Grid-Connected Stability Analysis And Control Of Virtual Synchronous Controlled DFIG Wind Turbine

The traditional vector control strategy is adopted in doubly-fed induction generator(DFIG)wind turbine to decouple the motor speed and frequency,which cannot provide inertia support for the power grid.For this reason,the virtual synchronous generator(VSG)control strategy is adopted to introduce the external characteristics of the synchronous generator,which can make the DFIG wind turbine have the voltage and frequency support capacity.In addition,considering that the DFIG wind turbine is connected to the grid through series-compensated line,it is prone to cause sub-synchronous oscillation(SSO)accidents,which poses a threat to the safety and stability of the system.Therefore,it is necessary to study the stability of DFIG wind turbine with VSG control in series-compensated power grid.In this paper,the eigenvalue analysis method is used to study the SSO problems of direct and indirect voltage type VSG control under series-compensated power grids for DFIG wind turbine.Furthermore,according to the characteristics of controller structure,the corresponding modified control strategies are proposed to improve the grid connected stability of DFIG wind turbine.Aiming at the stability research of direct voltage type VSG controlled DFIG wind turbine under the series-compensated grid,this paper firstly establishes a small signal state-space model including DFIG,VSG control and series-compensated line.According to the conversion relationship between the impedance model and the statespace model,the equivalent admittance of the DFIG wind turbine is obtained,and the frequency sweep method is applied to verify the correctness of the established model.Then,based on the model,the eigenvalue analysis method is used to study the influence of grid and controller parameters on the system stability.After that,according to the characteristics of the direct voltage type VSG control structure,a power proportional feedforward control strategy was designed to improve the stability of the grid-connected system and suppress SSO.Aiming at the stability research of indirect voltage type VSG controlled DFIG wind turbine under the series-compensated grid,this paper takes into account the deviation between the grid angle and the output angle of VSG control outer loop,and the state-space model is established under the unified grid coordinate system.According to the conversion relationship between the impedance model and the state-space model,the equivalent admittance of the DFIG wind turbine is obtained,and the frequency sweep method is applied to verify the correctness of the established model.Then,based on the model,the eigenvalue analysis method is also used to study the influence of grid and controller parameters on system stability.Afterwards,based on the characteristics of the indirect voltage type VSG control structure and the improved control method of traditional vector control,an additional damping control strategy in the stator voltage loop is proposed to suppress SSO.In the simulation environment,the models of two kinds of VSG controlled DFIG wind turbine connected to the grid through series-compensated line are built to verify the correctness of the analysis results.