A novel control strategy of a modular multilevel converter (MMC) based VSC-HVDC transmission system

In the conventional control strategy of the VSC-HVDC system based on the MMC, direct modulation was employed and the terminal behavior of the MMC was similar to that of the two-level converter. The DC bus voltage of the power dispatcher side was regulated indirectly by controlling voltage regulator side DC bus voltage, and the transmission line current was determined passively by the power flow. Fluctuation of the transmission line voltage would occur during rapid power flow variation due to the inherent capacitor-inductor coupling in the DC transmission line. In this paper, a new concept of the control strategy is proposed. At first, by the proposed control strategy AC grid current control, DC bus current control, and arm capacitor voltage balancing control of the MMC are fully decoupled. Secondly, the DC bus of the MMC operates as a controlled voltage source and the power flow is controlled by regulating the transmission current directly and actively, and the transmission line voltage fluctuation is fully suppressed during power flow variation. Validity of the proposed control strategy is verified by both full scale simulation and down scale experiment.