Impulse ageing of polymeric materials

Abstract

Impulse over-voltage is a common phenomenon in electric power systems. A switching impulse is created by a switching surge or local fault while a lightning impulse is due to direct lightning strike to high voltage plant such as an overhead line. Both impulse events create travelling waves in the system, damaging insulation components and equipment. This work is concerned with the hypothesis that lightning impulses can lead to accelerated ageing of extruded polymeric cables. The results show that there may well be a reduction in electric field strength of the insulation of a power cable that experiences a lot of impressed lightning impulse over-voltages. Pre-designed shaped polyethylene material sample discs have been manufactured using a mould tool. The samples then have been electrically aged using an impulse generator. A real-time software based monitoring tool has been designed to control the impulse wave-shape and process the measurement data. Sets of identical lightning impulses were applied to samples and this was then followed by ramped AC breakdown tests. The obtained results were analyzed using the Weibull distribution to identify any differences in lifetime between aged and un-aged samples. This thesis also provides insight into the dominant ageing processes through the analysis of dielectric spectroscopy and space charge measurement data. In order to quantify the effects of dielectric ageing due to impressed lighting impulse over voltages, experiments have also been undertaken using samples that have been aged under UV light and thermally. Analysis of obtained results reveals that mechanisms of these two ageing processes are significantly different from the mechanisms due to lightning impulse ageing

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

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