The small town of Carros in the south of France regularly struggles with power cuts. The reason for this is the municipality's location - a few kilometers from the border with Italy at the end of the only power line that supplies the Côte d'Azur and its hinterland. The power cuts usually occur in winter: When it gets uncomfortable without heating, even in Nice and Cannes, electric heaters are usually sufficient, as temperatures rarely fall below freezing. But: In Carros, the electricity sometimes goes off in such cases. However, the municipality has another resource at its disposal: the sun.

This is how the town became a pilot community for 'Nice Grid', the French contribution to the 'Grid4EU' smart grid project. As part of this project, six European electricity companies - financially supported by the European Commission - are investigating the functioning of smart grids and developing common standards. The French electricity grid operator Électricité Réseau Distribution France (ERDF), which is participating in the 'Nice Grid' project, is also part of the network.

The aim of testing the smart grid in Carros is to integrate decentralized energy generation into the local low-voltage grid, reduce peak loads and ensure the island operation of microgrids. Also involved in the project are manufacturers of technologies and products that are used for the generation of solar power as well as the storage and regulation of generation and consumption.

Smart meters for 2000 households

For the implementation of 'Nice Grid', project zones for the individual tasks were first designated in the municipality. The sub-area for the island tests - a street with eight commercial enterprises - can be disconnected from the rest of the grid in the event of a power outage. During this period, it will be supplied from its own resources. The infrastructure was set up in 2013 and around 2,000 households in the project zone received smart meters with communication functionality. The devices measure and transmit consumption to a central energy management system, which monitors and controls all of the smart grid's operating parameters. In particular, it provides forecasts on electricity production and peak loads in the smart grid, which households can call up and use to adjust their own electricity consumption to production and load peaks.

The 'Sunsys PCS²' battery inverter units from Socomec are used for decentralized energy supply in the Carros smart grid.

© Socomec

Photovoltaic systems with a total production capacity of 2.5 MWp were installed at around 200 locations. In addition, electricity storage systems in the form of lithium-ion batteries with a total capacity of 2 MWh were installed. A small part of the storage capacity was implemented as a home electricity storage system with 4 kWh. A large battery storage system was installed at the transition to the medium-voltage grid, and there are three further battery systems with a storage capacity of 620 kWh and four 'Sunsys PCS²' battery inverters from Socomec, each with an output of 66 kW, at the local distribution stations. The three battery systems and the inverters, together with three generator systems with 430 kWp, will supply the micro grid intended for island operation.

In normal operation, the Socomec systems convert the direct current generated into alternating current to supply the loads. As long as electricity production is greater than consumption, they charge the batteries and feed the surplus production into the higher-level grid. If energy generation is interrupted or production does not cover demand, the battery inverters convert the energy stored in the batteries into alternating current for the consumers.

The challenge of island operation

In stand-alone operation, the demands on the battery inverters increase: Firstly, they have to manage the start of the decentralized supply in the event of a grid supply failure without interrupting consumers. During operation, they are responsible for all the regulation tasks that are otherwise mainly the responsibility of the grid operator: Keeping energy generation and consumption in balance, stabilizing voltage and frequency and compensating for disturbances such as harmonics or voltage fluctuations. Once supply from the higher-level grid is possible again, reconnection to the grid must be trouble-free.

With Socomec devices, the black start function ensures that the inverter is synchronized and the loads can be supplied with power from the microgrid within a few seconds. In order to achieve the highest possible stability in the grid, each battery inverter works independently of the others in stand-alone mode. As excess energy cannot be fed into the grid during this period, the system throttles PV production wirelessly by slightly increasing the frequency value in the microgrid. This prevents power generation from having to be switched off and extends the runtime of the island phase. Before the grid is switched back on, the system control synchronizes the voltage, frequency and phase.

Five hours without power

As the island's power supply depends on the functionality and operability of the battery inverters, appropriate tests were required. In several runs, the systems had to demonstrate that they could handle the critical processes without errors. The highlight of the test runs took place in October 2015: The area was disconnected from the grid for five hours. The supply was taken over by the solar panels and storage units, which were around 75% charged at the time of disconnection. At the end of the test period, the charge was still 55% and would have been sufficient for a further four hours. The takeover of the supply when disconnecting from the main grid and the handover at the end of the island phase went smoothly.

The European smart grid project will run until the end of 2016 and will then be evaluated. It will then also be decided how to proceed with the pilot installation in Carros.

Author:
Steffen Breiter is Marketing Manager Germany at Socomec in Mannheim.