Electrical Tree Characteristics Of Epoxy Resin Under Superimposed DC Field

Epoxy resin is widely used in high-voltage direct current（HVDC）transmission equipment such as cable termination,valve bushing,cable joint,etc.During the operation of the power equipment,the epoxy resin insulation is subjected to the superimposed DC voltage generated by the switching operation,switching surge,lightning,etc.In addition,the polymer insulation not only withstands a superimposed voltage but is also subjected to various temperature environments.The electric tree is an important form of the polymer degradation,and also is the main cause of the insulation failure.The superimposed voltage and ambient temperature alter the charge transport process and influence the polymer degradation.This research focuses on the effect of superimposed DC field on the electrical tree growth under various temperature environments,and establishes a model for the influence of charge transport process on electric tree under the combination of a superimposed voltage and an ambient temperature.The main conclusions are as follows:1.Based on the growth characteristics of the electric tree with the liquid nitrogen under the impulse superimposed DC voltage,it was found that the electric tree was more likely to be initiated and grew under the same polarity superimposed voltage.With an increase in the impulse frequency,the probability of tree initiation,the tree length and the accumulated damage became larger.A charge transport model was proposed to describe the effect of charge transport process on the electrical tree degradation under different impulse superimposed DC voltages.2.Based on the growth characteristics of the electric tree with the room temperature under the harmonic superimposed DC voltage.With an increase in the harmonic order,the tree length reaches the maximum value under the 3^rd harmonic component.The DC component accelerated the tree initiation,growth and breakdown.Meanwhile,the positive DC component caused a shorter breakdown time.A charge transport model was proposed to describe the effect of charge transport process on the electrical tree degradation under harmonic superimposed DC voltage.Meanwhile,the reverse tree growth was found in this experiment,and the field-driven tree growth（FDTG）model was employed to interpret the tree breakdown process.3.Based on the growth characteristics of the electric tree with the temperature gradient under the harmonic superimposed DC voltage,it was found that the tree growth rate had a nonlinear trend with an increased temperature in the high voltage side,and showed a steady increase with an increased temperature in the ground side.A polarity effect was found under superimposed voltage,since the positive voltage had a more obvious acceleration on the tree growth.An isothermal surface potential decay（ISPD）experiment was designed to obtain the trap distribution of epoxy resin under different temperatures.A charge transport model was proposed to describe charge transport process and the tree behavior under the combined effect of superimposed voltage and temperature gradient.