C. CREUSOT, A. BERTINATO, P.-B. STECKLER, D. BRASILIANO, N. DEVEAUX – SuperGrid Institute
A. MORANDI, E. GUERRA, M. SIMONAZZI, M. FABBRI – University of Bologna Italy
G. ANGELI, A. MUSSO, M. BOCCHI – Ricerca Sistema Energetico

Abstract:

To cope with the transfer need of GWs level wind power to continental grid, the current solution is to use HVDC systems with the tendency to increase the DC voltage typically to 320 kV or 525 kV and employ an XLPE HVDC cable. The trend to operate at higher voltage levels is dictated by the converter technologies, typically using IGBT semi-conductors, and by the XLPE copper cable technologies. Both semiconductors and cables are limited in the amount of DC current they can handle, typically ranging from 2,000 A to 3,000 A. As a result, increasing the transmitted power generally requires increasing the voltage. Another option to overcome current limitations is to parallel semiconductor devices at different levels to increase the rated current. The same approach applies to cables: multiple cables can be installed in parallel to enhance the total current-carrying capacity. However, this solution significantly increases the footprint (right-of-way) and the copper usage of the installation and therefore its overall cost.

To address these challenges, several manufacturers have leveraged superconductivity, bringing superconducting cable technology to an industrial level. Superconducting cables can be designed for medium or high voltage applications and are capable of carrying extremely high continuous currents, exceeding 10 kA.

The SCARLET project, funded under European Union’s Horizon Europe research and innovation programme, started in September 2022, brings together industrial companies and research organizations to demonstrate the industrial feasibility of rethinking large-scale Renewable Energy (RE) transmission. Instead of relying on high voltage DC, the project explores the use of medium DC voltage enabled by superconducting (SC) cable as an alternative means of power transport.

Two different SC cables technologies are considered for both offshore and onshore applications, together with their respective protection schemes. They operate at a voltage level that can potentially eliminate the need for costly offshore conversion stations. High temperature SC, specifically using Rare-earth Barium Copper oxide (ReBCo) as the conductor material and cooled with liquid nitrogen is particularly suitable for offshore application. Alternatively, low temperature Magnesium diboride (MgB2) SC cable, cooled with liquid hydrogen, could also be adopted, particularly for in-land installations. In this case, the system is proposed as a bi-energy transport solution, combining both hydrogen and electricity.

Within this scope, an electrical system architecture and its protection scheme is elaborated.

As a reference case, a 1 GW system is described, operating at ± 50 kVdc, using a High Temperature SC to export power from an offshore wind farm to onshore grid. To reach this objective, the wind turbine generator conversion chain is redesigned to export power directly at DC medium voltage, eliminating the need for additional conversion between windfarm strings and the transmission cable. On the onshore side, a medium voltage converter architecture and associated AC switchyard are proposed.

The protection strategy is analyzed with the target of protecting the cable and converter against DC short circuit faults as well as maximizing the continuity of service. Moreover, a Resistive Superconducting Fault Current Limiter (RSFCL) used in coordination with mechanical DC Circuit Breakers is considered in the protection scheme. To analyze the interaction between the SC cable, converters, breakers and RSFCL, detailed multiphysics models of the SC cable and the RSFCL are implemented into EMTP-RV software as well as an Average Arm Model of the onshore converter.

Finally, after discussing the simulation results, an overview of the status of the two types of SC cable systems and the RSFCL demonstrators will be given.