To jump or not to jump

Our daily life depends more than ever on a reliable electricity supply.

However, the ongoing energy transition poses new challenges to the electrical power grid and its operators. For the integration of additional renewable generation into the power grid, it is often proposed to split the grid into smaller autonomous cells, also called “microgrids”.

Thereby, a village with a combined heat and power unit and added wind and photovoltaic generators could operate mostly independently without drawing energy from the grid. But how does splitting a large grid into smaller cells and adding more renewable generators affect the reliability of the electricity supply?

Benjamin Schäfer and Marc Timme, scientists at the Max Planck Institute for Dynamics and Self-Organization in Göttingen and the Technische Universität Dresden, Germany, analysed power grid frequency fluctuation in different regions in the world together with colleagues from the Forschungszentrum Jülich (Germany), Queen Mary University of London (Great Britain) and the University of Tokyo (Japan).

They formulated mathematical models to predict the effect of power fluctuations on the grid frequency. The results are now published in the article „Non-Gaussian Power Grid Frequency Fluctuations Characterized by Lévy-stable Laws and Superstatistics“, at Nature Energy.

How much do renewables impact frequency fluctuations?

The power grid in Germany and all of Europe operates at a grid frequency of 50 Hz. This frequency is mostly generated by turbines, e.g. in water or coal power plants, that rotate 50 times a second. If a consumer draws additional energy from the grid, for example because an aluminium factory starts a shift, the grid frequency drops briefly until additional power is provided by the generators. The deviations of the frequency from the reference of 50 Hz must never be too large since sensitive electronic devices may be damaged otherwise.

In addition to consumer behaviour, renewable generators also cause fluctuations of the grid frequency. For example, wind generation can only be as steady as the unpredictable wind and photovoltaic generation is fluctuating due to moving clouds. The international team of scientists asked: Do renewables impact the grid frequency as negatively as sometimes claimed by critics? How likely are large and therefore risky deviations from the reference frequency?

Two surprises in one analysis

To answer these questions, the researchers collected data from Germany, France, Great Britain, Finland, Mallorca, Japan and the USA. Note that Germany is not isolated but connected to most European countries via a joint European transmission grid. Similarly, Finland is part of the Nordic grid. Lastly, the USA is split into multiple synchronous regions and measurements from the largest region, the “Eastern Interconnection”, which also includes parts of Canada, were used.

A systematic analysis of the data revealed two surprises for the scientists. Firstly, the European grid displayed particular large fluctuations every 15 minutes. This is exactly the time period during which power plant operators agree on a new power dispatch via an energy spot market. Hence, the results point out that trading significantly impacts the fluctuations of the European grid.

Secondly, the stochastic deviations from 50 Hz do not follow a normal (Gaussian) distribution. Instead, extreme fluctuations are much more likely than expected based on a normal distribution. In order to understand these observations and potentially support the planning of a fully renewable power grid, the scientists formulated a mathematical model for the power grid. Using their model, they computed how large fluctuations are to be expected dependent on the size of synchronous region. Thereby, the impact of renewable generation should be made clearer.

Frequency fluctuations larger in small grids than in larger ones?

Small grids, in particular the one of Mallorca but also the British grid, displayed larger frequency deviations than larger grids, like the Continental European grid. “Our study indicates that splitting a large grid, like the European grid, into small microgrids will lead to larger frequency deviations. Microgrids are therefore only an option, if current frequency regulations are made less strict.” says Benjamin Schäfer, first author of the study. Before making a definitive statement, the researchers are collecting additional data, e.g. in Ireland and Iceland. In addition, the scientists are already planning experiments in microgrids.

Comparing different regions showed that regions with larger frequency deviations often also had a larger share of renewable generation. Britain, for example, has a much larger share of wind and solar generation compared to the USA. Similarly, frequency deviations in Britain are much larger than in the USA. To incorporate additional renewable generators anyway, the scientists recommend increased primary control and demand response, i.e., smart adaptation of producers and consumers following the grid frequency. (see also http://www.ds.mpg.de/2715432/smart_grid).

“Interestingly, frequency fluctuations caused by trading seem to be more relevant than fluctuations caused by renewables.” summarizes Prof. Marc Timme the study.