Andrea Kling

Status of high transport current ROEBEL assembled coated conductor cables

Assembling coated conductors (CC) into flat ROEBEL bars (RACC cable) was introduced in 2005 by the authors as a practicable method of reaching high transport currents in a low AC loss cable, which is a cable design suited for application in windings. The transport current of 1.02?kA in self-field at 77?K achieved so far, however, is still too low for several applications in electrical machinery such as larger transformers and generators/motors. A new cable concept for further increased currents was presented just recently. The goal of the new design was primarily to demonstrate the possibility of strongly increased transport currents without changing the important cable features for low AC losses. such as, for example, the transposition length of the strands. We present detailed investigations of the properties of this progressed cable design, which has threefold layered strands, an unchanged transposition pitch of 18.8?cm and finally the application of 45 coated conductors in the cable. A 1.1?m long sample (equivalent to six transposition lengths) was prepared from commercial Cu stabilized coated conductors purchased from Superpower. The measured new record DC transport current of the cable was 2628?A at 77?K in self-field (5??V?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 current redistribution effects in the cable, by monitoring a representative strand of each batch during the critical current measurement. Although current redistribution effects showed a complex situation, the behaviour of the cable was found to be absolutely stable under all operational conditions, even above the critical current. The high self-field degradation of the critical current reached the order of 60% at 77?K, and could be modelled satisfactory with calculations based on a proven Biot?Savart-law approach, adapted to the specific boundary conditions given in this new cable design.

Transport and magnetization ac losses of ROEBEL assembled coated conductor cables: measurements and calculations

Many superconductor applications require cables with a high current capacity. This is not feasible with single-piece coated conductors because their ac losses are too large. Therefore, it is necessary to develop superconducting cables with a high current capacity and low ac losses. One promising solution is given by ROEBEL cables. We assembled three ROEBEL cables from commercial YBCO coated conductors. The cables have the same width but a different number of strands, which results in different aspect ratios and current capacities. We experimentally studied their ac losses under a transport current or a perpendicular magnetic field. In addition, we performed numerical calculations, which agree with the experiments, especially for the transport case. We found that in the cables there is good current sharing between the strands. We also found that stacking the strands reduces the magnetization losses. For a given critical current, thicker cables have lower magnetization ac losses. In addition, a conducting matrix is not required for a good current sharing between strands.

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.

Investigation of a Rutherford cable using coated conductor Roebel cables as strands

Coated conductor applications such as fusion magnets, particle accelerator magnets and generator windings require high current-carrying capabilities. This requirement can be fulfilled by various cable concepts using commercial long length REBCO coated conductors with high current-carrying performance. In the past few years, our group has successfully developed the Roebel cable concept for coated conductors. The design advantages of such a cable are high current-carrying capability and low alternating current (AC) losses. Unfortunately, for large-scale applications, the possibilities of a simple scale-up of the Roebel geometry are limited and additional design ideas are needed. One way to reach the required high currents is the Rutherford cable concept. In this concept a conductor is wound with transposition on a flat metal former. In order to design the former, the bending properties of the Roebel assembled coated conductor cables (RACC) must be measured and characterized. This allows the identification of a destruction-free interval for the Roebel cable, in terms of bending angle and transposition length. In this work we designed and assembled a demonstrator of a coated conductor Rutherford cable (CCRC) with three RACC cables. We measured the critical current and the AC losses of the cable demonstrator. Our results show that, despite still needing efforts in terms of reproducibility of the assembly process and of AC loss reduction, this design is a promising and viable solution for high current-capacity cables made of coated conductors.

Investigation of the effect of striated strands on the AC losses of 2G Roebel cables

The assembly of meander shaped coated conductor tapes by the Roebel technique is a promising way to manufacture high current cables with low ac losses. The application of longitudinal striations to the single strands can be an option to create a filament structure for further possible reduction of the ac losses. Due to the complex Roebel strand geometry, it was important to identify a reliable technique to produce such structures using a picosecond-infrared (IR) laser for the groove etching process. We analyzed the effects of the filament structure on the magnetization ac loss behavior by comparing the losses of a cable with striated strands with those of a reference one with non-striated strands. The ac loss reduction in the Roebel cable with striated strands was confirmed. The measured magnetization loss of the 125 mm striated single strand is five times lower than that of the non-striated one. In the case of the cable sample the loss reduced by a factor of three, but not in the whole interval of amplitudes of the applied magnetic field. We also compared the results with those for a cable with insulated striated strands: they seem to indicate that the coupling currents occur mostly between the filaments, not between the strands.

