Energy beneath the waves

Keeping the country illuminated sometimes requires special connections. These connections overcome the complexities of distance and physical barriers to transport electricity from one electrical substation to another, invisible, below sea level: for this, we need submarine power lines. This infrastructure extends for many tens - hundreds even - of kilometres, transferring energy between the different zones of the electricity market in Italy and beyond. The submarine cables used to transport the electricity are complemented by overhead and underground power lines, and can serve to connect two distant areas of land which otherwise would not have any points of contact.

Their “job” is to transport high-voltage electrical current, either as alternating current, when the distance to be travelled is shorter, or direct current, if the kilometres increase and it becomes necessary to reduce transmission losses. Either end of the cable is directly connected to the electricity substations of the national grid. In the case of direct current connections, a “conversion” station is inserted into the system which transforms the current from direct to alternating, and vice versa.

Designing a submarine cable is no mean feat. First of all the route line must be studied in detail, assessing the geomorphological features of the seabed, the depths in question and distance to be covered. The most direct route is not always the optimum one. Once the project has been planned and the cable has been produced, special “cable-laying” vessels are brought in to lay the cable at the maximum depth.

The length of the line has a direct impact on the cable installation which, statistically, takes an average of around twenty days for each laying campaign. The cable laying inevitably begins at the landing points. This is one of the most delicate phases of the project; divers thread the end of the cable through ducts installed at the landing points to connect the cable from the vessel to the mainland. These ducts are creating using a technique called horizontal directional drilling which avoids the need for open excavations. Usually the landing points are located in remote areas in order to limit any inconvenience to the local community. Asides from the impacts of the installation works, once the project is completed the underground infrastructure is completely invisible.

The next step is to protect the cable. It is rare for submarine cables to suffer from faults or internal damage. Instead, this work aims to avoid damage to the line from external events such as trawling or anchoring. To this end, on sandy seabeds the jetting technique is used; this involves lowering custom-designed machinery from the vessel to the sea floor, where a jet of water and air is released to move the sediment and create a trench where the cables can be laid. And while it is true that faults can be very challenging to resolve, given the location of the cables and the professionals who must be involved, such problems are very rare: submarine power lines are very reliable and are essential when energy must be transported over long distances.

The constantly growing demand for energy, and the increasing focus on the environment, have resulted in the need to optimise electricity transmission using innovative solutions that are effective without disturbing the environment. Compared to the alternative of optimising existing overhead power lines or installing new ones, often cable transmission is the better, or sometimes only, possible solution. In general, submarine cables are the preferred solution both from a technical and an environmental standpoint, increasing the exchange capacity between different zones and reducing the impact of the power lines on the environment and the landscape.

In recent years, these types of connection have increased in all of the world's oceans. However, Terna holds the record. When it comes to depth and length, no other electricity transmission operator can boast similar performance credentials.

The Capri Torre-Annunziata and Capri-Sorrento cables were installed at a depth of just over 100 metres. Extending for 31 and 16 kilometres respectively, the two connections entered into operation at different times: the first in 2017, the second at the end of September 2020, together with the Sorrento substation. Both are part of the broader initiative to restructure the Sorrento peninsula, with a series of projects aimed at dismantling nearly 60 km of old and obsolete overhead power lines.

This was a historic moment for the island: before the installation of this infrastructure, Capri was supplied by a private diesel generator which was unable to meet demand and was highly polluting. In addition to connecting the island to continental Italy, the bidirectional connection will also allow Sorrento and the nearby area of Torre Annunziata to enjoy stable electricity and a reliable grid, while making use of renewable energy sources and reducing CO2 emissions by 130 thousand tonnes per year. The project has doubtless improved the lives of those who live on the island, as well as that of the tourists who visit, given that it has reduced the risk of blackouts, above all during high use periods such as the summer season. A concrete example of technological quality and efficiency, while respecting the local flora and fauna and the sea.

