Transgreen: The Trans-Med supergrid

Cable being shipped from a Nexans plant in Halden in Norway for the 580 km NorNed link. The success of this power link gives Transgreen partners confidence in the suitability of the technology for Mediterranean connections Source: Nexans
Pierre Kayoun of Nexans explains how the Transgreen initiative intends to export 5 GW of energy produced by renewable sources in the Mediterranean Basin to Europe.

Pierre Kayoun, Nexans, France

Transgreen is an unprecedented industry initiative that will study the feasibility of a power transmission grid between the northern and southern shores of the Mediterranean Sea based on the development of interconnections around the Mediterranean Basin.

The project was created in Paris in July 2010 with the signing of a draft agreement between 13 industrial partners, in the presence of Jean-Louis Borloo, the French minister of Ecology, Energy, Sustainable Development and Marine Affairs, and Henri Guaino, special adviser to President Sarkozy, and Pierre Lellouche, the French secretary of state for European Affairs.

Transgreen falls within the framework of the Union for the Mediterranean’s Mediterranean Solar Plan (MSP). This has the objective of establishing 20 GW of new renewable energy capacity by 2020 in the southern and eastern parts of the Mediterranean Basin, and especially North Africa, out of which 3″4 GW would be covered by solar photovoltaics (PV), 5″6 GW by wind and 10″12 GW by concentrated solar power (CSP). Around 5 GW would be exported to Europe to assist the viability of the power plant projects.

Transporting this power from the often remote areas where it will be generated to the places where the demand exists will require new transport and interconnection infrastructures. They will link production sites to the local grids and then across borders and into Europe, notably by undersea high-voltage direct current (HVDC) cable interconnections.

Industrial partners

The signatory companies in this industrial initiative are Abengoa, AFD, Alstom, Areva, Atos Origin, CDC Infrastructure, EDF, Nexans, Prysmian, RED Electrica de Espaàƒ±a, RTE, Siemens and Taqa Arabia.

This partnership comprises manufacturers and utility companies who are collaborating within a common legal entity with the remit to:

  • Propose the technical and economic blueprint for a trans-Mediterranean supergrid capable of exporting around 5 GW to Europe by 2020 and leading to concrete projects;
  • Promote an investment-friendly regulatory and institutional framework for viable projects on the southern shores of the Mediterranean;
  • Show the impact of the investment in infrastructure and electricity exchanges on growth, economic activity and employment;
  • Develop technical and technological co-operation with countries on the southern shores on the basis of projects for trans-Mediterranean power links;
  • Promote European technology and industry in global competition, and particularly the production of electricity from renewable energy, DC transmission and very-high-voltage undersea cables.

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The partnership has an initial horizon of three years, and it is open to other companies, particularly those from southern countries, with the aim of designing together the electrical grids that will help speed up the energy transition towards low-carbon electricity.

Transgreen and Desertec go hand-in-hand

Transgreen will work closely with the authorities of the countries concerned, the European Commission, the scientific community, development banks and non-governmental organizations. It will also co-ordinate its efforts with other Mediterranean Solar Plan projects such as Desertec.

It is important to emphasize that Transgreen is intended to complement the Desertec initiative, which intends to supply up to 15 per cent of the electricity used in Europe by 2050 from solar and wind power production facilities in North Africa and the Middle East. The primary focus of Desertec is on power production, while Transgreen is focused purely on transmission.

Conductors such as Nexans’ AAAC are expected to feature in Transgreen’s land connections Source: Nexans

The Transgreen partners view the creation of a trans-Mediterranean electrical supergrid as a vital contribution to the viability of projects for renewable electricity production in the Mediterranean Basin, as this will enable them to export part of their production to Europe and cut the cost of long-distance, high-power transmission by stimulating progress in advanced bulk power transmission technology.

Development of the interconnections will increase the security of the electrical systems of the partner countries as they adapt to meet the constant growth in demand for power. Transgreen will also create the conditions for building an electricity market in the southern Mediterranean, which will contribute both to the security of supply and competition.

