Roebel Strands Research Articles

The Importance of Geometrical Parameters on the Mechanical Stability of Roebel Cables

AC loss from Roebel cables with high-temperature superconducting strands are comparatively less, and they possess large current carrying capacity since they have a transposed structure. However, the strands should be strong enough to withstand various types of external loads during their operation. The present work simulates the performance of the Roebel strand against an external tensile load using 3-D finite element analysis, and the importance of geometrical parameters on the mechanical stability of Roebel cables is investigated. Results were then compared with another work where the analyses were performed on a monolithic Roebel strand. We concluded that the typical twisting of the strands is an important parameter that cannot be ignored when optimizing a Roebel cable. Hence, the stress induced in a Roebel strand against an external load depends not only on the geometrical parameters but also on the typical twisting of the strand.

Influence analysis of the geometrical parameters on the electro-mechanical stability of HTS Roebel cables

Under high-power applications, high-temperature superconducting (HTS) Roebel cables may subjected to heavy tensile load due to electromagnetic forces or due to external factors during the production process of these cables. The meander structure of Roebel strands is prone to these loads which mechanically weakens the stability of Roebel cables due to localized stress concentration. The applied transport current and exposure to an external or self-magnetic field induce AC loss in Roebel cables. In the present work, the influence of geometrical parameters on the electro-mechanical stability of Roebel cables is carefully investigated using 3D finite element tools. The parameters such as inner fillet radius, outer fillet radius, Roebel angle, strand width at crossing section and the gap between stacks of strand in the straight section are varied and its influence on mechanical stability and electromagnetic characteristics are observed. First a default geometry is modelled and its electro-mechanical simulation is performed against various load conditions. Each parameters of the Roebel cable are then altered on this default geometry. After each set of variation, electro-mechanical simulations are performed and its influence is compared with the default geometry.

3D Finite Element Electromagnetic Analysis of a 14-Strand HTS Roebel Cable

Roebel cables with HTS strands exhibit reduced AC losses and possess large current carrying capacity. Even though works on several 2D and 3D numerical modelling and simulations are available, little efforts are made on computer-aided design (CAD) modelling, finite element analysis (FEA) and simulation of Roebel strands and cables. In this paper, finite element electromagnetic analysis and simulations of a 14-strand HTS Roebel cable is performed. The results obtained are then compared with another work which uses an H-formulation technique to model the problem. Although several numerical simulations have been carried out to optimise the HTS Roebel cable, no FEA work has yet been conducted on the complete 3D structure of Roebel cable. This work can therefore be considered as an initiator of the 3D finite element electromagnetic analysis of HTS Roebel cables.

Magnetization Loss in REBCO Roebel Cables With Varying Strand Numbers

Assembled coated conductors are essential in many high-current high-temperature superconducting (HTS) applications. A promising option is to use Roebel cable, as it offers both low ac loss and mechanical flexibility. In this work, we measured magnetization loss at 77 K in REBCO Roebel cables with varying strand numbers from 6 to 14 to investigate the strand number dependence of ac loss characteristics of the Roebel cables. The 4-mm-wide Roebel strands were punched from nonstabilized 10-mm-wide Fujikura REBCO tapes. The applied field amplitude, frequency, and the field angle (the angle between the magnetic field and the tape wide surface) were varied. Three 10-mm-wide vertical stacks with conductor number of three, five, and seven were prepared as reference stacks using the same source Fujikura tapes. The measured magnetization loss values of the Roebel cables were compared with those of the equivalent stacks as well as with the results of COMSOL modeling of the stacks using the H -formulation. At magnetic-field amplitudes greater than the effective penetration field of the Roebel cables the cables have lower ac loss than equivalent stacks.

