Partial discharge of oil-paper insulation under DC voltage superimposed AC voltage

The insulation of converter transformer is stressed by AC and DC voltages. To investigate partial discharge (PD) characteristic of oil-paper insulation, this paper designed a power source system of DC superimposed AC voltage. The testing sample is an air gap model in the middle of an insulation cardboard. A broadband current transformer is used to measure PD current recorded by an oscilloscope with a bandwidth of 5 GHz. Experiments are performed under AC, DC only, and DC superimposed AC voltages with the sample immersed in transformer oil. using electrode configurations that is parallel plate. The single PD pulses are anglicized by ORIGIN software. It is realized that the impulse current signals generated due to partial discharges under AC and DC voltages are different with amplitude, rise time, counts, duration and energy content PD pulse under AC voltage is of long duration pulse compared to that generated under DC voltage, and the PD pulse under DC superimposed AC voltage is stochastic. The key point is the moment (phase position) PD occurring moves to the zero point of the applied voltage than the case only under ac voltage. These will contribute to the development of effective PD monitoring systems for oil-insulated converter transformer.