Ramp Technique for DC Partial Discharge Testing

A stepwise ramp technique has been used successfully in direct current (dc) partial discharge (PD) testing on high voltage components and on resin-packaged HV devices such as the Faint Object Camera for the Space Telescope. PD data is acquired separately on part-way ramps to rated voltage and on the intermediate voltage plateaus. For test specimen intended for dc service, this ramp method gives more information about insulation integrity than purely quiescent dc measurements. This is especially true for specimens of high resistivity which causes the discharge frequency to be deceptively low at constant dc voltage. By contrast, during ramping upwards the voltage distribution is capacitive, and the PD behavior resembles that of an ac test. This gives many more pulses in the voids, and without the undesirable heat produced if 60 Hz ac were applied. PD histograms are presented on various materials, with and without intentional defects. Influence of test parameters such as ramping speed are discussed. Experience with a variety of samples and commercial HV capacitors, as well as their life test performance, can lead to approximate acceptance/rejection criteria.