Analysis Of Sub-synchronous Oscillation Of Direct Drive Pmsg Based Wind Farm Dominated By Current Inner Loop

Large-scale wind power connected to power grid is an important way to realize the low-carbon transformation of national energy.Direct drive permanent magnetic synchronous generator(PMSG)has simple structure and has become one of the current mainstream models.However,due to its multi-scale response characteristics and complex dynamic interaction with power grid,even if there is no series compensation line in grid,direct drive PMSG based wind farm will also induce sub-synchronous oscillation(SSO),which endangers the overall security of generators and systems.Existing wind farm equivalent modeling methods cannot analyze the influence of the difference of wind turbine’s structure and operating parameters on SSO,and the interaction mechanism between wind farm and turbogenerator has not been clearly revealed.This paper focuses on the SSO caused by the direct drive PMSG based wind farm connected to the weak grid,the main research contents and innovative work include:(1)The equivalent aggregation model of direct drive PMSG based wind farm considering operation parameters and network structure is constructed.The mathematical model of each part of wind turbine generator(WTG)is established,and the linearized model of radial structure and chain structure wind farm is established by using graphical block modeling method.On this basis,K-Means aggregation method was used to cluster WTGs according to the dynamic output characteristics,and then the equivalent model of wind farm is constructed after collector network transformation and WTG parameters aggregation.Under the SSO scenario,the validity of the aggregation model is verified by model in EMTDC/PSCAD,which provides a model basis for the analysis of SSO characteristics.(2)Considering direct drive PMSG based wind farm connected to weak grid,the characteristics of SSO are analyzed based on eigenvalue analysis.For single-machine equivalent wind farm model and aggregate wind farm model,the eigenvalues and participation factors in the system are calculated,and the dominant factors corresponding to the SSO mode are determined.The influence of wind turbine generation(WTG)operating conditions,WTG number,current inner loop control parameters and grid strength on the SSO mode is analyzed.A time domain simulation model is built to verify the effectiveness of eigenvalue analysis.(3)In order to reveal the mechanism of SSO in direct drive PMSG based wind farms,based on the state space equation adopted by the wind farm,impedance model of direct drive PMSG based wind farm is constructed.The stability criterion of impedance determinant is used to judge the stability of system,and analyze the influence of different factors on the SSO characteristics.(4)After turbogenerators are connected to the interconnection system of the direct drive PMSG based wind farm,the dynamic interaction mechanism between wind farm and the turbogenerators is analyzed based on eigenvalue analysis and net damping analysis.When direct drive PMSG based wind farm is equivalent to the sub-synchronous power source,the excitation characteristics of the shaft torsional vibration mode and the sensitivity of each torsional vibration mode to the external power SSO are investigated;when wind farm controller is considered,the dynamic interaction between wind farm and turbogenerator is studied.The influence of wind farm operating conditions,WTG number,current inner loop control parameters and grid strength on the shaft torsional vibration of turbogenerator is analyzed.