Chopped partial discharge sequence

Abstract

In order to investigate partial discharge behavior, its mechanism and dynamics, a novel methodology – chopped partial discharges – has been elaborated. Typically, either during laboratory measurements or at on-site field testing, partial discharges are detected by applying a continuous sinusoidal excitation, most regularly 50/60 Hz or other frequency according to certain standards. A chopped partial discharge sequence consists of a number of packets of a base waveform, separated by a variable time delay. The base waveform may be sinusoidal, square-like, trapezoidal or with the different shapes reflecting various switching phenomena and stresses occurring in insulation systems. In this paper, a Chopped Partial Discharge approach is applied in the investigation of the internal mechanism inside voids, especially with respect to void surface conditions and charge decay phenomena. Two types of voids were investigated, the first having a semiconductive surface and the second embedded in epoxy resin. In order to properly understand the mechanism of partial discharge in a void, it is necessary to understand the field created by surface charges which remain after previous discharge events. Such deposited charges create an internal field, which interplays with superimposed external electric field in terms of the discharge inception conditions. The decay of the field created by charges results in a higher inception voltage, as this field component is in phase with the external background field immediately after the reversal of the polarity of the voltage. As a result of such compensation, partial discharge inception occurs at a different phase angle of the external background field relative to the equivalent inception angle for a continuous sequence. The chopped partial discharge method creates potential for various investigations in high voltage insulating systems.