Integrated approach to the Design and Control of Offshore HVDC Networks

The integration of large volumes of renewable energy systems into the existing power grid constitutes one of the main pillars towards the much needed low-carbon economy. Nonetheless, due to their intermittent nature and uneven geographical distribution, major developments in grid infrastructure are needed. In particular, to fully harness the potential of the North Sea offshore winds and convert it into useful electricity, it is necessary to transfer high amounts of power over very long distances, up to the places where this electricity is needed and consumed. Although Multi-Terminal High-Voltage Direct-Current (MT-HVDC) transmission systems have been identified as the main viable solution for this application, many questions on planning, analyzing and operating an HVDC complex network, connecting many terminals, are still open. Furthermore, since these terminals are expected to be multi-vendor by nature, the question of interoperability (stability & performance) naturally arises. Moreover, taking into account the uncertainty of the final state of the continuously growing interconnection, as well the confidentially agreements to which the vendors of HVDC terminal controllers are subject, Plug & Play operation of the power-electronic stations is likely required. It is in this context that the first research line of the IDeCON project has been developed. Particularly, the project aimed at showing that a minimum set of Plug & Play control requirements can be demanded from the vendors through future stability-grid codes, that would in turn guarantee an appropriate and safe operation of the MT-HVDC grid. As a first approach towards this ambitious objective, the research carried out in the IDeCON project focused on challenging conventional power converter stations inner loop controllers, by investigating control alternatives with plug-and-play features. The underlying control-theoretic framework of this research can be broadly referred as "energy modelling and control" of physical systems. The theory uses the physical structure of the system under consideration to design control laws that shape its energy flow, while providing the desired plug-and-play behavior described above. Within this research project, the theory has been successfully extended to the modular multilevel converter (MMC), the most promising power converter for the MT-HVDC application. As an additional research line of IDeCON, investigations have been directed to understanding if it is possible to obtain indications on the stability margins of HVDC grids, with focus on DC voltage stability, under the worst-case operational scenario that can be expected. This could provide important indications when designing the system, or planning its further expansions, thus bridging the domain of power system planning and stability assessment. Additionally, it can constitute the ground for designing advanced converter controllers that can actually maximize stability margins under such identified worst-case scenarios. Following this approach, the proposed integrated methodology for worst-case scenario identification, stability assessment and controller design has been positively tested on a representative test case. Such test case is based on MMCs for the interconnection of pre-existing point-to-point HVDC links into a multiterminal configuration.

IDeCON outcomes were a competence leap for the leading partner and transfer of knowledge to industrial partners (particularly, developed grid analyses and modeling methodologies) through dedicated events. IDeCON strengthened the exchange of best practices among all partners and NTNU's cooperation with Tecnalia, so that a further researcher's stay has already taken place and one is being planned. It also fostered a new and ongoing cooperation with ETH. IDeCON technical results set the basis for a change in planning and control of AC/DC high voltage networks, enabling the integration of more renewable energy and providing society with a more sustainable, efficient and reliable power supply. Moreover, developed tools and approaches could be exported to proximal fields, such as hybrid AC/DC distribution networks, thus broadening the project outcomes. All of this is perfectly in line with the European Green Deal and its quest for energy efficiency and zero net gas emissions by 2050.

The IDeCON project is developed in the field of offshore energy systems. It will create internationally competitive and highly multidisciplinary competence in the strategic area of design, control and optimization of offshore grids. The project has the potential to overcome the technical barriers that are presently hindering the practical implementation of High Voltage DC grids in the North Sea, which are necessary for the exploitation of the huge local wind resources offshore. The project is perfectly aligned with both the priority areas "Energy system of the future" and "Climate friendly (renewable) energy" of the Call for Proposals and it will provide qualifying research training to two fellows, through the funding of one PhD and one Post Doc position. The cornerstones of the project are scientific excellence and a highly-integrated approach to the complex, cross-disciplinary problems of offshore grid, tackled by exploiting both the in-house competence of the applicant institution and top-qualified international collaborations.