Characterization of Critical Current Distribution in Roebel Cable Strands Based on Reel-to-Reel Scanning Hall-Probe Microscopy

We have characterized the local critical current distribution in 2-mm-wide Roebel cable strands using reel-to-reel scanning Hall-probe microscopy (RTR-SHPM). Roebel cable is an equally transposed cable formed from coated conductor architectures that is promising for large current applications. Local spatial critical-current uniformity becomes a crucial issue for the thermal stability of high-current windings. In general, the one-dimensional longitudinal critical-current distribution in coated conductors has been characterized by TAPESTAR as a de-facto standard method. However, it has proved to be difficult to use this method to achieve sufficient quality control of Roebel strands because of the unavailability of information on current flow across the width at a transposition. Furthermore, the lack of spatial resolution across the width becomes a problem for narrower strands. In this study, we applied RTR-SHPM for the characterization of Roebel strands. By scanning a Hall senor across the width of a conductor that was moving in a longitudinal direction, a high spatial-resolution two-dimensional distribution of magnetic field can be obtained for long-length conductors. This enabled us to estimate the local critical current, including the dependence on effective strand width, as a function of a longitudinal coordinate. This information is critical to the optimization of the fabrication processes and for nondestructive quality assurance of long-length Roebel strands.

Mechanical and Thermal Properties of Central Former Material for High-Current Superconducting Cables

Ongoing projects to realize high-current superconducting cables for transport or magnet applications need to incorporate a high number of coated conductor tapes. Several design layouts published, such as conductor-on-round-core cable, stacks, or Roebel-Rutherford, can achieve this. Design layouts proposed by the research institutes ENEA and KIT use a central former having grooves along the length where stacks of tapes or Roebel strands can be embedded. The former material is a substantial fraction of the cable cross section influencing the overall performance of the cable. In this work, materials used as the central former are investigated after severe plastic deformation, using equal channel angular pressing (ECAP) and equal channel angular rolling (ECAR) processes. Applying these methods allows producing continuously long-length profiles. Materials under investigation are aluminum alloy EN AW 6063 and oxygen-free high-conductivity copper. The influences of substructural characteristic obtained by ECAP and ECAR technology on mechanical properties, as well as thermal and electrical conductivity, at operational cryogenic temperatures, at 77 K and 4.2 K, are observed.

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.

The scaling of transport AC losses in Roebel cables with varying strand parameters

A Roebel cable is a good candidate for low-voltage windings in a high-temperature superconductor (HTS) transformer because of its high current-carrying capability and low AC loss. Transport AC loss measurements were carried out in 1.8 m long 15/5 (fifteen 5 mm wide strands) and 15/4 Roebel cables. The results were compared with those in many Roebel cables composed of 2 mm wide Roebel strands. Comparison of the AC losses hinted that the intrinsic difference in normalized transport AC losses is due to differences in the g/w (ratio of the horizontal gap between the Roebel strands over the Roebel strand width) values. The intrinsic difference was confirmed by measuring transport AC loss in a series of horizontally arranged parallel conductor pairs with various g values. A method to scale transport AC losses in Roebel cables with varying strand parameters was developed. The scaling method will be useful for a rough assessment of AC loss in one-layer solenoid winding coils, such as in a HTS transformer.

Total AC loss measurements in a six strand Roebel cable carrying an AC current in an AC magnetic field

We measured the total AC loss in a 6/2 (i.e. 6 strands of 2 mm width) Roebel cable by incorporating a transport measurement method for Roebel cable into a total AC loss measurement system. The amplitude and frequency of the external magnetic field and applied transport current, and the field angle θ (the angle between the field and the normal vector to the cable surface) were varied. The results for θ ≤ 60° scale with the perpendicular magnetic field component, and increase with increasing transport current. In the high magnetic field region, the total AC loss values for a current amplitude I ≪ Ic agree with the Brandt model for a 2 mm-wide Roebel strand, not for a thin strip with the Roebel cable width. This shows the advantage of a Roebel cable over equivalent stacks composed of wider conductors. The total AC losses in a parallel magnetic field become more independent of the field amplitude with increasing current amplitude due to the increased dominance of the transport AC losses in the total AC losses. No frequency dependence was observed in the total AC loss data. A maximum entropy model was successfully constructed for the total AC loss results in a perpendicular magnetic field.

