ALTERNATING CURRENT (AC) INTERRUPTION OF POWER CIRCUIT BREAKERS BASIC INFORMATION



How Power Circuit Breaker Interrupt Alternating Current?

AC interruption occurs at current zero. During the following half-cycle, the recovery voltage will build up across the circuit breaker main contacts. The typical appearance of recovery voltage will differ in inductive, resistive, and capacitive circuits (see Fig. 10-62).

  
FIGURE 10-62 Typical shape of recovery voltage on interruption: (a) induction current; (b) resistive current; (c) capacitive current.

When opening an inductive circuit, the recovery voltage will rise suddenly at a high rate because current interruption occurs at the moment of system voltage peak. This case requires fast building of dielectric strength of the open contact gap.

When interrupting resistive load, current and voltage pass through zero at about the same moment. The recovery voltage will therefore rise at a moderate rate and no particular problems are imposed on the circuit breaker.

At the moment of interruption of capacitive current, the capacitance is fully charged. The recovery voltage rises slowly during the first half cycle but continues to rise to a value twice the system voltage.

This may lead to restrikes, undesired network oscillations, and over voltages. At the moment of fault current interruption the two sections—source side (S) and line side (L)—of the network are decoupled and oscillate independently about their driving voltage.

The difference of these two transients appears across the open contacts of the breaker pole. The behavior of this transient recovery voltage is determined by the circuit parameters.

The still-moving or already fully-open breaker contacts must be able to withstand the recovery voltage.
The most severe stress for the open contact gap is the initial peak and the rate of rise (kV/#s) of the recovery voltage.

If the recovery voltage exceeds the gap insulation, the arc will restrike and current will continue until the next current zero, when interruption will again be attempted. The rate of rise of recovery voltage is a function of the constants of the circuits which supply power through the breaker.

The larger the adjacent capacitance to ground before the major inductance limiting the fault current, the slower will be the rise of the recovery voltage. Some breakers modify the recovery voltage characteristics by limiting the current, modifying its power factor, and so on.

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