Early Detection of Normal Transitions in a High Temperature Superconducting Transformer Wound with a Plurality of HTS Tapes Using the Active Power Method

The authors have been developing an AC power source with a high temperature superconducting (HTS) transformer which is small in size and can supply large AC current. The HTS transformer needs a detection system of normal transitions for high reliability. The authors have reported the active power method as the detection system. However, the conventional method could not detect early the normal transition in a secondary coil of the transformer which was wound with a bundle of some HTS tapes since resistive voltage in the normal area is too small due to current sharing among the HTS tapes. In this paper, the authors propose a method to detect the normal transitions in the bundle conductor by improving the active power method. The early and correct detection can be achieved by measuring each active power of each HTS tape. In order to measure the active power, current leads are attached independently to all the HTS tapes. The current leads enable to measure the resistive voltage in each HTS tape, and each active power can be measured. The proposed method works well when even one of secondary tapes transits to the normal conducting state. Experimental results show the usefulness of the proposed method.