[time-nuts] One-night experiment: empirically verifying that the west coast power grid is actually interconnected

[Jeremy Elson]

There was an interesting thread on this list a few months ago ("Where do
people get the time?") that had some discussion about the history of
powerline timekeeping. I learned that the U.S. power grids monitor the
total phase error of a mains-driven clock to UTC, and will slightly slow
down or speed up the mains frequency to reduce the cumulative error when it
grows beyond a certain tolerance. This made me want to try measuring the
frequency of the mains power for myself, maybe to catch one of these time
error corrections in action.

I finally got around to doing that today: I bought a garden-variety
AC-to-AC wall wart (transformer) to bring mains power down to +/- 12V. The
transformer output was fed into my CNT-90 counter using an LPro-101
Rubidium oscillator as a frequency reference. I collected data every 3
seconds, using a 2 second gate time.

After getting some nice-looking data, I realized a nice extension would be
to compare the data from my home in Seattle to another vantage point
elsewhere on the West Coast. I convinced my good friend and occasional
time-nut Dave, who lives in Berkeley, California, to do a similar
experiment. Since Seattle and Berkeley are both part of the Western
Interconnection, the frequency data should match. He fed his mains power
into a DG1022 signal generator (which has a counter feature) and collected
30 minutes of frequency data. I wrote some software to compare them. Here
is the result (graph also attached, if that works):

https://www.circlemud.org/jelson/comparison-dave_1646804847-jer_1646764151-timeseries.png

Wow! I was not expecting the two curves to match up so well. What a
beautiful result!

The main difference between the curves seems to be a constant offset, which
is most likely explained by Dave using his counter's internal crystal
oscillator. The difference is about one part in 10^5, which is what you'd
expect from a garden-variety crystal. He's going to try using a GPSDO
reference on his counter next, and see if we can close the gap.

These curves represent rolling averages of about 30 seconds. I'd also like
to acquire high-resolution GPS timestamps of each individual zero crossing
and compute the phase offset between our power waveforms -- and try to use
that to measure the distance between us. This might not work since
transformers in the path between us can add phase delay, but I'll keep the
list updated on our results.

Overall, a really fun way to spend an evening!

-Jeremy N3UUO
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