Transgreen interconnections

The exact nature and location of the interconnections that will form the Transgreen supergrid is a key topic for the initial discussions. At the time of writing the Transgreen partners anticipate the creation of links between Tunisia and Italy, Libya and Italy, Algeria and Spain, Algeria and Sardinia.

There is already one subsea interconnection in place between Spain and Morocco. In fact, it is currently the only connection between the northern and southern Mediterranean systems. This interconnection, across the Straits of Gibraltar, consists of two 400 kV AC links (commissioned in 1997 and 2005 respectively) and comprises a total of seven cables (six plus one spare). At present, the link offers a total capacity of 1400 MW (2 x 700 MW) with an overload capacity of 1800 MW for 20 minutes.

Generally, the flow of electricity has been from Spain to Morocco. This will of course change when the renewable energy sources in North Africa start to come on-line.

Fortunately, the links were created with future developments in mind and it is possible to upgrade the current AC systems to operate as HVDC systems. This would effectively double the power capacity of the current links. However, the upgrade would involve the construction of suitable HVDC converter stations. Plans were also announced at the end of 2009 for a third link between Spain and Morocco to add an additional 700 MW of capacity by 2012.

Subsea cable technology

The Spain”Morocco links are based on oil-filled cables. However, a more advanced type of cable, featuring MIND (mass impregnated non-draining) construction is now well established for submarine HVDC applications. This type of cable features a copper conductor with insulation consisting of layers of paper impregnated with a high viscosity oil.

The cable requires no external pressurization, and even in the event of it being completely severed by accident only a very tiny amount of insulating medium will escape ” hence the term ‘non-draining’. There is a lead alloy sheath on top of the insulation, and mechanical protection is provided by steel tape and steel wire armouring.

A typical HVDC single core cable has a diameter of around 120 mm and weighs approximately 40 kg per metre in air. Cables for this type of project are not supplied ‘off-the shelf’. Instead, a comprehensive design, type-testing and approval process would be carried out to develop the ideal cable for each link.

At 580 km, the NorNed link between Kvinesdal in Norway and Eemshaven in the Netherlands (opened for commercial operation in 2008) is currently the world’s longest high-voltage submarine power cable. Nexans manufactured two 156 km lengths of MIND cable for the deep-water section ” laid at depths of up to 410 metres ” as well as a shorter length of cable to bring the link ashore in Norway.

The successful operation of MIND HVDC cables over the lengths and depths encountered in the NorNed project enables the Transgreen partners to have complete confidence in the suitability of this technology for the generally shorter interconnections required in the Mediterranean. MIND cables will also feature in the 500 MW COMETA interconnection between mainland Spain and the Balearic islands that is due to come on-line in 2011.

Land-based transmission technology

While the plans for the large subsea cables have grabbed the initial headlines, it should be remembered that the creation of a Mediterranean supergrid will also require considerable attention to the challenges of creating the land-based transmission links that will enable the cross-border interconnections to tie in seamlessly with the local grids.

Here there is considerable scope for the introduction of innovative transmission technology such as underground HVDC cables to reinforce local networks. However, many of the solutions will probably involve AC technology and overhead lines.

Nexans anticipates that we will see some of the newer smart grid technologies come into play, especially the introduction of sensors in both overhead and underground cables that will measure hot spots and humidity, and monitor current load and congestion in real-time to enable load to be safely increased to maximum capacity.

Furthermore, the introduction of new types of overhead cables using innovative designs and materials will enable existing links to carry more power. For example replacing traditional ACSR (Aluminium Conductor Steel Reinforced) conductors with AAAC-Z (All Aluminium Alloy Aero-Z) conductors could reduce overload and decrease losses by more than 20 per cent.

It is possible that the ripple effect from Transgreen could result in some profound changes to the nature of the whole European grid.

The next steps

Signing the draft agreement was just the very first step in a long and complex process. The Transgreen partners are now starting to form the working groups that will drive the three-year feasibility study. It is a great opportunity for everyone to work together to create a new energy future. A future that will increase security of supply for Europe while reducing carbon dioxide emissions and also contributing to new prosperity for North Africa.

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