Analysis On Influencing Factors And Restraining Measures Of Rail Potential In Metro System

At present,the metro system is generally powered by the DC traction system.On the one side,the rail needs to be used as the running rail of the train.On the other side,the rail needs to be used as the backflow rail to guide the current injected by the train into the rail to the traction power substation.Because the rail itself has a certain longitudinal resistance,current flowing through the rail will produce a certain potential on the rail.It may lead to high rail potential,and may threaten the safety of passengers and staff.And the rail is not completely insulated from the earth.The increase of rail potential increases the leakage current of rail into the earth,increasing stray current.In this context,in order to ensure the safe and reliable operation of metro system,it is of great significance to analyse the influencing factors and the restraining measures of rail potential.In this paper,based on the analysis of metro system,the traction power supply system,the backflow system,the over-voltage protection device（OVPD）and the drainage device in the metro system are analyzed and equivalent.The rail potential simulantion model is built by CDEGS software.The validity of the rail potential simulation model is verified by using the rail potential concentration parameter model,which provides a premise for the subsequent research on the influencing factors of rail potential.Based on the research of rail potential simulation model,the influence of track-to-track cross bond,traction power substation in the terminal of the line,train operating condition,OVPD operation and drainage device operation on rail potential distribution are analyzed.The results of the research show the setting of the track-to-track cross bond can decrease the rail potential to a certain extent.And when the longitudinal unit resistance of the track-to-track cross bond is set less than 2×10^-5Ω/m,the interval of the track-to-track cross bond is set below600m,and the uptrack and downtrack rail potential tends to be consistent.When the traction power substation is set in the terminal of the metro line,high rail potential in the power supply section in the terminal of the metro line can decrease.When there are small number of trains in the metro line,the condition that braking trains supply power to accelerating trains in different power supply sections will lead to rail potential increase in some positions in the metro line.OVPD operation can reduce the rail potential of the station,but it may increase the rail potential of other positions of the metro line.OVPD operation can also lead to a slight increase in the ground potential near the connected station grounding grid.When continuous OVPD operation occurs in the line,the current passing through the OVPD is larger when the operation OVPDs are closer,but the current flows through the distance of the backflow system is shorter.When the rail potential is unchanged before OVPD operation,the higher the equivalent resistance of OVPD is,the higher the rail potential after OVPD operation is,and the current flowing through OVPD is lower.When the drainage device operates,the rail potential near the traction power substation tends to 0.After OVPD operation,it is easier to reach the condition of the drainage device operation.Moreover,when OVPD and drainage device operate at the same time,the stray current will increase greatly.According to the analysis of the influence of OVPD operation on rail potential,the operation logic and equivalent resistance of OVPD are optimized.The threshold and delay time of OVPD operation are optimized according to the DC voltage that the human body can withstand and the changes of the rail potential of the station.Aiming at the phenomenon that OVPD continuous operation may occur in the metro line,the operation threshold and operation delay time of OVPD at different stations in the line are optimized to reduce the continuous operation of OVPD.Through the method of simple circuit equivalence to the simulation model or data fitting,obtain the relationship between the equivalent resistance of OVPD and the rail potential after OVPD operation and the current flowing through OVPD,and optimize the equivalent resistance of OVPD.According to the influence of train operation on rail potential,the train departure interval and station dwell time in the metro line are optimized.Take departure interval and stop time length as optimization objects.The running time of the train in the section between two stations can be obtained from the train running schedule.Reference traction calculation to estimate the length of acceleration time.The weighted sum of the simultaneous acceleration time of trains in the same power supply section is used to form a function.When taking the minimum value of the function,the departure interval or stop time obtained is the optimization result.The effectiveness of the optimization method is verified by comparing the rail potential distribution before and after the optimization of train operation arrangement.