Synchronized battery checks cause minute-by-minute grid dips

Grid frequency must stay within a controlled range. Large oscillations, or a mistaken response to an unexplained fluctuation, can contribute to blackouts; unchecked frequency dips can also damage power-plant equipment, particularly hydropower turbines. Researchers initially suspected hydropower, a known source of ultra-low frequency oscillations, but computer modeling did not explain why the dips recurred so consistently.

Dirk Witthaut of Germany’s Forschungszentrum Jülich and his team installed more than 80 power-quality meters around the Jülich campus. The devices sampled electrical measurements every three seconds. Readings from a transformer connected to a battery storage system showed the same once-a-minute pattern found on larger grids.

Many stationary batteries, including data-center uninterruptible power supplies (UPS) and grid-scale storage systems, check their state of charge every minute. Because internet-connected devices keep closely synchronized clocks, their checks can happen together. Each produces a tiny disturbance, but the combined effect appears on the grid.

Data from Ireland offered further evidence. Data centers accounted for 18% of the country’s electricity load by 2022, with each facility served by UPS systems. Between 2016 and 2021, the recurring frequency drop on Ireland’s grid rose from 3 milli-Hertz (mHz) to 12 mHz; some more recent dips have exceeded 20 mHz. The 50 Hz grid is designed to withstand swings of plus or minus 200 mHz, though the operator aims to keep them within 100 mHz. Researchers warn that UPS-related dips could add to disturbances from other loads.

Witthaut’s team recommends that battery manufacturers randomize the timing of charge checks rather than schedule them exactly on the minute. The researchers also detected periodic power fluctuations in Internet of Things devices, including smart ovens and Raspberry Pi clusters. They say software patches can address the issue, but a recent study found that less than 3% of IoT devices run the latest firmware. Any regulatory response would be limited by national boundaries, the researchers said, while the cause of the pattern is global.