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This paper presents the influence of superconducting tape insulation on the recovery time of superconducting fault current limiters. The analysis is based on the experimental results of short-circuit tests. The reduction in the thermal and dynamic effects of the passage of a fault current can be achieved by limiting the short-circuit time and the value of the surge current. An ideal fault current limiter is required to have almost zero impedance at operating currents and significant impedance at fault conditions. A superconducting fault current limiter (SFCL) meets these requirements under certain conditions. The recovery time—a very important parameter—shows the ability of the limiter to return to the superconducting state to be ready to limit the subsequent short circuit. The experimental results show that the recovery time can be significantly reduced with the application of thin-film insulation and an appropriate design of the conduction cooling of the HTS tape. [ABSTRACT FROM AUTHOR]

Numerical simulations of the electro-mechanical behavior in arrays of superconducting tapes are presented in this paper. The superconducting properties of tapes are influenced not only by the local magnetic field but also the mechanical strain. The mechanical strain will reduce the critical current density and lead to the degradation of superconductivity. Due to the nonlinear E − J behavior, it is difficult to analytically obtain the two-dimensional current and magnetic field distributions. We use the magnetic energy minimization method based on the field-dependent critical current density to determine the distributions of the current and magnetic field in arrays of rare-earth-barium-copper-oxide (REBCO) tapes. Next, the finite element method is employed to calculate the mechanical strain and stress distributions, and the anisotropic mechanical properties of the REBCO tapes are adopted. The numerical method is verified with the analytical solution for a one-dimensional single tape with transport current. The strain and stress distributions in superconducting tapes are calculated for different transport current/external magnetic field conditions. The relationship between the stress and the number of tapes in the arrays is discussed, and the effect of the distance between adjacent tapes on the stress is also investigated. The results may be beneficial to the design of superconductor magnets. [ABSTRACT FROM AUTHOR]

This paper presents the design of a Superconducting Fault Current Limiter (SFCL) and calculation results of forces and stresses in the windings of a resistive fault current limiter. The design of the fault current limiter consists of two parallelly connected and magnetically coupled windings, cooled by a single stage cryocooler. Magnetically compensated windings made of HTS tape give a very low voltage on the limiter at a nominal current. Limitation of the short-circuit time and the value of the maximum initial fault current reduces the thermal and dynamic effects of the passage of a fault current. Using devices which limit the value of a fault current can lower the level of required short-circuit capacity of the elements in a system. However, selected means of fault currents limitation must maintain the power quality standards. A perfect fault current limiter is required to have substantial impedance in fault conditions and zero impedance at work currents. Such requirements are met by a SFCL. An increase of current caused by the occurrence of a fault current results in the transition of the superconducting material from the superconducting state into the resistive state. This increases the impedance of short-circuit loop, allowing the fault current value to decrease. During a short-circuit, the forces generated from the short-circuit current also act on the limiter windings. Short-circuit current causes stresses in the superconducting tape. Exceeding the permissible stress value results in an irreversible reduction in the critical current of the superconducting tape. Calculations of the forces and stresses in the HTS tape for the maximum value of the short-circuit current were carried out using the finite element method. The constructed limiter was tested and the winding design ensures that the tape stresses are at a safe level even for short-circuit currents. [ABSTRACT FROM AUTHOR]

We have studied vortex pinning and dynamics in a Sr0.6K0.4Fe2As2 superconducting tape with critical current density Jc ∼0.1 MA/cm² at 4.2K and 10 T. It is found that grain boundary pinning is dominant in the vortex pinning mechanism. Furthermore, we observe large density of dislocations which can also serve as effective pinning centers. We find that the temperature dependence of critical current density is in agreement with the model of vortices pinned via spatial fluctuation of charge carrier mean free path. Magnetic relaxation measurement indicates that the magnetization depends on time in a logarithmic way. The relaxation rate in the low and intermediate temperature region is small, and it exhibits a weak temperature and field dependence. A crossover from elastic creep to plastic creep regime is observed. Finally, we conclude a vortex phase diagram for the high performance Sr0.6K0.4Fe2As2 superconducting tape. [ABSTRACT FROM AUTHOR]

