Questions on Insulator String

1. Insulator string Questions
1. I n a 6 unit suspension insulator string, the capacitance between each link and pin and earth is
0.1 of the self capacitance of each unit. Determine the voltage across the lowest unit as a
percentage of the total voltage and also the string efficiency. [58.29%]
2. Find the voltage distribution and string efficiency of a three unit suspension insulator string of a
three unit suspension insulator string if the capacitances of the link pins to earth and to the line
are respectively 20% and 10% of the self capacitance of each unit. If a guard ring increases the
capacitance to the line of lower link pin to 35% of the self capacitance of each unit, find the
redistribution of voltage and string efficiency.
[0.31V, 0.303V, 0.387V, 86.13%; 0.3299V, 0.328887V, 0.34123V, 97.68%]
3. Each conductor of a three phase overhead line is suspendedfrom a cross armof a steel tower by
a string of 4 suspension insulators. The voltage across the second unit is 14.2kV and across the
third 20kV. Find the voltage between the conductors and the string efficiency.
[131.79kV, 62.78%]
4. In a string of three insulator units, the capacitance of each unit is C, from each conductor to
ground is C/3, and from each connector to the line conductor is C/5. Calculate the voltage across
each unit as a % of the total voltage. To what value the capacitance between the connector of
the bottom unit and the line has to be increased by a guard ring to make the voltage across it
equal to that across the next higher unit? [31.5%, 28.3%, 40.196%; 0.6833C]