New Study Looks At Managing Grid Frequency Fluctuations From Renewables

January 12th, 2018 by Steve Hanley

The electrical grid is not something most people think about. Flip the switch, the lights come on. If they don’t, call the electric company. End of story. But the grid is actually a hugely complex organism, one that requires close attention to keep the electricity flowing at the right voltage and frequency.

Frequency is a concept unknown to most people. Alternating current flows first one way and then the other. The number of times a second that transition takes place is expressed in hertz. In Europe, the standard is 50 hertz. In North America, it is 60 hertz. The name comes from Heinrich Rudolf Hertz, the first person to provide conclusive proof of the existence of electromagnetic waves. Frequency stability is important because fluctuations can interfere with the proper operation of many electronic devices.

It is common to hear people say that renewables like solar and wind disrupt the grid because they vary in voltage and frequency depending on wind speed and cloud cover. There are also concerns about microgrids disrupting the stability of the larger utility grid. Researchers at the Juelich Research Center and the Max Planck Institute for Dynamics and Self-Organization in Germany have studied the electrical grids in Europe, Japan, and the US, and come up with some surprising findings.

“The first surprise was that the grid showed particularly strong fluctuations every 15 minutes,” says Dirk Witthaut of the Juelich Institute of Energy and Climate Research.”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.”

Second, the researchers found that frequency fluctuations did not follow the mathematically predicted model. Instead, they tend to be larger than expected. Are renewables responsible for that difference in behavior? The study found that renewables do in fact lead to greater grid fluctuations. “For example, the share of wind and solar generation in the United Kingdom is much higher than in the USA, leading to greater fluctuations in grid frequency,” explains Witthaut. Based on their research, the scientists recommend increased investment in primary controls and demand controls.

What surprised the researches was that fluctuations from power trading were larger than those from renewables. In other words, the grid is already dealing with frequency fluctuations that are larger than those caused by renewables.

Next, the scientists turned their attention to microgrids, and what they found is not such good news. “Our study indicates that dividing large and thus very slow grids — such as the synchronous grid of Continental Europe — into microgrids will cause larger frequency fluctuations,” says Benjamin Schaefer of the Max Planck Institute. “Technically, microgrids are therefore only an option if today’s very stringent frequency standards were to be relaxed.”

Relaxing those standards would require a significant redesign of many electronic devices and could damage existing equipment. That finding suggests better ways of managing frequency fluctuations will be needed before a distributed grid composed of many microgrids can become a reality.









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