An investigation into electrical degradation mechanisms within air-filled cavities in solid dielectric materials

Abstract

Degradation and failure mechanisms in electrical insulation systems (EIS) in high voltage apparatus are not only caused by the thermal and mechanical stresses but also by the electrical stress during operational service. Imperfections in terms of cavities, craters, contaminants and electrical trees might seriously affect in insulation material of HV equipment due to the changes in microstructural levels at these defects under the enhancement of electric fields. Air-filled cavities embedded in solid dielectric material have been considered to be one of the most complicated issues that partial discharge (PD) activity plays an important role, which induces progressive deterioration processes in the cavity from localised erosion to complete failure. Phase-resolved partial discharge (PRPD) analysis is an effective condition monitoring tool for diagnosing the identifiable stages of degradation mechanisms in the relationship between PD characteristics and morphological changes in an airfilled cavity within the insulation.

The main aim of this thesis is to experimentally investigate the progressive degradation mechanisms within an air-filled cavity embedded in polymers by PD analysis. The samples used in the PD experiment are made from two different shaped cavities embedded in solid polymers regarding a cylindrical flat-shaped cavity and a spherical-shaped cavity. Based on the results, the recognisable
variation in PRPD patterns and PD statistical quantities can be directly linked to distinguish the progressive levels of localised erosion on the surface wall inside an enclosed cavity in terms of surface erosion, erosion depth, electrical tree growth and upcoming failure. Interestingly, the results also show that the distinct PD characteristics can be analysed to accurately validate the presence of
corrosive by-products regarding micro-craters and electrical tree initiation within the cavity wall before the treeing growth progressively propagates towards the whole dielectric between parallel electrodes, leading to total failure. The findings from these studies are further examined to explore the microscopic structure on the cavity surface due to accelerated ageing processes by various
analytical techniques of microanalysis instruments used for additional prognoses in terms of physical, thermal and chemical properties. In particular, the noticeable PRPD patterns and PD diagnostic quantities, i.e. the average apparent charge, the maximum apparent charge and the average number of PDs per cycle are reproduced through a PD model with good agreement between measurement
and simulation results.

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Identifiers

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

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