Superconducting materials have excellent electrical and magnetic properties and have great development prospects. NbTi and Nb3Sn are currently the two most widely used low-temperature superconducting materials, but they require liquid helium refrigeration to be used at 4.2 K ultra-low temperature, which is costly. The critical superconducting temperature of Bi-2223 is as high as 110 K, and can use liquid nitrogen for cooling, showing good practical value. Bi-2223 is a compound with a layered structure, which is often prepared into tapes, so that the layer direction is parallel to the surface of the tape to exert its best superconductivity. However, on the road to its practical application, there are still problems such as insufficient purity of Bi-2223 phase, insufficient density, poor crystal grain connectivity, insufficient tensile strength and elongation. In order to be practical, factors such as electrical performance, mechanical performance, and refrigeration technology must be both considered. This paper summarizes the preparation technology and research progress of the practical Bi-based superconducting tape. The preparation process of Bi-2223 precursor powder, the preparation process and research progress of Bi-2223 tape, the progress of the practical application of Bi-2223 superconducting tape, etc. are introduced respectively, which provide references for further in-depth research on its preparation process and practical application. [ABSTRACT FROM AUTHOR]

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Copyright of Przegląd Elektrotechniczny is the property of Przeglad Elektrotechniczny and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A novel numerical method of calculating the current transfer length (CTL) from several matrices to superconducting region has been proposed in this paper, there are two ways to calculate the CTL corresponding to two criteria of CTL: E-based criterion and I-based criterion, and the relationship between these criteria have been given again. The method of calculating the CTL from several matrices to superconducting region has high accuracy compared with the method introduced by Stenvall, and simulation and experiments of measuring the CTL have been carried out to compare with the ones calculated by the numerical method. The results show that the novel numerical method has high accuracy in calculating the CTL of superconducting tape and is useful for generalizing to calculate the CTL of superconducting cable. [ABSTRACT FROM AUTHOR]

Highlights • The twisting chirality configuration obviously affects the bending strength of the HTS tape. • The optimum twist angle of the HTS tape is determined. • A numerical model is built to investigate the effect of the configuration on the properties of the tape. Abstract Seeking a geometry that can withstand greater transverse loads based on the electromechanical material properties of high-temperature superconducting (HTS) tape is an effective way of improving the transport performance of HTS cables. The cabling method requires the determination of the optimum twist angle of the HTS tape for withstanding transverse loads. This paper investigates the critical current characteristics of HTS tapes under combined deformation. The limit range of the twist angle under the combined deformation is measured and the optimum twist angle of the HTS tape is determined. The results show that the twisting chirality configuration obviously affects the bending strength of the HTS tape. In the elastic range, increasing the pre-twist angle increases the bending strength of the HTS tape, thereby improving the transport performance. In addition, a numerical model is built to further investigate the effect of the twisting chirality configuration on the electromechanical properties of the HTS tape, and the experimental results are explained. The experimental and simulation results generally agree well, and calculations show that there is always a sharp change in stress at the interface of different materials. These findings explain the mechanism of the effect of the twisting chirality configuration on the mechanical behavior and critical current of the HTS tape. They also provide a reference for cabling methodologies for the HTS cable configuration. [ABSTRACT FROM AUTHOR]

Highlights • A homogenization method for 1G multifilamentary tapes is revisited. • It is verified against AC loss results from published experiments. • Extension towards modelling losses in 1G power cables is performed. Abstract Challenges, linked with FEM modelling of HTS power cables, include the long computational time required to simulate the small domains that Bi-2223 superconductors possess in a large device. To decrease that time, in this paper the models of AC transport power losses for a tape and a simplified cable are achieved by a homogenization technique via a "filamentary-equivalent domain" approach. In it, a single homogenous domain with effective material properties is used to represent the superconducting material, where the domain would have the approximate shape of the multifilamentary region. In order to investigate its validity for a range of transport currents, including overcurrent, four tapes from the literature are modelled in three configurations. A simplified cable model is modelled via the homogenized model and an analytical equivalent circuit model. The homogenization technique can accurately predict the transport power losses of the majority of configurations while demonstrating fast computational times and is promising for simulation of real power cables. [ABSTRACT FROM AUTHOR]

With the high penetration of Renewable Energy Sources (RES) in power systems, the short-circuit levels have changed, creating the requirement for altering or upgrading the existing switchgear and protection schemes. In addition, the continuous increase in power (accounting both for generation and demand) has imposed, in some cases, the need for the reinforcement of existing power system assets such as feeders, transformers, and other substation equipment. To overcome these challenges, the development of superconducting devices with fault current limiting capabilities in power system applications has been proposed as a promising solution. This paper presents a power system fault analysis exercise in networks integrating Superconducting Cables (SCs). This studies utilized a validated model of SCs with second generation High Temperature Superconducting tapes (2G HTS tapes) and a parallel-connected copper stabilizer layer. The performance of the SCs during fault conditions has been tested in networks integrating both synchronous and converter-connected generation. During fault conditions, the utilization of the stabilizer layer provides an alternative path for transient fault currents, and therefore reduces heat generation and assists with the protection of the cable. The effect of the quenching phenomenon and the fault current limitation is analyzed from the perspective of both steady state and transient fault analysis. This paper also provides meaningful insights into SCs, with respect to fault current limiting features, and presents the challenges associated with the impact of SCs on power systems protection.

