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

 

What happens when you are working in countries where you are 'penalised' for having power factors less than say, 0.95 or 0.98, for supplies from public utility networks?

Your electrical system is then 'geared' around these power factors, protection system, cable ratings etc.

If you switch to a generator supply you would want it for those power factors.

To say a power factor of 0.8 is common for a process plant is true. But to say this acceptable for the rating of your generator I don't believe is a good thing to be advocating.

Most generator capabilities hit the rated VA curve only for an extremely limited power factor region. Around 0.75 to 0.8. After that you are guessing without the capability curve.

Generators should provide the VA rating written on the nameplate for at least 0.8-1 pf.

This would be based on a more 'realistic premise' for where 1 pf is the rule not the exception.

Agree?

 

[peterb]

Just a couple of points for you, iain1 -
1. Assume your plant is being supplied from the public supply. You have improved power factor to say 0.95 - very admirable. Further assume that your generator is rated to carry the real power load of the plant (say 325 kW at 480V) and has a rated PF of 0.8. This means that the generator is rated for real power = 325 kW, reactive power = 243 kVAR.

Now, you lose the public supply and switch on your standby generator. When you have switched on all the plant loads, 325 kW at 0.95PF, the generator will be operating at real power = 325 kW, apparent power = (325/0.95) = 342 kVA, reactive power = 105 kVAR.

As you can see, the load is well within the rated capacity of the generator. Increasing the power factor actually decreases the load on the circuit. When supplying an isolated load, the generator PF is set by the load PF.

2. The generators in question here (originally) are operated in parallel with the public supply. In this mode of operation, the power factor at which the generator operates is fixed not by the plant load, but by the excitation level of the machine - the bus voltage is largely fixed by the incoming public supply and variations in machine excitation will determine the reactive power supplied by the machine.
For parallel operation, you need to operate at a lagging power factor to avoid the possibility of pole slipping and out of step operation following system disturbances - for sure, you need to avoid a leading PF or one close to 1.0. This is a general statement that will apply to all "small" generators operated in parallel with the grid.

3. As stated above in several posts, generators willsupply their rated kW at rated PF. They will supply their rated kVA at power factors higher than rated, which actually results in reduced generator loading.

I hope this helps to bring us a little closer to closure on this item!

 

[bar01]

I think it is important to focus on the PF's for your facility. It depends on what kind of facilities you are designing. I have designed many high tech facilities. The power factor for these types of facilities is more typically in the .9 to .95 pf range. With energy efficient motors, and electronic lighting, computers, etc.. PF's are approaching unity power factor. This will only increase in the future.

For me the .8 pf rating on generators is outdated, unless I have an old site with a lot of old motors. I have been caught with someone reading the full load amps off a generator nameplate - and thinking this is the amount of amps he could get out of the gen set - but not thinking about the prime mover!

My generator spec says that the prime mover is to be rated (oversized) to handle the generator KVA rating I specifiy at .9 pf load. As electricals I think it is best to deal with KVA's and PF's - and leave KW out of it - after all we need to deal with total amps on the line. Let the manufacterer pick a prime mover to get you the amps you need at the highest PF you expect.

 

[jbartos]

Suggestion: One can look at the generator capability curve as a set of points that uniquely determine the allowed generator loading. E.g. under certain angle fi corresponding to the power factor=cos(fi), there will be a unique kVA value and the associated unique kW value from relationship PF=cos(fi)=kW/kVA.

 

[Dan76]

I think the confusion here lies in the difference between Real and Reactive power.
Real Power: Does work or Makes Heat (kW)
Reactive Power: Energy stored in Electric and Magnetic Fields (KVAR) +/- 90* from Real Power
Apparent Power: Vector Sum of Real+Reactive Power
This is why a Generator puts out more amps than a simple P=IE equation would suggest
Of course At 0 pf (impossible) a generator could theoretically deliver infinite KVA! If only we could eliminate resistance completely!
Then again to avoid consuming fuel the prime mover would have to be absolutely frictionless.

Apologies in advance for quoting textbook gibberish above, but I can confirm by personal experience that both capacitance and inductance are very real and can shock the heck out of you!

Also note: at very poor (ie <.8) power factors KVA can increase (as above), at least to the current limits of your cables, and/or windings. Output tends to drop to 0 when these are exceeded too much or too long, and the Fire department has to be called. So the capability curve is a composite of the electrical and mechanical capacities of the machine.

I really enjoyed this post, very lively, -Dan76

 

[jbartos]

Suggestion to the previous posting: If PF=cos(fi)=cos(90°)=0, then
kVA = kVA x PF + j kVA x sin(fi) = kW + jkVAR = kVA x cos(90°) + j kVA x sin(90°) = kVA x 0 + j kVAR = 0 + jkVAR, in kVARs
Therefore, the kVA can be infinite for kVAR equal to infinity, i.e. for kVA equal to infinity.
Remarks, any harmonics are not considered for simplicity.

 

note for peterb,

your message
&quot;3. As stated above in several posts, generators will supply their rated kW at rated PF. They will supply their rated kVA at power factors higher than rated, which actually results in reduced generator loading.&quot;

-This must be a typo or I am reading it wrong - I think we have made the point already that generators are rated at an 'assumed' power factor of 0.8. The name plate rating of 406kVA, 325kW, @ .8 power facter means it will give you 406kVA only at 0.8pf. At unity power factor it will give you 325kVA. Note that the 'rated' kVA (406kVA)is not supplied at power factors above 0.8.pf!

Generator manufacturer's don't even have to put 325kW, 0.8pf on their units - 406kVA will suffice! And you have to 'be in the know' that this really means at 0.8pf only. This is commercial mischief.

As we have already made the point 0.8pf does not reflect power factor in countries where the laws make it very costly to have poor power factors - public supply utilities do not want the current ratings of their networks increased by poor installation practices.

How can you people support/accept this practice?

 

[stevenal]

Consider the the kW and kVA ratings to be maximums. 0.8 pf just happens to be the point where both values can be at their maximum. Below 0.8, kVA sets the limit. Above, kW does.

 

[peterb]

Iain1 -
Let's try this one more time, then I promise I'm done with it.
Taking your example, and assuming 480V rated voltage:
406 kVA @ 480V gives rated current of (406/(sqrt(3)*0.48) = 488A. This is the rated current of the generator and this WILL NOT CHANGE with increased power factor.
What limits the generator output at 1.0 PF is not the generator capacity, which remains at 406 kVA or 488 A, but the engine output, which determines the rated 325 kW. The generator capacity is higher than the engine output at power factors greater than rated, as the limiting factor in this portion of the capability curve is the stator current heating (rated 488 A in this example). Below rated power factor, the limiting factor is the rotor current heating, which results in reduced KVA capacity at power factors lower than rated.
If you bought the above genset, then you must be aware that you will never get more than 325 kW out of it, no matter what power factor it runs at. This is a function of the engine output only, and there is nothing at all misleading if the [b[generator [/b] nameplate reads 406 kVA, 0.8 PF, 325 kW as this would be completely correct.
I realise that this may fail to convince you, but I sincerely urge that you do some further thinking and research on the matter. The website posted earlier in this thread by jbartos should help somewhat in understanding the subject.

 

[jbartos]

Suggestions:
1. Check with Manufacturer how the prime mover and generator have been married powerwise
2. Theoretically, the prime mover can have smaller, equal or higher shaft kW power output then the generator rated kW output at PF=1.0.
3. There are no rules how Manufacturer shall marry the genset. Things may be fairly free, and ultimately &quot;money talks.&quot;