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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. – Andy aka Sep 08 '18 at 08:48
  • https://pdfs.semanticscholar.org/dc77/0536ac70329938e55d1bffca4e581f4094a1.pdf –  Sep 08 '18 at 08:58
  • 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. – Tony Stewart EE75 Sep 08 '18 at 09:00
  • 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 – Tony Stewart EE75 Sep 08 '18 at 09:04
  • @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. – Andy aka Sep 08 '18 at 09:06
  • @TonyEErocketscientist I do not even understand the working of DFIG. I am unable to find a phasor diagram which helps me understand the interaction of magnetic fields and consequently the production of torque and induction of voltages in stator and rotor. If we assume that the rotor rotates at grid frequency (therefore no need to inject or draw power from rotor), what is the role of reactive power fed to stator and how does it affect the magnetic fields in stator and rotor –  Sep 08 '18 at 09:10
  • [This is a better document](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=2ahUKEwj1y7ONh6vdAhVMIcAKHWoAB4kQFjAEegQICRAC&url=https%3A%2F%2Fwww.ece.uvic.ca%2Fwiki%2F~elec365%2Flib%2Fexe%2Ffetch.php%3Fmedia%3Ddoubly-fed_induction_generators.pdf&usg=AOvVaw0du2ZzWyHVGle5DDFMToWE) – Andy aka Sep 08 '18 at 09:15
  • http://tinyurl.com/ycspwn8b The Generator produces AC and DC and then back to AC while the 2nd Gen phase error is powered by the chopped DC but control system must consider lag, hysteresis step loads, grid loads, temp, and dozen other variables. not trivial But my Falstad Sim is trivial – Tony Stewart EE75 Sep 08 '18 at 10:04
  • http://tinyurl.com/y7wnzzer Added 55 Hz Gen to other signals for 50Hz grid – Tony Stewart EE75 Sep 08 '18 at 10:10
  • @TonyEErocketscientist Thanks for answering. I am now clear on what happens in a DFIG. I will try to understand the simulations you sent. Can you please tell what software can I use for DFIG related simulations. Simulink is the only one I know. –  Sep 08 '18 at 10:14
  • Manitoba Hydro has a trialware small tool PSCAD for large system test simulation. Their motto is, **if you can imagine it, you can synthesize it** You may try it if you have the urge, but I haven't had the need to. It may be a supplement to Simulink https://www.google.com/search?num=50&client=firefox-b&ei=mqKTW9ypKpGtzwL-pYLYCg&q=Simulink+manitoba+hydro&oq=Simulink+manitoba+hydro&gs_l=psy-ab.3...22971.26527.0.27516.15.15.0.0.0.0.127.1478.10j5.15.0....0...1c.1.64.psy-ab..0.12.1186...0j0i67k1j0i22i30k1j0i22i10i30k1j33i160k1j33i21k1.0.PUkpSy2xy7I – Tony Stewart EE75 Sep 08 '18 at 10:25

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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: -

enter image description here

Picture source.

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.

Andy aka
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  • 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"? –  Sep 08 '18 at 09:35
  • @PMD is that mentioned in the document you listed and if so, on what page? – Andy aka Sep 08 '18 at 09:40
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    I have now understood the whole thing after you compared it with the synchronous generator. Thanks for answering. –  Sep 08 '18 at 09:55
  • @ Here's a quick and dirty simulation I just put together for 1 phase http://tinyurl.com/ycspwn8b Very simple compared to actual control system – Tony Stewart EE75 Sep 08 '18 at 10:00