Effect of self-field on the current distribution in Roebel-assembled coated conductor cables

Roebel cables are a promising solution for high current, low AC loss cables made of high-temperature superconductors in the form of coated conductors. High current creates significant self-field, which influences the superconductors current-carrying capability. In this paper, we investigate the influence of the self-field on the cables critical current and the current repartition among the different strands. In order to investigate the cables critical current, we analysed the influence of flux creep on the cable properties. Using the experimental materials properties derived from measurements on a single conductor as input for our calculations, we were able to predict the critical current of the cable in two limiting situations: good current sharing and complete electrical insulation among the strands. The results of our calculations show good agreement with the measured critical current of three Roebel cable samples.

Roebel assembled coated conductor cables (RACC): Ac-Losses and current carrying potential

Low ac-loss HTS cables for transport currents well above 1 kA are required for application in transformers and generators and are taken into consideration for future generations of fusion reactor coils. Coated conductors (CC) are suitable candidates for high field application at an operation temperature in the range 50-77 K. Ac-field applications require cables with low ac-losses and hence twisting of the individual strands. We solved this problem using the Roebel technique. Short lengths of Roebel bar cables were prepared from industrial DyBCO and YBCO-CC. Meander shaped tapes of 4 or 5 mm width with twist pitches of 123 or 127 mm were cut from the 10 or 12 mm wide CC tapes using a specially designed tool. Eleven or twelve of these strands were assembled to a cable. The electrical and mechanical connection of the tapes was achieved using a silver powder filled conductive epoxy resin. Ac-losses of a short sample in an external ac-field were measured as a function of frequency and field amplitude as well as the coupling current decay time constant. We discuss the results in terms of available theories and compare measured time constants in transverse field with measured coupling losses. Finally the potential of this cable type for ac-use is discussed with respect to ac-losses and current carrying capability.

Enhancement of the transverse stress tolerance of REBCO Roebel cables by epoxy impregnation

REBCO Roebel cables are considered for application in high-temperature superconducting inserts for accelerator magnets because of their fully transposed geometry, high-engineering current density, and adequate bending tolerance. In these magnets the cables experience Lorentz forces leading to transverse stresses up to 100–150 MPa. Previous reports have shown bare Roebel cables to degrade under such high stresses so that additional reinforcement is required. In this work, two identical Roebel cables are vacuum impregnated with a mixture of epoxy and fused silica in order to improve their tolerance to transverse stress. After impregnation, the critical current of the cables is measured under transverse mechanical loading at T = 4.2 K, T. A reference cable without impregnation is tested as well. Pressures up to 350 MPa are applied to a short (30 mm) section of each cable. No degradation was observed for pressures up to 250 MPa and 170 MPa in the two impregnated cables. The critical current of the non-impregnated cable, in contrast, started to decrease at stresses as low as 40 MPa.

Bending properties of different REBCO coated conductor tapes and Roebel cables at T = 77 K

Application of REBCO coated conductors in coils or cables involves deformation of the conductor in different modes, such as in-plane bending, out-of-plane bending and torsion. For example, the dipole magnet designs in the EuCARD-2 project require bending radii as low as 7.5 mm, inducing significant bending strain in the REBCO layer. In this paper, we investigate the effect of out-of-plane bending on the current-carrying properties of coated conductors from different manufacturers. The samples are manipulated by means of a Goldacker-type bending rig, which allows continuous bending at T = 77 K. By reversal to after each bending step, the reversible strain effect is separated from irreversible degradation. All tested conductors are found to tolerate compressive bending to a radius of 6 mm with less than 5% irreversible degradation of the critical current. The magnitude of the reversible strain effect shows a large variation among the samples. The effect of out-of-plane bending on Roebel cables is investigated as well, and the results are compared to the bending characteristic of single conductors. The results show no detrimental effect of the cable assembly on the bending properties within the constraints of the test.

Development and characterization of a 2G HTS roebel cable for aircraft power systems

Lightweight megawatt-range power cables in electric aircraft propulsion systems will allow reducing fuel consumption by more than a half. Taking into account the low voltage limitations for onboard use, superconductivity providing high current-carrying capacity becomes an enabling technology for this application. In the frame of a preliminary study of an onboard superconducting power distribution grid, we fabricated and tested a high-temperature superconducting (HTS) Roebel cable. The work was done by a consortium of institutions and supported by Airbus Group Innovations. SuperOx provided a 400 A-class 12-mm-wide 2G HTS wire. The team at the Karlsruhe Institute of Technology prepared 5.5-mm-wide Roebel strands with a transposition length of 226 mm and assembled the strands into a 6-m-long Roebel cable. The team at the Russian Scientific Research and Development Cable Institute performed characterization of the cable. Short samples of the cable were tested to determine their critical currents, mechanical properties, and stability during thermal cycling. For a 4-m-long section of the cable, we measured the critical current, as well as the transport ac losses at frequencies in the range from 50 to 400 Hz. This paper presents the cable design and test results. The feasibility of using 2G HTS Roebel cables in electric aircraft systems is discussed.