Generator P/Q diagram 1

[Denan]

Lately I got a question regarding capability diagram of a generator (gas engine driven).
The case is like this:

If prime mover is running with 100% load, say PF=1, does it get overloaded when PF is anything other than 1 (leg or lead)?
I don't think so, but am I wright?

Also, I appreciate any tips for proper reading of a capability diagram?

A client wants to run the generator in the whole area between 0,8lag - 0,9lead

 

[ausgen]

It's not so much that you'll overload your prime mover, but for (especially for leading loads) you might need to limit the demand to stay within the safe range of the P/Q curve.

If we compare supplying 1MVA at unity to 1MVA at 0.9 (leading or lagging), you're actually reducing the load on the prime mover from 1000 kW to 900 kW. The P/Q demonstrates the generator (alternator) limit, with the one you've attached being fairly standard. Lower than 0.8 lagging and you'll risk thermal rotor damage. Lower than 0.9 leading and your AVR will have excitation issues due to the capacitive discharge it will see from the load.

Cummins have a couple of whitepapers that might help explain it in more detail...

http://www.cumminspower.com/

http://www/literature/technicalpapers/PT-6001-ImpactofPowerFactorLoads.pdf

http://www.cumminspower.com/

http://www/literature/technicalpapers/PT-7007-SizingGensets.pdf

-ausgen

 

[Marmite]

I see it the other way around. The prime mover has to supply the real power (Watts) required by the load. The load is the load regardless of power factor. The generator has to supply the real power (Watts) plus the reactive power (VArs) consumed by the load, which summate vectorially to MVA.
Regards
Marmite

 

[Denan]

Thank you both for comments.

Since the prime mover gets a kW-signal it does not "see" any kVAr. But is it not "harder" for the prime mover to supply load with both P and Q, than only P? That was a question I got.

What you say Ausgen, then the load (kW) has to be reduced to be able to have more VArs. Did I understand it correctly?

Thanks for the attachments, I will read them.

 

[waross]

Let's go back to basics.
The engine or prime mover supplies Kilowatts. If the load is 1000 kilowatts the prime mover must supply 1000 kilowatts. (Plus any generator losses.)
If the load exceeds the capacity of the of the prime mover the prime mover will slow down.
The generator produces Kilo Volt Amps. The volts are set by the rated volts of the generator and the amps are limited by generator heating. If you exceed the rated amps you may expect the generator to overheat.
Now, when these limits are observed, check your capability chart for rotor heating and voltage stability issues.

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

 

[Marmite]

To use your terminology, it's not harder for the prime mover, it's the generator that feels the pain. Reactive power is power stored in the magnetic field of an inductor, or the charge of a capacitor. This energy is stored and released over the course of each half cycle, so neglecting losses there is no energy expended. The prime mover therefore doesn't "see" the reactive power. In terms of the alternator, it has to supply both the real power to the load, plus the reactive power in terms of the charging current of the inductance or capacitance which therefore heats up the windings more than if it was supplying a purely resistive load.
Regards
Marmite

 

[Denan]

Then it seems that I was right. It's just that I became uncertain, when a colleague here came with some comments about prime mover.

Thank you all again!

Regards,
Denan

 

[waross]

In practice, the reactive power and the power factor are determined by the load.
However, with more than one generator on a system, you may elect to supply more or less of the reactive VARs demanded by the load from the individual generators. The total VARs demanded by the load will be the sum of the VARS supplied by the individual generators.
If you are selling both Watts and VARs to the power company, the limit will be the maximum allowable current of the generator. If you wish to generate excess vars you must consider the effect on the generator current. If the machine is rated at a PF other than 1.0, you may increase VAR production within the limits of the capability diagram and produce VARs in addition to 100% Watts.

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

 

[ScottyUK]

To a first approximation, power delivered by the engine is not affected by the reactive load on the generator. However there are various second order effects such as the I²R losses in the stator which are influenced by the reactive load, and the power consumed by the excitation system in order to vary the reactive load (or maintain terminal voltage). When accurately modelling the engine-generator unit the power delivered by the engine does vary as reactive load changes.


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

 

[Denan]

Scotty, does it vary so much that it must be taken in consideration? I mean in this area between 0,8lag-0.9lead.

 

[rbulsara]

To rehash what waross and others have already indicated:

It depends how you will change the p.f., what is the definition of your 100% load (kw or kvA) and the rating of your alternator.