Copyright of Przegląd Elektrotechniczny is the property of Przeglad Elektrotechniczny and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In recent years, there are fast-increasing concerns on the utilizations of superconducting rotating electrical machines in different application areas, such as ship propulsion systems, aircraft drivers, and wind turbine generators, since these machines exhibit the merits of high current density, compact design, high power density, light weight, high torque density as well as high efficiency. One of the main limitations in front of the vast use of superconducting tapes in the fabrication of electrical machineries is AC magnetizing loss when tapes are exposed to an external magnetic field, which can decrease the critical current density of wires, as well. In the literature, most of the research works have been done on calculation of the AC magnetizing loss under a pure external magnetic field, while in reality, magnetic flux lines in AC electrical machines are usually distorted with harmonics because of different reasons such as distorted leakage flux, distributed coils of a winding in several slots, cogging fields, mechanical faults, etc. Since these distorted fields contain harmonics, then in this paper, the AC magnetizing loss of superconducting tapes has been electromagnetically modeled and calculated when they are subjected to nonsinusoidally distorted external magnetic fields. The magnetic field dependency of critical current density has been considered in a proposed finite element model. The results have shown that the AC magnetizing loss increases significantly under a distorted applied field compared with a sinusoidal one. In addition, the loss increase depends on the harmonic content which would increases drastically with total harmonic distortion of the applied magnetic field. [ABSTRACT FROM AUTHOR]

The home-designed metal organic chemical vapor deposition (MOCVD) system was applied to prepare the Gd0.5Y0.5Ba2Cu3O7-δ (GdYBCO) films at a high deposition rate in order to improve the production efficiency and reduce the preparation cost of high temperature superconducting tapes. Based on a simple self-heating method, the distance between the shower head and the substrate surface can be reduced effectively to increase the concentration of metal organic sources on the substrate surface, which can commendably improve the deposition rate of GdYBCO films and the utilization ratio of metal organic sources. At last, the GdYBCO films were successfully prepared on the LaMnO3 template at the high deposition rate of 1 μm/min by the MOCVD process based on the simple self-heating method and the critical current (Ic) was more than 220 A/cm-width (77 K, 0 T), corresponding to the critical current density (Jc) more than 4.4 MA/cm2 (77 K, 0 T). [ABSTRACT FROM AUTHOR]

In the feeder system of the Experimental Advanced Superconducting Tokamak (EAST), the high-voltage (HV) electrical vacuum feedthrough plays an important role in transmitting the voltage signals from one vacuum environment to anther vacuum environment under cryogenic temperature. Due to no commercial solutions, the feedthroughs for EAST were developed in the Institute of Plasma Physics, Chinese Academy of Sciences. In this paper, the structural design, electrical analysis, structural analysis and experiments involving thermal cycle, helium gas tightness, HV-leakage current and Paschen discharge are presented. The experimental result indicates that the leak rate of the developed feedthroughs was less than 1 × 10 −9 Pa·m 3 ·s −1 . After the 5 thermal cycles from room temperature (RT) to 77 K, the HV-leakage current experiment under 10 −3 Pa vacuum environment and the Paschen discharge experiments of feedthroughs were performed, the leakage current under 15 kV DC is lower than the allowable value 5 µA, and the leakage currents in helium gas and nitrogen gas from 1 Pa to 50 kPa are lower than the allowable value 10 µA. [ABSTRACT FROM AUTHOR]

Second-generation high-temperature superconducting (2G HTS) tape is used in magnets and cables because of its outstanding electromagnetic characteristics. However, with the development of winding technology, thinner tapes are required in the construction of magnets. The effect of using thinner substrates on the resulting mechanical and electrical properties of 2G HTS tapes must thus be urgently understood. The interfacial adhesive strength is an important index used to characterize the mechanical strength of 2G HTS tape. Previous experimental studies have shown that thermal stress is one of the major factors in the delamination of the component tape used for magnet winding or cable assembly. In this study, the effect of substrate thickness on the interfacial adhesive strength of 2G HTS tape was investigated using peel test modeling. The thermal residual stresses accumulated during tape synthesis and caused by altered temperature during tape preparation and application at 77 K were also considered. To address the geometrical, physical, and boundary nonlinear problem, the finite element method was used. The simulation results indicate that interfacial stress caused by thermal shrinkage may separate the tape near the superconductor layer at the outer edge; however, no significant effect was observed for the central part. When the thermal residual stress was considered, the peel strength was reduced by approximately 20%. The substrate thickness also played an important role in the magnitude of thermal residual stress, which resulted in an increase of the peel strength with decreasing substrate thickness. [ABSTRACT FROM AUTHOR]

