Tale of Two Power Systems – UPS Edition

This one nearly fooled us; we recalled the “two power systems” nature of a recent site and so when a second data set came in with somewhat similar characteristics, we thought it might be more data from the same facility. But this is a completely different site, and a completely different problem!

Looking over a power monitoring data set recently; we came across a site with a dual personality. The site in question had a marginally high total harmonic distortion (THD ~ 5.5%) from 5/15/17 up until 5/24/17 (specifically, at 3:50 pm). After that, the THD trended much higher, rising as high as 12% (with a large amount of visible high frequency noise).

A “before / after” look at the voltage and current waveforms provides more evidence, with visible notching on the voltage waveform “before”; and very high levels of high frequency noise (broad spectrum “after”.

Before 5/24/17 After 5/24/17

Individual harmonics similarly supported the findings of the THD log, with harmonics under 3% before 5/24/17, and very high harmonics across the spectrum after 5/24/17.

Before 5/24/17 After 5/24/17


Finally, the “before / after” affect was also seen in the Neutral-Ground voltage – with severely high NG voltages after 5/24/17; consisting mainly of high frequency components.

Before 5/24/17 After 5/24/17

The clue to understand this puzzle is that the RMS voltage of the device under test was very stable and well regulated (probably a UPS or power conditioner output) before the 5/24/17 date, and higher / less well regulated after the 5/24/17 date.

And here is the “moment of truth” when the voltage changes from moderately distorted / notched to severely distorted with high frequency noise.

Moment of Truth

So what’s the scoop? We’re not on site, but here’s our bet – that there is a UPS supplying 480Y/277 VAC power to the load, but is itself being fed 480 VAC Delta (ungrounded and/or no neutral). During Inverter operation, the UPS works fairly well (although we bet the notching and higher THD are not normal for this device). But when switched to Bypass, the load loses the neutral reference, and is picking up noise from the UPS rectifier and/or inverter circuitry.

The trip report notes “No power problem suspected, multiple tube failures, want to eliminate power as an issue.” They probably assume UPS installed = no power issues (and they are probably not experiencing sags / swells / etc. on other facility loads). But if they are operating a system requiring 480Y/277 VAC from 480 VAC Delta, and relying on the UPS to provide a neutral connection point, they are probably having some serious grounding, reference, and noise issues!