Research On Model Predictive Control Of Modular Multilevel Converter Based On Hybrid Topology

Modular multilevel converter(MMC)has the advantages of modular structure,high scalability and excellent harmonic performance,so it has been widely used in high-voltage and high-power scenarios such as flexible DC transmission and motor drive.In the scene of long distance and large capacity power transmission,overhead lines are usually used,which makes the DC side of the system prone to short circuit fault.However,the traditional half bridge MMC can not use the converter valve to block the short-circuit current path,and can not reduce the voltage in extreme weather conditions,which greatly reduces the stability and flexibility of power supply.Therefore,half bridge and full bridge sub module hybrid MMC with DC fault self clearing and buck operation capability emerges as the times require.There are some technical difficulties in hybrid MMC,such as the complexity of modulation strategy,the difficulty of voltage sharing of sub module capacitors and the difficulty of energy balance between bridge arms,etc.Firstly,this paper analyzes the sub module matching principle,system working principle and power characteristics of hybrid MMC,and deduces the corresponding mathematical model,which provides a theoretical basis for the research of model predictive control(MPC)strategy.Secondly,aiming at the problem of difficult parameter allocation and poor dynamic performance in the application process of traditional PI modulation,a hybrid MMC modulation control strategy based on MPC is studied.Aiming at the problem of large amount of calculation and difficult implementation of traditional MPC,this paper improves the traditional MPC algorithm,optimizes the network side current prediction model,makes the system rolling optimization under the condition of directly obtaining the near optimal solution,greatly reduces the amount of calculation of MPC,improves the dynamic performance of the system,and verifies the feasibility and effectiveness through simulation experiments.Thirdly,aiming at the problems of circulating current increase,output current distortion and sub module capacitor voltage instability caused by unbalanced bridge arm parameters in hybrid MMC system,a method of bridge arm energy balance and circulating current suppression is proposed.The unbalanced bridge arm energy component is introduced as the reference of cost function,and the optimal switching state is selected through rolling optimization to achieve stable operation of the system,The validity of the method is verified by simulation and experiment.Aiming at the DC short-circuit fault in flexible HVDC transmission,the DC fault ride through mechanism of hybrid MMC topology is analyzed,and the DC fault clearing control strategy suitable for hybrid MMC is proposed.In order to solve the problems of high demand of sensor acquisition and large amount of calculation of controller sorting caused by a large number of sub module sorting and voltage balancing,the existing sub module capacitor voltage sorting method is improved,and the grouping optimization sorting method is adopted to greatly reduce the calculation amount of sorting algorithm and reduce the calculation burden of controller.Finally,a lkW single-phase MMC functional power prototype based on ZYNQ controller is built for experimental verification and analysis,which verifies the feasibility and effectiveness of the proposed strategy.