Not a week goes by without media discussion on the cost of grid development in Germany. There are those who believe that this just means the addition of more cables. An alternative is now being proposed, not just in theory but also in practice as discussed at the "PV System Technology Forum", organized by Solarpraxis AG, publisher of pv magazine.

For several years now, the EnBW Regional AG, which operates the distribution network of the company EnBW (Energie Baden-Württemberg AG, a publicly traded electric utilities company based in southern Germany), has been conducting experiments in Sonderbuch, Germany. Instead of an additional local substation, which is a necessity at the moment, the company experimented with an adjustable local mains transformer and battery storage.

For every fourth PV system, the grid must be expanded

Sonderbuch has a population of 190, and 60 solar systems. The town was already producing more power than it consumes since 2008. In the meantime, the power production has significantly increased. Sonderbuch, thus, is a good example of a town with high PV development. Solar system installation and grid expansion go hand in hand here.

EnBW Regional expert Daniel Schölhorn stated, "We are at an expansion quota of one to four, meaning that for every fourth PV system that is installed, we need to expand the grid. A few years back this quota was at 10." It is, therefore, important to bring this quota down again.

On cloudy days in Sonderbuch, the capacity at the three local grid stations fluctuates between 80 kilowatts of feed-in from solar power to 20 kW of load. "It surprised us that the worse-case scenarios do come true," Schölhorn added. In the course of a day, the capacity at the local mains transformer can fluctuate more than once between maximum consumption and maximum feed-in. The capacity fluctuations can lead to voltage fluctuations that can eventually lead to the inverters shutting down.

In Sonderbuch, adjustable local mains transformers are already working, adjusting voltage in the local grid. The grid is fixed with multiple measurement points for the experts to examine the data. "We believe that in about half a year we can use the adjustable local mains transformers as the standard solution," said Schölhorn.

In order to quickly level off the fluctuations, EnBW will also add lithium-ion battery storage systems, which will function differently to those for self-consumption optimization. To compensate for the fluctuation, the storage system must be able to provide charge- and discharge- capacities of 28 kW and comparatively need only a 30kWh capacity.

Solar power with storage

As in Germany, a pilot project has been organized in Spain with the inverter company Ingeteam. One solar power plant in Navarra has been equipped with 150 kWh storage and another with a 1.1 MWh storage system. This way the production curve will not just be leveled, but also moved back to some extent.

In another project, a solar plant is combined with battery storage and a diesel generator. The battery storage is used to bridge the gap between a point where solar irradiation falls and the diesel generator starts. Such solutions are thus interesting for installations where a diesel generator is in use anyway.

At the moment, the experts are heavily concentrating on technical solutions. The next step would be to implement systems efficiently and to develop business models. What the cost would be for the grid connection of PV systems with new technologies in the free market remains to be answered.

Translated by Shamsiah Ali-Oettinger

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