Grid stability is likely to be increasingly challenged as power distribution moves from a centralized to a more decentralized model, new research has found. According to a paper published this week in the journal Nature Energy by researchers from Germany’s Max Planck Institute for Dynamics and Self-Organization and the UK’s Queen Mary University of London, integrating growing numbers of renewable power installations and microgrids onto the grid can result in larger-than-expected fluctuations in grid frequency.
The researchers collected data from grids of various sizes in Germany, France, the UK, Finland, Mallorca, Japan and the US. Based on this data, they developed mathematical models that “can establish the influence of making the grid smaller or of adding a bit more renewable energy” in order to aid in planning, said Professor Christian Beck of Queen Mary University, one of the paper’s co-authors. The team found that small grids like Mallorca’s displayed larger frequency deviations than larger grids, such as continental Europe’s. And comparing different regions showed that a larger share of renewable generation resulted in larger frequency deviations.
“The grid operators want the frequency to be 50 Hz, but it fluctuates a little bit around this all the time,” said Beck. “We can now establish the probability that the deviation is more than 2 per cent or so, which is a big deviation, and we found that the probability of that is higher than expected from pure random fluctuation.” Beck told PEi that the research team’s “first surprise was that energy trading had a significant impact on the grids studied” after Germany’s grid and others displayed particularly large fluctuations every 15 minutes, corresponding to spot market trading.
“The grid frequency had big jumps every 15-30 minutes,” he said, “and it wasn’t clear to us before that trading has such a big effect. Most people were worried about renewables because they are unpredictable and certainly produce fluctuations in frequency. Trading gives a similar order of, or stronger, fluctuation, which hadn’t been clear to us or, I think, to most people.”
Comparatively, the research showed that a larger share of renewable generation in a given region resulted in larger deviations from the standard 50 Hz. For example, the UK, with more renewables than the US, also had larger frequency deviations. To integrate more renewables onto the UK grid, the research team recommends increasing primary control and demand response. “The UK is somewhat special,” Beck said, “in that it has a much higher component of wind power contributing, and it also has an overall smaller grid than the rest of Europe. Still, frequency fluctuations caused by trading seem to be at least as relevant as fluctuations caused by renewables.”
Asked about the effects on microgrids, he said that “the maths allows us to extrapolate the effects depending on the size of the grid. If we extrapolate our results to smaller grids, then indeed we would be implying that the effects are more pronounced there, and if people wish to have a microgrid then they need to relax a little bit the conditions they demand on constant frequency.”
“I don’t think we are saying anything against microgrids,” he added. “You just have to complement them with suitable control strategies to make sure the frequency is constant enough.”