CONDITION MONITORING OF INSULATORS UNDER POLLUTION
ABSTRACT
Cheap and reliable supply of electrical energy has been identified as
an indicator of economic development and social welfare of any modern
society. With increasing demand of electrical energy extensive networks of
electrical power installations are being built at an ever increasing rate, fhe
major function of such power systems is to generate, transport and distribute
electrical energy over large areas which are different in topography, climate
and environment in an economical manner while ensuring a high degree of
reliability and quality of supply.
Increasing transmission voltages of EHV and UHV system place
utmost importance upon the insulation integrity and reliability. Thus
successful, reliable and uninterrupted operation of a modern power system
depends to a great extent on reliable insulation. This insulation must not
only withstand over voltages but must also provide satisfactory' performance
under different environmental conditions (pressure, temperature, humidity,
wind velocity etc.).
The insulation of an overhead electric power transmission line is
subjected to three basic types of abnormal conditions that may cause
(lashover and outages i.e. over voltages caused by lightning strokes, over
voltages caused by line switching and abnormal voltage gradients in the
insulation system caused by contamination of solid insulator surfaces. The
relative severity of these three conditions vary with system operating
vohage, with switching surges and contamination dominating on lines
operating above 500 kV or so [1][2] .
Contamination on outdoor insulations enhances the cliances o '
flashover Under dry conditions, contaminated surfaces do not conduct so
contamination is of little concern. Under conditions of light rain, fog or dew.
surface contamination behaves electrolytically and starts facilitating flow of
leakage current. High current density near the electrodes results in the
heating and drying of the pollution layer. An arc is initiated if the voltage
stress across the insulators dry band exceeds it's withstand capability.
Extension of the arc across the insulator ultimately results in tlashover. The
contamination severity determines the frequency and intensity of arcing and,
thus, the probability of flashover.
To study insulator behavior under pollution and for
improvement of insulator design proper testing under natural operating
conditions is required. Adequate resuks are difficult to obtain from tests in
natural conditions, and great efforts have been made to simulate pollution
conditions by artificial tests. Before the development of artificial pollution
test methods, the pollution characteristics of insulators could only be
determined by recording insulator performance in conditions of natural
pollution. Although such data are difficult to obtain, and are subject to
much larger inherent errors than are the results of artificial tests, they are
still essential for the validation and calibration of artificial test techniques,
especially for new voltage levels and unusual types of insulators[3].
The first overhead transmission line was designed in 1880's with 50
and 66 kV maximum operating voltage with porcelain insulators. Although
great advancements in electrical technology has taken place in the last
century but porcelain has remained as the main electrical insulator in large
part of the world even today. Nonetheless the shape and size of porcelain
insulators that are extensively used in majority of countries has seen
considerable modifications. It is clear that for the proper understanding and
design of insulators for different environmental condifions, it is necessary
that extensive studies are undertaken. There have been number of studies in
the last hundred years on ceramic as well as non-ceramic insulators. Almost
all the studies revolve around electrical parameters such as flashover
voltage, withstand voltage, leakage current, conductivity, chemical analysis
of pollutants and salt deposit density.
No doubt the parameters of interest have remained the same, but the
methodology involved in studying and understanding differed. Some
researchers performed their study under natural conditions and others under
artificial or simulated conditions. The studies under natural conditions have
marked advantages and disadvantages [4]. Keeping in view the rate at which
power transmission system is growing, depending solely on natural method
of testing will hamper the growth of insulation and its application
technology. Therefore, testing insulators with artificial methods that
approximates natural conditions have gained prominence. Also, since the
actual contamination conditions are very complicated, the artificial
contamination tests have made a greater progress than the study of field
contamination [5]. Thus tests are necessary for a proper judicious insulation
design.
Once a suitable level of insulation design is achieved for a particular
environment, it should not lead to complacency of the utility engineers
because the suitability of the obtained design is bound to change due to
increased population, industrialization and other factors. Moreover, ageing
also leads to deterioration in the insulation behaviour of the material.
It is not always that the environmental pollution level increase, there
are instances where due to, general awareness among the population towards
pollution and improved technology of pollution control devices, substantial
reduction in pollution levels have been achieved. For example reduction in
dust precipitation has been observed in Poland, Germany, leading to change
in leakage distance requirement from 2.6 cm/kV in 1987 to 1.3 cm/kV in
2002. It has also been reported that pH of rainwater has increased (from 5.6
towards 7) due to environmental improvement [6][7].Thus it is clear that if
pollution characteristics of 1987 is taken for design of insulation level for a
new installation in these countries, then it would result in over insulation. It
is clear from foregoing discussion that continuous condition monitoring of
line insulators should be done to arrive at an economical leakage distance
required.
Keeping in view the effect of pollution and its different nature, the
present study was taken up to identify the pollution indicators already in use
and suggest some new indicators/monitoring system to mitigate line failures
due to insulation failure. Periodic monitoring of pollution levels with the
help of new identified indicators can provide insight into the problem that
pollution poses.. This is necessary because insulation requirement must be
tailored to fit the needs of each specific case.
