Impulse withstand voltage characteristics of polluted insulation material surfaces under humid conditions

Recent research is indicating, that the impulse withstand voltage characteristics of insulation material surfaces are influenced by the relative humidity in a greatly differing extent. This influence mainly depends on the physical and chemical surface structure of insulation materials. In order to describe the impulse withstand voltage characteristics under humid conditions a test procedure called "Water Adsorption Test" has been introduced, which allows to classify various insulation materials into so-called water adsorption groups. The water adsorption test is based on a procedure to be performed on new test specimen. Previous papers have demonstrated numerous test results on different materials as epoxy resin, polyester resin and a laminate consisting of presspan and polyester foil. One of these papers has shown, that the water adsorption test is also suitable to determine the insulating properties of photodegraded test samples under humid conditions. The aim of the present paper is to find out, if the water adsorption test is suitable to describe the impulse withstand characteristics of naturally polluted insulation materials under humid conditions; hereby, the pollution layer consists of dust, soot and acids caused by exhaust fumes. For this purpose the water adsorption test has been performed on insulation materials, which had been exposed to indoor and outdoor conditions for several months. The present investigations have been carried out on two different glass fibre reinforced resins (epoxy resin and polyester resin). We obtained the result, that pollution does not lead to a degradation of the impulse withstand voltage characteristics under humid conditions in any case. Furthermore we found out, that the water adsorption test is not applicable in the presence of a pollution layer as dust or soot.