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KVA vs KW again!!! 5

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raithrovers1

raithrovers1

Industrial
Feb 10, 2009
85
Here we go again with another KVA vs KW problem!
As a UPS engineer, I was always told that the output rating of most UPS systems is given at 0.8pf. I obviously would always check the specifications but in general it is between 0.8 an 0.9 for the industrial systems that I work on.
When testing these systems with a purely resistive load bank I was taught way back in the day to only load test at 80% of the KVA rating to allow for the 0.8pf. I blindly followed what I was taught.
Now, as I get older (too quickly!), I question almost everything I do!
I have a 15KVA UPS (rated at 0.9pf), 115Vac out the specification says that the rated output current is 130AAC. If the rated output current is 130AAC then I would think that I should test this system at 130AAC for a full 100% load test. This is obviously 15KVA at approximately 1.0pf which is 15KW.
According to the way that I have been taught, I should test this at 117AAC (15KVA x 0.9pf / 115V).
Have I been taught incorrectly?

UPS engineer
 
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Why are you multiplying kVA by the power factor to get current? As far as I was aware, at 115V, a 15kVA source outputs 130A for any power factor...
 
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You need a reactive load, or maybe a rectifier and cap bank with a resistive load.

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As I said in my thread. I was instructed/ taught do this by my previous company. This was there procedure. If you had a 20KVA system it was load tested at 16KW. 15KVA tested at 12KW. And so on. I didn't say it was the correct thing to do!

UPS Service Manager
 
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In that case be careful of overloading the dc link components. The inverter will be fine but some dc links seem to be built with very little margin these days. What's the difficulty with a reactive loadbank? We were using these years ago.


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Scotty

It is always down to cost. With a purely resistive load bank you can test AC and DC systems easily. The people I have worked for think this is the best way to go and don't want to pay for additional equipment if at all possible!
I have never used a reactive loadbank. Can you switch in and out the reactive components like a resisitive load bank and therefore use it for DC also? This may help my case when asking people to procure one!

Thanks

UPS Service Manager
 
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Scotty

DC link is rated at nominal 220VDC 80A with the inverter efficiency at 85% at full load so we should be OK. Obviously actual float voltage on the link takes the power available over the required level (245VDC x 80A =19.7KW x 0.85 efficiency = 16.7KW).

UPS Service Manager
 
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Now that I think about it I wonder if the person who taught me got the reason mixed up for why they test it at 80%. Maybe it is as Scotty said down to the rating of the DC links in some older systems. If the DC link was rated for 15KW and the output of the Inverter 15KW then if the inverter had an effeciency of 80% we would need to de-rate the output power by 80%.
Kinda makes sense now!
Once again, probably a case of mis-information passed down through generations!!!!!!!

UPS Service Manager
 
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This is an old mistake in UPS power specifications. Output power of UPS are done in kVA because load are often reactive (pf<1). pf specified in UPS datasheet is minim accepted pf of load by equipment, not maxim active power; for above example, UPS have 15kVA nominal output power and is specified pf=0.8 (but nowhere and no one of manufacturers specify that pf is minim allowed), so maxim reactive power allowed is Q=(1-0.8^2)^0.5 x 15 = 9kVAr. You can put load at 15kW (pf=1; Q=0), it's nominal output power of UPS with pf=1 (allowed, it's above 0.8).
 
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I understand the power triangle and all of the math involved and the relationship of power factor, kva and kw. What I need clarified is comments like in this paper:
Here is an excerpt:
"we have a UPS rated for 10,000 VA, with an output power factor of 0.8..... Now we can calculate the total available wattage the UPS can provide, 10,000 VA*0.8=8,000 Watts."
This is where the confusion arises. I realise that if the full load had a power factor of 0.8 then the UPS would supply 8KW real power.
I would just like someone to explain why the statement says "the total available wattage the UPS can provide is 8000W". Which is similar to why I was taught to only load a UPS with 80% purely resistive load. What is limiting the Inverter to supply 80%? Do the UPS manufacturer's design the power tolerances of the IGBT's, inverter transformers and wiring to to the actual KW power ratings and therefore save money? Is it as simple as that?
Or is it due to the DC link tolerances that Scotty mentioned?
Or maybe a combination of the two?

UPS Service Manager
 
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I think you answered your question.
When it comes to capacity, watt-hourwise, there is a huge difference between a PF 1 load and a PF .8 load. There, it really matters.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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RaithRovers,

The active power determines the sizing of the dc link components and the battery. The inverter has to handle both active and reactive power. The manufacturers frequently assume a 0.8pf, which implicitly means that the dc link is rated at 80% of the inverter power rating. The key thing to remember is that reactive power is an AC-only quantity and it can't be 'seen' by the dc link.

Regarding test loads, ours was switched and could produce leading or lagging load. Main drawback was the weight - it was a big trailer mounted unit. Modern power electronics could achieve the same in a fraction of the volume and weight, and probably cheaper now that copper is so expensive.


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I'd agree with Scotty. The kW would be limited by the components from the AC power to the DC bus in the inverter. The components between the DC bus and the output set the maximum kVA. I suppose the manufacturers of larger units don't just rate the unit with both kW and KVA similar to smaller units just to make the unit appear better. Most small 1kW and down seem to have both the kW and kVA ratings given in their data.
 
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Nice write up... it was like Electro Magnetics course all over again! :p except of course not "purely academic" applications.
 
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