Control Of Converters For Megawatt Level DFIG

Energy supply and environment protection are important issues for sustainable development of human society. As one of the most important sources of renewable energy, wind power generation has received great attention all over the world nowadays. Doubly-fed induction generator (DFIG) is a special induction machine that is widely used in large wind plant because of advantages over other generators, such as decoupled power, variable speed constant frequency operation and small scale converter. In this paper, the grid-connected variable speed constant frequency DFIG system is specifically studied as research object. Vector control scheme applied to voltage-source dual PWM converters is deduced in detail.Several key technologies about DFIG rectifier control system are introduced firstly. Then, the focus of the study is confirmed, which including power control, maximum power point tracking and improvements for grid fault. The performance of the DFIG system depends not only on the generators but also on the back-to-back converters as well as how they are controlled. In general, each of these two PWM converters is controlled by using d-q decoupled control approaches. In order to decouple the power generated by the machine and control the power factor, a stator-flux-oriented vector control scheme is developed to control the rotor side converter (RSC) as well as the rotor speed to track the maximum wind power point. Correspondingly, in order to gain good performance in the stability of dc-link voltage and controllability of the reactive power absorbed from the grid, a grid-voltage-oriented control scheme is adopted to control the grid-side converter (GSC).According to different control objective of RSC, outer-loop variables could be both stator active power and generator mechanical rotational speed. Based on the principle of MPPT, calculation method of reference values is proposed about above-mentioned two modes. Finally, a detailed simulation model including wind turbine, generator, grid and PWM converter is built on MATLAB. The simulation results show power decoupled effect and good MPPT performances, and the speed mode has noticeably quicker dynamic response compared to current mode. Furthermore, fuzzy logic control is proved to be an effective method to improve torque and power pulse when speed or wind speed change.Dynamic response of the stator excitation current is neglected in traditional vector control scheme, this inaccuracy would result in terrible performance during grid fault. Therefore, the compensation term should be added into the model. Simulation results indicate effectiveness of the developed scheme to ride through grid fault after modification.