Curious Voltage Regulation

Came across an unusual case of voltage regulation for a CT Scanner site recently.

Site voltage appears to be well (and artificially) regulated, with no daily voltage fluctuations typical of utility power but with small load related voltage drops. Site had no significant outages, sags, swells, or transients over two weeks of monitoring.
Typical voltage harmonics supports some sort of static, inverter based power source, with broad band harmonics. Elevated 5th, 7th, 11th and 13th harmonics are curious.
No apparent voltage regulation related to step change in load current – which is unusual for a static UPS or power conditioning device feeding a medical imaging load.
Moderate level of increased voltage distortion related to nonlinear load current – again, not typical for a medical imaging device powered directly from a properly sized UPS or power conditioner.

So, what’s going on?

My first guess was a slow, electro-mechanical voltage regulator (such as a motor driven Variac or Powerstat) and which shows up occasionally for medical imaging or other sensitive loads. However, that sort of regulator would not eliminate short utility sags or drop-outs, or affect the voltage harmonics or THD.

My next theory is that this site has a large UPS or static power conditioning device feeding an entire medical imaging department. The voltage drop and increased voltage distortion under load would be the result of voltage drops in distribution (impedance calculated to be roughly 60 mohms @ 480 VAC) – so perhaps the power conditioning device is located in the basement with normal distribution voltage drops (primarily wiring, perhaps a transformer) from there to the equipment.

No significant power issues found, just like to understand what I am seeing in the data!