Research On The Control Strategy Of DFIG Wind Turbines In System Frequency Regulation

Wind power generations is the most well-developed and large-scale new energy generation, which is yearly increasing in the proportion of energy structure in our country. The development of clean energy, mainly consist of the wind power, would reduce the regional traditional energy consumption gradually, which is of great significance to the adjustment of the fossil energy sources mainly based national energy structure. What’s more, it’s also the effective method to fundamentally solve the air pollution, like haze and smog which are severe in parts of our country.Doubly fed induction generator (DFIG) is widely used in the newly-built large wind farms and its rotor speed is decoupled with grid frequency that it could no longer release or store rotor kinetic energy to damp frequency deviation, like the traditional synchronous generator does. As the penetration of wind power increasing in the power system, especially in the regions being rich in wind energy resource, it would significantly reduce the whole system frequency response. Therefore, the DFIG with frequency control strategy is further explored in this paper to relieve the impact of large-scale wind on grid frequency stability and main contents including:1. The foundation theories involved in this paper are introduced and expounded, including the maximum power point tracking (MPPT) principle of DFIG, operation control of DFIG in the different wind speed religions, the frequency regulation of power system and the existing frequency control strategy for wind turbines.2. The problems of rotor speed protection during DFIG releasing kinetic energy are analyzed. The available kinetic energy released by the DFIG rotor in process is quantitatively analyzed in different wind speed situations. Based on negative feedback of kinetic energy loss, an active rotor speed protection strategy is newly proposed to improve the existing one. The improvement will make DFIG actively exit frequency control according to its own condition, therefore maintain the stability itself during frequency regulation process.3. The problems of integrated frequency control strategy combining over-speed control and virtual inertia control are analyzed. As wind turbines using rotor kinetic energy can only support a short-time inertia and the exit would lead the grid frequency to a secondary drop, an integrated control strategy is newly put forward based on the analysis of MPPT theory. This strategy effectively combines the primary control with the inertia control and enhances the wind farm in the frequency regulation. The dynamic process of DFIG with proposed control and a reasonable speed adjustment setting method is further researched.4. The simulation system including wind farm is established based on the MATLAB/Simulink software, and the validity and effectiveness of the proposed protection strategy is illustrated by cases study.