Investigation On Key Technic For High Performance BDFIG-based Wind Power System

With the growing energy crisis and environmental pollution, more and more countries are looking for clean renewable energy source in steady of traditional thermal power generation. Wind power generation has become the most promising commercial renewable energy generation after developing in the last10years, and has occupied a certain percentage in national energy structure. In the near future, the offshore wind power generation will become a new hot topic in wind energy market. Due to the demanding nature conditions and weak grid connection conditions, the generator in offshore wind power generation requires high reliability and low cost, ability to adapt to varieties of grid faults as well.Brushless doubly fed induction generator (BDFIG) is a new type machine, whose stator is consist of two isolated windings of different pole numbers and rotor is of special structure. When both of the stator windings are excited by voltage source, BDFM behaves as a synchronous machine. In this condition, BDFM is in the mode of doubly fed operation and the speed of rotor can be adjusted via the frequency of control winding. BDFM is of high reliability and low cost for the absence of brushes and slip rings, which make it a promising candidate for offshore wind power generation.This paper studies the key technic for BDFM in offshore wind power generation, from the view of machine design and control strategy. Some solutions are proposed to provide a solid foundation and key technic for the future practical applications. The main research and key contributions are as follows:Firstly, the principle and features of BDFM are summarized, with the proposal of power delivery in stator windings and the limited operation circle. Analysis and comparison of nested-loop and doubly layer rotor structure are made based on the finite element method (FEM) in Ansoft, as well as on experimental results in the prototype machines. Harmonics in BDFM are studied based on the method of winding distribution functions. Further analysis on the torque ripple is given in the expressions of machine dimensions and winding distribution, which form the basis for the design and optimization in BDFM.Then, a generic vector model is established for BDFM, based on which the voltage oriented vector control is proposed. Decoupled control of active and reactive power is achieved by a feedforward method. Dynamic behavior of BDFM under the unbalanced grid conditions is analyzed in detail, with the proposal of four control targets. Considering the requirements of grid codes and wind power system, an improved vector control is proposed.Finally, in the applications of BDFM, the frequency of controlled quantities will vary with the rotor speed, making the conventional PR controller unsuitable. Investigations on the adaptive frequency estimation and controller are carried out. Adaptive vector filter and adaptive notch filter, as well as two type of adaptive controller are proposed to work for the AC components with frequency unknown.