Recovery in superconducting fault current limiters at low applied voltages

We investigated recovery of superconductivity in superconducting fault current limiter elements based on Au/YBa2Cu3O7 (YBCO) films after the release of faults at low applied voltages. The elements were fabricated by patterning Au/YBCO films. They were subjected to simulated AC fault current, and then to small current for recovery measurements. The samples were immersed in liquid nitrogen during the experiment. After the fault, the resistance decreased monotonously to zero. The decrease was originated from the transition from the normal state to the superconducting state. The applied voltage before the fault release affected the recovery time, but not the decrease rate. The recovery speed depended on the size of samples and was faster in smaller samples. The experimental results were analyzed quantitatively with the concept of recovery propagation. A heat balance equation was derived and solved to explain the linear decrease of resistance during the recovery. The result agreed with data well.