Analysis Of Voltage Rise And Suppression Scheme Of Regenerative Braking When Electric Multiple Units Pass Long Downhill Section

With the development of the western region in China,a large number of mountain railways are constructed.Among them,Sichuan-Tibet Railway,as the "most difficult railway to build",has a large terrain drop along the way.The highest elevation of railways is 4400 meters,and the elevation drop is more than 3000 meters,so the process of electric multiple units(EMUs)passing the long downhill section is inevitable.In this process,the huge regenerative braking energy will be sent back to the traction network and may threaten the voltage safety.This paper will study the voltage safety problem caused by regenerative braking energy in the long downhill section.At present,some scholars have analyzed regenerative braking energy by power flow calculation or establishing equivalent models to perform simulations.However,the multiple components are usually simplified in the existing vehicle-grid equivalent model,which is not accurate for the simulation analysis.Therefore,this paper attempts a detailed analysis scheme for the influence of regenerative braking energy on mountain railways.Firstly,based on the analysis of traction network models and EMUs models in the focused scenario,the principle and process of the modeling design are given,and the integrated model is established,which considers detailed components including complex traction network,high voltage cable,on-board converters,train body,grounding system,and rails in the current loop.This proposed scheme increases the accuracy of the model.Secondly,the slope parameters are introduced into the vehicle-grid model via the traction calculation.The speed limitation is analyzed to ensure the operation safety of EMUs under different slope parameters,and the force is investigated to calculate regenerative braking power.Through analyzing the vector relationship between the phase voltage and the phase current of motor,the mathematical model of dual Pulse Width Modulation(PWM)under braking condition is extended,and the mathematical relationship between the regenerative braking power and the target instruction parameters is deduced.Then,by substituting the calculated results into the extended dual PWM mathematical model,the target instruction parameters are solved.The accurate electrical characteristics including catenary voltage and the intermediate Direct Current link voltage are simulated,to estimate the voltage safety.By comparing the simulation results and the experimental testing results at Chinese Cheng-Yu railway line,the feasibility and practicality of the modeling method are verified.Next,focus on the downhill section where the catenary voltage exceeds the safety range under braking condition,Modular Multilevel Converter STATCOM(MMC_STATCOM)is used to dynamically compensate for the vehicle-grid system.Based on the principle of MMC_STATCOM,the capacitance parameters of the sub-module,the inductance parameters of the bridge arm,and the parameters of the LC filter circuit are calculated.The capacitor voltage control strategy and the dq cross decoupling control strategy are studied,and the simulation model is established.Finally,based on the electrical relationship between MMC_STATCOM and vehicle-grid system,the compensation device is put into the vehicle-grid integrated model.The results show that the compensation scheme has good voltage stabilization effect and can optimize the harmonic problems.In conclusion,by establishing the accurate vehicle-grid integrated model and the extended dual PWM mathematical model,this paper simulates the overvoltage caused by regenerative braking energy under different slope parameters,to estimate the voltage safety.According to the evaluation results,MMC_STATCOM is used to suppress the voltage for downhill section with high voltage rise.This study can provide reference for the mountain railways preliminary design in future.