Anlysis And Mitigation Strategy On Sub-synchronous Oscillation Of Dfig Grid-connected System

With the increase of the grid-connected capacity of doubly-fed wind turbine(DFIG),the sub-synchronous oscillation(SSO)caused by DFIG and series compensation grid becomes more and more prominent.Therefore,based on impedance analysis method,the sub-synchronous oscillation of DFIG-based wind turbine under the condition of series compensation is studied in this paper.Aiming to improve the grid-connected stability of DFIG-based wind turbine,the main influencing factors of the SSO are analyzed and the mitigation strategies considering the control characteristics of DFIG-based wind turbine itself are studied.In this dissertation,the complex vector theory is used to establish the impedance model of DFIG-based wind turbine.The dq impedance model of DFIG and the positive and negative sequence impedance model of DFIG in two-phase stationary coordinate system is unified.The influence of rotor side current inner loop,power outer loop,grid side current inner loop,DC voltage outer loop and phase-locked loop were considered in the modeling.Based on the established impedance model,the influences of rotor current loop gains,series compensation level and rotation speed on the stability of the system are analyzed by using generalized Nyquist criterion(GNC).Considering the control characteristics of the DFIG system,the stator current disturbance feedback is used in the rotor-side converter(RSC),and the grid-side converter(GSC)current disturbance feedback is used in the GSC to design the damping controller.The influence of damping control on the equivalent admittance of the DFIG and the stability of the whole grid-connected DFIG system is analyzed.Based on the impedance model,the influence of active and reactive power output of DFIG-based wind turbine on the stability of the grid-connected system is studied.It is concluded that the increase of the output active power value of the DFIG is beneficial to improve the system stability whether the DFIG is in sub-synchronous operation or super-synchronous operation.Under the sub-synchronous operation condition,the stator of DFIG generate inductive reactive power or the GSC absorb inductive reactive power can improve the stability of the system.The inductive reactive power emitted by the stator side will increase the point of common coupling voltage,and the inductive reactive power absorbed by the GSC will reduce the point of common coupling voltage.For this reason,a reactive power coordination control strategy is proposed to increase the stability of the grid-connected system.On the basis of analyzing the influence of power control strategy on system stability,combined with complex vector impedance model and RLC equivalent circuit,the influence of stator output active power and the DFIG system reactive power output on system equivalent damping and SSO frequency is further quantitatively analyzed.The increase of stator output active power,inductive reactive power generated by stator and inductive reactive power absorbed by GSC will increase the equivalent resistance of the system,thus improving the stability of the system.The reactive power coordination control strategy can further increase the equivalent resistance of the system,and has universal applicability under different working conditions,so as to effectively suppress the sub-synchronous oscillation.The simulation model was built in Matlab/Simulink,and the correctness of the DFIG-based wind turbine model derived in this paper was proved by frequency scanning method.The effectiveness of the coordinated damping control and reactive power coordination control strategy to mitigation SSO was verified by simulation.