Generator uprate - Class B to Class F? 3

[ScottyUK]

Apologies for posting a generator question in the motors forum but at this size the generator is constructionally no different to a small synchronous motor and I want the motor rewinders to share their thoughts.

We have a small (290kVA) standby generator which has a steam turbine prime mover and which we're considering as a candidate for uprating. The prime mover is anecdotally good for about 200% of present output based on similar machines on site, and the limiting factor is likely to be the generator. It's an IC01 air-cooled brushless design with a Class B stator and Class F rotor. I'm trying to get a feel for how much we might be able to uprate the machine if we rewind it as Class F / Class H respectively, and where we might hit the next thermal limit. My guess is that stator lamination insulation might be the next problem. Any of the rewinders here have any gut feel for this?


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

 

[rhatcher]

ScottyUk, the beauty of it is that that a motor and a generator are the same, just operating in different quadrants. So, you are in the right place.

In general, I would think you could probably upgrade by 10-15% with a rewind. Usually, changing the rating would involve changing the number of turns and the wire size. For a synchronous machine, this would most likely be required on the rotor as well as the stator. While upgrading the insulation class does offer desireable benefits, you do not normally increase the rating by doing this alone.

The limiting factor for changing the rating is flux density and saturation of the iron (stator and rotor). Once you get past the linear portion of the iron's magetization curve, additional excitation produces very little additional flux but much more heat. Normally the winding insulation of any class will fail before the stator iron lamination insulation fails.

I hope this helps.

 

[edison123]

Scotty - You have left out the voltage.

If it is under 600 V class, can't do much in conductor size increase but can do a thermal increase with class H Nomex slot insulation and class H magnet wire. I would say about 10 to 20%.

If it is a MV winding and if it is really with class B insulation (i.e. bituminous insulation not an original class F epoxy-mica insulation derated for class B operation), then there is a good possibility of conductor increase as well as a thermal increase. But then your rating is small, so I am not sure how much cushion the original design has in to increase the copper area. In bigger MW machines, I have done uprating from 15 to 33%.

Stator core is not a big factor in uprating (unless the machine is really, really old like early 30's) because the core plate varnish typically has a thermal rating over 200 deg C.


Muthu

http://www.edison.co.in

 

[ScottyUK]

Hi Muthu,

Sorry: it's a 415V machine from the early 1970s. It is listed as a class B stator although the blueprint says it is a PU insulated winding. I'm not that familiar with 35-year-old winding materials but I'll upload the drawing tomorrow.

rhatcher,

Thanks for the advice so far. I hadn't expected to increase the flux density much because it is a young enough machine that the iron is probably quite close to saturation. I doubt it has the design margin that was common in the fifties or earlier. Do think there may more scope for pushing the core a little harder?


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

 

[edison123]

BTW, this forum is better suited for generator questions since the generator constructional details and problems are very similar to motors. IMO, this forum title should be changed to Electric Motors & Generators.

Muthu

http://www.edison.co.in

 

[edison123]

Hi Scotty - By early 70's, class F insulation (class F magnet wire and class F varnishes for LV machines and epoxy-glass-mica for MV machines) had become the norm. So, I suspect yours is already a class F stator derated to class B, quite common practice in 70's. In your case, a class H rewind of both stator and rotor might get you about 10% more KW.

AC generators (LV or MV) are typically designed to operate very near the knee of the magnetic curve for a good voltage regulation. So, I don't think you'll gain much there by pushing up the flux density.

Muthu

http://www.edison.co.in

 

[ScottyUK]

That kinda confirms my thoughts: in the big machine world we frequently see specs requiring a class F stator running at a class B temp rise. Looks like the same sort of philosophy may have existed in the small generator world too back then.

If the best we can squeeze out of the machine is another 10% then it's probably not economically worthwhile: we're desperately undersized for the loads of today but I guess we have the option of fitting a bigger alternator if the mechs are correct with their estimate of uprating the turbine.

Thanks for the advice, both of you.

Oh, and yes, I agree that this forum would be better named "rotating electrical machines" or similar. That would confuse the hobbyists no doubt.


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

 

[rmw]

How about Electric motors, generators & THEIR controls?

rmw

 

[itsmoked]

"Electric" would have to be traded for "their". "Electric" is more important for search engines.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rhatcher]

A star each for Edison123, ScottyUk, and rmw for inspiring a new and more appropriate name for this forum.

