The electricity grid of the future rests on HVDC technology

At present, the European transmission grid mainly works with alternating current. Yet the profound transformation of the markets and of energy sources, with non-programmable renewable systems becoming widespread, is making it necessary to use HVDC (High-Voltage Direct Current) connections more and more often. This is an established technology, in use for over 60 years, which has recently undergone significant progress. It enables the connection of offshore systems, such as wind farms located out at sea, in order to establish interconnections between electricity grids. In addition to all this, it offers excellent energy efficiency, system stability and a reduced environmental impact.

During the recent Technology Watch organised by Elettricità Futura (the leading Confindustria association from Italy’s electricity sector) in partnership with CESI (the Centro Elettrotecnico Sperimentale Italiano), entitled "Infrastrutture elettriche, accumuli e flessibilità: soluzioni e tecnologie per la transizione e la sicurezza energetica" (Electrical infrastructure, storage and flexibility: solutions and technologies for energy transition and security), Bruno Cova, the Power System Excellence Manager at CESI, stated that 58% of Europe’s new extra-high-voltage transmission lines will be in direct current. This figure reflects the converging pathways of technological evolution, environmental needs and sustainability constraints.

Time has therefore become a crucial factor in Italy's energy transition. Decarbonisation is no longer a future prospect, but a process which has already begun, and which requires modern, digital infrastructure which is compatible with our environmental challenges. In this context, Terna — as the national electricity grid operator — is a key player in steering the electricity system towards a zero-emissions future, leveraging strategic innovations that combine sustainability and technology. And one of these innovations is HVDC: the technology by now considered an essential element of large energy corridors, thanks to its ability to transport large quantities of electricity across very long distances with lower losses than alternating current.

While HVDC has become more widespread over the past 15 years, the earliest connections using this technology actually date back to the 1960s, like the Sa.Co.I. direct-current line between Sardinia, Corsica and Italy (Tuscany), currently the focus of a huge modernisation project. The HVDC interconnection with Greece, on the other hand, dates from 2002, while the SA.PE.I., running between Sardinia and the Italian mainland (Lazio) since 2009, set the most recent record for the depth of undersea cable installation. Meanwhile, in more recent terms, electrical interconnections have been activated with Montenegro (2019) and France (2023). All these projects confirm Italy’s leadership at continental level, with their innovative, strategic works of engineering excellence

«HVDC technology — an area in which Italy has always been at the cutting edge, from designing to constructing and commissioning cables and systems — is an immense engineering challenge of crucial importance for the energy transition. In fact, infrastructural projects using direct current are strategic for the interconnection of grids, including those which are very far apart, and for the improvement of energy transmission,» explains Maria Rosaria Guarniere, who leads the Engineering and Project Execution department at Terna. «From this perspective, the major marine works that we are building or which we have planned are Terna’s sustainable response to the constantly growing demand for energy, using innovative and effective solutions that minimise interference with the environment,» adds Guarniere.

Major projects. The Italian strategy for the development and strengthening of the electricity grid is based on many projects, which are important at the national level and beyond. The most ambitious of these is undoubtedly the Tyrrhenian Link, a submarine HVDC connection with a length of around 970 kilometres which will link Sicily to Sardinia and Campania. In addition to strengthening the interconnection between the islands and the mainland, the project will pave the way for greater integration of production from renewable sources, increase grid security, and reduce congestion. All of this will be achieved while minimising the environmental impact, thanks to submarine and overland cables plus cutting-edge technologies for the electrical substations. Similarly, the Adriatic Link, which will connect the regions of Abruzzo and Marche will rely on direct-current transmission to strengthen the transport of energy between the Centre-North and Centre-South of Italy. Both projects are concrete examples of how Terna is building a high-capacity, low-impact electricity highway, taking action in advance of European directives on grid sustainability.

And that’s not all: on the topic of the Adriatic Link, Terna launched the Adriatic Terna Innovation Zone in June 2025. This is the new innovation hub which the company has decided to open in Marche, with the aim of paving the way for the transformation of the region, and of the Adriatic area in general, into a centre of excellence, developing technological innovation to support the energy transition and bolstering the innovative entrepreneurial ecosystem throughout the Adriatic. Its initiatives include the OpenLAB: a laboratory for experimenting with advanced engineering solutions, particularly in the field of high-voltage direct current cables and technologies in the maritime sector, which are closely intertwined with the issues of sustainability and energy transition.

