Research On The Distribution Characteristics And Protection Strategies Of Stray Current In Urban Rail Transit

With the rapid development of urban rail transit,the problem of stray current has become increasingly serious.Stray current has the characteristics of instantaneity and randomness,and it is difficult to measure directly.The size of stray current is usually from a few milliamperes to a few amperes,but in some old lines with long operation time and degraded insulation protection,its instantaneous peak value can reach more than 2000 A.Stray current will also cause corrosion and perforation of the buried metal pipes,DC bias of grounding transformer,harmonic increase of reactor and other problems,bringing many hidden dangers for the normal operation of urban rail transit power supply system and urban power grid.In this thesis,firstly,the resistance network models of rail-ground two-layer and rail-drainage net-ground three-layer stray current distribution are established respectively,and the analytical formula of stray current is solved,and simulation models is built to analyze the static distribution characteristics of stray current.Secondly,an improved stray current experimental platform is built,the experimental results are compared with the simulation results,and the influence of traction current,drainage net structure and other factors on the stray current is analyzed.Then the dynamic distribution model of stray current is obtained by combining the calculation results of train traction and traction power supply with the static distribution model of stray current.Besides,the dynamic distribution characteristics of stray current under three different traction conditions of fast,economic and comfortable before and after considering the additional resistance are solved and analyzed.Finally,the traditional and new stray current protection strategies are studied.The research results show that the stray current is in direct proportional to the train traction current and the length of the power supply section,and it is inversely proportional to the rail-ground transition resistance or the rail-drainage net transition resistance,while the longitudinal resistance of the drainage net has little influence on the stray current.The drainage net can reduce the generation of stray current,and its suppression effect on stray current is directly related to the traction current.According to the analysis of the dynamic distribution characteristics of the stray current,during the complete train operation,the peak value of the stray current is mainly concentrated in the two sections of the final stage of starting acceleration and the initial stage of braking deceleration,and the increase of the stray current in the braking deceleration stage is more obvious.The influence of the three additional resistances,namely wind resistance of annular gaps in tunnels,curve additional resistance and ramp additional resistance on the stray current is mainly concentrated in the starting and braking stages.After considering the additional resistance,the stray current value changes the most under the economic traction condition,the second under the fast traction condition,and the smallest under the comfortable traction condition.The traditional stray current protection strategy can restrain the stray current to a certain extent,while the fourth rail power supply technology,traction power supply system based on DC auto transformer(DCAT),traction power supply system based on negative resistance converter(NRC)and other new stray current protection strategies can fundamentally eliminate the generation of stray current.