Vacuum SwitchgearMV AC Switchgear

Non-sustained disruptive discharge (NSDD) definition in switchgears according to IEC

NSDD has been seen on vacuum circuit breakers, may occur during capacitive current switching and short-circuit breaking current tests,but also at lower currents and voltages. NSDDs have not been identified in actual service.
Disruptive discharge associated with current interruption, which does not result in the resumption of power frequency current or, in the case of capacitive current interruption does
not result in current in the main load circuit.
An NSDD exhibits itself as a partial voltage change. Such changes can sometimes be clearly seen with normal time resolution measurements. This is particularly true in three-phase tests when the same polarity and magnitude of voltage change is observed in all three phases as the result of a shift in the voltage of the parasitic neutral capacitance to earth of a non-solidly earthed load produced by an NSDD. Example shown in Fig(left figure).
In other cases, a clear identification of an NSDD may require a high time resolution measurement to observe the high frequency current or voltage pulse of an NSDD as seen in
Fig(right figure)
NSDDs does not affect the performance of the switching device.


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