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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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By which of the following methods string efficiency can be improved?
Using long cross arm
Grading the insulator
Using a guard ring
Any of the above
Any of the above
String efficiency in a suspension insulator string is a measure of the uniformity of voltage distribution across the individual discs. Since the discs near the cross-arm have higher shunt capacitance, the voltage is not distributed equally; the mentioned methods are standard techniques to equalize this distribution.
String efficiency in a suspension insulator string is a measure of the uniformity of voltage distribution across the individual discs. Since the discs near the cross-arm have higher shunt capacitance, the voltage is not distributed equally; the mentioned methods are standard techniques to equalize this distribution.
η=n×Vdisc(max)Vstring×100% — Definition of string efficiency where n is the number of discs
Iguard≈Ishunt — Principle of guard ring to nullify shunt capacitance effects
The unequal voltage distribution is caused by shunt capacitance between the metal links and the grounded tower. By using longer cross-arms, we reduce this shunt capacitance. Grading insulators means using discs of different capacitances, and a guard ring compensates for the shunt capacitance by providing an opposing charging current, thereby balancing the potential gradient.
The disc nearest to the conductor experiences the maximum electrical stress.
String efficiency is always less than 100% in practice.
A perfectly uniform distribution results in 100% efficiency.
Reduces the risk of flashover across the bottom disc.
Increases the lifespan of insulators by preventing localized dielectric breakdown.
Guard rings add mechanical weight to the cross-arm.
Grading requires maintaining a inventory of different types of insulator discs.
High Voltage (HV) and Extra High Voltage (EHV) transmission lines.
Suspension insulator strings for long-span transmission towers.
String efficiency tends to decrease as the number of discs in a string increases.
Option A reduces Cshunt, Option B modifies Cself, and Option C provides a capacitive compensation current.
D is correct — All listed methods help in neutralizing or minimizing the effect of shunt capacitance, which is the primary cause of uneven voltage distribution across the insulator string.
Always remember that the disc closest to the transmission line conductor carries the maximum voltage; anything that balances the potential across all discs increases string efficiency.