What Are Doubly Fed Induction Generators?

Doubly fed induction generators (DFIGs) are wound-rotor induction machines configured so that electrical power can enter or leave through both the stator windings and the rotor windings simultaneously, rather than only through the stator as in a standard squirrel-cage induction generator. The stator is connected directly to the AC grid at grid frequency, while the rotor is connected to the grid through a back-to-back power converter that can supply or absorb a fraction of the machine's rated power. By controlling the frequency and phase of the rotor current, the converter allows the mechanical rotor speed to vary over a wide range while the stator output remains locked to grid frequency. This variable-speed capability makes DFIGs the most widely deployed generator topology in modern variable-speed wind turbines.

The technology draws on induction machine theory, power electronics, and power systems engineering. The back-to-back converter typically handles only about 25 to 30 percent of the machine's total power rating, which keeps the converter cost substantially lower than the full-rated converters used in permanent-magnet synchronous generator systems. The MIT literature review on DFIG systems for wind turbines provides a detailed treatment of the machine equations and control architectures that underpin DFIG design.

Operating Principle and Variable-Speed Control

The central idea behind DFIG operation is slip power recovery. In a standard induction machine, the difference between the rotor mechanical speed and the synchronous speed of the rotating magnetic field, called the slip, results in rotor currents at slip frequency. In a DFIG, instead of dissipating this slip power as heat in rotor resistance, the converter captures it and delivers it to the grid. When the rotor spins faster than synchronous speed (super-synchronous operation), both the stator and the rotor export power to the grid. When the rotor spins slower (sub-synchronous operation), the stator exports power while the converter supplies power to the rotor from the grid, with the net result still being generation.

The rotor-side converter controls the active and reactive power delivered by the stator independently of rotor speed, using vector control techniques that resolve rotor currents into components aligned with the stator flux. This decoupled control allows the wind turbine to maximize aerodynamic energy capture across a wide range of wind speeds by adjusting rotor speed, and to provide reactive power support to the grid as required by grid codes.

Power Converter and Grid Interface

The back-to-back converter in a DFIG consists of two voltage-source converters sharing a DC link. The rotor-side converter regulates rotor current to control machine torque and power factor, while the grid-side converter maintains the DC link voltage and controls the reactive power exchange with the grid at the converter terminals. Pulse-width modulation at switching frequencies typically between 2 and 4 kHz produces low harmonic distortion in both the rotor and grid currents.

The direct stator connection to the grid means that grid voltage disturbances appear directly across the stator terminals without any converter buffering. Voltage dips cause large transient currents in the stator, which in turn induce large currents in the rotor through transformer action, potentially damaging the converter. Modern DFIGs incorporate crowbar circuits or advanced control strategies for low-voltage ride-through (LVRT), allowing the turbine to remain connected and support the grid during fault conditions. NREL documentation on doubly fed induction generators in offshore wind systems describes the grid code requirements and control strategies applied in large offshore installations.

Performance and Wind Energy Integration

DFIGs improve overall wind farm energy yield compared to fixed-speed generators by allowing each turbine to operate near its optimal tip-speed ratio as wind speed changes. The variable-speed range typically spans roughly 70 to 130 percent of synchronous speed, corresponding to a two-to-one range in rotor mechanical frequency. PMC research on DFIG wind turbines with advanced controllers documents how modern control strategies further improve energy capture and grid support.

Applications

Doubly fed induction generators have applications in a range of disciplines, including:

• Onshore and offshore variable-speed wind turbines in utility-scale wind farms
• Grid-connected tidal stream and run-of-river hydrokinetic turbines
• Wound-rotor induction machines used in adjustable-speed pump-storage hydroelectric plants
• Frequency regulation and reactive power support services to electrical grids