Degradation process of silicone-gel by internal surface discharges

Silicone gel is widely used to encapsulate power electronic circuits. It is well known that surface discharge in the module is one of the weakest points in insulation. These discharges cause the growth of cavity in silicone gel. We have observed the cavity dynamics in order to investigate the degradation process. The results indicated that streamers propagate through the gaseous phase in filamentary channels from the electrode and reaches the tip of the cavity, and leaves charges until the next discharge occurs. Moreover it was revealed that the propagation of the cavity was caused by the positive discharges. The self-healing ability of gel determined whether the propagation succeeds or not.