Partial Discharge Characteristics in an Artificial Air-filled Void under Superimposed Sinusoidal Voltages at LN₂ Temperature

Partial discharge (PD) characteristics within an artificial air-filled void contained in a solid insulator were experimentally investigated under superimposed sinusoidal voltages of 60 and 600Hz at liquid nitrogen temperature (77K) to find the effects of the waveforms of distorted sinusoidal voltages on PD characteristics at 77K. The results show that PD inception voltage at 77K is independent of the applied voltage waveform. The results also show that, when the 600Hz component of the superimposed voltage is increased with its constant peak value, PD intensity seems to be independent of the voltage waveform, whereas PD pulse number increases. Since PD charge magnitude and pulse number at 77K were remarkably different from those at 298K, their mechanisms are discussed based on the observation results of residual charge distributions remaining on a void surface subjected to PDs under 60Hz ac voltages at both temperatures.