The quest for stability in microgrids

According to the International Energy Agency, global energy demand could increase by 40% by 2040. In light of this, power distribution and access must be improved. Microgrids, composed of consumers and local sustainable energy producers (using photovoltaic panels or wind turbines, for example) are among the solutions proposed. These miniature power grids are better suited to more small-scale use, such as in a small town or village, and supply no more than a few megawatts. Although their development and installation are making progress, they have one weakness: they are less tolerant of variations in load, production and frequency.

To remedy the issue, Inria launched the SyNPiD project (for "self-synchronisation in power grids with periodic dynamics”). The aim is to study these smaller and harder-to-stabilise electrical systems in detail and use mathematical modelling to optimise their operation.

Grids easily affected by variations

“Our systems for modelling energy use and understanding and managing frequency variations are adapted to large-scale networks. We need to reinvent everything and find new approaches”, says Denis Efimov, coordinator of SyNPiD and head of the Valse project team at the Inria centre at the University of Lille (joint with Centrale Lille and CRIStAL). In collaboration with a team from the Brandenburg University of Technology Cottbus-Senftenberg (BTU) in Germany, the researcher and his colleagues have designed a methodological framework for mathematical analysis to manage frequency variations in microgrids.

For example, in a village of just a few hundred inhabitants, if someone arrives and wants to charge their electric car, this causes a major variation and upsets the microgrid, which is not used to this type of output. Such an operation requires around 30kWh, which is considerable on this scale... As a result, the network automatically short-circuits.

A key role in the energy transition

This problem should be taken very seriously, because microgrids have an important role to play in the energy transition. Compared with large grids, they are better suited to using renewable energy sources, which is what everyone is looking for. In France, photovoltaic solar power currently generates 21.1 GW and wind turbines generate 23.8 GW. These figures are rising steadily, but still fall far short of the government's target of 101 to 113 GW of renewable energy capacity by 2028.

Another advantage of microgrids is that they distribute electricity locally, which reduces transmission costs and cuts losses, not to mention the social benefits of a system that encourages local energy production, initiatives that include smaller structures, and partnerships between entities based in the same area.

Microgrids are particularly recommended on islands that are currently heavily dependent on oil, but also in isolated rural areas, such as on the African continent, where many communities have no access to electricity. They are also well suited to supplying energy to a group of buildings such as a campus, or even apartment blocks, but their system still has to be reliable, with no risk of tripping at the slightest variation!

At the crossroads between specialities

How have the SyNPiD researchers gone about improving their reliability? “The difficulty lies in modelling these small networks, delving into the details and understanding just how resilient these systems are”, explains Denis Efimov. On the French side, the Valse project team headed up the theoretical part in terms of dynamic analysis and estimating frequency changes, while the German researchers at BTU had a suitable platform for carrying out full-scale tests. “The project lay at the crossroads of our specialities", says Denis Efimov. “We had a lot of very fruitful meetings because there was lots to discuss and many difficulties to overcome, but together we managed it!”

The scientists at BTU used a platform with a total power of 110kW. Half was provided by a conventional generator, and the rest by four decentralised energy production units (equivalent to solar panels or wind turbines). All this over a distance of around 15 km, which could be compared to a small village of about 50 inhabitants. In reality, microgrids are generally a little larger, but the simulation nevertheless offered a fairly realistic order of magnitude.

© Brandenburg University of Technology Cottbus-Senftenberg

Grids are more resistant than expected

The teams then studied in detail how the electric current behaved in this system in order to learn from it and design an algorithm capable of measuring the whole phenomenon. “We had some surprises", admits Denis Efimov. “The trajectories of the current were not what we expected and were very different from what happens in a large conventional network".

The researchers identified frequency variations which, in practice, would cause the whole grid to shut down... However, the simulation showed that these could be tolerated by the system. “These networks are more resilient than expected. When there’s a variation, sometimes all you have to do is wait and everything returns to normal after a short transition phase. You don’t necessarily have to have a shut-down each time”.

Next step: roll-out in real microgrids?

The challenge now is to analyse the SyNPiD results in detail to identify with certainty when and how this type of microgrid needs to shut down. “It's very tricky because there are so many different types of behaviour", says Denis Efimov. “Our analytical framework is still under construction and all this information will feed into it”.

Now completed, SyNPiD project and its results are being studied by two post-docs. Further trials on other platforms are due to take place in the near future with the BTU, within the new ANR project SyNNuM.

Will the SyNPiD methodological framework one day be used in a “real” microgrid? Denis Efimov answers in the affirmative: “We’re using power grids that were built decades ago and are still working. Our aim is to solve the problems without undoing everything. With SyNPiD, we propose to adjust existing grids, so yes, it’s possible!”