SUMMARYThe partial discharge (PD) activity during propagation of electrical treeing in epoxy resin is described. The electrical trees grew in needle-plane samples without and with an internal barrier up to the final breakdown. The simultaneous tree growth and discharge activity show a correlation between the propagation state of the tree and the thereby measured PD. Especially, the changes in the tree structure can clearly be detected. Based on these characteristics a new model describing tree growth is presented. It appears that measurements from machine insulation can be interpreted in terms of the PD pattern.
The growth of electrical trees in heterogeneous solid dielectrics was investigated by means of two-dimensional numerical simulations. In contrast to conventional approaches the introduced newly developed algorithm is based on a deterministic model together with a spatial distn'bution of the electrical breakdown strength. With standard needle plane geometries, two main types of treeing, bush-like and branch-like trees were simulated correctly in respect to their shape and the general voltage dependency. The tree propagation in homogeneous dielectrics as well as the eflect of barriers upon treeing was simulated. This tree formation was compared to tree growth in epoxy resin with an embedded glass-mica layer. Moreover the efect of electrically weak interfaces between the polymer and the barrier was investigated and the mean time to breakdown was compared for different set-ups. From this comparison it is concluded that only barriers with intact integace zones, whose breakdown strength is not weakened, lead to an optimum of tree resistance.
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