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[Jonathan Johnson]

A couple of weeks ago, I had one of my computer network customers suffer a power outage. Being a construction company, they figured that they could just plug the UPS for the server into one of their construction gennys (specificallly, a generator for a light tower).

The UPS DID NOT like the power. Once the battery output died, the UPS shut down and would not power on, probably because of incorrect frequency. (After utility power was restored, everything was happy.)

This was a fairly inexpensive UPS, probably of the "line interactive" variety.

Had they an "online" UPS that was tolerant of frequency and voltage variation, it might have worked, and provided clean power to the servers -- though I probably would hesitate to recommend that as a solution.

Thankfully, none of the servers or other devices connected to the gennys suffered any obvious problems.

I recommended an inverter-type generator for backup purposes.

[Scott Holtzman]

A couple of weeks ago, I had one of my computer network customers suffer a power outage. Being a construction company, they figured that they could just plug the UPS for the server into one of their construction gennys (specificallly, a generator for a light tower).

The UPS DID NOT like the power. Once the battery output died, the UPS shut down and would not power on, probably because of incorrect frequency. (After utility power was restored, everything was happy.)

This was a fairly inexpensive UPS, probably of the "line interactive" variety.

Had they an "online" UPS that was tolerant of frequency and voltage variation, it might have worked, and provided clean power to the servers -- though I probably would hesitate to recommend that as a solution.

Thankfully, none of the servers or other devices connected to the gennys suffered any obvious problems.

I recommended an inverter-type generator for backup purposes.

If you have an old 15k gennie and weight is not a problem find an old 10kva telco 48v rectifier, grab four small lead acid batteries (set the float voltage right) and then a 7.5kva inverter like a Lorain Power Constac.  There are piles of this stuff in electronic surplus shops and it is built like a brick shithouse.  You could send that rectifier anywhere from 90v to 160v and it would not give a crap and keep the batteries charged to float the inverter.  The inverter was clean enough power for a Bell digital central office 3B Unix computer so it's good enough for your console.

[David Buckley]

Had they an "online" UPS that was tolerant of frequency and voltage variation, it might have worked...

"Might" being the operative word here.

By default, all UPS systems track the incoming mains frequency, and thus an online UPS might be just as intolerant of unstable mains as a cheapo line-interactive unit.  The problem usually isn't the absolute frequency, its the rate-of-change of frequency, casued by the generator frequency wandering around the setpoint.

However, some online UPSs can be configured to always output the nominal frequency irrespective of the input frequency. 

The reason UPSs track the input frequency is for downstream fault clearing.  UPSs have very little capability to supply power above the rated output, and thus if the UPS is overloaded, or subject to a short term transient load (eg motor starting, old school CRT starting) or short, the UPS cant do anything about it other than shutdown.  So what they do is to use the static switch to flick back to utility power to clear the fault, as the utility power is close to infinite.  One can certainly get many hundreds of amps to clear a dead short by opening a 20A breaker.

because the static switch is selecting between the street power and generated UPS power, the two power sources have to be synchronized.  So the inverter tracks the incoming mains.

[Scott Holtzman]

"Might" being the operative word here.

By default, all UPS systems track the incoming mains frequency, and thus an online UPS might be just as intolerant of unstable mains as a cheapo line-interactive unit.  The problem usually isn't the absolute frequency, its the rate-of-change of frequency, casued by the generator frequency wandering around the setpoint.

However, some online UPSs can be configured to always output the nominal frequency irrespective of the input frequency. 

The reason UPSs track the input frequency is for downstream fault clearing.  UPSs have very little capability to supply power above the rated output, and thus if the UPS is overloaded, or subject to a short term transient load (eg motor starting, old school CRT starting) or short, the UPS cant do anything about it other than shutdown.  So what they do is to use the static switch to flick back to utility power to clear the fault, as the utility power is close to infinite.  One can certainly get many hundreds of amps to clear a dead short by opening a 20A breaker.

because the static switch is selecting between the street power and generated UPS power, the two power sources have to be synchronized.  So the inverter tracks the incoming mains.

Great explanation David and all true.  That's why I brought up the rectifier/inverter solution.   The inverter is run off the -48v supply and the regulator only has to get the power under control enough to charge the batteries and cover the load.  The frequency is load invariant (I think that is the correct term).

[Geoff Doane]

The UPS DID NOT like the power. Once the battery output died, the UPS shut down and would not power on, probably because of incorrect frequency. (After utility power was restored, everything was happy.)

It might be a case of the generator not liking the UPS either.

We run into this with the mobile production truck that I look after at my day job.

The electrical loads inside the truck are separated into technical and utility loads, with the technical circuits being fed by a pair of Eaton 3 KVA double conversion UPS units.

Normally, the truck is powered from the venue's power, with a 200 ft., 2 ga. cam-lock feed.  There are two hots and a ground feeding an isolation transformer with multiple primary taps.  A new neutral is derived on the secondary to give standard single phase 120-0-120 power for the equipment in the truck.  If we can't get shore power for some reason, we can run on a 12 KVA Onan/Cummins diesel generator that is built into the truck.  The generator always puts out 120-0-120V, so it doesn't feed through the transformer or tap switches.

If we try to power only the technical equipment, the generator hunts badly, even though the two UPS units are only running at about  50% capacity.  I suspect this has something to do with a high in-rush current to the charging circuits.  When the UPS sees the input frequency (and probably voltage) drop, it disconnects from the line, and the generator goes over-speed slightly.  The fix is to make sure the generator is also feeding the utility loads (air conditioning and lighting), and then the cycling of the UPS doesn't cause the speed issues.

Even at full load in the truck, we're only drawing a bit over 30A at 240V, so the generator is nowhere near its rating, but the sharp fluctuations in current seem to cause problems.

In the case of the original problem, adding a couple KW of lighting load, or "ballast", to the generator itself might have solved the problem.  A construction company likely has some halogen work lights lying around.  It might be worth experimenting with in case of future outages.

GTD

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