In order to study the influence of DC pre-stress time on the characteristics of electrical tree initiation under DC superimposed impulse voltage

±50 kV lightning impulse voltage is superimposed to DC bias voltage with amplitude of 0 k V

±10 kV pre-stress time of 1 min

10 min

60 min and 180 min

respectively. In addition

the bipolar charge transport model is used to simulate the space charge and electric field distribution near the needle tip under composite voltage. The results show that

with increasing DC pre-stress time

the electrical tree initiation length increases gradually under a DC superimposed impulse voltage

and the concentration and distribution range of space charge increase. Moreover

the initiation length of electrical tree is related to the concentration of space charges

injection depth of space charges

and the development process of streamer. The results show that the electrical tree initiation length under the composite voltage and the space charge distribution under the DC pre-stress have an obvious polarity effect. The electrical tree initiation length under the positive impulse voltage is significantly longer than that under the negative. When the DC pre-stress time and amplitude are the same

the space charge concentration and distribution range of negative DC pre-stress are larger than those of positive DC pre-stress.