Frequency: Island vs. Mainland

We were looking at some power monitoring data this week for an MRI (Magnetic Resonance Imaging) site located in Puerto Rico. The site power was generally pretty good (in terms of things like voltage regulation, harmonics, sags and swells, etc.) but we were struck by the difference in frequency regulation at this site as compared to a mainland site.

We look at a lot of mainland US power data sets (~300 per year) and frequency is almost never anything but rock stable. Frequency fluctuation is often a good indication of an alternate power source (such as a free-running UPS or back-up generator)

Island Frequency

Frequency at an MRI site in Puerto Rico (one week monitoring, 480 VAC, with good voltage regulation)

Mainland Frequency

Frequency at an MRI site in New Jersey (one week monitoring, 480 VAC, with good voltage regulation)

The frequency was technically “acceptable” at both sites – most equipment is spec’d for 60 Hz +/-1% (59.4-60.6 Hz) or +/- 1 Hz (59 – 61 Hz) and some is spec’d much wider. Switch mode power supplies might work fine from 47 – 63 Hz, for instance.

However, some older equipment does use the line frequency for timing or for regulation, might control output or dose using phase-controlled regulators, or might have some protection or control circuits that trigger off the incoming AC waveform – and these might act up a bit if the frequency varies. So frequency is a good thing to keep an eye on when working with equipment installed on an island or other smaller power grid.