Dual Harmonic Injection for Reducing the Submodule Capacitor Voltage Ripples of Hybrid MMC

Reducing the capacitor voltage ripples of the half-bridge submodules (HBSM) and full-bridge submodules (FBSM) in a hybrid modular multilevel converter (MMC) is expected to reduce the capacitance, volume, and costs. To address this issue, this article proposes a dual harmonic injection method which injects the second harmonic circulating current and third-order harmonic voltage into the conventional MMC control. First, the mathematical model of the proposed control is established and analyzed. Then, the general strategy of determining the amplitude and phase angle of each injection component is proposed to suppress the fluctuations of the fundamental- and double-frequency instantaneous power. The proposed strategy can achieve the optimal power fluctuation suppression under various operating conditions, which also has the advantage of reducing the voltage fluctuation difference between HB and FB SMs. The correctness and effectiveness of the proposed strategy are verified in simulations in PSCAD/EMTDC.