Research On Sub-synchronous Oscillation In Wind Farm Integrated Power System Considering Static Var Compensator

Since the 21 st century,the widespread application of new type renewable energy power generation technology represented by wind power brings new challenges to the stability of power system,such as the long-standing sub-synchronous oscillation(SSO)phenomenon in Hami district,Xinjiang,which mechanism is yet unclear and theoretical research is relatively lack.Large-scale wind farms consist of various types of wind turbine generators(WTGs)and dynamic reactive power compensation devices,making the problem more complicated.This paper based on the practical phenomenon for further study,focuses on the interaction among direct-drive permanent synchronous generators(D-PMSGs)based wind farm,static var compensator(SVC)and connected grid.1)The time-domain mathematical model of equivalent D-PMSGs based wind farm and SVC are established.Both frequency scanning method and eigenvalue analysis method are used to study the oscillation mechanism of SVC.The results obtained indicate that the weak grid strength,the large proportional gain and the small integral time constant of SVC lead to the negative resistance characteristic of SVC causing the instability of system2)Based on the above models,the small signal model of D-PMSGs based wind farm equipped with an SVC is further established.Through eigenvalue analysis,the physical dynamic links and control loops involved in sub-synchronous interaction come to light.The results prove that the sub-synchronous interaction is related to the DC capacitance and the filter inductance of D-PMSGs based wind farm,the equivalent inductance and the fixed capacitance of SVC,the equivalent inductance of grid,the control of current inner-loop of grid-side converter(GSC)and the control of SVC.Moreover,the weaker the grid strength is,the smaller the proportional gain of current inner-loop of GSC is,and the larger the proportional gain of SVC is,the worse the stability of system will be.3)Considering the frequency characteristics of grid impedance,the interaction among D-PMSGs based wind farm,SVC and connected grid is studied.The results obtained prove that the natural resonant frequency of grid has an important influence on the stability of wind power integrated power system.Further more,the mutual effects between critical control parameters and natural resonant frequency are analysed.SSO problem caused by the natural resonant frequency can be avoided by optimizing those parameters.