Research On Operation And Control Of Direct-driven Permanent Magnet Wind Power Generation System Under Unbalanced Grid Voltage Conditions

As most of China’s wind farms are installed in remote areas where rural grids aremostly weak, grid voltage unbalance caused by asymmetrical loads or grid faultsusually occurs. If direct-driven permanent-magnet synchronous generator (PMSG) windturbine is still controlled by conventional control strategy under unbalanced grid voltageconditions, the grid currents could be highly unbalanced, and grid reactive power andactive power and DC-link voltage could have double supply frequency oscillations.Consequently, in order to enhance the operation ability of PMSG system connected grid,the operation and control of PMSG system under unbalanced grid voltage conditionsshould be further investigated.The mathematical model of PMSG system is deduced in the thesis firstly. Then theoperation behavior of the direct-driven PMSG system under unbalanced grid voltageconditions is investigated in detail. Based on the different operation requirements for thewind turbines under unbalanced grid voltage conditions, the dual current closed-loopcontrol schemes of the grid-side converter with three selective control targets areproposed. Meanwhile, the simplified positive and negative sequence current referencevalues are achieved based on the positive sequence grid voltage orientation. Accordingto result of simulations, the dual current closed-loop control schemes can achieve theobjectives including eliminating the double supply frequency oscillations of activepower, or restraining the double supply frequency oscillations of reactive power, orsuppressing the negative sequence current injected into the grid. As a result, theoperational stability and reliability of the PMSG system connected grid during thenetwork unbalance can be further improved. To further improve the operationperformance of the whole system, a quasi-proportional-resonant (quasi-PR) controller isproposed based on the simplified positive and negative sequence current reference inorder to enhance the dynamic response of PMSG system. The quasi-PR control schemedoesn’t need the positive and negative current decomposition, thus, the grid currentcontrol can be simplified. With the proposed quasi-PR control scheme, the doublesupply frequency oscillations of active power or reactive power, or the negativesequence grid current can be effectively eliminated. The simulation results are used toverify the proposed control strategy. Finally, an experimental platform of the grid-sideconverter is developed based on the TMS320F2812digital signal processor (DSP) andthe intelligent power module (IPM). The conventional control scheme and dual current closed-loop control scheme are compared by the experiment. The feasibility and validityof dual current closed-loop control strategy of grid-side converter has been furtherdemonstrated.The work of this thesis lays the theoretical foundation for enhancing the operationability of PMSG system and expands the application scope of PMSG system used in thepower system.