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

Abstract

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 (IQ~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.