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

A hybrid superconducting fault current limiter (HSFCL) has been developed c onsisting of a transformer whose secondary is connected to a series of para llel YBCO superconducting bars. The transformer is used as a coupling imped ance of the protected line with a superconducting switch. We show that very high quench currents (I-Q similar to 3.000 A) may be achieved at 77 K with YBCO single domain samples grown by Bridgman, but that the transition is i nitiated 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 e lectrical contacts through the increase of the critical currents and (ii) t o 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 i sostatic pressure post-processing treatment, a high oxygen pressure anneali ng and, finally, including Ag additives in the 123/211 composites. Post-pro cessing processes promote the generation of 1/[031] partial dislocations wh ich behave as Aux pinning centres. On the other hand, we will show that an enhanced spreading of the quench transition may be achieved by decreasing q uench currents, either by increasing the measuring temperature or by MgO do ping the MTG YBCO bars. This last approach of decreasing T-c and hence J(c) appears very promising in order to develop effective elements for a SFCL.