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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