Study On Partial Discharge Characteristics Of Vehicle Cable Terminal Under Impulse Voltage

On-board cable is one of the important electrical equipment of Electric Multiple Units（EMU）.Due to the harsh and changeable operating environment of EMU,it is necessary to understand the influence of each factor on the cable termination in order to ensure the stable operation of EMU.Among the factors,the impact of the operating impulse voltage on the cable termination cannot be ignored,but the research on the impact of the operating impulse voltage on the cable termination is rarely reported;At the same time,various local defects are left in the manufacturing process of the cable termination,and the defects during operation will discharge and deteriorate the insulation,affecting the safety of the EMU.This article focuses on the characteristics of the operating impulse voltage on the normal cable termination and the partial discharge characteristics of the cable termination with tip,impurity and knife mark defects.It provides a theoretical basis for the research on the ability of insulation of vehicle terminal to withstand switching impulse voltage and partial discharge of impulse voltage diagnosis defect terminal,In this paper,a test platform for partial discharge after multiple cumulative effects of impulse voltage and cumulative effects of impulse voltage is built to study the partial discharge characteristics of normal on-board termination and on-board termination with different defects under different impulse times,the maximum discharge q_max and average discharge q_ave,the number of discharges n and the phase interval φ are obtained,and the characteristics of discharge and the mechanism of insulation degradation of cable termination under impulse voltage are studied with the scanning electron microscopy and molecular dynamics.First,according to the discharge development characteristics of the cable termination under different shock times,its development is divided into three stages:the initial discharge period（500 times,1000 times）,the discharge development period（1500 times,2000 times）,and the late discharge period（2500 times）.Under the impact of the impulse voltage,the charged particles continuously bombard the surface of the ethylene-propylene rubber to produce dents.The third body of the ethylene-propylene rubber molecule ·CH₂ falls off to generate H₂,and micropores are formed on the surface of the ethylene-propylene rubber.The roughness increases slightly because of the micropores and dents on the surface of the ethylene-propylene rubber,which increase the electron creepage distance,reducing the multiplication of the secondary electrons and suppresses the termination discharge;As the number of impacts increases,the molecular chain of ethylene-propylene rubber breaks and reacts to form free radicals,and lumps appear on the rubber surface.At this time,electrons are weakened and easily excited to escape.The collision increases and promotes discharge.At this time,the termination discharge volume gradually increases.In the later stage of the test,the ethylene-propylene rubber molecules were further broken to generate free radicals and recombined to generate H₂,C₂H₆,C₂H₄,and C₃H₆,which resulted in more obvious surface dents and pores,and increased insulation degradation.According to the three periods of the development of termination discharge under impulse voltage,the development of discharge with three types of defects under 500,1500,and 2500 tests was carried out respectively,and it is found that:At the beginning of the test,the knife mark defect is obviously affected by the impulse voltage,the initial discharge is the largest,followed by the impurity defect,the tip defect is the smallest,and the discharge is the least obvious;With the increase of the number of tests,the impurity defect discharge develops quickly,followed by the tip defect,and the knife mark defect discharge develops slowly and may even weaken;In the later stage of the test,the defect discharge is obvious,but the tip defect discharge increases the most.The test shows that the number of shocks received at the initial stage of operation of the termination is small:the impulse voltage easily affects the deterioration of the insulation of the termination with knife mark defects;The number of impulse voltage shocks accumulated for a long time in the termination operation:Impulse voltage has the greatest impact on impurity defects,and it is easy to degrade the insulation at the termination containing impurity defects;when the impulse voltage accumulates enough,the insulation deterioration of the termination with tip defects is the most serious.