Apart from contamination, weather also plays a significant role in
deciding the flashover performance of the insulators. A very recent grid
failure of northern grid of India in March 2008 can be explained on the basis
of dubious nexus of contaminants and prevailing weather condition. Normal
winter rains did not occur during the period December 2007 and March
2008, thereby self cleaning of insulator surface did not occur leading to
substantial deposits on the insulator. Apart from this, the relative humidity
was very high during the first week of March 2008 and unusual fog
conditions prevailed on 6'^ 7th and 8"^ March 2008[8],[9].
In addition to the conclusions that will be given in paragraphs to
follow, the present study proposes pH of the contaminants as a new
effective pollution severity indicator. The effect of contaminant pH on
flashover performance for both natural and artificial contaminant has been
found to be quite significant [10], [11]. The pH effect on flashover
perfomiance has not yet been investigated in detail; reports by Ramos [5]
and Chrazn [7] have only slighfly dealt this parameter.
Taking into consideration the North India grid failure of March 2008,
the Power Grid Corporation of India started washing of power transmission
lines using helicopters in November 2008. This hot line washing procedure
was started as the winter rain forecast was not very encouraging. Around 82
million Indian rupees were spent for 300 flying hours for washing [12].
As already stated, the pH of the contaminant can be used as indicator
of pollution severity. This pH can be used for continuous monitoring of
overhead line insulators. It is suggested that hot line washing should only be
started after the pH of the contaminant deposit exceeds the predetermined
value. This predetermined value can be arrived at after detailed chemical
analysis of the locational pollutants. Thus electric utilities should develop
pH monitors that can relay signals to control stations so that maintenance
engineers devise a proper maintenance schedule that is economical and
serves the purpose of maintaining the reliable operation of the grid.
Based on extensive experimentation, both with natural and artificial
pollutants the conclusions drawn are as given under:
1) For a fair and judicious insulation design/level, accurate knowledge
of locational pollutants is important. Thus insulation requirement
must be tailored fit for every location.
2) Detailed qualitative chemical analysis of pollutants in the area
surrounding Aligarh lead to presence of Na^, K^ , Ca"" . Mg"^" .
HCOj" , C03^', C r , N03^" and S04^" (in dust and ground water), Cd.
Cr, Pb, Fe, Cu, Ni, Zn, Mn (in soil due to ground water
characteristics), fly ash and oxides of sulphur and nitrogen (due to
thermal plant, brick kilns), particulates, CO, oxides of sulphur and
nitrogen (due to vehicular emissions), phosphate and nitrates(due to
agricultural fertilizers)
3) Insoluble component of deposits does not directly contribute to
conductivity but aids in impediment of ionic movement, thereby
leakage current.
4) The pH of the deposit and solvent affects the flashover
characteristics.
5) Temperature, high RH affects withstand and tlashover
characteristics. Pressure in the present study remained almost
constant in the range 742-745 mm of Hg.
6) Temperature affects viscosity of electrolytic solution and thus has an
effect on withstand and flashover characteristics.
7) Higher RH leads to increase in availability of dissociated water
molecules, thus affecting withstand and flashover characteristics.
8) Deposits with univalent cations have higher speed of movement (due
to smaller ionic cloud density) than bivalent cations.
9) Ionic and hydrated radii affects withstand and flashover
characteristics.
10) Solubility of salt also plays major role in deciding the withstand and
flashover characteristics.
11) Transportation number affects leakage current flow.
12) For multi contaminant deposit, interaction of multi cation/ anion
decides behaviour of insulation.
13) Activity coefficient of ions is also responsible for change in withstand
and flashover characteristics due to addition of sparingly
soluble/insoluble salt.
14)The flashover characteristics obtained after extensive experimentation
with various soluble and insoluble salts leads to proposition of a
model that can be used for semi-arid/sub-tropical climatic zone
such as Aligarh.
The details of what has been summarized in the preceding paragraphs
are detailed in the main body of the thesis.
Chapter 1 of the report deals with brief description of types of
pollutants, insulating materials and the various prevalent standard
tests. The chapter also details literature survey of relevant studies
during last hundred years.
In chapter 2, the statement of the problem has been presented.
Detailed work plan has been prepared for conduct of numerous
experiments necessary to arrive at useful conclusions.
The experimental set up, choice of contaminants, sample
preparation, various measurement systems required for conduct of
experiment has been detailed in chapter 3.
Chapter 4 deals with the reporting of various experimental
values obtained. Also number of graphs and bar charts showing
variation of different parameters has also been compiled in this
chapter.
In chapter 5 detailed analyses and discussion on natural tests,
artificial (semi-natural) tests is given. Also a model has been
proposed for predicting flashover voltage values for insulators
installed in semi-arid/sub tropical climatic zone. A comparison
between experimental obtained values and predicted values is
presented.
In chapter 6, the summary of results obtained, conclusions,
research contribution and future work suggested has been reported.
It is hoped that this work will be of great use to practicing
engineers and large power utilities.
REFERENCES
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8. The Hindustan Times, Delhi Edition, 8"" March 2008.
9. The Hindustan Times, Delhi Edition, 9'^ March 2008.
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Proceedings of 2008 International conference on condition
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371,
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Accepted for publication and presentation at CEIDP 2009 to be held 18-21
October 2009, Virginia Beach USA.
12. The Hindu, Delhi Edition, 17* February 2009.
