Propagation characteristic study of partial discharge UHF signal outside transformer

In the UHF detection of power transformers, the main method is detecting the UHF signals through UHF sensor in the transformer. The sensibility of this method is high but mounting way is trouble. Field detection and lab testing show that the UHF signals can diffract from the transformerpsilas joint. Some PD detector using external detection has used in field. But, the difference of signals in transformer and external transformer is not clear. In this paper, the feasibility of using external UHF sensor for detecting partial discharge activities is discussed, and then external propagation characteristic of UHF E-M wave is studied by lab experiments. Wavelet packet analysis of signals show the high frequency of external signals is bigger than inside signals. The relationship of distance of joint and UHF sensor is studied in this paper. The results show that UHF signals could leak outside the transformer through the joints after multiple refraction and reflection in the transformer tank. An antenna outside the tank could receive the signals. Study result will supply some signal acquisition direction for the UHF detection transformer PD.