Some Results of the EU Project FASTGRID

The EU FASTGRID project aimed at improving the REBCO conductor in order to enhance its economical attractiveness for the Superconducting Fault Current Limiter (SFCL). Two approaches were simultaneously investigated: i) the reduction of the tape length through the increase of the electric field during limitation, ii) the reduction of the tape cost through improved yield and higher critical current, including the lowering of the liquid nitrogen bath temperature to 65 K. We carried out a base line using an upgraded THEVA tape. Different approaches have been tried to improve the shunt layer, both with metallic and non-conductive materials. As a first solution, we developed and successfully tested a conductor with a Hastelloy shunt able to withstand 130 Vrms/m during 50 ms at 65 K and low prospective current faults. This conductor was used in two pancake coils. These have been successfully tested at IPH in Berlin. The second shunt solution developed consisted of a ceramic in epoxy matrix coating. Another significant improvement of the tape is the successful implementation of the Current Flow Diverter (CFD) concept with a simple sulfidation process or a deposition of an insulating yttria nanolayer by Inkjet Printing. An increase by one order of magnitude of the Normal Zone Propagation Velocity with respect to tapes without CFD has been measured. A low cost and fast hot spot detection system has also been developed. To reach much higher electric fields under limitation, the FASTGRID team also developed advanced tapes based on a sapphire substrate, which can tolerate ultra-high electric fields up to about 1 kV/m. Validated at laboratory scale, this game-changing technology needs to be implemented in long lengths with an industrial process. One pancake was integrated and tested with a low-cost optical fiber and fast hot spot detection system, developed and patented within the FASTGRID project. This manuscript will showcase most of these FASTGRID results.