05-05-2014, 12:12 PM
Study Of Diffferent Types Of Insulator
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Introduction :
Electrical Insulator must be used in electrical system to prevent unwanted flow of electric current to the earth from its supporting points. In transmission and distribution system, the overhead conductors are generally supported by supporting towers or poles. So there must be insulator between tower or pole body and current carrying conductors to prevent the flow of current from conductor to earth through the grounded supporting towers or poles.
Properties of Insulating material
The materials generally used for insulating purpose is called insulating material. For successful utilization, this material should have some specific properties as listed below,
1. It must be mechanically strong enough to carry tension and weight of conductors.
2. It must have very high dielectric strength to withstand the voltage stresses in High Voltage system.
3. It must possesses high Insulation Resistance to prevent leakage current to the earth.
4. The insulating material must be free from unwanted impurities.
5. It should not be porous.
6. There must not be any entrance on the surface of electrical insulator so that the moisture or gases can enter in it.
7. There physical as well as electrical properties must be less effected by changing temperature.
Characteristics of Polymer Insulator
The main element of polymer insulator displays very excellent properties in terms of heat proof
performance, chemical stability, electrical insulation, wear resistance and glossy performance compared
to general organic rubbers thanks to its both sides nature-organic and inorganic properties expressed simultaneously by silicone.
Pin Insulator
Pin Insulator is earliest developed overhead insulator, but still popularly used in power network up to 33KV system. Pin type insulator can be one part, two parts or three parts type, depending upon application voltage. In 11KV system we generally use one part type insulator where whole pin insulator is one piece of properly shaped porcelain or glass. As the leakage path of insulator is through its surface, it is desirable to increase the vertical length of the insulator surface area for lengthening leakage path. These rain sheds or petticoats are so designed, that during raining the outer surface of the rain shed becomes wet but the inner surface remains dry and non-conductive. So there will be discontinuations of conducting path through the wet pin insulator surface.
In higher voltage like 33KV and 66KV we use multiple part pin insulator, where a number of properly designed porcelain shells are fixed together by Portland cement to form one complete insulator unit. For 33KV two parts and for 66KV three parts pin insulator are generally used.
Designing consideration of Electrical Insulator
The live conductor attached to the top of the pin insulator is at a potential and bottom of the insulator fixed to supporting structure of earth potential. The insulator has to withstand the potential stresses between conductor and earth.
1. When insulator is wet, its outer surface becomes almost conducting. That is why the upper most petticoat of a pin insulator has umbrella type designed so that it can protect, the rest lower part of the insulator from rain. The upper surface of top most petticoat is inclined as less as possible to maintain maximum flash over voltage during raining.
2. To keep the inner side of the insulator dry, the rain sheds are made in order that these rain sheds should not disturb the voltage distribution they are so designed that their subsurface at right angle to the electromagnetic lines of force.