The performance of a multi-layer composite superconducting tape depends on operating conditions, such as the applied field, temperature and stress. Therefore, accurate analysis of the electromagnetic-thermo-mechanical coupling behaviors of the high-temperature superconducting (HTS) cable is important to understanding the mechanism of interaction among different physical fields and the success of optimizing the design of the HTS cable structure. This paper develops a three-dimensional finite element model based on the twisted-stack slotted-core HTS cable structure for electro-thermo-mechanical analysis. Critical currents of a single multi-layer composite superconducting tape and the whole HTS cable structure are calculated. Profiles of the magnetic field, current density, temperature and stress are also obtained. It is found that the selection of tapes having higher critical current enhances the transport performance of the cable. Current densities are nonuniformly distributed in the stack. From the bottom to the top of the stack, the current density first increases and then decreases, in inverse relation to the magnetic field. Cooling with liquid helium improves the cable transport capacity. The temperature of the superconducting tapes is slightly higher than that of the aluminum core, but the stress in the aluminum core is greater than that in the superconducting tapes. The results imply that the transport capacity of the cable can be improved by selecting superconducting tapes with high critical current density and arranging them in stacks in a manner consistent with the electromagnetic field distribution. The model also explains the mechanism of interaction between different physical fields. It is a useful tool that clarifies the multiphysics coupling behaviors of an HTS cable with complex structure, and provides a theoretical reference for the design of novel HTS cable structures. [ABSTRACT FROM AUTHOR]

Superconducting tapes made of REBa2Cu3Oy (REBCO; RE: rare earth, i.e., Y, Gd, or Eu) exhibit superior performance at the critical current density, even at liquid-nitrogen temperatures. However, the current capacity of these tapes must be enhanced for use in large-scale applications. We have proposed a method of constructing parallel conductors, which must be insulated and transposed to obtain uniform current sharing and low ac loss. If the transposition points deviate from the optimal ones, a shielding current is induced to maintain a constant interlinkage magnetic flux, which produces an additional ac loss. In our previous studies, the theoretical expressions for the additional ac loss were derived on the basis of a critical state model. In this study, by using an n-value model to express the I-V characteristics of REBCO superconducting tapes, we theoretically investigated the additional ac loss properties of two-strand transposed parallel conductors wound into solenoid coils. The results showed that under the nonsaturation condition, the additional ac loss increases with decreasing n value. This suggests that in parallel conductors composed of REBCO tapes that operate at liquid-nitrogen temperatures, transposition must be performed with high accuracy at the optimal points. To avoid increasing the additional ac loss, the n value of the tapes must be consistently larger than approximately 10, and the deviation of the transposition point should remain below a few percent of the conductor length. [ABSTRACT FROM AUTHOR]

High-temperature superconductor (HTS) rare-earth-barium-copper-oxide (REBCO) tapes are very promising for use in high-current cables. The cable geometry and the layout of the superconducting tapes are directly related to the performance of the HTS cable. In this paper, we use numerical methods to perform a comparison study of multiple-stage twisted stacked-tape cable (TSTC) conductors to find better cable structures that can both improve the critical current and minimize the alternating current (AC) losses of the cable. The sub-cable geometry is designed to have a stair-step shape. Three superconducting tape layouts are chosen and their transport performance and AC losses are evaluated. The magnetic field and current density profiles of the cables are obtained. The results show that arrangement of the superconducting tapes from the interior towards the exterior of the cable based on their critical current values in descending order can enhance the cable’s transport capacity while significantly reducing the AC losses. These results imply that cable transport capacity improvements can be achieved by arranging the superconducting tapes in a manner consistent with the electromagnetic field distribution. Through comparison of the critical currents and AC losses of four types of HTS cables, we determine the best structural choice among these cables. [ABSTRACT FROM AUTHOR]

Eddy current loss in metal stabilized layers of a hightemperature superconducting coated conductor is affected by the superconducting layer. We present ac loss measurement results on a 10-mm-wide nonstabilized Fujikura tape, on 10-mm-wide, 0.1-mm-thick copper strips, and on the Fujikura tape stacked vertically with the copper strips in a perpendicular applied field at 77K. The applied magnetic field amplitude was varied up to 100 mT at 67.9, 87.6, and 107 Hz. The eddy current loss in the copper strips was estimated by taking the difference between the loss in the stack and the isolated Fujikura tape. The results show approximately complete shielding of perpendicular magnetic fields in samples at low magnetic field amplitude, and there is little dependence of the eddy current loss on the copper strip thickness up to 300 μm. We also present the shielding of perpendicular magnetic fields by copper strips stacked with a 30-cm-long 15/4 (15 4-mm-wide strands) Roebel cable. The shielding of perpendicular magnetic fields in the Roebel samples was observed even at high field amplitudes. [ABSTRACT FROM AUTHOR]

