Wacky Weekend: NERC to relax power grid frequency strictures

Leo Bicknell bicknell at ufp.org
Sun Jun 26 12:27:10 UTC 2011

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

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

   Utility  Generator
      |   \/   |
      |   /\   |
   ATS #1a  ATS #1b
      |        |
   UPS #1    UPS #2
      |        |
       \      /
        \    /
        ATS #2

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

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

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 at ufp.org - CCIE 3440
        PGP keys at http://www.ufp.org/~bicknell/
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