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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 ____________.82

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Solution

## The correct option is A 82Number 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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