AC Transport Current Loss Characteristics of REBCO Coated Conductors Subjected to Bending Strains

We experimentally investigated dependence of AC transport current loss characteristics of HoBCO on bending strains. The losses were compared with the losses in the straight conductors. The measurement results showed that values of the AC losses were affected by the bending strains. However, when the losses and the amplitudes of the transport currents were normalized by the strain dependent critical current, curves of the losses vs. the amplitude of the currents fell into one curve unless superconducting layers of the conductors get damages caused by bending strains. Based on these results, it can be considered that the AC transport current loss characteristics of HoBCO subjected to bending strain can be estimated by knowing values of critical currents of bent conductors and the loss characteristics of straight ones in the range where the bending strains do not damage the superconductor layers.     

Losses in a BSCCO/Ag tape carrying AC transport currents in ACmagnetic fields applied in different orientations

In most prospective electric power applications of high-temperature superconductors (HTSs), the conductor is wound in a coil configuration. For a coil of finite length, the magnetic field orientation is axial in the middle of the coil, while the field contains a substantial radial component at the coil ends. In a superconducting BSCCO/Ag tape, the AC losses depend strongly on the orientation of the magnetic field. In this study, we present experimental results of the AC losses in a multifilamentary silver-sheathed Bi-2223 HTSs tape, carrying alternating transport currents in externally applied alternating magnetic fields at different orientations, with respect to the face of the tape and perpendicular to the transport current. The AC losses were measured calorimetrically, at fixed temperature and frequency. The results are compared to semi-empirical models of the AC losses in HTSs tape. We present a more general model of the angular dependence of the AC losses     

Geometry considerations for use of Bi-2223/Ag tapes and wires withdifferent models of Jc(B)

In typical power applications, Bi-2223 conductors carrying AC current will be subjected to external magnetic fields whose orientation and conductor's geometry are of major significance for the AC loss magnitude. This paper investigates the influence of the geometry and aspect ratio of nontwisted Bi-2223 conductors in reducing the AC loss for such applications. A numerical model of high-T_c materials has been used in finite-element-method (FEM) simulations. The model incorporates power-law E-J characteristics with J_c and n defined by both parallel and perpendicular local magnetic field components. It allows computations of field and current distributions with transport current and/or applied field of any orientation. Monofilamentary tapes of rectangular and elliptical geometry with anisotropic J_c(B), as well as square and round wires with isotropic J_c(B) have been used for simulations under various operating conditions. A comparison between AC losses, magnetic field, and current distributions in the tapes and wires is presented     

Characteristics Measurements of HTS Tape with Parallel HTS Tapes

Many high temperature superconducting (HTS) tape manufactures make an effort to reduce the transport current loss of HTS tapes. The knowledge of critical current and self-field in an HTS tape is very useful to compute the transport current losses. The spatial distribution and magnitude of self-field are variable due to the neighboring materials. In this paper, the critical currents and the transport current losses of BSCCO and YBCO tapes with paralleled magnetic material (Ni tape) and/or diamagnetic material (BSCCO tape) are experimentally investigated to improve the AC loss properties. The critical currents of HTS tapes with paralleled Ni tape are slightly decreased and the transport current losses are markedly increased. However, the critical currents and transport current losses of HTS tapes with paralleled BSCCO tape have not current carrying are more improved than single HTS tape.     

Electromagnetic Field Analysis of YBCO Coated Conductors in Multi-Layer HTS Cables

Electromagnetic field analyses of YBCO coated conductors in mono-layer and multi-layer high$ T_ c$superconducting (HTS) cables with practical configurations were carried out to calculate their AC losses. Numerical models employing one-dimensional FEM and two-dimensional FEM were used in the analyses. The error of the 1D-model was assessed by comparing the loss calculated by the 1D-model and that calculated by the 2D-model. The gap between the conductors in each layer, the relative position of conductors in the inner layer and in the outer layer, and the number of layers were varied to study their influence on the AC loss. The numerically calculated AC losses were compared with the analytical values.     

Finite-element method modeling of superconductors: from 2-D to 3-D

A three-dimensional (3-D) numerical modeling technique for solving problems involving superconducting materials is presented. The model is implemented in finite-element method software and is based on a recently developed 3-D formulation for general electromagnetic problems with solid conductors. It has been adapted for modeling of superconductors with nonlinear resistivity in 3-D, characterized by a power-law E-J relation. It has first been compared with an existing and verified two-dimensional (2-D) model: Compared are the current density distribution inside the conductors and the self-field ac losses for different applied transport currents. Second, the model has been tested for computing the current distribution with typical 3-D geometries, such as corner-shaped and twisted superconductors. Finally, it has been used with two superconducting filaments in the presence of external magnetic field for verifying the existence of coupling currents. This effect deals with the finite length of the conductors and cannot be taken into account by 2-D models.     

