The Analysis And Restrain Strategy Research On Resonance Of Grid Connected DFIG Wind Turbine

Doubly-fed induction generator(DFIG)wind turbine is one of the most widely used types of wind turbines,and the stability of grid-connected DFIG wind turbine has attracted much attention.Because DFIG wind farms are generally built in areas far away from the load center,there are generally two kinds of grid connection states when resonance occurs,one of which is connected to a weak grid which the grid impedance cannot be ignored.In this case,the interaction of grid impedance and the equivalent impedance of DFIG wind turbine may cause oscillations.The other is to connect with the series compensation grid with a series compensation capacitor.In this case,there is a risk that sub-synchronous resonance(SSR)may occur in the grid-connected system.Therefore,the impedance model is applied to analyze the grid-connected oscillations of DFIG wind turbine under these two states.Furthermore,improved controllers to suppress potential grid-connected oscillations and improve the operation stability of DFIG wind turbines are designed.In this dissertation,impedance analysis method is applied to analyze the stability of grid-connected DFIG wind turbine.Firstly,a small-signal synthetical impedance model for a DFIG wind turbine is established in the dq coordinate system.Compared with existed impedance model,the synthetical model in this dissertation includes the inner and outer loops of both the rotor-side and grid-side controllers,what’s more,the DClink part is also considered in the model to combine the rotor-side and grid-side through power coupling effect.For the weak grid condition,the influence of the controller parameters and the grid strength on stability of the DFIG wind turbine is analyzed based on the generalized Nyquist stability criterion.Based on the established impedance model of DFIG wind turbine in dq coordinate system,the small-signal transfer relationship between the voltage at PCC(point of common coupling)to the output of the rotor-side and grid-side controllers is deduced.Based on the transfer relationship,a novel controller which introduces the feedforward of voltage disturbance at PCC into both rotor-side and grid-side controllers is designed to improve the operation stability.Because the physical meaning of the impedance model in the dq coordinate system is not clear,it is difficult to directly use this model to perform sub-synchronous resonance analysis under the series compensation grid condition.Therefore,this dissertation transforms the impedance model in the dq coordinate system into a twophase stationary coordinate system through frequency transformation and expressed by complex vector.The positive and negative sequence impedance model of DFIG wind turbine is obtained in a two-phase stationary coordinate system.Not only the coupling degree of the model is reduced remarkably,but also the physical meaning is much clearer.Based on this model,an equivalent RLC series resonant circuit is used to analyze the effects of controller parameters and grid parameters on the sub-synchronous resonant frequency and system equivalent damping.Because the analysis shows that the system equivalent damping among the sub-synchronous frequency band is decreasing with frequency,this dissertation introduces the additional damping and virtual inductance in the rotor-side controller simultaneously to increase the system equivalent damping while reducing the system’s inherent sub-synchronous resonance at the same time.The suppression effect of sub-synchronous resonance is more efficiently compared with traditional sub-synchronous damping controller.The frequency sweeping of the DFIG wind turbine equivalent impedance is performed by using a detailed simulation model in Simulink to verify the correctness of the proposed models.The correctness of the stability analysis and the effectiveness of improved controllers is verified through simulation under the conditions of the weak grid and series compensation grid.Experiments on a laboratory experimental platform verify that the stability analysis is correct and improved controller is effective under weak grid condition.