Characterising Resistance to Electrical Treeing in New XLPE-Based Materials for High-Voltage Cables

Doktorsavhandling, 2015

This work looks into electrical treeing in cross-linked polyethylene-based materials (XLPE), where the ability to resist tree development is considered as an indicator of improved applicability for insulation in high voltage power cables. The thesis presents results of a twin PhD project involving joint work of two students. One of the students (author of this thesis with a background in electrical engineering) concentrated on developing new methodology for testing the resistance to electrical treeing in XLPE modified by addition of voltage stabilising agents, while the design and syntheses of these agents was the domain of the second student, Markus Jarvid (having his background in chemical engineering). The latter work has been published in a separate thesis. In contrast to the traditionally used needle-electrode test objects, a wire-electrode geometry has been introduced for creating the highly divergent electric field distribution in the insulation, necessary for initiation of an electrical treeing process. The wire-electrode object has shown a benefit of producing several trees during each individual test, therefore providing more data for further analyses. These objects also allow for exposing a larger volume of the material and let trees incept in weak-spot locations. How to conveniently analyse the resulting treeing data from performed experiments is further analysed in the thesis. To increase the applicability of the elaborated methodology for testing materials characterised by different degree of transparency, optical observations of the treeing process were complimented by simultaneous detection of partial discharges (PDs). Analyses of the latter have allowed an interpretation scheme practical for measuring resistance to treeing in non-transparent materials. To evaluate how voltage stabilisers influence the resistance to treeing in XLPE, a broad range of stabilisers have been identified and tested. A detailed analysis demonstrates a positive effect imposed by addition of 4,4’-didocyloxybenzil. This compound is thereafter compared with the effect of various other stabilisers, including benzil-, thiaxanthone- fullerene- and melamine-types. Finally the stabiliser efficiencies are correlated with molecular ionisation potential and electron affinity, where it has been found that the stabilising efficiency increases with electron affinity of the added molecules. The results from this twin PhD project opens up a possibility to design new types of practically useful voltage stabilisers and test their suitability for improving the insulation in future high voltage cables.