Research On Control Methods Of Modular Multilevel Converter

In recent years,with the rapid development and wide application of power electronics technology in New and Renewable Energy,High-Voltage Direct Current power transmission(HVDC)and other fields,Modular Multilevel Converter(MMC)with its unique advantages of modular design,high-quality output voltage,applicable to high voltage,redundant characteristics has attracted more and more attention.This paper is devoted to some problems on start control,capacitor voltage balancing and hybrid MMC topology control.First,based on the detailed analysis of the MMC topology and operation principle,the mathematical model is derived.At the same time,the relationship between unbalanced voltage and circulating current of MMC is studied,and the circulating current can be controlled through the internal imbalance voltage,preparing for the design of the circulating current controller.To suppress the current impact of DC bus in the process of MMC start-up,a DC side voltage based MMC start-up method is analyzed,the method can suppress impact current in the controlled charging stage.Therefore,a MMC soft start-up has been realized.Finally,based on the analysis of hybrid MMC topology,a CLS-PWM based open-loop capacitor voltage balancing strategy is studied.To extend the linear operating range of the SPWM strategy and increase the amplitude of the fundamental wave of ac-side line voltages,a third-harmonic component is also included in the modulating reference signal.Simulation results in MATLAB/Simulink verify the validity of proposed control strategy,which can effectively reduce capacitor voltage ripple,as well as the output voltage THD by increasing the output voltage levels.