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Ageing in Epoxy Resin as a Precursor to Electrical Treeing

Mcdonald, Harry

[Thesis]. Manchester, UK: The University of Manchester; 2020.

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Abstract

A common picture of electrical treeing is of a tree forming; generating channels which then steadily grow until it reaches a runaway stage of rapid growth as the tree approaches the ground conductor. In numerous cases however stages of treeing have been identified which do not conform to this picture. Properties such as tree chemistry, partial discharge magnitudes and phase dependency, channel widths, tree structure and growth speeds are regularly found to be dependent upon the radial extent of the trees. Also identified are changes of channels with time; such as when radial extension of channels is halted, giving way to channel widening, darkening or even the formation of new micro-channels. Each time the properties of the trees change, we are observing differences in the underlying process or processes which produce the electrical trees. This may be a temporary change, such as when channel growth halts only to restart after a waiting period. Or it can indicate a more fundamental shift in the nature of tree growth has occurred. The objective of this project was to better understand electrical treeing at a more fundamental level. Electrical treeing tests are typically performed using needle-plane electrodes however this configuration will influence and change the properties of growth. To recreate the conditions of electrical treeing in cables, a plane-plane electrode configuration is tested; utilising a number of sample ageing techniques in attempt to accelerate tree initiation and growth. A localised breakdown was formed using these methods; however it was not repeatable nor was tree growth found to occur following it. These tests identified the need to determine ageing prior to tree initiation in long term tests in which channels do not quickly form. A new chemical analysis technique is applied to electrical treeing for the first time, developing new insight into the nature of this ageing. Atomic force microscopy with infrared spectroscopy (AFM-IR) allows chemical characterisation with spatial resolutions of 50 nm. Using this an early stage channel is found to have formed without producing chemical degradation; based on this it is proposed tree initiation in this case was most likely the result of electromechanical fracturing. Meanwhile the ageing which precedes initiation is found to be more heterogeneous than previously believed. In the study of mature channels it was identified that channels at different distances from the needle tip have distinct chemical signatures. Using these results, along with results and discussions from the literature, electromechanical fracturing is proposed to be active in electrical tree growth in epoxy. The power of the AFM-IR, demonstrated by these results, along with a large range of potential applications means its use is highly recommended in the future study of electrical treeing. The results obtained here should also be considered in the understanding of tree formation and growth.