Study On Dynamic Stray Currents In Traction Power Supply Circuits Of Metros

In recent years,with the rapid development of urban rail transit,the problem of stray currents corrosion occuring in the operation of the subway has become more and more serious.At present,most of the attention is focused on the corrosion of DC stray current.The stray current is no longer a pure meaning of DC due to the current harmonics produced by the traction converter and load variation during the train start-stop.It is a dynamic AC and DC mixed current.Therefore,the study of stray currents should not only consider the corrosion of DC,but also the distribution characteristics of stray current under the dynamic condition,its corrosion and electromagnetic interference to surrounding pipelines.In order to investigate the distribution characteristics of AC and DC stray currents in metros and take corresponding protective measures.Firstly,the current harmonic characteristics of traction converter are theoretically analyzed,and the track current of Metro Traction Substation varying with time is measured,and the harmonic components of stray current source are analyzed by the measured results.In terms of the theoretical investigation of the current distribution path,the current distribution model of the subway is constructed by the professional software CDEGS.An efficient method is implemented into the model,in which the transition resistance of the travel conductor is equivalently considered on the cable sheath.With regard to the factors such as the transition resistance of traveling track and soil resistivity which affect the stray currents distribution,the distribution characteristic in frequency domain and time domain is analyzed detaitedly for the stray currents by using a single variable method.The connection and grounding mode of drainage network and structural steel bars is changed,and the stray current prevention approach is proposed from the viewpoint of the tunnel structure.The influence of stray currents on the pipeline with insulating anticorrosive coating has been tested by combining moment method with circuit analysis.A comparison is also made between measured and simulated results for exploring the the influence of several pipeline protection measures on the pipeline-to-ground potentialsOn the basis of the above research,the following conclusions are obtained.The harmonics of the traction converter penetrate into the traction power supply system,so that the dissipating path of the stray currents includes not only the resistive coupling but also inductive coupling.As the resistivity of the tunnel soil structure increases correspondingly,the scattered current flow path is blocked and the currents are reduced.The variation of the locomotive position can change the impedance distribution of the track and the earth circuit,which in turn affects the distribution characteristics of the stray currents.The larger the harmonic frequency is,the stronger the electromagnetic coupling between the tracks becomes.Moreover,the AC stray currents increase with the increase of induced potential difference.The uniform connection of the drainage net can improve the ability of the drainage net to collect stray currents and reduce the corrosion of the surrounding pipeline.The structural steel bars are connected and grounded at the station,which can raise the soil potential and reduce the potential difference between the track and the ground,thereby reducing the stray currents.Laying the shield wire parallel to the pipe and connecting it to the pipe can reduce the maximum pipe-earth voltage and thus protect the pipeline.