Partial Discharge analysis in a metal-dielectric air gap on machine insulation at arbitrary testing voltage

Partial Discharge (PD) tests were performed in a metal-dielectric air gap between a spherical electrode and the surface of machine insulation, which consists of mica, epoxy resin and glass-fiber. The PD activities were investigated at three types of applied voltages: triangular, square and trapezoidal waveform. In the trapezoidal waveform, the duration of the linearly rising part of the applied voltage was varied. The effect of the time derivative of the applied voltage dU/dt on the PD activities was studied. The PD activities were studied at a power frequency (50 Hz) and at a lower frequency (10 Hz) of the applied voltage. The results show that the time derivative of the applied voltage has a significant effect on the PD behavior. In each waveform, most of the PD pulses occur during the linearly rising period, but never occur during the linearly falling period. At lower frequency, compared with the positive pulses, the number of negative ones decreases dramatically with the increasing rising time, and PD activities even disappear at the triangular waveform.