Simulations of partial discharges of small microcracks parallel to the electrical field in polymeric materials

Gaseous voids in polymeric dielectric materials are likely locations for partial discharges (PD). Numerous parameters influence the development of the charge avalanche. To date, various experiments and simulations have been carried out to investigate the PD behaviour as a function of the geometry of disc-shaped or spherical voids. In contrast, this paper deals with the restricting effect of the walls of cylindrical voids parallel to the electrical field on the avalanche. Simulations of the inception voltage and the apparent charge in longish cylindrical voids of constant length show increased inception voltage and decreasing apparent PD charge when the diameter of the void is reduced. These simulation results could be supported by experiments. To assess this, the diameter of drilled holes in epoxy resin was increased stepwise from 50 to 300 pm with a constant hole depth of 300 pin. For each diameter partial discharges were measured optically and electrically. The knowledge of the critical (sufficient) size of a void for PD to take place under given field conditions helps to formulate the requirements for the manufacturing processes for improved dielectric materials.