Research On Control Strategy For Stand-Alone DFIG-based Wind Power Systems

With the shortage of global energy and the prominent environmental problems caused by conventional energy generation, wind power generation gets more and more attention. Most of current research on wind power generation focus on grid-connect systems, but stand-alone systems have more advantages to solve power supply problems for the areas which grid can not covered. The doubly-fed induction generator (DFIG)adopt rotor excitation technology which makes it more competitive in wind power systems of variable speed constant frequency. For this reason, the thesis do research on control strategy for stand-alone DFIG-based Wind Power Systems.The thesis first introduces the structure of stand-alone wind power system, then analyses and models the two main parts——DFIG and back-to-back converter, and discuss its control strategy under balanced loads. Back-to-back converter has two parts, rotor-side converter (RSC) and load-side converter (LSC), they decoupled via intermediate DC capacitor. RSC controlling is to build DFIG stator voltage which is also the bus voltage of stand-alone systems; LSC controlling is to build intermediate DC voltage. Owing to the frequently unbalanced loads, this thesis detailed analyzed the impact of unbalanced loads on the system, and discussed the system control strategy in this case, and then proposed that the ultimate control purpose is to build balanced three-phase stator voltage. For the protection of DFIG, this thesis employs the strategy that use LSC to compensate negative sequence component of current. Additionally, it make use of PI-Resonant controller to control positive and negative sequence component simultaneously at the forward rotating coordinate system. Taking account of the redundancy of current sensor which brings adversely affect on system’s economy and stability, this thesis proposed a strategy with reduced current sensors. This strategy can avoid load current sampling and reach the same control effect. At last, it was verified effective and feasible through simulation and experiment.