Acceptance Tests of REBCO Wires for Field Coil of 10 kW High-Temperature Superconducting Generator Cooled by Liquid Hydrogen

A prototype high-temperature superconducting (HTS) field coil for a liquid hydrogen-cooled HTS generator presented a current–voltage (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I-V</i>) curve that was lower than that predicted from the critical current (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_C</i>) characteristics of the REBCO wires. It was assumed that wire degradation occurred between the shipping and winding process. Therefore, this study aimed to verify the quality of the REBCO wires (approximately 140 m × 11 rods) used to manufacture field coil by investigating the differences between shipping and acceptance tests. A section lower than the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_C</i> specification value of 150 A was detected. Additionally, small <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_C</i> drops were also observed in this wire. Moreover, there was a two-step slope observed in the Weibull plot of the acceptance test of this wire, which was different from the shipping test result. This wire also had visual imperfections. Consequently, this wire was not used for the manufacturing of the field coil. A total of four racetrack double-pancake coils were manufactured using wires that passed the acceptance test, and its <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I-V</i> characteristics were measured under liquid nitrogen cooling. The measured <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I-V</i> characteristics of the coils were in good agreement with the calculated curves incorporating the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_C</i> value, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_C-B</i> characteristics of the wire, and the three-dimensional magnetic field analysis result. It was confirmed that the coils present the transport current characteristics as designed by conducting the acceptance test and selecting the wires.