“Nonsustained disruptive discharges (NSDD)”-new investigation method leading to increased understanding of this phenomenon

NSDD are an intrinsic feature of vacuum circuit breakers and their distinctive properties are due to the excellent ability of a vacuum gap to interrupt high frequency currents. Until recently they could not be detected during short circuit testing due to their frequency band being beyond that of phenomena associated with standard 50 Hz testing. However, once discovered their occurrence has been interpreted as a sign of excessive distress of a vacuum interrupter. This paper discusses work carried out to improve the understanding of this phenomenon so that an informed judgement of the real effects of this may be made. The first part describes a simple test methodology, which has been developed to enable systematic investigations of NSDD without requiring expensive testing at high power test laboratories. The second part describes and discusses voltage and time dependencies of NSDD on a large variety of circuit breaker and interrupter related properties. As a result it can be stated, that the NSDD observed are caused by particles released due to mechanical vibrations during the opening operation of the circuit breaker, and that their probability increases significantly with increasing system voltage. However, there is no direct connection between NSDD and the short circuit performance of a vacuum interrupter. The NSDD behaviour is instead strongly dependent on material, surface finish and cleanliness conditions of the individual interrupter components as well as on the manufacturing method of the vacuum interrupter itself