Low Voltage Ride-through Control Strategy For Direct-drive Wind Turbine Generator

In the background of global climate change and increasingly prominent energy problems,wind power as a renewable energy has been developed rapidly.With the increase of wind power penetration,wind power generation systems will also face more stringent requirements,and the new version of the grid-connected standard makes new requirements for the power characteristics of wind turbines under lowvoltage ride-through conditions on the basis of the original.In this paper,the control strategy of direct-drive wind turbines under low-voltage ride-through conditions is analyzed and studied in depth,and the main research contents are as follows.(1)The mathematical model of direct-drive wind turbine is analyzed;the pitch control and converter control strategies of direct-drive wind turbine in steady state are studied;the traditional control strategies of unloading circuit and grid-side converter of direct-drive wind turbine under low-voltage ride-through conditions are studied.(2)The control principle of finite-set model predictive control(FCS-MPC)is studied with a three-phase voltage-source converter as the control object,including its basic idea,prediction model,and cost function;the original model prediction algorithm is improved for the delay problem,and the improved multi-step model predictive control is proposed.The improved algorithm is verified by MATLAB/Simulink simulation platform to have better tracking characteristics and can significantly reduce the harmonic content of the current waveform under the consideration of time delay.(3)The model predictive control is applied to the grid-side converter of directdrive wind turbine,and the LVRT control strategies based on the model predictive control are proposed for symmetrical and asymmetrical faults of the grid,and the positive and negative sequence component extraction techniques and phase locking techniques are investigated under the asymmetrical grid voltage.The proposed control strategy is verified by MATLAB/Simulink simulation platform to effectively improve the power response speed of the converter under symmetrical faults;under asymmetrical faults,it can lift the positive sequence component of grid voltage and suppress the negative sequence component of grid voltage.