AC loss measurements in pancake coils wound with 2G tapes and Roebel cable: dependence on spacing between turns/strands

AC loss measurements in five single pancake coils wound with 4 mm wide commercial high temperature superconductor wires were carried out to investigate the dependence of coil AC loss on separation between the superconductor layers in the neighbouring coil turns (g) and coil turn number (N) for a given number of ampere-turns NI. The highest frequency was set at approximately 1 kHz. The AC losses measured at different frequencies agreed well with each other. AC loss in the coils with the same N increases with decreasing g. When g is increased to 1.5 times the tape width (6 mm), the loss level is similar to that in an isolated wire. Transport AC loss per unit length in the pancake coils increases with increasing turn number. However, when g is increased to 1.5 times the tape width, the loss level in the coils with different turn number is almost the same. This indicates that, at the same NI, a coil with greater N is advantageous, even considering the conductor length difference. Therefore to achieve a given level of NI for a coil and minimize AC loss, we should favour coils with more turns. Coil AC loss can be measured using voltage taps attached to copper blocks outside a coil with reasonable accuracy. This is important for measuring AC loss in a coil with a complicated structure where voltage taps are not able to be attached inside the coil. AC loss in a nine-turn pancake coil wound with a 5 m long 9/2 Roebel cable (i.e. with 9 mm × 2 mm width strands) was measured using voltage loops arranged in each Roebel strand in the central turn of the coil. The AC loss in the coil was compared with two straight 9/2 Roebel cables with and without spacing between the strands. The combination of inter-strand spacing and turn spacing is an effective way to reduce AC loss in a single pancake coil wound with a Roebel cable.

Coated Conductor Rutherford Cables (CCRC) for High-Current Applications: Concept and Properties

In the past few years the Roebel technique has been established as a method for cabling of coated conductor tapes to achieve high current carrying capabilities for low ac-loss applications. We have successfully developed Roebel cables consisting of up to 50 tapes and with current carrying capabilities up to 2.6 kA (77 K, self-field). However, for applications like busbars or fusion magnets current carrying capabilities of more than 10 kA are required. Such high current carrying capabilities cannot be reached by a simple scale-up with additional tapes. We present a concept for Coated Conductor Rutherford Cables (CCRCs) for currents exceeding 10 kA using Roebel subcables as strands. The first steps towards a short subsize CCRC demonstrator cable with electrical and mechanical characterization of commercial coated conductor tapes, strands and a first Roebel strand have been performed and will be discussed.

Current and Field Distribution in Meandered Coated Conductors for Roebel Cables

Alternating currents or fields in coils, transformers or generators may lead to high losses in superconducting cables. The main contributions to these ac-losses are hysteresis losses of the superconductor and coupling losses between the strands. The hysteresis losses can be reduced by decreasing the width of the tapes. The resulting smaller cross section of the individual filaments could lead to a strong reduction in performance, if the tape is not homogeneous and the current cannot flow around local defects any longer. Hence, detecting local defects before cutting and assembling becomes very important. With our scanning techniques, we visualize the field penetration and the current flow within different two-dimensional structures like Roebel strands. The effect of different local defects on the current distribution in Roebel single strands is examined. Direct measurements of the conductor performance with respect to its geometry are important for finding the best solutions for future cable designs.

Finite Element Analysis of Torsion Experiments on HTSC Tapes

During the past years the application of 2G HTSC material is growing. However, to achieve high currents single HTSC tapes must assembled in a Roebel-cable geometry. Going to even larger cable concepts like Rutherford cables, twisting of HTSC tapes is required. To examine the influence of twisting or torsion on critical current of Coated Conductor tapes or Roebel strands, systematic experiments were carried out giving results for critical twisting lengths. The results have to be examined considering the different strain states within the tape under torsion. On one hand longitudinal strain plays a role, but on the other hand shear strain has to be addressed also. To understand these results theoretical assessment of the stress strain situation within the tape under torsion was performed. Together with finite element analysis the effect of longitudinal and shear strain was systematically examined to give an understanding of the critical current behavior under torsion.

