Space Charge Simulation of Electric Tree Before Germination in XLPE at DC Voltage Based on Improved Bipolar Carrier Transport Model

The existence of electrical tree (ET) will reduce the insulation performance of XLPE cables, and long-term development may lead to cable breakdown. The impact of space charge is one of the causes of ET, the phenomenon of charge accumulation in high voltage DC (HVDC) cables is more common. In this paper, based on the bipolar carrier transport (BCT) model, the improved carrier dynamic equilibrium model is used to simulate the charge distribution before the generation of ETs. The results show that the maximum charge density accumulated at the negative voltage is larger than that at the positive voltage, besides, the process of charge accumulation reaches the steady state faster. The charge density at the tip is the highest. Because the electron mobility is larger and the injection barrier of needle/XLPE is lower, the area of charge accumulation is larger and the steady state can be achieved faster. The accumulation of charges with the same polarity near the tip of the needle will weaken the field strength, and the reduction of the field strength is greater under the negative voltage. Therefore, under the negative voltage, the impact of the space charge on the material is more severe, and the probability of producing ET is greater than that under the positive voltage.