Sizing transformer for UPS

[mull982]

I'm trying to determine the size of a 480V isolation transformer to feed an existing 250/500kW rated UPS. The isolation transformer is required because the existing 480V distribution system is being converted to an HRG system.

The information I have on the UPS show it as a 250/500kW rated UPS however it appears that it is supporting far less load than its rating based on the existing UPS feeder circuit. The existing feeder circuit consists of a 100A fused feeder and a 1/0 feeder cable. Based upon this feeder circuit this tells me that the UPS is limited to supporting just under approximately 100kW. The customer had also previously commented that he believes it supports 100kW worth of load.

In this situation would you size the isolation transformer for the 250kW rating of the UPS or just based upon the existing connected UPS load of 100kW? Would there be an issue of providing a 250kW rated transformer on the existing 100A feeder circuit in the event customer wanted to increase load in future and upgrade feeder circuit?

One thing about the UPS that has me a bit confused is that the bypass switch appears to have an overall 500kW rating but shows only a 250kW rating for the static switch. Is that common for the bypass switch to have an overall available frame rating with a lesser rated installed static switch (similar to breaker frame and trip unit as a comparison)? UPS is an APC UPS.

 

[brian john]

So a 250/500 can be sized to achieve 500kw at some future date. What is the OCP and conductor size?

What is the projected future growth?
Is this a dual source UPS?

 

[mull982]

So a 250/500 can be sized to achieve 500kw at some future date. What is the OCP and conductor size?

What is the projected future growth?
Is this a dual source UPS?

Currently the Upstream OCP is 100A with a 1/0 feeder cable.

Not exactly sure what projected future growth is, perhaps something I need to look into.

Not technically a dual source UPS but it does have to source feeders from upstream MCC’s that feed an ATS which in turn feeds the UPS. UPS input and Bypass input are jumpers internal to the UPS.

My initial thought was that the isolation transformer creates a single point of failure?

 

[Ingenieur]

a single line would help, kind of hard to follow what you describe
ups, ats, mcc, etc
why does a hrg necessitate an iso xfmr

as far as sizing future load is critical

what does this mean?

Not technically a dual source UPS but it does have to source feeders from upstream MCC’s that feed an ATS which in turn feeds the UPS

the ups supplies feeders from an upstream mcc?
and they supply an ats?
and the ats feeds the ups?

 

[mike_kilroy]

Currently the Upstream OCP is 100A with a 1/0 feeder cable.

Not exactly sure what projected future growth is, perhaps something I need to look into.

Not technically a dual source UPS but it does have to source feeders from upstream MCC’s that feed an ATS which in turn feeds the UPS. UPS input and Bypass input are jumpers internal to the UPS.

My initial thought was that the isolation transformer creates a single point of failure?

I buy oversized UPSs. That way, when it is speced to 30 min on outage it will run longer.

If POWER Feed SHOWS 100kw, that is the xfmr size I would pick. If UPS Is typical near 1.0 PF, 100kva xfmr is right size.

Sent from my SM-G900V using Tapatalk

 

[mull982]

I tried to attach One-Line but file is too large so I'll try to explain.

Two upstream 480V MCC's each have a breaker which feed an ATS. The output of this ATS currently feeds the UPS with the UPS input and bypass input jumpered inside the UPS. Each of the upstream MCC's has a 100A feeder breaker and 1/0 cable which feeds the ATS. Because the existing 480V system is being converted to an HRG system we are proposing to put an isolation transformer between the existing ATS and UPS.

I guess it really may come down to what type of future load growth is needed. If we only provide a transformer to match the current loading and circuit ampacity of 100kW then if they ever want to add load in the future they would need to replace the UPS.

I'm thinking the best option may be to provide a larger UPS perhaps 225kVA and install on the existing circuit with the understanding that they would have to upgrade the feeder circuit for any required future load. Is there any issue with providing a transformer on a circuit that has a smaller ampacity (225kVA unit on 100A circuit)? The only thing that comes to mind is perhaps issues with the breaker tripping on energization/inrush.

 

[brian john]

a single line would help, kind of hard to follow what you describe
ups, ats, mcc, etc
why does a hrg necessitate an iso xfmr

as far as sizing future load is critical

what does this mean?

Not technically a dual source UPS but it does have to source feeders from upstream MCC’s that feed an ATS which in turn feeds the UPS

the ups supplies feeders from an upstream mcc?
and they supply an ats?
and the ats feeds the ups?

The UPS logic needs a solidly grounded system for proper operation. We had a system with an HRG and when the static switch would attempt to operate to bypass the load the system would dump. The installation of a transformer will give them that solidly grounded system.

 

[Ingenieur]

you might find this interesting
http://www.rpm-eng.com/publications/2009pubjun.pdf