UPS (Uninterruptible Power Supply) Failing

Q: I have a situation where our UPS servicing data center rack systems are failing repetitively after replacements.

We know our mains transformer is undersized in terms of load capacity and under voltages are measured going to the input of the UPS that is providing back up power to data center in a rack mount. However these voltages measured were 110VAC and still within operating range. In another facility with the same rack systems, colleagues are saying they're seeing "brown outs". I don't know to what voltage level they are dropping down to be called a brown out.

The questions are the following:

  1. Can brown outs cause power supplies to fail? If so, how?
  2. What other anomalies can cause failures?
  3. What is the best solution to protect our UPS?

A: Check the type of UPS (ferro-resonant or dual conversion) and the "limits" under which it will trigger operation. Most UPS units act to mitigate some level of harmonic content on the bus - so having a "dirty" line will require the UPS to be online more frequently and for longer periods, leading to battery and/or component failure. Likewise, voltage and/or frequency issues will also bring the UPS online. Some UPS units recharge very slowly (trickle charger approach); in this case, excessive operation will eventually drain - and ultimately deteriorate - the battery. Others recharge more quickly - but still slower than they drain.

If the "normal" condition (caused by the undersize transformer?) is to be near the low end of the acceptable voltage, an otherwise insignificant disturbance (including harmonic content) will initiate cycling of the UPS.

And put some surge suppression upstream of the UPS units to alleviate potential damage from line surges (lightning strikes).

An 110V unloaded voltage is too low. This could be due to other upstream loads or it may be because your utility is supplying an undesirable low voltage at your connection point. In either case the first thing to do is to adjust the internal taps upward, closer to 120V nominal or above say 125V. This will give you much greater margin and nearly all dry type and most large oil immersed transformers will have these taps.

My best guess for the failing UPS units is likely due to cycling. As the voltage drops below the acceptable range when loads are added the UPS will cycle on and raise the voltage or carry the entire downstream load, depending on the type of UPS. If these units cycle repeatedly 10s or 100s of times a day then it would explain the repeated failures.

Finally I wouldn't worry so much about the overloaded transformer. Transformers generally overload very gracefully.

If you can take a measurement of the transformer temperature, either on the core steel (best) or on the external winding, this will give you a better idea of the margin remaining. If the temp is unreasonably high say 25% greater than the insulation class rating I would add a fan to the unit to provide some force cooling until a scheduled replacement can be made. In reality you could operate for months like this and no real damage would be done.

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