Research On The Mechanism And Mitigation Strategies Of Sub-synchronous Oscillation In DFIG-Based Wind Power Integration System

In recent years,with the large-scale integration of renewable energy sources represented by wind power,several new type sub-synchronous oscillation(SSO)accidents caused by wind power have occurred in domestic and foreign grid operations.Compared with the traditional SSO in power system,the SSO caused by wind power presents new characteristics,specifically as follows:(1)Oscillation is related to the dynamic interaction between the wind turbine controller and the grid,and has nothing to do with the mechanical shaft of wind turbine.(2)There is a morphological evolution of oscillation,SSO could enter constant-amplitude sustained oscillation originated by small-signal negative-damping instable operation.(3)After the occurrence of the SSO,multiple sub/super-synchronous frequency interharmonics are coupled into the electrical quantities of wind turbines.In view of the above characteristics,the specific problem of SSO in doubly-fed induction generator(DFIG)wind farms connected to series-compensated system is studied in this thesis.The main research contents include:(1)The time-domain model and frequency-domain model of DFIG are derived respectively:A complete time-domain model of DFIG-based wind turbine,including wind turbine,mechanical shaft system,induction generator,and power electronic converters,is established to study the oscillation form and complex characteristics of SSO.Based on the method of harmonic linearization,the equivalent frequency-domain impedance model of DFIG-based wind turbine considering the dynamic process of rotor side converter(RSC)and phase-locked loop is established.(2)The in-depth analysis on the two modes of new type SSO is launched.Firstly,the mechanism of negative-damping divergence oscillation is summarized,and then it is proved by time-domain simulation that the transition of SSO from negative-damping divergence to constant-amplitude sustained oscillation is caused by the output limitation of the current inner loop of RSC.Furthermore,the influence of correlation factors on constant-amplitude sustained oscillation is further studied,and the relevant theoretical analysis is improved.(3)The multi-frequency coupling phenomenon of the new type SSO is analyzed theoretically.The multi-frequency response process of the controller of wind turbine under the sub-synchronous disturbance is deduced,the influence of multi-frequency coupling characteristics on the output power of wind turbine is studied,and the multi-frequency coupling regularity of DFIG-based wind turbine under sub-synchronous disturbance is summarized.(4)Based on the frequency-domain impedance model of wind turbine,the problem of SSO in DFIG-based wind power integration system is studied by impedance analysis.This thesis proposes a closed-loop pole stability criterion suitable for system SSO stability analysis,by comparing with the widely used damping stability criterion based on equivalent circuit,its validity is verified.In the application of the impedance analysis method,the concept of frequency-domain sensitivity is introduced.The stability criterion is combined with sensitivity analysis to define the corresponding parameter sensitivity,and the influence of control parameters of wind turbine on the SSO characteristics is studied.In the end,based on the analysis results of closed-loop pole sensitivity of parameters,two SSO mitigation strategies are proposed,which are RSC parameter optimization and additional damping controller.