Low temperature partial discharge properties of silicone gels used to encapsulate power semiconductors

Soft silicone gels are frequently 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. The objective of this paper is to study PDs occurring on ceramic AlN metallised substrates, embedded in silicone gel, with a special attention paid to the behaviour at low temperature (down to -60°C). Such extreme temperatures can be found at different locations for instance in aircraft electrical systems. The investigations carried out at ambient temperature show that a stable and reversible PD regime of small amplitudes (a few pC) is reached above a critical inception voltage (PDIV). Short duration exposure to low temperatures (down to -60°C) has a small effect on PDIV and PD charge. On the other hand, sample exposed for hours to low temperatures (-20 to -60°C) show a large decrease of PDW, correlated to a large increase of PD charge. This effect is linked to the appearance of degradations (cracks) induced in the gel by thermo-mechanical stress.