Non-sustained disruptive discharges: test experiences, standardization status and network consequences

In the test practice of KEMA 32% of the vacuum circuit breakers offered for testing in 1999 have shown non-sustained disruptive discharges (NSDD), mostly within 300 ms after interruption. This implies, that although current has been interrupted, very brief periods of conduction occur, which seems to be an accepted inherent property of vacuum switching devices. NSDDs occurred in the complete range of rated voltages (12 kV-50 kV) and short-circuit breaking current (12 kA-40 kA) of the tested devices. No clear correlation of NSDD occurrence with arcing current could be established. In 79% of the cases, less than four NSDD occurrences per test report were counted. It is demonstrated from short-circuit current test data that during the occurrence of NSDD, a much larger part of the circuit is contributing to transient phenomena than just the "local" parasitic in the immediate vicinity of the breaker. This is derived from the measured duration of the conducting period during NSDD and contradicts the general notion that NSDD is just a local phenomenon, the duration of which is entirely determined by the parameters of the switching device itself. The new standard IEC 62271-100 for circuit breakers makes no distinction between (a) capacitive re-strike (late breakdown leading to polarity reversal of the load capacitor) and (b) NSDD in the case of a vacuum switching device breaking down more than half a power frequency cycle after capacitive current interruption. It is demonstrated that over-voltages that accompany such "capacitive NSDDs" certainly can not be neglected.