A New Control Scheme for a Single Phase Modular Multilevel Converter Connected to the Grid Under Unbalanced Arm Power Conditions

Abstract

Single phase modular multilevel converters (MMCs) are usually controlled by using two DC voltage sources in order to avoid unbalanced power conditions between the upper and lower arms. This paper proposes a method for controlling the single phase MMC connected to the grid by using one DC voltage source instead of two. In the proposed topology, the grid is connected in parallel with the lower arm, thus, creating a power mismatch between the upper and lower arms. The proposed control method injects a DC circulating current for the power transfer between the DC and AC sides and a fundamental frequency harmonic that is able to balance the capacitor voltages of the upper and lower arms under unbalanced arm power conditions. This method ensures the stability of the MMC operation and the grid while reducing the number of the employed DC voltage sources. The efficiency of the proposed method is revealed by simulations conducted on a single phase MMC connected to a low voltage grid using MATLAB/SIMULINK.