As an emerging technique for surface smoothing, solution deposition planarization (SDP) has recently drawn more attention on the fabrication of the second generation high temperature superconducting (2G-HTS) tapes. In our work, a number of amorphous oxide layers were deposited on electro-polished or mirror-rolled metallic substrates by chemical solution route. Topography evolution of surface defects on these two types of metallic substrates was thoroughly investigated by atomic force microscopy (AFM). It was showed that root mean square roughness values (at 50 × 50 μm 2 scanning scale) on both rough substrates reduced to ∼5 nm after coating with SDP-layer. The smoothing effect was mainly attributed to decrease of the depth at grain boundary grooving on the electro-polished metallic substrate. On the mirror-rolled metallic substrates, the amplitude and frequency of the height fluctuation perpendicular to the rolling direction were gradually reduced as depositing more numbers of SDP-layer. A high J c value of 4.17 MA cm −2 (at 77 K, s.f. ) was achieved on a full stack of YBCO/CeO 2 /IBAD-MgO/SDP-layer/C276 sample. This study enhanced understanding of the topography evolution on the surface defects covered by the SDP-layer, and demonstrated a low-cost route for fabricating IBAD-MgO based YBCO templates with a simplified architecture. [ABSTRACT FROM AUTHOR]

Copyright of Przegląd Elektrotechniczny is the property of Przeglad Elektrotechniczny and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A mechanical lap joint of REBCO high-temperature superconducting (HTS) tapes with an indium foil inserted between joint surfaces has been proposed for segment-fabrication of HTS magnet of an advanced fusion reactor and various HTS applications. In a previous study, we successfully achieved a joint resistivity of about 3.5 pΩm2 by heat treatment during fabrication of the joint. In this study, improved heat treatment method was developed by controlling contact pressure and combining bake-out process. We evaluated joint resistance and critical current of the joint depending on the heating condition and bake-out condition. The joint resistivity after heat treatment was 2.5 pΩm2 without decreasing critical current with conditions of heating temperatures of 90–140 °C, a heating time of 30 min, a contact pressure of 100 MPa, a bake-out time of 30 min, and a bake-out temperature of 150 °C. Owing to applying the contact pressure when the indium becomes softer, the true area of contact increased and the thickness of the indium decreased greater than those in the previous method. In addition, it was shown that voids appearing by heating were removed by bake-out process from thermal desorption spectroscopy test. [ABSTRACT FROM PUBLISHER]

High-temperature superconductors (HTS) have the significant potential of magnetic-force-based applications. Nowadays, wide spread of coated conductors tapes (CC-tapes) enables the use of HTS stacks in levitation applications. For the development of CC-tapes-based bearings and transportation systems, it is important to have information on stability of the system in response to the lateral displacements. This study continues series of studies on the levitation properties of CC-tapes stacks and primarily focuses on the magnetized stacks. In our study, we present new results on investigation of the magnetic levitation force between the permanent magnet and magnetized stack of HTS tapes containing from 100 and 200 tapes of 12 mm × 12 mm. Stacks were magnetized in 8-T superconducting magnet and placed in a levitation force measurement system. Levitation force and trapped flux were studied for various magnetizations of the stack and for various numbers of layers in the stack. In addition, stability of the system was studied. The experimental investigations on the influence of lateral displacement on the levitation performance of stack with different trapped fluxes and stack thicknesses were processed in this study. Levitation force decay during the lateral displacements above a permanent magnet was observed. [ABSTRACT FROM PUBLISHER]

We have developed and installed an 11 T Bi2Sr2Ca2Cu3Oy (BSCCO) insert coil for a 25 T cryogen-free superconducting magnet (25 T-CSM). The insert coil composed of a stack of 38 double pancakes was wound with DI-BSCCOTM Type HT-NX conductors which had high critical tensile strength of 400 MPa at 77 K. The insert coil is cooled by two two-stage Gifford-McMahon cryocoolers and the operation temperature of the insert coil is designed to be below 10 K. The insert coil has successfully generated 11.48 T without a background field and 24.57 T with a 14.0 T background field produced by outer low temperature superconducting coils in a 52 mm room temperature bore. Before the performance test, we fabricated and tested an R&D insert coil consisting of two double pancakes, which had the same configuration as the actual insert coil pancakes do, to confirm mechanical strength of the conductors [ABSTRACT FROM PUBLISHER]

