Research On Analysis And Suppression Strategy Of Subsynchronous Oscillation In DFIG-based Wind Farms

With the increasing wind power penetration in power systems,the operation of large-capacity wind farms will increase the risk of sub-synchronous oscillation and bring aboutnew challenges to the safety and stability of power system.In this paper,the sub-synchronous oscillatory characteristics of the DFIG-based wind farm are analyzed,and the sub-synchronous oscillation suppression strategy is put forward by using the flexible and rapid control capability of the wind turbine.The main contents are as follows:The dynamic mathematical models of DFIG-based wind farms and grid-connected system are established,which include aerodynamics,drive chain,doubly fed induction generator,converter control system and series compensation line model.The dynamic model lays the foundation for the analysis of the sub-synchronous oscillatory characteristics of the DFIG-based farm and grid-connected system.The eigenvalue analysis method is used to analyze the oscillation characteristics of the DFIG-based wind farms and grid-connected system and the key parameters of the sub-synchronous oscillation mode under different line topologies.For the sub-synchronous oscillation mode,the state variables are most related to the rotor current by participation factor.The key factors influencing the sub-synchronous oscillation mode are analyzed.The results show that the wind speed of DFIG,the proportional parameters and the integral parameters of the inner loop of the rotor-side converter and the network topology exert a significant effect on the sub-synchronous oscillation characteristics of system.A sub-synchronous oscillation suppression strategy of DFIG is proposed based on virtual impedance.Firstly,the impedance model of the system is established.Secondly,the correctness of the impedance model is verified and the stability of the system under different control parameters is analyzed by frequency scan and Nyquist stability criterion.Then,the virtual impedance controller is designed based on the impedance model.The suppression effect is studied by the Nyquist stability criterion and Bode plot.Finally,the time domain simulation is used to compare the suppressing effect of controllers on sub-synchronous oscillation.A sub-synchronous oscillation suppression strategy based on the rotor phase shift is proposed in view of the doubly fed wind farm grid-connected system parameter identification is prone to bias and the detailed parameters related to the doubly fed induction generator controller are difficult to obtain.The suppression strategy is on the basis of the analysis of the rotor current component.Through phase shift and calculating avarege value of the rotor current,a fast stabilization of the system sub-synchronous oscillation is achieved.The effectiveness of the proposed suppression strategy and the robustness to the measurement frequency error are verified by time domain simulation.The theoretical research and simulation results show that the eigenvalue analysis method can be used to analyze the operation characteristics of the system.By using the flexible control characteristics of the DFIG,the design of an effective controller can realize the rapid suppression of the system synchronous oscillation and provide important theoretical support for the safe and stable operation of the power system with wind farms.