Hybrid Circulating Current Suppression Strategies for AC/AC Modular Multilevel Converter Combining Repetitive Control and Feedforward Compensation

ABSTRACT

The full‐bridge submodule‐based modular multilevel converter (MMC) can achieve three‐phase 50 Hz to single‐phase 16.7 Hz direct AC/AC conversion, making it suitable as railway power supplies. However, the presence of 16.7 Hz complicates the coupling between the single‐phase and three‐phase sides, causing odd‐frequency circulating current components, which influence power quality and increase power losses. To address this issue, this paper proposes hybrid circulating current suppression strategies combining repetitive control and feedforward compensation. First, the improved odd‐harmonic repetitive control (ORC) and improved 4k ± 1‐order harmonic repetitive control (4k ± 1‐RC) are proposed to suppress the circulating currents. Each consists of a proportional (P) controller connected in parallel with the repetitive controller, followed by a proportional‐integral (PI) controller to accelerate the dynamic response. Second, to further improve the dynamic response of strategies, a common‐mode (CM) insertion indices compensation method is introduced as a feedforward component for the strategies. Finally, comparative simulation and HIL experiments conducted under identical parameter settings demonstrate that both proposed CCSCs achieve satisfactory circulating current suppression performance. The 4k±1‐RC‐based CCSC exhibits a smoother transient response, whereas the ORC‐based CCSC provides better harmonic suppression.