Research On Grid-connection Control Of Doubly-fed Generating System

With the development of the energy application, the researchers focus their attention on the renewable energy, such as the wind,the hydro , the solar energy. Doubly fed induction generator (DFIG) is widely applied in the wind power generation and the pump livestock energy, which have high efficiency, low cost and the characteristic of variable speed constant frequency (VSCF). Foreign and domestic researchers have already worked much more on DFIG, especially employed in the wind-powered electricity generation. It could improve the wind energy efficiency, reduce the system capacity, lower the cost as well as make the flexible connection between the mechanical and electrical system by maximum power point tracking (MPPT). VSCF doubly fed wind energy generation system and its control scheme are studied.In this paper, the structure and the principle of DFIG, the difference and the relation between the Synchronous generator and asynchronous generator are analyzed. The DFIG's mathematical model under synchronized (MT) coordinate system is established, its equivalent circuit and the vector chart are also introduced. The relations of the essential physical quantity, such as the power, the torque as well as the energy flow relations, are analyzed.The grid-connected doubly fed generation system using bidirectional PWM converter is mainly studied. The grid-side PWM converter block is controlled with three-phase high frequency PWM vector control scheme with direct current hysteretic loop. The converter maintains a high power factor when the energy flows into or out of the converter. The rotor-side PWM converter block is controlled by stator-flux oriented vector scheme. The active and reactive power can be controlled respectively. Moreover, the Grid-connection methods and the power characteristic are analyzed, the active power computation based on the wind turbine model and reactive power limit rule are also explicated.The whole simulation module of the doubly fed generation system is set up on the platform of the commercial MATLAB/SIMULINK software package. The simulation with no-load grid-connection control theory and grid-connection control theory validated the accuracy of the system's control strategy. Finally, the hardware design and the software programming are presented. The whole experimental platform of the doubly fed generation system is built and the correlative experiment is done. The test result shows the feasibility of the no-load grid-connection control strategy.