Generator serving UPS - problems?

[catchtwentytwo]

I haven't see these units personally but you might want to read about a similar type system by Hitec and a recent problem at a major data center. Some times I wonder if we're making things too complicated.

ZERODOWNTIME Magazine

Root Cause Analysis of Power Outage

 

[tom baker]

I have seen problems with on line UPSs (ie that take the incoming power and condition it) when run with a generator. If the generator runs out side of the freq tolerance of say 59.5 to 60.5, the UPS kicks in and corrects the frequency to 60, and then, the UPS battery runs out.
I have to go in to the UPS settings and change the frequency to 57 to 63 hz, plus a few other changes.

 

[dbuckley]

Some times I wonder if we're making things too complicated.

The most reliable power systems are the simple ones. I think the design of the Piller units is excellent, as they are really simple, and there is less to go wrong than the traditional arrangement.

Heres some things I know:

Any emergency power system that relies on paralleling generators will go wrong one day. Add bonus points if the cooling systems are powered from the output busbars, as then you can be certain that when a genset does hiccup whilst starting and thus the gensets fail to syncronise, the diesels will all shut down from overheat before you crack the syncronisation problem.

So divide and conquer; if the load is two big for one genset (over 2MW) then run multiple emergency power systems, rather than trying to parallel it into one big system.

Electrical engineers like to put breakers and isolators where they aren't needed, because thats the way its always done. Every additional element is a risk of failure. Use the absolute minimum number of things between source and load.

If there is a reasonable cable run between the UPS and the final PDUs, then put static switches right by the PDU, and dual supply the switches. Cyberex make the best (read - most reliable, and longest serving) static switches, and will build a combo static switch with PDU, which is the best approach.

If the data centre is biggish, use dual UPSs and dual distribution, and make sure the IT guys know to plug their stuff into both distributions, so if one does go the IT stuff keeps going. This is a 'Tier 4' technique, and gives the advantage you can work on power kit (and specifically the UPS) whilst keeping the centre running and protected against utility power failure.

Finally, the most important service is cooling. Power to the IT equipment is important, but keeping the cooling running is more important.

 

[dbuckley]

Years ago, I saw a backup generator feeding a big bank of UPSs get into that surge and stall cycle. It was quite unnerving to watch as the lights and AC in the computer room would dim and brighten with the generator, the UPSs meeping plaintively as they switched from full battery to generator power every few seconds.

There something you gotta know about UPSs; they're wimps. Most of them can manage something like 200% load for one cycle, so if you get a bolted fault downstream the UPS cant clear it, or rather the UPS can't supply enough fault current to clear it quickly.

Obviously the manufacturers of UPSs know this, so what do they do? If you get a large step current change the UPS basically give up, and (assuming incoming power is present) switches to static bypass, so the upstream supply (normally the utility, with it's infinite grid) can provide serious clearing current, and thus pop the downstream breaker. A few cycles later, the UPS switches the loads back to the inverter output.

For this trick to work, not only do you need upstream power, but the UPS output has to be in phase with the input power. The inverter will track the incoming waveform, but it wont track it too quickly. So if you have a droopy genset, then the rate of change of frequency will be too great for the inverter to track, and the UPS will do - something. What exactly it does depends on the design, it may go totally off-grid, or it may switch to incoming power. But it cant operate as normal.

 

[catchtwentytwo]

".......and make sure the IT guys know to plug their stuff into both distributions, so if one does go the IT stuff keeps going."

Amen to that. We also find that the IT guys have problems understanding the "twist" in Twist Lock receptacles. I've started to save the "Righty-Tighty....Lefty Loosey" notices on those Dunkin Donuts "Box o' Joe" coffee containers as handouts.

Seriously, I've had over 11 years experience with Cyberex, primarily their 1997 vintage STS/PDU's and agree they have a great product (saved my job several times). A well engineered data center needs them, not only to protect against failures, but to permit concurrent (AKA transparent ) maintenance activities.

However, take a look at LayerZero Power Systems Static Switches. The guy who invented that Cyberex Static Switch (Static Switch Method and Apparatus ) and several colleagues started their own company and have a product with a lot of improvements, including more redundancy in the power supplies, fiber optic communications, and an audio-visual prompted bypass procedure. It is an impressive product.