Suppression Of Capacitor Voltage Fluctuation In MMC Sub Module Based On Circulating Current Control

Modular Multilevel Converter(MMC)system has been widely used not only for rectifying the grid side,but also for inverter driving three-phase AC load with its unique topology,modularization,good scalability and easy to realize redundant configuration.When the MMC system works,there will be circulating current between phases.If it is not controlled,the normal operation of the system will be affected,so it is necessary to control the circulating current between phases.In addition,when using MMC system to control the motor,if the motor runs in low frequency condition,the fluctuation of sub module capacitor voltage is obvious.If the fluctuation is not suppressed,the capacitor loss will be increased and its service life will be affected.Therefore,it is particularly important to adopt effective capacitor voltage fluctuation suppression method to suppress the fluctuation.The following work has been done in this paper,mainly focuses on the circulating current between phases and the voltage fluctuation of sub module capacitor:Firstly,aiming at the problem of circulating current between phases,the mathematical model of MMC system is established,carrier phase shifted pulse width modulation strategy is adopted,the simulation circuit is built,and the voltage balance control of the system is carried out.At the same time,by analyzing the generation mechanism of circulating current,the circulating current controller based on PIR control is established after comparing with the traditional PI controller,so as to realize the whole MMC system with the combination of voltage balance control and circulating current control based on PIR control strategy.Then,aiming at the problem of sub module capacitor voltage fluctuation,the mathematical analysis of sub module capacitor voltage fluctuation is carried out,and the second frequency circulating current injection method is applied to suppress the fundamental frequency fluctuation of sub module capacitor voltage;after analyzing the advantages and disadvantages of the second frequency circulating current injection method and high frequency component injection method,a new compound injection method combining the two methods is proposed,at the same time,the frequency selection of high-frequency component injection is analyzed through many experiments.The optimal high-frequency component injection frequency is determined;two methods are used to suppress the capacitor voltage fluctuation of the sub module in the MMC system of the load side and the grid side.Finally,the simulation results show that the control strategy combining voltage balance control and circulating current control based on PIR control can effectively control the circulating current between phases and make the system operate stably.The capacitance voltage fluctuation experiments of sub modules in two application scenarios of load side and grid side show that the second frequency circulating current injection method and composite injection method can effectively suppress the capacitance voltage fluctuation.The fundamental frequency fluctuation of voltage,especially the fundamental frequency fluctuation of capacitor voltage under low frequency condition,is restrained more greatly,and the overall effect of capacitor voltage fluctuation is more significant.