Research On Control Strategy Of Doubly-fed Induction Generator Participating In Primary Frequency Modulation Of Power System Under Power Limitation

The energy crisis has promoted the rapid development of wind power generation.With the large-scale wind power Grid-connected worldwide,the frequency stability of power system has become increasingly prominent.As the mainstream of wind power generation,doubly fed wind turbines(DFIG)are widely used in wind power generation because of their variable pitch adjustment,improved utilization of wind energy and variable speed and constant frequency operation.But because the rotor side of DFIG is connected to the power grid through the frequency converter,the rotor speed of DFIG is decoupled from the power grid frequency,and it can not respond to the change of system frequency.After large-scale wind power is connected to the power grid,the inertia of the system will be reduced,which is not conducive to the stable operation of the system.Therefore,it is of great significance to study the frequency modulation capability of DFIG after it is connected to the grid.In this paper,the DFIG is operated under rated wind speed by over-speed load reduction and power limitation,and above rated wind speed by pitch angle to control power limitation,reserving active reserve,which makes DFIG have primary frequency regulation capability when disturbance occurs.On the basis of traditional pitch angle control,the relationship between pitch angle change and frequency change is improved.A frequency modulation control method is designed respectively above and below rated wind speed,and is verified by simulation.In the wind turbine layer,the limited power load reduction rate is defined in the whole wind condition interval to ensure the stable operation of the wind turbine while reserving the corresponding active reserve.The adjustment coefficient of DFIG is adjusted to improve the sag characteristic of wind turbines.Improvement on the basis of traditional frequency modulation control.The virtual inertia control coefficient and sag control coefficient curves and fitting expressions are formulated for different wind speed ranges.The primary frequency modulation capability of DFIG in power grid is effectively improved by using power-limited downconversion coefficient control strategy.The simulation results are validated under slope wind speed and variable wind speed by using MATLAB/simulink 4 machine 2 area simulation system model.In the control of wind farm layer,the influence of wind power permeability on grid frequency is analyzed.On the basis of reserved reserve for limited power operation of wind turbines,the allocation strategy of active power output of high wind speed units is put forward to reduce the FM output of low wind speed units,and DFIG is involved in frequency modulation as much as possible through coordinated control among units.At the same time,reasonable adjustment coefficients are set up under different wind speed conditions to satisfy the system frequency stability and reduce the frequency modulation pressure of synchronous machine.Finally,in the 4-machine-2-area system model built by MATLAB/simulink,the wind farms based on high wind speed and low wind speed are simulated and analyzed respectively.