PROPAGATION OF ELECTRICAL TREE-STRUCTURES IN SOLID POLYMERIC INSULATION

Two alternative theoretical approaches to electrical tree propagation exist. Stochastic models attribute tree structures to random probabili stic factors, whereas in the discharge-avalanche model mechanism-drive n field fluctuations are responsible. Here we review the predictions o f these approaches in the light of the available experimental evidence . It is shown that both models give the fractal structures and the for m of structure distribution observed experimentally The width of the d istribution functions predicted are, however, less than those found ex perimentally The quantitative formulation available to the physical mo del also enables it to reproduce several other features of tree propag ation such as voltage dependence, growth laws, and discharge behavior patterns. This is not possible in the stochastic approach without mech anistic assumptions which are difficult to relate to the stochastic pr ocess. The connection between the discharge-avalanche model and determ inistic chaos is explored. Experimental evidence is presented supporti ng the contention that the electrical treeing phenomenon is the result of a deterministic breakdown mechanism operating in a chaotic regime at fields lower than those required for runaway breakdown. Space-charg e deposition and re-arrangement is proposed as the physical origin of the chaotic field fluctuations. Tree shapes are shown to be related to the variation in the fluctuation range available as the tree grows in accord with the predictions of the discharge-avalanche model.