Stability Analysis Of DFIG System Considering Switching Under Large Disturbance

In response to the energy and environmental crisis,countries around the world have changed the traditional energy pattern and vigorously developed renewable energy power generation,with wind power generation as the representative.As the penetration rate of wind power is increasing year by year,the stable operation of power grid is also facing new challenges.In particular,the dynamic interaction between power electronic converters or between power electronic converters and the network brings new sub/super synchronous oscillation risks.However,most of the existing researches adopt analysis methods based on small-signal models,focusing on negatively damped oscillations under small disturbances,but there are relatively few studies on the mechanism of special nonlinear switching participating in oscillations.In view of this,this paper studies the influence of the nonlinear control switching under large disturbances on the stability characteristics of the DFIG grid-connected system.The main work is as follows:(1)When the DFIG grid-connected system is positively damped,the subsynchronous frequency band oscillation phenomenon with limit saturation during the large disturbance period was discovered.The oscillation corresponds to the non-smooth oscillation of the non-smooth system and belongs to the category of global dynamics mathematically,and the influence of limit and the non-smooth bifurcation characteristics of different parameters are analyzed.First,the non-smooth bifurcation theory and switching oscillation are introduced;secondly,the d/q axis current limit in the power outer loop of the DFIG grid-connected system and the rotor-side converter(RSC)is introduced,and the switching sub-synchronous oscillation phenomenon caused by the above d/q axis current limits is analyzed.The influence of short-circuit capacity,fault ground resistance,PI parameters of the power outer loop on the oscillation is further analyzed,and the non-smooth bifurcation of the oscillation relative to one parameter is revealed.Finally,the non-smooth bifurcation characteristics of the PI parameters of the power outer loop are given on the plane,indicating that the switching oscillation may be related to the saturation of different limits.(2)When the DFIG systems is positively damped,the switched chaotic instability form induced by limit after large disturbance was discovered,which is mathematically corresponding to the non-smooth bifurcation occurs from the convergence to the stable equilibrium point to the chaos of the trajectory of the non-smooth systems,and the non-smooth bifurcation characteristics of multiple parameters are analyzed.Firstly,the non-smooth bifurcation theory and switched chaotic oscillation of switched dynamic system are introduced.Secondly,the non-regular oscillation phenomenon in the DFIG system after the AC fault is analyzed,and it is confirmed as a chaotic oscillation with the power spectra and the Lyapunov exponent,furthermore,its pseudo-period frequency drift characteristics are analyzed.And then,the relationship between the chaotic oscillation and the limits saturation is analyzed,revealing that the chaotic oscillation is induced by saturation of multiple limits,which is essentially a switched chaotic oscillation.Finally,the non-smooth bifurcation characteristics of the switched chaotic oscillation under large disturbance are analyzed with the viewpoint of SCR,fault condition and wind speed,indicating that the chaotic oscillation are more likely to occur under the condition of large disturbance and weak interconnected network.(3)A simplified state-space model of the grid connected(VSC)is established,and the small disturbance stability of the model is analyzed.Then a type of switched oscillation caused by the current limit saturation was revealed,and its possible causes of oscillation is analyzed by combining its physical reduced-order system(PROS).First,the simplified state-space model of VSC is deduced,and its small disturbance characteristics and the influence of parameters are analyzed.Secondly,the influence of d-axis current limit saturation on the dynamics of the system with positive damping is analyzed,it is found that a type of switched oscillation with continuous limit saturation occurs under large disturbances,and the equilibrium of mathematical reduced-order system(MROS)and existing problem are analyzed.Thirdly,from the perspective of the PROS obtained by continuous limit saturation,the reason for the switched oscillation of original system(OS)is analyzed,that is.the negative damping oscillation of the PROS;Finally,the relationship between the parameter conditions of the PROS’s oscillation and the OS’s switched oscillation is discussed.(4)For the sub-synchronous oscillation phenomenon of the VSC system induced by the asymmetric unilateral d-axis current limit(AU-d-CL),an approximate analysis method based on the describing function of unilateral limit and generalized Nyquist criterion is proposed.In detail,firstly,a simplified model of the grid-connected VSC system is established.And then,small-signal stability property of the grid connected VSC system is analyzed,and the phenomenon of negative damping SSO induced by AU-d-CL is presented.Furthermore.the frequency-domain model of VSC system and describing function of AU-d-CL are deduced.Finally.combined with the describing function and the generalized Nyquist criterion,the amplitude and frequency of SSO with different parameters and limit values are approximately estimated,which are basically consistent with the simulation results.It could explain the phenomenon that the amplitude of SSO varies according to the limit value,while the frequency remains unchanged.