Degradation of silicone gel by partial discharges due to different defects

In order to evaluate the influence of partial discharges on the lifetime of power electronic circuits models of different defects are developed. These defects cause PD within the silicon gel used to embed the circuit because of electrical and mechanical reasons. The first model is a spherical void in the middle between two spherical electrodes. Shortly after registering the first PD within the void, these PD lead to a breakdown of the entire geometry at a field strength that can be found in power electronics under working conditions. The ageing of the material takes place very rapidly and parallels to the ageing behaviour of epoxy resin can be found. The second model is an inhomogeneous field of a needle-plane geometry with a needle's tip radius of about 5 /spl mu/m. This is a model for inhomogeneties in the electric field distribution. PD occuring here lead to the development of electrical treeing at the needle tip which causes as well breakdown within a short time. In both cases PD occur inside the bulk of the material. It can be concluded that any defects inside the encapsulation of power electronic circuits, such as voids inside the silicone gel originating in the filling process or small radii of components limit the lifetime of power electronic circuits because of the strong degradation of silicon gel in the presence of partial discharges.