Deep sea grid interconnectors – The vital link between renewables and energy security

Renewable sources of electricity become an increasing and exceedingly important factor in the energy equation. Global annual renewable capacity additions increased by almost 50% to nearly 510 gigawatts (GW) in 2023, the fastest growth rate in the past two decades, marking a major step forward in the reduction of fossil fuel power. Moving to renewable energy is vital to decreasing greenhouse gas emissions and aligning with the 1.5 Celsius climate target set in the Paris Agreement.

Even as renewable energy generated a record 30% of global electricity in 2023, furthering the growth of renewable energy relies heavily on grid interconnection.

Interconnections provide the optimal way to ensure energy security across regions and continents:

• When grids are interconnected, surplus energy from one region is easily transported- via the interconnected power grid – to where electricity is needed. This ensures energy security across the interconnected grids and avoids reliance on fossil fuels when renewable energy cannot meet local demand.
• By equalizing demand and supply across interconnected grids, excess power is more readily available to areas with increased demand, thus ensuring energy price stability and future renewable investments.
• The harnessing of electricity across grids goes beyond adjacent regions to include the interlinking of power grids across continents and islands and offshore renewable energy sources. Deep water, once a barrier to interlinking grids, is less of an issue thanks to innovations in cable design, new materials and alloys, and cable installation and maintenance advancements.

1. Deep sea grid interconnections – from megawatts to gigawatts

The interconnection of grids is not only reaching impressive depths, but their capacity has also gone from hundreds of megawatts to gigawatts.

To date, the deepest installed HV cable system is the SaPeI interconnector, stretching 435 kilometers, linking Sardinia and mainland Italy, reaching 1,640 meters below sea level.

One revealing example of this revolution is the Tyrrhenian Links project, currently under construction. It will connect Sicily with Sardinia and the Italian peninsula. It will be installed at a record-breaking 2,200 meters deep, for a transmission capacity of 1,000 MW. Achieving this is possible thanks to advances in high-voltage direct current (HVDC) systems, which can transmit larger amounts of power over long distances.

If this technology is already available for shallow waters, engineering challenges arise when increasing the water depth.

2. Mass-Impregnated (MI) undersea cables – decades-long reliability track record

Mass-impregnated cables’ first commercial use was in 1954 for the Gotland HVDC Link to connect the Island of Gotland to mainland Sweden. Since then, mass-impregnated cables have been the primary choice for subsea HVDC interconnector projects requiring voltages surpassing 500 kV over long distances and extreme depths.

Simply put, a MI undersea cable is a type of HVDC cable specifically designed for underwater applications. Here is the breakdown:

• Construction: it is made with layers of high-density paper tapes wrapped around the conductor. These tapes are then impregnated with a special, high-viscosity compound. This compound is key – it’s thick and doesn’t flow easily, even if the cable is damaged.
• Application: it is used for transmitting large amounts of electrical power over long distances underwater. They are particularly useful for applications exceeding 500 kV DC and long distances.

MI cables offer three main advantages that make them innovative for undersea applications:

1. Reliability: The high viscosity compound prevents leakage even if the cable is damaged, making it more reliable for underwater use compared to older designs.
2. Durability: cables installed decades ago are still operational today, demonstrating their long lifespan.
3. Depth capability: they can be used for extreme depths with proper design features.

Overall, MI cables are a well-established and trusted technology for transmitting large amounts of power underwater, making them a key innovation for subsea power transmission.

Breaking even new barriers to deep sea cable depths will be the Great Sea Interconnector project. Reaching depths of up to 3,000 meters in some areas, the project will connect Israel, Cyprus, and Greece via Crete. Stretching 900 kilometers from Crete to Cyprus, the 1,000-megawatt, bi-pole cable will bolster energy security and facilitate electricity exchange between the countries.

3. Overcoming subsea cable challenges – new design approaches

The main challenge in using MI technology in subsea cables however is the elongation of the insulation system during deployment and retrieval.

There are many ways this can be overcome. The most prominent is through cable design, conductors, materials, or installation methods.

Additionally, 500+ kV extruded cable designs are also being developed. A major advantage of extruded cable design is its ability to sustain greater elongation compared to MI cables. A challenge for extruded design is the need for effective water blocking in the conductor in the case of damage to the cable. At 3,000 meters below sea level, the pressure is so great that if the water is not blocked in the conductor, it can easily penetrate tens of kilometers into the cable, leading to costly repairs.

Nexans is deeply involved in the electrical transmission revolution that is going on.

In fact, the Group has been playing an essential role for a long time. In 1977, Nexans deployed its first HVDC MI cables for the Skagerrak subsea interconnector between Denmark and Norway. Almost 50 years later, the original cable system is still in use. Today, its expertise in building, installing, and repairing deep sea HVDC systems spans MI and extruded technologies.

Its most monumental undertaking is currently crafting the world’s longest and deepest subsea interconnection: The Great Sea Interconnector. This colossal project symbolizes a new era in energy interconnections and demanded immense resources and logistical mastery.

Innovation is the lifeblood of deep sea grids. For this sustainable future revolution to reach its full potential, robust and far-reaching interconnectivity is paramount. Deep sea grid interconnectors are the invisible threads weaving a global energy tapestry. Challenges remain, but solutions are on the horizon.

Deep sea grids are not simply cables on the ocean floor; they are the lifelines of a safer energy landscape. They are the physical manifestation of a global commitment to a sustainable future, a future powered by the boundless potential of renewable energy.