The pattern showed up in data from Europe to Australia and from the US to Malaysia. Germany's largest transmission operator, Tennet Germany, brought the puzzle to Dirk Witthaut of Forschungszentrum Jülich. The first suspect was hydropower plants, whose vibrations can cause ultra-low frequency oscillations, but the model could not explain why the pattern repeated every minute without fail.
The researchers reasoned that something so regular needed a clock, and installed more than 80 power quality meters around their campus. The readings peaked every minute on the transformer attached to the battery storage system. The cause: many stationary batteries, from data center uninterruptible power supplies (UPS) to large grid batteries, check their state of charge once a minute, and because they are internet-connected, their clocks are almost perfectly synchronized.
According to TechCrunch, data from Ireland show the dips quadrupled from 3 millihertz in 2016 to 12 in 2021, with some recent drops above 20 millihertz. That is still well within the 200 millihertz the grid there is built to withstand, but combined with other loads it could push the grid toward blackout conditions.
The fix is simple: the researchers recommend that UPS and battery makers add randomness to the check schedule. They also found periodic fluctuations in IoT devices, from a smart oven to Raspberry Pi clusters, where the fix is a software update, but many such devices are never updated.