Researchers trace worldwide grid disturbances to synchronized battery checks

Grid operators on several continents have spent years watching their network frequency dip slightly every 60 seconds — always at the same moment, everywhere. Germany's largest transmission system operator, Tennet, eventually brought the puzzle to Dirk Witthaut, a researcher at Forschungszentrum Jülich. After years of investigative and modelling work, his team has traced the pattern — visible in grid data from Europe, Australia, the United States and Malaysia — to an unexpected source: batteries checking their own charge.

Frequency is what keeps a grid orderly, and it must stay within a narrow range. Oscillations that grow too large, or an operator's wrong response to an anomaly nobody can explain, can end in blackouts. Even when they don't, the sags can damage equipment in power plants — especially hydropower turbines, which have historically been the main driver of similar ultra-low frequency oscillations.

The Jülich team first tested the obvious suspect. A computer model of a hydropower plant control system on the Scandinavian grid could produce a resonance peaking once a minute — but not one that persisted, since mechanical resonances come and go. Anything operating with such clockwork regularity, the researchers concluded, required a clock.

The Jülich campus runs its own distribution network — transformers, solar panels, battery storage serving as uninterruptible power supplies, a high-performance computer and offices full of smaller UPS units. More than 80 power quality meters installed across the site, sampling every three seconds, found the same minute-by-minute peak at the transformer attached to the battery storage system. Many stationary batteries, from data-center UPSes to grid-scale installations, check their state of charge every minute, and because the devices are internet-connected, their clocks keep near-perfect synchronicity. Each check is imperceptible on the wider grid. All of them together are not.

Grid operators' own data confirmed the finding. In Ireland, where data centers had grown to 18 percent of the national load by 2022, the once-a-minute sag quadrupled between 2016 and 2021, from 3 milli-Hertz to 12. Some recent Irish drops have exceeded 20 milli-Hertz.