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Re: UPS and generator interaction?

  • From: David Lesher
  • Date: Tue Mar 30 10:31:20 2004


Oh, another detail.

Some 98% of the UPSi around are "standby" units. They sit and
trickle their batteries until the line fails, then quickly kick in.
They take 'n' hours to recharge when the line returns.

But there exist another genus. These 'full time' units ALWAYS
run the load from the UPS inverter; and have a big AC line->DC
"battery charger" -- big enough so as to keep up.

The advantage is a very high degree of line isolation. Any surge,
sag, glitch, spike may affect the AC->DC side of the equation,
but will have to get past the battery plant and inverter for the
load to see it. 

Note it does not even care what the input frequency is. I know
of one large unit that was sent to Mexico City. At the time, it
was 50Hz, but there was some announced plan for the city to go
to 60, Real Soon Now. The UPS "battery charger" ran on anything
between say 40-70 Hz, but the inverter made 60.0 Hz, period.

Such units are not common or cheap. In the low end, Sola Corp
used to make some in the low (1-3?) KVA range. Top end, how many
KVA do you need? I think the one going to Mexico City was 500KVA.

If your power is really rotten [Here, Guyana comes to mind...]
you may want to spend more up front.

Side thought, but not a NANOG topic. What in your data center
really cares if your generator puts out 57 or 63 Hz, not 60.0?
Why?




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