[AusNOG] Automatic transfer switches and generators

Matthew Perkins matt at spectrum.com.au
Fri Feb 26 16:05:59 EST 2021


That's called a Static bypass and most commercial UPS setup's 
incorporate one. These days its the only way you can legally do 
maintenance inside the UPS without dropping the load. It's also another 
reason why the UPS inverter stays insync with the mains. So the static 
bypass can be switched without a big bang.  Most Static Bypass systems 
also incorporate an interlock that stops you switching to bypass unless 
the inverter is in sync with the load or the inverter is shut down and 
internal bypass is activated.

It's also sometimes refereed to as an external bypass. Unlike an ATS 
they are not usually Automatic and they do not switch the neutral and 
earth which remain in parallel only the Active phases are switched.

Matt


On 26/2/21 2:30 pm, John Edwards wrote:
> 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.
>
> John
>
>
>
>
> On Fri, 26 Feb 2021 at 13:33, Matthew Perkins <matt at spectrum.com.au 
> <mailto: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 <mailto:AusNOG at lists.ausnog.net>
>     http://lists.ausnog.net/mailman/listinfo/ausnog
>     <http://lists.ausnog.net/mailman/listinfo/ausnog>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ausnog.net/pipermail/ausnog/attachments/20210226/3f722d56/attachment.html>


More information about the AusNOG mailing list