Partial discharges and streamers in silicone gel used to encapsulate power electronic components

Silicone gels are widely used to encapsulate IGBT power electronics modules. The role of the gel is to protect components and connections against moisture, pollution, and also to avoid partial discharges that would occur in air without encapsulation. The objective of this paper is to study the partial discharge mechanism (PD) in silicone gel in point-plane geometry, using either impulse voltage or ac. Under impulse voltage, high-speed visualization of single discharges shows that they are due to the initiation and propagation of a "streamer". Most of characteristics of streamers in gels are similar to those previously reported in liquids. A limited self-healing capability of the silicone gel is observed. After several PD's, permanent degradation of the gel occur. Under ac voltage, a stable "self sustained" PD regime is seen, due to the long lifetime of cavities in the gel. These phenomena allow us to interpret PD patterns recorded under ac.