Influence of Local Inhomogeneities in the REBCO Layer on the Mechanism of Quench Onset in 2G HTS Tapes

The resistive-type of superconducting fault current limiters (R-SFCL) using the second generation of high-temperature superconductors (2G HTS) are well adapted to power grid protection. Almost electrically invisible in normal operation, it quenches and becomes highly resistive in the event of a fault. The tape operation at the transition from the superconducting state to the resistive state is the basis of a R-SFCL. In this scope, we studied the onset of a quench occurring in 2G HTS tapes using high-speed imaging to record the bubble generation in liquid nitrogen. In the first milliseconds of operation, the bubble generation appears highly inhomogeneous. Dissipation is initiated on multiple spots over the tape surface, which first expands in the direction of the width of the tape and then along its length. A detailed analysis of the REBCO layer is performed by means of scanning Hall probe microscopy (SHPM) after the optical study of the tapes is completed. Analysis of the local current density distribution demonstrates that the bubble generation occurs at positions of local inhomogeneities in the REBCO layer. These inhomogeneities, well recognized through SHPM, are not reliably described by the critical current as a function of the position derived magnetically (e.g., by traditional TapeStar measurements). Similar results were obtained on samples from Superpower and SuperOx.