A model for bipolar charge transport in insulators

Failure of electrical cable insulation is still a major problem for the electricity generation and supply industries. Under high field conditions (>10⁸ Vm^-1), space charge and the concomitant electric field dynamics are involved in the initiation of dielectric breakdown. Space charge is established either by charge transferred across electrode-insulator interfaces, or by field assisted ionisation of centres within the bulk of the dielectric. The action of high fields upon charge provides mechanisms by which electrical energy is transformed into potentially destructive mechanical energy. Measurements of the dynamic space charge distribution within plane parallel samples of insulators contained between aluminium electrodes under high voltage DC stress have been previously obtained using a pulsed electro-acoustic (PEA) apparatus. The pulsed electro-acoustic technique has been developed to provide a high resolution and reliable method of measuring the space charge density dynamics within insulators. It is a flexible alternative to the pressure wave propagation method. In this paper, experimental data for space charge dynamics within degassed cross-linked polyethylene (XLPE) is compared to a simple numerical simulation, which has been derived from concepts first utilised in semiconductor physics