Generator Output vs. Power Factor. 13

I work for a small municipal light Dept. in Massachusetts, USA. We have three 3412 Catarpillar generator sets (325kW each) for peak shaving. The 3 phase generators are rated at: 406kVA, 325kW, @ .8 power facter. The generators are brushless, revolving field, with a solid state exciter. When we run these units the plant power factor jumps from .85 PF to .99PF. I run these units at 325 kW each for a total of 975kW going back on to the grid. The mechanic that we are interviewing to maintain these units says we are overloading each unit by 20%. He claims that if the power factor is above the nameplate 0.8, you have to de-rate the output kW. Now this seems backwards to me, if anything I would think you could run one of these units at 406kW due to the .99PF. He insists that I'm wrong and because this is generation it is different. Also note that, the name plate on each unit specifies that 489amps at 480 volts is the maxumum. When I run them at 325 each, the amperage per phase is approx. 405 via the fluke 43 meter.

Any Thoughts...

Thanks all.

Chris

 

[SooryaShrestha]

I am also interested to see the Cat. capability curve. I am planning to collect such capability curves of different makes, specially small ones.If possible, please email in my address soorya9@hotmail.com.

 

Generator manufacturer's incorrectly 'badge' their units with an incorrect usage of the term power factor. A genset that will deliver 1760kVA of power at its terminals at full load and unity power factor is often 'badged' as a 2.2MVA genset (at 0.8pf). This is commercial mischief by gen set makers. The unit will never safely deliver 2.2MVA of power.

Furthermore the term 'watt' as applied' to electrical systems is meaningless. If you have a purely reactive load, no power(watts) is produced. However, a generator will still suck away your fuel!

Gen set makers need a rap over the knuckles, for the 'bastardisation' of the term power factor (with no reference to load).

 

[jbartos]

Suggestion to the iain1 May 19, 2001.
1. Whatever happened to the readings of test meters from which various curves are created?
2. What about witnesses present at the testing, e.g. government witnesses?
3. What about many Customers who come back for the same product and were satisfied by the product including the check of the generator capability curve, and other curves?
4. What about the competition in the generator manufacturing industry?

 

[don01]

hi jbartos
I haven't purchased too many gen sets in my time (in fact not one) so I can't speak for the voracity of those suppliers but I have had more than one ocasion where even government certs don't seem to match reality. What you say is true about points 1,2,3,4 but hey they aint gurantees.
I got a stereo rated at 250 watts on a real big label (and australia has the toughest consumer laws going) ~ yeh you guessed it "peak" in little letters

Wasn't there a guy called Nader and a motor car once.

regards Don

 

[jbartos]

Suggestions:
1. Statistically, there are isolated events that are not counted.
2. Believe it or not testing reveals a lot, which is incidentally somewhat implied in Don01 May 21, 2001 posting (How else would he know? What did he believe then?).

 

[SooryaShrestha]

I have sympathy to the feelings of iain1 May 19 posting. But we are power professionals and not philosophers or poets. We firmly believe on facts, figures and data and proper interpretation of them. So I think if somebody says the term "watt"is meaningless, then we need to think that it must have been said by non-professional or with genuine reason. I think iain1 must be a power professional. I like to bring this again to the notice of the forum that there are facts in his posting, but some of the unexplained facts can be explained like this.
That under reactive power drawn from a generator will suck fuel, means the fuel has been used for some active power. From no equipment can we draw pure reactive power, because the noload losses are always there.It is immaterial whether the generator is supplying active or reactive power. the windage losses, the magnetizing eddy current and hysteresis losses will always be there.
The term power factor is user friendly version of the relation between instataneous values of the current and voltage. So I do not think it is a bad idea of representing the active power with a factor and the apparent power. Besides, the 2.2 MVA @ o.8 pf has the same current as 1.76 MVA @ unity pf. It is the current limitation of the generator characteristics, that makes them as equal.
So in this case the manufacturer will not be at fault, though I fully agree with iain1 regarding the technicalities that might be used by the manufacturer to shut the consumers' mouth. So why are we power professional here, other than make aware the consumer, of the repercussion of any spec.?

 

Ask any generator manufacturer if they divide the true VA rating by 0.8 and badge it.

Read BS 5514.

Watts is meaningless for pure electrical systems learn to think for yourself rather than sprout textbook gibberish.

