Energy

Innovative cable technology to meet the EU’s energy transition goals

Nexans Blog Jul 13, 2021

Maxime Toulotte, Head of Technical Marketing, Nexans Subsea and Land Systems (SLS) Business Group and Frédéric Lesur, Senior Engineer for high voltage cable systems and power grids, explain the critical role that innovative cable technology will play in the transition to carbon-neutral energy.

The EU has set the ambitious goal of becoming the first climate-neutral continent by 2050. This would require major changes in the energy mix and present significant technical challenges for power grids. To solve these challenges, ongoing R&D is focused on increasing transmission capacity and reducing the cost of submarine power cable projects.

Submarine power cables form an integral part of offshore wind projects and transcontinental interconnections to secure supply and maximize capacity. Several large-scale offshore wind farm interconnection and connection projects have been undertaken in recent years, and more are planned in the coming decade.

Floating platform technology

Europe is the undisputed leader in offshore wind energy, accounting for almost 80% of global capacity. Most of these wind farms are constructed in shallow water closer to shore. The wind energy industry is now casting its net further offshore into deeper waters, where winds are both stronger and more consistent, to meet increasing demand.

Conventional wind turbines installed directly on the seabed are only suitable for waters up to 50 meters deep. Beyond that, the technology becomes financially unviable. While a potential solution for greater depths would be to use bigger turbines on floating platforms, this creates a technological challenge for the cables that carry the energy.

Most wind farms have used electric static subsea cables to connect the turbines to one another and the offshore substations to the land. But for deep-water wind farms, the solution is to build wind turbines on floating bases that are anchored to the seabed. In this instance, “dynamic” subsea power cable systems collect and export the energy.

The waves and currents subject the cables that connect the turbines to the grid to significant and dynamic stresses. Consequently, the dynamic cables must withstand both hydrodynamic movement and weight of the floating platform.

Fortunately, Nexans, can rely on a decades-long experience developing and manufacturing dynamic cables and umbilicals for customers in the marine oil and gas industry. We use this experience in combination with knowledge from designing and installing high-voltage subsea cables as the foundation for new research.

Floating wind farms will consist of enormous turbines that will eventually reach power levels of around 20 MW. Current power levels are about 7 to 10 MW. The thicker cables required for such high-voltage levels cannot tolerate any water penetrating the insulation. They need a water barrier which, in static subsea cables, comes in the form of a lead sheath that coats the insulation. However, this type of sheath cannot withstand the waves’ dynamic movements and bending stresses.

An alternative barrier must be thick enough to provide adequate protection yet possess a good level of flexibility. We subsequently developed cables, using metallic foil and polymer insulation layers, which are more suited for extremely long cables.

By adopting a holistic approach to engineering design, we cover factors such as resistance, flexibility, flotation, and temperature regulation. It is also possible to incorporate fiber optics in the dynamic power cables at a minimal cost. This allows end-to-end communication, which provides an excellent cable condition monitoring system for operators.

Interconnections

To limit the risk of blackouts, interconnections allow the connection of various electrical networks, even if they have different energy mixes. This connection enables renewable energies to penetrate power grids more effectively and neutralize the wind power’s intermittent nature, both onshore and offshore.

Historically, HVDC (high-voltage direct current) subsea interconnections have utilized mass-impregnated (MI) paper-insulated cables, and is still the technology of choice for large interconnectors. The insulation comprises several hundred layers of paper tape wrapped around the conductor and impregnated with thick oil. Although reliable and long-lasting, the complexity in manufacturing is given by the need of controlled tension in the paper tapes and even impregnation of the insulation – over hundreds of kilometers –.

HVDC cables have no technical limits on their length and can be used for the longest links both onshore and offshore (up to 700 km). For some years, a technique known as XLPE (cross-linked polyethylene) insulation has been used for HVDC cables. These cables are in high demand in the offshore wind farm market and will feature in the construction of several new wind farms and interconnections in the UK.

Superconductivity

Simultaneously, superconducting links – discovered more than a century ago – can also provide an appropriate solution in some cases. No longer a niche technology limited to dense urban areas, they now play a continental-scale role in renewable energy.

Superconductor cables increase the delivered power supply by using existing ducts at lower voltage levels and without any Joule losses. This eliminates the need for disruptive civil engineering works and minimizes the land occupied by transformer stations. For our Ampacity link project in Essen, it was even possible to free up a substation’s right-of-way.

A new breed of cable-laying vessels

A new generation of innovative, technically advanced cable-laying vessels is currently under construction, capable of installing high-voltage subsea cables in depths beyond 2,000m. These vessels will minimize the number of offshore HVDC connections and potential alterations to the seabed.

Almost 150 m long and 31 m wide, the “CLV Nexans Aurora” vessel features dual cable-laying lines capable of handling a payload of 10,000 tonnes. The innovative, concentric design of the turntable, based on proven technology, enables two 5,000-tonne cables to be laid simultaneously at a rate of up to 20 m/min.

The hull has been designed to reduce the vessel’s movement in dynamic positioning (DP) mode and is managed by a fully redundant system that operates at the highest level – DP3. Six thrusters provide high-propulsion capacity of more than 20 MW. Other innovations include the regeneration of cable-laying braking power and the use of power from shore during cable loading operations.

Once launched, the Nexans Aurora will contain the largest offshore split turntable in the world. It will be our largest, most versatile and environmentally friendly cable-laying vessel to date. The vessel will enter active service in summer 2021.

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