Working of a Doubly-Fed Induction generator (DFIG) in wind turbines

I am unable to understand how a Doubly-Fed Induction generator works (in Wind Energy conversion system)? I do not understand how the reactive power fed into the stator interacts with the rotor so as to produce active power in the stator. It would be helpful if somebody is able to explain the phenomenon with the help of a phasor diagram showing the voltage, current and the field directions.

• Have you found an on-line web site that states what you cover in your question? I ask because your words appear confusing to me i.e. you feed reactive power to the stator but get real power in the stator. That bit seems confused. Sep 8, 2018 at 8:48
• pdfs.semanticscholar.org/dc77/…
– user172541
Sep 8, 2018 at 8:58
• When you have a motor generator with a double full bridge to compensate for the difference in phase or speed to the grid you generate this difference with a 3 phase PWM control IGBT dual bridge for bi-directional control of power factor phase and frequency error due to step load changes. Sep 8, 2018 at 9:00
• Since phase load shifts are equiv. to reactive loads, the PWM conversion allows the power input to be more compliance to rotation changes in phase and frequency over a limited range to reduce mech. stress. Thus 2 sets of windings. Main and phase compensation generated power is combined to result in unit power factor to the grid but with 6th harmonic distortion. This looks like a Generator driving a motor to another generator to provide the error voltage in phase amplitude and frequency. Used in Mines , rolling Mills and large ship engines with full torque from 0 RPM Sep 8, 2018 at 9:04
• @PMD section 1 starts fine then section 2 (allegedly a generator) appears to be talking about a motor i.e. refers to torque being generated by the stator and rotor fields. At this point I lost interest because the explanation seemed inappropriate for a generator. Sep 8, 2018 at 9:06

I am unable to understand how a Doubly-Fed Induction generator works (in Wind Energy conversion system)?

I think the document you linked is a little confusing and I would recommend this better document instead. Entitled "Principles of Doubly-Fed Induction Generators (DFIG)" and produced by Lab-Volt.

However, in simple terms, think about a regular 3 phase synchronous generator first; DC is applied to the rotor, the shaft is rotated at synchronous speed and the generator produces an AC output of the correct frequency (50 or 60 Hz).

However, if the shaft isn't rotated at synchronous speed but a somewhat slower speed (as what usually happens in a wind generator), you can still produce a synchronous output frequency (50 or 60 Hz) by feeding the rotor with an AC current of "so many hertz" instead of DC. The rotor frequency now "makes up the difference" between shaft speed and synchronous speed: -

The important things to notice is that the rotor voltage and current is derived from an AC/DC converter (rectifier) followed by a DC/AC convertor. The DC/AC converter is an inverter whose frequency can be set to a value that "makes up the difference" between shaft speed and synchronous speed.

Not shown in the diagram is that the control block needs to accurately measure shaft speed in order to calculate the rotor frequency required to produce a synchronous AC output from the stator.

It would be helpful if somebody is able to explain the phenomenon with the help of a phasor diagram showing the voltage, current and the field directions.

If you understand the phasor diagrams for a standard synchronous generator and the relationship between slip and driving frequency for a 3 ph inductor motor, then it will be clear. If you don't understand these then you need to ask a more appropriate question.

• I understand how the rotor current frequency compensates for the difference in "some many hertz". What I don't understand is that why "we need to inject reactive power into stator"?
– user172541
Sep 8, 2018 at 9:35
• @PMD is that mentioned in the document you listed and if so, on what page? Sep 8, 2018 at 9:40
• I have now understood the whole thing after you compared it with the synchronous generator. Thanks for answering.
– user172541
Sep 8, 2018 at 9:55
• @ Here's a quick and dirty simulation I just put together for 1 phase tinyurl.com/ycspwn8b Very simple compared to actual control system Sep 8, 2018 at 10:00