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  • 1
    Language: English
    In: Superconductor science & technology, 2020-07-31, Vol.33 (9), p.94013
    Description: When exposed to time-dependent magnetic fields, REBCO Roebel cables generate AC loss resulting from both magnetic hysteresis and induced inter-strand coupling currents. Until now, the AC loss has been computed in a two-dimensional approximation assuming fully coupled or decoupled strands, and a finite inter-strand resistance could be simulated only with three-dimensional models. In this work, we propose a homogenization procedure that reduces the three-dimensional geometry of the Roebel cable to two dimensions, without ignoring connections between the strands. The homogenized cable consists of two parallel 'monoblocks' with an anisotropic resistivity. The proposed model enables computation of AC coupling loss without the need for complex three-dimensional simulations. For experimental validation, a Roebel cable with soldered strands was prepared. The inter-strand resistance was determined by applying a transverse current and measuring the voltage profile. Additionally, the AC magnetization loss of the cable was measured in fields of 1 to 50 mT with frequencies of 1 to 2048 Hz using a calibration-free technique. With the measured inter-strand resistance as input parameter, the monoblock model gives a good estimate for the AC loss, even for conditions in which the coupling loss is dominant.
    Subject(s): ac losses ; roebel cable ; coupling losses ; hts coated conductors ; Condensed Matter Physics ; Materials Chemistry ; Electrical and Electronic Engineering ; UT-Hybrid-D ; Metals and Alloys ; Ceramics and Composites
    ISSN: 0953-2048
    E-ISSN: 1361-6668
    Source: IOPscience extra
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  • 2
    Language: English
    In: IEEE transactions on applied superconductivity, 2016-06, Vol.26 (4), p.1-5
    Description: Roebel cables made of high-temperature superconducting coated conductors are regarded as promising cables for winding applications in virtue of their large engineering current density and low losses. The composing meander-shaped strands are assembled very tightly into the cable, which results in a strong electromagnetic interaction between them. This interaction profoundly influences the effective self-field critical current (I c ) of the cable, which is much lower than the sum of the critical currents of the composing strands. In addition, the AC losses are influenced by the material's properties and by the geometrical configuration of the cable. Being able to predict the effective critical current and ac losses of such cables is very important for a proper design of applications: Due to the complexity of the cable's geometry and of the material's properties, this prediction can only be performed with advanced numerical tools. In this paper, we use finite-element-based models to compute the effective I c and the ac losses of Roebel cables composed of 31 strands using tapes from two manufacturers. The AC losses are analyzed in the simultaneous presence of transport current and background perpendicular field proportional to the current, which mirrors the situation occurring in a winding. Our models include the angular dependence of J c (B,θ) at 77 K, which is very different for the two materials. By means of a successful comparison of the simulation results to experimental data obtained with a calorimetric method measuring the evaporation of liquid nitrogen, this work confirms the applicability and efficiency of our numerical techniques for simulating the electromagnetic behavior of Roebel cables and devices thereof.
    Subject(s): Superconducting cables ; Integrated circuits ; Numerical modeling ; Critical current density (superconductivity) ; Power cables ; Current measurement ; Roebel cables ; HTS applications ; Superconducting magnets ; Loss measurement ; AC losses
    ISSN: 1051-8223
    E-ISSN: 1558-2515
    Source: IEEE Electronic Library (IEL)
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  • 3
    Language: English
    In: IEEE transactions on applied superconductivity, 2014-06, Vol.24 (3), p.1-5
    Description: In this contribution, we develop a refined numerical model of pancake coils assembled from a coated conductor Roebel cable, which includes the angular dependence of the critical current density J c on the magnetic field and the actual (three-dimensional) shape of the current lead used to inject the current. Previous works of ours indicate that this latter has an important influence on the measured value of the AC losses. For the simulation of the superconductor, we used two alternative models based on different descriptions of the superconductor's properties and implemented in different mathematical schemes. For the simulation of the current lead we use a full three-dimensional finite-element model. The results of the simulation are compared with measurements and the main issues related to the modeling and the measurement of Roebel coils are discussed in detail.
    Subject(s): Superconducting cables ; numerical simulations ; Power cables ; coils ; Computational modeling ; Roebel cables ; Superconducting magnets ; Loss measurement ; AC losses ; Superconducting coils ; Electrical engineering. Electrical power engineering ; Applied sciences ; Various equipment and components ; Electric connection. Cables. Wiring ; Exact sciences and technology ; Finite element method ; Usage ; Innovations ; Superconductivity
    ISSN: 1051-8223
    E-ISSN: 1558-2515
    Source: IEEE Electronic Library (IEL)
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  • 4
    Language: English
    In: IEEE transactions on applied superconductivity, 2019-01, Vol.29 (1), p.1-9
    Description: One of the most significant and basic parameters connected with the performance of rare earth-based coated conductor is the angular dependence of their critical current. Because of the complex tape architecture, complicated production process and long length of coated superconducting tapes, the angular dependence homogeneity of the superconductor along the conductor length I c ( B ) needs to be evaluated. In this paper, the homogeneity of several samples of SuperPower tapes manufactured between 2008 and 2014 was examined for fields up to 600 mT. It was found that, for a given sample, the variation of results of the critical current is at most 6.7%. In addition, the uniformity is dependent on the angular arrangement of the tape in the external magnetic field.
