as a continuous
surface film. Hydrophobicity can be lost due to different ageing mechanisms - heavy wetting, blown sand, corona and spark
discharges and possibly solar radiation. For the same pollution surface density, the surface resistance of a polymeric insulator
is generally some orders of a magnitude higher than that of a similar porcelain or glass insulator. It also follows that the
leakage currents associated with polymeric insulator discharges are generally some orders of magnitude lower than the
corresponding levels for ceramic insulators.
Due to the dynamic nature of a polymeric surface and the resulting complex interaction with pollutants and wetting agents,
there exists today no quantitative model of pollution flashover for polymeric insulators that is similar to the one expounded in
Section 2.2.1. for ceramic insulators. However, a qualitative picture for the pollution flashover mechanism is emerging 30. It
involves such elements as the migration of salt into water drops, water drop instability, formation of surface liquid filaments
and discharge development between filaments or drops when the electric field is sufficiently high.
2.3 Environmental Aspects
From the discussion of the previous sections, it is clear that there exists a direct relationship between the likelihood of
flashover and the conductivity of the polluted surface layer. In this section, attention will now turn to the formation of this
conducting layer on the insulator surface and the important aspects that determine its conductivity. These aspects are:
• The quantity of pollutants on the insulator surface; this is determined by the contamination deposit-process.
• The types of pollutants present, plus the wetting conditions.
• The natural cleaning properties of insulators.
• Whether the polluted surface layer is in the form of distinct droplets or as a continuous film.
An influence common to all of the above is the climate in which the insulator is installed.
2.3.1 Climates or atmospheric variables and typical environments
The conditions surrounding a H.V. insulator leading to the pollution deposit, and the wetting or cleaning of the insulator, are
caused by a set of atmospheric variables which interact among themselves and with the insulator surface. The most important
atmospheric variables are: wind, rain, humidity, temperature and pressure. Atmospheric conditions can vary in both time and
space. Similar identifiable patterns of occurring atmospheric conditions may be grouped into climates.
Climate is, therefore, the result of the interaction of atmospheric conditions with the surface of the earth and may be classified
as local, regional or global. A pertinent feature of meteorological information is that it is expressed in average values,
obtained from statistics taken over a long period of time (e.g. 30 years).
A general classification of climates is given in Table 2-1.
1999-09-01 11
Table 2-1 A general classification of climates 31.
TYPE OF
CLIMATE
DESCRIPTION 1 DESCRIPTION 2 DESCRIPTION 3
Tropical Often called Equatorial climates. Here the
weather is hot and wet around the year.
These climates are found within about 5°
of latitude North and South of the
Equator
Hot tropical climates with a distinct wet
and dry season. They occur roughly
between 5° and 15° North and South of the
Equator. In parts of South and South-East
Asia the division betweenthe wet and dry
seasons is so clear that they are called
Tropical Monsoon Climates
Dry Hot deserts with little rain at any season
and no real cold weather although
temperature drops sharply at night. The
Sahara desert and much of the Arabian
peninsula are the best examples of this
type.
Tropical steppe or semi-desert with a short
rainy season during which the rains are
unreliable and vary much from place to
place. Good examples are found in parts of
India and the Sahel region of Africa.
Deserts with a distinctly cold season.
These occur in Higher Latitudes in
the interior of large continents. The
best examples are parts of central
Asia and Western China.
Warm
Temperate
Rain occurs at all seasons but summer is
the warmest time of the year and
temperatures range then from warm to
hot. Winters are mild with occasional
cold spells. Much of Eastern China and
the South Eastern States of the USA fall
in this category.
Winters are generally mild and wet,
summers are warm or hot with little or no
rain. This type of climate is often called
“Mediterranean” because of its wide extent
around that sea. It occurs in smaller areas
elsewhere, for example central Chile,
California and Western Australia.
Cold The cool temperate oceanic types of
climate: Rain occurs in all months and
there are rarely great extremes of heat or
cold. This climate is found in much of
Northwest Europe, New Zealand and
coastal British Columbia.
