Initially FASTGRID aimed to reduce the superconducting conductor cost by a factor 5 by lowering the operating temperature and increasing the electric field under limitation. We succeeded nearly to double this ambitious goal by achieving a cost reduction of 9.75 using a thick Hastelloy® shunt. The additional costs of the conductor due to the thick Hastelloy® shunt and of the cryogenics are not considered, but limited anyway. A complete techno-economic analysis confirmed that the conductor cost dominates the total device cost and so the high interest of FASTGRID. Some measurements have shown that still higher values are possible (+ 30 % in terms of electric field). This result is due inter alia to the work of THEVA about the super¬con¬duc¬ting tape with the thorough analysis of the whole elaboration process. It led to critical current per unit width much higher than foreseen at the beginning (+ 30 %).
The conductor has been used to wind two 5 kV coils fully representative of future SFCL basic elements. These two coils were successfully tested in Berlin at IPH.
A thorough and unique Life Cycle Analysis (LCA) was carried out on the THEVA and OXO tapes. This LCA has quantified the large environmental advantage of a SC conductor compared to a copper one for most categories, especially acidification (20 %), freshwater ecotoxicity and eutrophication (30 %), marine eutrophication (25 %), particulate matter (20 %), photochemical ozone formation (30 %)…
Even if the developed conductor shows secure operation whatever the fault conditions are, FASTGRID has implemented the CFD (Current Flow Diverter) proposed by EPM in the tape architecture. This patented configuration makes possible to significantly enhance the Normal Zone Propagation Velocity (NZPV) and so the security of operation by spreading the energy dissipation. The CFD is based on a thin electrical insulating layer over a large part of the interface between the superconducting and metallization layers. Several strategies have been investigated. The most promising is based on a silver sulfide insulating layer whose industrial implementation is rather easy. This time the CFD effect was clearly experimentally demonstrated. The exploitation of a CFD tape may be broad: the CFD process could be included in the future for all Coated Conductor REBCO tapes.
Beside the Hastelloy® shunt route, conductive polymer-based shunts were investigated to reach even higher electric fields. This high cp shunt is made of epoxy resin combined with ceramic fillers (Al2O3, SiC, glass fibers) to fit the thermal expansion of the tape. Reduction of the maximum temperature reached during limitation process has been confirmed experimentally. FASTGRID has demonstrated the possibilities of this emerging solution.
Sapphire substrate tape is a total game-changing technology to reach ultimate electrical fields, at least one order of magnitude higher (2 kV/m experimentally measured) than with Hastelloy®-based tapes, while presenting intrin¬sically high NZPV. This route was at a low TRL level at the beginning of FASTGRID which aimed to develop this breakthrough route and increase its TRL. The studied architecture is sapphire/YSZ/CZO/YBCO; it has been thoroughly investigated. FASTGRID made possible a lot of advances about the sapphire route both from theoretical and elaboration points of view. Several phenomena are much better understood. Pieces of 1 m have been produced. The next challenge is the reel-to-reel manufacturing but FASTGRID paved the way in this direction.
A highlight of FASTGRID is the patented optical fiber sensing technique to quickly detect a hotspot whereas the detection system is very low cost with a simple implementation. This new technique, based on Mach-Zehnder technique, was validated on a few meters (2 x 6 m). The hot spot detection time is within 10 ms. As for the CFD implementation, this hot spot detection system has a great potential of exploitation for most SC applications.
Thanks to FASTGRID, recommendations were developed to test DC SFCL and we think that these recommendations will become standards soon.
