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True/false: cross-arm length and string efficiency

Short answer

TRUE. Increasing cross-arm length reduces pin-to-tower shunt capacitance C1, lowering the ratio m=C1/C, making voltage distribution more uniform and thus raising string efficiency.

State whether the following statement is TRUE or FALSE. Also give the justification: Increasing the cross-arm length, string efficiency can be increased.

TRUE.

Justification

String efficiency is

η=Voltage across stringn×Voltage across the disc nearest the conductor\eta = \frac{\text{Voltage across string}}{n \times \text{Voltage across the disc nearest the conductor}}

Unequal voltage distribution, and hence low efficiency, is caused by the shunt capacitance C1C_1 between each link pin and the earthed tower or cross-arm. The ratio

m=C1Cm = \frac{C_1}{C}

(shunt to self capacitance) decides the unevenness. As mm decreases, the voltage across the units becomes more uniform and the efficiency rises. For example, for a 3-disc string:

mmString efficiency
0.2≈ 78%
0.1≈ 87%
0.05≈ 93%

Increasing the cross-arm length places the string farther from the tower body. The pin-to-tower capacitance C1C_1 falls, so mm falls and the string efficiency increases.

Limitation: a much longer cross-arm makes the tower heavier, costlier and wider, so it is used only within practical limits. Other methods, such as capacitance (static) grading and guard rings, are also used.

✓ Answer checked by Claude Opus 5.5 (high effort)

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