Cold continental climates with a warm
summer and cold winter. Much of Eastern
and Central Europe and Central and Eastern
Canada and the USA have this type of
Climate
Sub-Arctic
or Tundra
The winters are long and very cold.
Summers are short but during the long
days temperatures sometimes rise
surprisingly high. This type of climate
occurs in Central and Northern Canada
and much of the Northern and Central
Siberia.
Arctic or Ice
cap
In all months temperatures are near or
below freezing point. Greenland and the
Arctic continent are the best examples of
this type but it also occurs on some
islands within the Arctic and Antarctic
circles.
High
mountain
and Plateau
Where land rises above or near the
permanent snow line in any latitude the
climate resembles that of the Sub-Arctic
or Arctic. The largest extent of such
climate is found in Tibet and the great
mountain ranges of the Himalayas
2.3.1.1 Local climate
For its general characteristics, the local climate depends on the regional climate and - ultimately - upon the global climate-
system. It is, therefore, useful to remember that the local climate of a particular place is a variation on the regional climate.
Indeed, the mechanisms acting to create a local climate are essentially the same as those creating the global climate32. This
means, that it is possible to apply this knowledge to create models to:
a) Understand the physical process of the interaction between climate and insulator.
b) Predict the pollution phenomena.
Work has been done to correlate the general climatic specification and meteorological data with the pollution flashover
performance of insulators, as is reported in Section 7, where the impact of climate on selection and dimensioning is discussed.
1999-09-01 12
The aim of such a study is to find the basic relationship between the atmospheric variables and the pollution phenomena.
Information on the time-variation of atmospheric variables is necessary. The information sources will, of course, vary as
needs differ. The Meteorological Service usually only provides general information, i.e. average values; However, when an
application is submitted to its research department, specific information can be obtained.
Depending on the study being made, either regional or local climate data will be used. For example, to study the insulation
design or maintenance of a transmission line 100 km long (place) and for an expected life of 50 years (time), regional climate
information will be used.
2.3.1.2 Typical environments
To assist in the selection and design of external insulation, typical environments have been defined. Some examples are:
• Marine: Areas where the insulator pollution is dominated by the presence of the sea. The pollutants present on the
insulators are, therefore, mostly NaCl and other marine salts that are easily soluble. On insulators close to the coast it is
generally found that the inert component of the pollution is low.
• Industrial: Areas in close proximity of polluting industries - such as steel mills, coke plants, cement factories, chemical
plants, generating stations or quarries - are classified as industrially polluted. In these areas, the pollution types can be
very diverse. The pollutants present may vary from dissolved acids - such as found close to power stations or chemical
plants - to slow dissolving salts - such as gypsum or cement - found close to quarries or cement factories. Generally, the
pollution has a high inert component in areas close to industries.
• Desert: In desert environments, the pollution tends to be sand based. The desert sands may contain high amounts of
salt, e.g. 18 % in Tunisia 62, resulting in a very conductive layer when wetted. The pollution on the insulator tends to be
hygroscopic with a very high inert component. Inland desert areas are typically very dry, dusty, windy and hot. The
large fluctuations between day and night raises the relative humidity to levels as high as 93% during early morning up to
sunrise thereby leading to very heavy dew that causes frequent flashovers in some cases. If desert areas are close to the
coast, the pollution problems are compounded61.
• Mixed: If industrial areas are situated close to the coast or desert, then the pollution can be described as mixed.
• Agricultural: Localised insulator pollution may also be caused by agricultural activities such as crop spraying,
ploughing etc. When lines cross land ready for harvesting the structures may serve as perches for large birds - thereby
leading to flashovers due to bird streamers.
The environment may also be classified according to the nature of the contamination-source, as was done in a survey33 on
insulator in-service performance. The classification was as follows:
• Areas with no signs of pollution-related problems. These areas are defined as ‘clean’ areas.
• Areas with isolated pollution problems of limited extent that can usually be traced to a particular pollution-source.
These areas are defined as experiencing ‘local’ pollution. Local pollution is often found in areas where the general
atmospheric condition is pollution free but local industrial or agricultural activities cause the problem.
