Research On Equivalent Modeling Of Doubly-fed Wind Farm Andsub-synchronous Oscillation Of Its Grid-connected System

With the development of wind power generation,large-scale wind energy resources have been developed.Uneven geographical distribution characteristics of wind power and load centers in China restricts the consumption of wind power,and long-distance transmission is an important technology to solve this problem.Series compensation can significantly improve the transmission capacity,increase stability margin of system,and has been widely used in longdistance transmission but brings sub-synchronous oscillation.However,large-scale wind farm with small capacity of a single machine and the complexity of modeling are bottlenecks for studying the dynamics of wind power integration such as sub-synchronous oscillation.To study sub-synchronous oscillations,it is inevitable to establish an equivalent model of doubly-fed induction generator(DFIG)-based wind farms.In this context,studying the equivalent modeling of power systems with DFIG-based wind farms and its sub-synchronous oscillation characteristics has important theoretical and social values.The main research in this paper is listed as follows.For the equivalence modeling of DFIG-based wind farms,an equivalence modeling method based on equivalent power angle coherence and similar coherence is proposed.The coherence criterion of DFIG-based wind turbines is given from different perspectives,which effectively solves the complexity and limitation of clustering.In addition,based on the external characteristics of DFIG,a power aggregation algorithm is proposed.The problem of equivalent modeling of large-scale DFIG-based wind farms is systematically solved from the aspects of coherent cluster identification,coherent bus aggregation and parameter aggregation of coherent clusters.In order to study the sub-synchronous oscillation characteristics of the doubly-fed wind farm grid-connected system,an equivalence model of the doubly-fed wind farm grid-connected system is established based on the equivalence modeling method of the doubly-fed wind farm,and the small-signal model of the equivalent system is derived.The eigenvalue method deeply analyzes the effects of wind speed,series compensation,converter parameters,phase-locked loop parameters and different control strategies on sub-synchronous oscillation of the doubly-fed wind farm grid-connected system.Furthermore,to more accurately describe influence of control parameters on the sub-synchronous oscillation,control parameters are normalized,and gain parameter is defined analogous to series compensation.The influence mechanism of the rotor-side converter on the sub-synchronous oscillation of the doubly-fed wind farm grid-connected system is studied and the quantitative indicator is proposed.It is proved for the first time that the influence of the rotor-side converter on the subsynchronous oscillation of the system can be characterized by equivalent resistance and equivalent capacitance.The equivalent resistance and equivalent reactance expression of the whole system at the sub-synchronous frequency are derived.Based on the equivalent resistance and equivalent reactance expression,the influence of the inner and outer loop control coefficients of the rotor converter on the sub-synchronous oscillation is explained.The concept of interval normalized sensitivity is defined based on normalized sensitivity.By solving the normalized sensitivity and the interval normalized sensitivity of the equivalent resistance and the equivalent reactance with respect to the inner and outer loop parameters,the difference between the inner and outer loop proportional and integral coefficients on the sub-synchronous oscillation is quantified.Preliminarily discusses the sub-synchronous oscillation suppression measures for power systems with DFIG-based wind farms.Based on the influence of series compensation on the sub-synchronous oscillation,a sub-synchronous oscillation suppression method based on segmented switching capacitor is proposed.Based on the inner and outer loop proportional coefficients of the rotor-side converter on the sub-synchronous oscillation,the proportional weakening compensation strategy of the inner and outer loops on the rotor side is proposed.The feasibility of the proposed suppression measures is verified by simulation.The above research results are of great significance for the safe operation of power systems with doubly-fed wind farms.On the one hand,the doubly-fed wind farm equivalent modeling method provides a modeling basis for large-scale wind power analysis of electromagnetic phenomenon mechanism.On the other hand,sub-synchronous oscillation research have theoretical and socioeconomic value for ensuring safety and stability of large-scale wind power grid-connected systems.