Research On Suppression Of DC Voltage Fluctuation Of Direct Drive Wind Turbine During Asymmetric-Fault Ride-Through

As fossil energy is gradually depleting globally,people are paying more and more attention to renewable energy.With the continuous improvement of wind power generation,its proportion in the power grid is increasing year by year.Whether the wind turbine can run stably will have a greater impact on the stability and safety of the power grid.At present,the low-voltage ride-through control technology for direct-drive wind turbines under symmetrical grid faults is relatively complete.However,long-term grid operation experience shows that asymmetric faults account for a large proportion of grid faults.Direct-drive wind turbines will suffer from grid-connected current distortion,double-frequency fluctuations of grid-side outputted power,and double-frequency fluctuations of DC voltage,which increase the difficulty of low-voltage ride-through control for direct-drive wind turbines.In this paper,based on this background,the research on the double-frequency fluctuation suppression strategy of DC voltage for direct-drive wind power system under asymmetric fault is carried out.First,the forward and reverse synchronous rotating coordinate system is introduced.The mathematical model and equivalent circuit of grid-connected converter is given.Based on the theoretical derivation of power outputted by the wind power system when the grid-connected voltage is asymmetric,the mechanism about the double-frequency fluctuation of the DC voltage is explained.Secondly,the instantaneous symmetrical component method based on the rotation vector and the instantaneous symmetrical component method based on the notch filter are analyzed in this paper.The theoretical derivation of power fluctuated at double-frequency output from the stator of synchronous generator is found to find the variable that determines the double-frequency power.Based on this,a double-frequency fluctuation of DC voltage suppression strategy for a direct-driven wind power system based on the measurement of grid-side double-frequency power is proposed.Thirdly,the working characteristics of the PI controller,the PR controller and the Quasi-PR controller are analyzed and compared,and the reason for using the Quasi-PR controller in this paper is obtained.According to the equivalent circuit of synchronous generator,the relationship between electromagnetic power output from the stator and the axis component of stator current is deduced,and then a double-frequency fluctuation of DC voltage suppression strategy for a direct-driven wind power system based on a Quasi-PR controller is proposed.Finally,a mathematical derivation about the increasing on rotor speed caused by the application of this strategy in this paper during a grid fault.It is proved through simulation that the increase of rotor speed is small and will not cause adverse effects on the wind turbine.This paper analyzes the feasibility of proposed two strategies,and based on this,uses the Matlab/Simulink system as a simulation platform to conduct relevant simulation research on the transient process of the wind turbine during asymmetric fault.The experimental verification and discussion of the control strategy are carried out,and the simulation results prove the feasibility and effectiveness of proposed two control strategies.