Research Of AC/DC Converter System Based On Modular Multilevel Converter

For AC traction drive system,AC/DC/AC system without power frequency traction transformer is a hot spot in the field of locomotive industry at home and abroad.Because it can effectively reduce the weight of the locomotive,reduce the wheel to rail wheel impact,prolong rail service life,reduce the maintenance and failure probability of the line,and improve the reliability and overall efficiency of the traction system.One typical scheme of the AC/DC converter system is using modular multilevel converter(MMC)and medium frequency transformer as core component.MMC system uses a direct AC/AC/DC transformer mode and one medium frequency transformer,which enables it a simpler structure and higher power transmission efficiency.With deepen research of MMC technology,research on MMC topology shows essential theoretical value and engineering application value.But present analysis about work principle and control technology are still not enough.To analysis MMC system,the main contents of this paper are as follows:First,principle of the main circuit is analyzed based on AC/DC system of MMC,including the operation mode,energy flows,power analysis,the voltage fluctuation of the MMC sub-module capacitor and the circuit current between MMC arms.Then,dynamic mathematical model of the system is established.Second,main circuit parameters,including the grid side inductance,MMC related parameters,medium frequency related parameters,and phase-shifting transformer related parameters,is designed on the basis of rated parameters of a power unit in CRH5 type train.Third,control circuits of each part of the system are analyzed and the control shemes are designed respectively,including the control strategy of the front four quadrant converter,the average and balance control of the MMC sub-module capacitor,control method of the isolated bi-direction AC/DC converter.Further,an open-loop control strategy based on the principle of harmonic injection towards the MMC output voltage is proposed,and a balance control algorithm based on sorting method towards MMC sub-module capacitor voltage is designed.Forth,the simulation model is built in the Matlab/Simulink platform with the rated parameters.The result proves the correctness and effectiveness of the control strategy proposed,as well as verifies the stability of the system on all operation conditions.The simulation results show that the DC side of the system can achieve stability with full power factor 1,and bidirectional energy flow.Moreover,the designed control method is flexible,which can give a full play of the advantages of the MMC,making it more convenient in redundant design and more applicable to any kinds of different voltage levels.