Frequency dependence of magnetic ac loss in a five strand YBCO Roebel cable

Abstract

We have measured the frequency dependent magnetic ac loss in a five strand Roebel cable with2 mm wide non-insulated strands and a transposition length of 90 mm. The cable is made from12 mm wide SuperPower YBCO (yttrium barium copper oxide) coated conductor tape with a20 µm thick surrounding copper stabilizer layer. The ac loss of the Roebel cable is measured in asinusoidally varying external magnetic field in perpendicular orientation, with amplitudesup to 150 mT, in the frequency range of 30–200 Hz, and at a temperature of 77 K. Nomeasurable loss is observed in the parallel orientation. In the perpendicular orientation, thecable loss is predominantly hysteretic in nature. The loss either increases or decreaseswith frequency, in different field ranges. To resolve the origin of the frequencydependence, we measure the loss in a single strand and in standard tape with similarcopper stabilization. The cable and sum of strand losses converge in the highfield limit, indicating that strands are not electrically coupled. We attribute theobserved frequency dependent increase in loss to a sum of the eddy current loss inthe copper stabilizer and the intrinsic frequency dependence of the hysteresisloss. Experimental results and finite element method calculated values for thefrequency dependent contributions to the magnetic ac loss agree well with each other.