Effects of Surface Charge on Partial Discharge Characteristics in a Cavity at Very Low Frequency Excitation

Partial discharge simulation is an alternative approach to investigate physical discharge phenomena in a dielectric material. In this paper, measured experimental results of partial discharge events are used to fit simulated discharge data developed in a finite element method based software at very low frequency and power frequency. Simulation results show that, at very low frequency, charge decay has significant effects on discharge behaviors although the cavity surface conductivity is relatively low compared to that for power frequency. On the other hand, under power frequency, accumulated charges are hardly decayed between consecutive discharges even though the conductivity of cavity surface is relatively high.

I. Introduction

Partial discharge (PD) tests are considered an effective diagnostic tool for assessment of the health of electrical insulation [1 , 2] . This test is generally performed at power frequency (50/60 Hz) to emulate normal high voltage operating conditions. It is, however, problematic to conduct such a test onsite as a significant reactive power is required for testing high capacitance objects such as high voltage cables. An alternative approach to overcome this obstacle is to conduct the test at very low frequency (VLF) [3 , 4] . Measured PD data have shown that the discharge characteristics at VLF were quite different from those at power frequency [5 –10]. Therefore, a detailed research of PD characteristics at VLF excitation by comparing measured results and simulated data facilitates development of a realistic model to describe physical activities in the insulation material during the course of a PD occurrence.