Question

In a 3 unit insulation wire, the voltage stress is 20 kV and the wire efficiency is 75% in the entire bottom most unit. What will be the total voltage across the entire wire?
1. 180 kV
2. 45 kV
3. 90 kV
4. 22.5 kV

Answer 1

Correct Answer - Option 2 : 45 kV

Concept:

String Efficiency (η):

The string efficiency is defined as the ratio of voltage across the string to the product of the number of strings and the voltage across the unit adjacent string.

\( \eta= \frac{V}{{n\; \times\; V_n}}\)

Where,

V is the voltage across the string.
Vn is the voltage across the bottom disc near to conductor
n is the number of the disc in a string

Calculation:

Given that,

Let the total voltage = V 

Voltage across the bottom most unit = 20 kV

Number of insulators (n) = 3

String efficiency = (conductor voltage)/(number of discs × voltage across the disc nearest to the conductor)

String efficiency \( = \frac{V}{{3\; \times\; 20\times 10^3}} \times 100 = 75\% \)

V = 45 kV