Distinction of the partial discharge source in oil by two frequency correlation method

A method based on frequency characteristics to distinguish partial discharge (PD) source in oil filled transformer is proposed. Frequency spectrum of electromagnetic wave from two typical PD sources, i.e., protrusion of metal and air void in oil was investigated. It was found that metal protrusion gave PD signal with stronger component in the frequency range above several hundreds MHz compared with air void. Since the acquisition of frequency spectrum is difficult in practical transformer test condition, we tried to apply the two frequency correlation method to distinguish the two PD sources. In the method, intensity of two frequency components of each PD signal is plotted on the x-y plane, where the horizontal axis represents the intensity of low frequency band and vertical axis represents that of high frequency band, which is a method to characterize a PD source on the x-y plane. The two PD sources were plotted clearly on the different area of the two frequency correlation graph, that is, metal protrusion yielded PD signal with higher frequency component than air void. We concluded that the method is effective to identify the PD source in oil filled transformer.