100% load at 1.0 pf=100%rated kW. Assuming your alternator is rated at powerfactor around 0.8pf, your kVA at this time would only be 80% of its rating.

If you keep the kW constant and add reactive load to change the pf, the KVA (amps) will invariably increase. Prime mover only supplies real power so it will not get affected. As long as the kVA (amps) is within rated kVA (amps) of the alernator you are fine. At some point (usally below .8pf) the alernator would get overloaded.

You can only reduce the kW, if you try to change the pf from unity and keep kVA ( amps) the same. This in fact will reduce the load on the prime mover and alternator will remain unaffected.

 

[slavag]

Hi.
Possible also say:
I=sqr(I^2active+I^2reactive)
I is constant , max generator current. Of course need check according to stability curve.
Regards.
Slava

 

[ScottyUK]

Denan,

Probably not: depends really on how accurate a result you need. Unless you have some pretty good instruments your measurement error is likely to be at least as big as the variation in field power and stator losses combined over the operating range you are considering.

On a small machine it is probably irrelevant in most circumstances; on a large utility machine the field itself may consume several MW, which is worth accounting for as fuel has to be burned to produce it.


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

 

[rmw]

Denan,

My answer to your question is probably not. If the designer did a proper job of marrying the prime mover and the generator then he/she will have sized the prime mover for 1.0 PF which is but one point on any generator's capability curve. Any other operation at any other PF, lagging or leading will require less MW than the MW necessary for 1.0 PF.

If, on the other hand the generator is rated for X MW at some PF less than unity and the designer only designed for the MW required at that specific point, then it will be overloaded as the generator tries to operate between the lagging and leading rated PF point toward the center of the curve.

I would think it would be an odd happenstance that a gen set would be designed that latter way.

rmw

 

[waross]

Diesel gensets in the range that I am familiar with, about 25 KW to 1200 KW are typically rated at a power factor of 0.8
That is, a generator end that will deliver 100 KVA will be driven by a diesel engine of 80 KW output.
Universal by all the major builders.
Spend some time checking specs on some web sites.
Try Caterpilar, Cummins, F G Wilson, Kholer, and any other major builders.
One caveat, a stand-by set will be de-rated, both KW and KVA, so as to allow a 10 % overload one hour in 10 (or 12).
(Google this site for more information on the difference between standby ratings and prime ratings.)
I have spent my hours on those sites comparing and reconciling ratings and cross referencing by engine model and generator end number.
Remember KVA describes the generator end. KW describes the prime mover.
An exception. Single phase generators above about 15 KW at 1800 rpm are not generally manufactured. Three phase gen-sets are re-connected and re-rated. The engine is not changed, but it may be tuned down a little but not always.
The re-connection results in a 1/3 loss in KVA capability.
The excess engine power allows the power factor to be re-rated to 100%.
A three phase, 60 KW, 75 KVA, 0.8PF set will be re-connected and re-rated to a 50 KW, 50 KVA 1.0 PF single phase set.

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

 

[davidbeach]

rmw,

Your methodology wouldn't be very practical. Consider a 100kW, 1.0pf gen set; a prime mover capable of up to 100kW coupled to a 100kVA generator. You could only get 100kW at 1.0pf as that is the only point at which 100kW can be produced and delivered without exceeding 100kVA. For a load of 100kW at 0.8pf your generator needs to produce 125kVA, but that is beyond its capability.

Now consider a 100kW, 0.8pf gen set. You still have the same 100kW prime mover but now the generator is rated 125kVA. The best you can do at 1.0pf is the 100kW the prime mover can produce, but you can get that 100kW all the way down to 0.8pf.

Far better to get full prime mover output over a range of load pf values than to be restricted to full output only at 1.0pf.

 

[Denan]

Davidbeach,

we do have gensets rated that way, with PF=0,8, either customer will use it or not. Most often not, but it's possible.

Scotty, it is not big generator, the biggest is about 10,5kVA (Ie is less than 4A).

Thanks for all the comments, I have a better understanding of this now.

 

[cranky108]

If you consiter how some prime movers are rated for power output, it might be better in the long term to look at a higher rated prime mover. My $.02.

Haveing said that, recently I looked at some small steam units, and the generator specs. And because the two were manufactured by different companys, The prime movers were rated less than the geneators. I believe this has to do with the standard product sizing, rather than the designed sizing. In both cases the units produced less than the rated prime mover because of the available excess process steam.

I guess at the lower cost end a little extra machine metal dosen't cost that much more.