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Question

In a string of three identical suspension insulator units supporting a transmission line conductor. If the self capacitance of each unit is denoted as C farad. The capacitance of each conductor pin to ground can be taken as 0.15 C farads. If the maximum permissible voltage per unit is 20 kV. Then the string efficiency (in percentage) is ____________.
  1. 82

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Solution

The correct option is A 82
Number of units, n = 3
Ratio of shunt capacitance to mutual capacitance,

K=0.15 CC=0.15
Voltage across bottom most unit

V3= safe working voltage of the unit = 20 kV
So voltage across top most unit,

V1=V31+3K+K2=201+3(0.15)+(0.15)2=201.4725

V1=13.58 kV

Voltage across middle unit V2=V1(1+K)
= (13.58 kV) (1 + 0.15)
= 15.617 kV

Maximum safe working voltage of the string,

V=V1+V2+V3
= 13.58 +15.617 + 20
V = 49.197 kV

String efficiency =VnVn×100=49.1973×20×100=81.955=82%

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