Status of high transport current ROEBEL assembled coated conductor cables

Abstract

Assembling coated conductors (CC) into flat ROEBEL bars (RACC cable) was introducedin 2005 by the authors as a practicable method of reaching high transport currents in a lowAC loss cable, which is a cable design suited for application in windings. The transportcurrent of 1.02 kA in self-field at 77 K achieved so far, however, is still too lowfor several applications in electrical machinery such as larger transformers andgenerators/motors. A new cable concept for further increased currents was presented justrecently. The goal of the new design was primarily to demonstrate the possibilityof strongly increased transport currents without changing the important cablefeatures for low AC losses. such as, for example, the transposition length of thestrands. We present detailed investigations of the properties of this progressed cabledesign, which has threefold layered strands, an unchanged transposition pitchof 18.8 cm and finally the application of 45 coated conductors in the cable. A1.1 m long sample (equivalent to six transposition lengths) was prepared fromcommercial Cu stabilized coated conductors purchased from Superpower. Themeasured new record DC transport current of the cable was 2628 A at 77 K in self-field(5 µV cm−1 criterion). The use of three slightly different current carrying batches of strand material(± 10%) was a special feature of the cable, which allowed for interesting investigations of currentredistribution effects in the cable, by monitoring a representative strand of each batchduring the critical current measurement. Although current redistribution effects showed acomplex situation, the behaviour of the cable was found to be absolutely stable underall operational conditions, even above the critical current. The high self-fielddegradation of the critical current reached the order of 60% at 77 K, and couldbe modelled satisfactory with calculations based on a proven Biot–Savart-lawapproach, adapted to the specific boundary conditions given in this new cable design.