A string insulator having 4 units. 33.33% of voltage of the total voltage is across the bottom most unit. Its string efficiency is

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 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 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 = 0.33V

Number of insulators (n) = 4

String efficiency \( = \frac{V}{{4\; \times\; 0.33V}} \times 100 = 75\% \)