Power Ratings

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UPS:
http://www.power-solutions.com/watts-va
From the above link, I have gathered that a UPS has 2 different ratings: for apparent power (KVA) and real power (KW). The KW rating is often less than KVA rating. Can anybody tell me why those ratings are different? Is it because the peak instantaneous power resulting from a power factor of 60% would be lower than the peak power when pf=unity?

Power Station:
Why are power stations rated in MW and not MVA? Do they assume that the load will be purely resistive, in which case the MW rating is actually a MVA rating?
 

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  • #2
UltrafastPED
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You seem to already understand the essence of the problem:

kW is the power available; kVA is reactive power.
 
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KVA are the units of apparent power, not reactive power. So the "power available" should be expressed in KVA..or shouldn't it?
http://www.justgenerators.co.uk/pages/FAQ.htm [Broken]
it says 3-phase generators have KVA rating because their loads do not usually have unity pf. Why, then, do power stations not also have KVA rating?
 
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UltrafastPED
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KVA or apparent power is the sum of 2 quantities: KW(real power) and KVAR (reactive power).

During the operation of your loads, take motor for example, the horse power that the motor can produce (and that you can feel) is the real power consumed by this motor, while this motor is also consuming another type of power which is reactive power but you don't have any return even it is consuming power.

So real power produce work, and is reflected on some action by the machine.

In power generation facilities, of course there is apparent power for the generation set, but your client (suppose you are the engineer designing the station) needs this amount of real power, because he needs work and he needs a power that he want to utilize, not just apparent power, so for this reason they mention this power generation is 300 MW or some thing else.
 
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