Influence of Shear Stress on Current Carrying Capabilities of High Temperature Superconductor Tapes

To achieve high currents single HTSC tapes or Roeble strands must be combined into a cable. Many concepts like Rutherford cable, coated cable in conduit or full size cable model of Roebel strands require twisting of the HTSC tapes. Twisting or torsion exerts longitudinal and shear stress on the tapes which degrade the current carrying capabilities. Finite element analysis was done to explain this behavior. However, the influence of longitudinal stress in HTSC tapes on critical current is well documented, but there is no experimental data for pure shear stress available. To verify the finite element analysis shear stress experiments are of significant interest. With a three point bending method shear stress can be applied to a single HTSC tape. Critical current measurements with varying shear stress are conducted for several HTSC tapes. With the experimental results and the longitudinal stress dependence a correlation between torsion experiments and finite element analysis is possible.

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.

Magnetic AC Loss Characteristics of 2G Roebel Cable

Roebel cable has been successfully manufactured using second generation (2G) YBCO wire. Roebel shaped strands of width 2 mm and a transposition length of 90 mm were cut from 40 mm wide tape supplied by American Superconductor Corporation. The tape has a weakly ferromagnetic Ni-5 at%W alloy substrate. Eleven insulated strands were assembled to form the cable. This paper reports results for the magnetic AC loss as a function of field amplitude for an insulated 2G Roebel strand and insulated multi-strand Roebel cable in perpendicular and parallel field orientation. In parallel field, the loss at the highest field amplitude measured is reduced by a factor of approximately 30 below the losses in the perpendicular field. In perpendicular field, the losses at the low field amplitude of the Roebel cable sample is influenced by the magnetic substrate. At high field amplitudes ( >0.1 T), the cable and total strand loss converge as expected for an insulated cable, where coupling currents are absent.

Scan measurements on ROEBEL assembled coated conductors (RACC)

The application of RE-BCO coated conductors (cc's) in high field magnets as well as in electrical machinery decisively depends on the ability to produce conductors with high current densities and small losses. The production process of the conductors restricts the thickness of the superconducting layer and therefore the current. It is possible to enhance the current carrying capability by combining several conductors to a cable. The ROEBEL bar concept is the most suitable solution because of reduced ac losses. Cc's are suitable for shaping ROEBEL strands and to perform the cabling. We employed different scan techniques to get an overview of the conductor properties. The magnetoscan, which was successfully applied to cc's, was optimised for scanning a prototype of such a RACC-cable. The resulting map of the magnetic field reflects the superconducting properties of the conductor and allows analysing the defect structure and the critical currents. Due to the complex configuration of the RACC-cable some considerations concerning the effects at conductor edges and the transitions between the strands were necessary. The field map of the ROEBEL-cables can be used to optimise them.

High current DyBCO-ROEBEL Assembled Coated Conductor (RACC)

Low AC loss high transport current HTS cables (>1 kA) are required for application in transformers, generators and are considered for future generations of fusion reactors coils. 2G coated conductors are suitable candidates for high field application at quite high operation temperatures of 50-77 K, which is crucial precondition for economical cooling costs. As a feasibility study we present the first ROEBEL bar cable of approx. 35 cm length made from industrial DyBCO coated conductor (THEVA GmbH, Germany). Meander shaped ROEBEL strands of 4 mm width with a twist pitch of 180 mm were cut from 10 mm wide CC tapes using a specially designed tool. The strands carried in average 157 Amps/cm-width DC and were assembled to a subcable with 5 strands and a final cable with 16 strands. The 5 strand cable was tested and carried a transport current of >300 Amps DC at 77 K, equivalent to the sum of the individual strand transport critical currents. The 16 strand cable carried 500 A limited through heating effects and non sufficient stabilisation and current sharing. A pulse current load indicated a current carrying potential of >1 kA for the 16 strand cable.