Charge storage on insulation surfaces in air under unidirectional impulse conditions

In many insulation systems, overvoltages below the flashover level can give rise to discharges which produce free charge carriers. These charges may be deposited on insulation surfaces, resulting in a high surface field and consequent distortion of the normal field. This may result in increase, or decrease, in flashover voltage. Investigations were undertaken to examine the factors and mechanisms governing the production of surface. charge and the resulting field distortion. The experimental equipment consisted of a simple point-insulation-plane electrode assembly, mounted in a glass test cell filled with air, to which unidirectional impulse voltages were applied. Polycarbonate and polymethylmethacrylate samples were tested. Measurements of the discharge currents, residual surface field and resulting dust figure patterns were carried out. Under both positive and negative impulse voltages, discharge current pulses occurred resulting in high surface fields on the insulation sample. On the wavetail of the applied impulse discharge, currents occurred in a reverse direction to those originally produced by the applied impulse. With repeated impulses, the stored surface charge produced by one impulse significantly affected the discharge activity of the following impulse. Under flashover conditions, a substantial surface field remained following the flashover.