    Subject(s): Integrated circuits ; 〈inline-formula xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"〉 〈tex-math notation="LaTeX"〉 n〈/tex-math〉 〈/inline-formula〉-value correlation ; High-temperature superconductors ; Magnetic field measurement ; Critical current density (superconductivity) ; Critical current ; Current measurement ; Conductors ; REBCO tapes ; Magnetic fields ; high-temperature superconductors (HTSs)
    ISSN: 1051-8223
    E-ISSN: 1558-2515
    Source: IEEE Electronic Library (IEL)
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  • 5
    Language: English
    In: Superconductor science & technology, 2016-07-05, Vol.29 (8), p.83002
    Description: Second-generation high-temperature superconductor (HTS) tapes, known also as coated conductors, are very promising tapes for HTS applications, in virtue of their extremely high critical current density, in-field behavior, and mechanical strength. Yet, the extremely large (typically in the range 1000-10 000) width-to-thickness ratio of the superconducting material makes them prone to high power dissipation in the presence of time-varying magnetic fields perpendicular to their flat face-a condition frequently met in several applications. Since the dissipation is directly proportional to the square of the superconductor's width, an obvious way of reducing it is by striating the superconductor into narrow filaments (stripes): in that case, provided that the filaments are electromagnetically uncoupled, the losses are reduced by a factor equal to the number of filaments. In the past two decades, many researchers from groups around the world have tried to apply this idea to practical conductors using different techniques. The aim of this paper is to provide a review of such efforts, focusing on the different approaches to make filamentized conductors, on the effectiveness of the AC loss reduction, and on the applicability of those techniques to long lengths.
    Subject(s): AC losses ; HTS coated conductors ; filaments
    ISSN: 0953-2048
    E-ISSN: 1361-6668
    Source: IOPscience extra
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  • 6
    Language: English
    In: IEEE transactions on applied superconductivity, 2016-04, Vol.26 (3), p.1-4
    Description: Stacks of high-temperature superconducting tape can be magnetized to produce a variety of different trapped field profiles in addition to the most common conical or pyramidal profiles. Stacks of tape using discarded Roebel cable offcuts were created to investigate various stacking arrangements with the aim of creating a stack that can be magnetized to produce a uniform trapped field for potential applications such as NMR. A new angled stacking arrangement proved to produce the flattest, most uniform field of all the overlapping stacking arrangements and has the potential to be scaled up. FEM modeling in COMSOL was also performed to complement the measurements and explain the limitations and advantages of the stacking arrangements tested.
    Subject(s): Superconducting cables ; tapes ; superconducting tapes ; Magnetic field measurement ; Superconducting films ; stack of HTS ; Hall probe scans ; Stacking ; Superconducting magnets ; Surface roughness ; Rough surfaces ; Roebel cable
    ISSN: 1051-8223
    E-ISSN: 1558-2515
    Source: IEEE Electronic Library (IEL)
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  • 7
    Language: English
    In: IEEE transactions on applied superconductivity, 2014-12, Vol.24 (6), p.1-8
    Description: Striation of high-temperature superconductor coated conductors as a way to reduce their magnetization ac losses has been the subject of intense worldwide research in the past years by several groups. While the principle of this approach is well understood, its practical application on commercial materials to be used in power applications is still far to be implemented due to manufacturing and technological constraints. Recent advances in tape quality and striation technology are now enabling systematic investigations of the influence of the number of filaments on ac loss reduction with a consistency that was not available in the past. In the present work, we demonstrate the technological feasibility of importantly reducing the magnetization losses of commercially available coated conductors by striating them into a high number of filaments (up to 120). The loss reduction exceeds one order of magnitude and does not come at the expense of current-carrying capability: samples with 10 and 20 filaments are unaffected by the striation process, whereas samples with 80 and 120 filaments still retain 80% and 70% of the current-carrying potential, respectively. We also investigate the transverse resistivity between the filaments in order to understand the paths followed by the coupling currents: we found that the coupling current prevalently flows in the metallic substrate, rather than in and out of the filaments. Finally, we use oxidation as a method to reduce the coupling currents and the corresponding losses. The contribution of this work is threefold: 1) it describes the know-how to produce a large number of high-quality striations in commercially available coated conductors, greatly reducing their losses without extensively degrading their transport properties; 2) it provides a comprehensive characterization of the said samples (e.g., measurements in a wide frequency range, transverse resistance profiles, influence of oxidation on dc and ac behavior of the sample); and 3) it provides new insight on the patterns of the coupling currents.
