Protection System for Normal Transitions in a Single-phase Bi2223 Full Superconducting Transformer by the Active Power Method under Flowing Currents of Various Frequencies

The authors have developed a large AC current supply with a single-phase Bi2223 full superconducting transformer. The supply is very small and light and can output currents of various values and frequencies. In order to operate the supply safely, a protection system for normal transitions in the transformer is essential. The authors have proposed the system based on the active power method, which detects the normal transitions as dissipated active power in the normal area. Previous studies have shown that the protection system can work well for the transformer in transporting constant current of 60 Hz. The supply is used under transporting currents of various values and frequencies. AC loss and iron loss in the transformer are increased by increasing the value and frequency and then they are detected as active power in a superconducting state. Therefore these losses cause incorrect recognition of the normal transitions. In this paper, the authors propose the system which can detect correctly the normal transitions regardless of the losses. The experimental results show its usefulness for the transformer.