Research On Distribution Characteristics And Simulation Of Stray Current In DC Transit System

Most of urban mass transit is supplied by DC transit system,and rails are used as return rails.It is inevitable that a small part of the current flowing in rails will leak into track bed,tunnels and bridges,which is called stray current,because the rail cannot be completely insulated from the track bed structure.The direct-current has the characteristic of strongly electrochemical corrosion,and will cause corrosion on metal structures in the return system and buried metal pipelines.It must be given adequate attention,because the corrosion is long-term,hidden and unexpected,and it must be serious when an accident caused by corrosion has happened.With the development of urban rail transit,the hazards of stray current have gradually highlighted.For the conditions of non uniform rail-to-ground resistance,multi-interval and multi-locomotives,rail insulation,conductor parameters etc,the study of stray current distribution can provide theoretical basis for the design of stray current protection and monitoring in urban rail transit system,and has great significance in reducing stray current in practical engineering.In this paper,the traction power supply system,traction load,stray current corrosion and collection system are briefly introduced.In order to analyze the distribution of stray current in traction return system with different structure,a variety of different structural models are established.Analytical formulas are detailedly derived,and some defects in formulas which widely exist in documents are fixed.Furthermore,a method is presented,which used to calculating stray current in two-way feeding.It is found that the stray current and rail potential in two-way feeding are obviously less than one-way feeding,and the distribution of stray current is no longer ideal when resistance to ground are existed at the position of traction substation and locomotive.A method to calculate stray current is presented by using basic formulas,when the rail-to-ground resistance for different segments in interval is a fixed value.The distribution of stray current is analyzed under two conditions,one is that each segment has different track-to-earth resistance values in a permanent position,another is that the value of rail-to-ground resistance in each segment and the length of mutational segment are fixed,but the position of mutation is locomotive.It turned out that the stray current would increase when the average track-to-earth resistance of the interval was decreased.The simulation models are created by using a MALZ module of CDEGS,which are corresponded to the previous calculative models,and the simulation results are compared with calculation results.Simultaneously,Simulations for single interval with two trains and double intervals with two trains are conducted.The results show that regularity of distribution of CDEGS simulation is the same as calculation.It has a good match when the structure of simulation is simple,but the results have a wide deviation both structural steel and drainage grid for a complex structure with three-layer conductors.The reason is that the resistance of structural steel bars is much smaller than the net which is installed in the same concrete.The simulation results of uneven distribution of rail-to-ground resistance in an interval are consistent with the calculated,and the rail-to-ground resistance calculated by simulation results coincides well with the setting value.It is found that there is an equivalent resistance to ground existed in each traction substation and locomotive after conducting simulations for multi-intervals or multi-locomotives.For some factors which have impact on stray current,an analysis for stray current is conducted through changing model parameters or the arrangement of conductors.Results indicate that the resistivity of rail coat is the main factor affecting track-to-earth resistance.It has non influences on rail potential if the resistance decreases,but the leakage current and net potential will increase.The spatial structure of steel bars has few effects on collection effect.Rail resistance can greatly affect leakage current,which bigger cause less stray current,but do not affect the collection effect of drainage grid.In addition,increasing the cross-sectional area of drainage grid or setting up a dedicated stray current collection cable can efficiently reduce the net potential.