What is DFIG

Rotor and Stator are both connected to grid and both feed power to grid hence the name Doubly Fed Induction generators( DFIG)

Basically, a Slip Ring Induction Generators, similar to the generator used Other Wind Turbines

Stator is connected to grid circuit similar to any other induction generator

Rotor windings are also connected to grid, but through an AC-DC-AC converter circuit.

Different Types Of DFIG Operating Speed

The operating ranges results from the specified slip limits. In general, the following relationships are

in effect :

Synchronous speed (Ns) = 120*f / P

Slip (S) = (Ns-N) / Ns

Sub-synchronous operating range (S > 0)

 Synchronous operating range (S=0)

 Super-synchronous operating range (S < 0)

Rotor Current Frequency (Fr) at different mechanical speeds (N)

Sub-synchronous operating range (S > 0)

Fs=Fr + (PxN) / 60

Ex. For N=1000

Fr = 50 -33.33 = 16.66Hz (Fr is Positive)

Rotor RPM Approx  900 to 1000 This condition Stator Give Power to Grid.

Rotor Take Power to Grid And Rotor Frequncy 33.3Hz As per Above formula

This Speed Very useful During Low Wind Condition Around 4m/s to 5m/s wind speed   

Synchronous operating range (S=0)

Fs=Fr + (PxN) / 60

Ex. For N=1500

Fr = 50 -50= 0Hz

Super-synchronous operating range (S < 0)

Fs=Fr + (PxN) / 60

Ex. For N=1600

Fr = 50 -53.33 = -3.33Hz (Fr is Negative)