Propagation Characteristics of Partial Discharge Radiated Electromagnetic Waves for Insulation Diagnosis of Oil-Filled Bushing

Abstract

Currently, as a failure event due to deterioration of the oil-filled transformer, the existence of defects due to oil-in bushing has been confirmed. On the other hand, insulation diagnostic techniques for oil-filled bushings have not yet been established. The purpose of this research is to develop reliable insulation diagnostic technique for the oil filled bushing by measuring partial discharge (PD). Focusing on electromagnetic wave (EMW) radiated by partial discharges generated in oil-filled bushings, we have studied its propagation characteristics using finite-difference time-domain (FDTD) analysis software. PD source can be located from the frequency characteristics of EMW leaking to the outside of the condenser type oil bushing. Simulation shows that PD source located at the top of the bushing emits EMW leaking to the outside the bushing as if the bushing acts as a coaxial cylindrical waveguide consisting of center conductor, capacitor foil and porcelain insulator. On the other hand, PD source at the bottom of the bushing causes the resonance frequency to appear as a coaxial cylindrical waveguide inside the bushing, whereas the resonant frequency appears strongly in the rectangular cavity resonance arising from the tank size. The above results suggest that PD signal and external noise can be distinguished by estimating the frequency maximum of the EMW leaking strongly to the outside.