Development of kA Class AC Superconducting Conductors and Fabrication of a Coreless Superconducting Autotransformer

Abstract

Recently, multifilamentary superconducting wires with very low AC losses have been produced and those practical applications are going to be realized. To realize the actual-size power machines and apparatuses, it is necessary to develop 1_??_10 kA AC conductors. However, the critical currents of multifilamentary wires at I T are several-tens A, and therefore it is necessary to use the multistrand conductors consisting of several-tens or -hundreds of strands. Such conductors sometimes show AC current degradations because of some sources as (1) imperfect transposition, (2) wire motion, (3) temperature increase, (4) longitudinal magnetic field component effect and so on.
Formerly, a coreless transformer had been considered to be unpractical because of its large exciting current. However, Yamamoto et al. proposed that a coreless superconducting transformer should be possible as a step-down autotransformer at the receiving side utilizing its large exciting current as the reactive power source to cancel the charging current of an underground transmission line or UHV line, and therefore the shunt reactors could be eliminated.
In this paper, about the configurations and characteristics of AC conductors which are removed of some causes of AC degradations are mentioned, and then the quench test results of two small coils made with these conductors are reported. In these conductors, low AC loss strands with ultra-fine NbTi filaments are twisted around central bundle of stainless steel wires. One of the coils has been designed as a model coreless autotransformer, and its test result is also described.