Circulation Analysis and Suppression Methods of Modular Multilevel Converter
Due to the inconsistency of capacitor voltage caused by the charging and discharging of submodule capacitor in modular multilevel converter(MMC),there is a circulation between the three phases of MMC,which not only increases the system loss,but also affects the stable operation of the system.By establishing the mathematical model of MMC,the formation mechanism and components of MMC circulation under unbalanced grid are analyzed,and a circulation suppressor combining second-order generalized integrator(SOGI)and quasi-proportional resonant(QPR)controller is designed.Firstly,the double frequency component in the circulation is extracted by SOGI,and then sent to QPR controller after the difference with the reference value,so as to suppress the inter-phase circulation of MMC.The MMC simulation model is built in MATLAB/Simulink to verify the feasibility and effectiveness of the proposed method.The results show that the control strategy can effectively suppress the arm circulating current,the peak value of the circulating current decreases by about 41%,the capacitor voltage of the submodule decreases by about 20%,and the total harmonic distortion of the bridge arm current decreases by 93.91%.
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