Potential distribution over a string of suspension insulators:

In suspension insulator, it consists of connecting number of porcelain discs in series. So, due to the porcelain portion between two metal links, each disc forms self or mutual capacitance C, which is same in every disc. But due to the shunt capacitance, between metal link and tower or earth, the voltages across discs are different. The disc nearest to the live conductor has maximum voltage and maximum possibility of puncture. The voltage across disc decreases with the distance from live conductor. So, electrical stress is maximum at the nearest disc and gradually decreases.

String efficiency:

As the voltage across the suspension insulator is not uniformly distributed, the unequal distribution measured by string efficiency which is denoted by the ratio of voltage across the whole string to the product of number of disc, used in insulator and the voltage across the nearest disc of the conductor.
So, String efficiency = voltage across the string/ (n X voltage across the disc nearest to the conductor.)
Where, n = number of disc in the string.

Methods of improving string efficiency.

To make uniform voltage across the disc of suspension insulator, we have to improve the string efficiency. The methods are,
(a) Using long cross- arm: We can improve the string efficiency by minimizing the shunt capacitance. If we increase the distance between disc and tower, that means use a longer cross arm, shunt capacitance may be decreased. Here cost and strength of tower is the major drawback.
(b) Correct grading: In this process different mutual capacitance with different dimension are chosen. The discs are graded by capacitance, that is capacitance is minimum at top unit whereas, maximum to the unit nearest to the conductor. By this method various numbers of different sized insulators are required.
(c) Using guard ring or static shielding: The guard ring is a large galvanized steel ring surrounding the bottom unit which introduces capacitance between metal fittings and the line conductor. These capacitances are greater for lower units and voltage across the unit is reduced. By this way, potential across each unit in a string equalize by the guard. Guard used with arcing horn to protect the insulator string from damage due to over voltage. It is used mainly in 220 KV and 400 KV systems.

Arcing horn: Due to lightning or switching surges, line voltage increases. This voltage sometimes may raise higher value than insulator flash over voltage, which may damage the insulator. For decreasing the damaging probability, one pair galvanized steel rod installed at the top and bottom of insulator string. This rod is known as arcing horn. The gap between the pair of rod is less than insulator string length due to which, flash over voltage bypasses between arcing horn and protect the insulator.

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