Research On Transient Stability Of DFIG Connected To Power System Based On Virtual Inertial Control

With the goal of carbon peak and carbon neutralization put forward,to comply with the development trend of green and low-carbon economy and reshape the energy system has become the trend of global development in the future.To promote the clean and low-carbon transformation of energy,the key is to speed up the development of non-fossil energy,especially wind power,solar power and other new energy.Wind energy resources are very abundant in China,such as the eastern coast and northwest regions.Nowadays,with the gradual increase of wind power permeability in the power system,more and more wind power generation is bound to bring new problems to the security and stability of the power system.Therefore,the impact of wind power access on the transient stability of power system has been widely concerned.At present,the dominant variable speed wind power system usually works in the maximum power tracking mode in order to obtain the maximum wind energy.This makes the speed of the variable speed wind turbine decoupled from the system,and its rotational kinetic energy is hidden,resulting in the wind turbine can not provide inertia support function to the system.If there is a large disturbance in the power grid,the frequency and transient power angle of the power system have a high probability of instability,which will affect the safety and stability of the wind turbine.In order to solve this problem,many countries have stipulated in their wind turbine grid-connected specifications that wind farms need to have inertial support functions similar to those of conventional power plants.Virtual inertial control can provide certain inertia response capability for wind turbine through additional control.Most of the existing researches focus on the transient power angle or frequency stability of wind turbine system after adding virtual inertia control.Therefore,this paper focuses on the influence of Doubly-Fed Induction Generator access to power system transient stability after taking into account virtual inertial control.In this paper,firstly,based on the operation principle,basic structure and mathematical model of DFIG,the control principles of pitch angle control,maximum power tracking control,rotor-side converter control and grid-side converter control in doubly-fed induction machine and DFIG integrated control system are analyzed in detail.Then,aiming at the problem of doubly-fed wind turbine responding to the dynamic change of power grid,three different operating states of doubly-fed wind turbine are analyzed based on the operation characteristics of doubly-fed wind turbine.The operation characteristics of doubly-fed wind turbine analog synchronous units responding to system changes by adjusting output power are studied.By analyzing the principle and deficiency of virtual inertia control,an optimized inertia control strategy based on rotor kinetic energy is proposed,which dynamically adjusts the gain coefficient according to the rotor kinetic energy.the inertia control gain is set for doubly-fed wind turbines running under different working conditions,and then the dynamic inertia control is realized.Finally,in view of the influence of large-scale wind farms connected to the power grid on the inertia characteristics and transient stability of the power grid,the influence of wind farms connected to the grid on the inertia characteristics of the system under different wind power access ratios is studied.according to the frequency characteristics of the system,the frequency response model is established,and the change of the frequency characteristics of the system after additional inertia control is analyzed.Based on the extended equal area criterion,the relationship between the frequency modulation characteristics and power angle characteristics of wind turbines connected to the grid due to additional virtual inertia control is analyzed.Based on the optimal inertia control method considering the rotor kinetic energy of DFIG,a dynamic adaptive virtual inertia control strategy is proposed.The wind farm grid-connected simulation system is built on DIg SILENT/Power Factory,and the simulation verification is carried out under load change and three-phase fault respectively.