J964e: Single doubly fed induction machine with two fully controlled ac–dc power converters…


Single doubly fed induction machine with two fully controlled ac–dc power converters

Variable-Speed Concept Utilizing Doubly Fed Induction Generator (DFIG):In a variable-speed turbine with DFIG, the converter feeds the rotor winding, while the stator winding is connected directly to the grid. This converter, thus decoupling mechanical and electrical frequencies and making variable-speed operation possible, can vary the electrical rotor frequency. This turbine cannot operate in the full range from zero to the rated speed, but the speed range is quite sufficient. This limited speed range is caused by the fact that a converter that is considerably smaller than the rated power of the machine is used. In principle, one can say that the ratio between the size of the converter and the wind-turbine rating is half of the rotor-speed span. In addition to the fact that the converter is smaller, the losses are also lower. The control possibilities of the reactive power are similar to the full power-converter system. For instance, the Spanish company Gamesa supplies this kind of variable-speed wind turbines to the market. The forced switched power-converter scheme is shown in Figure. The converter includes two three-phase ac–dc converters linked by a dc capacitor battery. This scheme allows, on one hand, a vector control of the active and reactive powers of the machine, and on the other hand, a decrease by a high percentage of the harmonic content injected into the grid by the power converter.

Source:
Juan Manuel Carrasco, Leopoldo García Franquelo, Jan T. Bialasiewicz, Eduardo Galván, Ramón C. Portillo Guisado, Ángeles Martín Prats, José Ignacio León and Narciso Moreno-Alfonso “Power-Electronic Systems for the Grid integration of Renewable Energy Sources: A Survey”. IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, August 2006




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