PARASITIC ARCING IN EHV CIRCUIT-BREAKERS

The normal operation of an EHV circuit breaker involves interrupting a fault current (less-than-or-equal-to 63 kA) at power frequency (50 or 60 Hz) while having the capability to withstand rapid voltage restrikes between the interrupter contacts when fully open. Also, on closing the contacts, some limited prestriking may be tolerated. There is increasing evidence that departures from such operation may occur under some conditions in service. For instance, high-frequency arcing may occur when interrupting inductive loads, and late breakdowns across the PTFE nozzle have also been observed. The paper describes some basic experiments to investigate conditions within an interrupter unit which might explain some of these phenomena. It is demonstrated that charge may be induced on the surface of PTFE nozzles used in such circuit breakers and that this charge invariably appears to be of a negative polarity and accumulative. It is shown that the presence of the surface charge can lead to a reduction in the effective dielectric strength of the circuit interrupter. More seriously, there are situations where there is a probability of the breakdown path not being directly between the interrupter contacts within the nozzle, where proper discharge control might be executed, but rather external to the nozzle with potentially disruptive consequences.