Dielectric breakdown phenomena pose major scientific and technological problems. The degradation of polymer insulation under high electric stress is associated with space charge formation. In fact, space charge accumulation in dc regime is the main reason that prevents the use of polymers as insulation for high voltage direct current cables. Whilst the Laplacian field, may be adequate representation of that which is experienced by an insulator over a short time, it is well appreciated that, over longer time scales, this becomes modified through the injection of charge carriers from the electrodes or the dissociation of neutral species within the bulk. These charged species subsequently migrate through the dielectric before becoming trapped at specific sites within the material. The net result of these processes is that the local field at particular sites within the insulation may be substantially different from that which would be experienced in the absence of such phenomena. In particular, the local field may greatly exceed that the threshold stress above which the degradation takes place, so causing accelerated electrical ageing of the material and eventual failure. Whilst this may occur through the action of both ac and dc fields, space charge is intrinsically more problematical under dc conditions. The development of polymer-insulated dc cables therefore represents a particularly challenging technological problem. Consequently, recently research into space charge formation and its effect on the ageing and failure mechanisms of insulating materials has intensified, not only because of its engineering importance, but also through technical advances made in instrumentation.
Attempts have been made to investigate the electric field distribution in polymeric power cables under dc conditions. Previous attempts were less significant as little knowledge on space charge distribution in the cable. Cable space charge measurement system has been set up in the High Voltage Lab and electric field in the presence of space charge has been estimated using empirical formula. This unique cable space charge measurement system enables us to investigate space charge distribution as a function of electric field, temperature and even polarity reversal, an operation often used in dc power transmission. However, for a practical dc power cable the electric field is affected by conductivity of the material which is a function of both temperature and electric field. The coupled problems impose significant difficulty to know electric field distribution in high voltage dc power cables therefore a serious thread to the reliable operation of dc power cables.
The project concerns the accurate determination of electric field distribution in the presence of space charge where electrical conductivity of the material will be determined as a function of both electric field and temperature. The coupled equations will be solved using a numerical software package. An algorithm will be produced so that the future experimental data can be loaded and electric field distribution is ready to be displayed.