 

[peterb]

I have a few problems with the direction being taken here - am I the only one?

Generator manufacturer's incorrectly 'badge' their units with an incorrect usage of the term power factor. What is the correct usage of the term in this context? A genset that will deliver 1760kVA of power at its terminals at full load and unity power factor is often 'badged' as a 2.2MVA genset (at 0.8pf). This is commercial mischief by gen set makers. The unit will never safely deliver 2.2MVA of power. Which reputable manufacturers are doing this? Is there any evidence of this practice, any documentation?

Furthermore the term 'watt' as applied' to electrical systems is meaningless. If you have a purely reactive load, no power(watts) is produced. However, a generator will still suck away your fuel! Even without any electrical load, the engine-generator set will certainly suck away your fuel just idling along. There are always losses and they are real. There is no such thing as a PURELY reactive load - there is always going to be some resistance involved in the real world.

Gen set makers need a rap over the knuckles, for the 'bastardisation' of the term power factor (with no reference to load). The genset makers that I am familiar with rate their sets in terms of rated kilowatts at rated power factor.

Watts is meaningless for pure electrical systems learn to think for yourself rather than sprout textbook gibberish. I'm afraid that you will have to explain your meaning a bit here - "watts is meaningless"?? "textbook gibberish"??

By the way, Soorya, you are a little off in your calculation - 1760 kVA at unity PF & say 4.16 kV equates to 244.3A, while 2200 kVA has a rated current of 305.3A. What is the same is the real power of 1760 kW in each case (1760 kVA @ unity PF, 2200 kVA @ 0.8 PF)

 

[electricpete]

my head hurts ;-)

 

[SooryaShrestha]

Thank you peterb for the correction. It is my mistake.

 

All generator manufacturers divide the true output rating of their sets by 0.8 and badge this as the 'rating' of the generator.

All world standards ratify this practice (this does not make it correct).

If BS 5514 is not good enough for you, you could try IEC 34.

If the International Electrotechnical Commission is not good enough for you, then I can't help you. Since the IEC reached agreement with CANENA this applies to people living and working in America. Although it was common practice for generator manufacturers to 'make up' the rating of their generators even in North America (where I don't live or work) long before this.

I wanted to 'challenge' this forum with questioning the terms watts and power factor because it is what generator manufacturer's do. Referencing power factor without referencing load is unacceptable. Applying a mechanical 'wattage' to a generator is ok, but it confuses issues such as efficiency and power factor in mechanical-electrical and electrical to mechanical energy conversions.

This is what I was hoping people in this forum would discuss.

 

[SMLD]

I'm confused with iain1's posts....

I have been taught that kVA (Apparent Power) already has the power factor calculated in. Secondly, kW ( true power) does not reference PF.

2200kVA and 1760kW@.8pf being synonomous.

I do agree with iain1 that Gen mfgs want to make their gen sets look as beefy as possible, but is this just one of the many marketing ploys that we are subjected to each day universely, that challenge us to see the real picture. For example, does anyone really believe that "no one" knew the west coast would have a shortage of power? Why is gasoline priced at $1.9899 instead of $1.99. Does Viagra really "turn back the clock" or are we pumping air into a tire with a hole into it.

I view this just as the many thing we as a race do collectively to catch the consumers eye. Don't forget, thats the first step in marketing. Whether it's the gen mfgers', gas comp., or car dealers. We have to keep our minds sharp to see through the smoke and mirrors.

CAC

 

[peterb]

Iain1 -
In my experience, the limiting rating factor in all engine generator sets is the output power (kW) of the engine. The rated real power (kW) output divided by the rated power factor will always give the rated apparent power (kVA) of the genset.
The package assembler (who may or may not be the engine manufacturer) will start by selecting an engine from the range available to him. The next step is to purchase a generator (as the engine manufacturerer is not a generator manufacturer). There are some prime considerations in selecting the appropriate generator -
- Ability to accept the engine rated output power (this is rated kW, not kVA)
- Ability to provide the rated power at rated power factor (this is not universally 0.8; could be 0.85 or I am presently working with steam turbogenerators rated for 0.95 and 0.93 PF)
The generator manufacturer builds a machine that is based on a kVA rating. The capability curve (see above discussion, particularly the post from jack6238 on Mar 25) follows from the machine impedances and the excitation, as well as the rated stator and rotor limits.