The Sorgente-Rizziconi cable is located at a depth of more than 350 metres. Operational since 2016, this power line is a unique infrastructure. One of the most innovative projects ever designed and built in the energy sector, it is a triumph for both Italy and Terna. Indeed, its 105 kilometres, which unite Sicily and Calabria to the Italian peninsula, set a new record for alternating current undersea cables. The project benefited from the financial support of the European Union as part of the European Energy Programme for Recovery and, to bring it to fruition, Terna engaged over 150 suppliers and manufacturers to ensure an extremely high level of expertise and technology. From an environmental perspective, its entry into operation enabled 114 of old power lines to be decommissioned (reducing the impact for over 200 hectares) and led to savings of 700,000 tonnes of CO2 emissions each year. The project also enables Sicily's renewable energy production - particularly wind and solar power - to be used to best effect, mitigating outages and reducing the cost of electricity in Sicily.

Meanwhile, the SA.CO.I reaches a depth of 500 metres. This is Terna's “oldest” power line, operational since 1967 and extending for approximately 385 kilometres, 120 km of which are underground and submarine cables. For around twenty years, the SA.CO.I connection was bi-terminal and was used exclusively to transmit energy from Sardinia to Tuscany, using Corsica as a physical bridge for the transfer. In 1987 a conversion terminal was constructed in Lucciana, enabling Corsica to use some of the energy transferred by the connection; the Sa.Co.I thus became the world’s first high-voltage direct-current three-terminal connection, and was renamed SA.CO.I 1. The need to use the best technologies of the time and to transition onto the 400 kV grid meant the creation of two new substations to replace the original ones: one at the same site in Codrongianos and the other moved from San Dalmazio to Suvereto in the province of Livorno. SA.CO.I 2, which entered into operation in 1992 and is still in use, uses the same connections installed in 1967 but continues to guarantee the safe operation of the Sardinian and Corsican networks, helping to regulate the frequency of the two island grids. SA.CO.I 3, currently in the authorisation phase, will enable the renewal and optimisation of the underground and submarine power lines to bring greater stability and reliability to the national electricity system.

With the Italy-Greece connection, Terna descended to 1,000 metres. This power line is one of the ten longest and most powerful submarine high-voltage direct-current connection, with a maximum transmission capacity of 500 MW and a maximum voltage of 400 kV. Opened in July 2002, it extends for 160 kilometres from the electrical substation in Galatina, in the province of Lecce, to the Greek substation in Arachthos, Epirus, connecting the Italian and Greek electricity grids via the Otranto channel. The project was conducted in collaboration with the Greek electricity provider PPC (Public Power Corporation) through financial support from the European Union to enable the exchange of electricity between the Italian and Greek grids.

The Italy-Montenegro power line is even deeper, with a maximum depth of 1,215 metres. The connection represents an essential and international link, providing the first “electricity bridge” between Italy and the Balkans, and is the longest HVDC connection installed by Terna. Its four hundred and twenty-three kilometres of cable run below the waters of the Adriatic, accompanied by 22 km of underground cables: 16 in Italy, from the coastal landing point to the Cepagatti substation, and 6 in Montenegro, from Budva to the Kotor substation. It is another record-breaking infrastructure in terms of technology and innovation, and is of strategic importance for the safety of the two electrical systems and for the integration of renewables. In fact, this submarine power line enables the bi-directional exchange of electricity between the two countries, offering Italy the possibility to consolidate both its national system and its role as a European and Mediterranean electricity transmission hub.

The record, however, goes to the SA.PE.I: the world's deepest submarine cable, laid up to 1,640 metres below sea level. Its 420 km unite Sardinia and Lazio through two undersea cables, able to support 500 kV voltage for 1,000 MW of power; its landing points are two electricity conversion stations in Italy, located in Fiume Santo (Sassari) and Borgo Sabotino (Latina). The entire infrastructure became operational in 2011, with the aim of increasing the safety of the Sardinian system and the reliability of electricity coverage in central Italy.

All of the installation phases of this electricity connection were planned with a view to sustainability, monitoring the sensitive marine environments affected by the line, from the Cetacean Sanctuary to the Posidonia oceanica and Cymodocea nodosa seagrass beds. The installation - which followed the same no-impact line - was conducted by the Giulio Verne, the largest cable-laying vessel in the world. Measuring 125 metres long and 31 metres wide, it is one of only a few ships equipped with the necessary advanced technology to operate up to depths of 2,000 metres, even in adverse weather conditions. With investments totalling € 750 million, the SA.PE.I connection is one of the most important electrical infrastructure projects ever constructed in Italy, and represents an important step towards the creation of an increasingly connected national grid, uniting the north and south.