CIRCUIT BREAKERS EQUIPPED WITH PARALLEL IMPEDANCE

Impedances in parallel with the arc may be either capacitors or resistors, or both, in various combinations. Although such impedances modify the shape of the specified inherent transient recovery voltage, the type and degree of modification in the synthetic test should be the same as in the direct test.

For example, the insertion of a resistor equal to the surge impedance of the line will reduce the line side rate-of-rise to half value. The effect is not as pronounced for a bus fault where a large number of lines are in parallel because their combined surge impedance is much lower than the resistance in parallel with the arc.

Where the shunt impedance is a resistor, particularly if the ohmic value of the resistor is low, the actual peak transient recovery voltage (TRV) in a synthetic test may not attain the value it would in a direct test because of the limited energy available from the voltage source.

Furthermore, the shunt resistor may cause a too rapid decay of the dc voltage following the TRV crest.

In some cases, to meet the TRV requirements of ANSI/IEEE C37.09-1979 [3], it may be possible

1) To adjust the parameters of the voltage circuit to provide the necessary additional energy absorbed by the shunt resistor

2) To switch over to an additional ac voltage source capable of maintaining voltage across the resistor.

An equivalent transient recovery voltage waveform across the terminals of the test circuit breaker can be produced by replacement of resistance at other appropriate places in test circuits.