REBCO superconducting tapes have excellent critical current properties even at high temperature and high magnetic field. Many projects for the research and development of superconducting machines and devices with REBCO tapes are in progress worldwide. The important problem to realize them is the ac loss reduction of REBCO superconducting tapes since the most part of thermal load of cryocoolers is the ac loss. The evaluation of the induced ac loss in the windings is also quite important to design the superconducting system, especially the cooling system. In this study, we investigated the applicability of the scaling law to the stacked tapes using the currently fabricated REBCO superconducting tapes. We defined the number of stacked tapes as n, and the temperature as T . The ac loss for \rmB{\rm{m}}\,{\rm{< \,B}}{\rm{pe}} decreased with increasing n due to the influence of diamagnetic field, whereas the ac losses for \rmB{\rm{m}}\,{\rm{> B}}{\rm{pe}} agreed with each other regardless of n. By defining Ic around zero field at the respective temperature, in the respective number of stacked tapes as {\rm{Ic0(T,\,n)}}, {\rm{I}}_{{\rm{c}}}-B normalized by using \rmIc0(T,\,n). The normalized \rmI{\rm{c}}\-{\rm{B}} curves coincided with one master curve for every $n$ regardless of T$. The normalized M-H curves and ac loss curves against the field amplitude by using {\rm{Ic0(T,\,n)}} also coincided with one master curves for every n regardless of T. It was verified that the scaling law of M and ac loss was available even if the REBCO tapes stacked into multilayers. [ABSTRACT FROM PUBLISHER]

This paper presents the design and experimental results of a coreless superconducting fault current limiter (SFCL). The limiter has a rating of 15 kV and 140 A and was tested with a 2.5 GVA short-circuit generator. During the occurrence of a short circuit, the elements of the system are less exposed to electrodynamic and thermal overloads. The application of an SFCL leads to an increase of the capability of the power network for connecting distributed generation energy sources. Compact design of the coreless superconducting fault current limiter consists of three magnetically coupled windings cooled in liquid nitrogen. A high magnetic coupling between the superconducting windings gives a very low voltage on the limiter at a nominal current. The presented solution reduces size and weight of the device. The paper summarizes the design of the coreless limiter and presents in detail the results of tests performed at a high-power test facility. [ABSTRACT FROM PUBLISHER]

FeSe Superconducting tapes were fabricated with powder in tube (PIT) process. Both in-situ and ex-situ PIT processes were performed with Se and sintered Fe-Se powders as precursor powders, respectively. The influences of different fabrication techniques on the phase evolution mechanism, morphology and superconducting properties of these tapes were systematically investigated. It was noticed that during the in-situ PIT process, large content of tetragonal β-FeSe phase could be formed under low temperature. However, the obtained β-FeSe phase was not stable with increasing temperature and decomposed into δ-FeSe and Fe. Although with increasing temperatures, β-FeSe phase content increased again, the final β-FeSe phase content could not reach 100%, with relatively large content of δ-FeSe particles distributed uniformly in β-FeSe matrix. On the other hand, during the ex-situ PIT process, higher temperature was necessary for the formation of higher ratio of β-FeSe phase from δ-FeSe phase. With the increasing temperature, β-FeSe phase content increased monotonously, and the β-FeSe content of nearly 100% was obtained under the sintering temperature of 1000 °C. Due to the different Fe/Se ratio in the obtained β-FeSe phase, a superconducting transition at 8.3 K was obtained on the ex-situ FeSe tape. Further optimization of this PIT process for larger superconducting phase content is on the way. [ABSTRACT FROM AUTHOR]

This review shows the highlights of a 4-year-long research project supported by the Japanese Government to explore new superconducting materials and relevant functional materials. The project found several tens of new superconductors by examining ∼1000 materials, each of which was chosen by Japanese experts with a background in solid state chemistry. This review summarizes the major achievements of the project in newly found superconducting materials, and the fabrication wires and tapes of iron-based superconductors; it incorporates a list of ∼700 unsuccessful materials examined for superconductivity in the project. In addition, described are new functional materials and functionalities discovered during the project.

