| Under the global energy and environmental crisis,wind power,as the most developed and economic renewable energy,its installed capacity in the grid is increasing.Due to the intermittent and random fluctuation of wind speed,the output of wind turbine generator fluctuates greatly.At the same time,the rotor speed of direct drive permanent magnet wind turbine connected to the grid through power electronic devices is decoupled from the grid frequency,thus it reduces the inertia and damping of the grid,so the large-scale grid connection of wind power will affect the stable operation and power quality of the grid.In order to solve the above problems,this paper proposes to apply the virtual synchronous generator(VSG)control technology to the grid side converter control system of direct drive permanent magnet wind turbine,so as to ensure that the wind turbine has the ability to respond to the system frequency change and provide active power support for the grid.At the same time,flywheel energy storage device plays an important role in making up for the lack of reserve capacity of wind turbine and restraining the fluctuation of wind power.Therefore,the coordinated control method of wind turbine and flywheel energy storage system based on VSG control is studied in this paper.Firstly,by analyzing the structure and working principle of direct drive permanent magnet wind turbine,the mathematical model including wind turbine and permanent magnet synchronous generator is established,and then the influence of wind turbine on grid inertia under the traditional grid connected mode is analyzed.Compared with the inertia response principle of synchronous generator,the virtual inertia of wind turbine is derived,and the influence of the virtual inertia of wind turbine participating in system frequency modulation on the stability of power grid is analyzed,which lays a theoretical foundation for the following study of VSG control strategy.Secondly,in view of the problem that the wind turbine can not provide inertial support for power grid,the VSG control strategy is proposed for the grid side converter of direct drive permanent magnet wind turbine to respond to the system frequency change,and the constant voltage control is adopted for the motor side converter to maintain the bus voltage stability.The exciter,frequency regulator and inner loop controller of virtual synchronous generator are designed by simulating the control system of synchronous generator.The parameters of VSG controller are adjusted by combining the small signal closed-loop transfer function of VSG active power and the theory of optimal damping ratio.The effectiveness of VSG control theory is verified by simulation.Finally,aiming at the problem of insufficient reserve capacity of wind turbine based on VSG control strategy,the coordinated control method of wind turbine and flywheel energy storage system is proposed.In order to solve the problem that the inertia response of grid side converter based on VSG control will affect the stability of bus voltage,a speed tracking control strategy is proposed for the converter of flywheel generator side in the grid connection start-up phase,a constant voltage control strategy of DC bus is adopted in the stable operation phase,and the zero pole elimination method is used to adjust the controller parameters.Based on the analysis of the moment of inertia of wind turbine and the limited range of flywheel rotor speed,from the perspective of power dispatching instructions of power grid,under the condition that the wind power system and flywheel energy storage system meet the constraints,the flywheel energy storage system is mobilized to cooperate with wind turbine to provide active support for the grid.A simulation model of wind storage joint system is built in Matlab/Simulink,the simulation results show that the proposed control strategy not only improves the ability of suppressing wind power oscillation,but also achieves the purpose of maintaining bus voltage stability. |
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