Short Circuit Current Limitter for 480V ATS

[Paul the BIG Finn]

I have a problem with a short circuit analysis which requires me to the compute the short circuit rating of my synchronizing switchgear and distribution switchgear usi9ng both utility and generator fault contributions because the hospital I am working for uses closed transition ATSs. rbalex, who signs as "Bob, Robert B. Alexander, P.E." wrote a very informative post on that topic:

http://forums.mikeholt.com/showthread.php?t=110459

In the hospital we are working in there are ~80 ATSs distributed throughout the million square foot facility. It is only when a failure occurs in one of the four ATSs located in the Penthouse between Substation #2 and the syncronizing switchgear AND the ATS is in transition between utility and generators with fault contributions coming from both sources that the fault levels of ~85 kAIC occur (the rating of the switchgear is 65 kAIC).
The longer feeder runs to the other ATSs burn off the fault current before it can reach the ATS and return to the switchboard. Right now I am looking at ways to reduce the fault contribution from the utility to these four ATSs. I don't have any experience with current limiting reactors and the ones I found are for use with higher voltages. I need to protect 1,200A and 2,000A ATSs at 480V. Any suggestions?

 

[ron]

In the research that I've done, an air type (no solid core) will have magnetics radiating around for far distances and would need its own room with a lot of clearance with steel nearby or else liquid steel will turn up. A core type doesn't have the sever magnetic problems like the air core, but it will have voltage drop issues.
So ... in my opinion, reactors are not the way to go.
Look for a series rating with upstream devices, replace the equipment or re-run the incoming conductors to that equipment and make them longer.

 

[rcwilson]

Paul- I bought current limiting air-core reactors in 480V MCC's to drop a 50 kA fault current below the 25 kA short circuit level the client required. Westinghouse (now Eaton) was able to provide them in a standard vertical section. It was not a catalog item, but it was something they had done before and had designs to accomodate.

No problems occurred with heating under normal loading. Most MCC's were 800A or 1200 A main bus, probably operating at 30-50% loading. Current limiting low voltage reactors are availalbe.

 

[nakulak]

being that this is a hospital, would there be some shielding requirements for the use of this equipment ?

 

[steve66]

Did you assume infinate primary on the line side of the service transformer? I think this usually makes the calculation overly conserative, unless of course, you have something like a nuclear power plant in your back yard.

Steve

 

[beanland]

Air Core Reactors

Air Core Reactors

I agree with RCWilson. Air core reactors would be the logical choice. The magnetic flux escaping the coils is minimal during normal use. Not much impedance is required to drop fault currents that high.

Your available fault current at 480V seems high. A 3000kVA transformer with 5.75%Z will pass only 63kA with infinite bus source.

 

[jim dungar]

Your available fault current at 480V seems high. A 3000kVA transformer with 5.75%Z will pass only 63kA with infinite bus source.

The OP said the problem was when his generators are being operated n parallel with the utility(closed transition switching). Co-gen applications often produce very high SCA.