Structural finite element analysis (FEA) has been used as an insightful tool to investigate the electromechanical behavior of HTS REBCO tapes and twisted stacked-tape cables under tension, torsion, bending and combined loads. A novel technique was developed for modeling the layered composite structure of the 2G tapes with structural solid-shell elements in ANSYS®. The FEA models produced detailed strain information for the REBCO superconducting layer which was then paired with an analytical model to predict the critical current performance of the 2G HTS tapes under various loads. Two commercially available HTS tapes (SuperPower and SuNAM) under tension, torsion and combined tension–torsion were first analyzed with FEA and compared with available experimental results at 77 K. A sharp critical current degradation was experienced at the yield strength of the tapes under tension and below a 100 mm twist-pitch under torsion. Combined tension–torsion loads had a more gradual degradation of critical current for twist-pitches of 115 mm or shorter but had a negligible difference compared to pure tension for longer twist-pitches. Using the structural solid-shell technique for modeling 2G tapes in ANSYS®, an FEA methodology for simulating full scale three-dimensional HTS stacked-tape cables under pure bending was created. A model of a Twisted-Stacked Tape Cable (TSTC), a configuration first proposed at MIT, was initially developed and then adapted to the slotted-core HTS Cable-In-Conduit Conductor produced by the ENEA laboratory in Italy. The numerical axial strain of the HTS REBCO tapes within the cables as calculated by FEA were found to agree with an analytical model for two cases: perfect-slip (frictionless) and no-slip (bonded). The ENEA CICC model was also compared with recent experimental critical current data at 77 K and was found to match best using a low friction coefficient of 0.02 indicating that the tapes within the cable freely slide with respect to each other helping to reduce the axial strain during bending. [ABSTRACT FROM AUTHOR]

An extensive characterization of commercially available High-Temperature Superconducting (HTS) REBCO tapes has been recently performed at KIT. The main thermo-physical properties of the tapes have been measured, and heat slug and quench propagation have been investigated in vacuum at LN 2 temperature, using a resistive heater as driver and recording the voltage and temperature evolution after the pulse at several locations along the tapes. In this paper, we present a study of thermal disturbance propagation in a HTS tape with brass stabilizer. The experimental data are analyzed first, to identify the phenomena that influence heat propagation in the tape, and namely the heat loss to the sample holder and the non-ideal efficiency of the resistive heater. A numerical tool is then developed, which solves the 1D transient heat conduction equation in each layer of the tape and accounts for the thermal coupling between layers. The heat loss to the sample holder and the non-ideal efficiency of the resistive heater are taken into account in the model. A first validation of the thermal part of the model against an extended database of heat slug propagation tests is then performed: the comparison between simulation and experiment confirms the very good capability of the model to reproduce the measured temperature evolution. Finally, the results of the simulations of quench propagation are compared with experimental data, showing the capability of the model to reproduce the experiment, within the uncertainty in the input parameters. [ABSTRACT FROM AUTHOR]

In several HTS applications, the superconducting tapes experience the simultaneous presence of DC and AC excitations. For example in high-current DC cables, where the transport current is not perfectly constant, but it exhibits some ripples at different frequencies introduced by the rectification process (AC–DC conversion). These ripples give rise to dissipation, whose magnitude and possible influence on the device’s cooling requirements need to be evaluated. Here we report a study of the AC losses in a HTS coated conductor subjected to DC currents and AC ripples simultaneously. The modeling approach is based on an integral equation method for thin superconductors: the superconducting tape is modeled as a 1-D object with a non-linear resistivity, which includes the dependence of the critical current density J c on the magnetic field. The model, implemented in a commercial finite-element program, runs very fast (the simulation of one AC cycle typically takes a few seconds on standard desktop workstation): this allows simulating a large number of cycles and estimating when the AC ripple losses stabilize to a constant value. The model is used to study the influence of the flux creep power index n on the stabilization speed and on the AC loss values, as well as the effect of using a field-dependent J c instead of a constant one. The simulations confirm that the dissipation level should not be a practical concern in HTS DC cables. At the same time, however, they reveal a strong dependence of the results upon the power index n and the form of J c , which spurs the question whether the power-law is the most suitable description of the superconductor’s electrical behavior for this kind of analysis. [ABSTRACT FROM AUTHOR]

The magnetoresistance and irreversibility fields of commercial bismuth-based 1G tape were studied on the basis of the temperature dependencies of the magnetoresistance at the two relative orientations of magnetic field and superconductor plane. The critical temperatures of this tape are the following: T = 110.3 K and T = 109.9 K, and the width of superconducting transition is Δ T = 0.5 K. The widths of the transition to the superconducting state versus applied magnetic fields were derived for both orientations. The experimental data were fitted using the formula Δ T = C H + Δ T . The irreversibility field values were obtained and successfully fitted as a function of temperature. At 77 K, they were found to amount to H = 72.8 kOe and H = 5.5 kOe for the parallel and perpendicular directions, respectively. [ABSTRACT FROM AUTHOR]

