The Study Of Initial Characteristics Of Electrical Tree In Silicone Rubber Under Polarity Reversal Voltage

HVDC systems have many excellent features such as large transmission capacity,high stability,low power consumption,and high speed of regulation,and are widely used in transmission lines.However,when the flow reversal occurs in the HVDC transmission system,the breakdown phenomenon of the electrical branches will occur,which is a great challenge for the insulation of the HVDC transmission cable.Silicone rubber(SIR),as the main insulation material of HVDC transmission cable accessories,is a very weak link in the system.Therefore,the in-depth study of the initial characteristics of silicone rubber under the polarity reversal voltage has important guiding significance for studying the insulation tolerance and key technologies of HVDC transmission cable accessories.The cable insulation will accumulate space charge under the polarity reversal voltage,which will cause breakdown of the electrical branch.In this paper,the initial characteristics of the electrical conductivity of the silicone rubber under the polarity reversal voltage were studied,and the initial characteristics of the electrical branch in different temperatures(30-120°C)and after 180°C thermal aging were studied.The results of the study indicate that the initial polarity of the electrical branches under the polarity reversal voltage has a significant polarity effect.When the negative voltage is pre-stressed,due to the small size and light weight of the electrons,it is easier to inject into the sample,and thus the When the negative voltage is pressed,the starting probability of the electrical branch is larger;with the increasing of the pre-voltage,the starting probability of the electrical branch is gradually increased;with the increase of the reversal time,the starting probability of the electrical branch is reduced.The result shows that the starting voltages at 30°C,60°C,90°C and 120°C are 19.7 kV,18.8 kV,17.2 kV and 16.2 kV,respectively.Compared to 30°C,the starting voltage at 120°C decreased by 17.8%.The analysis shows that as the temperature continues to increase,the silicon-silicone chain segments become severely slack,resulting in a decrease in the starting voltage.Under the polarity reversal voltage,the higher the temperature is,the more the space charge at the needle tip is injected,which increases the strength of the reversal electric field and promotes the initiation of the electrical branch.In addition,the internal electric field distortion of the silicone rubber sample under high temperature is more serious,and the local electric stress is strengthened,thereby promoting the development of micropores and cracks in the amorphous region,and the macroscopic appearance of the electric branch length is larger.After long-term heat aging,re-crosslinking occurs first in the silicone rubber,its crystallinity increases,and then as the aging time prolongs,the molecular thermal motion continues to increase leading to the destruction of the physical crosslink layer,and its crystallinity decreases.Therefore,the initial probability of electrical branching appears to increase first and then decrease.