Effects Of Temperature Gradient On Electrical Tree Growth In Silicon Rubber Of Cable Accessory Under Dc Superimposed Impulse Field

Cable accessories are the importance components of DC cable transmission,the performance of the insulation material is directly related to the safe and stable operation of the cable.Because the cable accessories mostly use multi-layer composite insulation medium,and work in the environment of temperature gradient and compound field voltage at the same time,the deterioration of the main insulation of silicone rubber(SIR)electrical tree seriously threatens the cable insulation safety.In this thesis,the high temperature vulcanized silicone rubber(HTV SIR)electrical tree is taken as the research object to study the characteristics of electrical tree under impulse voltage and DC-impulse compound field voltage under temperature gradient environment,and to reveal the mechanism of electrical tree degradation under charge transfer operation under temperature gradient and compound field voltage.It provides experimental basis and theoretical support for the insulation of HVDC cable accessories in China.The main work and conclusions are as follows:(1)Based on the initial characteristics and growth characteristics of the SIR electrical tree under impulse voltage and temperature gradient,it was found that the local temperature rise of the insulating material will cause the initial voltage of the electrical tree to drop;during the growth of the electrical tree,the temperature gradient affects the length,accumulated damage distribution of the electrical tree.According to the conductivity test and trap level distribution characteristics,it is found that the conductivity of SIR increases at high temperature,and the charge is more easily trapped from the trap with a higher energy level,which promotes the injection and migration of charge,thereby accelerating the breakage of the silicone rubber molecular chain and further Impact ionization promotes electrical tree growth.(2)Based on the growth characteristics of electrical tree under DC-impulse composite voltage and temperature gradient,it is found that under the same type of composite voltage,with the increase of temperature gradient,the length,fractal dimension and accumulated damage of electrical tree increase;under the same temperature gradient The length,fractal dimension,and accumulated damage of the electrical tree under the compound voltage of different polarities are greater than those of the same polarity.Studies have shown that temperature gradients and recombination voltage affect the trappted and detrapped behavior of electric charge,and the periodic movement of charge accelerates the growth of electric tree.Combining the characteristics of trap distribution and conductance,the influence law of temperature gradient and combined voltage on charge transport behavior and electrical tree degradation is obtained.(3)Based on the DC-impulse composite voltage and the electrical tree growth characteristics at normal temperature,it was found that the composite voltage waveform affects the electrical tree growth characteristics.The jungle-shaped electrical tree grows slowly,but the accumulated damage is large;when the same polarity composite voltage is applied,The length of the electrical tree increases with the increase of the DC voltage amplitude and the probability of electrical tree breakdown is large;when the heteropolar composite voltage is applied,the length of the electrical tree varies with the amplitude of the DC voltage.From the microscopic morphological characteristics of the electrical tree,it is verified that the charge accumulation,recombination discharge,and the combination of positive and negative ions release energy are the main reasons for the growth of the electrical tree.