Electrical Tree Progression in the Presence of Micro-voids due to Thermal Aging

Thermal aging irreversibly changes the chemical structure of the polymer, causing the formation of oxidized carbonaceous layers and micro-voids. Voids also increase in size and number due to thermal aging. The presence of voids is known to affect electric tree growth inside the epoxy resin. The current work aims to understand the effect of thermal aging on electrical tree growth using a stochastic 3D model. The numerical stochastic model is developed based on the Weismann and Zeller (WZ) model for electrical tree growth. Spherical voids of varying diameters are inserted into a computational sample. The tree is assumed to initiate from a needle tip of a needle-plane electrode configuration and grow in a step-wise manner from the existing tree structure. The location of a new branch is chosen stochastically, with the probability depending on the local electric field. After the addition of each tree branch, the electrical field is recalculated with appropriate boundary conditions. Partial discharges (PD) within the tree tubules result in charge redistribution along the channel walls and on the tips of tree tubules. Similarly, pd within existing voids cause charge deposition on the void walls. The effect of charge on tree tubules, tips, and void walls is taken into account in simulating tree progression. The effect of the size and distribution of the voids is investigated in this work. Computationally generated trees are compared with experimentally obtained trees.