AC Losses in YBCO Coated Conductors Subjected to Tensile Stress

Stress dependencies of AC transport current losses, magnetization losses and total losses were investigated applying tensile stresses to an YBCO (IBAD/PLD) conductor. Measured transport current losses showed slight stress dependence in the range of 0$sim$1 GPa. That can be explained by stress dependence of the critical current of the conductor. The magnetization and total losses are less sensitive to the stress than the transport current losses.     

Fabrication of BiSCCO 2212/MgO conductors by melt processing

We have investigated the possibility of fabricating robust BiSCCO 2212/MgO composite conductor structures which can carry significant currents. These have been prepared by a simple melting process and final long term annealing, and the transport critical currents measured between 77 and 4 K. We have successfully created metre long conductors which carry more than 10 A supercurrent at 4 K, and have been able to integrate four individual spiral conductors together into a coil structure. The durability of this kind of composite conductor has been demonstrated by repeated testing and remounting of individual samples.     

Transport AC Losses in YBCO Coated Conductors

Modeling and design of superconducting power devices (e.g. fault current limiters), based on second generation HTS tapes, requires accurate evaluation and prediction of AC losses. Transport AC losses measurements have been performed on samples of YBCO coated conductors at 77 K, as a function of current. The results have been compared with the classical analytical model for self field AC losses calculation, taking into account the role of the magnetic substrate.     

Analysis of AC Loss and Current Distribution Characteristics of Multi-Layer Triaxial HTS Cable for 3-Phase AC Power Transmission

This paper presented numerical studies on layer-to-layer current distributions and AC losses in triaxial multi-layer three-phase cable conductors. The simulations were performed for triaxial cables of 2-layer/phase configurations with several combinations of twist pitch to investigate the influence of the cable twist configuration on the layer-to-layer current distribution and the AC loss under 3-phase AC operation. From the numerical results, it was shown that the twist pitch combinations for uniform layer-to-layer current distributions could be found. The numerical results also showed, however, that the AC loss can not be reduced by equalizing the layer-to-layer current distribution. The reason is clarified from the numerical data of the current density profiles across the superconducting layers. For the cable design, it is necessary to select proper twist configuration considering not only the equalization of current distribution but also the minimization of AC loss.     

Numerical Evaluation of Total AC Loss in Parallel Tape Conductor With Transport Current in Oblique Magnetic Field

We numerically calculate AC losses in a three-strand parallel tape conductor with optimum transposition by means of the finite-element method that directly analyzes the magnetic field distribution. The parallel conductor carries an alternating transport current, whose amplitude is 70% of the critical current, in an in-phase external AC magnetic field that has an arbitrary angle to the flat face of Bi-2223 tapes. It is confirmed that the AC losses in the range of very small and large field amplitude are almost equal to those in cases of applying only a transport current to the parallel conductor and only an external magnetic field to a single Bi-2223 tape, respectively. However it is found that the AC losses are affected by the interaction among the transport current, external magnetic field, and strand number in the range around the full penetration field. Our proposed finite-element analysis helps us to evaluate the AC loss generated in such the middle range of field amplitude quantitatively.     

AC losses in HTS tapes in applied longitudinal magnetic fields at variable temperatures

In most power devices, the conductor is carrying an ac transport current while it is exposed to an ac magnetic field transverse to the current path. In certain applications, such as power cables or a control winding in a controllable reactor, the conductors are exposed to a magnetic field component longitudinal to the tape axis that is parallel to the current path. To create an improved base for the design of such power devices it is of interest to study the losses in high-temperature superconductor tapes due to longitudinal field in detail. We have investigated the losses at several temperatures of a nontwisted multifilamentary Bi-2223 tape when it was exposed to a longitudinal magnetic field. The losses were measured with a calorimetric method and the results were compared with the critical state hysteresis loss model. The hysteresis losses are dominating at power frequencies (50, 60 Hz) in the investigated field range 2-200 mT and are accurately described by the critical state hysteresis model.     

Numerical analysis of the effects of the magnetic self-field on the transport properties of a multilayer HTS cable

In this paper, the effects of the magnetic self-field on the transport properties of a multilayer high-T/sub c/ superconducting (HTS) cable are investigated by means of two-dimensional finite-element method (FEM) simulations. Analyzed is a three-layer HTS cable, but the developed methods can be used for a different number of layers. The superconductor is described by the nonlinear power-law relation E=E/sub c/(J/J/sub c/)/sup n/, where the parameters J/sub c/ and n depend on the magnetic field experienced by the material. This dependence decreases the global transport capacity of the superconductor, enhancing its AC losses. It is shown that, especially at high transport currents, the AC losses are considerably higher than in the case where the dependence on the magnetic field is neglected. A simple electrical model, considering the cable from macroscopic point of view, has been proposed for finding the optimal winding pitches, leading to a uniform current repartition. The use of this electrical model allows to overcome the difficulties of direct three-dimensional FEM computations. In addition, the rapidity of solutions by the electric model gives the possibility of testing quickly many geometrical configurations in order to find the ones leading to an even current repartition. This optimization process would not be possible with detailed FEM simulations.     

