Space Charge Characteristics in 160 kV DC XLPE Cable under Temperature Gradient

A temperature gradient (TG) will be formed across a high voltage cable insulation resulting from Joule heat in the conductor. Therefore, it is important to measure space charge profiles in a full-scale cable under TG. In this study, an improved pulsed electro-acoustic system used for coaxial cables was established by injecting nanosecond pulses into the outer semi-conductive layer. An induced current heating and a water circulation system was used to regulate the temperatures of a cable conductor and the the outer semi-conductive layer. By this method, a TG between the cable conductor and the outer semi-conductor layer was controlled and set at various temperatures and the TG was formed across the radial insulation so as to avoid the influence of heat from the cable conductor. Space charge profiles in a 160 kV DC coaxial XLPE cable was measured under 10 kV/mm stress with polarity reversal. According to CIGRE TB496, the load cycle operation of the cable under 15 kV/mm with temperature difference of 40 degrees C was performed for 48 and 72 h, including cable conductor cooling and heating. The results show that there is little space charge injection in the bulk at room temperature, but some remnant hetero-charges accumulates near the outer semi-conductive layer during voltage reversal. Under TG, hetero-charges appear near the outer semi-conductive layer on the low temperature side and hetero-charges increased with increasing TG. In addition, the hetero-charges increase during cable conductor heating and decrease during cooling.