Electrical treeing test of DC cable XLPE insulation under DC voltage and high temperature

Along with its increasing application in high-voltage DC transmission network, XLPE insulated cable has become a global research hotspot. To study the cable endurance to DC operation, the initiation characteristics of electrical trees in XLPE cable insulation need to be investigated under DC voltages and design temperatures. A novel treeing test configuration, consisting of a heat oil bath, a DC voltage source and a micro-imaging system, is set up for the research on the treeing characteristics of XLPE samples under DC voltage and high temperatures. Treeing tests were performed on XLPE specimens made from a 110 kV DC cable, and each specimen has a pin-plane electrode system within. Under a continuously applied DC voltage of 60 kV for 20 days, the specimens, immerged in silicon oil heated up to 90°C, showed no electrical trees. Application of 60 kV DC voltage on specimens at room temperature for 10 days could not trigger a tree either. Under the condition of grounding after pre-stressing of a DC voltage, 20 kV was high enough to initiate a tree in the XLPE DC insulation. It is concluded that electrical treeing cannot be easily caused by constant DC voltages, even under the combined action of a high voltage and an elevated temperature. The detectable damage in XLPE insulation is a joint result of space charge's injection and extraction, and the deterioration caused by charge extraction will be more serious than injection. That is why grounding after pre-stressing can trigger a tree much easier.