Thermal behavior and resistance against electrical discharges of modified silicones for outdoor application

Silicone composite insulators are state of the art for outdoor applications. The development of these insulators has reached a high level so in a next step the weight of these insulators can be reduced. One way to do so is by adding a low-density filling material. In this paper, the thermal stability of silicones filled with hollow microspheres (HMS) with diameter in the micrometer range is examined. High thermal stability is a key property of outdoor insulation materials as it determines the resistance against arc discharges as well as erosion due to dry band discharges. Therefore, arc resistance tests, inclined plane tests and thermogravimetric analysis are used to investigate the influence of HMS on thermal stability of silicone. The influences of thermal material properties like heat conductivity, heat capacity and temperature conductivity are examined and a method to calculate heat conductivity of porous composite materials is discussed. Results show no influence of thermal material properties on thermal stability, thus observed decrease of thermal stability of silicone filled with glass HMS cannot be explained by thermal characteristics. Instead, increase of surface area of the silicone is identified as key parameter, which causes a stronger thermal degradation. In addition, application of glass HMS in liquid silicone rubber (VMQ1) hinders the curing and reduces thermal stability further. Adding of ceramic HMS and silanization of glass HMS decreases erosion and increases arc resistance. This can be explained by an increased interphase quality between the silicone and these fillers, which prevent accelerated thermal degradation due to an increased thermally active surface area of the silicone.