Equivalent Modeling Of Wind Farm And Study On Subsynchronous Oscillation Characteristics Of Grid Connected System

With the rapid growth of wind power integrated capacity,the dynamic coupling interaction between wind farm and system has triggered many new subsynchronous oscillation events,which exerts a serious threat on the power system in terms of safe operation.The subsynchronous oscillation caused by the interconnection of wind farms is a comprehensive effect that combined "field" with "grid",which is not the simple superposition of a single machine effect.The accurate description of the effect of "field" is the basis of analyzing and restraining the subsynchronous oscillation.The mathematical modeling of wind farms is a process of establishing the mathematical model which can reflect the function of wind farms by the study of the interaction characteristics and internal laws of the actual field network.The existing wind farms equivalent method ignores the coupling between generators and the power grid,which improves the simulation difficulties on the accurate reflection of the subsynchronous oscillation characteristics caused by the interaction between the actual field and the power grid.Aiming at the problem of subsynchronous oscillation in grid-connected wind farms,the dynamic characteristics analysis of wind farms,the equivalent modeling of wind farms,the mechanism of subsynchronous oscillation in grid-connected wind farms,and the influence of wind farm access on the subsynchronous oscillation characteristics of the system are studied systematically and deeply.The main innovative works are as follows:The graphical modeling method of wind farms and analysis of grid-connected dynamic characteristics.A graphical modeling method of wind farms is proposed.A wind farm is composed of a cluster of wind turbines connected with a collection network,the dynamic characteristics of wind farms are affected not only by the operating conditions,control parameters,collection network and other factors,but also by the power grid(when the share of wind power grid capacity reaches a certain degree,its influence cannot be ignored).A full order linear model of wind farm is constructed using the idea of block modeling and the effect of the collector network.The variation trend which reflects wind farm dynamic characteristics under different operating conditions,control parameters,collector network,power grid strength and other factors are analyzed.It can be revealed that the operating conditions and control parameters of wind farms are the two key factors influencing the dominant dynamic characteristics of the system under the condition of weak power grid.A wind farm equivalence method based on cluster clustering of unit operation characteristics is proposed.Wind farms present the characteristics of "large number,complex model and high order",hence it is an inevitable choice to conduct the reasonable and effective equivalence for wind farms.The traditional single machine equivalent model of wind farm does not consider the influence of the steady-state operation point on the dominant eigenvalue,meanwhile it is lack of the strict theoretical basis for equivalence,and it is difficult to ensure the consistency of the dominant dynamic characteristics before and after the system equivalence.Considering the coupling effect between wind farm and power grid,combined with K-means clustering method,a wind farm equivalent method based on the clustering of unit operation characteristics is proposed under non-ideal conditions(there are differences in operating conditions and control parameters between units),and a multi-unit equivalent model is constructed,which can effectively overcome the shortcomings of a single unit equivalent model without considering the differences of unit operation characteristics.Therefore,considering the aforementioned demerits and based on the principle of similarity transformation,that is,the similarity matrix has the same eigenvalue,and the mathematical basis for single machine equivalence of wind farm then can be analyzed.The equivalent resistance model of DFIG wind farms considering the difference of operation characteristics is proposed.Focusing on the subsynchronous oscillation problem of DFIG wind farms connected to the system with fixed series compensation,the "resistance" model of wind farms is established based on the state space equation to clarify the relationship among operating conditions,control parameters,resonance frequency and negative resistance,and reveal the conditions of system oscillation induction.Because of the differences in operating conditions and control parameters for each wind farm,the equivalent resistance model of the DFIG wind farm considering the differences of operation characteristics is established,which can realize accurate and quantitative analysis on negative resistance characteristics of wind farms.A generalized complex torque coefficient method is proposed so as to analyze the subsynchronous oscillation mechanism of PMSG wind farms.Aiming at the subsynchronous oscillation problem of PMSG wind farms connected to weak AC system,the generalized complex torque coefficient method is proposed.The interconnected system is divided into the "mechanical" and the "electrical" subsystems,and the interaction between subsystems is studied.In terms of the two scenarios of single equivalent PMSG wind farm and multi parallel equivalent wind farm,the calculation methods of damping coefficient of the frequency domain and time domain are constructed respectively.The influences of operating conditions,control parameters and power grid strength on damping coefficient are analyzed,and the mechanism of system negative damping instability is revealed.The damping characteristics of the system are identified by the net electrical damping method and the method of additional excitation signal injection.The torsional vibration of synchronous generator shafting induced by wind farm connection is manifested in rotor speed oscillation,which contains abundant electromechanical coupling information.The contribution of wind farms which exert the torsional vibration on synchronous generators shaft system is identified by applying AC voltage signal at both ends of excitation winding to excite speed oscillation,observing the influence of wind farm access type,control parameters,operating conditions and other factors on the free response characteristics of speed disturbance.