Suppression Of Circulating Current Harmonics And Capacitor Voltage Fluctuations In Modular Multilevel Converters

Modular multilevel converter(MMC) is an innovative multilevel power topology and the most competitive topology for high- and medium-voltage applications, due to its remarkable features such as modularity, easy scalability and perfect voltage waveform quality. MMC has been firstly applied in high voltage direct current(HVDC) transmission, in which MMC is able to achieve high voltage level and easy to install and assemble. Moreover, MMC is shown to be of great potential in the field of medium-voltage motor drive applications. Compared to traditional cascaded H-bridge(CHB) converter, MMC eliminates the bulky and costly phase-shifting transformer. However, serious voltage fluctuation of sub-module capacitor is a problem challenging MMC at low-speed operation.Capacitor voltages of SMs must be balanced for stable operation of MMC. This paper analyses the operating principle, presenting whole control method for MMC, which achieves the power regulation and capacitor voltage balancing control. Actually, capacitor voltages are controlled by regulating circulating current. The even-order ac circulating current harmonics of MMC will increase the power loses and current stresses on the semiconductor devices. This paper proposes a circulating current harmonics suppression method based on proportional repetitive controller(PRC), which has the ability of tracking and elimiting circulating current harmonics.This paper analyzes the causes and nature of capacitor voltage fluctuation, which consist of second-order common-mode fluctuation and fundamental frequency differential-mode fluctuation. The suppression scheme based on a proportional integral resonant(PIR) controller is proposed for common-mode fluctuation. This paper proposes the differential-mode voltage fluctuation suppression method, establishing a high-frequency power interaction between the upper- and the lower-arm SMs by means of simultaneously injecting a common-mode voltage at the ac terminals and a circulating current between the upper and lower arms. Moreover, optimal circulating current injection scheme is presented, which decreases the amplitude of circulating current. Finally, the trapezoidal circulating current takes the place of sinusoidal circulating current to be injected to cut down the magnitude of circulating curr ent.Ultimately, validity of the proposed control approaches has been proved by experimental results based on a three-phase MMC prototype.