Download The effect of contamination on insulator flashover characteristics

The effect of contamination
on insulator flashover
characteristics
Done by
Hamid Al-Dhaferi
For
Dr.M.Shwehdi
Introduction
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With the ever increasing demand for electrical
power, there has been a steady growth in high
voltage transmission lines required for optimum and
economic transfer of large blocks of power over
long distances
As the level of transmission voltage is increased,
switching and dynamic over vlotages and withstand
ability of the insulator under polluted conditions are
important factors which determine the insulation
level of the system
The reliability of the system mainly depends on the
environmental and weather conditions which cause
flasover on polluted insulators leading to system
outages.
It is generally recognized that the main
events leading to flashover of polluted
insulators under service voltage are :
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the formation of a conductive layer on
the insulator surface
leakage current surging with associated
dry band formation and partial arc
development along the insulator surface
eventually spanning the whole insulator
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The problem is more complex in case
of DC systems because of large
contamination due to electrostatic
forces and longer duration of partial
arc. Therefore the design of external
insulation under polluted conditions
is very critical and lots of
investigations are under way to
arrive at safe creepage distances for
systems operating in different
environments.
Insulators
For example:
suspension insulators
as shown in figure
The basic parts are:
 porcelain
 pin,
 cap
Pollution Type
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Dust, micro organism, bird secretions,
flies
Desert pollution
sand and dry wind
Ice & Fog deposits at high altitudes
Coastal pollution
• corrosive and hygroscopic salt layers are
deposited on the insulator surfaces
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Industrial pollution
• smoke, petroleum vapours, dust
CONTAMINATION TESTS
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(1) salt-fog
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(2) wet-contaminant,
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(3) clean fog test.
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The Salt-Fog Test:
In the method, the insulator is
energized at the service voltage
which is held constant through the
test, and subjected to a salt fog
whose salinity, ranged from 2.5 to
160 g/m3.
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Wet-Contaminant Test Method:
In this test, the insulators are
contaminated by spraying the
contaminant mixture. Voltage is
applied 3 to 5 minutes after the end
of the contamination procedure while
the insulator still wet. The test
voltage is raised until flashover
occurs
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Clean fog test method:
This test method can be separated into
two types:
(1) In some fog test, insulators are
contaminated, dried, and then wetted
by clean-fog. Test voltage is applied
to the insulators when the leakage
resistance has reached its lowest
value. This method has been mainly
developed in Germany, under the
name “pre-deposit method”.
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(2) In the second type of test :
voltage is applied to dry
contaminated insulators and then a
wetting condition is applied. This can
be regarded as a reasonable
simulation of natural conditions;
however, it is more complicated than
other methods.
Pollution monitoring
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Generally speaking there are three
methods currently in use for
monitoring the pollution on electric
insulators for power distribution
lines. These can be classified as
either direct or indirect methods
depending on whether the pollution
measurement is performed in the
power network or in a special
location away from the power
network.
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Equivalent salt deposition density (ESDD)
method:
The insulator is washed with a given
quantity of distilled water. Then, the
electrical conductivity of the solution
obtained is measured. The equivalent
weight of NaCl that yields the same
conductivity is determined and the ESDD is
the equivalent amount in milligrams of NaCl
per square centimetre deposited on the
insulator surface.
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In the second method, the insulator
is placed in a special electric circuit
to measure its leakage resistance
after having sprayed its surface with
distilled water.
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The third method consists of a
special test ring suspended between
insulator chains and the tower. These
chains are permanently energized
from high-voltage power lines. A
monitor is connected in series with
each insulator chain to measure the
leakage current
Process of pollution and start of flashover
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1) Deposit of pollution on the
insulator surface.
2) The surface layer is moistened
3) The surface layer is heated and
causes an increase in the
conductivity and the leakage current.
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4) The heating results in local drying
of the surface layer and so-called dry
bands occur.
5) Partial arcs occur across the dry
bands on such contaminated
insulators.
6) The partial discharges increase
7) Finally the partial streamer
discharges (partial flashovers) are
“connected” in serious and a
complete flashover occurs.
These figures show insulator flashover
Protect insulator from contamination and flashover
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