Small Signal Stability Analysis And Improved Control Of DFIG-based Wind Turbine System In Weak Grid

With the large-scale development of wind energy,the penetration of wind power into power system is continuously increasing,which will affect the stable operation of power system.The wind farms are usually located in the remote areas where is far away from the load center.Therefore,wind power needs to be delivered through a long-distance transmission line.With the increased capacity of wind power integration,the short circuit ratio(SCR)at the point of common coupling(PCC)will become lower,which will make the grid weak.Under weak grid condition,the interaction between wind turbines(WTs)and the grid will cause that the whole system cannot operate normally.Among several tapes of WTs,the doubly fed induction generator(DFIG)is the most widely employed one.Therefore,the small signal stability of DFIG-based WT system in weak grid should be paid high attention.This paper studies the small signal stability and improved control strategy of DFIG-based WT system in weak grid,and the main contents are follows:(1)The static admittance model and small signal admittance model of DFIG-based WT are developed in the synchronous rotating frame.Considering the current loop and control delay,the static admittance model of grid-side converter system is built.Moreover,the small signal admittance model is derived considering the effect of phase locked loop(PLL).The influence of the stator flux transient components,the current loop and control delay are taken into account to build the static admittance model of machine-side system,and the small signal admittance model are developed considering the effect of PLL.Based on the admittance model of grid-side converter system and machine-side system,the static admittance model and the small signal admittance model of DFIG-based WT are obtained.The effects of PLL on the admittance characteristics of grid-side converter system,machine-side system and DFIG-based WT are analyzed in detail.Moreover,the influences of grid-side converter system and machine-side system on the small signal admittance characteristics of DFIG-based WT are also analyzed.(2)The admittance model of DFIG-based WT is the second-order matrix,that is,DFIG-based WT system is Multiple Input Multiple Output(MIMO)system.Therefore,the generalized Nyquist criterion is employed to analyze the stability of DFIG-based WT system.The analysis indicates that the admittance of DFIG-based WT is changed because of the effect of PLL disturbance,which will reduce the small signal stability of the system in weak grid.This paper detailedly analyzes the impacts of the control bandwidth of PLL,SCR,grid-side converter system,PI controller parameters of the current loop,system operation point and control delay on small signal stability of the system.(3)Under weak grid condition,DFIG-based WT system is easy to be unstable.Therefore,this paper proposes a voltage feedforward control strategy to reshape the admittance of DFIG-based WT and reduce the negative effects of PLL disturbance,which can significantly improve the small signal stability of the system.The proposed control strategy doesnot need to change the PI controller parameters and system operation point.Moreover,the proposed strategy is easy to implement.The 1.5MW DFIG system is built in MATLAB/Simulink and 2.5kW experimental platform is established.The simulation and experimental results verify the theoretical analysis and the effectiveness of the proposed control strategy.