Degradation of YBCO coated conductors due to over-current pulse

For HTS (High-Temperature Superconductor) applications to electric power devices, superconductors are subjected to short-circuit fault currents 10 to 30 times the normal operating current. These over-currents will drive the HTS conductors normal and degrade or burn-out the HTS conductors. Therefore it is important to establish a novel thermal stability criterion for over-current pulse drive. In this study, we carried out preliminary experiments on the damage caused by over-current pulse drive focusing on the limitation of the temperature rise without degradation. We prepared five YBCO sample tapes fabricated with IBAD/PLD and IBAD/ MOCVD methods. Measurements were performed at 60 K and 80 K (Conduction cooling with Gifford-McMahon cryocooler was adopted) in self-field. The experimental results indicated that the degradation of the sample tapes for the over-current pulse with a duration of 1 second depended on not the initial critical current I_c, but the peak temperature Tpeak. The permissible peak temperature without degradation existed at about 400-600 K. The sample tapes were also observed before and after the experiments by magneto-optic (MO) imaging to find the distribution of degradation area.