• Areas with widespread pollution problems that can not usually be traced to a localised pollution-source. These areas are
defined as experiencing ‘regional’ pollution. Regional pollution can often be found in extended industrially developed
areas - typically with numerous chemical plants, steel mills, and cement or fertiliser factories. Regional pollution may
also be found along coastal areas, especially if the weather pattern includes a dry season that allows the accumulation of
pollution on the insulators.
These types of classification can only be used to describe the environment in general terms. Therefore, a detailed study of the
actual pollutants present is required to achieve an optimal insulator selection. Service experience has demonstrated that the
performance of ceramic insulators - in all but the severest environments - is adequate if the insulators have been properly
dimensioned. However, several factors may adversely influence performance even though insulator selection was appropriate
at the time of design.
Firstly, the environment may change during the lifetime of the insulators. This can be particularly troublesome in
industrialising areas, where the region may have been classified originally as ‘clean’ and then - at some point - additional
sources of pollution become located near an installation. This could be the caseif new factories are constructed, or if an area
becomes developed for agriculture after the insulators have been selected.
Secondly, it has been observed that - in some areas thought to be clean - pollution effects become apparent several years after
the insulators have been installed, even if no new industrial or agricultural activity takes place. This is simply a matter of the
insulators gradually accumulating pollution with time, often on a time-scale of several years. Other changes in the
environment could be related to changes in the nesting habits of birds, which have been known to cause pollution flashovers.
1999-09-01 13
Third, extreme changes in weather have been known to cause major outages because of unusual meteorological patterns.
Major storms that may occur with relatively low probability can suddenly cause severe coastal pollution. In inland areas, long
dry periods with little rain may also cause an unusual build-up of pollution.
2.3.2 Type of pollution
To degrade the service strength of an insulator, the pollution must either form or adversely influence a conductive layer on its
surface. Pollution can, therefore, be classified either as being an active type - i.e. pollution that forms a conductive layer - or
as being an inert type - i.e. pollution that adversely influences the conductive layer4.
The amount, or severity, of the pollution layer on an insulator is normally expressed in terms of the Equivalent Salt Deposit
Density (ESDD). This quantity is obtained by measuring the conductivity of the solution containing pollutants removed from
the insulator surface and then calculating the equivalent amount of NaCl having the same conductivity 322. ESDD is expressed
in mg salt per cm2 of the insulator’s surface area.
2.3.2.1 Active type of pollution
Active pollutants are classified according to the ease by which the conductive layer is formed. Two types are apparent:
1. Conductive pollution
2. Pollution that must dissolve in water to become conductive
Typical examples of each of these pollutant types are given in Table 2-2.
Table 2-2: Examples of the different active pollution types 4 36 37 39 69.
CONDUCTIVE POLLUTION DISSOLVING
Metallic deposits such as
Magnitite, Pyrite
Gasses in solution:
SO2, H2S, NH3
Salt Spray
Bird Streamer
Ionic Salts: 
NaCl, Na2CO3,
MgCl2, gypsum CaSO4
Others,
Fly ash, cement
2.3.2.1.1 Conductive pollution
Metallic deposits
Metallic deposits are normally found close to mining activity and related industries. The electrical strength of the insulator is
severely affected if the density of the pollutants on the insulator surface is such that the individual particles are in contact or if
the gaps between the particles are bridged by an electrolyte.
Bird Droppings
It has been reasoned that bird droppings can explain a large number of unidentified outages of transmission lines with system
voltages up to 500 kV 34 35 36. When large birds release their excrement, a long continuous length of highly conductive fluid
droppings (volume conductivity 10 - 30 mS/cm) can shorten the air gap between the tower structure and the conductor. Then
the remaining air gap is too small to withstand the phase-to-earth voltage. Most of these flashovers occur during the time
period prior to birds’ commencement of daily activity.
A secondary effect is that the insulators are covered with the bird excrement, which is a pollution layer with a very high salt
content. If the birds utilise the tower frequently, this may become a very thick layer.
