Why does Grading Capacitor use in Circuit Breaker?

Grading capacitor is commonly used in High Voltage Circuit Breaker for uniform voltage distribution across the Breaker contacts at CB open position. In a multi-break Circuit Breaker, Grading capacitors are connected in parallel with every break of the CB.

Reasons for using Grading Capacitors in Circuit Breakers.

When the capacitors are connected across the breaks of a Circuit breaker, there is no current flow through the capacitor units. It will equally distribute the voltage across the breaks. The Grading capacitors are used in Circuit Breaker for the following reasons.

Grading Capacitor in Circuit Breaker
Grading Capacitor in Circuit Breaker
  1. If any of the breaks are not closed in the multiple breaks circuit breaker, the system voltage can appear across the open break and it will fail due to the designed voltage of the break. By using a grading capacitor, the failure of the break can be avoided in that condition due to the uniform distribution of voltage across the breaks.
  2. During switching of Reactor or any inductor unit, Restriking voltage will generate across the breaks of Circuit Breaker. Restriking over voltage may lead to failed Circuit Breaker. By using the Grading capacitor in Circuit Breaker, over-voltage stress can distribute across the breaks and it will save the Breaker from failure.

Use of Grading Capacitor in Circuit Breaker

Grading capacitors are generally used in 400KV and above voltage level circuit breakers. In the 765KV Circuit breaker, always grading capacitors are used. There are 04 nos. of Breaks available in 765KV Circuit Breaker and Grading capacitors are used for the equal voltage distribution to avoid failure of the CB.

At the 400KV level, generally grading capacitors are used for switching the Reactor units. During the opening of the circuit breaker of the reactor, there is high restriking overvoltage generated across the CB and to minimize the effect, gradin capacitors are used.

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