Power grid

Injecting power into the grid: the purpose of European offshore ORE farms

The challenges related to the grid integration of ORE can be broken down according to a scale criterion. A distinction is therefore made here between so-called local grids and the large-scale grid. Today, the manager of the French high-voltage power grid considers that the integration of renewable energies is possible according to the objectives of the French Programmation Pluriannuelle de l’Energie, without major modifications to the existing grid. It is therefore locally that the current challenges of the ORE sector related to power grids are located. At the farm scale, the subsystems of the power grid are essentially composed of:

• cables between turbines, known as Inter-Array Cables (IAC), of medium voltage (33 to 66 kV) and the associated connectors,
• the electrical substation, the convergence point of the IAC which raises the voltage to minimise losses when exporting to shore,
• the export cable(s) whose voltage is generally between 125 and 225 kV. These cables connect the ORE farm to the delivery point on land to make the connection to the terrestrial grid.

Farm topology and innovative substations: two major challenges

The first challenge lies in the design of the IAC grid connecting the turbines to the substation. This is a topological optimisation problem aimed at minimising investment costs, maximising grid efficiency, optimising reliability and thus minimising operating costs. This subject is studied at France Energies Marines through several collaborative R&D projects (VALARRAY, DTOCEANPLUS and MOSISS projects).

The electrical substation represents another major challenge for ORE, specifically in the context of commercial floating wind farm development. Indeed, its foundation is a controlled aspect in shallow water, but its transposition in water heights above 80 m makes the bottom- fixed solution economically impractical. The transition to innovative floating or submarine alternatives is the subject of an ongoing project coordinated by France Energies Marines, which brings together the major players in the sector (LISORE project). The latter deals with issues related to the foundation itself, as well as the potential impacts on on-board electrical equipment.

The stability of cables in high-current environments: a specific issue of tidal turbines

Due to their high voltage and high-power characteristics and the associated high costs, the redundancy of export cables is not easily envisaged. Once the ORE farms are in operation, these cables represent a significant proportion of incidents. This experience feedback from offshore wind sector in the North Sea is reinforced for the tidal stream sector, which is planning to deploy ORE farms in high-current environments that put more constraints on the export cable. This topic has been studied in the framework of a project on the development of tools for assessing cable stability in such environments (STHYF project).

Connectors and energy storage: the next power grid challenges

Other issues related to grids are included in the France Energies Marines’ roadmap, although there are no R&D projects on these subjects yet. Electrical connectors represent a technological and operational challenge in terms of optimising their installation and operation at sea. As the power levels required are steadily increasing, they will require innovative alternatives to the solutions currently in use, whether they operate in air (dry-mate) or water (wet-mate). Although energy storage as a technical response to grid stability does not seem to be a topical issue in the short and medium term, it may nevertheless provide added value from an economic point of view in response to the challenges of the energy transition in other sectors such as maritime transport (hydrogen/ammonia recovery, use in island environments…).

Photo credit: StekrueBe

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Neil Luxcey

Farm Optimisation R&D Manager

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