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FASTGRID Report Summary

Project ID: 721019

Periodic Reporting for period 1 - FASTGRID (Cost effective FCL using advanced superconducting tapes for future HVDC grids)

Reporting period: 2017-01-01 to 2017-12-31

Summary of the context and overall objectives of the project

Sustainable development goes through the transmission of huge volumes of renewable electricity over long distances and the multi-terminal HVDC (MTDC) grid appears as one of the best solutions. But the full selective protection of these MTDC remains an issue because current solutions aren’t satisfying. Superconducting Fault Current Limiters (SCFCLs) bring an innovative solution but they have to be safe and cost effective: these are the objectives of our project FASTGRID (Cost effective FCL using advanced superconducting tapes for future HVDC grids). FASTGRID seeks to develop and provide a superconducting REBCO conductor suitable from economic and technical points of view.
The today REBCO conductors are not properly suitable for operation at high voltages (> 100 kV): the electric field developed in the tape during the current limitation is still too low (approx. 50 V/m for 50 ms) which means that the FCL requires prohibitive lengths of tape. FASTGRID aims to improve the properties of the REBCO conductor in order to enhance significantly, by 2 to 3 times, the electric field limit and its intrinsic protection to provide a cost effective and safe solution for HV applications. The cost reduction indispensable to enlarge the spreading of SCFCL is obtained by three ways: i) the cost reduction of the tape length through the increase of the electric field under limitation ii) the cost reduction of tape through production with improved yield and higher critical currents iii) the lowering of the temperature of the liquid nitrogen bath at 65 K. Lower temperature allows reduction of tape quantity and overall cost as compared to 77 K operation. Liquid nitrogen is advantageous: industrial low cost coolant, good exchanges, satisfying dielectric properties.
We plan to start from THEVA tapes and upgrade them in order to reach a high critical current (> 1000 A/cm-w @ 65 K, sf) with improved homogeneity over the length (less than 10% of fluctuations in critical current) and with enhanced Normal Zone Propagation Velocity (NZPV) through Current Flow Diverter (CFD) innovative and unique concept. Substantial work is also planned on the shunt layer to reach high electric fields under limitation and exclude destructive hot spots. A functionalization of the conductor surface will improve the exchange properties. The conductor will be used in a SCFCL module (1.2 kA – 50 kV) tested at 65 K. This module will be designed with the high voltage requirements and will include a quench detector based on optic fibre. A dedicated DC fast breaker will be developed. The SCFCL module will be extensively tested in an industrial environment as a sub-element of a real HVDC device and the conductors will be brought to the next level of technology readiness.
FASTGRID also focused on advanced tapes based on sapphire substrate, which can tolerate ultra-high electric fields (in the range of kV/m). Here we seek to bring this breakthrough sapphire route to the technology development stage (TRL5). This game-changing technology needs to be implemented at long lengths with an industrial process.
Succeeding in implementing these innovations at an industrial scale promises real breakthroughs in the SCFCL business. Availability of such advanced conductor will have an impact on virtually all other applications of HTS tapes.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

WP1 Advanced and emerging REBCO Tapes
Activities were focused on the implementation of CFD through Ink Jet Printing (IJP) method, which perfectly fits. CFD is based on a connection of the superconducting layer to the Ag layer only on the edges, where the central part is electrically isolated thanks to 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 (fluorescent imaging) has been developed to measure the NZPV. Electrically insulating shunts (e.g. Stycast) have been investigated to reach high electric fields under limitation. Sapphire substrates with YSZ/CZO on top are investigated also. Difficulties to use unpolished sapphire substrates have been identified. The substrate waviness degrades epitaxy. The CZO spin coating and IJP deposition conditions have been adapted. Two approaches are being explored to tackle this isssue: i) increase the thickness of the CZO cap layer ii) perform the CSD-YBCO growth on a sputtered YBCO seed layer/YSZ/EFG sapphire received from Tel-Aviv University (TAU). Results confirm the proof-of-concept of double-sided REBCO sapphire substrate to be economically very attractive because the length of substrate needed for the FCL is halved. Simulations have confirmed the NZPV enhancement even for thick Hastelloy® shunt and the interest of the CFD even for sapphire substrates.
WP2 Long length REBCO tapes for demonstration
A lot of effort has been dedicated to improve the Ic amplitude and homogeneity. The origins of the Ic local have been systematically 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 reduced by 2 to 3 and that is now lower than 5%. The initial objectives of 500 A/cm-w at 77 K and a deviation lower than 10% are already overcome.
WP3 Functional material for devices for FCL smart Module
The FCL module will operate under 50 kV DC with an operating current of 1.2 kA DC. It will be immersed in subcooled liquid nitrogen at 65 K with a pressure of 3 bar absolute. The module will be composed of 10 sub-modules called pancakes and connected in series. The pancakes use the bifilar configuration. The implementation takes into account the high voltage constraints and a specific electrical isolation was designed and qualified. The implementation of an optical fibre using the optical time domain reflectometry technique to detect a hot spot is under study. The cryostat for the module including the instrumentation has been specified. A dedicated DC circuit breaker is under design to quickly isolate the module. The clearing time including a back-up sequence is 50 ms.
WP4 Validation and demonstration tests
This task has started in June 2017. The first tests have been performed on short samples from THEVA and TAU (sapphire substrate). Tests have shown that the quench is less homogeneous at 65 K compared to 77 K and confirmed very high electric fields on sapphire substrate. In addition AC losses measurements were made.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

FASTGRID will significantly improve the state of the art of REBCO tapes suitable for SCFCL. “Fast” charging magnets will also benefit from the project and the implementation of SCFCL in the grids will speed up thanks to more economical and robust conductors. In addition, several material development routes will be explored including new advanced shunt and thermal stabilization concepts.
After the FASTGRID project, the SCFCL devices will reach the requirements for commercialisation. The industrial manufacturing industries will be the providers of the advanced REBCO tapes. The market is estimated at several hundred millions of Euros only for Supergrids.

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