Research On Variable-Frequency Control Strategy Of Modular Multilevel Converter

With the wide application of electric drive in the field of engineering,it is of great significance to study high-performance power electronic converters and their control strategies.Compared with traditional power electronic converters,modular multilevel converter(MMC)is considered as one of the most promising power electronic converters due to its high modularity,easy redundancy design and high-quality output,which is very suitable for variable-frequency drive occasions under medium-high voltage and high power.However,the MMC variable-frequency drive system is a multivariable,nonlinear,and strong coupling system,which causes certain difficulties in its control.In this paper,researches are carried out around the key issues in MMC variable-frequency drive control.The topology structure of MMC and the working mode of submodule(SM)are introduced,the operation principle of MMC is analyzed,and the basic mathematical models of MMC are established.On this basis,the construction of the three-phase MMC experimental platform is completed.Aiming at the output current and circulating current control problems of MMC,the model predictive control(MPC)method is employed,and the prediction model and cost function of each control target are established.The N+1-level and 2N+1-level rolling optimization methods are discussed.Due to the large computational burden of MPC optimization,an MPC optimization method based on simplified finite sets is proposed,which avoids the rolling optimization of all finite sets.Aiming at the problem of SM capacitor voltage balance,a SM voltage equalization method based on FPGA parallel comparison and sorting is proposed,which can save the sorting time and improve the real-time performance of the system.The above methods are verified by simulation and experiment.Aiming at the problem of the low-frequency SM capacitor voltage fluctuation,the reasons for the SM capacitor voltage fluctuation are analyzed in detail,and the SM capacitor voltage fluctuation equation is calculated according to the energy change.A low-frequency suppression strategy is proposed.This method achieves the purpose of suppressing the SM capacitor voltage fluctuation by accelerating the charging and discharging process of the SM capacitor,which avoids the calculation process of the high-frequency circulating current in the traditional method.Simulation and experiment are carried out to verify the effectiveness of this strategy.Taking the permanent magnet synchronous motor(PMSM)as the driving object,the current prediction model of PMSM based on MMC is deduced,and the MMC variable-frequency drive control system is designed.The effectiveness of the MMC variable-frequency drive control strategy is verified by simulation and experiment.