Magnetization Loss and Shield Effect in Multi-Stacked Tapes With Various Stacking Configurations

AC loss is one of the major topics in large AC power applications using high temperature superconductor such as power transformers, transmission cables and fault current limiters because it is closely related to operation efficiency. Multi-stacked tape conductors should be used to transport the large current in those power applications. A research of various arrangements of HTS tapes for multi-stacked tapes has been performed to increase the capacity of transport current in HTS power applications. In this paper, we studied magnetization loss and shield effect from several different arrangements of BSCCO tapes such as Face-to-Face type, regular matrix type (m$times$2) and irregular matrix type. The results show that the magnetization loss of the Face-to-Face arrangement was lower than those of the other matrix types for the same stacking numbers. We think that the result was due to the shield effect by demagnetization of adjacent HTS tapes which were located as face to face.     

Computationally efficient winding loss calculation with multiplewindings, arbitrary waveforms, and two-dimensional or three-dimensionalfield geometry

The squared-field-derivative method for calculating eddy-current (proximity-effect) losses in round-wire or litz-wire transformer and inductor windings is derived. The method is capable of analyzing losses due to two-dimensional and three-dimensional field effects in multiple windings with arbitrary waveforms in each winding. It uses a simple set of numerical magnetostatic field calculations, which require orders of magnitude less computation time than numerical eddy-current solutions, to derive a frequency-independent matrix describing the transformer or inductor. This is combined with a second, independently calculated matrix, based on derivatives of winding currents, to compute total AC loss. Experiments confirm the accuracy of the method     

A.c. loss measurements in high-tc superconductors

Low a.c. loss is a key issue for electrical engineering applications of high-T_c superconductors at industrial frequencies. Different material options are characterized by three different a.c. loss measurement techniques which approach as much as possible the application conditions of the conductors. Melt-textured-growth YBa₂Cu₃O_7−x bulk samples, suitable for magnetic bearings, are characterized in an external a.c. field by magnetization measurements. Melt-casted Bi₂Sr₂Ca₂Cu₃O_8−x samples and powder-in-tube Bi₂Sr₂Ca₂Cu₃O_10−x tapes are suitable for current leads or power cables. In these applications the conductors are exposed to a.c. transport currents. The associated losses in self field are measured by a very sensitive electrical measurement technique. Finally, a calorimetric method is necessary when larger conductors (for instance Bi₂Sr₂Ca₂Cu₃O_8−x tubes) are tested under transport currents I_t, generating a transverse magnetic field H α I_t, as encountered in magnet windings for SMES, transformers or generators. The results show that the a.c. losses are sufficiently low for self field applications at industrial frequencies and a comparison of the different high T_c superconductors is given. The results show further that the a.c. losses are essentially hysteretic and can be modeled using the Bean model.     

Self-field and geometry effects in transport current applications of multifilamentary Bi-2223/Ag conductors

This paper describes quantitatively the influence of the self-field and the cross-sectional geometry on the effective critical current and the ac losses in transport current applications of nontwisted multifilamentary Bi-2223/Ag conductors. The results are obtained with finite-element method simulations. The numerical implementation includes an anisotropic model for the dependence of the critical current density J/sub c/ and the power index n on the local parallel and perpendicular magnetic field components. The relation is given between the intrinsic critical current density and the effective critical current for different multifilamentary conductors. Shown are examples of the current and magnetic flux density distributions in order to demonstrate their effect on the ac losses in self-field.     

Quench Behavior of YBa2 Cu3O7 Coated Conductor With AC Transport Current

Alternating current (ac) loss and quench behavior measurements were performed on a YBa₂Cu₃O_x coated conductor at 45 K. The minimum quench energies (MQEs) and the quench propagation velocities were measured as a function of transport current and frequency. AC losses were measured at the corresponding temperature and frequencies to quantify the internal thermal load during ac quench measurements. It was found that the direct current (dc) and ac quench behavior are similar, but that the MQE for dc experiments is higher than the corresponding ac cases. This difference is not seen in the normal zone propagation velocity data, indicating that the difference in MQE is minimal if the ac losses are included in the energy calculation. The ac experiments show that the MQE and propagation velocity varies weakly with frequency.     

Mechanical Losses Due to Frictional Heat in Bi-2223 Tapes With Structural Materials of Various Thermal Expansion Properties

We have experimentally studied relation between mechanical losses and thermal expansion properties in superconducting coils. For the experiments, three kinds of structural materials (GFRP, DGFRP, and ZFRP) were used. A glass fiber reinforced plastic (GFRP) contracts with decreasing temperature; on the contrary, a Dyneema and grass fiber reinforced plastic (DGFRP) and a Zylon fiber reinforced plastic (ZFRP) expand with the decrease of temperature. We used sample holders made of those plastics and measured AC current losses of a Bi-2223 tape in liquid nitrogen. Even if the magnetic field applied to the tape and an AC transport current of the tape were same, the loss depended on the sample holder materials. When the sample holder is the GFRP, the loss was largest in the three kinds of plastics, and in case of the ZFRP, the loss was smallest. The ZFRP has the largest expansion property of the three materials, that is, the loss decreased with expansion of the sample holder's materials.