Partial Discharge Detection and Analysis of Oil-paper Insulation under DC Voltage Based on UHF Method

Partial discharge (PD) was an important factor of insulation deterioration and breakdown. UHF signals with frequency ranking of 3GHz would be produced when PD occured. Five typical PD geometries including needle-plate, suspended particle, surface discharge 1, surface discharge 2 and bound cavity had been designed to investigate the characteristics of PD under DC stress. And a UHF signal detection system of PD under DC stress had been established in the experiment. A series of UHF signals of PD in oil-paper insulation were recorded by the detection system. Characteristics in time domain, frequency domain and time-frequency domain of UHF signals based on different geometries and different polarities of dc were analyzed and compared through Fourier transform and wavelet transform. Frequency bandwidths with predominant content seem to be different for UHF signals generated by different geometries. While spectrums appear similar for UHF signals generated by the same PD geometry with different polarity of DC and different scales of voltage. The distribution of signal energy looks different as time and frequency.