The Research On Direct Power Control For The Grid Connected Converters Of The Double Fed Wind Power Generator

Growing energy problems in the world, people’s attention to renewable energy are increasing. Wind power as a new way of renewable energy power generation has made a rapid development. With large-scale wind turbines connected to the electricity grid, the lack of its own brings to the grid security problem is more and more obvious. Therefore, how to improve the control effect of wind turbines is imperative.Firstly, this thesis introduces the basic principle of voltage source PWM rectifier and gives the mathematical model of the coordinates. Based on the theory of instantaneous power,the power mathematical model of PWM rectifier is established. And, these theoretical basics are used for the futher study.Secondly, this thesis introduces the grid side converter based on traditional switch table direct power control(DPC) strategy. Some disadvantages are found including reactive power loss of control and current distortion through studying traditional switching table. In order to improve this solution, the concept of power loop duty cycle control with the voltage loop sliding mode control is introduced in direct power control to allocate only a fraction of control period to the voltage vector selected from conventional switching table in DPC, while a null vector is applied for the rest of time. The main advantages of the proposed duty cycle control are strong robustness against inductance and capacitance variations and more sinusoidal grid current. The simulation results verify the feasibility and robustness of the proposed control strategy.Finally, according to the harmonic problems caused by unfixed switching frequency, the thesis presents a DPC scheme based on the predictive control strategy, including the voltage loop sliding mode control strategy. This kind of control strategy based on predictive control algorithm solves the traditional fixed frequency direct power control problems, eliminates the tracking error of active and reactive power at the end of switching period and improves the quality of the grid side current. Constant switching frequency facilitates the design of the filter.Using Matlab/Simulink software platform, a model of fixed frequency direct power control strategy is built. The simulation results verify the effectiveness of the new strategy and the robust of capacitance parameter change.