Ageing and degradation mechanisms of silicone polymers used for outdoor electrical insulation

Summary

Silicone is an organic material largely used in outdoor electrical insulation. Performances and results have shown to be largely depending upon chemistry and applications. It is used either in the form of silicone rubber moulded over a fiberglass core in composite insulators or as a coating over ceramic insulators in overhead line or substation applications. Corona activity has been identified in the last fifteen to twenty years as a major threat for silicone rubber, and besides the erosion pattern often visible in such cases there are other signs of degradation of the polymer. Progressively, experts have pointed out the necessity to be more stringent in the evaluation of maximum acceptable stress levels allowed on silicone housings [1]. Among the typical degradation seen on the housing material, whitening and hardening of the silicone compound in its structure after exposure to corona and the acidic by-products remain incompletely described even today. This paper will provide some keys explaining the chemical degradation of silicone rubber under electrical stress with correlations to field reports and actual chemical investigations on failed composite insulators. Likewise, silicone coatings on ceramic insulators which are being used for their hydrophobic properties can be degraded over time. Ageing tests on samples covered with silicone coatings of various chemistries are presented. The main degradation patterns are described in this paper as well as field results from actual insulators returned from service. Based on numerous observations and testing, an ageing classification chart is proposed combined with the hydrophobicity of the material and recovery considerations. Periodic Soxhlet analyses over years of field observation described in this work are supporting this evaluation.

Additional informations

Authors

Simka Philipp, Ranjan Nitesh, Suwanasri Thanapong