Asynchronous generators 1

[lz5pl]

My current project is a small hydro power plant - one 600 kVA, 6.3 kV generator, with step-up transformer 6.3/20 kV. My problem is that the generator is asynchronous type. I have to propose single line diagram (the project is on conceptual stage), but I have totally forgotten how such generators are excitated and paralleled with the grid. I have some experience with synchronous machines, but asynchronous disappeared from my mind in all these 25 years after university. :~/
Could you give me a link to some materials on this matter?

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[Skogsgurra]

How asynchronous generators are exited? They are not. At least not separately. They get their exitation current from the grid.

How to switch on to the grid? Just bring the machine near sync speed and connect. There is no phase and synch speed is not very critical, you can be off a few percent without too much trouble. There is not even a voltage to care about. It is very much like starting an asynch motor - which a generator also is - only running a bit faster than grid. That is what turns it into a generator.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[slavag]

If I remember right, it was common practic for the small synch generation too. Run prime mover up to subsynch speed, connect to grid and after connect AVR.
Regards.
Slava

 

[dpc]

If this generator is going to run for long periods of time, you will want to include power factor correction capacitors, probably connected via their own contactor. Induction generators have horrible power factors since they must draw their excitation current from the system. If the output of the generator will vary, you may want to switch in multiple levels of capacitors roughly corresponding to the output of the generator.

Google for "induction generator" - you should find plenty of reference material.

 

[waross]

Hi dpc. I was under the impression that the excitation current of either an induction motor or an induction generator may vary with the voltage but was independent of the load. Is the switching of capacitors on an induction generator to compensate for voltage fluctuations?
Thanks.

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Bill,

The caps makes the combined generator and local cap bank look like a load of reasonable PF to the system beyond. The caps don't influence the excitation current drawn by the induction machine, they just hide its effect from the system by drawing leading VArs which locally compensate for the lagging VArs drawn by the induction machine.


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If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

What has been said above can be summed up like this:

True: you need a separate contactor for the capacitors.
True: reactive load is independent of generator load.
Not True: need to change capacitor value with load.
True: an induction generator with capacitors may selfexcite when separated from grid and still running. May cause overvoltage.

That's why you need two contactors or circuit breakers, one to switch capacitors and one to switch generator to grid.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[lz5pl]

Thank you all for the support. Meanwhile I found a sample design (just single line diagram) of similar HPP, but 250 kVA, 0.4 kV. Capacitor bank is provided also and I wondered where it has some different purpose than power factor correction. But now after all your explanations it is clear.

Well, in summary my HPP will look like this:
20 kV switchgear, consists of 4 cubicles: 6/20 kV trafo bay, outgoing to the grid, measuring panel and auxiliary services panel (some 25 kVA 20/0.4 kV transformer should be sufficient). For economic reasons only one circuit breaker will be installed - in transformer panel. Outgoing to the grid and auxiliary services will be via load-break switches. Relay protection of the outgoing feeder will trip the same circuit breaker as generator protection.
On 6 kV side we will see in Monday with our MV switchgear supplier what is available, but I suppose we could avoid cubicles and connect generator straight to step-up transformer. Generator circuit breaker will be on 20 kV side. From the 6 kV side of trafo I could take connection to the capacitor bank cubicle with integrated contactor and fuses.
Some questions are still not clear regarding generator protection, but at the moment I have no idea whether generator will be star or delta connected.
Anyway this evening the concept is looking more clear for me. Thank you again and enjoy your weekend!

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[dpc]

Maybe I'm looking at this incorrectly (and it certainly would not be the first time), but my understanding was that the power factor of an induction generator remains fairly constant regardless of the kW output of the machine. There is some variation - the power factor is a function of the machine design and the slip, just like an induction motor.

So to maintain this constant power factor, as the generator kw output goes up, the kvars into the generator must go up. This is consistent with the direct-connected wind turbines we have worked with - they had switched capacitors with 2 or 3 levels of capacitance. It probably is not directly proportional to kW output, since the power factor is not truly constant, but I believe the var requirements would change with load.

Induction motors have different var requirements depending on the load, so I guess I don't see why induction generators would be any different.

I await enlightenment....

 

[waross]

Hi dpc. No question that the power factor of a motor varies with the load.
However I was under the impression that the magnetizing VARs drawn by a motor were not greatly affected by the load on the motor.
The VARs drawn during motor starting are definitely much greater than the running VARs but I have only one reference in my library that suggests offsetting the starting VARs with capacitors. In the proposed scheme the capacitors were disconnected in several steps as the motor accelerated.
Thanks Scotty and Skogs for the information.

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

 

[cranky108]

I think some thing was lost here.
This is a hydro unit, so runing the unit below sync speed and closing it in is bad. You want to have it runing above sync speed before closing it in, or it will try to act as a pump. You want it to start as a generator.

From what I've seen on wind generation, the VAR draw will follow with the power output. So if this were a larger unit I'd be concerned about following the VAR requirments. But a one or two step capacitor bank probally fits here.

Another concern is what happens during a fault? In some wind farms the capacitor banks will trip off, hitting the system with a larger VAR requirment while the system is undergoing an inrush condition.

 

[Marmite]

The reactive power import of an induction generator increases as the real power export increases. A plot of reactive power import against power import/export is a circle diagram similar to that of a synchronous machine. The main difference is that an induction machine can only operate on the circular locus, so there is always a defined relationship between real and reactive power. The PFC capacitors have the effect of shifting the circle down the y (Reactive power) axis. It is conventional to compensate for all of the no load reactive power demand, although as more power is exported there is more reactive power drawn from the network.
When an induction generator is connected to the network there is an initial magnetising inrush, similar to that when a transformer is energised, followed by a transfer of real and reactive power to bring the generator to its operating speed. For a large induction generator it is common to use a soft start circuit to control both the magnetising inrush and the power transfers to accelerate or decelerate the machine. The soft start is a pair of back to back thyristors in series with the generator connection. Usually after some seconds a bypass contactor is closed to shunt the soft start thyristors and minimise losses.
Regards
Marmite

 

[lz5pl]

After "google"-ing on the subject I still cannot understand up to which power factor we should compensate VARs - to power factor 0,95-0,98 or to 1.05 (old joke from the University - to solve the problem we assume cos phi=3!).
Seriously speaking: should we compensate asynchronous generator as usual induction motor, or we should overcompensate it a bit? According to me if the machine needs VARs just to have excitation it should be compensated as a motor, but I am still unsure.

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[dpc]

I would not overcompensate - there's no benefit in that. The power factor correction depends on what the connecting utility requires. The generator will run fine with no compensation. The correction is done to improve the power factor to avoid penalties or charges from the utility.

 

[lz5pl]

Additional "google"-ing lead me to the following link:

http://www.calomat.de/en/news/articles/asynchronous_generator.pdf

It could be useful for somebody else.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant