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DSSSB JE EE 2019 Official Paper (Held on 25 Oct 2019)

Option 2 : 45 kV

__Concept:__

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.

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

For the adequate performance of the transmission line, it is essential that the voltage distribution across the line should be uniform. This can be achieved by the following methods.

1) Use of the longer cross arm

2) Capacitive grading

3) By using grading rings or static shielding

__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**