Simulation of partial discharges in conducting and non-conducting electrical tree structures

Abstract

Electrical treeing is of interest to the electrical generation, transmission and distribution industries as it is one of the causes of insulation failure in electrical machines, switchgear and transformer bushings. Previous experimental investigations of electrical treeing in epoxy resins have found evidence that the tree structures formed were either electrically conducting or non-conducting, depending on whether the epoxy resin was in a flexible state (above its glass transition temperature) or in the glassy state (below its glass transition temperature). In this paper we extend an existing model, of partial discharges within an arbitrarily defined non-conducting electrical tree structure, to the case of electrical conducting trees. With the inclusion of tree channel conductivity, the partial discharge model could simulate successfully the experimentally observed partial discharge activity occurring in trees grown in both the flexible and glassy epoxy resins. This modelling highlights a fundamental difference in the mechanism of electrical tree growth in flexible and glassy epoxy resins. The much lower resistivities of the tree channels grown in the glassy epoxy resins may be due to conducting decomposition (carbonized) products condensing on the side walls of the existing channels, whereas, in the case of non-conducting tree channels, subsequent discharges within the main branches lead to side-wall erosion and a consequent widening of the tubules. The differing electrical characteristics of the tree tubules also have consequences for the development of diagnostic tools for the early detection of pre-breakdown phenomena.

Partial discharge patterns in conducting and non-conducting electrical trees

“…1c) the conducting tree structure acts as an extension of the pin tip and PD occur only in localized regions at the tree ends. Similar results were reported in [6]. Deterministic simulations of the partial discharge activity are also shown in figure 1 (b) and (d) for comparison with the experimental images.…”

“…2c and 2d, it is noteworthy to emphasize that the partial discharge rates are significantly higher while the magnitudes of the measured discharges are approximately 2-3 orders of magnitude smaller. The deterministic simulation model of PDs in electrical trees proposed in [4,6] was used to simulate the PD activity in conducting and non-conducting electrical trees and to obtain the corresponding PD apparent charge magnitudes. The PDs were simulated over 50 cycles of the applied sinusoidal voltage, so that the results of the simulation can be directly compared to the experimental data.…”

“…The apparent charge was measured using a RCL resonant circuit connected in series with a high voltage discharge free coupling capacitor. Simulations of nonconducting electrical trees, as grown in the flexible resin, were found to have typical partial discharge (PD) apparent magnitudes in the range 1 pC to 10 nC [6]. In this case, PD magnitudes were detected experimentally using a resonant RLC circuit of frequency 200 kHz and amplitudes recorded by a digital storage oscilloscope (DSO).…”

“…Deterministic simulations of the partial discharges within electrically conducting and non-conducting tree structures were based on the techniques described in references [4,6]. The simulations were based on using spheres of charge to describe the pin electrode and the induced charge at the pin-tip from charge contained within the tree structure.…”

“…Recent computer simulations of the partial discharge activity in the two types of tree structure have demonstrated a significant difference in the measured partial discharge amplitudes [6]. In this work we compare the different apparent charge measurements obtained from experiment with simulations of the partial discharge activity in both conducting and non-conducting electrical trees to obtain a better understanding of the relationship between the local extent of the partial discharge phenomena and the recorded partial discharge patterns.…”