Study of space charge effect on dielectric DC breakdown of synthetic insulators with the pressure wave propagation method

It is well known that the existence of space charge in polymeric insulators leads to a modification of their dielectrical behaviour, in particular their dielectric strength in D.C. voltage domain. The aim of this paper is to propose a correlation between space charge and dielectric breakdown in the case of short time tests. Low Density Polyethylene samples (LDPE), 1mm thick with semi-conductive electrodes were first submitted to a D.C. voltage of 50kV and 100kV for one hour. After space charge measurement using the pressure wave propagation method (PWP), a ramp voltage of +/-5kV/s was then applied up to breakdown. A first explanation of the breakdown process is given, assuming that the breakdown is initiated at the interfaces and depends on the nature of space charge. A simulation based on the thermal breakdown theory has shown a good agreement between theoretical and experimental results.