Study on Improvement of the Stability for the Superconducting Current-Limiting Element Using Cores and Coils

The stability test is one important test of the viability of using YBCO-coated conductors as superconducting current-limiting elements apart from the current-limiting capacity test. A decrease in stability will degrade the current-limiting performance of the current-limiting element and eventually damage the element. In this paper, a method of ensuring the stability of the superconducting current-limiting elements, which are made using YBCO-coated conductors, and the results of the method analysis are presented. The method of ensuring stability was applied to obtain the results using the relationship between the superconducting current-limiting element and the electrical coupling condition. The condition was a parallel connection of the superconducting current-limiting element to the secondary coil. With this method, superconducting current elements can be tested in a stable condition. In addition, the winding direction (the additive/subtractive polarity) can be changed, and diverse test conditions can be provided by adjusting the number of turns of the secondary coil. The additive and subtractive windings were used in the test with 3 : 1 and 3 : 2 primary-to-secondary turn ratios, and the test period was set at 5.5 cycles to correspond to the breaker operation time. The applied voltage and current were determined considering the minimum voltage level of the YBCO-coated conductor. The test results showed that with the additive polarity winding, the superconducting current-limiting element was thermally stable regardless of the turn ratio and the specific resistivity characteristic of the YBCO-coated conductor.