RESEARCH AND DEVELOPMENT OF SUPERCONDUCTORS WITH LARGE CURRENT CAPACITY FOR SUPERCONDUCTING TRANSFORMERS INSTALLED IN UNDERGROUND SUBSTATIONS

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We first analyzed the test results of a 500kVA liquid-nitrogen cooled oxide superconducting transformer, which we previously designed and built, and then extracted the research subjects necessary for the development of superconducting transformers with larger capacity. According to the results, we made the following basic research with the aim of the realization of oxide superconducting transformer windings with large current capacity and high voltage withstand level ; (1)investigation of electromagnetic properties of silver-sheathed Bi-system superconducting wires, (2)study on the optimum configuration of transposed parallel conductors with large current capacity, which are composed of oxide superconducting strands, (3)study on the stability of superconducting parallel conductors at liquid-nitrogen temperature, (4)evaluation of the condition of the bubble formation in saturated and sub-cooled liquid nitrogen in the case where superconducting windings are changed into normal state, (5) … More study on the insulation strength of subcooled liquid nitrogen and the applicable insulation structure, and (6)design and test of oxide superconducting model coils which simulate the actual condition of superconducting transformer windings in the electric power system. Furthermore we studied the practical use of oxide superconducting transformers in the electric power grid. On the basis on the obtained results, we entirely supported the development project of a 1000kVA-22kV/6.9kV oxide superconducting transformer cooled by liquid nitrogen and produced it successfully. We performed the field-test of the superconducting transformer first in Japan. In the test, we observed and evaluated the electromagnetic properties of superconducting parallel conductors, and the insulation strength and cooling properties of sub-cooled liquid nitrogen in the case that the superconducting transformer windings was exposed to the actual condition in electric power grid. Therefore we verified the obtained results in this study. Less