[AusNOG] Automatic transfer switches and generators

John Edwards jaedwards at gmail.com
Fri Feb 26 14:30:42 EST 2021

For a truly paranoid setup, you can put an additional ATS *after* the UPS.

This potentially lets you replace a failed or failing UPS without downtime.

One time, this saved me because the UPS took a catastrophic surge to its
input side, but the inverter and batteries kept powering the network.

We were able to switch over to mains without downtime and install a new UPS.


On Fri, 26 Feb 2021 at 13:33, Matthew Perkins <matt at spectrum.com.au> wrote:

> A generator ATS and UPS setup are not trivial configurations. The whole
> system needs to be engineered by a qualified engineer with experience in
> those sorts of setups.  Both for Australian Wiring rules and also to
> ensure all your earth/neutral bonds are at the same potential.  If you
> end up with a floating neutral loosing power will be the least of your
> worries. Replacing all the PSU's in all your equipment will be.
> Typically in a Generator / ATS / UPS setup the UPS is of the double
> conversion type. The UPS output is always in sync with the mains and the
> output of the inverter is always running the load. This is to cover the
> situation where the inverter fails and the UPS goes into bypass.
> If the mains fail the UPS Sends a signal to the ATS that sends a call
> for the generator to start. The Controller in the ATS then executes a
> per-progamed set of make/break commands
> With respect of output of ATS
> The Mains active is broken
> The Generator Neutral is made
> The Mains Neutral is broken
> The Generator Active is made
> In that sequence you will note the mains and generator neutrals overlap
> this is extremely important if you dont want to let the smoke out.
> The UPS  then detects input from the generator and that the inverter is
> unsynchronized  It then starts to slowly adjust the phase to align the
> inverter output within 1% of the generator and and at that point the
> whole thing is shoved back into line.
> When mains is restore the process above is reversed.
> Key things are Earth bonding of generator and neutral overlap is of
> extreme importance a floating neutral even on single phase UPS systems
> can see potential differences of many hundreds of volts to earth.
> take care
> Matt.
> On 26/2/21 1:41 pm, James Andrewartha wrote:
> > Has anyone fed a generator into an automatic transfer switch? We got a
> > generator recently, which is hooked up to some circuits in our server
> > room, that can be manually switched between mains and a generator. We
> also
> > have an existing UPS that most of the room runs on, and has a runtime of
> > about an hour at the moment. I want to have the ATS fed by the UPS and
> the
> > mains/generator circuit, so they will be able to use the UPS until we get
> > the generator running.
> >
> > I ask because I was reading APC's documentation which has these notes:
> >
> >> Do not plug an ATS into a generator without the protection of a Double
> >> Conversion On-Line Uninterruptible Power Supply (UPS).
> >> **Using the Rack ATS with a Generator**
> >> APC recommends placing a UPS between the generator and the ATS input.
> >> The UPS will condition the input voltage to the ATS to prevent
> >> thrashing. A second UPS should be used between the power utility and the
> >> other input of the ATS. This UPS will power the load while the generator
> >> starts and stabilizes. The UPS should be sized to allow time for the
> >> generator to be started and the time needed for the UPS attached to the
> >> generator to operate normally. While the generator is starting, the
> >> generator voltage and frequency are not stable. If the ATS is connected
> >> directly to the generator, the ATS can switch to the generator input
> >> before the generator has stabilized. The load of the ATS will cause the
> >> generator voltage to dip. This dip can also shift the frequency of the
> >> voltage. Either of these issues can cause the ATS to switch to the other
> >> input.
> > Whereas Eaton says:
> >> However, the STS module can also be supplied by one UPS and another type
> >> of source, or by two non-UPS sources providing a sinusoidal output (AC
> >> system, engine generator set, etc.).
> > I'm thinking the scenario listed by APC is less likely in my situation
> > since the switch to generator power will be manual, only after we start
> it
> > up, and that even if the ATS does switch back to the UPS then the other
> > loads (dual-input ones) will remain on the generator, so the ATS load
> > won't be as significant the second time it tries to switch back.
> Thoughts?
> >
> >
> _______________________________________________
> AusNOG mailing list
> AusNOG at lists.ausnog.net
> http://lists.ausnog.net/mailman/listinfo/ausnog
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ausnog.net/pipermail/ausnog/attachments/20210226/91d0783a/attachment.html>

More information about the AusNOG mailing list