Question

A string of \(3\) discs can withstand a total voltage of \(25.76\ \text{kV}\). If each unit can individually withstand a maximum of \(11\ \text{kV}\), then the string efficiency is

• A. \(68.75\%\)
• B. \(65\%\)
• C. \(78\%\)
• D. \(85\%\)

Hint:

String efficiency compares actual string voltage with the ideal voltage sharing capacity of all discs.

Answer 1

The Correct Option is C

Concept: String efficiency is: \[ \eta=\frac{\text{Voltage across whole string}}{n\times \text{maximum voltage across one disc}}\times100 \] where \(n\) is number of discs.

Step 1: Given: \[ n=3 \] Total voltage across string: \[ V=25.76\ \text{kV} \] Maximum voltage per disc: \[ V_{\max}=11\ \text{kV} \]

Step 2: Apply string efficiency formula. \[ \eta=\frac{25.76}{3\times11}\times100 \] \[ \eta=\frac{25.76}{33}\times100 \] \[ \eta=0.7806\times100 \] \[ \eta\approx 78\% \] Therefore: \[ \boxed{78\%} \]