UPS Sizing

[SAkieran]

i am currently sizing a UPS system.

the supply voltage is 525V, it goes through a transformer before it goes through the online double conversion UPS.

the output voltage is 200/115VAC, i have worked out the load to be 150A and the backup standby is 2 hours.
what is confusing me is, do i size the UPS on the total output at 200/115VAC or must i calculate back to the current drawn off the system at the 525V board. if so how do i calculate back, i have read several books and i was under the impression that i size at the final load but now i think i could be wrong.

all help is appreciated.

 

[waross]

I would use the peak loading of the heaviest loaded phase at 115V, times three.

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

 

[SAkieran]

ok i was thinking that would have been the case.

well i have balanced the loads on the UPS, we were given 13kVA for the DCS and instrumentation as we are not involved in that design, my senior thinks that this is single phase but i don't think that should matter as it should be the single phase from the 115VAC so it should be a matter of balancing it correctly, would you agree with this.

so in general, you would size the UPS from the 115VAC heaviest load by 3 and not fromt the 525V board.

also this UPS provides 3 phases to 3 DBs, do i take the overall heaviest single load. i have them fairly well balanced but i was just wondering.

 

[rbulsara]

my senior thinks that this is single phase but i don't think that

.

I would suggest you discuss this with your senior and not here. You may get technically correct but wrong advice for the project from those not involved in the project.

Rafiq Bulsara

http://www.srengineersct.com

 

[SAkieran]

yeah thats a good point, was just looking for an alternative opinion, on the sizing

 

[EricAXEWC]

If you are calculating the standby time you will need to account for the DC/AC conversion efficiency from the battery bus and of course you have to ensure the DC bus can supply enough power during outages, although if the UPS is sized correctly this should already be accounted for.

 

[PHovnanian]

When sizing the UPS and the input feeders, you will have to consider a few details that the UPS manufacturer is best able to assist you with.

Consider the demand that recharging batteries will place on the source circuit in addition to the load being fed. This will depend on the charging system being used. For fast recharging following an outage, this can represent a significant additional load.

Also, check the effect of load unbalance on the UPS rating and conversion efficiency. They may be 'fairly well balanced' but it would be better to have an actual mfgs number within which to work where they will stand behind their performance numbers.