A study of the erosion suppression mechanisms of nanofilled silicone rubber composites for HVDC outdoor insulation applications

Abstract

This thesis presents a study on the erosion suppression mechanisms of nano-sized inorganic fillers in silicone rubber composites for high-voltage-direct-current outdoor insulation applications. The DC inclined plane tracking-erosion test and the dry-arc resistance test are employed to evaluate the erosion performance of the composites in the study. Thermal and surface morphology analyses are utilized to investigate the effect of nano fillers on the DC erosion resistance of silicone rubber. This novel approach in using these techniques collectively is defined as the mechanistic-based framework for investigating the erosion suppression effects of nano fillers as critical material design aspects of DC silicone rubber outdoor insulators. The study begins by investigating the role of the filler-silicone interface on suppressing the DC erosion of silicone rubber. The influence of the interface on suppressing erosion is comparatively analyzed against the influence of enhancing the composite thermal conductivity with the inclusion of thermally conductive fillers. Fumed silica, nano alumina tri-hydrate and thermally conductive sub-micron hexagonal boron nitride are filled in silicone rubber as part of the study. Fumed silica in silicone rubber is found to have a more influential effect on improving its DC erosion resistance as opposed to incorporating the other fillers, despite the composites having comparable thermal conductivities. Fumed silica’s favorable interaction with silicone rubber suppressed silicone depolymerization and promoted crosslinking during DC erosion. Moreover, leakage current data indicates the slowest inception for the stable eroding arc for the fumed silica composite. This highlights the significance of the nano filler’s interface in suppressing the erosion of silicone rubber, which is more important than improving the composite thermal conductivity with thermally conductive fillers. The study investigates the correlation between the role of silica-silicone interaction and the residue shield of the composites during the DC dry-band arcing using fumed silica and ground silica fillers as part of the study. Thermal analyses indicating higher additional crosslinked residue with more filler polymer interactions with the fumed silica composite as compared to ground silica. Surface morphology of the eroded residue of composites tested under the inclined plane test indicate a vital role for fumed silica on improving the residue’s integrity and coherency, which is the residue barrier effect shielding the bulk silicone against progressive erosion. A conclusion which is supported by the dry-arc resistance test showing similar residue characteristics to those obtained under the DC inclined plane test. The aforementioned findings are verified for a more practical application of using nano fillers in silicone rubber outdoor insulation composites as additives to improve the DC erosion resistance. In this approach, fumed silica and nano alumina trihydrate are added as additives, of less than five percent by weight, to base micro fillers of either ground silica or alumina trihydrate which are of much higher weight fractions. The viability of the filler barrier effect is synergistically achieved by adding fumed silica to the main ground silica filler, thereby maintaining the residue integrity and achieving a superior DC erosion resistance as compared to the other tested composites. This is an important finding which confirms the barrier effect as an important DC erosion suppression mechanism of nano filler additives aiding the volume effect of micro fillers replacing the depolymerizable silicone rubber content. To further investigate the role of the shield effect in the suppression of stable dry-band arcing, the dry-arc resistance is introduced as a quick testing method. Alumina trihydrate filled composites of different filler levels and sizes are examined with the dry-arc resistance test and the inclined plane test. The ranking order of the composites in terms of the eroded quantities coincide under both tests. This ranking order is also consistent with the levels of silicone rubber combustion found under thermal analysis. The consistency in the outcomes indicate the possibility of using the dry-arc test as quick testing method for analyzing the erosion resistance of silicone rubber and investigating erosion produced outcomes such as the shielding effects of inorganic fillers.

Date	20 Jan 2022
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Éric David (Supervisor) & Refat Ghunem (Co-supervisor)