Modular Low-frequency Transmission Converter for Wind Farm Integration and System Frequency Regulation Strategy

With the rapid development of deep-sea offshore wind power, Voltage Source Converter-based High Voltage Direct Current (VSC-HVDC) systems using modular multilevel converters have been successfully applied in long-distance offshore wind energy transmission. However, the construction costs of offshore platforms are relatively high. In this regard, low-frequency power transmission systems based on modular multilevel matrix converters (M3C) have emerged as a recent research focus, eliminating the need for offshore commutation valves. In this paper, we propose the integration of hydrogen storage modules with M3C power modules using high-frequency isolation windings and current-doubler rectifier circuits, enabling the low-frequency power transmission system to have energy storage capabilities. We establish simplified circuits for electrolysis cells and hydrogen production modules and provide a detailed control structure for the hydrogen storage modules and M3C. A switching logic is proposed based on the frequency deviation of the AC system to switch between hydrogen storage mode and fuel cell discharge mode for the energy storage modules. This facilitates the adjustment of AC system frequency during transient processes and enhances power system stability. We construct a system simulation model based on the proposed control methods and simulate various operating conditions such as steady-state wind power grid connection, AC system faults, and AC frequency excursions to validate the correctness and effectiveness of the proposed converter structure and control strategy.