Partial discharge phenomena in composite insulation materials

The partial discharge behaviour of vacuum impregnated and air impregnated composite insulation materials was investigated when they were exposed to electrical stress and simultaneously to mechanical stress. By means of phase resolving PD analysis typical PD patterns were found, which allow insight into the type of imperfection and the physical phenomena going on as well. For various materials PD occurred in different modes, clearly distinctive by the PD impulse amplitude. For some materials PD activity became detectable above the sensitivity level of the equipment (100 fC) only by application of mechanical stress. In general a clear influence of the mechanical stress on the PD activity was observed. A hypothesis for the mechanism of PD in composites is introduced. Conclusions concerning the aging mechanism of the material are drawn