Circulating Current Control for a Three-Phase Modified Modular Multilevel Converter with a Reduced Number of Current Sensors

The high capacitance requirement becomes a significant problem when modular multilevel converters (MMCs) are applied to medium or low-voltage systems. This paper proposes a capacitor voltage control scheme for a three-phase modular multilevel converter (MMC) with optimized capacitor sizing. Unlike the existing circulating current control, the proposed controller uses the middle capacitor voltage to control the second-order harmonic component of the circulating current. Therefore, it is unnecessary to measure the two arm currents; only one voltage sensor is required, which reduces the number of current sensors. The proposed controller can effectively control the circulating current to be constant or inject second-order harmonics to reduce the capacitor sizing. The detailed analysis, controller design, and performance of the proposed scheme are presented in this paper. The effectiveness of this study is confirmed by simulation and experimental results.