Minute-by-minute battery checks are causing synchronized grid sags

Ireland’s 50 Hz grid recorded minute-by-minute frequency dips that quadrupled from 3 millihertz in 2016 to 12 millihertz in 2021, according to data reviewed by Dirk Witthaut’s team at Forschungszentrum Jülich in Germany. Some more recent dips have exceeded 20 mHz. Small as that sounds, the timing gave grid operators little comfort.

Witthaut’s team describes the current mechanism as entirely preventable. Its recommendation to UPS and grid-scale battery manufacturers is to “add some randomization to the schedule,” rather than check state of charge exactly on the minute.

What brought the grid puzzle to Jülich?

Germany’s largest transmission system operator, Tennet Germany, brought the recurring disturbance to Witthaut. Years of investigation and modeling followed before the team identified a cause. The same one-minute pattern appears in grid data from Europe, Australia, the United States and Malaysia.

Every 60 seconds, grid frequency would fall by a tiny amount on synchronized schedules. Such sags are not unheard of, but their regularity made them difficult to dismiss. Grid balance depends on keeping frequency within a prescribed operating range.

If oscillations grow too large, or operators respond without understanding them, blackouts can follow. Left unchecked, repeated excursions can also damage power-plant equipment, particularly hydropower turbines.

Why did hydropower fail to explain it?

Hydropower has historically been a main driver of what researchers call ultra-low frequency oscillations, or UFLOs. Every turbine vibrates, and vibrations matching a resonant frequency can magnify to dangerous levels. The researchers used the familiar image of a wine glass breaking under a powerful voice to explain that mechanism.

Witthaut and his team first modeled a hydropower plant control system on the Scandinavian grid. The model produced conditions capable of generating a resonance that peaked once per minute, but it did not explain the signal’s persistence.

Mechanical resonances normally come and go. This electrical signature continued minute after minute, so the investigators turned to an obvious requirement for such regularity: a shared clock.

What did the campus meters reveal?

The team installed more than 80 smart power-quality meters around the Jülich campus. The site contains medium- and low-voltage distribution lines, transformers, solar panels, battery energy storage systems serving uninterruptible power supplies, a high-performance computer system and office computers with smaller UPS units. The meters sampled frequency, voltage, current and power once every three seconds.

Measurements from the transformer attached to the campus battery energy storage system peaked every minute, matching the larger grid disturbance. Many stationary batteries, from data-center UPS systems to grid-scale installations, check their state of charge every minute.

Internet connectivity keeps their clocks in near-perfect synchronization. Individually, each disturbance is imperceptible on a wide grid; together, the aligned checks become visible.

How large is the disturbance?

Ireland provides the clearest field evidence because data centers had grown to consume a large share of the country’s electricity. By 2022, data centers represented 18% of Irish grid load, and each facility used a fleet of UPS systems.

Between 2016 and 2021, the minute-by-minute frequency drop on the Irish grid quadrupled from 3 mHz to 12 mHz. Some recent drops have passed 20 mHz.

Ireland’s 50 Hz grid can withstand swings of plus or minus 200 mHz, although the operator would prefer to limit them to 100 mHz. A 20 mHz disturbance is modest within that range. Other loads also sag the frequency, so the synchronized contribution from every UPS can push the grid toward blackout conditions when margins narrow.

Can randomized battery checks stop the sags?

Randomizing the schedule breaks the alignment that turns negligible single-battery disturbances into a recurring grid signal. The approach changes timing rather than battery hardware, which makes it a practical remedy. Software, at least, is willing to cooperate.

What risk remains after the UPS fix?

UPS systems drive the problem today, but the phenomenon can persist elsewhere. The researchers also tested internet-connected devices, including a smart oven and Raspberry Pi clusters, and detected periodic power fluctuations.

They found a simple software patch for the behavior, yet many IoT devices remain unpatched. A recent study found that less than 3% of IoT devices run the latest firmware.

Who has to act?

Regulators may soon have to require changes, although each can address only its own corner of the grid. The worldwide nature of the issue ultimately requires coordination across borders.

The mystery is solved; the remediation timetable is not. Unless manufacturers randomize battery checks and IoT suppliers improve patch adoption, the synchronized 60-second disturbance is likely to persist for years.