| Modular Multilevel Converter(MMC)is a kind of multilevel converter that has been widely used in the field of high voltage direct current transmission and mediumvoltage variable-speed drive system in recent years.It has some advantages such as high degree of modularity,low switching frequency,low harmonics and low loss.However,when applied in the field of variable-speed drive system,especially under low-speed and high-torque conditions,there is a problem of large fluctuations in the capacitor voltage of the sub-module,which limits its application.High-frequency injection method can solve the problem,but it will bring high-amplitude common mode voltage on the motor side,which will damage the motor insulation and cause safety hazards.Therefore,the Flying-Capacitor Modular Multilevel Converter(FC-MMC)is proposed.This thesis first describes a series of basic theories of FC-MMC,including the topological structure of FC-MMC and its sub-modules,working principles,common modulation strategies,and control strategies.It focuses on the principle of FC-MMC to simultaneously reduce the voltage fluctuation of the sub-module capacitance and the common mode voltage of the motor side.A simulation model was built based on the MATLAB/Simulink platform,and comparisons were made between the sub-module capacitor voltage fluctuations and common-mode voltages when outputting lowfrequency large currents when MMC without high-frequency injection,MMC with high-frequency injection,and FC-MMC were used.On the basis of submodule of each half-arm capacitor voltage has been balanced,this thesis established the state space model of three-phase FC-MMC.On this basis,a new type of control strategy-state — error feedback control strategy is proposed,which optimizes the tuning gain.This thesis also proves the stability of the proposed control strategies and their design methodology based on Lyapunov theory.Compared with the traditional multi-loop PI control,the proposed method only needs one tuning gain for each phase unit,which avoids the cumbersome parameter tuning process.Aiming at the problems of large calculation amount,high switching frequency,and large loss in traditional bubble sorting capacitor voltage balancing strategy,a group switching capacitor voltage balancing strategy based on sub-module capacitor voltage threshold is proposed.This strategy pre-sets the capacitor voltage threshold and applies the switching state vector of the sub-module in the previous control period to this control period.According to the different switching states of the sub-module in the current period,it is compared with the upper and lower thresholds set.Group all submodules to determine the priority order of the current control cycle of each sub-module.A simulation model was built in MATLAB/Simulink,and the control strategy and capacitor voltage equalization algorithm proposed in this thesis were simulated and verified.The results showed the feasibility of the proposed strategy.Finally,a single-phase FC-MMC experimental platform based on dSPACE is built,and the proposed control strategy is verified in this experimental platform.All experimental results are consistent with the simulation results,which proves that the control strategy proposed in this paper has a good control effect. |
