Tuning the critical currents in bulk MTG YBCO for current limiting devices

• Published in: Superconductor science & technology Vol. 13; no. 6; pp. 879 - 885
• Main Authors: Obradors, X, Puig, T, Mendoza, E, Plain, J, Figueras, J, Granados, X, Carrillo, A E, Varesi, E, Sandiumenge, F, Tixador, P
• Format: Journal Article
• Language: English
• Published: IOP Publishing 01.06.2000
• ISSN: 0953-2048; 1361-6668
• DOI: 10.1088/0953-2048/13/6/353

A hybrid superconducting fault current limiter (HSFCL) has been developed consisting of a transformer whose secondary is connected to a series of parallel YBCO superconducting bars. The transformer is used as a coupling impedance of the protected line with a superconducting switch. We show that very high quench currents (I sub(Q) approx. 3.000 A) may be achieved at 77 K with YBCO single domain samples grown by Bridgman, but that the transition is initiated at the electrical contacts. To overcome this problem two different strategies concerning the tuning of the critical currents of the materials have been envisaged: (i) to promote an enhanced quenched volume from the electrical contacts through the increase of the critical currents and (ii) to initiate an homogeneous quench through a decrease of the overall critical currents of the material. Several methodologies have been envisaged to enhance the critical currents of Y-123/211 composites: an optimized 211 content, a low temperature-high isostatic pressure post-processing treatment, a high oxygen pressure annealing and, finally, including Ag additives in the 123/211 composites. Post-processing processes promote the generation of 1/6 < 031 > partial dislocations which behave as flux pinning centres. On the other hand, we will show that an enhanced spreading of the quench transition may be achieved by decreasing quench currents, either by increasing the measuring temperature or by MgO doping the MTG YBCO bars. This last approach of decreasing T sub(c) and hence J sub(c) appears very promising in order to develop effective elements for a SFCL.