Theoretical analysis on ac loss properties of two-strand parallel conductors composed of superconducting multifilamentary strands

Abstract

The ac loss properties of two-strand parallel conductors composed of superconducting multifilamentary strands were theoretically investigated. The constituent strands generally need to be insulated and transposed for the sake of uniform current distribution and low ac loss. In case the transposition points deviate from the optimum ones, shielding current is induced according to the interlinkage magnetic flux of the twisted loop enclosed by the insulated strands and the contact resistances at the terminals. It produces an additional ac loss. Supposing a simple situation where a two-strand parallel conductor with one-point transposition is exposed to a uniform ac magnetic field, the basic equations for the magnetic field were proposed and the theoretical expressions of the additional ac losses derived. As a result, the following features were shown. The additional ac loss in the non-saturation case, where the induced shielding current is less than the critical current of a strand, is proportional to the square of the magnetic field amplitude and the square of deviation length in transposition and has a Debye-type frequency dependence. On the other hand, in the saturation case, the additional ac loss is nearly proportional to the field amplitude and the deviation length in transposition. It has no frequency dependence at a higher frequency than the critical frequency.

Experimental study on the ac loss properties of two-strand parallel conductors composed of superconducting multifilamentary strands

“…We used NbTi superconducting multifilamentary strands to form two-strand parallel conductors. The primary purpose of the present study is to verify the theoretical expressions of the additional ac losses which were derived on the supposition that two-strand parallel conductors are composed of superconducting real multifilamentary wires without the interfilamentary bridging [6].…”

“…Therefore, before the fabrication of the transformers and the pulse coil, we started the study of the ac loss and current distribution properties of superconducting parallel conductors [1,5]. We also made a detailed theoretical analysis of the ac losses for a simple case when a two-strand parallel conductor with one-point transposition was exposed to a spatially uniform magnetic field [6]. We proposed the basic equation for the magnetic field and derived theoretical expressions of the additional ac losses due to the formation of parallel conductors by classifying them into the non-saturation and the saturation cases.…”

“…We proposed the basic equation for the magnetic field and derived theoretical expressions of the additional ac losses due to the formation of parallel conductors by classifying them into the non-saturation and the saturation cases. As a result, the following electromagnetic behaviour of superconducting parallel conductors was predicted [6]. In the case where the strands are evenly transposed so as to be inductively equivalent to each other, shielding current is not induced at all in a parallel conductor.…”

“…The above-mentioned features were only theoretically predicted and not proved. In this paper, we carried out the verification experiment on the theoretically predicted ac loss properties of superconducting parallel conductors in [6]. We measured the ac losses in two-strand parallel conductors by a pick-up-coil method and made a quantitative discussion on the full particulars of ac loss properties in both the non-saturation and saturation cases and also on the threshold criterion between them in comparison to the theoretical prediction in [6].…”

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The ac loss properties of two-strand superconducting parallel conductors were experimentally investigated by using NbTi multifilamentary strands with a rectangular cross section. We prepared the simple situation that was supposed in the previous theoretical study (M Iwakuma et al 2002 Supercond. Sci.

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