Pollutants in dissolved state
A more common conductive pollution type is where the pollutants are already dissolved in the wetting agent, as in acid rain
and salt-fog conditions. Some of these pollutants - such as gasses dissolved in water, e.g. SO2 - are difficult to detect by
taking measurements from the surface of the insulators, because this contaminant returns to the gaseous state as soon as the
insulator surface dries4.
1999-09-01 14
2.3.2.1.2 Pollution that needs to dissolve
Various studies have been made to find a relationship between the dissolving characteristics of salt contaminants and the
insulator flashover voltage 52 39 37 69. From these studies, the following parameters have been identified as being important:
• The solubility of the salt.
• The rate at which the salt goes into solution.
Figure 2-5 39 shows the effect of salt-solubility on the limiting flashover voltage of an insulator for three different equivalent
salt deposit densities (0.01 mg/cm2, 0.03 and 0.10 mg/cm2). The limiting flashover voltage is the minimum value achieved
under a cold fog test for a polluted insulator. Eight salts were investigated. From this figure, it is clear that there is very little
dependence of pollution flashover voltage on the solubility of the contaminating salt.
0
2
4
6
8
10
12
14
0 20 40 60 80 100
Solubility (g/100 g H2O)
Li
m
iti
ng
 F
la
sh
ov
er
 V
al
ue
 (
kV
, r
m
s)
Na2SO4 MgSO4
NaCl
Mg(NO3)2
MgCl2
Ca(NO3)2
CaCl2
NaNO3
ESDD= 0.01 mg/cm2 
0.03 mg/cm2 
0.10 mg/cm2 
Figure 2-5: Relationship between Salt solubility and limiting flashover values (LFOV) 39.
Different salts also have different rates at which they go into solution; generally the higher the solubility of the salt the quicker
it will go into solution - but this is not always the case. This is shown in Table 2-3 where the salt is classified according to its
solubility and speed by which it goes into solution.
Table 2-3: General classification of salts according to their solution properties.
LOW SOLUBILITY SALTS HIGH SOLUBILITY SALTS
FAST DISSOLVING SALTS MgCl2 , NaCl
SLOW DISSOLVING SALTS MgSO4, Na2SO4, CaSO4 NaNO3, Ca(NO3)2, ZnCl2
Highly soluble salts that dissolve quickly need a short time in contact with water to go into solution. Therefore, a highly
conductive layer can form quickly on the insulator during all wetting processes. However, with higher wetting rates - e.g. rain
etc. - the pollution will also be purged more easily from the insulator due to its high solubility.
Low solubility salts that also dissolve slowly need a large quantity of water to speed up the solution process. This is illustrated
in Figure 2-638. The relationship between ESDD and the quantity of distilled water used to make the measurement is shown
for insulators that came from two environments; one in an agricultural area, Huang Du, and another is from an environment
close to a steel plant. In both of these areas, the main pollutant is gypsum.
1999-09-01 15
Figure 2-6: Relation between ESDD and Quantity of distilled water 38.
This figure shows that for the naturally polluted insulators, an increase in ESDD occurs for an increase in the quantity of
distilled water used for making the measurement. This is in contrast to an insulator polluted artificially with NaCl - i.e. a fast
and highly soluble salt - that does not show the same tendency.
Various studies have shown that insulators contaminated with highly soluble and fast dissolving salts - such as NaCl - have
lower clean-fog withstand voltages than insulators contaminated with low solubility salts which are slow dissolving39 40 41 -
such as gypsum (CaSO4.2H2O) - in spite of them having the same contamination severity (see Figure 2-7).
Figure 2-7: Influence of various salts in the contamination layer on the insulator fog withstand voltage41.
It was also shown that the relationship between the flashover voltage in a steam fog test and the steam input-rate was
dependent on the type of salt on the insulator. A comparison was made between insulators naturally polluted - mainly gypsum
- and insulators artificially polluted with NaCl and kaolin 42. The results are presented in Figure 2-8, which show that the
flashover strength of insulators polluted with mainly gypsum have a greater dependency on steam input-rate than do insulators
polluted with NaCl.
