Electrical Insulator Material
In an overhead transmission line, the conductors are bare and don’t have insulated coating over it. The insulator used to maintain safety and the necessary clearance between live conductors and support of the metal structure.
The insulators are also used to provide support to the bus bar conductors. To manufacture the insulators, in most of the applications, the following materials are used.
- Synthetic Resin
This type of insulators made from the ceramic material. The good quality of porcelain is free from holes, cracks, and laminations. The porcelain made from a powdered mixture of plastic clay, silicon, and feldspar.
This entire process done in the mill and gives appropriate shape. The insulators covered with the glaze. The glaze is used to keep free from dust and moisture. After that, the heat applied to it. The metal parts of insulators are made of malleable cast iron with galvanizing.
Glass also used as insulating material. This type of insulators used for extra high voltage AC and DC transmission systems. By the heat treatment, the glass toughened.
The glass is a transparent material. So, by visual inspection, we can find cracks or any other defect on it. The outer surface is more affected by moisture, this is the disadvantage of glass insulators.
Because of the moisture on the outer surface, the leakage current is more. The glass insulators has less mechanical strength compared to a porcelain insulators. Therefore, it can not sustain high weight and tension.
This type of insulators used for the protection of indoor equipment only. The compound of silicon rubber resin used to make the synthetic resin insulators.
This type of insulators is cheaper compared to other types of an insulators. But the lifespan is less and the leakage current is higher. The main application of this insulator is in bushings.
Configuration of Insulator Strings for Extra-high Voltage AC and HVDC Transmission System
Instead of one insulator, a number of insulators are connected and make a string in the extra high voltage transmission line.
There are four types of configurations;
- Vertical string:
- Horizontal string configuration:
- Special configuration for extra-high voltage requirement:
Flashover is a common reason for insulation failure. In the case of flashover, an arc produced through the air surrounding the insulators. The insulator may get failure by abnormal stress, cracks, and aging.
The insulators selected by electrical characteristics to work at the best. The switching and lightning surge are also responsible for insulators flashover.
Pollution, raindrop, mist, and dart are also responsible for the deposition of material on the insulators. In this way, the dielectric strength of insulators reduced.
It is tedious work to find out the voltage across each unit of the insulator when the number of insulators in the string is large.
If the string of insulators has n units, the maximum voltage across string is E, the voltage across insulator unit is E1, E2, E3, …., En, the capacitance of each unit is mC.
The voltage distribution across the xth unit is given by;
The highest voltage across the insulator unit nearest to the conductor is as below equation,
And string efficiency is as below equation,
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