For years, mysterious electrical anomalies have been flummoxing grid managers around the world. Every 60 seconds, the frequency on the grid would drop a tiny bit. 

Even weirder: The sags happened at the exact same time on power grids across countries and continents.

It was a mystery that Dirk Witthaut, a researcher at Forschungszentrum Jülich in Germany, couldn’t pass up.

Germany’s largest transmission system operator, Tennet Germany, brought the puzzle to Witthaut, and it took years of investigative and modeling work to determine the cause. From Europe to Australia, the U.S. to Malaysia, the same pattern shows up in the data.

Such frequency drops aren’t unheard of, but they are troubling. And the regularity with which they have been occurring can make the phenomenon feel even more ominous.

The grid’s orderly operation relies on the frequency being kept within a certain range. If the oscillations get get too large, or if grid operators don’t know the cause and respond improperly, they can lead to blackouts. Even if the sags don’t cause blackouts, they can damage equipment in power plants if they go unchecked, especially in hydropower turbines.

Hydropower plants have historically been a main driver of so-called ultra-low frequency oscillations (UFLOs). Every turbine vibrates, and if those vibrations occur with a certain regularity, known as the resonant frequency, they can magnify, sometimes to dangerous levels. Think of a wine glass breaking before an opera singer’s mighty pipes — that’s resonance.

So Witthaut and his team started investigating hydropower, the most obvious potential culprit.

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