Research On Insulation Breakdown Mechanism And Partial Discharge Characteristics Of Emu Cable Termination In High-speed Railway

With the rapid development of China Railway High-speed,high-speed electric multiple units(EMU)trains are increasingly used in harsh environments.The use of train-mounted ethylene-propylene rubber(EPR)cables(including cable terminations and connectors)are prevalent,since they are key components in the power transmission of EMU trains and the connection of various electrical equipment.During actual operation,there have been many accidents in northwestern China due to the explosion of train cable termination.It seriously affects the safe operation of the EMU.Therefore,investigating the insulation failure mechanism of train-mounted ethylene-propylene rubber cable terminations and studying the characteristics of partial discharge(PD)inside the cable terminations under low-temperature environments is of great significance to fundamentally solve the hidden dangers in the safe operation of high-speed EMU trains.In this paper,ethylene-propylene rubber insulation film materials were prepared,ethylene-propylene rubber cable termination models were made,and actual running ethylenepropylene rubber cable termination entities were selected as experimental samples in turn.As the insulation of termination will easier to breakdown when they operated in the repeated cycling process at "low temperature to operating temperature",an experimental platform for partial discharge which can also simulate rapid temperature changes,is established.The mechanism and development process of insulation damage of train-mounted ethylenepropylene rubber cable terminations under rapid temperature changes are studied.This paper combines experimental research and results analysis.The main research work is as follows:(1)Based on the established high-frequency partial discharge test experimental platform,the discharge spectrum characteristics and surface damage of ethylene-propylene rubber materials at low temperatures were first studied.The results show that as the temperature decreases,the carbon traces on the surface of rubber materials gradually increase and the breakdown phenomena begin to appear,but the partial discharge spectrum does not show obvious regularity.Then,the discharge development process and surface micromorphology of the ethylene-propylene rubber cable termination model and entity were studied,and the differences were compared.It is found that when the experimental temperature is below-30°C,the discharge phenomena gradually become obvious,and the discharge spectrum is mainly characterized by the gradual overlapping of the turtle's back and the wings.Band-shaped protrusions and depressions appear on the rubber insulation surface.As the temperature continues to decrease,the erosion effect of the discharge phenomenon on the insulation surface is more obvious.Puncture holes appear in the depression area,and the insulation surface shows obvious burning erosion.(2)The differential scanning calorimetry(DSC)method was taken to test the temperature of state transition of the EPR material and the stress control tube(SCT)material,and a test platform for the pressure between the EPR/SCT interfaces was established to detect and calculate the interface pressure under different low-temperature conditions.By analyzing the changes in the cable termination model and the physical discharge spectrum characteristics at the corresponding temperatures,it is found that as the temperature in the low-temperature part of the experiment continues to decrease,the SCT will take the lead in molecular chain motion below-26.5°C.The glass transition process which includes the frozen of molecular chain,the decrease of elasticity and property transition of EPR material will appear later,which causes a structural mismatch problem at the interface between the EPR insulation layer and the SCT layer.It leads to a sharp drop in the pressure between the interfaces,which in turn triggers the interfacial discharge.Defects such as interstices occur.And then,the wing shape and the turtle's back shape overlap in the discharge spectrum.(3)The discharge between the EPR/SCT interface in the train cable termination is an important cause of the insulation failure phenomenon.Except for the interfacial defect caused by the interstices at low temperatures,other types of defects between the EPR/SCT interfaces are also not ignored.In this paper,four kinds of common defect problems during the installation and operation of train cable terminations are selected.Typical defect models such as residual defect of conductive materials between interfaces,insulation scratch cavity defect,semi-conductive layer burr defect,and the stress control tube failure defect were designed.The collection and processing of discharge information have been carried out under lowtemperature conditions.Through a multi-scale image decomposition process of the discharge spectrum,combined with a variety of image texture feature extraction methods,the characteristics that can describe various types of defect models were obtained.The parameters are finally tested by the identification algorithm,and the characteristic parameters that can effectively reflect the partial discharge characteristics of the defect model were selected.In the end,the method for judging the insulation status of the cable terminations was proposed.