Nature Energy: 1-minute grid frequency pulse traced to clock-synchronized battery self-tests in UPS units

A team led by physicist Dirk Witthaut of the University of Cologne, Germany, has reported a likely origin of a 1-minute oscillation in grid frequency seen worldwide. The work was published in Nature Energy. According to the authors, clock-synchronized control software in digitally controlled energy devices, particularly uninterruptible power supplies (UPS) and battery energy storage systems, produces periodic power-demand peaks that can give rise to the pattern nature.comNature Energynature.comOpen source ↗.

The pattern appears in frequency time series of grids on five continents, including systems that are physically isolated from one another. Minute-scale frequency fluctuations were documented in Scandinavia in 2014 at the latest. They had been linked to hydropower systems, but hydropower alone did not explain the pattern in other grids, including continental Europe, Britain, Australia and the United States.

Measurements at Forschungszentrum Jülich

The researchers analyzed high-resolution electricity measurements from buildings on the Forschungszentrum Jülich campus in Germany. The 60-second pattern appeared in four buildings, all of which contained UPS units. Buildings without such systems did not show it.

The team detected short bursts of current from the batteries at one-minute intervals and traced the resulting fluctuations through the electrical system of one building to its connection with the public high-voltage grid. According to the authors, the peaks propagate across voltage levels. The manufacturer later confirmed that the cause was routine battery self-tests run automatically by the management software nature.comNature Energynature.comOpen source ↗.

The synchronization is attributed to clocks: digital devices can align their internal clocks with external time signals, so independently operating systems may run scheduled tasks at the same moment. The authors note that the explanation still needs to be tested at other sites.

Amplitude, wear and mitigation

The amplitude of the pattern has increased substantially in several grids in recent years. The authors link it to wear and tear in hydropower systems and to calls for stricter frequency-control requirements. Typical amplitudes are a few millihertz, which the researchers describe as relatively minor.

They warn that the growth of battery storage, data centres and Internet-of-Things devices may amplify such cyber–physical power oscillations. As a remedy, the authors propose improved control design at the source, such as small variations in software schedules. They contacted the manufacturer of one of the identified systems, which released a software update that relaxed the strict periodicity of its system checks nature.comNature Energynature.comOpen source ↗.