Non-linear fillers in electrical insulation

It is shown that in unfilled polyester, and that filled with Al ₂O₃, the conductivity is Ohmic at low field strengths and space charge limited (SCL) at high fields. With ZnO, however, its non-linear, semi-conducting characteristic changes the conductivity of the filled polyester at sufficiently high concentrations. At low concentrations the characteristics of the polyester dominate but for higher concentrations there is a transition from SCL at low fields to tunnelling or Schottky emission across the inter-granular boundaries at high fields. Treeing tests are a practical demonstration of this phenomenon. Unlike Al₂O₃, which provides a barrier to tree growth due to its high adhesive bond and fracture toughness, ZnO provides an additional enhancement to tree resistance because of its non-linear properties and its ability to redistribute accumulated space charge