Grid integration and fault ride-through of fractional frequency offshore wind power system based on Y-connected modular multilevel converter

Fractional frequency transmission is a promising solution to offshore wind power integration. This paper proposes a new grid integration scheme of fractional frequency offshore wind power system (FFOWPS) based on Y-connected modular multilevel converter (Y-MMC). To get better power quality, a novel Y-MMC modelling and control method considering frequency leakage is presented. Besides, in order to achieve satisfactory fault ride-through (FRT) performance for FFOWPS, this paper proposes a novel FRT strategy. The strategy makes use of the onshore grid voltage and offshore grid frequency droop control to coordinately control the power of Y-MMC and wind turbines. This strategy achieves power balance on both sides of Y-MMC through coordinated control, thereby suppressing the increase of DC capacitor voltage during faults. It is also able to support the grid by injecting reactive power during voltage sags. Additionally, a new pitch angle control algorithm is designed to keep the balance between input mechanical power and output active power of wind turbines during FRT, avoiding the rapid rise of rotor speed. The simulation results in MATLAB/Simulink demonstrate good performance of FFOWPS during wind speed variations and validate the effectiveness of the proposed frequency leakage suppression strategy and FRT strategy.