A Multiharmonic Injection Method for Reducing Peak Arm Current of MMCs With Limited Capacitor Voltage Ripple Increase

In modular multilevel converters (MMCs), injecting second-order harmonic circulating current (SHCC) with a specific initial phase can effectively reduce the peak arm current, which enables MMCs to handle larger output currents under the same current-rated switches, enhancing the overall current-handling capability of the system. However, the injected SHCC inevitably increases the submodule capacitor voltage ripple (CVR). This paper analyzes the relationships between the injected SHCC, CVR, and the peak arm current. It is found that when the amplitude of the injected SHCC exceeds a critical value, the effectiveness of further injecting SHCC to reduce the peak arm current diminishes significantly. Consequently, a new competitive SHCC injection method is proposed. On the basis of the proposed SHCC injection method, zero sequence voltage (ZSV) is also adaptively injected to further reduce the CVR cost without the need for look-up tables or offline calculations. Ultimately, a multi-harmonic injection (MHI) method is proposed, which effectively reduce the peak arm current while minimizing the SM CVR. The simulation and experimental results verify the effectiveness of the proposed control methods.