[142447] in North American Network Operators' Group
RE: Wacky Weekend: NERC to relax power grid frequency strictures
daemon@ATHENA.MIT.EDU (Alex Rubenstein)
Sun Jun 26 10:30:10 2011
From: Alex Rubenstein <alex@corp.nac.net>
To: NANOG list <nanog@nanog.org>
Date: Sun, 26 Jun 2011 10:28:28 -0400
In-Reply-To: <20110626122710.GA88850@ussenterprise.ufp.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
> I think we're missing something, which is where these ATS's are
> installed.
>=20
> 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:
>=20
>=20
> Utility Generator
> | \/ |
> | /\ |
> ATS #1a ATS #1b
> | |
> UPS #1 UPS #2
> | |
> \ /
> \ /
> ATS #2
> |
> Load
IMHO, this design is overly complex, and will lead to the most usually form=
of failure - human. There is lot to think about in the above, especially d=
uring maintenance.
You also have an interested situation on retransfer (gen -> normal) when 1a=
and 1b do not transition at precisely the same exact moment, which almost =
never happen. And if one of 1a or 1b gets 'stuck', you have UPS's parallele=
d being fed from unsynchronized sources, which leads to other problems (suc=
h as bypass not being available, etc).
> 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.
That is not typical. The normal contactor/breaker doesn't usually open unti=
l the emergency source is available. It remains closed to the dead normal u=
ntil emergency is ready to be transferred to.=20
>=20
> 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.
So are you saying that the load was directly connected to utility (no UPS p=
rotection) until utility had a problem?=20
> 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.
Closed transition should not really be thought of as syncing to the grid. C=
losed transition infers a very, very short overlap. A few cycles. Mainly so=
that downstream load does not get interrupted on the retransfer.
