Electrical Tree Characteristics in Polypropylene Under Impulse Superimposed DC Voltage in LN2

During the operation of the high-temperature superconducting (HTS) dc cable, due to the operation of the power electronic device, the cable insulation is subjected to the repeated impulse overvoltage while being subjected to the rated dc voltage, which easily generates the electrical tree and causes insulation failure. Polypropylene, the primary substrate for polypropylene laminated paper, faces a working environment in liquid nitrogen (LN
₂
). The insulation performance of polypropylene in a low-temperature environment may change and affect its degradation process, threatening the safe and stable operation of power system. In this paper, a needle-plane electrode system was used to study the electrical tree characteristics in LN
₂
. The results show that the tree morphology changes with different impulse frequency under dc-impulse voltage. With the increase in the positive dc voltage, the treeing process is accelerated. However, with the increase in the negative dc amplitude, the tree length decreases. With the impulse frequency increasing, the tree length increases significantly.