New Control Strategy Of Variable-speed Constant-frequency DFIG Wind Turbines’Participation In System Frequency Regulation

With the power system’s integration of wind power increasing, especially a large number of DFIG-based wind farms in place of conventional synchronous generators, which will decrease the controllability of the system frequency. Therefore, enhancing the controllability of the DFIG so as to make a contribution toward ancillary services becomes more significant and necessary. The primary contents of this thesis can be summarized as follows:Firstly, the DFIG operation theory is analyzed, including the mathematical model of DFIG and wind turbine. An islanded electrical system is created in DIgSILENT/PowerFactory.Secondly, based on the islanded electrical system model, the influence of DFIG penetration and the response of DFIG to frequency variations are investigated. Simulation results show that, DFIG wind turbine would make no response to the frequency variations without ancillary control loop.Thirdly, the "virtual inertia" control strategy is investigated, and it can’t improve the steady-state error of the system frequency. And then, the pitch angle controller used for primary frequency regulation is proposed, which could improve the error. Finally,a new composite frequency control strategy is proposed, which could improve the initial rate of change and the steady-state error of the system frequency. The simulation results demonstrate the effectiveness of the approach.Fourthly, the influence of the controller parameters on the frequency regulation is analyzed and it’s of value in parameter selection.Fifthly, the composite frequency control strategy is simulated in a multi-machine system.