The decrease in flashover voltage with increasing steam input-rate is ascribed to the greater amount of pollutionthat is
dissolved at the higher wetting-rate. To achieve the same flashover voltage during the test as that applied in-service
conditions when flashovers occurred, the steam input-rate had to be an order of magnitude higher than that recommended by
IEC 507 22.
1999-09-01 16
Figure 2-8: Flashover voltage of naturally and artificially polluted insulators as a function of steam input rate42.
Flashovers have been reported on insulators polluted by slow dissolving salts - such as gypsum (CaSO4) - but they generally
occurred during extended periods of wetting; i.e. dense fog, heavy rain storms lasting longer than three hours or live spray
washing 42 43.
Other factors that complicate the relationship between the type(s) of salt and the flashover voltage are when:
• The solubility of a salt is affected by the existence of other salts; e.g. the solubility of CaSO4 is inhibited by the presence
of NaCl 37.
• The process by which a salt goes into solution can be either exothermic (temperature rises) or endothermic (temperature
lowers). Any temperature change will greatly influence the conductivity of the solution that forms 69.
• The wetting process of the insulator is influenced by the hygroscopic properties of the salt. Therefore, different wetting-
rates will occur for different salts - even though the ESDD values may be the same 69.
2.3.2.2 Inert pollution
Inert material deposited on an insulator surface has, until now, been considered to give an indirect and relatively small
influence on the withstand voltage. The greater the inert material deposit, the thicker will be the water film retained on the
insulator surface - and so the amount of soluble material dissolved in the water film will be higher.
Recently, significant differences have been found in the d.c. withstand voltage between insulators contaminated artificially
with Tonoko and kaolin under the same ESDD conditions 44 45. In addition, it has also been reported that there is an influence
of the amount of the inert material on the hydrophobicity and the withstand voltage of polymeric insulators 46 47.
The amount of inert material found in the pollution deposit on an insulator is expressed as the Non-Soluble Deposit Density
(NSDD) given in weight of the non-soluble deposit per unit surface area of the insulator 322. NSDD is expressed in mg/cm2.
In this section, the influence of both the type and the amount of inert material on the contamination performance of insulators
will be discussed.
2.3.2.2.1 The influence of inert material type
In the conventional clean-fog procedure for insulator artificial contamination tests, a constant amount of inert material and a
variable amount of salt are included in the solution for contaminating a specimen insulator. Kaolin and Tonoko are typical
inert materials for artificial contamination tests and so they will form the basis of this discussion. Although the shape of a
specimen insulator and the contamination method may influence NSDD, 40 g of inert material per 1 litre of water has been
specified in IEC 507. This amount is regarded as giving approximately 0.1 mg/cm2 of NSDD on the insulator surface 22.
1999-09-01 17
However, the experimental results given in Figure 2-9 show that the deposit density on a specimen disc varies with the type of
inert material - in this case, Tonoko and Roger’s kaolin - when the specimen is contaminated with a solution having the same
‘concentration’ of inert material. The results presented in Table 2-4 are, therefore, given for the same inert material deposit
density.
Figure 2-9: Relationship between NSDD and the quantity of inert materials in the contamination suspension 45.
Comparative test results of d.c. and a.c. contamination withstand voltage with Tonoko and Roger’s kaolin are also shown in
Table 2-4 45. Significant differences - 20 to 25% - can be seen in the d.c. withstand voltage between Tonoko and Roger’s
kaolin although the salt deposit density (SDD) is the same.
Table 2-4: Results form flashover voltage tests45.
