Research On Control Strategy Of Modular Multilevel Converter(MMC)

Because of its advantages in modular structure,flexible extension,less output harmonic,easy for redundancy,and its characteristics of efficient and reliable operation,modular multilevel converter is suitable for medium and high voltage power conversion applications.Therefore,the related research of MMC has important significance both in theory and practice.In this paper,the basic model structure and operation principle of MMC,and the modulation mode and capacitor voltage balance strategy,circulating current analysis and control are introduced.And the main innovation of this paper is the design of capacitor voltage observer.The number of voltage sensors in the system is effectively reduced by using the observer,and the above control strategy can be well realized by using the estimated value of the observer.The main contents are listed as follows:1)By comparing the commonly used modulation modes(including nearest level modulation,carrier phase shift modulation,phase disposition modulation and sub-module unified PWM),and introducing various balancing control strategies,this paper uses sorting algorithm with specified sorting frequency,which greatly reduces switching and ensures output performance.2)By analyzing the circulating current and specified sorting frequency,the actual sub-module capacitor voltage approximation strategy is adopted,which reduces the deviation between the actual output voltage and the expected value of the arm.Results of simulation verify that the control strategy can effectively reduce the high frequency circulating current and sub-module capacitor voltage fluctuations.3)In control system of MMC,capacitor voltages are necessary for voltage balancing and circulating current control.However,with the increasing of voltage and power level,the number of cascaded modules and voltage sensors increases,and thus,it results in increased cost and hardware complexity.To reduce the number of sensors,a reduced-order observer based on passivity theory and condition of persistent and sufficient excitation was proposed.The asymptotic stability of observers is proved by theory.The research has good engineering practical value.