Rail Potential Mitigation Research Based On Moving Returning Equi-point

The urban rail transit generally adopts DC traction power system,in which running rails are also used as the traction current returning rails for economical reasons.When the traction current returns to the traction substations,it causes voltage drop on running rails,as well as the rail potential between running rails and the earth.Due to the rail potential,part of the return current leaks into the earth and forms the stray current,which causes electrochemical corrosions on buried metallic pipelines.The serious electrochemical corrosions cause obvious or potential safety accidents frequently.However,the existing measures do not reduce the rail potential or mitigate the stray current from the causal sources,and the mitigation performance is also limited.Both stray current amplitude and corrosion time influence the electrochemical corrosions on buried metallic pipelines.Therefore,this thesis proposes to reduce the stray current amplitude and corrosion time together for mitigating the stray current corrosion.To realize the amplitude reduction and corrosion time shortening for both rail potential and stray current,this thesis proposes to establish and move the returning equi-point(REP)through power electronic technology.The details are as follows:For the establishment and movement of REP,as well as the zero-resistance converter system(ZRCS),the definition of REP is proposed: returning points on running rails which have the same rail potential with the traction substation.The REP is established and moved in successive steps on running rails.Then the return path on the rail(namely current-carrying section)of train current are also shortened and moved in successive steps.Hence,the amplitude and corrosion time of rail potential are reduced.Comparing typical realizing methods,ZRCS,which is consisted of negative resistance converter(NRC),switch unit(SU),and zero feeder(ZFE),is proposed to establish and move REP according to its operation principles and zero-resistance regulation strategy.The mathematic models of rail potential,stray current and leakage charge are established for the traction power system with and without ZRCS,respectively.The effect of SU quantity in ZRCS is discussed,and the mitigation performance of ZRCS is also validated.For the circuit topology and control strategy of ZRCS,non-isolated circuit topology of NRC is proposed.The NRC circuit topology has only one power electronic switch that the train current flows through,so that the on-state loss of NRC is reduced maximally.According to the structural characteristics of ZRCS,the double-capacitor train location detection is proposed,which is sample,reliable,and cost-effective.The control strategy based on the double-capacitor train location detection is also adopted to ZRCS.The train rail-section is obtained by determining the amplitude ratio and direction of the double-capacitor voltage.Then,through regulating NRCs and SUs,REPs are always established at both sides of the train current-carrying section.To verify the operation of ZRCS in the traction power system experiments in the laboratory,a non-isolated circuit topology of train emulator is proposed,which can emulate the train characteristics under multiple conditions continuously.The source power of train emulator is reduced by energy feedback.The modelling and experimental results prove that the proposed circuit topology and control strategy of ZRCS can establish and move REP in successive steps under different grounding schemes and train conditions.For the optimization,reliability and fault-tolerant of ZRCS,the SUs location optimization method is proposed.The SUs locations are designed optimally to reduce the amplitude and corrosion time of rail potential in the traction and braking conditions,which improves the comprehensive mitigation effect of rail potential,stray current and leakage charge.According to the influence of ZRCS faults in establishing and moving REP,the circuit equivalent method of NRC,SUs and ZFE faults is proposed.The faults are classified,and their occurrence and operation principles are analyzed,respectively.The fault-tolerant control strategies are proposed for improving the operation reliability of ZRCS.Through simulations and experiments,the proposed optimization method and fault-tolerant control strategies are validated.For the control strategy coordination and optimization of ZRCS with multiple trains in typical urban rail transit traffic lines,ZRCS is proposed to share in both of the up-traffic and down-traffic lines.It reduces the cost and installation volume of ZRCS,and ensures similar mitigation performance for the up-traffic and down-traffic lines.The optimization control strategy based on the maximum rail current detection of rail-sections is proposed with the above double-capacitor detection units.REP is always established at both ends of the rail-section with the largest current,so that more current returns through REP,and the rail current is reduced.Then the rail potential amplitude and corrosion time are reduced together,which mitigates the rail potential and stray current corrosion from the causal sources.According to the proposed SUs location optimization method,the SUs locations are designed for multi-train operation,which improves the comprehensive mitigation performance of rail potential,stray current and leakage charge in the up-traffic and down-traffic lines.In order to verify the multi-train operation,the equivalent superposition method of ZRCS is proposed.With the same SUs sequence,the linear sum of rail potential when each train operates separately,leads to the same rail potential distribution when multiple trains operate in coordination.Through simulation and experiments,the share of ZRCS,optimization control strategy and multi-train equivalent superposition method of ZRCS are validated.This thesis has 160 figures,27 tables,and 149 references.