750kW standby generator loading
750kW standby generator loading
(OP)
How many amps can I put on this 750kW cat? Manufacturer uses 0.8 as power factor. My 480V building is at 0.95 power factor.
750kW/0.8 = 937kVA
750kW/0.95 = 789kVA
Building Load is 1020A.
1020 X 480V X root3 x 0.95 = 806kVA
So, one guy says use the 937kVA rating because the bldg pf is .95.
The other says use 789kVA rating because the 750kW rating is based on 0.8, independent of the bldg pf.
Which is correct? Can I put the full 1020A on this cat or not?
Thanks!
750kW/0.8 = 937kVA
750kW/0.95 = 789kVA
Building Load is 1020A.
1020 X 480V X root3 x 0.95 = 806kVA
So, one guy says use the 937kVA rating because the bldg pf is .95.
The other says use 789kVA rating because the 750kW rating is based on 0.8, independent of the bldg pf.
Which is correct? Can I put the full 1020A on this cat or not?
Thanks!






RE: 750kW standby generator loading
RE: 750kW standby generator loading
The generator has a range of kVA and power factor that it will operate in without a big problem - this is the generator capability curve. Normally, I'd expect the **generator** to be able to operate at its rated amps at unity power factor without a problem. But you also need to look at the energy/power side. That 969 kW has to come from somewhere, namely the engine.
RE: 750kW standby generator loading
I think that your calculation is incorrect. If your building load is 1020Amp @ 480V then the total apparent power is 848kVA. Since the power factor is 0.95 then the total real power is 806kW.
Whilst the Cat Generator is capable of delivering 937kVA, the diesel engine driving the generator is only rated to deliver 750kW of real power. If you apply your building load to this generator it will over load the diesel engine. You need to check the continuous and standby ratings of the generator.
RE: 750kW standby generator loading
So you would be overloading the engine, but not nearly as badly as I indicated. Sorry for the confusion.
But basic thrust of all these responses is the same - the generator can't put out kW that the engine cannot provide - at least not for long.
RE: 750kW standby generator loading
Genny rating-
750 kW @ 0.8pf = 938 kVA
Load-
1020 A * 480 V * sqrt(3) = 848 kVA
848 kVA * 0.95 = 806 kW
Your load is within the kVA capacity, but exceeds the kW rating.
RE: 750kW standby generator loading
All calculations and opinions above are correct. I would add one more practicle aspect. In your case the alternaotr is most likey the limiting case and not the engine, because the engine is being applied at low end of the range of its capacity. A given size engine is typically combined with a "range" of alternators to provide a "range" of (KW/KVA) rating of engine-generator sets.
For example, the engine used for 750kW set is normally the same as that would be used for say a 1000kW unit. (This probably is a 8 cylinder engine). A 12 cylinder engine would be used for sets in 1000-1500KW range and 16 cylunder engines are used for 1500-2250kw range or even 2500kw with some modern fuel and turbo charge systems.
Provided the engine is adequate, the the limitation of an alternator is its ampere rating. Look for rated amps on it. As long as you are within that limit, regardless of the power factor, the "alternator" will not overheat.
So it is very likely that your engine is adequate for 806 kw output, at alternator is capable of supporting 937kva or 1125 amps. But this would be only be a coincidence would void any warranties and if something goes wrong insurance may find reasons not to compensate.
RE: 750kW standby generator loading
RE: 750kW standby generator loading
If the generator end is nameplated for .8 power factor, then that is what you can load the unit to, the horsepower (or kW) rating on the engine nameplate, and the generator rating will determine you maximum available kW and kVA loads.
Note, some standby rated machines today are going out with gen ends rated at .90 pf.
And the comments regarding capability curves is important, those in the business will find those curves have changed shape in the last few years, mainly as iron and copper has been reduced in the stators and rotors, so has the reactive capability been reduced.
RE: 750kW standby generator loading
RE: 750kW standby generator loading
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Sometimes I only open my mouth to swap feet...
RE: 750kW standby generator loading
I agree. That is what I meant in the follow up post.
RE: 750kW standby generator loading
If the set will support the load then your engine has adequate power but you may be voiding your warranty.
Some sets are rated to allow a 10% over load 1 hour in 10 or something similsr. It appears that you will be exceeding that limit with your proposed loading.
However if this is an existing set that you have to live with and the set is off warranty and you want an appraisal of the
possible problems I will make some suggestions.
If the engine is in good shape and it slows down it is overloaded and will not carry the load. Period.
If it carries the load, but blows clouds of black smoke, check the engine. This may be a sign of a malfunctioning turbo, a plugged air filter or other malfunction.
I understand from one of the automotive forums that a diesel may be blowing a lot of smoke when it is developing peak power. This is great for a dragster but not for a generator set with neighbours phoning the EPA.
Most engines that I have encountered in your size range will not be damaged by power outputs below the black smoke limit.
I would suggest watching the coolant temperature to check that the radiator is adequate for higher power outputs.
If there are reliability, warranty, insurance, and/or code compliance issues then you probably want a larger set.
If you have none of these issues and the set is supporing the load, it will probably continue to do so.
Your minor and major overhaul hours may be a little less than the old rule of thumb of a minor overhaul at 15,000 hours and a major overhaul at 30,000 hours.
Oh and by the way, I have encountered over the years, a couple of prime power sets that had the engines specified and set up at 125% of required power, so that they could lose 25% of their output due to age and wear before they couldn't carry the load. These engines would run a long time, 24/7 between overhauls. If you have lucked into one of these, you are good to go as is.
respectfully
RE: 750kW standby generator loading
The bottom line, as it was so succinctly stated, is that that by design neither the KVA nor the kW rating should be exceeded. For short bursts, I'd probably be OK, but for new building design purposes, I need to meet both the kVW and the kW rating.
The 750kW, BTW, is a standby rating. And the motor loads are sequenced to minimize inrush. Not sure about the non-linear loads.
How do I find capability curves? Looked all over the CAT site with no luck.
Thanks again!
RE: 750kW standby generator loading
Usually this info is provided to the contractor or electrical engineer for the project in the submittal package, sometimes on standby jobs this get's overlooked.
Hope that helps.