Study finds problems with frequency fluctuations in power microgrids

January 19, 2018 //By Nick Flaherty
Frequency measurements from 2015 (data: 50Hertz): the power grid frequency fluctuates around 50 Hz in the European grid and exhibits large jumps particularly in the trading intervals of 15 minutes. Usually, the grid frequency is within the yellow area but upward and downward deviations (grey) are particularly likely every 15 minutes. Copyright: MPI für Dynamik und Selbstorganisation / Benjamin Schäfer
Researchers from Jülich and Göttingen have analysed different types of fluctuations in several power grids in Europe, Japan, and the USA and found that microgrids could be a major problem.

The team from Forschungszentrum Jülich and MPIDS analysed the grid frequency fluctuations in power grids in different regions of the world to predict potential vulnerabilities and were surprised by the results.

“Our power grid works at a frequency of 50Hz, usually generated by turbines, for example in hydro- or coal power plants, which rotate at that speed.  When a consumer uses more electrical energy from the power grid, the grid frequency drops slightly before an increased energy feed-in re-establishes the original frequency," said Benjamin Schäfer from the Max Planck Institute for Dynamics and Self-Organization (MPIDS) in Göttingen. "Deviations from the nominal value of 50 Hz must be kept to a minimum, as otherwise sensitive electrical devices could be damaged."

Renewable energy generation also causes grid frequency fluctuations because the wind does not always blow at the same speed and clouds constantly alter the feed-in from photovoltaic systems. A frequent suggestion for integrating renewable energy generators into the power grid involves breaking the grid down into small autonomous cells known as microgrids. This would allow a community with a combined heat and power unit and its own wind and photovoltaic generators, for example, to operate its energy systems in an autonomous manner.

"The first surprise [we found] was that the grid showed particularly strong fluctuations every 15 minutes," said Dirk Witthaut from Jülich’s Institute of Energy and Climate Research and the Institute for Theoretical Physics of the University of Cologne. "This is the exact time frame during which generators on the European electricity market agree on a new distribution for the electricity generated – this alters how much electricity is fed into the grid, and where. In Europe at least, power trading therefore plays a key role in balancing grid frequency fluctuations."

The second surprise was that statistical grid fluctuations around the nominal value of 50Hz do not follow the expected Gaussian distribution, which is a symmetrical distribution around an expected value.

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