Dynamics of voltage polarity reversal as the controlling factor in space-charge induced breakdown of insulating polymers

Our investigation is focused on the understanding of high-field phenomena in polymers. In highly insulating materials such as polyethylene, space charge has a strong influence on both the short and long term breakdown strengths. The aim of this report is to demonstrate and discuss the importance of the dynamics of voltage polarity reversal at the injecting needle electrode on space charge induced breakdown. Evidence of space charge injection is provided by the observation of local breakdown generated by grounding of the sample after the polarization (so-called `grounding´ tree). The effect which is polarity dependent, can be observed only when the sample is short-circuited immediately after charging. Field computation is performed versus injected charge and a critical charge density is derived in order to explain polymer breakdown upon grounding. The time dependence of the effect is explained by space charge relaxation. Polymer degradation under ac voltage is discussed on the basis of this interpretation