Grid Connected Control Research Of Wind Farm Based On Separate Hybrid Energy Storage System

With the fast development of wind power in several years,the grid-connected capacity of wind power has been increasing,but the random fluctuation of wind power has a greater impact on the stability of power systems.The application of energy storage technology to wind power generation provides a solution to the problem of wind power grid connection.Therefore,this paper adopts a separate all-vanadium flow battery-super capacitor hybrid energy storage system to control the wind farm's grid connection from two aspects: smoothing the output power fluctuation of the wind farm and improving the low voltage ride through(LVRT)capability of the wind farm.The main work of this paper is as follows:The basic principle of direct-drive permanent magnet synchronous wind turbine is analyzed.Based on the characteristics and working principle of wind turbine,permanent magnet synchronous generator and full power converter,their mathematical models and control strategies are established.A grid-connected simulation model of 6MW direct-drive wind farm was established on Matlab/Simulink simulation platform.In order to make better use of hybrid energy storage to improve wind farm LVRT capability,based on the characteristics and advantages of traditional centralized energy storage and distributed energy storage,a separate hybrid energy storage access method is proposed,that is,the energy storage energy is centralized at the exit bus of the wind farm by using centralized energy storage;the power storage element is distributed at the DC bus of the single wind turbine by using distributed energy storage.The vanadium redox flow battery(VRB)is selected as an energy storage element,and the super capacitor(SC)is used as a power storage element.Based on the working principle of VRB and SC,their equivalent circuit model and simulation model are established.The charging and discharging simulation experiments are carried out to verify the correctness of the model.Based on the four-quadrant working principle of the bidirectional DC/AC converter,a improved control strategy of steady-state unity power factor control,active power smooth is limited and reactive support priority under grid transient faults,which is proposed for the converter.This strategy enables the centralized energy storage system to stabilize the active fluctuation of the wind farm when the grid is steady,and output reactive power to the grid when the grid voltage drops,to help the grid voltage return to normal as soon as possible.In addition,the control strategy of the bidirectional DC/DC converter of the distributed energy storage system is improved.The improved strategy can make the distributed energy storage system smooth the output power fluctuation of a single wind turbine during the steady state of the grid,and absorb the excess energy on the DC side of the wind turbine when the grid voltage drops..Finally,a grid-connected simulation model of wind farm system based on separate hybrid energy storage is established on Matlab/Simulink simulation platform.Under the condition of variable wind speed and different degrees of grid voltage drop,the effects of energy storage systems at different locations on wind power fluctuation and LVRT performance are studied.The simulation results show that the hybrid energy storage system adopts the proposed access method and control strategy,which can effectively smooth the fluctuation of wind power in the steady state.Under the transient fault of the power grid,it improves the LVRT capability of the wind farm from two aspects,including raising the wind farm's grid voltage and ensuring the stability of the wind turbine DC bus voltage.