Research On Excitation Control And Optimal Operation For Doubly-Fed Wind Power Generator

Due to continually diminishing reserves of normal energy sources and environment pollution, it is very necessary to develop wind power as its renewable and clean characteristic. Variable-speed constant-frequent (VSCF) wind power generation system with doubly-fed induction generator (DFIG), that excited by inverter located at rotor loop, possesses characteristics of smaller capacity, lower cost, flexible connection of mechanical-electric system, higher wind power capturing capability. Therefore, it becomes the mainstream developing type, and its AC-excitation control technology becomes an important research direction. Supported by the National Science Foundation of China Project 'Research on optimizing control strategy overall operating condition for variable wind turbine hybrid system (NO: 50677021)' and as a part of it, this thesis studies AC-excitation control and optimal operation technology for VSCF doubly-fed wind turbine based on the full analysis on the references from domestic and aboard. Improvements on net-side converter control strategy, the maximum wind energy capturing control strategy and converter system modeling method are taken; primary contents and original contributions of this thesis can be summarized as follows:Firstly, system structure, control targets and mathematic model of AC/DC/AC converter employed by wind power generation are analyzed. Aimed at improving active current tracking speed and enhancing DC-bus anti-disturbance ability and based on the ordinary direct current control strategy for net-side inverter, a novel control strategy with a comprehensive reactive current adjusting target of both power factor keeping and DC-bus voltage maintaining, using fuzzy logic controller, is originally proposed. Simulation results indicate its efficiency.Secondly, operation function, equivalent-circuit model, power relation and mathematic model of DIFG are analyzed. In order to study DIFG stability characteristic during VSCF operation, the small disturbance analysis method is adopted to establish and study DIFG linear models on several typical operating points. Results show that DIFG is stable in normal operating speed range, and has the same stability varying trends as operating from sub-synchronize or sup-synchronize to synchronize speed.Thirdly, power characteristic, operating field and control target of wind turbine are analyzed. DIFG double closed-loop vector control strategy is established; regulating principle and dynamic characteristic in active power control mode or speed control mode is discussed. To solve the problems of stable and dynamic control ability excited in the normal maximum wind energy capturing strategies, and wind velocity measuring inaccuracy, a novel fuzzy optimal speed tracking strategy is originally proposed, in which variations of rotating speed, generation power and wind velocity are input into fuzzy logic controller together, in order to enhance fuzzy deduction efficiency for wind that gradually changed. Simulation results with natural wind model indicate the wind generation capturing ability improvement of the novel strategy, during the operating field that below rated wind velocity. After that, aimed at the lowest loss operation for DIFG, an optimal reactive power reference control strategy using fuzzy logic controller is presented, based on relation between loss characteristic and reactive power of DIFG. Simulation results indicate its effectiveness for adopting DIFG characteristic varying, maintaining lowest loss and improving operating efficiencyFourthly, considered with wind turbine pitch control strategy, cutting-in and cutting-out control strategies for DIFG are analyzed. Simulation results validate the effect on improving operating performance by mechanical and electric corresponding action.Fifthly, normal modeling methods for wind power generation system simulation are summarized and analyzed. As AC/DC/AC converter model built by Matlab/Simulink/PSB library blocks needs lots of time for calculation during simulating, a novel simplified model of AC/DC/AC converter system is originally proposed. It can not only discribe the work principle and outer characteristics of the converter system reasonablely, but also save calculating time, which is suitable for simulation in wind power generation system level. After that, a modeling method for DIFG whole process simulation across cutting-in operation is discussed and accomplished, which supplied a simulating platform for cutting-in control strategy validation.