I don't have either BS5514 or IEC34 available to me. Can you please quote relevant extracts that speak to your point? I am sure that all would be interested.

When you purchase a relatively large genset, it would be prudent to specify a works and/or site load test, with appropriate instrumentation, to verify the contracted rated output, efficiency, temperature rise etc. This should go a long way to alleviating your concerns - there are many ways to keep manufacturers honest and we engineers have a primary duty to ensure that equipment is supplied in accordance with specifications. Performance of this duty may include some or all of the following - preparation of the specification, evaluation of tenders, review of supplier documentation (which should include a generator capability curve and full rating data), inspection during manufacture, witness of works tests, inspection of installation and performance of commissioning tests on site.

 

If a generator manufacturer deems that his/her machine has a 'mechanical power rating' of (using the previous example) say 1760kW, the manufacturer is allowed to 'badge' this as a 2200kVA generator (I'll dredge out the clauses, including Amaerican stds, when I'm next in the office). The generator will be viewed to have a 'safe capacity' to deliver around 1760kVA at unity power factor (although it is not advisable to regulate the load power factor at unity with generator supplies - for risking leading power factors). This is overloadabilty traded against efficiency, and would suggest a higher 'true' kW rating of the actual machine.

This calls into question what 'safe capacity' and 'true output' from a generator means.

Capability curves never reflect what large process loads do (in terms of starting, harmonics etc).

Specifying a site load is half the answer, specifying starting requirements for large loads coming on-line with process running and for 'black start' is just as important, as is the general type of the process load.

 

[SooryaShrestha]

I insist that technically, noting 1760 kW at 0.8 pf means same as 2200 kVA. In the case of generator drawing 1760 kW at 0.8 pf, it actually means it drew 2200 kVA. But it does not mean it drew 2200 kW at unity pf. If the name plate indicates 2200 kVA and the pf is 0.8, it provides active power equal to the product. But if it indicates 2200 kW at unity pf, then it precisely meant something different. Thus 1760 kW at 0.8 pf and 2200 kW at unity pf, as they both draw the same current, may mean the same thing. I have seen generator rated 14200 kVA at 0.86 pf. It not only means providing 12212 kW at 0.86 pf, it also means it will provide 14200 kW at unity pf. We have tested the machine also. This machine is made by ABB ENERGI, Norway. I have got capability curve also. Generally from the point of the rated pf and rated active power the curve will be parallel to reactive power axis, which means the active power does not increase with the increase of pf. But in the curve which I am referring it is not parallel but it is a circle with the origin as the center. If somebody is interested to see I can send it.
If some manufacturer indicates its rating as 2200 kVA and unity pf, which normally nobody does and if somebody does it it must be able to generate 2200 kW. If it can not be generated it means manufacturer's warranty not being fulfilled.
As far as the black start of the generator is concerned, it is the part of the magnetic property of the core. I do not know how it can be read though the capability curve. If some body knows how it can be noted from the capability curve, it will be worth a star. I am myself trying to see documents with such stuffs as the inertial effect of prime mover and the generator rotor and flywheel if any.

 

1760kW is the 'prime mover' mechanical rating at its 'peak'.

How can you get 2200kVA out of a 1760kW machine?

The most you can get is 1760kVA (and this is pushing safe limits - overloadability against efficiency). If you had a load running at 0.8pf the 'absorbed power' is 1408kW, this would then give you around 1250kW of output process power.

Is there a flaw in this argument?

btw the 'base' load of the gen set would be 1600kW.

What is the correct 'wattage' of the prime mover?

 

(For peterb's information) - the IEC 34 definition is -

Rating - VA output at assumed 0.8 pf.

Which lets the cat out of the bag.

The original question is that the 325kW generators are rated for 406kVA at 0.8pf.

However the IEC definition does not (by itself) take account of VA ratings above 0.8 pf.

At a pf of 1 your VA rating could (and normally) is 325kVA.
At a pf of 0.9 your VA rating could (and normally) is inbetween 325kVA and 406kVA. Your VA rating decreases with increasing pf. It is a 'flatline'.

So while your VA rating at 1 pf is 325kVA the name plate rating is allowed to be 406kVA.

Make sense?
Do you agree this is commercial mischief?

