Reactive Power Compensation Control Of DFIG And Stability Analysis During LVRT Process

At present,the development of wind power generation technology is in a period of rapid rise.With the wind energy characteristics of the randomness and uncertainty,the influence of large-scale wind farm connection on power grid can not be ignored.According to the wind farm codes,the doubly-fed induction generator(DFIG)is required to possess low voltage ride through(LVRT)capability during grid faults,to keep DFIG grid-connected operation and provide reactive power compensation performance.However,in the process of injecting reactive support current from wind farm to the fault grid,sub-synchronous oscillation may occur in the grid connected system under some parameter conditions,which will seriously threaten stability of system and safety of DFIG.Therefore,in this paper,the control result of voltage support during grid fault and the mechanism of system sub-synchronous oscillation phenomenon caused by LVRT reactive power compensation are investigated,and the influence of main factors are analyzed.Firstly,based on the basic structure and operation principle of DFIG,establishing the mathematical models of DFIG system main modules,including wind turbine model,DFIG model,rotor side and grid side converter(RSC&GSC)model based on the stator voltage oriented vector control method.Secondly,the electromagnetic transient response of DFIG in the process of voltage sag is analyzed,and the limit of reactive power compensation capability of DFIG stator and GSC is deduced.Combined with the grid code requirements for LVRT capability of wind turbine,making full use of the reactive power compensation capability of DFIG and SVC,the phased control strategy of LVRT reactive power compensation is formulated.A complete grid-connected doubly fed wind power system model is built in the DIgSILENT/PowerFactory simulation software.Then the voltage support result of the LVRT reactive power control strategy is theoretically derived and simulated.Based on the vector control,considering the transient process of wind turbine,the closed-loop controller of RSC and GSC and the external grid conditions,the closed-loop transfer model of LVRT reactive power compensation control system of DFIG and grid-connected SVC are constructed,and the transfer functions are derived.Var eigenvalue stability analysis,Nyquist stability analysis and quasi-steady state model analysis,the mechanism and the influence factors of sub-synchronous oscillation in the process of LVRT reactive power compensation is discussed,including.The results confirm that,with the increase of LVRT compensation gain value,the increase of delay time constant of RSC controller,the increase of grid impedance and the increase of SVC voltage loop gain value,the conjugate eigenvalues in the sub-synchronous frequency range of grid-connected power system will shift to the right,which means that the system damping will become weaker and the system stability will be deteriorated causing stronger sub-synchronous oscillation.Finally,the validity of theoretical analysis is verified var simulation results.