Research On Secondary Drop Optimized Control Method Based On Primary Frequency Control Of Doubly-Fed Wind Turbines

Wind power has become one of the most potential new energy generation methods because wind energy has many advantages such as clean,wide source and renewable.However,with the large-scale grid connection of wind power,the stability of grid frequency is facing increasingly severe challenges.On the one hand,the application of the doubly-fed asynchronous generator completely decouples the power from the frequency,which causes the wind turbine to fail to track the frequency fluctuations in time,and the equivalent inertia of the system is 0.On the other hand,the wind turbine generally adopts the maximum power tracking control strategy that additional active power cannot be provided to account for fluctuations in grid frequency.This paper mainly studies the primary frequency control strategy of doubly-fed wind turbines.Firstly,this paper introduces three main methods for the existing wind turbines to participate in primary frequency control:rotor inertia control,over speed control and pitch control,and analyzes the principle of participating in the system's primary frequency modulation control.Based on the situation of wind power grid-connected in Jibei Power Grid,the primary frequency control method with reserved standby and no standby is compared.Secondly,based on the RT-LAB platform,a semi-physical simulation model is built to compare and analyze various control methods.The simulation results show that compared with the standby frequency modulation method,the inertia control of the rotor without power reserve ensures the economical operation of the unit and effectively improves the problem of inertia reduction caused by grid-connected wind turbines.Thirdly,based on the simulation verification,this paper chooses to study the wind turbine's participation in primary frequency modulation control based on rotor inertia control.Aiming at the secondary drop of the system frequency caused by the sharp drop of the torque at the moment of exiting the primary frequency regulation based on the rotor inertia control,the causes of the secondary drop of the system frequency are analyzed from the two points of the drop of active power and the release of rotor kinetic energy.Based on this,the key parameters affecting the secondary drop such as inertia time constant,active frequency modulation coefficient and lower speed limit are studied.Finally,in order to optimize the secondary frequency drop of the system caused by rotor inertia control,this paper proposes a new speed recovery strategy from the perspective of power drop speed.The strategy optimizes the control output power based on the variable power curve to mitigate the secondary frequency drop due to the sudden drop in active output when the speed is restored.On this basis,the particle swarm optimization algorithm is used to optimize the parameters of the variable power curve.Based on MATLAB/Simulimk,the simulation model of the doubly-fed wind turbine is built and the primary frequency modulation strategy based on rotor inertia control is designed.The simulation results show that the speed recovery strategy based on the variable power curve can effectively optimize the secondary drop of the system frequency and improve the smoothness of the electromagnetic power change.