Control Strategy Of DFIG-based Wind Turbines For Power System Frequency Regulation

With the world's aggravating energy crisis, vigorously developing renewable energy sources has been becoming a powerful way to resolve the energy problems. As the most mature new energy power generation technology, wind power generation has attached great attention from all over the world. As the continuous developing of wind power, more and more wind turbines access to grid which leads to th decreasing of the traditional synchronous generator. Thus, the system frenquency control ability must be affected. Therefore, on the basis of analyzing the effects of doubly-fed wind power grid unit on power system frenquency, study the strategy of doubly-fed induction generators taking part in power system frequency modulation has important theoretical significance and use value.Firstly, the thesis analizes the basic operation principle of double-fed inductive wind generators and gives out the basic structure of double-fed wind generation system. Then it symply analysize the operating principle of double-fed induction wind power system and established wind speed model, aerodynamic model, shafting model and generator model. According to controlling method of the stator-flux oriented vector,it works out the mathmatical model of line side converter and the rotor side converter and the principle of the range of fan operation as well as maximum power tracking.Secondly, the thesis introduces the static and dynamic characteristics of line frenquency, and analyzes the reasons of the grid frequency flutuation. Focus on the frequency fluctuation, the thesis introduces methods of system frenqency fluctuation and the machanism of frequency response. On the basis of these analyzation, the author puts forward that as the conventional unit continuously replaced by double-fed induction wind turbines, the rotational inertia of the whole electrical power system will decrease while the reserve power capacity will increase and the ability of the frequency modulation of the grid will also decrease. The thesis also introduces the process of inertial response of synchronous generator and the process of primary frequency control which provides a theoretical basis for the study on control strategy in the post-study.At last, through the simulation studies of the inertia switch control ring of double-fed induction wind power unit, the thesis puts forward that it can decrease the rate of system frequency but it cannot decrease the frequency of steady-state deviation. The author further studies the pitch angle frequency control ring, it puts forward that it can decrease the frequency of steady-state deviation but it cannot decrease the rate of system frequency. Combining the advantages of these two ways of control methods, the author comes up with a new hybrid frequency control strategy. Through the commom works of the curve of inertia switch control and pitch angle control, the new strategy decreases the intial rate of the system frequency and the steady state error. The author also verifies the effectiveness of hybrid frequency control strategy in the simulation study in four machines and two districts.