Question

Two insulator discs of identical capacitance value C make up a string for a 22kV, 50Hz, single-phase transmission line. If the pin-to-earth capacitance is also C, then the string efficiency is:

• A. 50%
• B. 75%
• C. 90%
• D. 86%

Hint:

The string efficiency depends on the capacitance of the insulator discs and the pin-to-earth capacitance. With equal capacitance values, the string efficiency is typically 75%.

Answer 1

The Correct Option is B

The string efficiency is defined as the ratio of the total voltage across the string to the sum of the voltages across each disc. When insulator discs of identical capacitance are used, the string efficiency is given by:
\[ \eta = \frac{1}{\sqrt{1 + (2 \cdot C / C_1)}} \] where \( C_1 \) is the pin-to-earth capacitance, and \( C \) is the capacitance of the discs. For this case, the string efficiency turns out to be 75% when the pin-to-earth capacitance is equal to the disc capacitance. Thus, the correct answer is 75%.