Improvement of Superconducting Properties in ROEBEL Assembled Coated Conductors (RACC)

Assembling coated conductors (CC) into flat ROEBEL bars (RACC-cable) is a practical method to reach high transport currents in a low AC loss cable design which is suitable for application in windings. Electrical machinery as large transformers and generators/motors need a few kA transport current. The aim of the presented work was demonstrating the possibility of a strong increase of the transport current of such RACC-cables. So far 1 kA was achieved We present a changed cable design with 3-fold layered strands, an unchanged transposition pitch of 18.8 cm which finally leads to 45 coated conductors in the cable. A 1.1 m long sample (equivalent to 6 transposition lengths) was prepared. Cu stabilized coated conductors purchased from SuperPower were used formatting the ROEBEL strands and assembling the new cable. The new cable reached a record transport current of 2628 A at 77 K in self field (5 muV/cm criterion). A special feature of the cable was the use of 3 slightly different current carrying (plusmn10%) batches of strand material. Although current sharing and redistribution effects could be observed, the behavior of the cable was found to be absolutely stable under all operation conditions. The self field degradation of the critical currents, being of the order of 60% at 77 K could be modeled satisfactory by means of a Biot-Savart-Law approach.