DC and AC Characterization of Pancake Coils Made From Roebel-Assembled Coated Conductor Cable

Abstract

Abstract-Roebel cables made of HTS coated conductors cancarry high currents with a compact design and reduced AC losses. They are therefore good candidates for manufacturing coils for HTS applications such as motors and generators. In this paper we present the experimental DC and AC characterization of several coils assembled from a 5 meter long Roebel cable built at KIT, which differ in the number of turns and turn-to-turn spacing. Our experiments, supported by finiteelement method (FEM) calculations, show that a more tightly wound Roebel coil, despite having a lower critical (and therefore operating) current, can produce a higher magnetic field than a loosely wound one. For a given magnetic field produced at the coil's center, all the coils have similar AC losses, with the exception of the most loosely wound one, which has much higher losses due to the relatively large current needed to produce the desired field. The experiments presented in this paper are carried out on the geometry of pancake coils made of Roebel cables, but they are exemplary of a more general strategy that, coupling experiments and numerical simulations, can be used to optimize the coil design with respect to different parameters, such as tape quantity, size, or AC loss -the relative importance of which is dictated by the specific application.

Impact of critical current fluctuations on the performance of a coated conductor tape

“…Treating this case requires the results obtained in section 4 to be incorporated in the analysis performed in the previous section. Several concepts of cabled conductors have been developed for the purpose of winding superconducting coils, with the most relevant for CC tapes being the cable from the stacked tapes [78,[91][92][93][94][95][96][97], Roebel cable [98,99] and round CORC [74,100,101] or CORT cables [75,102]. One can compute the distribution of the magnetic field in the winding and utilize the data obtained on a short sample of tape to predict the local critical current of each tape in various cable locations in different turns.…”

A self-consistent model for estimating the critical current of superconducting devices

“…The error with respect to experimental data is small (below 4%). This close estimate is remarkable considering that the model considers only the geometrical properties of the cable and the 𝐽 𝑐 (𝑩) dependence which was obtained for a single 12 mm tape [2], [26]. The numerical simulation using the MAX criterion yields a 𝐼 𝑐 of 1005 A.…”

“…The coils were wound using the Roebel cable described in the previous section. Five different coil were assembled from the same Roebel cable as reported in [3], [26]. These corresponded to coils with different inter-turn gaps.…”

“…A second local maximum is reached in the last turn of the coil. Just like in [26], a magnetic field dependent critical current density 𝐽 𝑐 (𝑩) was used to model the self-field effects. This effect can be observed by comparing Figure 6a and Figure 8.…”

“…Since the transposition length of the cable is much shorter than its length, this corresponds to the AVG criterion described above, as all strands experience the same electromagnetic conditions. One voltage tap pair per strand was placed in the cable, yielding uniform results [29]. Using a criterion of μ − 1 V cm , 1 it was found experimentally that the cable had a I c of 1002 A.…”