Research On Stability Analysis Of Double Fed Induction Generator Based On Impedance Modeling

With the continuous expansion of the scale of wind farms and the continuous increase in the capacity of wind turbine assembly machines,it has brought many challenges to the security and stability of power system and wind power equipment.Wind power in China is mainly developed in the form of large-scale concentration and the load center is far away from the wind farm.Such a power transmission mode will introduce a certain line impedance between the wind farm and the power grid.In such a situation,the power grid is regarded as a weak electric network.The large-scale wind power grid connected with the weak power network can interact and influence each other,causing the grid system is unstable.For the instability problem caused by system interconnection,the impedance stability analysis method can accurately analyze the causes of instability,and provide a solution for the stability of grid connected system.Doubly fed wind power generator as the most extensive type of wind turbine,this paper carries out research on the problem of stability of doubly fed wind generator system under the condition of weak network,the main contents include: double fed motor system of positive and negative output impedance modeling,design grid strength and various control parameters on the stability of the system and virtual impedance controller.The modeling of equivalent output impedance of doubly fed asynchronous motor system is the focus of this paper.The paper uses the method of harmonic linearization,deduces the positive and negative sequence impedance model of the double fed motor system grid side converter,motor and rotor side converter and the whole system,in the process of modeling considering the influence of the motor structure,current regulator and PLL of impedance modeling,and the characteristics of the output impedance of analysis.Combined with the model and Nyquist stability criterion impedance established,respectively analyzes the stability in weak network under the network side converter,motor and rotor side converter and grid connected DFIG system,study the different effects of grid impedance value and control parameters on the stability of the grid system,which is validated by simulation.The unstable phenomenon for doubly fed generation system under the condition of weak network,this paper puts forward a design method based on virtual impedance control.The proposed controller is easy to implement without additional sensors.It can effectively improve the stability of the system without changing the parameters of the control system and the working condition of the system,and its effectiveness is verified by simulation.