Enhanced Control of Direct-Drive Permanent Magnet Wind Power Generation System Containing Flywheel Energy Storage Unit During Asymmetrical Fault in Power Grid

The mechanism of DC bus voltage fluctuation in permanent magnet direct-drive wind power system containing flywheel energy storage unit during asymmetrical fault in power grid is analyzed,and on this basis an enhanced control strategy suitable to low-voltage ride through(LVRT) in such a wind power system is researched.Three selectable independent operating modes can be provided when the wind power system provides power grid transient reactive power support during asymmetrical fault in power grid by the proposed control strategy.The proposed control scheme surmounts the limitation of restraining DC-link voltage fluctuation by grid-connected converter under traditional control modes and leads proportional integral-resonant(PI-R) controller into closedloop current control of flywheel motor to absorb double power frequency pulsating power in DC-link capacitor by flywheel energy storage unit,thus the stable control of DC-link voltage free of fluctuation during asymmetrical fault in power grid can be effectively implemented and further enhance LVRT ability of wind power system under asymmetrical fault in power grid.The correctness and effectiveness of the proposed control strategy are verified by Matlab/Simulink-based simulation results.