Research On The Shafting Oscillation Characteristics And Control Strategy Of Grid-connected DFIG-based Wind Power Generation System

Wind turbine based on doubly fed induction generator(DFIG)has been most widely used in the large-scale wind farm because of its significant advantages including operation under a wide rotor speed range,excitation with small capacity converters,and decoupled control of active and reactive power,etc.In addition,with the wind energy penetration levels rapidly increasing,the impacts of DFIG-based wind power generation system on the power grid have become much more significant than ever before.Under external disturbances such as wind speed sudden change or short-circuit fault,the shafting oscillation as well as the oscillation of the output active power generally occur in a grid-connected DFIG-based wind power generation system.It might lead to shafting damage and the low frequency oscillation in the connected power grid,which would weaken the stable operation capability of the DFIG system and the connected power system.In this dissertation,for improving the stability of a grid-connected DFIG-based wind power generation system,the mechanism of shafting oscillation is analyzed by using the damping tourque analysis method.Furthermore,the influence of system parameters on the shafting oscillation characteristics and the shafting oscillation mitigation control strategy are deeply studied,which are the key points of the dissertation.The main work is carried out as follows.(1)The detailed mathematical model of the grid-connected DFIG-based wind power generation system is firstly established.Based on the linear system theory,the corresponding linearized model which applies to the small signal stability analysis is deduced.Furthermore,the small signal stability of the studied system under a certain operation condition is analyzed,in which the conclusion illustrates the necessity of the study on shafting oscillation characteristics and the control strategy.(2)The coupling relationship between the mechanical subsystem,electromagnetic subsystem and control subsystem in a grid-connected DFIG-based wind power generation system is analyzed.Meanwhile,the action mechanism of the DFIG electromagnetic torque on shafting oscillation is presented.In addition,the influence of system parameters including wind speed,DFIG parameters and control parameters on the damping of the shafting oscillation is investigated in detalis,and the simulation results are used to verify the conclusions.The analysis results lay the fundamental theory on the appropriate parameter selection and shafting oscillation damping control method design in a grid-connected DFIG-based wind energy conversion system.(3)Based on the influence of the DFIG electromagnetic torque on the shafting oscillation and the characteristics of generation and transfer for the torque and active power in the DFIG-based wind power generation system,four shafting oscillation mitigation control strategies are mainly investigated,and the effectiveness is demonstrated by using the simulation results,respectively.Moreover,comparative analysis among the four shafting oscillation control strategies under different operation conditions is implemented,and the method of selecting the appropriate controller in a grid-connected DFIG-based wind power generation system is proposed,which may further improve the practicality of the proposed control strategies.The work of this dissertation provides the theoretical analysis for improving the stability of the DFIG-based wind power generation system and the connected power grid,which provides a novel solution to integrate large scale wind power into the power system.Additionally,the study in this dissertation may lay the foundation for the construction of the power grids with wind energy conversion system and the design & control of the DFIG-based wind power generation systems.