Differential Flatness Based Circulating Current Controller for Single-phase MMC

The existing research of circulating current suppressing controller for modular multilevel converter(MMC) are mainly focused on three-phase system. A novel circulating current reducing method based on differential flatness theory is proposed for single-phase MMC. Firstly, the second-order generalized integrator is used to obtain the double frequency circulating current component. Secondly, all pass filter(APF) is introduced to produce the orthogonal virtual vector. The proposed controller is composed of feed-forward control and dynamic error feed-back control. Feed-forward control, as the dominant control variable, consists of the functional relationship between input variables, and state variables and the desired flat output reference trajectory of the system. The feed-back error control is used to eliminate errors between desired and actual outputs, and impact of internal and external disturbances. As a result, the controller dynamic response speed is enhanced. Finally, a simulation model is built on PSCAD/EMTDC and simulation results verify the effectiveness of the proposed circulating current control strategy.