[AusNOG] Automatic transfer switches and generators

[Matthew Perkins]

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?
>
>