Full Degree of Freedom Based Control Scheme of the Single-Phase Direct AC–AC Modular Multilevel Converter for Railway Power Conditioning Under Asymmetric Branch Conditions

This paper conducts a thorough analysis on the circuit dynamics of the single-phase direct ac–ac modular multilevel converter (MMC) to address the branch energy divergence problem in the railway power conditioning application scenario. Based on the analysis, a full degree of freedom (DOF) based control scheme is proposed. By controlling all the DOFs in a coordinated manner, extended branch power controllability can be achieved, which enables each branch's energy to be regulated individually without deteriorating the terminal currents. As a result, the control flexibility and reliability of the MMC can be effectively enhanced in the presence of asymmetric branch conditions. The proposed control scheme is implemented in a cascaded control structure with well-established algorithms. The frequency-domain stability analysis is also presented to guide controller parameter design. The effectiveness of the proposed control strategy is validated through the experiments under various conditions.