R for Rectiformer

Hands up, everybody who’s heard of a rectiformer before. You almost certainly haven’t, unless you’re an employee of Terna, the company that manages the Italian national transmission grid, and a member of the work group currently studying and developing this piece of equipment, which has the potential to revolutionise the electricity sector. Even in Italian, the name they have chosen for it — rettitràfo — is quite unheard of in everyday vocabulary. In both languages, it is a portmanteau of two other words: "rectifier" and "transformer" ("rettificatore" + "trafo" in Italian). Though not the most elegant name, with a rather old-fashioned ring to it, the piece of equipment to which it refers might prove to be a real shot in the arm for the development and further spread of direct-current lines. In simple terms, a rectiformer is an innovative electrical machine capable of combining the functions of an electrical transformer with those of an AC/DC converter.

The project started out in Terna in 2017, thanks to a brainwave from Vincenzo Agnetta (of the Process Technologies and Systems team, in the Grid Development Strategies and Dispatching department). He realised the potential benefits of an integrated solution for the management of energy conversion and transformation. Next came the decision to invest significant resources in the development of the project, including a collaboration with the Politecnico di Milano University: a concrete example of how a synergistic, multidisciplinary approach can see an idea transform and blossom into a tangible innovation for the energy sector.

As Agnetta, the inventor of the rectiformer, explains, what makes it special «is its unique design, which integrates power conversion modules directly into the internal structure of the transformer. By doing so, the same device can handle the stages of both transforming and converting electricity, ensuring optimum usage of space and reducing the complexity of the infrastructure needed».

The project was born out of the need to find solutions to overcome the challenges posed by the energy transition, which is characterised by a progressive, inexorable increase in the production of energy from renewable sources, particularly wind and solar power. However, these sources are intermittent and non-programmable by their very nature. As a result, it is necessary to create new electrical interconnections in order to transport and integrate more and more green energy from the zones with the greatest production potential, thereby increasing the sustainability and resilience of the entire system. Indeed, this power must often travel long distances, via submarine cables and cross-border connections. Therefore, in technical-economic terms, it is preferable to use so-called High-Voltage Direct Current (HVDC) infrastructure, which requires complex and costly conversion stations.

This is where the rectiformer may prove to be a game-changer in enabling the energy transition, in a number of ways. First of all, it allows for optimum usage of space: by combining the functions of transformation and conversion in a single device, the rectiformer may actually reduce the need to construct large buildings and dedicated infrastructure. This undoubtedly results in a reduced impact on the landscape, as well as simplifying the conversion stations. It also means greater sustainability: by reducing the visual and environmental impact of the infrastructure, the rectiformer could support Terna’s ESG (Environmental, Social and Governance) goals, helping to pave the way for a more sustainable electricity system.

Another advantage consists of the reduction of costs and times: by eliminating the need to build separate structures to hold the HVDC converters and the so-called electricity filtration systems, the rectiformer could lower the total costs of civil engineering works and speed up the construction and authorisation of infrastructure. Moreover, the rectiformer would also have a role to play in the management of faults: in fact, its specific design would make it easier to intervene in the event of malfunctions on direct-current lines.

Mattia Amatruda, engineer and project manager on Terna’s innovation team, reveals that the machine has been designed «to temporarily halt the conversion of alternating current to direct current, allowing the alternating current to enter the HVDC line. Thanks to this option, normal alternating-current switches could be used, cutting out the need to develop expensive and complex direct-current switches, which are not currently available at commercial scale».

Project stages, from design to patent. The process of developing the rectiformer involved various different work groups within Terna, with the participation of the Innovation and HVDC Technologies areas in particular. By combining Terna’s skills with those of the professionals at the Politecnico di Milano University, it was possible to develop a detailed software model of the machine, used to conduct advanced simulations of its behaviour. These studies enabled the definition of a low-voltage prototype design, and the validation of the operating principles behind the rectiformer.

Subsequently, the low-voltage prototype was built and underwent testing in the laboratories at the Politecnico di Milano University. This collaboration with a research centre of excellence played a vital role in confirming the soundness of the rectiformer in terms of physics and mathematics, allowing Terna to file a first patent on the grounds of the machine’s novelty, inventive step and industrial application. A second patent is already in the works, regarding the rectiformer’s control methodologies.

«One of the most significant aspects of this project is the collaboration between the worlds of industry and academia. The partnership between Terna and the Politecnico di Milano University has made it possible to combine advanced technical skills with operating capacity, speeding up the process of developing the prototype», notes Carla Napolitano, Head of Innovation at Terna.

Francesco Castelli Dezza and Nicola Toscani, full professor in the Department of Mechanical Engineering and postdoc at the Politecnico di Milano University respectively, also commented on this factor: «One of the university’s fundamental missions is to make a significant contribution to industrial research, supporting the practical development of solutions which can be applied in the field in relatively short times. We are very grateful to Terna, therefore, for choosing us as their partner in conducting research on the rectiformer. The converter is not only the result of a brilliant brainwave, but it might prove to be a real shot in the arm for the development and further spread of direct-current lines».

If the rectiformer is found to be scalable for medium- and high-voltage — the latter being the one managed by Terna — it may have a significant impact not only on the Italian electricity grid, but at international level too. Because transmission grids all over the world share similar operating principles, the rectiformer may become a globally implemented solution for the management of direct-current infrastructure. The adoption of this technology could simplify the construction of major works, such as international interconnections and submarine power lines, improving the resilience and efficiency of electricity grids.

In this sense, the rectiformer can also be seen as a symbol of a strategic approach to innovation, based on investment in research and development and fully harnessing human capital. Once again, Terna has shown its ability to anticipate future challenges, combining sustainability, technology and collaboration. And the rectiformer is just one of the most intriguing among the projects which the Italian national transmission grid operator is carrying out in this field: with around 2 billion euros in investments in innovation and digitalisation, of the 16.5 billion euros total contained in the 2024-2028 Industrial Plan presented last March, Terna demonstrates an outstanding commitment to sustainability and technological innovation, thanks also to a detailed R&D strategy.