Polymeric materials like mineral filled and fibre reinforced epoxy resins were applicated in many power apparatus for electric insulation. Compared to ceramic and glass the composite insulators based on mineral filled polymers have many advantages. Bu these materials for outdoor and indoor use show ageing phenomena under environmental stress. To simulate environmental and operation stress, in a first step mineral filled epoxy test specimens are aged by water immersion for different lengths of time. Afterwards test specimens are exposed to high electric stress of varied levels. Partial discharge activity is measured in selected time intervals. The time until the breakdown occurs is determined (so-called lifetime). Apparent charge magnitude and distribution changes significantly at high electric stress. It is shown that water immersion influences partial discharge activity, which is an ageing mechanism itself. The longer pre-ageing through water immersion and higher electric stress the shorter is lifetime of the test specimen.
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