[AusNOG] Automatic transfer switches and generators

[James Andrewartha]

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?


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