Space charge accumulation in XLPE insulation at high temperature and water content

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Space charge accumulation in extruded DC cable insulation can cause local electric field enhancement. For subsea cables with polymeric or damaged metallic water barriers, water will migrate into the insulation system. As high water content has been found to increase conductivity, absorbed water is expected to affect the transport and accumulation of charge in XLPE. This paper presents results from pulsed electro-acoustic measurements of space charge accumulation in XLPE insulation with low and high water content. The water content in the samples was controlled by a climate chamber, allowing the water content to vary in the range of 5-95 ppm. The measurements were performed at 20, 40 and 60 °C, with an applied DC voltage of 10 kV. The DC voltage was switched off during the measurements. The results show that increased water content at 20 °C leads to formation of positive hetero space charge near the cathode, while for the dry samples homocharge formed at both electrodes. At higher temperatures, increased water content decreased the magnitude of accumulated charge in the wet samples compared to the dry samples. Both high- and low water content samples showed accumulation of positive charge at both the anode and the cathode. The results could partly be explained by conductivity increasing with increasing water content, but also showed evidence of additional mechanisms occurring.