The dependence of AC loss characteristics on the spacing between strands in YBCO Roebelcables

Abstract

Transport AC loss in a short length of 9/2 YBCO Roebel cable, (i.e. with9 × 2 mm width strands), with 0.25 mm spacers between the strands is measured. Thefrequency varies from 59 to 354 Hz. The result is compared with the loss for acable without spacers between the strands. Transport AC loss is decreased by thepresence of the spacers. The AC loss reduction due to the extra spacing is moresignificant when the amplitude of the cable current is small compared to the cableIc.The losses in the cable with spacers normalized by the square of the cable currents plotted againstIt/Ic approximately agree with those in a cable without spacers. Electromagnetic modelling wascarried out for 9/2 and 8/2 cables, modelled as parallel stacks, to assist in understandingthe above experimental results. The 8/2 configuration allows the greater use of symmetryto speed computations. Reasonable agreement between the numerical results and themeasured results was obtained. This supports suggestions made in previous publicationsthat the transport AC loss in a Roebel cable is roughly equivalent to the loss in twoparallel stacks carrying the same current in each tape. The electromagnetic analysis in the8/2 stacks shows the flux lines are more perpendicular to the strand face when the verticalspace between strands is smaller, and this leads to a larger induced electricalfield and larger AC loss. At small current amplitudes, the modelling shows thespacing has a strong effect on the AC loss in the surfaced part of the strands.