Dynamic model for bipolar charge injection during electrical tree initiation

We have recently investigated the process of electrical tree initiation in crosslinked epoxy resin samples having a pin-plane electrode arrangement by quantitative measurement of the light emission using a phase resolved synchronous photon counting system and CCD imaging of the spatial distribution of the emitted light. The light emission behaviour was studied as a function of a step ramped 50Hz alternating electrical stress, which was increased until defect formation or tree growth occurred. Before the onset of resin damage, electroluminescence was detected. In this work, the electroluminescence was assumed to be proportional to the injection current. This made identification of the injection law possible, and it was found to be either Fowler-Nordheim or Schottky, depending on the magnitude of the field at the pin-tip. In this paper, a physical model is developed for charge injection from a metal electrode into the resin insulator. Its implications for electrical tree initiation are briefly discussed