Generator Sizing

[jcole]

Hello guys and gals

I have a 1500 KVA 480V transformer that feeds a 2500A rated main on a drawout switchgear.

I am wanting to figure out what size, in kw, generator I would need for standby power to replace the transfomer in case of failure, power loss, etc.

I know that to find kw the formula would be VA(1.732)PF. I also know I should size the generator for the loads but I would like to size it for the current available at the transformer. The primary of this transformer is fused at 150A at 12470V.

My calculation:
A=kva(1000)/V(1.732)
A=1500(1000)/480(1.732)
A=1804

Kw=VA(1.732)PF
Kw=480(1804)(1.732)(.9)
Kw=3560

How would you have went about finding this?

All comments apprecitated.

 

[ron]

If the transformer is 1500kVA, measure the power factor of the load on the secondary, then multiply 1500 times the power factor, then choose the next standard size generator higher than the calculated result.
Power is power, no matter what the voltage.

 

[Rick Christopherson]

My calculation:
A=kva(1000)/V(1.732)
A=1500(1000)/480(1.732)
A=1804 Amps per phase

Kw=VA(1.732)PF
Kw=480(1804)(1.732)(.9) = 1349 KW
Kw=3560 I have no idea how you got this.

Why make this so complicated (and incorrect)? If you have a 1500 KVA transformer, then size the generator to 1500 KVA. Generators are rated by KVA, not KW. The KW sticker on the side of them is for convenience. A diesel engine would be rated by KW, because that is the Real Power it can produce, but the windings in the generator are limited by their ampacity, and that is where the KVA rating comes from.

By the way, if you simply took 1500KVA * 0.9 = 1350 KW

 

[rcwilson]

Get a one line or a listing of the major loads (>100KW) and talk to your generator sales reps about generator sizing. You need to consider starting load inrush as well as the running load.

Maybe there is a 250 HP chiller motor, 100 HP air compressor, 500 KVA of lighting load and a 500 KVA UPS that are all going to try and start at once on the generator. The generator is going to die even though load is less than its 1500 KVA rating. Sequencing of the loads or a larger generator may be needed

If it is just a bunch of lights and heat, an 800 kW/1000 KVA geenrator might do quite well.

 

[jcole]

Thanks for the replies.

Obviously I put way too much thought into this. Its been a while since electric 101.

Our engineering department proposed this. A coworker and I was just pondering what size generator would be needed. We obviously will not be on the design side of things. I work for a waste water treatment facility so the loads are mostly motor w/ VFD and lighting.

I also understand that load shedding would need to be done on start up. 99.9% percent of the time the load is probaly around 500VA but if flow was to get up and all loads were to come on I still dont believe we would be at 1000VA. But this is a municipal and the penalty for enviromental disruption is not good. So I know they will size to "What if..."

Thanks again for the replies.

 

[ron]

Rick,
Since generally the engine offers the kW rating as you mention, a low factor application (<0.8), will require sizing the gen set from that kW value, to be sure the engine is made big enough.

 

[art82]

gen set

gen set

take alll the load that you know will be continous heavy loads of course add them up. take in consideration your others loads then come up with a kw if this is not a life critical facility put time delay controls on your heavy loads so they wont bogg down the generator

 

[Goroon]

....I work for a waste water treatment facility so the loads are mostly motor w/ VFD and lighting. ....

I also work for one of them places.. good thing I am not a mechanic.

An Up-Grade (one site of many) has a "like for like" replacement new Generator for expansion. Red flagged due to location change.

Generator, due to closer location, has required new service equipment due to AIC rating

 

[jcole]

"A diesel engine would be rated by KW, because that is the Real Power it can produce, but the windings in the generator are limited by their ampacity, and that is where the KVA rating comes from. "-Rick

So I am understanding that the kw rating on a generator is the mechanical power of the prime mover?

And kva is maximum power that the generator can withstand before winding failure?



Thanks again for replies.

 

[Rick Christopherson]

Rick,
Since generally the engine offers the kW rating as you mention, a low factor application (<0.8), will require sizing the gen set from that kW value, to be sure the engine is made big enough.

Ron, it is actually the opposite. With a low power factor, the "Real Power" is very low compared to the amperage in the windings. Only with a unity powerfactor will the KW and KVA ratings be the same, and KW can never be greater than KVA.

When you have a motor that is running idle without a load, the amperage will be about 50% of the nameplate FLA, even though the output shaft of the motor is doing no real work. This relatively high current for no actual work is because the PF drops very low. The reverse is true in the generator. It does not take as much horsepower to create the current when it has a very low powerfactor.

 

[Rick Christopherson]

"A diesel engine would be rated by KW, because that is the Real Power it can produce, but the windings in the generator are limited by their ampacity, and that is where the KVA rating comes from. "-Rick

So I am understanding that the kw rating on a generator is the mechanical power of the prime mover?

And kva is maximum power that the generator can withstand before winding failure?

Yes, that is correct. (Sorry, your post came in while I was typing my last post.)

 

[jcole]

Rick

When I see kw I always assume electrical power. Thanks for clearing things up for me.

And about that calculation, I dont have any idea where I came up with it either.