750kW standby generator loading 3

[julesm]

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!

 

[davidbeach]

The engine can put enough mechanical power into the generator to come out the leads as 750kW of electric power. The generator can supply the currents associated with 937kVA. On the capability curve, 937kVA would look like a semicircle, but the portion of the circle that would extend beyond 750kW is not available because the prime mover can't supply it.

 

[dpc]

The generator can probably operate at 1020 A at 0.95 power factor. But this corresponds to 969 kW and your engine is rated for a maximum generator power output of 750 kW. So you probably are not going to get that amount of power out of the engine for very long.

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.

 

[niallnz]

jamdore

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.

 

[dpc]

Oops, my calculation is incorrect. 1020 A at 0.95 pf = 806 kW.

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.

 

[alehman]

The simplest way to think about it for me is that there are two limits for the generator - kVA and kW. You have to satisfy both. If the load power factor is higher than the generator rated power factor, kW will be the constraint. If less, kVA will be the constraint.

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.

 

[rbulsara]

The folloing view is not legally recommended view, but just an insight to some realities. At the end of the day stick with nameplate ratings:

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.

 

[rbulsara]

Another caution: Some vendors may limit the Kw rating by throttling or setting kW limits in their governor and/or smart voltage regualtors which has some built-in protective funtions. So you may want to check with manufactures.

 

[catserveng]

If you bought a 750 ekW standby, you will likely have very little overload capacity on the engine. As engine technology has evolved with the pressures of cost and emissions, you pretty much get what you pay for and not much else with the electronically controlled engines.

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.

 

[davidbeach]

rbulsara, I certainly wouldn't try to push it, particularly with an engine with electronic controls. It is highly likely that even if the engine iron is capable of higher output the controls are locked into the rated power output and the governor won't take you above nameplate power.

 

[ScottyUK]

Most mid-size sets have dual or multiple ratings based on prime (continuous) generation, standby generation, and emergency. Which is the 750kW rating based upon? I would be surprised if the building load is steady at 789kVA: is this a theoretical maximum or an averaged load? Have you considered the effect of motor starting if you are loading the machine to the limit? Have you considered the non-linear loads which are produced by all manner of modern electronic equipment? There are a lot of considerations if you are trying to squeeze every last VA out of the genny. Anyway, if this genny is holding up an important facility do you really want it working at the end of its tether trying to support the load? I wouldn't!


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Sometimes I only open my mouth to swap feet...

 

[rbulsara]

davidbeach:

I agree. That is what I meant in the follow up post.

 

[waross]

The alternator should be safe up to 1128 amps ov normal loads.
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

 

[julesm]

Thanks everyone for the helpful discussion. (I now realize that I am supposed to be judicious with the star ratings. I truly appreciate everyone's feedback. I could star you all....)

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!

 

[catserveng]

You will need to contact your CAT dealer for the capability curve for your unit. There is an on-line system called TMI the dealer sales and service people have access to, and you should request the generator performance info, which will include the reactive capability curve. You will need to provide the serial number of either the prime mover or the generator for him to access the info for your unit.

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.