DFIG --- does Converter control shaft spped OR only mag. flux rotation speed ?

Thread Starter

seandeg

Joined Apr 25, 2013
2
Question relates to a DFIG with partial scale power converter which enables operation at +/- 30% target generator shaft rotational velocity.

In either subsynchronous or supersynchronous mode does the system physicall slow down or speed up the generator shaft, and via the gearbox slow down or speed up the blades shaft ?

I do not believe that it does, however have read in an published paper that this indeed happens.

I believe the item that speeds up or slows down as end result of the power converter action is the composite magnetic flux of the generator rotor until gets to target ( be it 1500RPM or 1800RPM )

Thanks
seandeg
 

MisterBill2

Joined Jan 23, 2018
18,928
This question needs a whole lot more explanation, because probably nobody is aware of what you are asking. WHAT is a "DFIG", and what is a "partial scale power converter"?
And how does a gearbox fit into the discussion??? You need to explain a lot more so that we can understand what it is that you seek to find out. Almost none of the folks who participate here are mind readers, especially over long distances. But there are a very few exceptions.
 

Alec_t

Joined Sep 17, 2013
14,370
In either subsynchronous or supersynchronous mode does the system physicall slow down or speed up the generator shaft, and via the gearbox slow down or speed up the blades shaft ?
I would say yes. If 'the system' extracts energy from the generator, that exerts a counter-torque on the blades' shaft, via the gearbox, and hence slows the shaft.
 

MisterBill2

Joined Jan 23, 2018
18,928
In an open loop variable speed drive, with either an induction or synchronous motor, the drive sets the rotation speed of the magnetic field rotation.
 

Thread Starter

seandeg

Joined Apr 25, 2013
2
In an open loop variable speed drive, with either an induction or synchronous motor, the drive sets the rotation speed of the magnetic field rotation.
Thanks MisterBill2 and Alec_t for your replies.

Apologies for my poor explanation of the question.

The context is a Wind Turbine, using a Dual Feed Induction Generator ( DFIG ) combined with a Partial Power Converter ( that is a power converter that does not attempt to supply all the output power - rather supplies at most approx. 30%. The flow of energy goes from the Blades via a shaft into a Gearbox ( with in the reqion of 1:90 ratio), then to shaft of the Generator.

Any Power Converter in the Wind Turbine context consists of a capacitor bank sandwiched between two Rectifier/Inverter units. In a Full Power Converter the output of the Generator ( variable voltage and frequency ) feeds into the Converter, gets converted to DC and then that DC gets inverted to AC at correct amplitude and frequenct ready to be connected to the grid.

However with a Partial Power Converter the output from the Generator Stator is controlled by current supplied from the Converter into the Generator's Rotor windings. The frequency of this current from the Converter is such that the resulting frequecny at the Stator output is at the desired grid frequency. Typically a Converter can achieve this objective only within +/- 30% of the target frequency.

Ideally the physical rotor shaft rotates at 1800rpm, and so does the magnetic field. In this case ( synchronus operation) the Converter has no work to do, other than DC current supply to the Generator's rotor windings.

When the shaft rotates at less than 1800rpm ( the subsynchronous case ) the Converter should inject current at a frequency such that the resulting composite rotating magnetic gets back to the target of 1800rpm. My question is --- is it only the rotating magnetic field that gets adjusted back to where it should be OR does the actual physical rotating velocity of the shaft get adjusted ? ( which implies that working back via the gearbox the rotation velocity of the blades shaft also gets adjusted up ).

In the supersynchronous case ( physical shaft of generator rotating above the target ) the Converter should act to slow down the rotating magnetic field by injecting current at appropriate frequency - but 180 degrees out pf phase. In this case does the physical rotation velocity of the shaft gets reduced ?
 

MisterBill2

Joined Jan 23, 2018
18,928
OK, and thanks for the explanation. Now the question makes a lot more sense. What I can offer is that to raise the mechanical rotation speed would be a backwards flow of power, and thus a negative efficiency. So it is not reasonable that it would be done. but now I am thinking that even delivering a rotating magnetic field will consume a fair amount of power and so it is also a questionable action. Either mode seems like it would require a very big addition to the complexity of the converter system.
Several years back I had suggested that for the very large wind turbine installations it would make a lot more sense to have simply a large variable displacement hydraulic pump at the top and a variable displacement hydraulic motor at ground level, driving the generator. This combination would allow holding a constant generator speed at all usable wind velocities, and also put the generator in a place much easier to service. It would also eliminate the need for a gearbox completely. And since hydraulics is a very mature technology it is reasonable that it would not require any great amount of new inventions to start off.
But I am not aware of anyone actually doing such an installation. Oh Welll!!
 
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