Ultra-high frequency spectral characteristics of partial discharge in insulation oil

We have designed and manufactured four types of typical defects found in power transformers to simulate partial discharge (PD) in oil insulated power transformers. Stable PD signals were generated by applying AC voltages on to the PD cells. We measured spectral response of PD signals from these defects using our Ultra-high frequency (UHF) PD measuring system and apparent discharge using IEC60270 PD detector simultaneously. Partial discharges from the defects in oil generated well detectable UHF signal up to about 2 GHz. UHF PD patterns were recorded and analyzed for each defect types. The phase resolved partial discharge (PRPD) patterns of defect obtained from UHF range were different from each other. The patterns of UHF PD signals were also different from those obtained from that of conventional method. Type of defect could be identified by obtaining PRPD patterns.