Wacky Weekend: NERC to relax power grid frequency strictures

Jussi Peltola pelzi at pelzi.net
Sun Jun 26 14:08:35 UTC 2011

On Sun, Jun 26, 2011 at 05:27:10AM -0700, Leo Bicknell wrote:
> 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.

It makes little sense to sync to the grid when the generator is only
used when the grid is down - and unless you run your generators 24/7
your UPS will have to make up for the comparatively long time it takes
for the generator to start, so it's rather useless to sync the generator
when the power comes back so you can avoid a sub-second break when
transferring back to utility power.

UPS's shouldn't mind a break-before-make transfer, and motor loads are
more and more often inverter/VFD driven types that should have their own
delayed start logic that will hopefully handle most power glitches
gracefully. Power distribution downstream of UPSs is a different animal
with different goals, and there synchronized UPSs and make-before-break
makes sense.

To at least pretend to be relevant, the absolute frequency of the grid
is, again, not relevant at all - and when the power is out, you won't
be able to sync to it anyway.

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