Current distribution in parallel-connected YBCO high-temperature superconducting elements with different critical currents

A high-temperature superconductor is investigated for application to a fault-current limiter. However, the current-carrying capacity of a single element is too low for practical use. This parallel connection of superconducting elements is required to increase the current capacity. This paper discusses an ac current distribution between two YBa₂Cu₃O_7-x superconducting elements connected in parallel that differ in electrical aspects such as a critical current level and voltage-current characteristic. As the instantaneous value of the current rises from zero to peak, the following processes are observed: (1) the transition from superconducting to normal conducting state is initiated in the element with the smaller critical current; (2) the current value in the element is kept constant until the other element with the larger critical current level becomes normal conducting state; and (3) only when the instantaneous value of the ac current exceeds the summation of the individual critical current values of each element does the voltage across parallel-connected elements appear. These results are caused by the quick recovery characteristic of YBCO superconductors from the normal conducting state to the superconducting state.