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Cost effective FCL using advanced superconducting tapes for future HVDC grids

Periodic Reporting for period 2 - FASTGRID (Cost effective FCL using advanced superconducting tapes for future HVDC grids)

Reporting period: 2018-01-01 to 2019-06-30

Today's REBCO conductors are not properly suitable for SuperConducting Fault Current Limiters (SCFCL) at high voltages. FASTGRID aims to reduce the cost by 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 iii) the lowering of the liquid nitrogen bath temperature.
WP1 Advanced and emerging REBCO Tapes
The general aim of this task is to develop REBCO tapes fully suitable for SCFCL. Three main approaches are investigated:
• CFD (Current Flow Diverter) implemented in commercial REBCO tapes to increase the NZPV (Normal Zone Propagation Velocity).
• Development of high thermal capacity shunt to enhance the electric field under limitation.
• Development of high Ic RECO tapes on sapphire substrates, a breakthrough solution.
The implementation of CFD is performed through the Ink Jet Printing (IJP) method, which perfectly fits to deposit nanocoatings on long lengths. CFD is based on a connection of the SC layer to the Ag layer only on the edges, where the central part is electrically isolated thanks to a dedicated layer and we chose Y2O3. Work has been carried out in order to optimize the thickness homogeneity of the Y2O3 layer and the REBCO oxygenation. The current density Jc is now approaching the original one. A new experimental visualization method has been developed to measure the NZPV and with other measurements, it shows the effective NZPV enhancement of the CFD tapes in agreement with the simulations.
Electrically insulating shunts (Stycast® composite) have been investigated to reach high electric fields under limitation. The Stycast® composite has been studied to adapt its coefficient of thermal expansion to the tape. Simulations show the decrease of the temperature rise under limitation.
Sapphire substrates with YSZ/CZO on top are investigated. This solution is a breakthrough for SCFCL since electric fields higher than 1000 V/m are then possible. Difficulties to use unpolished sapphire substrates have been identified. The CZO spin coating and IJP deposition conditions have been two adapted approaches. FASTGRID develops that game-changing technology. Ic-w as high as 390 A/cm-w (77 K, sf) and 750 A/cm-w (65 K, sf) have been recorded. Different thicknesses of the sapphire substrate have been investigated. 2000 V/m has been recorded with 1 mm thick substrate.

WP2 Long length REBCO tapes for demonstration
A lot of efforts have been dedicated to improve the Ic value and homogeneity. At the beginning of FASTGRID the critical current was about 330 A/cm-w (77 K, sf) with a Ic homogeneity of 10 %. The origins of the Ic local decreases have been systematically and thoroughly investigated and corrected in part. A new process with a 3 µm thick REBCO layer and the optimization of the tape temperature with its strict control during the deposition of the HTS led to an Ic 30 % higher with a standard variation lower than 5 %. The initial objectives of 500 A/cm-w (77 K, sf) and a deviation lower than 10 % are already overcome.

WP3 Functional material for devices for FCL smart Module
This task aims to i) design and optimize a SCFCL, ii) manufacture the smart conductor (tape + Hastelloy® shunt + quench detection), iii) realize the FCL module including the cryostat iv) design the DC breaker associated.
The FCL module will operate under 50 kV DC with an operating current of 2 kA DC. It will be immersed in subcooled liquid nitrogen at about 67 K. The module will be composed of 10 sub-modules (bifilar pancakes). The pancakes have been thoroughly designed and qualified.
The adopted conductor uses a 500 µm thick Hastelloy® shunt soldered on the THEVA tape. This bonding is performed by dedicated lamination process. This architecture ensures the safe operation whatever the faults and including a failure of the main switch. Optical fibers are put on the substrate to detect the quench even localized (“hot spots”) making the conductor smart. EPFL has introduced within FASTGRID a new detection concept using the phase shift of the signal due to a temperature change. Demonstrated on mock-ups this technique makes the quench detection very easy and low cost.
A dedicated DC circuit breaker has been fully designed to quickly isolate the module. The clearing time including a back-up sequence is 50 ms. All
FASTGRID has several impacts:
• OXO has now a dedicated pilot line for CFD implementation on SC tapes.
• The THEVA production line of SC tapes has progressed a lot with an increase of Ic and its homogeneity.
• THEVA has now an advanced production facility to bond a thick shunt on a SC tape.
• A full LCA (Life Cycle Assessment) for the SC conductor helping to make the best choices.
• New method of KIT using fluorescent coating of HTS to investigate the quench propagation.
• SGI has a production line for SC pancakes from several conductors.
• EPFL has developed a simple and low cost detection method for quench using fiber optic.
• SGI has a production line to lay the optical fiber on the conductor.
• SGI has a new innovative and competitive product: SCFCL for high DC voltages.
• SGI has now a dedicated DCCB.
• NEXANS has gained in competitiveness in SCFCL thank to the optimized lower price conductor.
• Development of recommendation to test DC SCFCL.