Dielectric diagnostic of moisture induced degradation processes in mineral reinforced high-voltage composite insulation

The application of composite insulations based on mineral filled polymers has increased drastically during the last twenty years. There are many new possibilities in designing electrical insulations by using polymeric materials as well for indoor as for outdoor applications, especially in high-voltage power apparatus. The disadvantages of polymeric materials are aging effects which are not understood in detail so far. Penetrating water leads to complex physical and chemical interactions at the microscopic interface between filler and polymer matrix. The results of the interactions at the large internal filler surfaces are an increase in the conductivity and an intensification of the Maxwell-Wagner effect. These effects combined with the electrical service stress can lead to the detachment of the filler-resin bond which results in an increase of the dielectric properties. Simultaneously a decrease of the electrical strength and life-span is observed. The present paper describes a dielectric diagnostic method in the frequency range between 0,01 Hz and 1 GHz for detecting moisture induced degradation processes in mineral filled epoxies. A correlation between the dielectric parameters and the electrical strength will be derived, based on variations of the water immersion time and the electrical testing frequency. Finally an attempt to explain the degradation and deterioration physical and chemical effects at the interface is given.