As per the concept of induction machine operation, it is supposed to generate active power when operating in super-synchronous mode, and consume active power when in sub-synchronous mode. Still, how is the DFIG able to generate active power in sub-synchronous operation? Pardon me if the question sounds basic, but I am looking for some clarification on this matter. Thanks!
Due to the currents and frequencies injected to the rotor of the DFIG by the back-to-back connected converter, the standard torque-speed curve of the uncontrolled induction machine can be shifted to the right, or to the left. Consequently, the operation zone of the machine as a generator can be extended to the subsynchronous speed zone with these speed defined by the uncontrolled machine; however, in fact, it is a supersynchronous in the controlled machine operational characteristics. You may read the following paper, and the relevant part of the following book for more details:
Article Steady state modeling and analysis of DFIG for variable-spee...
Book Dynamic Security of Interconnected Electric Power Systems - ...
The stator winding in DFIG injects active power either in sub-syncrhronous and super-synchronous operation condition. But only in super-synchronous operation both stator and rotor contributes to total active power injected to the grid. The power electronics converter connected in Back to Back configuration and its four-quadrant operation capabilities, manages the power exchange between the rotor DFIG and the grid. DFIG technology allows variable-speed operation of wind-mills, and power balance exchange is a key factor to take into account.
The power is decided by phase angle of the inner EMF. In DFIG, we can easily change the angle by adjusting the injected rotor current's phase angle. Note that the rotor speed is decoupled from the grid frequency, but the induced EM field (EMF) is still synchronous.
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