Single grid connection of Doubly-Fed Induction Generator for wind turbines

This paper proposes a new configuration of wind energy conversion system, called Single External Feeding of Doubly-Fed Induction Generator (SEF-DFIG), and its control scheme. In the proposed configuration, the stator windings are directly connected to the grid, and the rotor windings are fed by an isolated inverter with no power source. Since the power electronics including dc link capacitors can be integrated into the rotor, it is possible to eliminate slip rings and brushes, which are requisites in the conventional DFIG system. The isolation of the rotor circuits requires controlling the dc link voltage by manipulating the rotor power. For this reason, the controllability of the power factor is limited, so it is important to optimize the operating points depending on the generated power and speed. Several considerations on the characteristics of the proposed system; torque, power factor, possible operating area and efficiency for wind turbine applications are analyzed. And where the SEF-DFIG is applied in wind turbine system, the considerations for the performances are also discussed. The experimental results of a 2kW DFIG system are presented to prove the feasibility of the proposed system and method.