KVA vs KW again!!!
KVA vs KW again!!!
(OP)
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?
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 http://www.calibrepower.co.uk





RE: KVA vs KW again!!!
RE: KVA vs KW again!!!
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If we learn from our mistakes I'm getting a great education!
RE: KVA vs KW again!!!
UPS Service Manager http://www.nolanpower.com
RE: KVA vs KW again!!!
In the ideal world that would be great. But we only have resistive load banks and this is not going to change soon.
UPS Service Manager http://www.nolanpower.com
RE: KVA vs KW again!!!
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If we learn from our mistakes I'm getting a great education!
RE: KVA vs KW again!!!
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 http://www.nolanpower.com
RE: KVA vs KW again!!!
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 http://www.nolanpower.com
RE: KVA vs KW again!!!
Kinda makes sense now!
Once again, probably a case of mis-information passed down through generations!!!!!!!
UPS Service Manager http://www.nolanpower.com
RE: KVA vs KW again!!!
RE: KVA vs KW again!!!
www.rittal-corp.com/literature/download.cfm?id=307&sbu=05
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 http://www.nolanpower.com
RE: KVA vs KW again!!!
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
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: KVA vs KW again!!!
RE: KVA vs KW again!!!
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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If we learn from our mistakes I'm getting a great education!
RE: KVA vs KW again!!!
RE: KVA vs KW again!!!
UPS Service Manager http://www.nolanpower.com
RE: KVA vs KW again!!!