Polarity Effect on Electrical Treeing in Silicone Gel under Repetitive Voltage Impulses

Silicone gel is widely used to encapsulate power electronic modules. The weakness of the electrical insulation is surface discharges initiated at the gel-substrate-electrode triple junction and the subsequent formation of cavities; so called electrical trees. The propagation characteristics of cavities under high frequency and fast rise time voltages, which are typical waveforms formed in power modules, have not been fully understood. Thus, polarity effect on cavity shape in silicone gel under repetitive voltage impulses were investigated. The results show that the branch number hardly increases in the earlier stage, but once the cavity length is saturated, the branch number increases rapidly. The branch number increases in order of the following conditions: under negative polarity, under bipolar impulses, under positive polarity. Under bipolar impulses the cavity length is shorter than the one under unipolar impulses. Branching mostly occurs from voids.