Why are generators rated in kW and transformers in kVA 1

[brian1334]

I have a general idea of why generators are rated in kW and transformers are rated in kVA but I am having a hard time truly understanding it. I figured since you dont know what load the generator is driving you would rate in kVA?

 

[waross]

When you get into the actual power of the diesel engine of a generator there are so many ratings and exceptions that it is difficult to describe the engine capability with a short statement. The engine is selected to give rated performance at reasonable atmospheric conditions, at a reasonable elevation and with normally available fuel.
Consider an alternator that may safely handle 100 amps.
At 250 Volts that's 250 KVA.
Now we are going to add an engine, but what size?
Standby power;
Normally a 200 kW engine will be used for a PF of 0.8
Prime power;
This rating often allows 10% overload for one hour in 12.
First the gen-set may be re-plated with a nameplate showing the capacity as 227 KVA (May be rounded off to 225 KVA or 230 KVA).
Note, to make it more confusing, there may be one nameplate for the generator and a second nameplate for the total gen-set.
The prime mover will be 80% of 250 KVA, not 80% of 227 to provide enough power for the allowed overload.
So, same engine for the 225 KVA prime power set as for the 250 KVA standby set.
This may be the same physical set with only the nameplates changed or the standby set may have added accessories such as an oil cooler, and/or a larger oil sump.
Then if a lot of run time hours are anticipated, some consultants may spec an engine that it 25% oversize on the theory that there will not be a loss of set capacity should winter diesel fuel be used and the set may run more hours before a rebuild is done on the engine.
Continuous power, it depends, but multi generator N+1 installations are often best described as continuous duty applications.
Then the same set may be rerated for use at 208 Volts. Smoe sets will have a nameplate showing the ratings at all normal voltages for which the set will be used. Some use the nameplate that matches the Voltage that the set was ordered for. As the Voltage drops (for example from 240 Volts to 208 Volts) the KVA drops and the rated kW drops but the actual available kW often stays the same.
One size DOES NOT fit all!

Bill
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"Why not the best?"
Jimmy Carter