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Analysis of Electrical Tree Growth Through Dielectric Interfaces

Pattouras, Michalis

[Thesis]. Manchester, UK: The University of Manchester; 2016.

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Abstract

Electrical trees have long been the interest of the electrical insulation community due to their role in power systems equipment failure at locations where high divergent fields might arise due to impurities, contaminants or voids. Even through trees take a long time to grow in real life, they can be grown experimentally in shorter times under various conditions so that their growth characteristics can be investigated. Different samples have been fabricated to investigate the effects of interfaces in electrical tree propagation. Initially, the impact of an interface perpendicular to the electric field, and the interface position, thickness and/or composition on the polymer’s lifetime was investigated. In the results acquired, the positive impact of interfaces positioned perpendicular to the electric field was evident: increasing the samples’ time to breakdown as well as the electrical tree inception time. Due to the encouraging results, further investigation has been focused on interface modification and how this might be used to control the electrical tree growth as well as the samples’ time to breakdown. Altering the interface’s surface roughness using a number of different methods was carried out. Results were graphically and statistically analysed so that the any conclusions are robust, and uncertainties clear. The statistical analysis used by generating regression model equations was a novel method to predict how different electrical tree parameters were affected/affecting by others. In this way the dielectric’s lifetime could be predicted with a certain level of confidence. The modification of the interface by coating the surface with either a thin layer of pure or nano-filled (hexagonal Boron Nitride) epoxy resin resulted in it being impervious thus preventing the electrical tree to propagate through it. This was a novel method that showed that specific modification methods can significantly enhance the dielectric’s lifetime when applied appropriately. Details of new sample fabrication techniques are described which enable better control of the materials and interfaces, and data on tree length growth characteristics are discussed.