Analysis of Chopped Partial Discharge sequence

In order to investigate partial discharge 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. 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. The void embedded in epoxy resin was investigated. 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. As a result partial discharge inception occurs at a different phase angle of the external background field relative to the equivalent inception angle for a continuous sequence.