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 n 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 : n n 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

n 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: n porcelain n pin, n cap

Pollution Type n n Dust, micro organism, bird secretions, flies Desert pollution sand dry wind Ice & Fog deposits at high altitudes Coastal pollution • corrosive and hygroscopic salt layers are deposited on the insulator surfaces n Industrial pollution • smoke, petroleum vapours, dust

CONTAMINATION TESTS n (1) salt-fog n (2) wet-contaminant, n (3) clean fog test.

n 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/m 3.

n 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

n 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”.

n (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 n 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.

n 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 Na. Cl that yields the same conductivity is determined and the ESDD is the equivalent amount in milligrams of Na. Cl per square centimetre deposited on the insulator surface.

n 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.

n 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 n n n 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.

n n 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

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