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Insulators Used in Overhead Transmission Lines
What are Insulators?
Insulators are the elements of transmission system which provide necessary insulation between line conductors and supports and hence, prevent any leakage current from the conductors to the earth.
The line conductors in the overhead transmission lines should be supported on the poles or towers in such a way that the current from the conductors do not flow to the earth through the supports which means the line conductors must be properly insulated from the supports. This is achieved by using the insulators between the line support and conductors.
The commonly used material for manufacturing the insulators of overhead transmission line is porcelain. Although, glass, polymer and steatite and many other special composition materials are also used for manufacturing the overhead line insulators. The porcelain is stronger mechanically than glass, gives less trouble from leakage and is less effected by variation of temperature.
Desirable Properties of Insulators
The insulators of overhead transmission lines should have the following desirable properties −
Insulators should have high mechanical strength so it can withstand conductor load, wind load, etc.
Insulator material should have high electrical resistance to avoid the leakage currents to the earth.
Insulator material should have high relative permittivity so that its dielectric strength is high.
The insulator material should be non – porous, free from cracks and impurities so that it has high relative permittivity.
The ratio of puncture strength to flashover should be high for the insulator.
Causes of Insulator Failure
The insulators in overhead transmission lines are required to withstand both mechanical and electrical stresses. The mechanical stress is due to the conductor load, wind load, etc. while the electrical stress is mainly due to line voltage and it may cause the breakdown of the insulator. The electrical breakdown of the insulator can occur either by flash-over or puncture.
In flash-over, an arc occurs between the line conductor and the insulator pin and the discharge jumps across the air gaps, following the shortest distance. It is because the insulator is generally dry and its surfaces have proper insulating properties. Thus, the arc can only occur through the air gap between conductor and insulator pin. In case of flash-over, the insulator can continue to work in its proper capacity unless extreme heat produced by the arc destroys the insulator.
In case of insulator puncture, the arc discharge occurs from the conductor to insulator pin through the body of the insulator. Therefore, the insulator is permanently destroyed due to excessive heat. In practice, a sufficient thickness of porcelain is provided in the insulator to avoid puncture by the line voltage.
The ratio of puncture strength to the flash-over voltage is called the safety factor of the insulator, i.e.,
$$\mathrm{Safety \: factor\mathrm{\, =\, }\frac{Puncture \: strength}{Flash \: over\: voltage}}$$
Hence, from the definition of safety factor, it is clear that the value of safety factor of an insulator should be high so that the flash over occurs before the insulator gets punctured.
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