Charge transport dynamics and the effects on electrical tree degradation under DC voltages in thermally aged silicone rubber

Electrical tree is one of the major issues for insulation failure in high voltage power cables. In this paper, we investigate electrical tree degradation under DC voltages in thermally aged silicone rubber (SIR). Charge transport dynamics are revealed by means of space charge measurement and simulation to give a better insight into the electrical tree initiation and evolution. The results show that a thermal oxygen reaction enhances the SIR cross-linked networking and thus gives rise to a higher tree initiation voltage in the early stage of aging. Subsequently intensified thermal degradation and cracking reaction accelerate SIR cross-linked network breakdown and generate micro-cracks. The increased deep traps and significantly decreased charge carrier mobility after thermal aging result in greater homo charge accumulating near the needle tip. The mechanism of DC electrical tree initiation in SIR is clarified based on the trap theory. It is argued that more trapped charges in the low-density region near the tip in the late thermally aged stage would decrease the tree initiation voltage, while the resultant homo charges would in turn reduce the electric field around the tip. Therefore, after thermal aging, the reduction of tree initiation voltage under DC voltage is less than that under AC voltage.