In a message written on Sat, Jun 25, 2011 at 03:32:09PM -0700, Seth Mattinen wrote:
Most of these come in open, delayed, or closed transition models: http://www.gedigitalenergy.com/powerquality/ATSHome.htm
I think we're missing something, which is where these ATS's are installed. I don't think most utilities allow (largeish) ATS's to do a closed transition from a genset to the utility grid, but I may be wrong. There may be other ATS's in your facility that do a closed transition though. For instance, consider this (somewhat simplified) dual UPS design: Utility Generator | \/ | | /\ | ATS #1a ATS #1b | | UPS #1 UPS #2 | | \ / \ / ATS #2 | Load ATS's 1a, 1b, sense utility power for quality. Should the utility power quality not meet specs (e.g. go out), they disconnect from utility, tell the generator to spin up, wait 5-15 seconds for the generator(s) to spin up and then close to the generator. They are in an open state for perhaps 20+ seconds, generators are never closed to the utility. Going back the drop may be shorter, perhaps 10 seconds, but there's still a long-ish open gap. Definately not sub-second. ATS #2 takes the dual UPS output (from synchronized UPS's) and does a closed transition between the two sources. Indeed, a previous employer had ATS's at this location that could switch between sources in less than 1/4 wave, the equipment never knew the differenece. Very impressive. It's not that you couldn't install a closed transition ATS in the ATS 1a/1b location from an electrical point of view, but I don't think codes, power companies, or common sense make it a good idea. As others have pointed out, the grid can do weird things because your neighbors did something stupid, or a car hit a power pole and shorted 3 phases together. Syncing to it is, well, crazy. Maybe small plants are different, but I've never seen a 1MW+ plant where the generators synced to utility. I can imagine CoGen might have some different requirements, and I've never worked with that, but I don't think that's what we are discussing. -- Leo Bicknell - bicknell@ufp.org - CCIE 3440 PGP keys at http://www.ufp.org/~bicknell/