Test
Voltage
Specimen
Insulator
Quantity of Salt / Non-
soluble Contaminant
g/l
SDD
mg/cm2
NSDD
mg/cm2
50% FOV
kV/unit
Corrected
50% FOV
kV/unit
Max. Leakage
current
mA
13/40
Tonoko
0.068 16.7
[100]
15.8
[100]
250
13/60
kaolin
0.03 0.079 14.8
[89]
14.4
[92]
430
250S 133/40
Tonoko
0.079 11.0
[100]
10.6
[100]
850
a.c. 96/60
kaolin
0.025 0.135 10.0
[91]
10.5
[99]
1200
15/40
Tonoko
0.076 26.6
[100]
25.5
[100]
200
320DC 16/60
kaolin
0.03 0.113 22.2
[83]
22.6
[89]
550
13/40
Tonoko
0.068 16.3
[100]
15.4
[100]
230
250S 13/60
kaolin
0.03 0.085 12.8
[79]
12.5
[81]
350
d.c. 13/40
Tonoko
0.16 25.0
[100]
26.8
[100]
80
320DC 13/40
kaolin
0.03 0.082 20.9
[84]
20.3
[76]
160
Note 1: SDD and NSDD values show average values measured on more than 10 insulator units for individual cases.
Note 2: Maximum leakage current shows the average maximum value for individual cases.
Note 3: Corrected 50% FOV value was the one corrected to NSDD = 0.1 mg/cm2.
Note 4: [ ] shows the percentage ratio of 50% FOV for the case of kaolin relative to that of Tonoko.
Note 5: Insulator types are specified in the paper45.
1999-09-01 18
Table 2-4 shows that a 5-10% difference in the a.c.-contamination withstand voltages was found between Tonoko and Roger’s
kaolin when the NSDD was adjusted to the same level. The variation of the surface resistance of the contaminated insulator
during the tests is shown in Figure 2-10, which illustrates that the surface resistance of an insulator contaminated with Roger’s
kaolin reduces faster and is much lower than that of an insulator contaminated with Tonoko.
Tonoko
Brazilian kaolin
Mexican kaolin
Georgia kaolin
Italian kaolin
Roger’s kaolin
10
1.0
0.1
0 10 20 30 40 50 60
Time lapse, min
Su
rf
ac
e 
R
es
is
ta
nc
e,
 M
oh
m
/u
ni
t
Figure 2-10: Time variation of surface resistance during the course of clean fog tests of contaminated insulator units
polluted with a combination of salt and various types of kaolin and Tonoko 48 .
The very wide variations in the physical and chemical properties of the various kinds of kaolin used internationally in
insulator contamination tests are shown in Table 2-548.
Table 2-5 : Physical and chemical properties of common inert materials used in insulator contamination tests 48.
kaolin
Item Measuring method Tonoko Roger’s Georgia Italy Mexico Brazil
Particle Size, µm
(50% value)
Laser Light Scattering 6.2 5.8 6.3 4.5 13.5 25.9
Main Constituents
of material
X-ray Diffraction Quartz
Muscovite
Quartz
Kaolinite
Quartz
Kaolinite
Quartz
Kaolinite
Quartz
Kaolinite
Cristobalite
Quartz
Kaolinite
Chemical Loss on Ignition 4.8 14 14 12 6 13
Composition, X-ray SiO2 67 46 46 48 77 48
Percentage by Fluorescence Al2O3 16 37 38 37 16 36
Mass Fe2O3 5.8 0.9 0.7 0.7 0.2 1.0
The surface resistance and the withstand voltage characteristics of an insulator artificially contaminated with these types of
kaolin, together with the Tonoko, are shown in Figure 2-10 and Figure 2-11 respectively 48. A large variation is apparent,
even among the various types of kaolin 10 49.
The main minerals of Tonoko and kaolin - as determined by the X-ray diffraction method - are Muscovite (Al2Si2O5(OH)4)
and Kaolinite (KAl2Si3Al10(OH)2) respectively, together with Quartz (SiO2) that is common to both.
The different surface resistivities of Tonoko and the various types of kaolin that apply under artificial fog conditions can be
explained by the different crystal structures of these materials. Hydroxyl groups [OH]- are located inside the crystal structure
in the case of Muscovite, whereas they are located outside the crystal structure in the case of Kaolinite. Kaolin consisting of
Kaolinite is, therefore, much more hydrophilic than Tonoko consisting of Muscovite.
Recently it was confirmed that the type of inert material had a similar influence on the contamination withstand voltage of
silicone rubber polymeric insulators 50.
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