Doubly Fed Induction Generator (DFIG) in Wind Power Plant

Main components of wind power conversion to electric power :

1. Blades and turbine rotor
2. Gear box
3. Induction generator
4. Rectifier / Inverter unit (DC transmission link)

• Energy conversion system with D.C. link is shown above
• Wind energy received by blades rotates the turbine shaft and through gear box the rotor of induction generator starts rotating converting mechanical power into 3-phase electrical power.
• Stator winding of induction generator is connected to the grid circuit.
• Rotor winding is connected to grid using a power converter unit.
• This unit is a 4 quadrant power converter. It has two back to back PWM-VSI.
• Rotor side converter : It regulates electromagnet torque and also for maintaining excitation of machine supplies small amount of reactive power.
• In synchronous and sub-synchronous mode, it can transfer maximum power. Secondly as converter is connected to rotor, power rating of converter is reduced. This allows whole power flow through stator.
• Inverter controls supply side.
• Diode bridge rectifier and D.C. link capacitor are also sometimes used.

Advantages of Doubly Fed Induction Generator (DFIG) in Wind Power Plant

1. It is suitable for high capacity power applications.
2. Without bridge over-loaded it is able to produce more output.
3. Inverter cost is not much more.
4. Converter cost is also not much more.
5. Machine is robust and gives stable response.

Disadvantages of Doubly Fed Induction Generator (DFIG) in Wind Power Plant

1. It requires a complex control.
2. Machine is not that rugged.
3. As there are slip-rings, periodic maintenance is a must.
4. In case grid fault condition arise and as stator is directly connected to grid circuit, it is sensitive in that situation.
5. As gear box is provided the cost is more, so also maintenance of gear box is periodic, not be neglected.