Research On The Harmonic Characteristics Of A DFIG-based Wind Generation System And Its Suppression Strategy

With the worse and worse conditions such as energy crisis and environment pollution, wind power has been attached more importance to, which is a clean and pollution-free renewable energy. In recent years, wind power generation has been developed rapidly all around the world, and its scale continues to expand. With the installed capacity of the wind farm and the proportion of wind power in the whole power system increasing constantly, the influences of wind power generation on power system are gradually highlighted. Among them, the impact on the power quality is an important problem which is caused by wind power connected to the grid. In order to ensure the safety and reliability of the power grid, and to use wind energy more efficiently, it is very necessary to study the problem.In this paper, the wind power grid connected system with doubly fed induction generator is analyzed. The research is mainly about the harmonic characteristics of doubly fed induction generator, the effects of long distance transmission lines on power quality and harmonic suppression strategy.As the single unit capacity of wind generation is required more and more, doubly fed induction generator because of its own advantages, has become an important type of unit. Firstly,based on the doubly fed induction generator, the basic principle of its operation is analyzed, and its mathematical model is established. Besides, under the vector control strategy, the decoupling control of active power and reactive power of wind power generation is realized. And the performance of the system is simulated by PSCAD/EMTDC simulation software. Secondly, the principle of SVPWM modulation of the converter of the generator is introduced, and the wide frequency harmonics from the modulation are analyzed. Finally, coupled with the harmonic equivalent model of doubly fed induction generator, a mathematical algorithm is used to describe the harmonic characteristics of the generator.In a wind power grid connected system, the high voltage transmission line is an important part. However, as the harmonic of the generator presents a wide frequency characteristic, influences of distributed capacitance of the high voltage transmission line will not be ignored. In the paper, the distributed parameter model of the high voltage long distance transmission line is established, and the resonance mechanism about it is analyzed. In addition, the equivalent model of wind power grid connected system is established, and its influences on the power quality of the grid are analyzed.At last, as to the wide frequency domain harmonics and the resonances of the wind power grid connected system, a suppression strategy applying hybrid active power filter with injection circuit is proposed. And the effectiveness of this method is verified by simulation.