I am wondering how everyone else is handling modeling and calculations for closed transition ATS switches. For the scenario of re-transiting from generator power back to utility power, I am presently modeling the ATS as a bus with both the utility and generator on at the same time. I believe that this is an accurate way to model this using ESA.

My main question is dealing with equipment duty of the rest of the system during this re-transition. Is this something that I should be concerned with or because this transition happens so quickly it is something that I shouldn't be concerned with? Any help is appreciated.

Thanks
Mike

Excellent question, and one that will spawn a good debate I am sure. Mr conservative Zog is going to shock everyone by saying I think it can be ignored for arc flash due to the brief period of time for the transition (Still need to consider AIC though).

I would consider both sources in parallel when evaluating the withstand rating. I have modeled it that way in the past for data centers and similar facilities. Although the transition / overlap is very brief, it could be quite bad if Murphy (and his law) entered the picture.

So I want to make sure that I am on the same page with both. I should be looking at the unit substations (Upstream) and the down stream equipment (panel, transformer, motors, etc..) for equipment duty during this re-transition period.

Thanks for the help

MJ

Why AIC? The ATS doesn't interrupt fault current. Neither single source OCPD will see the contribution from both sources if the ATS faulted during the transition. If you had a fault downstream of the ATS at the instant of closed transition, you would would have fault current both sources, but what is the probability of a fault occurring during the transition? There's no reason to believe that a downstream fault would not be an independent event from the switching, so the probability would be infinitesimal.

This is a very good point - I know some facilities that look at this similarly, examining whether the risks are related or not. As an example, for a main tie main 480V Switchgear fed with 2500 kVA transformers, with the tie open each bus has approx 40 kA available from the transformer and motor loads combined. When the tie is closed (for 4 seconds in this particular scheme) there is 80 kA available on the Switchgear bus. There are several feeder breakers in the Switchgear that feed MCCs through about 5 feet of hard bus, so the MCC has about the same short circuit availability as the switchgear. The Switchgear is rated 100 kA, so it is fully rated for the short 4 second closed transition period. It was judged that the likelihood of an incident occurring during the 4 second closed transition period was sufficiently high to warrant Switchgear fully rated for that situation. The MCCs that see the same fault current are rated 65 kA because it was judged that the likelihood of an incident in the MCC at the same time that the switchgear was in a closed transition state was extremely remote. The switchgear typically undergoes closed transition switching a handful of times per year.

The approach above seems to be a little more conservative than most other folks in the petrochemical industry that I talk to. Most of them would still buy 65 kA gear in the situation described above, and most of them have the same 4 second closed transition period that we do.