The influence of thermal aging on AC dielectric strength of transparent silicone rubbers for HV insulation

Optical methods and monitoring systems for the PD detection in GIS, GIL and oil-immersed type transformers have been demonstrated. Very recently these methods have been extended to the detection of PDs in solid insulation materials of cable accessories. This novel application in high-voltage (HV) cable terminations consists of integrated fiber-optic sensor which requires transparent insulation materials. Therefore, transparent silicone rubbers are considered as replacement material for currently used opaque elastomeric insulation material in stress cones of HV cable accessories. For this purpose a preliminary study on the long-term stability of two commercially available fully transparent silicone rubbers was performed. Accelerated thermal aging tests were carried out for 4 hours up to 100 hours at 200 °C under a normal air atmosphere and normal atmospheric pressure. The impact of thermal aging on the AC dielectric strength of the silicone samples was investigated and additionally the optical transmittance was determined using UV-Vis spectrophotometer. The investigation indicated that the dielectric strength value of the silicone samples was seriously reduced for long-term exposures to elevated temperatures. In addition, the UV-visible transmittance of thermally aged silicone samples was also decreased. The influence of these phenomena must be taken into account for the design of HV insulation materials, particularly under extremely high temperature operating conditions.