Mechanism Analysis And Suppression Of Subsynchronous Oscillation Of Direct Rive Wind Power Grid-Connected System

In recent years,with the increasing scale of wind farms and the installed capacity of wind turbines,the interaction between wind farms and power systems is increasing.In July2015,a wind power base dominated by direct-drive wind turbines in northwestern China.In the case that there is no series compensation in the nearby power grid,a sub-synchronous oscillation accident spanning five voltage levels occurs,resulting in the severe consequences of full cutting of three 660 MW units of a power plant.Therefore,it is of great significance to study the sub-synchronous oscillation mechanism and its suppression method of wind power grid-connected systems.In this paper,the system's sub-synchronous oscillation caused by the integration of large-scale wind farms is mainly based on the system of wind farms with permanent magnet direct-drive synchronous wind turbines integrated into the weak AC grid.From the perspective of impedance stability,the analysis leads to the analysis.Secondly,the mechanism of synchronous oscillation,and based on the eigenvalue sensitivity analysis of the system small interference model,a method of suppressing sub-synchronous oscillation is designed from the perspective of grid-connected inverter control strategy.The main work is as follows:Firstly,the research methods of traditional sub-synchronous oscillations of power systems at home and abroad and the generation mechanism of different types of sub-synchronous oscillations are summarized.The research methods applicable to new sub-synchronous oscillations are briefly described,which lays a foundation for subsequent research.Secondly,the electromagnetic transient detailed mathematical model of the aerodynamic system,wind turbine,machine-side converter and grid-side converter of the direct-drive wind turbine is established,and the MATLAB/SIMULINK platform is built according to the relevant parameters and control strategies.The wind turbine grid-connected model of the wind turbine provides simulation support for subsequent analysis and verification.Then,aiming at the characteristics of synchronous sub-synchronous oscillation of direct-drive fans under weak grid,the output impedance model of grid-connected inverter is established according to the harmonic linearization theory,and the causes of system instability under weak grid are analyzed from the perspective of impedance stability.Andcompare the impact of access to the AC grid strength,the number of different fans,different system control parameters on the stability of the system.Finally,according to the mechanism of sub-synchronous oscillation caused by the interaction between the above-mentioned fan and weak AC grid,an oscillation suppression method based on additional damping control is designed.The modal analysis and sensitivity analysis of the system small interference model are used to determine the converter control at the grid side.The link is connected to the damping controller to suppress the sub-synchronous oscillation of the system.The impedance model and MATLAB/SIMULINK time domain simulation are used to compare the impedance of the system at the sub-synchronous frequency before and after the additional damping control and the output power variation to verify the effectiveness of the method.