Online Harmonic Injection for MMC over a Wide Frequency Range in DC Offshore Wind Farms

Modular multilevel converter (MMC) based wind power converter exhibit large sub-module (SM) capacitor voltage fluctuation due to the low-frequency output characteristics of wind turbines. Harmonic injection (HI) can effectively suppress the SM capacitor voltage fluctuation, but its injection value should be adjusted according to the change in MMC operating conditions. This implies that it is challenging to determine the optimal injection value. To address these challenges, we propose an online harmonic injection (OHI) strategy for the MMC operating in a wide frequency range. The strategy comprises second-order harmonic current injection (SHCI) and third-order harmonic voltage injection (THVI), where the amplitude of the second-order harmonic current is calculated automatically to dynamically change with wind speed changes in offshore wind farms. The fluctuation characteristics of the SM capacitor voltage over a wide frequency range are evaluated. The mathematical model of the proposed OHI strategy is then established as a unified calculation principle. The proposed strategy can suppress SM capacitor voltage fluctuations and reduce the MMC power loss over a wide range of wind speeds. The SM capacitance can also be reduced by 27% using the proposed method. The effectiveness of the proposed OHI strategy is validated by Simulink simulations and hardware experiments.