Study On Stray Current And Rail Potential Of Urban Rail Transit

Safety and reliability in the operation of urban rail transit has become the primary concern of urban rail transit practitioners,with the rapid development of urban rail transit in China.Domestic subway lines usually adopt DC traction power supply system,which is an ungrounded system,and rail to backflow.Traction return current flows through the rail,causes a tail to ground voltage,called rail potential.Rail current leaks to the earth,called stray current.High rail potential and large stray current will threaten the safe operation of subway.At present,the problems caused by rail potential and stray current frequently occur in domestic subway lines,which need to be solved urgently.At first,this paper briefly describes the structure of DC traction power supply system,especially the structure of traction current return system,and introduces the causes of rail potential and stray current and its influencing factors,as well as the existing preventive methods against stray current.In addition,this paper introduces several accidents caused by rail potential and stray current occurred in domestic subway lines.The parameters and states of traction current return system are the main factors affecting rail potential and stray current.In this paper,the longitudinal resistance of the rail with joint bars is measured and analyzed.Compared with original rail,the joint bars connected with bulging nail makes the longitudinal resistance of the rail increased 56%~80%,the joint bars connected with good exothermic welding makes the longitudinal resistance increased31%~55%,the joint bars connected with poor exothermic welding makes the longitudinal resistance increased 47%~159%.On the basis of measurement and combined with the actual situation of a certain line in Beijing subway,the track models are built with CDEGS,and the distributions of rail potential and stray current under different conditions are calculated.The simulation results show that the joint bars will increase the rail potential and stray current leakage,and the installation mode will also affect rail potential and stray current.Compared with distribution installation,centralized installation has a greater influence on rail potential.Rail grounding often exists in subway lines,and the grounding state of traction current return system will affect rail potential and stray current.This paper monitored a unidirectional conduction device in front of a railway depot in Chengdu Metro,and found that unidirectional conduction device will provide a flow path for stray current and bring rail potential and stray current problems to the depot.Monitored the Over Voltage Protection Device(OVPD)of a certain line in Beijing metro,explored the path of traction return current when OVPD is closed.Based on that,CDEGS software is used to simulate and calculate the effects of unidirectional conduction device and OVPD on rail potential.The results show that unidirectional conduction device and OVPD could change the rail potential of the entire line,and the variation values are contingent on the change of rail potential in grounding point before and after grounding,and the distance from grounding point is farther,the variation values is smaller.Besides,unidirectional conduction device and OVPD will aggravate the stray current leakage.Finally,aiming at the problem that the calculation formula in available measurement method of rail transition resistance is not accurate,this paper proposes an improved formula.By calculating and comparing the errors in transition resistance calculation under a variety of situations of the two formulas,get the conclusion that,when the length of measured section is less than 4 km,the maximum calculation error of transition resistance can be reduced from 1.20?km to 0.48?km by using the improved formula.The correctness of the above conclusions is verified by CDEGS simulation.In addition,this paper presents an on-line monitoring system for rail to drainage net transition resistance,which can automatically monitor the transition resistance,and has the advantages of accurate monitoring,simple operation,high reliability,easy maintenance,and can be integrated into the existing stray current monitoring system and drainage cabinet.