Impact of mechanical stress on the dielectric behaviour of fibre-reinforced insulation materials

The present work was carried out in order to obtain a better insight into the dielectric behaviour of fibre-reinforced insulating materials, when they have been heavily stressed mechanically. Virgin and mechanically prestressed material was exposed to a high dielectric AC field employing high resolution PD-measurement. Optical investigation shows that mechanical prestress may produce a high number of microcracks of the order of a few tens of micrometers. In consequence, these samples show partial discharges (PD) up to 0.5 pC before breakdown. This result corresponds to theoretical estimates for the dimensions of the cracks necessary for PD-inception. As these effects occur far above service conditions, it may be concluded that the material employed for the time being might be used at higher service stresses