Q13Wind and Solar Energy Systems

Doubly Fed Induction Generator (DFIG) in Wind Generator Topologies

A Doubly-Fed Induction Generator (DFIG) is a wound-rotor induction generator whose stator winding is connected directly to the grid, while its rotor winding is connected to the grid through a partial-scale, back-to-back AC-DC-AC power converter consisting of a rotor-side converter (RSC) and a grid-side converter (GSC) sharing a common DC link. This arrangement allows the generator to be 'doubly fed', receiving (or supplying) power through both the stator (directly, at grid frequency) and the rotor (through the converter, at a variable, controllable frequency).

The rotor-side converter controls the rotor current's magnitude and frequency, allowing the generator to operate at variable rotor speed while still delivering power to the grid at the fixed grid frequency through the stator - the rotor-side converter effectively injects a rotor voltage/current at slip frequency, compensating for the difference between the mechanical rotor speed and the synchronous speed dictated by the grid frequency, allowing operation both above synchronous speed (super-synchronous, where the rotor circuit exports power to the grid through the converter) and below synchronous speed (sub-synchronous, where the rotor circuit imports power from the grid through the converter), typically across a range of approximately plus/minus 30% around synchronous speed.

The grid-side converter maintains the DC link voltage at a constant level and can additionally provide reactive power support to the grid independent of the rotor-side converter's operation. Because only the rotor power (a fraction of the total generator power, corresponding to the slip range) passes through the power converter, while the majority of power flows directly from stator to grid without conversion, the DFIG topology achieves variable-speed operation using a converter rated for only about 25-30% of the total generator rating, giving DFIG-based wind turbines (Type C configuration, as discussed in relation to another question in this examination) a significant cost advantage over full-scale-converter (Type D) designs while still providing substantial variable-speed operational flexibility, which is precisely why DFIG has become the most widely deployed generator topology in modern utility-scale wind turbines, particularly for onshore installations where its cost-effectiveness is especially valued.