The propagation characteristics of UHF partial discharge in power transformers with complex winding structure

The detection of Ultra high frequency (UHF) Electromagnetic wave(EM) induced by partial discharge (PD) has been applied in power transformer to find insulation defects due to such features as interference-free and abundant discharge information. However, the propagation mechanism of UHF EM in power transformer has not been detailed investigated, and there are few effective results to instruct the field detection of UHF PD and fault diagnosis. In this paper, a simulation model based on Finite-difference time-domain (FDTD) was proposed, and the propagation characteristics of UHF EM from three PD locations (interturn, oil clearance and outside winding) were investigated. The magnitude, frequency and attenuation of the UHF EM were compared after it passes through the winding. In this model, Gaussian pulse with different amplitude and width is used for equivalent PD current. Results show that the UHF EM from different positions will be damped oscillation at some characteristic frequency when it passes through the winding. The UHF PD signal from interturn has more oscillation duration than other locations. Otherwise, the UHF PD oil clearance and outside windings contain more spectrum information. A simple transfer function between the amplitude of UHF EM and that of PD current was got. It indicates that the shape and spectrum of UHF EM are independent with the amplitude of PD current. The size of winding and oil clearance is the main factor to affect the frequency of UHF signals.