In most cases a three-phase energization or reclosing of a
power line produces switching overvoltages on all three phases. The overvoltage
generation depends on the circuit breaker; some of the circuit breakers may not
generate any overvoltages.
A typical example is the circuit breaker that closes when
the voltage across the contact is zero. The overvoltage calculation has to
consider trapped charges left on the phases without fault in case of high-speed
reclosing.
In the worst case each switching operation produces three
phase-ground and three phase-phase overvoltages. Two methods are in use for
characterizing the overvoltage probability distribution function; the case-peak
method and the phase-peak method are described as follows:
Case-peak method:
From each switching operation, the highest crest overvoltage
of the three overvoltages is selected and included in the probability
distribution. Each switching operation contributes one value to the overvoltage
distribution.
This method is based on the observation that the inclusion
of the other two values does not significantly affect the probability of
flashover. This results in the distribution of switching surge overvoltages per
three-phase energization or reclosing operation, and is used to calculate the
probability of flashover per three-phase switching operation..
Phase-peak method:
From each switching operation, the crest switching
overvoltage on each of the three-phases is included in the probability
distribution. Each operation contributes three crest values to the probability
distribution.
This results in a per phase distribution of overvoltages
that can be used to calculate a per phase probability of flashover for the
switching operation. These methods are used to determine both a phase-ground
and phase-phase overvoltage distribution.
Because the probability of positive and negative polarity
overvoltages is equal, these methods consider only the absolute value of
overvoltages, regardless of the polarity. The effect of negative and positive
polarity is considered in the calculation of the switching surge flashover
rate.
Line switching overvoltages may be reduced through the use
of
a) Preinsertion resistors on the circuit breakers,
b) Controlled closing of the breaker, or
c) Surge arresters.
A preinsertion resistor and controlled closing reduces the
overvoltage along the entire line. Surge arresters only reduce the overvoltages
close to the arresters.
However, within a substation, arrester separation effects
may be neglected, i.e., the switching impulse voltage is approximately the same
throughout the substation. Thus, an arrester provides protection to all
connected equipment.
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