DFIG generators.

Status
Not open for further replies.
P

Paul.Downie

Member
Hi can anyone explain with a basic explanation and onwards how a Ac DFIG generators works. How do the IGBT's interact with the generator. What is ment by supersynchronous and subsynchronous. Thanks paul
 
You are going to have to help me understand your abbreviations. Please start with "DFIG" and "IGBT."
 
The descriptions make it sound like a rotary transformer that amplifies power by using the mechanical energy input.

Does that even make sense? :-?

What it really sounds like is that the output frequency is determined by the field frequency rather than the RPM.

Now, that's gotta make sense! :roll:
 
With a normal generator you have a static magnetic field, and the action of the copper moving within the static magnetic field causes voltage to be generated. The AC waveform generated is in strict relationship to the pole configuration of the genny, so typically a generator will output two complete waveforms per phase, so a genny rotated at 1800RMP outputs 60Hz juice.

With a DFIG genny the exciting field is not fixed and static, but is rotated electrically. Thus there is a third element to the relationship the output voltage has to the rotational position. Imagine it as having the effect of the generator rotating. if the field rotates in the same direction as the rotor, then you get less output cycles per revolution.. Rotate it the other way you get more.

Traditionally, generators that are connected to a line must be syncroniosed to that line, the generator not only has to rotate at the same frequency, but in phase. With a DFIG the relationship is broken, the generator can rotate at any speed, and the electronics driving the field makes the generator output power at the right frequency and in phase.

A Siemens innovation, I believe.

IGBT (Insulated Gate Bipolar transistor) are a relatively new type of semiconductor device, which posesses some of the best fatures of bipolar devices and MOSFETs, and are now seen in many applications, train traction control, inverters, DFIG field drive and dimmers to name but four. One very cool feature is that in a properly designed circuit you dont need to fuse the things; the controlling electronics detects the over-current and simply turns the IGBT off for a couple of cycles, then tries again. Having stage lighting dimmers that dont pop fuses when the lamp goes short is a very nice feature, and once the patents expire, one we'll all benefit from.
 
If I'm understanding, I've mostly heard these called an Induction Alternator instead of an Induction Generator. With that disclaimer in mind, IGs are similar construction to induction motors. An induction motor motor develops torque when the rotor slips (runs slower than the stator rotating mag field) generally 3 to 5 percent - a 2 pole motor (synchronous at 3600 rpm) would develop full torque at 3490 to 3420 rpm.

So, to make an IG, connect an induction motor shaft to a driver, and overspeed the motor to get the same amount of rated slip - say 4%, or 3740 rpm. The driver is pumping power in to the shaft, porportional to the rated torque, and electric power is being fed to the grid, porportional to the rated horsepower.

The tricky part is IGs need to be connected to a grid to work. They won't self excite. Although I have wondered if an inverter couldn't excite an IG.

The problem is the speed range for generation is fairly narrow - should be about the same speed percent of slip if the IG were operating as a motor.

I've already told you more that I know about induction alternators, the rest is conjecture:

carl
 
Starting the conjecture part:

Double Fed Induction Generator: I had not heard of these before, but what I am reading makes sense. If the driver speed is not stable - a wind turbine for example - then the answer is to use a wound rotor with slip rings, and feed a low frequency current into the wound rotor. So now a slow rotor speed has the appearance of turning faster (or slower):: shaft rpm + (or -) rotor current frequency X 60 = electrical rotor RPM. Now a slow turbine can make the electrical rotor RPM overspeed enough to generate power. You just need a VFD to power the rotor.

What part comes next - after the conjecture?

carl
 
I think after the conjecture comes the si-fi/fantasy part.

Again, If I am understanding what I am reading, this almost makes sense. Any of you motor guys feel free to jump in and straighten me out.

A standard induction motor has quite a bit of current circulating in the shorted rotor bars. As I understand, the frequency is equal to the slip frequency. Using the same 4%, slip, it would be 2.4 Hz.

Here is where it gets weird: Again if I am reading/understanding correctly, This low-freg current to the wound rotor is actually pumping power out of the rotor - remember, there is an induction between the stator and the rotor cause of the slip frequency. So, if this is the case, then the IGBTs are the semiconcuctors that make up the power sections of the VFD that powers the rotor.

I'm working on one now that has identical rectifier and inverter sections. Changing the gating to the semiconcuctors will cause the drive to regenerate, sucking the energy out of the motor, braking it very quickly.

I'm thinking you get power out of the stator and the wound rotor.

carl
 
Interestingly, I know of a variable speed diesel gen that does quite well.

It is a 7.5kW, that drives a rectified three-phase alternator. The DC output is fed to an inverter, 120/240 single phase. The engine just runs fast enough to produce the power required. If the power requirements go down, the gen slows right down. If it had a pm exciter, the alternater wouldn't even have to have sliprings. It makes me wonder if this technology won't give Double Fed Induction Generators a run for their money.

carl
 
Status
Not open for further replies.
Top