Wide Frequency Domain Control Strategy of Large Capacity Modular Multilevel Wind Power Converter

With offshore wind power technology development, wind power generation equipment presents the trend of high voltage, high power, and high power density. Modular multilevel converter (MMC) has a broad application prospect in the offshore wind power field. Due to the uncertainty of the offshore wind speed, the operating frequency of the modular multilevel converter fluctuates from low frequency (0~20Hz) to high frequency (40~50Hz). In order to support the stable operation of the system in the full-speed domain, the MMC controller needs to solve the sub-module capacitor voltage fluctuation problem in the low-frequency domain. The control strategy designed in this paper adopts a high-frequency injection method to control the voltage fluctuation of the sub-module capacitor in the low-frequency domain and uses a circulating current controller to maintain the energy stability of the phase unit. Furthermore, a limiting factor is designed to connect two frequency bands. The switching process is smooth and stable and shows good continuity in the entire speed domain. Finally, the effectiveness of the proposed strategy is verified by simulation.