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.

Surge arresters are usually installed phase-ground. If metal-oxide arresters are used to limit phase ground switching overvoltages to a level lower than about 70% of E2 (2% probability), the phase-phase overvoltage level will reach about twice the protective level of the arrester.

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