Europe’s power grid runs at 50 Hz, and that figure tells grid operators, second by second, whether supply and demand are in balance. The large conventional generators that keep the grid stable spin in step, and that shared rhythm makes every jump in demand visible across the network. When demand rises suddenly, the rotating mass at the heart of these machines gives up a little energy and slows down, pulling the frequency with it until control systems step in and bring it back. Small wobbles are normal, but they are usually messy and random.
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When TenneT Germany, the country’s largest transmission system operator, noticed a dip that came back every 60 seconds, regular as clockwork, it had a puzzle on its hands. The drops posed no threat to security of supply, but they were clearly measurable and had been showing up for years. A team led by Dirk Witthaut at Forschungszentrum Jülich, working with researchers from the University of Cologne and the Karlsruhe Institute of Technology (KIT), has now tracked down the cause and published the results in Nature Energy.
A suspect with a watch
Grid frequency carries the fingerprints of almost everything connected to the network. Power trading leaves sharp steps as plants switch on and off with each trading period, wind and solar add fluctuations of their own, and countless smaller disturbances pile on top. “That really was years of detective work,” says Witthaut, a professor at the University of Cologne’s Institute for Theoretical Physics.
The exact 60-second rhythm narrowed the search quickly. “It was soon clear to us that the cause had to be something controlled by the clock,” comments Witthaut. Nor could a single power plant or factory be behind it. “The effect we observed can only be produced if many devices draw power at the same moment.”
Caught on campus
The team found its breakthrough at the Living Lab Energy Campus in Jülich, a research facility for testing highly integrated energy systems, where it fitted 80 measuring points to individual devices. The same minute-long pattern turned up exactly where large batteries provide uninterruptible power supply (UPS), the backup that keeps servers and sensitive equipment running through a power cut. “We took a closer look at one of them and found a spike in current at the start of every minute, caused by the power query,” Witthaut says.
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Each UPS checks its battery once a minute, and the check itself draws a brief burst of power. On its own, a single check is negligible, but multiplied across many units whose clocks tick over to the new minute together, it becomes a short, sharp jump in demand, which the whole synchronous grid registers as a dip in frequency. The researchers also found the same signal on grids outside Europe, making it a global phenomenon.
An accidental standard
There is no binding standard that decrees how often or when these kinds of checks should run. Witthaut suspects most manufacturers simply decided that a 60-second interval seemed sensible, and simply timed the check to the start of each minute. He also thinks batteries may not be the only culprits. “It can’t be ruled out that other digitally controlled devices, such as air conditioners, heat pumps, washing machines or dishwashers, show similar patterns,” he says.
Small dips, real wear
Each brief jump in demand slows hydropower turbines slightly before they speed up again, adding to mechanical wear, and the effect is likely to grow as more storage comes online. In Ireland, for example, where data centres with large battery backup account for around a fifth of national electricity demand, the amplitude of the one-minute signal quadrupled between 2016 and 2021.
But as it turns out, a fix for the problem would cost next to nothing and be quick to put in place. “You would simply have to spread the queries out at random, so they no longer all start at the same moment,” says Witthaut. Because no rule sets the timing today, a small change in manufacturers’ firmware could quietly remove a signal that has run through the grid for years. (TF)
The study, “One-minute frequency disturbances from unintended synchronization in cyber–physical power systems”, is published in Nature Energy.
