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Cost effective FCL using advanced superconducting tapes for future HVDC grids

Periodic Reporting for period 3 - FASTGRID (Cost effective FCL using advanced superconducting tapes for future HVDC grids)

Reporting period: 2019-07-01 to 2020-11-30

REBCO (REBa2Cu3O7, RE=Rare Earth) conductors were not economically suitable for SuperConducting (SC) high voltage Fault Current Limiters (SFCL). FASTGRID reduced the cost by nearly one order of magnitude using three ways: i) the tape length reduction through the increase of the electric limitation field ii) the tape cost reduction through production with improved yield and higher critical currents (+ 80 %) iii) the lowering of the liquid nitrogen bath temperature (77 K to 65 K). FASTGRID enhanced the security of operation of SFCL but also most HTS applications thanks to the successful implementation of the Current Flow Diverter (CFD) by a simple and versatile way. FASTGRID explored other innovating directions. A new and low-cost hot spot detection patented system is an important outcome of FASTGRID, useful for all HTS applications. FASTGRID made advances on new REBCO tapes based on a sapphire substrate, a real breakthrough in the SFCL technology. A new conductor called high cp (specific heat) shunt (polymer reinforced with ceramic) brings an attractive option for SFCL conductors.

In short, FASTGRID made possible a leap forward in the performances and cost of SFCL besides the numerous advances about CFD implementation, quench detection, sapphire substrate tapes, high cp shunt, simulation and experimental tools… All the more there is a clear and real demand today for SFCL. Many transdisciplinary works and great collaborations between partners were carried out. Most of these advances are valid for other SC applications so the exploitation of FASTGRID goes beyond SFCL. The works do not stop with the end of FASTGRID. Many of us, companies inter alia, are convinced about the interest of the developments that will be pursued. FASTGRID was a fantastic and essential trigger.
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.
The cost reduction of nearly one order of magnitude for the REBCO conductor is a clear progress beyond the state of the art. FASTGRID showed the strong reduction of the capital cost (factor 4.4) as well as the capital plus operational costs (factor 2.9) of a 320 kV DC SFCL based of the conductor developed within FASTGRID compared to a “conventional” one. Works comparing different solutions within another EC project (PROMOTIoN) showed a real and clear interest of the SFCL for HVDC Supergrids from technical and economical points of view.

Thanks to FASTGRID and a better understanding of the process, the THEVA production line of REBCO SC tapes has progressed a lot with an increase of the critical current (+ 80 %) and its homogeneity (from 10 % down to 3-4 %) along its length. The impact of these significant advances oversteps the SCFCL domain. These advances are very important for nearly all applications of the tapes and they enhance THEVA’s competitiveness at international level.

The successful implementation of the Current Flow Diverter to enhance the Normal Zone Propagation Velocity experimentally by one order of magnitude in another important impact and outcome of FASTGRID, all the more the CFD tapes are of large interest for most SC applications, not only for SFCL.

NEXANS, as SGI industrial shareholder, has gained in competitiveness in SCFCL thanks to the optimized lower price conductor developed within FASTGRID for medium voltage SCFCL on the shelf today.
FASTGRID Project Meeting 7 in Milan