Study On The Impact Of Direct-Drive Permanent Magnet Wind Turbines Virtual Inertia Control On Power System Small Signal Stability

With the rapid development of the wind power industry and the great progress of wind power technology, the direct-drive permanent magnet wind turbines with its advantages of simple structures and high reliability become the important direction of the wind power development in the future. Large-scale wind power integration brings severe challenges to the safe and stable operation of power systems. Due to the wind power characteristics of centralized development and long-distance transmission, the small signal stability of the power systems face bigger threats. And the decoupling control of direct-drive permanent magnet wind turbines makes wind turbines externally show "no inertia". When wind turbines connecting to the power system, it reduces the inertia of the whole system in some degree. The method to solve the above problem is adding the inertia control to wind turbines. However, the addition of inertial control changes the coupling relationship between wind turbines and the power system and it has a certain influence on the small signal stability of the power system. Therefore, the limitations of the power system small signal stability should be considered in the choice of inertia control strategies and parameters. The dissertation studies the impact of direct-drive permanent magnet wind turbines inertia control on power system small signal stability.Firstly, the direct-drive permanent magnet wind turbines mathematical models and basic control strategies are revealed under the operating conditions. The basic components of direct-drive permanent magnet wind turbines mathematical model include the wind mill, the shaft and the back-to-back converter. And the basic control include the maximum power point tracking control, pitch control and converter control.Secondly, based on the eigenvalue analysis method, the simplified model of the direct-drive permanent magnet wind turbine under different operating conditions are studied. The simplified model retain dominant state variables of wind turbines, and are mainly applied to the analysis of the power system small signal stability. The calculation results of eigenvalues and the simulation results of the dominant state variables are compared between the detailed model and the simplified model under different operating conditions. The comparison results show that the eigenvalues and the dominant state variables of the detailed model and the simplified model are relatively close, which proves that the simplified model are reasonable.Finally, the influence characteristics are studied in simulation that the inertia control have on the small signal stability of the power system. The droop control and the virtual inertia control are analyzed. The four-generator two-area simulation system with direct-drive permanent magnet wind turbines is built in PSCAD/EMTDC. And under the premise that wind power does not change initial power flow, the influence characteristics are studied in simulation that the addition of the droop control and virtual inertia control have on the small signal stability of direct-drive permanent magnet wind turbines. Furthermore, the influence characteristics are studied in simulation that the active power, connected location and control parameters of the direct-drive permanent magnet wind turbines have on the small signal stability of the power system.