Characterization of a severely degraded silicone elastomer HV insulator-an aid to development of lifetime assessment techniques

A high voltage composite insulator showed severe degradation and failure of the silicone elastomer sheds following service in a substation environment. Sensitive analytical techniques were used to investigate the extent of degradation of the silicone elastomer (SiE). The polymer content of some surface regions decreased as degradation increased. This was quantified through observation by scanning electron microscopy (SEM), weight loss upon heating using thermogravimetric analysis (TGA), determination of surface "silica" levels in damaged areas by X-ray photoelectron spectroscopy (XPS) and Raman mapping through a cross-section of the elastomer producing a degradation depth profile. It is believed that due to the environmental conditions at the site of installation, the electric field may have reached corona onset threshold on parts of the elastomer surface. Because of the moist conditions the corona was maintained, effectively preventing the elastomer from regaining hydrophobicity and leading in time to severe degradation. This investigation has also identified techniques applicable to the assessment of condition of other, less damaged SiE insulators which may in turn lead to the prediction of the remaining useful lifetime.