Development of REBCO superconducting power transformers in Japan

In Japan we started a national project to develop a 66/6.9 kV-20 MVA transformer with REBCO superconducting tapes in 2006. This paper gives an overview of progress of the development of superconducting transformers in Japan and also describes the fundamental technologies studied before now to realize a 66/6.9 kV-20 MVA transformer as follows. To reduce the ac loss in REBCO superconducting thin tapes, authors proposed a new method different from the conventional technique of reducing the ac loss in superconducting multifilamentary wires. It consists of scribing process into a multifilamentary structure by laser or chemical etching, and a special winding process. Making a multilayered solenoidal coil with laser-scribed REBCO tapes, we verified the ac loss reduction in proportion to a filament width even in coil configuration. In addition, to realize a current capacity more than the rated secondary current of 2.4 kA, we first investigated the workability of REBCO tapes in the actual winding process with forming a transposed parallel conductor, where REBCO tapes were bent edgewise at transposing points. Making a test coil of a 24-strand parallel conductor, we verified no degradation of the critical current and nearly uniform current distribution among the tapes. The result suggests the applicability of the method of enhancing the current capacity by forming a parallel conductor with REBCO tapes. Further, to realize the dielectric strength regulated for the Japanese standards, i.e. lightning impulse withstand level of 350 kV and excess ac voltage of 140 kV, we made test coils and carried out dielectric breakdown tests. As a result, we got hold of the required insulation distance at the important points from the viewpoint of insulation design.