Research On Control Strategy Of Modular Multilevel Converter

The modular multilevel converter(MMC)is a potential converter technology for high voltage direct current(HVDC)transmission system,flexible alternative current transmission system(FACTS)and electric power drive system.MMC has received extensive attention in the academia and engineering circles.This article has conducted in-depth research on the control strategies of the MMC.First,the topological structure and working principle of MMC are analyzed,carrier phase-shifted modulation,nearest level modulation and carrier disposition modulation are studied.The parameters of sub-module capacitor and bridge arm inductance are design,the specific calculation method of two parameters are given.The voltage fluctuation of the sub-module capacitor severely affecting the normal operation of the MMC,There are many problems when the traditional voltage balancing control method based on the nearest level modulation is adopted.In order to control the voltage fluctuation of the capacitor,an improved capacitor voltage balancing control strategy based on carrier phase-shifted modulation is proposed.The simulation results show that after using the balancing control strategy,the voltage fluctuated near the reference value is small,the capacitor voltage fluctuation is under control.Then,the mechanism and hazards of circulating current are analyzed,the circulating current mainly contained the DC component and the double fundamental frequency component,the traditional circulating current suppression method is analyzed.In order to suppress the phase-to-phase circulating current and reduce the internal loss of MMC,an improved circulating current suppression strategy based on low-pass filter is proposed.After the improved circulating current suppression is added,the circulating current fluctuation is significantly reduced and the circulating current amplitude decreased,the THD value of the bridge arm current decreased,a better the circulating current suppression effect is obtained.Finally,the hierarchical control principle of MMC-HVDC system is analyzed,the mathematical model of MMC-HVDC in different coordinate systems is established,the control strategy of MMC-HVDC system is studied,the control methods used on both sides of the MMC are determined.According to the mathematical model of active power and reactive power in the dq coordinate system and the current control loop,constant power control and constant voltage control are determined to control MMC.The simulation results show that the strategy can realize the independent control of active power and reactive power,the purpose of high-efficiency commutation for MMC in HVDC transmission system is achieved.