A Novel High-Capacity Cascaded Wind Turbine Converter with High Frequency Link for Fractional Frequency Offshore Wind Power Application

In order to adapt to the increasing capacity of single wind turbine, the internal converter as the key equipment of fractional frequency transmission system (FFTS), has attracted more and more attention. Based on modular multilevel technology and cascaded H-bridge technology, in this paper, a novel cascaded wind power converter is proposed. The new topology uses an isolated high-frequency transformer, which replaces the expensive and large transformer used in the low-frequency transformation of wind turbine, which is suitable for medium-voltage and large-capacity wind power system. In the meantime, the overall control scheme is designed. The d-axis zero current control is adopted on the wind turbine side, and the feedforward decoupling control is adopted on the grid side. Considering the secondary voltage fluctuation in the DC side, the control strategy is improved by injecting zero sequence voltage and reforming split capacitors. Finally, simulation model of offshore wind farm composed of permanent magnet direct drive wind turbines using the proposed converter is built in the MATLAB/Simulink. And the feasibility of this proposed topology has been verified through operation performance in different scenarios.