EDSA Arcflash, Fault Calculations and the UPS

[Snowjob]

I am using EDSA to do some calculations on the Arcflash and Faults on some downstream critical panels below the UPS.

I was getting some funny results. When I looked closer I realized that the 100 KVA UPS was blocking EDSA from seeing the protective devices on the upstream Main panels.

Then I got to thinking , just how does a 100 KVA UPS behave during downstream fault conditions? Does the UPS' sense the fault and its internal breakers open to protect the UPS and pass the fault up the Staic Bypass?

If so that would certainly change my EDSA model! Or does the fault on the lower panels actually pass up the output side of the UPS and through its inetrnals.

That would sure change my EDSA model!

 

[ron]

I model UPS's in static bypass mode for short circuits.
A UPS generally will put out approx 4-6 times FLA then transfer to bypass to clear a fault.

 

[__dan]

UPS rated for fault clearing

UPS rated for fault clearing

Look at the operating manual. The UPS has a rating for fault clearing. For a Liebert 600 kw (750 kva) module the factory manual says it will deliver 200% of nameplate current for 1 second into a fault to clear the fault (not in bypass).

Semiconductor driven output devices are generally current limiting, they can be designed to be naturally current limiting. The UPS has local input and output breakers and the UPS's IGBT output semiconductors can blow like fuses (they will blow open, catastrophically for the IGBT), if for example they close into an unsynchronized bus.

If it transfers to bypass, the current is probably flowing through SCR device static switches. The amount of current the SCR's can handle in a fault is not a huge number. I don't know the range but there's a physical limitation of available charge carriers in the semiconducting material. There's also OCPD breakers on the static switches.