Researchers Solve Mystery of Minute-by-Minute Frequency Dips on Power Grids Worldwide
Researchers in Germany found that the synchronized once-a-minute frequency dips on grids worldwide come from batteries checking their charge level every minute. The fix is a software change, but the problem could persist for years.
Researchers at Forschungszentrum Jülich, a research centre in Germany, have solved a years-old puzzle: why power grids across countries and continents see a slight frequency drop every 60 seconds, all at exactly the same moment. The pattern appears in data from Europe to Australia and from the U.S. to Malaysia. Grid operation depends on keeping frequency within a set range; if oscillations grow too large, or operators respond wrongly because they don't know the cause, blackouts can follow. Unchecked sags can also damage power plant equipment, especially hydropower turbines.
Hydropower was not the culprit
The puzzle was brought to researcher Dirk Witthaut by Tennet Germany, the country's largest transmission system operator. His team first examined hydropower, historically a main source of ultra-low frequency oscillations. Using a computer model of a hydropower plant control system on the Scandinavian grid, they found conditions producing a resonance that peaked once per minute, but this did not explain why it persisted so long.
A clock inside the batteries
The team then concluded that such regular timing implied a clock. On the Jülich campus they installed more than 80 smart power quality meters, sampling frequency, voltage, current and power every three seconds. Measurements at the transformer connected to a battery energy storage system peaked every minute. Many stationary batteries, from data center uninterruptible power supplies (UPSes) to grid-scale batteries, check their state of charge once a minute. Because the devices are internet-connected, their clocks are almost perfectly synchronized, so their combined effect becomes visible.
Ireland's grid data supported this. Data centers made up 18% of the grid's load by 2022. Between 2016 and 2021 the minute-by-minute frequency drop quadrupled, from 3 milli-Hertz (mHz) to 12 mHz, and some recent drops have exceeded 20 mHz. Ireland's 50 Hz grid is built to withstand swings of plus or minus 200 mHz, though the operator would prefer to keep them within 100 mHz.
The fix
The researchers recommend that UPS and grid-scale battery makers add randomization to the schedule rather than checking exactly on the minute. Periodic fluctuations were also detected in Internet of Things (IoT) devices, including a smart oven and Raspberry Pi clusters. The fix is again a simple software patch, but many IoT devices go unpatched: a recent study found fewer than 3% run the latest firmware. Regulators may have to demand changes, but they can only act on their own part of the grid, so global coordination will ultimately be needed.
