Degradation by partial discharge in cavities under AC electric field

Abstract

Partial discharge (PD) is a localized electrical breakdown event inside solid or fluid insulating materials. PD does not link the two conductors directly. However, it can cause damage to the insulating material, and eventually build up a damaged channel leading to final breakdown.

In general, PD events can be classified based on their discharge mechanisms, such as surface discharge, corona discharge, and free space cavity discharge which is the main focus of this work. Cavity discharge studied in this work is generated by an air-filled spherical cavity inside the insulating material, which is epoxy resin. Such small cavities exist in many high voltage equipment where polymeric insulating materials are involved. Detection of cavity PD is generally believed to be very concerned by industrial operators, and sometimes an immediate replacement of the plant is required.

However, with the growing understanding of material degradation induced by cavity PD, it is possible to assess the degradation level by PD behaviour. By understanding the link between PD behaviour and degradation level, equipment’s’ service life may be extended to their maximum.

Cavity PD in epoxy resin was found to develop itself through five stages in this work, defined by clear variations in terms of their phase resolved partial discharge (PRPD) patterns. Optical and scanning electron microscopic observations at degradation surfaces also confirmed the physics behind PD stage transition. Previous researchers usually focus on PRPD patterns against cavity air consumption for short life materials such as polyethylene, or singular pulse characters against degradation surface with long life materials such as epoxy resin. In this work, the feedforward (usually termed as feedback by other researchers, however, considering the fact that after each discharge event the cavity suffers deterioration and become a slightly different system, feedforward is a more accurate term from system control theory) or memory effect between adjacent pulses, short term surges, and long term behaviours were the main focus of discussion. The cavity losing its feedforward capability or memory capacity, as well as PD events getting localized onto specific damaged points, are experimentally shown to be the driving force from one degradation stages to another showed by PRPD pattern transitions.

Results achieved in this work successfully established the link between various aspects of information which were usually studied in a less interactive manner. The results suggest that accumulative PD behaviour transition along degradation time is governed heavily by the cavity surface condition, the two of which form a closed loop. Pulse train analysis studies the capability of feedforward capability of degraded cavity surfaces. Both PRPD analysis and pulse train analysis take step changes in a synchronized manner with the discovery of new-born surface condition features. Some of the surface condition features can induce later stage discharge patterns such as swarming and treeing (“wing-like”) patterns, feedforward capability study of such patterns can directly be utilized to determine degradation level at such surface locations.

Such information could be used to qualitatively, and quantitatively determine the level of degradation induced by cavity PD. At this stage, PRPD patterns can be used to predict degradation surface conditions, and a number of factors related to the PD feedforward capability of such degraded cavities are found. The feedforward capability was never investigated to such depth, and was never used as a condition monitoring tool in the past. This thesis contributes towards a new way of thinking for cavity PD degradation assessment, and if materialized, a longer and more reliable service life of polymeric insulating materials if cavities are involved.

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Identifiers

ORCID for Paul Lewin: orcid.org/0000-0002-3299-2556

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