The effect of contamination The effect of contamination on insulator flashover on insulator flashover

characteristicscharacteristicsDone byDone by

Hamid Al-DhaferiHamid Al-Dhaferi

For For

Dr.M.ShwehdiDr.M.Shwehdi

Introduction Introduction With the ever increasing demand for electrical With the ever increasing demand for electrical

power, power, there has been a steady growth in high there has been a steady growth in high voltage transmission linesvoltage transmission lines required for optimum and required for optimum and economic transfer of large blocks of power over long economic transfer of large blocks of power over long distances distances

As the level of transmission voltage is increased, As the level of transmission voltage is increased, switching and dynamic over vlotages and switching and dynamic over vlotages and withstand withstand ability of the insulator under polluted conditions are ability of the insulator under polluted conditions are important factors which determine the insulation important factors which determine the insulation level of the system level of the system

The reliability of the system mainly depends on the The reliability of the system mainly depends on the environmental and weather conditions which cause environmental and weather conditions which cause flasoverflasover on polluted insulators leading to system on polluted insulators leading to system outages.outages.

    It is generally recognized that the main It is generally recognized that the main events leading to flashover of polluted events leading to flashover of polluted insulators under service voltage are :insulators under service voltage are :

the formation of a conductive layer on the formation of a conductive layer on the insulator surface the insulator surface

leakage current surging with associated leakage current surging with associated dry band formation and partial arc dry band formation and partial arc development along the insulator surface development along the insulator surface eventually spanning the whole insulatoreventually spanning the whole insulator

The problem is more complex in case The problem is more complex in case of DC systems because of large of DC systems because of large contamination due to electrostatic contamination due to electrostatic forces and longer duration of partial forces and longer duration of partial arc. Therefore the design of external arc. Therefore the design of external insulation under polluted conditions insulation under polluted conditions is very critical and lots of is very critical and lots of investigations are under way to investigations are under way to arrive at safe creepage distances for arrive at safe creepage distances for systems operating in different systems operating in different environments.environments.

InsulatorsInsulators

For example: For example:

suspension insulatorssuspension insulators

as shown in figureas shown in figure

The basic parts are:The basic parts are: porcelainporcelain pin,pin, cap cap

Pollution TypePollution Type Dust, micro organism, bird secretions, Dust, micro organism, bird secretions,

fliesflies Desert pollutionDesert pollution

sand and dry windsand and dry wind Ice & Fog deposits at high altitudesIce & Fog deposits at high altitudes Coastal pollution Coastal pollution

• corrosive and hygroscopic salt layers are corrosive and hygroscopic salt layers are deposited on the insulator surfacesdeposited on the insulator surfaces

Industrial pollution Industrial pollution • smoke, petroleum vapours, dustsmoke, petroleum vapours, dust

CONTAMINATION TESTSCONTAMINATION TESTS

(1) salt-fog (1) salt-fog

(2) wet-contaminant, (2) wet-contaminant,

(3) clean fog test.(3) clean fog test.

The Salt-Fog Test:The Salt-Fog Test:

In the method, the insulator is In the method, the insulator is energized at the service voltage energized at the service voltage which is held constant through the which is held constant through the test, and subjected to a salt fog test, and subjected to a salt fog whose salinity, ranged from 2.5 to whose salinity, ranged from 2.5 to 160 g/m3.160 g/m3.

Wet-Contaminant Test Method:Wet-Contaminant Test Method:

In this test, the insulators are In this test, the insulators are contaminated by spraying the contaminated by spraying the contaminant mixture. Voltage is contaminant mixture. Voltage is applied 3 to 5 minutes after the end applied 3 to 5 minutes after the end of the contamination procedure while of the contamination procedure while the insulator still wet. The test the insulator still wet. The test voltage is raised until flashover voltage is raised until flashover occurs occurs

Clean fog test method:Clean fog test method:

This test method can be separated into This test method can be separated into two types:two types:

(1) In some fog test(1) In some fog test, insulators are , insulators are contaminated, dried, and then wetted contaminated, dried, and then wetted by clean-fog. Test voltage is applied by clean-fog. Test voltage is applied to the insulators when the leakage to the insulators when the leakage resistance has reached its lowest resistance has reached its lowest value. This method has been mainly value. This method has been mainly developed in Germany, under the developed in Germany, under the name “pre-deposit method”.name “pre-deposit method”.

(2) In the second type of test :(2) In the second type of test :

voltage is applied to dry voltage is applied to dry contaminated insulators and then a contaminated insulators and then a wetting condition is applied. This can wetting condition is applied. This can be regarded as a reasonable be regarded as a reasonable simulation of natural conditions; simulation of natural conditions; however, it is more complicated than however, it is more complicated than other methods.other methods.

Pollution monitoringPollution monitoring 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.

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.

In the second methodIn the second method, the insulator , the insulator is placed in a special electric circuit is placed in a special electric circuit to measure its leakage resistance to measure its leakage resistance after having sprayed its surface with after having sprayed its surface with distilled water.distilled water.

The third methodThe third method consists of a consists of a special test ring suspended between special test ring suspended between insulator chains and the tower. These insulator chains and the tower. These chains are permanently energized chains are permanently energized from high-voltage power lines. A from high-voltage power lines. A monitor is connected in series with monitor is connected in series with each insulator chain to measure the each insulator chain to measure the leakage current leakage current

Process of pollution and start of flashoverProcess of pollution and start of flashover 1) Deposit of pollution on the 1) Deposit of pollution on the

insulator surface. insulator surface.

2) The surface layer is moistened2) The surface layer is moistened

3) The surface layer is heated and 3) The surface layer is heated and causes an increase in the causes an increase in the conductivity and the leakage current.conductivity and the leakage current.

4) The heating results in local drying 4) The heating results in local drying of the surface layer and so-called dry of the surface layer and so-called dry bands occur.bands occur.

5) Partial arcs occur across the dry 5) Partial arcs occur across the dry bands on such contaminated bands on such contaminated insulators.insulators.

6) The partial discharges increase6) The partial discharges increase 7) Finally the partial streamer 7) Finally the partial streamer

discharges (partial flashovers) are discharges (partial flashovers) are “connected” in serious and a “connected” in serious and a complete flashover occurs.complete flashover occurs.

These figures show insulator flashoverThese figures show insulator flashover

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