Surface discharge measurements of polymeric materials

The measurement of surface charge is an important parameter for evaluating the electrical performance of high voltage insulation materials. The method proposed allows on line measurement of charge and can generate two-dimensional images that represent the charge behaviour on the surface of the material under test. The measurement system utilises Pockels effect by using a wafer of bismuth silicon oxide (BSO) as the sensing element. This method allows non-destructive measurements since no direct electric signal or component interferes with the surface charge. Unlike conventional surface charge measurement systems, this approach can dynamically observe and simultaneously quantify surface charge. This allows the surface charge distribution under the influence of a time varying electric field to be studied. The system´s accuracy and sensitivity have been improved through the use of an optical phase modulator. The system uses a high speed CCD camera and a purpose built synchronisation control circuit to obtain a high temporal resolution. This has allowed the study of charge behaviour at mains power frequencies. Surface charge distributions have been observed by applying AC voltages to a needle electrode in direct contact with the BSO. AC surface discharge behaviour of polymeric materials bonded to the back-face of the BSO has also been investigated.