DFIG mode of operation

[seandeg]

Dual Feed Induction Generators ( DFIG ) are used in variable speed Wind Turbine systems. The "variable speed" means that, between lower and upper limits of wind speed, the turbine drive shaft rotational velocity will track the wind speed. So the shaft rotates at a variable speed into the Gearbox. The Gearbox has a fixed ratio ( lets say 1:90 )the output shaft of which drives the rotor of the the Generator. A DFIG in this case.

The Stator of the DFIG connects to the grid, so has imposed grid frequency. The frequency of electricity produced by the Wind Turbine system at the Stator must match that of the grid. If the rotor is rotating at anything but the perfect 1800RPM that will not be the case. This is where the "dual feed" part of DFIG comes in - the rotor is fed an AC signal ( current ) such that the composite signal built from rotational velocity and now current is at the target grid frequency. The current injected into the Rotor windings comes from a Power Conveerter.

The above is my understanding of how the DFIG/Converter type variable speed Wind Turbine operates.

My question is does the physical rotating shaft of the Generator gets speeded up or slowed down by the process ? My guess is that it does not - because if it did them the blades shaft would also gets sppeded up or slowed down via the gearbox operating in reverse, and that means the Wind Turbine IS NOT "variable speed". Which defeats the purpose !

NOTE: I have searched extensively in the multidude of academic papers on DFIG but unfortunately have always lost my way at some point in the math

Any insights or explanations will be much appreciated.
Thanks

 

[davidbeach]

Shaft speed is what it is. These are induction, not synchronous, machines. The amount of power generated is a function of slip speed while running at a supersynchronous speed. In a normal induction machine the rotor speed is the air gap speed and the shaft has to turn at a supersynchronous speed. In the DFIG the rotor and stator together define the air gap speed.

At a constant shaft velocity the field frequency can cause the machine to be subsynchronous (motoring) or supersynchronous (generating).

Anyway that’s how I understand it.

 

[waross]

The part that you may be missing is that the current fed to the rotor is variable frequency. As the shaft speed changes due to loading and or wind speed variations, the frequency of the rotor current is varied accordingly.
As the shaft speed varies, the rotor feed frequency varies.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

DFIG by the way stands for "Doubly Fed Induction Generator".

I know, pedantic...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[seandeg]

Thanks for that correction jraef.
Somewhere or other I mistakenly picked up the " dual-feed".

Certainly the shaft speed varies with wind, blade pitch, yaw angle.
My basic question is --- does the physical mechanical shaft of the Generator, on which the Rotor is mounted, ALSO vary its speed in response to current supplied at the Rotor terminals ?

seandeg

 

[davidbeach]

The amount of electrical power produced is a function of the frequency of the field (rotor). An increase the electrical power produced, with all else equal, and the shaft will slow, decrease the electrical power and the shaft will speed up. Shaft speed is determined by mechanical power in minus electrical power out minus losses.

 

[Strong]

"My basic question is --- does the physical mechanical shaft of the Generator, on which the Rotor is mounted,"
The first paragraph of the OP gives the answer! The generator shaft has to be variable speed, since it is connected to a fixed ratio gearbox that is connected to a variable speed turbine.

Walt

 

[LionelHutz]

It's not really rotor current, but rotor frequency that is of concern with a wound rotor machine.

If you stay within the limits of the machine, then the operating speed will always be dictated by the frequency applied to the rotor.

You gave 1800rpm as the synchronous speed. Lets include slip by assuming the rated slip of the machine is 50rpm. If 30Hz is applied to the rotor, the speed of the machine would vary from 850rpm to 950rpm as it goes from 1/2 nameplate power motoring to 1/2 nameplate power generating. So the answer is yes, the operating speed absolutely depends on the frequency applied to the rotor.

 

[seandeg]

Thanks Strong,

.... yes I know the shaft speed varies - by wind/pitch. The question is wheter or not its speed ALSO varies in response to current at the Rotor terminals ?
There should be a yes/no answer to this.
I believe the answer is NO

seandeg

 

[LionelHutz]

You're asking a bad question. Rotor current and rotor speed are not directly related to each other, so you can not simply correlate a rotor speed with a rotor current.

Rotor current is approximately proportional to the torque passing through the machine. In theory, you could have any current at any rotor speed. In practice, the blade performance curve will likely set the practical speeds vs currents that the machine might operate at.

 

[pwrengrds]

The speed of the shaft does change. The stator has a fixed electrical speed, the slip difference is the physical speed of the shaft + rotor electrical speed, which can be negative or positive.

If the rotor is fed to create a magnetic field that rotates at the stator speed but in reverse direction the speed of the shaft would be zero (control is only practical within 30% of stator speed). That's essentially what is done, vary the speed of the rotor magnetic field to allow the speed of the physical rotor to spin at multiple speeds while maintaining the slip at a relative fixed value. Once the rotor gets to full speed, the operational mode is changed, and power is pulled out of the rotor (up to 30% of the total turbine power) as well as the stator.