    Subject(s): High-temperature superconductors ; Critical current density (superconductivity) ; Current measurement ; Conductors ; Loss measurement ; Oxidation ; Yttrium barium copper oxide ; Measurement ; Magnetization ; Analysis ; Innovations ; Barium compounds ; Voltage ; High temperature superconductors ; Copper ; Electric properties ; Magnetic properties
    ISSN: 1051-8223
    E-ISSN: 1558-2515
    Source: IEEE Electronic Library (IEL)
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  • 8
    Language: English
    In: IEEE transactions on applied superconductivity, 2013-06, Vol.23 (3), p.5900205-5900205
    Description: Roebel cables are a promising solution for high-current, low ac loss conductors for various applications, including magnets, rotating machines, and transformers, which generally require the cable to be wound in a coil. We recently assembled and characterized a 5 m long sample and wound it into pancake coils. In this contribution, we investigate the ac loss behavior of such pancake coils by means of numerical simulations based on two complementary models: the finite-element model based on the H -formulation and the minimum magnetic energy variation method based on the critical state. These two numerical models take into account the axis-symmetric geometry of the coil and its detailed structure, simulating each strand composing the cable. The local current density and magnetic field distributions are shown and the ac losses for various current amplitudes are computed. The influence of the number of turns and of their separation on the ac losses is investigated. The results of the computations are compared with the measurements and the main reasons for the observed discrepancy are discussed.
    Subject(s): Coils ; numerical simulations ; Power cables ; Computational modeling ; Magnetic separation ; Current measurement ; Roebel cables ; Ac losses ; Loss measurement ; Numerical models ; Transformers and inductors ; Electrical engineering. Electrical power engineering ; Special rotating machines ; Exact sciences and technology ; Applied sciences ; Electromagnets ; Electrical machines ; Various equipment and components
    ISSN: 1051-8223
    E-ISSN: 1558-2515
    Source: IEEE Electronic Library (IEL)
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  • 9
    Language: English
    In: IEEE transactions on applied superconductivity, 2015-02, Vol.25 (1), p.1-4
    Description: Roebel cables made of high temperature superconducting (HTS) coated conductors can carry high currents with a compact design and reduced ac losses. Therefore, they are good candidates for manufacturing coils for HTS applications such as motors and generators. In this paper, we present the experimental dc and ac characterization of several coils assembled from a 5-m-long Roebel cable built at Karlsruhe Institute of Technology, which differ in the number of turns and turn-to-turn spacing. Our experiments, which are supported by finite-element-method calculations, show that a more tightly wound Roebel coil, despite having a lower critical (and therefore operating) current, can produce a higher magnetic field than a loosely wound one. For a given magnetic field produced at the coil's center, all the coils have similar ac losses, with the exception of the most loosely wound one, which has much higher losses due to the relatively large current needed to produce the desired field. The experiments presented in this paper are carried out on the geometry of pancake coils made of Roebel cables, but they are exemplary of a more general strategy, based on coupling experiments and numerical simulations, that can be used to optimize the coil design, with respect to different parameters, such as tape quantity, size, or ac loss, the relative importance of which is dictated by the specific application.
    Subject(s): Coils ; Superconducting cables ; Critical current density (superconductivity) ; Power cables ; Conductors ; Loss measurement ; Magnetic fields ; Finite element method ; High temperature superconductors ; Usage ; Numerical analysis ; Innovations
    ISSN: 1051-8223
    E-ISSN: 1558-2515
    Source: IEEE Electronic Library (IEL)
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  • 10
    Language: English
    In: Superconductor science & technology, 2014-08-13, Vol.27 (9), p.93001
    Description: Energy applications employing high-temperature superconductors (HTS), such as motors generators, transformers, transmission lines and fault current limiters, are usually operated in the alternate current (ac) regime. In order to be efficient, the HTS devices need to have a sufficiently low value of ac loss, in addition to the necessary current-carrying capacity. Most applications are operated with currents beyond the current capacity of single conductors and consequently require cabled conductor solutions with much higher current carrying capacity, from a few kA up to 20-30 kA for large hydro-generators. A century ago, in 1914, Ludwig Roebel invented a low-loss cable design for copper cables, which was successively named after him. The main idea behind Roebel cables is to separate the current in different strands and to provide a full transposition of the strands along the cable direction. Nowadays, these cables are commonly used in the stator of large generators. Based on the same design concept of their conventional material counterparts, HTS Roebel cables from REBCO coated conductors were first manufactured at the Karlsruhe Institute of Technology and have been successively developed in a number of varieties that provide all the required technical features such as fully transposed strands, high transport currents and low ac losses, yet retaining enough flexibility for a specific cable design. In the past few years a large number of scientific papers have been published on the concept, manufacturing and characterization of such cables. Therefore it is timely for a review of those results. The goal is to provide an overview and a succinct and easy-to-consult guide for users, developers, and manufacturers of this kind of HTS cable.
    Subject(s): Roebel cables ; high current ; low ac loss
    ISSN: 0953-2048
    E-ISSN: 1361-6668
    Source: IOPscience extra
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