 

[peterb]

iain1 -
I think that there is a basic misunderstanding here. The generators are rated 325 kW, 0.8 power factor, from which the 406 kVA rating follows. There is no question that the real power rating is anything other than 325 kW. This is the rated power output which can be delivered at the terminals of the genset.
The 406 kVA rating is the capacity of the generator, without regard to the prime mover. In other words, if the engine were capable of producing 406 kW, the generator would be rated to supply that power at 1.0 power factor.

The only problem that I can see here is that you are apparently confusing real power (kW) with apparent power (kVA).

Soorya is of course perfectly correct in stating that 1760 kW at 0.8 PF is the same as 2200 kVA. The flat line mentioned is the rating of the prime mover - the actual generator capability curve is an arc of a circle whose radius is the rated kVA of the machine, but the combined characteristic of the generator + prime mover limits this to a straight line. Please refer to the link provided much earlier in the thread by jbartos for info on this topic.

 

[rhatcher]

iain1, I do not agree. This standard rating is based on the realistic premise that the load served by the generator will have a power factor of less than 1.0. I understand that you are advocating that the prime mover output should be matched to the generator output so that full kva rating can be achieved at pf = 1.0. For example, a 406kw prime mover powering a 406kva generator so that the output can be rated at 406kva at pf=1.0. (note that I am assuming perfect efficiency to keep thing simple)

This would be good if you are powering a purely resistive load as the prime mover and generator run at full load and highest efficiency. Realistically though, you will be supplying power to a load with mixed real and reactive power requirements. Using the above generator with a 0.8 pf load will obviously allow an output of 365kw and 244 kvars for a total of 406kva. Here is the problem with that. You are using a 406kw prime mover to provide 365kw of power to the load. This is pretty inefficient.

The fact is that the efficiency of a generator is pretty high over wide range of load when compared to that of a prime mover. That, coupled with the fact that pf < 1.0 is the rule, not the exception, leads to the standard practice of sizing the prime mover for desired kw and then oversizing the generator to allow for the inevitable reactive power requirements of the load. In fact, this practice is so prevalent and makes such good sense that, as you pointed out, the IEC has defined the rating of a generation unit as KVA at pf = 0.8. Generators are, as pointed out already in this thread, rated a pf's other than 0.8, but the idea is the same and the following guidelines still apply.

The following guidelines apply when looking at generator ratings. For example: the rating 406kva at 0.8 pf provides enough infomation to know the following:
- will produce 365kw and 244 kvars at 0.8 pf
- might not produce more than 406 kva at any pf (armature current limitation)
- might not produce more than 365 kw at any pf (prime mover output limitation)
- might not produce more than 244 kvar at any pf (excitation limitation).

Now, when I say &quot;might not&quot; above, I mean that the basic rating doesn't give you enough info to know so you must assume that the rated outputs (365kw and 244kvars) are limiting. To know more, you must refer to the generator cabability curves to see specifically how the various limitations of the system affect output for a given pf.

I hope this clears things up a little.

 

[jbartos]

Suggestion/Answer to iain1 (Visitor) May 25, 2001
1760kW is the 'prime mover' mechanical rating at its 'peak'.
<<O.K.>>
How can you get 2200kVA out of a 1760kW machine?
<<Simply, mathematically by
1760kW/2200kVA=0.8 = power factor
Physically, by connecting 1760kW prime mover to the genset rated 2200kVA, and 0.8 PF and neglecting smaller kW losses. This does not mean that the prime mover cannot be more powerful to satisfy the maximum generator kW resulting from the genset capability curve, e.g. 1850kW or so. It is necessary to remember that the kVARs in kVA**2=kW**2+kVAR**2 are inactive or reactive power, which may be hard to understand without adequate amount of education in electrical engineering.>>
The most you can get is 1760kVA (and this is pushing safe limits - overloadability against efficiency).
<<Please, could you prove it or provide some references?>>
If you had a load running at 0.8pf the 'absorbed power' is 1408kW, this would then give you around 1250kW of output process power.
<<Please, could you prove it or provide some references?>>
Is there a flaw in this argument?
<<Yes, missing proofs or references.>>
btw the 'base' load of the gen set would be 1600kW.
<<Please, could you prove it or provide some references?>>
What is the correct 'wattage' of the prime mover?
<<See above.>>