For several years TenneT, Germany's largest transmission system operator, watched something odd: the frequency in the European power grid dips briefly — every minute, on a precise 60-second cycle. The deviations were never critical to grid security, but they were measurable and unusually regular. Nobody knew why.

A team at Forschungszentrum Jülich led by Dirk Witthaut, who is also a professor at the Institute for Theoretical Physics at the University of Cologne, has now found the cause, together with researchers from the University of Cologne and the Karlsruhe Institute of Technology. It is published in Nature Energy: large battery storage systems check their state of charge once a minute, and because they all do it at the same moment, they produce a measurable spike in demand.

Years of detective work

"It really was years of detective work," Witthaut says. The frequency pattern is shaped by many things: electricity trading, with power stations switched on and off abruptly; fluctuating feed-in from renewables; assorted disturbances. Two observations narrowed the search. First, a cycle that exact can only come from something controlled by a clock. Second, the effect can only be produced if very many devices draw power at once — a single consumer or generator is ruled out.

The clue came from measurements on the team's own campus. In the "Living Lab Energy Campus" at Jülich, a research infrastructure for highly integrated energy supply, the team installed measurement points on end devices at 80 locations. The same periodic pattern in electricity consumption appeared precisely where large battery storage systems provide uninterruptible power. "We looked more closely at one of them and found a spike in consumption at the start of every minute, caused by the power check," Witthaut says.

A standard nobody set

There are no binding standards for such power checks. Witthaut suspects most battery manufacturers assumed a 60-second interval was sensible — and simply chose the start of each minute as the test time. The result is a herd behaviour in electrical engineering: thousands of devices making the same reasonable decision, together producing a rhythm nobody planned.

The effect is not European. The researchers found the pattern worldwide. In Ireland, where data centres with large battery storage currently account for around a fifth of electricity demand, the amplitude of the one-minute signal quadrupled between 2016 and 2021.

Why it is not harmless

The periodic fluctuations already affect hydroelectric stations. The brief demand spikes slow the turbines momentarily before they spin back up to speed, which increases wear. As storage capacity grows, the problem is likely to grow with it.

The fix would be simple: distribute the checks randomly instead of always starting them at the same time. That is the catch. It requires a change in firmware, in configurations and in standards — on devices that sit in the field for years, run by operators with no reason to change anything as long as the deviation goes unnoticed. A random start time costs nothing but the realisation that it is needed.

Witthaut himself dampens the tidy answer: "It is entirely possible that battery storage systems are only part of the problem here. It cannot be ruled out that other digitally controlled appliances — air-conditioning systems, heat pumps, washing machines and dishwashers — may also exhibit similar patterns."

That is the more general point. When millions of devices align their work to the same clock, a synchronised system appears that nobody designed. Grid operators see the result in the frequency, but the cause lives in the software of devices that look unremarkable one at a time. Anyone specifying storage or flexible load this year should add one line: randomise the check interval.

Sources