An approach based on the insertion of a CuO layer in YBaCu O (YBCO)-coated conductor was proposed and demonstrates high current-carrying ability. The multilayer architecture was deposited on oxid-buffered Hastelloy tapes using a metal organic deposition (MOD) process (low-fluorine solution route). It was revealed that the introduction of a CuO layer was effective to avoid the presence of a-axis grain and pores of the YBCO films, which were frequently observed in multilayer films. Presently, the thickness of the multilayer films was almost 1.5 µm. Based on the improvements of the surface quality and c-oriented texture which were proved by X-ray ( ??-2 ??, f, and ?)-scan, AFM images, and SEM (surface and cross-section) images, the critical current density (Jc) of YBCO films with middle CuO and bottom CuO are 2.0 MA/cm and 2.5 MA/cm (at 77 K, self-field), respectively. [ABSTRACT FROM AUTHOR]

The recovery time is a crucial parameter to high temperature superconducting tapes, especially in power applications. The cooperation between the reclosing device and the superconducting facilities mostly relies on the recovery time of the superconducting tapes. In this paper, a novel method is presented to measure the recovery time of several different superconducting samples. In this method criterion used to judge whether the sample has recovered is the liquid nitrogen temperature, instead of the critical temperature. An interesting phenomenon is observed during the testing of superconducting samples exposed in the liquid nitrogen. Theoretical explanations of this phenomenon are presented from the aspect of heat transfer. Optimization strategy of recovery characteristics based on this phenomenon is also briefly discussed. [ABSTRACT FROM AUTHOR]

We construct a visible instrument to study the mechanical-electro behaviors of high temperature superconducting tape as a function of magnetic field, strain, and temperature. This apparatus is directly cooled by a commercial Gifford-McMahon cryocooler. The minimum temperature of sample can be 8.75 K. A proportion integration differentiation temperature control is used, which is capable of producing continuous variation of specimen temperature from 8.75 K to 300 K with an optional temperature sweep rate. We use an external loading device to stretch the superconducting tape quasi-statically with the maximum tension strain of 20%. A superconducting magnet manufactured by the NbTi strand is applied to provide magnetic field up to 5 T with a homogeneous range of 110 mm. The maximum fluctuation of the magnetic field is less than 1%.We design a kind of superconducting lead composed of YBa2Cu3O7-x coated conductor and beryllium copper alloy (BeCu) to transfer DC to the superconducting sample with the maximum value of 600 A. Most notably, this apparatus allows in situ observation of the electromagnetic property of superconducting tape using the classical magnetic-optical imaging. [ABSTRACT FROM AUTHOR]

This paper presents a simplification method for finite element models for the calculation of ac losses in stacks of superconducting tapes. A fast and accurate model of these losses can be exploited in the design of large-scale superconducting applications, where the coils are represented as stacks of tapes in 2-D models. The simplification method aggregates the superconducting, substrate, and copper layers of several windings of the superconducting tape to form bulk elements. The resulting model is able to evaluate the ac losses in the superconducting layer faster than the full model. The accuracy of the proposed method and decrease in computation time are shown. [ABSTRACT FROM PUBLISHER]

The powder-in-tube method is widely used in fabricating iron-based superconducting wires and tapes. To make tapes, a multi-pass rolling process is usually adopted. However, the multi-pass rolling process limits the efficiency of tapes. In this work, rolling deformation technique was studied systematically by fabricating Sr 1 − x K x Fe 2 As 2 superconducting tapes. The total rolling reduction ratio is about 80% and the difference of superconducting performance of tapes rolled by 2, 3, 5 and 7 passes has been investigated. The critical current density J c , Vickers micro-hardness and microstructure of the superconducting core indicate that tapes after 2, 3, 5 and 7 rolling passes exhibit a similar trend. The width of the tapes and the area of superconducting cores increase with decreasing the number of rolling passes, but the transport J c of tapes after different rolling passes seems to be the same, except for the tape rolled by 2 passes, whose transport J c is lower than the other tapes. Concerning the geometry uniformity for the superconducting cores, the sausaging phenomenon was not observed from the photograph of longitudinal cross-section of all the samples. “Lobes” phenomenon on transverse cross-section can be suppressed through decreasing the rolling passes. Therefore, we can obtain uniform and high-performance Ag-sheathed iron-based superconducting tapes by cutting the number of rolling passes down to 3, which is more advantageous to the large-scale producing in the future. [ABSTRACT FROM AUTHOR]

The experimental determination of the AC losses in a superconducting material or structure has been of long-standing interest to the academic community. The purposes of the current experiments are to explore the influences of the position of temperature sensor heating time and limit temperature rise on the measurement results with the calorimetric method. The optimization heating time and the limit temperature rise of this method are obtained. The present results are beneficial for the more precise measurements of the AC losses in hightemperature superconducting materials and structures. [ABSTRACT FROM AUTHOR]