I just noticed the change today. Of course, sometimes I am a little slow. If this happened before today, please just pat me on the head and accept me as the sometimes bright, sometimes slow, guy that I am. No hard feelings ever.

 

[rhatcher]

I will add the following thought.

ScottyUk said "I hadn't expected to increase the flux density..." and edison123 said "I don't think you'll gain much there by pushing up the flux density."

Keep in mind that the output of the generator is proportional to air gap flux density (before the knee). When you increase or decrease the generator output, you do this by increasing or decreasing the flux density. This is accomplished by increasing or decreasing the field excitation.

Obviously, to increase the output rating beyond the nameplate value you must increase the flux density in the stator beyond the rated value. To do this, you must increase the flux provided by the rotating fields beyond the rated value.

In either case, iron saturation is a concern. Assuming the prime mover can supply the additional power, the iron is the limiting factor when increasing the rating of a generator.

 

[edison123]

rhatcher

Thanks for the LPS. If someone doesn't like the change, Scotty started it.

Now coming to your last post, the generator output is not controlled by the excitation. It's done by the primemover. Adj using the excitation only changes the generator PF if it's running alone or controls the VAR's if it is connected to the grid.

As for the flux density in a generator under various loads, the flux density practically stays the same from no-load to full-load. When the load is applied, the flux is demagnetized by the armature currents and hence the excitation is increased (by the AVR) to offset this demagnetization and to bring back the original no-load flux density.

As I said earlier, the generators are designed to operate just near saturation point and hence the flux density cannot be a factor in an uprate. An uprate is mainly decided by how much more copper can be added and the temperature class of insulation.


Muthu

http://www.edison.co.in

 

[rhatcher]

Edison123,

Adjusting the excitation control changes the voltage when operating solo, not the power factor. But that is a minor point. What I have been thinking about is your explanation of the flux density under various loads. It sounds as if you agree that increased excitation is needed for increased load but you disagree that the increased excitation causes increased flux density.

I have to disagree. In my understanding, the minimum flux is required at no load and the amount of flux required to maintain constant terminal voltages increases as load increases.

When you look at the equivalent model of a synchronous generator, you will see that Vt = Ea -(Ra +jXs)Ia. Vt is terminal voltage, Ea is induced armature voltage proportional to flux density, and Ra + jXs is the output impedance.

When there is no load, Ia is zero and Vt equals Ea. When a load is then applied, voltage drop across the output impedance causes Vt to drop to a lower value. In order to bring Vt back to the original, no-load value, Ea must be increased. This is done by increasing the excitation and therefore the air gap flux density. If the stator coils do not see increased flux, Ea cannot be increased. This implies that the stator iron will see more flux. All of this can only be accomplished if more flux is produced by the field poles, so the rotor iron sees more flux as well.

There is nothing that I know of in the equivalent model or otherwise to explain the idea that the flux density is constant and is somehow counter balanced by the stator flux.

 

[edison123]

Well, let's agree to disgree.

Muthu

http://www.edison.co.in

 

[rhatcher]

Agreed.

 

[Hoxton]

It was quite common in the 70's for consultants to specify "class B" rises with "class F" insulation. So they would accept the new material, but not the new temperature.

Note that the the increased temperature rise would result in increased stress due to increased differential expansion btween copper and insulation. So perhaps they were wise....

 

[ScottyUK]

Hi Hoxton,

That's still a very common spec requirement on big machines today. The newer class H and class 220 insulations are making slight inroads but are still largely ignored with class F insulation and class B rise being the norm.


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

 

[wolf39]

Hi:

Just came across this one by accident.

Increase of flux means proportional increase of terminal voltage. The terminal voltage cannot be increased because proper synchronization of the generator with the system would not be possible. If the unit is synchronized to the grid, increase of excitation (flux) does increase reactive power (over excited).

Regards

Wolf

http://www.hydropower-consult.com

PS.
Excactly when was this forum re-named to "Electric motors, generators & control engineering"? I'm happy that generators became part of this forum.
Wolf

 

[ScottyUK]

Hi Wolf,

Name changed about a month ago. I agree - generators and motors are so similar that they belong in the same forum.

This is a small islanded set, but terminal voltage does need to be held reasonably steady at the nominal 415V value. Increasing flux would also increase the machine internal voltage 'E' which would allow a heavier load to be connected and be the j[ω]X_s volt-drop to be compensated for.


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