Research On Control Strategy Of Railway Power Conditioner With Super-capacitor Based On MMC

Electrified railway plays an important role in the development of national economy.With the rapid development of electrified railway,not only the power quality of traction power supply system but also the power supply quality of users who surround the railway lines has been damaged due to the nonlinearity,asymmetry and volatility of electric locomotives.Therefore,it is necessary to take effective measures to control the negative sequence current,reactive power,harmonics current and other power quality problems of electrified railway.The Railway Power Conditioner(RPC)can solve the problems such as negative sequence current,reactive power and harmonics current of electrified railway traction power supply system,and it can enhance the regenerative braking energy utilization of high-speed railway.However,the traditional two-level RPC technology does not have great advantages in solving the above problems.Therefore,the RPC with super-capacitor based on modular multilevel converter(MMC-RPC)is deeply studied in this paper.The main researches contents of this paper are as follows:The current power quality problems and the solutions in the conventional electrified railway are discussed and the domestic and foreign research progress of RPC of recent years is summarized.The research of the regenerative braking energy of traction supply system is introduced.Then the structure and compensation principle of traction transformer and the performance of RPC are analyzed.Compared with the traditional RPC,the MMC-RPC shows great advantages.But it still cannot make effective use of the regenerative braking energy of high-speed railway and reduce the influence of the traction load peak value on the traction transformer.Firstly,the properties of different energy storage media are compared and analyzed.And supercapacitors are selected as energy storage devices in this paper due to their advantages such as long cycle life and fast charging and discharging speed.Then the system structure of energy storage MMC-RPC and four typical energy flow modes are proposed.Then,the operation condition of energy storage MMC-RPC is analyzed based on three control modes,which are the regenerative braking energy storage priority control mode,the peak shaving discharge priority control mode,and the negative sequence compensation priority control mode.The equivalent model diagram and equivalent phasor diagram under different working conditions are drawn and the equivalent circuit diagram is established according to different workingconditions and control modes.Finally,an energy storage MMC-RPC adaptive control system is proposed.In four different typical working conditions,the proposed system can choose control mode from three aforementioned control modes according to the different power index requirements of the traction system.Simulation results prove that if the regenerative braking energy storage priority control mode is selected,the advantage of this control mode lies in the high utilization rate of regenerative braking energy,which can reduce the impact of regenerative braking energy on the power grid.But this condition will cause negative sequence problems of the traction power supply system.If the peak shaving discharge priority control mode is selected,the system peak shaving rate is higher than other modes and the utilization rate of traction transformer can be improved,but it will also cause negative sequence imbalance of three-phase traction system.Considering that different working conditions have different characteristics under different control modes,an energy storage MMC-RPC adaptive control system is proposed.In four different typical working conditions,according to the different power index requirements of the system,the system can be switched between three control modes: regenerative braking energy storage priority,peak-cutting discharge priority and negative sequence compensation priority.MMC-RPC can effectively solve the problem of negative sequence,but it cannot completely solve the regenerative braking energy problem under braking condition and the internal sub-module voltage fluctuation problem of MMC.In order to solve the above problems,an energy storage device is connected to the MMC-RPC system to realize the efficient utilization of regenerative braking energy.a sub-module capacitance voltage fluctuation control strategy of energy storage MMC-RPC is introduced.This strategy can realize the voltage restraints of internal sub-module capacitance voltage fluctuation.Firstly,the principle of sub-module capacitance voltage fluctuation is analyzed in this paper,and then a control strategy of sub-module capacitance voltage fluctuation is proposed to control the energy flow of energy storage system and eliminate the voltage fluctuation of sub-module.Finally,the feasibility and effectiveness of the control method are verified by comparing and analyzing the voltage fluctuation of the sub-module before and after adding the novel control strategy.