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?

 

[rcw retired EE]

A generator is rated in KW at a certain power factor so it too is rated in kVA.

A generator is converting mechanical energy from the prime mover to electrical energy. The engine, turbine, windmill, waterwheel, or kid on a bicycle spins the generator is rated in horsepower = kW. The generator cannot produce any more kw than is supplied by the prime mover. That given, specifying its size in kW makes sense.

A transformer can handle its rated kVA at any power factor. A 100 kVA generator, rated at 0.8 pf = 80 kW cannot supply more than 80 kW. A 100 kva transformer can deliver 100 kw at 1.0 power factor.

 

[brian1334]

Thanks RCWILSON, your explaination of mechanical energy being converted to electrical makes sense why it would be rated in kW as it relates to horsepower.

 

[cranky108]

The KW and PF ratings on a generator mark the crossing of two capability curves.
Where for transformers the KVA rating is at a voltage, marking the heat disapation capability of the tramsformer.

 

[rbulsara]

Generator is power generator (coverts the form of energy) and its ability to deliver the real power is limited by its prime movers capacity in kW or HP. Where as the reactive power that makes up total KVA can be almost anything by sizing the alternator accordingly. The kVA rating is limited by the alternator. So it has both ratings, kW and kVA and neither should be exceeded.

The transformer is only a pressure (voltage) changer and does not care whether power passing through itself is reactive or resistive or its combination. Its ability is limited only by amperes it can pass through at a given voltage, hence only kVA rating matters. Most kW it can pass thorough would be equal to its kVA rating at unity power factor.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

KVA describes the thermal capacity of either a generator or a transformer. Actually the first limit is the current. Heating is dependent on the current and both generators and transformers have a safe maximum current. This is then multiplied by the rated voltage to determine the KVA.
A generator rated at 200 Volts or 208 Volts will have a higher KVA rating if it is dialed up to 240 Volts.
The kW rating of a generator is mos often 80% of the KVA rating and is an indication of the minimum power capacity of the prime mover. It starts to get complicated when comparing standby ratings with prime ratings and different available voltage setings.
But, bottom line, KVA describes the generator end. kW describes the minimum power of the prime mover.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sibeen]

kW describes the minimum power of the prime mover.

...or maybe even the maximum

 

[GTstartup]

" A 100 kVA generator, rated at 0.8 pf = 80 kW cannot supply more than 80 kW."

Why not? at unity it should supply 100KW no?

 

[rbulsara]

GTstartup:

Only if the prime mover is rated for 100kW plus losses, which usually is not the case.


Rafiq Bulsara

http://www.srengineersct.com

 

[cranky108]

The generator rating is a point where the excitation curve meets the thermal capacity curve on the generator. 80KW @80% pf, dosen't mean the generator can't handle a higher load, or lower power factor.
The untimate capacity of the generator is the capacity of the prime mover, or heat disapation ability, which ever is lower.
At the same time below the rating power factor, full kva can not be achieved because of the excitation limits.

 

[waross]

Hi cranky108.You may be correct as far as larger machines are concerned. However for smaller machines, below about 1 megawatt, the PF rating at 80% is an industry standard. With the different voltages and excitation schemes that may be used with the same alternator in different applications, the capability curves are not usually a limit.
For instance with just a change in model number the same physical gen-set may be rated at 80KVA/100kW prime power, or 90KVA/110kW standby power.
If that set is rated at 240 Volts, the same set will be rated at 85KVA/70kW at 208 Volts. The dead-rack setting may be trimmed back to 70kW or it may be left at the 80kW. If this is a prime rated set, it will then be capable of outputting 88kW, despite the rating of 85KVA/70kW.
Then we come to excitation. Basic AVR, PMG, load current boost module, saturable PT/CT excitation. These are all rated at 0.8 PF and the capability curves will all be different.
I have probably answered sibeen's comment to some extent.

kW describes the minimum power of the prime mover.

...or maybe even the maximum

I have encountered a couple of sets where this was true. Usually there is a little extra capacity in the prime mover to allow for changing atmospheric conditions and engine aging. In Canada a diesel engine may produce 10% more power on summer fuel than it does on winter fuel.
In some installations, for many years old crankcase oil was disposed of in the fuel supply. This increased the SG and as a result improved the power and fuel economy of the prime mover.
With the new generation engines, the fuel filters are much finer to adequately protect the new generation injectors with a much finer spray. With these the cost of replacing fuel filters is greater than the saving in fuel cost.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[GTstartup]

rbulsara

Agreed, but that is a prime mover restriction not a generator restriction. We should be careful not to give the impression that a 100KVA generator cannot supply 100KW under any circumstances

 

[waross]

Hi friends; we may have some confusion of terms. If "generator" means the bare alternator or generator end, one set of conditions apply. If "generator" means a complete diesel engine and alternator assembly then different conditions and capabilities apply.
Gt and Rafiq, you may both be correct.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

I think we all are in agreement.



Rafiq Bulsara

http://www.srengineersct.com

 

[wolf39]

brian1334:

Objection!

Generators and transformers are always rated in kVA. Just have a look at name plates.

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[waross]

Exceed the current as determined by the KVA rating and you may damage the alternator by overheating.
Exceed the real kW rating,(which may be greater than the nameplate rating) and you may stall the prime mover and drop the frequency.
Severe overloads on diesel sets often exceed both the KVA and the kW limits.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

wolf39:

You are perhaps more familiar with the large alternators for hydro units, which can almost have endless prime mover power behind it and there just the KVA rating makes sense.

For the rest of us mortals (industrial and commercial applications), a generator is a generator-engine set. Those units are invariably labeled with both kW and kVA rating. Where the engines are picked based on kW rating (as small as possible) and the alternators are typically rated 0.8 pf of that kW rating. In other words, there is no guarantee that the engine behind those alternators can provide kW more than 80% of the alternator KVA rating.

Rafiq Bulsara

http://www.srengineersct.com

 

[wolf39]

Hi Bill, hi Rafiq:

Yes, you are right, I'm not familiar with generator-sets. As both of you pointed out convincingly, exceeding the kW rating can be a problem or even catastrophic for the prime mover, even if the kVA generator output is within its thermal limits.

As you can see, despite my experience and age, I'm still in the learning curve. Thanks for the comments.

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[cranky108]

So what is behind the KW or HP rating of a prime mover?
Rated in new condition, at sea level, at 20 degree C?

Then it would make since for them to have a derating factor outside those conditions.
It also makes since that a manufacturer would put the smallest rated prime mover as possible to reduce cost.

 

[edison123]

Hi Wolf - Though generators (both thermal and hydro units) have MVA and MW ratings, I have always seen them called out by their MW ratings only.

Muthu

http://www.edison.co.in