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The authors have been developing a large AC current source with a single-phase Bi2223 superconducting transformer. The authors aim to develop the source of supplying current with the maximum frequency of 1 kHz and the maximum amplitude of 1 kA. However, the conventional transformer has some leakage inductance resulting in large voltage drop, and therefore it was unable to transport the current with high frequency and large amplitude. In this paper, the authors propose a configuration of the transformer with less leakage inductance than inductance of the conventional one. Improved current source has output current with the maximum frequency of 1 kHz and the maximum amplitude of 900 A. In addition, a protection system for normal transition in the transformer is proposed. The authors have proposed the active power method to detect the normal transition. For high frequency and large current, however, the normal transition may be incorrectly detected by using the method due to large AC loss. The authors show a new method to avoid the incorrect detection by an empirical formula based on measurement results of the AC loss. From the experimental results of the normal transition, the authors show that the proposed method is more correct to detect the normal transition than the conventional method.

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