Experimental and numerical transport AC losses in a four-strand Roebel cable bifilar stack

Abstract

REBCO Roebel cables are promising candidates for bifilar coils (or non-inductive coils) in large scale high temperature superconducting resistive type fault current limiters (SFCLs), as they enable large current-carrying capability in conjunction with low AC loss. Here we report transport AC loss measurement results in two Roebel cable bifilar stacks (RBS) with different vertical spacing between the turns assembled from a straight 4/2 (four 2 mm width strands) Roebel cable. We also report two-dimensional finite element method AC loss simulation results using the H-formulation in the RBSs, in two parallel bifilar stacks (TPBS) which have the same geometrical dimensions as the RBS with 0.284 mm spacing (g) but have unequal current distribution between the conductors composing the stacks, and in an equivalent bifilar stack (EBS) comprising four 4 mm wide conductors while keeping identical vertical geometrical dimensions with the RBS with g = 0.284 mm but has unequal current distribution between the conductors. We show that the measured transport AC loss values in the RBS with g = 0.284 mm are approximately one fifth of those in the parent straight 4/2 Roebel cable due to cancellation of the perpendicular magnetic field components in the bifilar stack, and also show the AC loss in RBSs increases with increasing g. We then show the calculated transport AC loss values in the RBS with g = 0.284 mm are approximately one third of the values in the TPBS and EBS when It,turn/Ic0,cable > 0.6, where It,turn is the current amplitude for upper or lower turn of the bifilar stacks and Ic0,cable is nominal Ic value for the 4/2 Roebel cable. We numerically demonstrate that the result is caused by flux flow loss in the inner conductors in the TPBS and EBS due to unequal current distributions in conductors. Roebel cables show clear advantage over simple vertical stacks for SFCL applications in terms of AC loss.