Abstract
The HVDC power transfer scheme has become more prevalent in recent times as it is the preferred scheme for long range power transfer. As the cable is stressed based on local fluctuations such as ambient temperature and field enhancements, the need for a greater understanding of the underlying mechanisms affecting the cable becomes more emphasized. Flat, layered samples are used for measurements as they can be used to emulate cable joints/terminations which are considered weak points of a cable. This project will investigate the space charge accumulation and resulting field enhancement of layered polymeric HVDC insulation. The samples are comprised of two layers of cross-linked polyethylene (XLPE) where one layer is cross-linked once while the other is cross-linked twice. This is to create a discontinuity of the conductivity between the layers which should facilitate charge trapping. The measurements are performed using the Pulsed Electro-Acoustic method (PEA) with an applied voltage of 10 kV and with temperatures of 20, 40 & 60oC. Finally, the viability of using an equivalent RC circuit as a prediction of the field development has been investigated. The results show that the greatest charge accumulation occurs at 40oC, while the amount of accumulated charges are quite similar at 20 & 60oC. Heterocharges was observed at the cathode for all samples, while homocharges was present at the anode for all samples. At the dielectric interface charges of negative polarity was observed at all temperatures, but more positive charges was observable with higher temperatures. The heterocharges at the cathode lead to a field reduction in the range of 9.1-47.4% while the homocharges at the anode lead to a field enhancement in the range of 6.7-40.4%. While the system is quite complex and many mechanisms are influencing the results at the same time, a possible reason for the observed results is that the space charge formation increases slower with increasing temperature than the charge detrapping in the samples, possibly as a result of the increasing mobility of the charge carriers. Other influential mechanisms may be the electrode materials, sample morphology, charge transport in the sample and charge injection.
Representing the samples in terms of an RC circuit was discovered to be viable at low temperatures as fewer mechanisms were present. At higher temperatures more mechanisms were observed and the accuracy of the model decreased. More RC branches could be added in order to rectify this, however at the cost of making the model significantly more complex.