Power grid operators across Europe, Australia, the United States and Malaysia have spent years puzzling over a rhythmic anomaly: ultra-low-frequency oscillations that cause grid frequency to dip slightly every 60 seconds, simultaneously across networks on different continents. Now, researchers have identified the culprit — uninterruptible power supplies (UPS) and stationary batteries that verify their charge level once per minute using clocks synchronized over the internet. Each individual check is imperceptible, but when millions occur at the same instant, they produce a measurable disturbance on the grid. The findings were published in Nature Energy.
- The pattern: Grid frequency drops slightly every minute, in lockstep across separate national networks.
- The cause: UPS units and grid-scale batteries run charge checks on internet-synchronized clocks, so their checks coincide.
- The fix: Randomizing the timing of those checks would eliminate the coordinated disturbance.
What the oscillations are and why they matter
The phenomenon consists of ultra-low-frequency oscillations (ULFO): the grid frequency falls slightly every minute, at the same time in networks on different continents. The pattern was observed in data from several countries, and Germany’s largest transmission operator, Tennet, brought the problem to researcher Dirk Witthaut at the Forschungszentrum Jülich research center.
The issue matters because, although each dip is small, larger oscillations can damage equipment — particularly hydroelectric turbines — and, if operators fail to identify the cause or respond poorly, could contribute to a blackout. The Irish case illustrates the escalation: the minute-by-minute frequency drop rose from about 3 millihertz in 2016 to 12 in 2021, and some recent dips have exceeded 20, on a 50-hertz grid with a tolerance of ±200 millihertz. It is yet another pressure on systems already under strain, mirroring the transformer shortage affecting Europe.
How the researchers found the culprit
The team first ruled out the usual suspect and then installed sensors until the pattern emerged. According to the research, they began with hydroelectric power, the classic cause of such oscillations, using a model of a hydropower control system on the Scandinavian grid. That model could generate resonance, but it did not explain the steady one-minute cycle. The team then installed more than 80 smart power-quality meters across the Jülich campus, sampling every three seconds. A transformer connected to the campus battery storage system showed spikes every minute.
That was the clue. UPS systems and stationary batteries — including those in data centers and grid-scale batteries — check their state of charge every minute, and because they are connected to the internet, their clocks stay tightly synchronized, so all the checks coincide. The team also detected periodic fluctuations from Internet of Things devices, including a smart oven and Raspberry Pi clusters. It is no coincidence that the focus points to digital infrastructure: the growing electricity demand of AI data centers multiplies precisely this type of equipment.
A surprisingly simple solution
The fix is to desynchronize. Manufacturers of UPS systems and grid-scale batteries simply need to schedule those charge checks at random moments rather than all at once. IoT devices can be fixed with a software update, though many never receive one: a cited study estimates that fewer than 3% run the latest firmware. The article suggests regulators may need to intervene and that coordination will have to be global.
The broader lesson is that this problem is not caused by a fault or an attack but by success: millions of reasonably designed devices all doing the right thing — checking their battery — at exactly the same instant because they share the same internet time. It is a synchronization effect nobody planned, one that only emerges at scale, and exactly the kind of risk that becomes more likely as the grid fills with batteries, data centers and connected gadgets. The good news is that the solution is trivial and cheap. The bad news is that it requires thousands of manufacturers to agree to make their clocks a little less precise — and that, like protecting devices from voltage surges, rarely happens on its own.