Research On Power Quality Control Of Traction Network Based On MMC-RPC

The negative sequence,reactive power and harmonic problems brought to the traction power supply system are increasingly serious in pace with rapid growth of Chinese electrified railways,threatening the safe and stable operation of the traction network and the public power grid.Japanese experts first proposed the Railway Static Power Conditioner(RPC),which has made a significant contribution to solving the power quality issues in the railway traction power supply system.The RPC of Modular Multilevel Converter(MMC)structure can effectively solve some problems such as negative sequence,harmonics and imbalanced reactive power in the traction network,and overcome the shortcomings of traditional RPC.This paper takes MMC-RPC as the research object to realize the research on the governance of the power quality problem of the electrified railway traction network,and the following work is carried out:Firstly,the research analyzes the topological structure of MMC-RPC and the working principle of single-phase MMC,studies the modeling of MMC-RPC in deep,and establishes its conventional model,which provides theoretical support for MMC-RPC’s controller design.Furthermore,the dynamic phasor method is used to derive the voltage and current harmonic relations generated by the converter itself under the commonly used double closed-loop control,which theoretically determines the harmonics generated by MMC-RPC during operation,thereby the harmonic model of MMC-RPC is established.Secondly,the research on the control strategy of MMC-RPC is carried out.From the perspective of current and power,it analyzes the compensation command principle of V/v traction transformer,and provides compensation objectives for MMC-RPC to improve the negative sequence,harmonics and reactive power problems of traction network.A power decoupling droop control method for modular multilevel converter is proposed.The control method considers the influence of transmission line loss on the power quality control effect of traction network,which can effectively reduce the reactive power allocation error.And to solve the problem that the traditional double closed-loop controller cannot guarantee the accuracy of the system output,the internal loop parameters are corrected through differential flat error feedback control,which accurately follows the reference value and has a good dynamic response.The simulation experiment in Matlab/Simulink shows that the negative sequence current of the primary side of the traction transformer can be eliminated well,and the unbalance degree is greatly reduced,the output power of MMC-RPC can follow the expected power value well,and the power fluctuation is small.The power quality of the traction power supply system has been further improved.Finally,according to the harmonic model based on the dynamic phasor method,taking into account the co-treatment of the harmonics generated by the converter and the harmonics in the traction network,a harmonic control strategy is proposed for voltage loop proportional resonance control and current loop proportional complex integral control,through simulation waveform analysis in Matlab/Simulink,it is verified that the controller can eliminate harmonics effectively.After MMC-RPC is used,the THD content of the system is greatly reduced,it achieves a good harmonic suppression effect;by comparing with the traditional double closed-loop PI control,the effectiveness of the proposed harmonic suppression strategy for the current balance of supply sector and the management of negative sequence current in traction transformer primary side is verified.