DFIG

Doubly-fed induction generator (DFIG) is a type of electrical generator commonly used in wind turbines to enable variable-speed operation with a reduced-power electronic converter. In a DFIG, the stator is connected directly to the grid at the grid frequency, while the rotor is connected to the grid through back-to-back grid-side and rotor-side converters linked by a DC intermediate bus. This arrangement allows the rotor electrical angle and frequency to be controlled independently of the grid, enabling efficient operation over a wide range of wind speeds.

The electrical topology consists of a wound rotor connected via slip rings to two voltage-source converters that share a common DC link. The rotor-side converter controls the rotor current and torque, converting a portion of the mechanical power into electrical power at slip frequency. The grid-side converter regulates the DC link voltage and can inject or absorb reactive power, delivering the remaining power to the grid. The net effect is that the stator supplies the majority of the real power to the grid while the rotor-side converter handles slip power, allowing variable-speed operation without a full-scale converter for the entire machine rating.

Control objectives include maintaining desirable rotor speed, optimizing energy capture, and providing grid support through active and reactive power control. Advantages of the DFIG approach include efficient variable-speed operation with reduced converter rating and good dynamic response. Drawbacks involve mechanical complexity due to slip rings, higher maintenance, and the need for sophisticated protection and grid-code compliance. DFIG-based wind turbines have been widely used since the late 1990s, though newer designs increasingly employ full-scale converters.