Terna’s vision extends beyond individual projects to set its sights on the development of an all-round model. In fact, in its 2025 Development Plan, Terna set out a series of projects, some with timelines extending beyond 2034, which will enable a significant increase in energy exchange capacity between market zones, bringing the current 16 GW to around 39 GW. Taken all together, they make up a "smart" grid integrating HVDC and alternating-current solutions, sensors, digitalisation and dynamic flow management. The model will transform the electricity grid from a simple physical infrastructure into an interactive cybernetic system, capable of adapting to an increasingly variable and decentralised energy mix in real time.

Making the most of what’s already there: the "non-new grid solutions". One of the strengths of Terna’s strategy is its adoption of solutions that can increase the capacity and resilience of the grid without building new lines. This is the case of the so-called “non-new grid solutions”, a set of technologies that strengthen existing infrastructures and infrastructural corridors. Particularly noteworthy among these is "reconductoring”: the replacement of traditional conductors with those of the latest generation, which can transport more power without invasive interventions on the landscape. There is also the modernisation of existing backbones, to improve operating performances in alternating current as well as increasing transport capacity thanks to direct-current operation.

Another crucial element is Dynamic Thermal Rating (DTR), a real-time monitoring system which, thanks to the use of sensors installed on pylons and conductors as well as meteorological models, optimises the load of overhead power lines based on environmental conditions as well as certain parameters like temperature and wind. Essentially, the system makes the grid more “smart”, flexible and high-performing.

Cables laid at extraordinary depths. Supporting this infrastructural and technological transformation are immense efforts in research and development, with trials carried out in highly technologically specialised laboratories, which allow components and systems to be tested prior to their installation. In fact, each of these must be able to operate with the utmost reliability under stress for decades. Italy has claimed its place as a global pioneer in laying submarine cables at extreme depths, with historic records: -1,000 metres for the Italy-Greece connection in 2000 and -1,600 metres between Sardinia and the mainland in 2008. Work is already underway on projects at even greater depths, as far as 2,150 metres below sea level in the plans for the Tyrrhenian Link, with the use of synthetic fibre reinforcements like kevlar and advanced laying systems. In this area, one aspect which is overlooked but crucial concerns the logistics of installation: new cable-laying ships have been designed to install HVDC cables at extreme depths, with a precision that minimises the impact on the seabed and reduces build times.

«At present, HVDC is the only technology that can guarantee the transport of large quantities of energy across long distances using submarine cables», notes Riccardo De Zan, head of Project Management Centre of Expertise at Terna. «It’s a very complicated solution, including in terms of obtaining and procuring the components, but it offers enormous electrical advantages over alternating current. It’s an immense technological challenge, not just in terms of the depth at which the submarine cables are laid, but also regarding the use of materials and production techniques which, as progress continues, pave the way for innovative and sustainable solutions for the protection of the cables too.»

«The submarine cable is, without doubt, the most fascinating part of this technology, but the electrical substations are also highly complex and just as much of a challenge. Today, these are advanced, digitalised systems, but there are still a few suppliers capable of planning, producing and constructing them», continues De Zan. «Another aspect of HVDC which is not to be underestimated», he tells us, «is that while the technology is used in Northern Europe and other countries primarily to transport large quantities of energy, we have adopted the solution in Italy also thanks to its ability to regulate grid flows and for greater stability in the grids on the larger islands. What’s more, it also has a role in the "recovery" of structurally weak grids, such as those in Sardinia and Sicily.»

A strategy with an eye on Europe and the Mediterranean. Terna's strategy is part of the broader European plan for decarbonisation. The Grid for Speed plan launched by the EU Commission sets out investments of 60 billion euros per year in strengthening grids, over 6 billion of which are allocated to Italy. Italy, with its geographical position and industrial skills, can therefore consolidate its role as an energy hub for Europe and for the Mediterranean area, thanks to the 30 interconnections which are already active with France, Austria, Switzerland, Greece, Slovenia, Malta and Montenegro, as well as those which are planned or being constructed, including the connection with Tunisia (the Elmed project) and the doubling of the Italy-Greece line.