Partial Discharge Characteristics when Applying Ramp Up and Down Voltage Waveform to an Oil-Gap Electrode Simulating Oil-Immersed Film Capacitors

Oil-impregnated film power capacitors are made of polypropylene (PP) films co-wound with aluminum foil electrodes, impregnated with insulating oil. In a gas void, if any, or around the electrode edge, partial discharges (PDs) occur, leading to PP film deterioration and eventually to dielectric breakdown. To elucidate the PP film deterioration mechanism, we have investigated PD characteristics using the electrode system that simulates an oil-impregnated film capacitor (oil-gap electrode system). In this study, by applying a ramp up and down voltage waveform to an oil-gap electrode system, we have successfully observed that gas voids form in an oil gap when PDs occur and then PD continues inside the remaining voids, without reaching immediate dielectric breakdown. The observation results clarified the temporal changes in PD numbers and the gas void size within 400 ms of the ramp up and down voltage application.