Second-generation High-temperature Superconducting Research Articles

COMPUTER SIMULATION OF A SUPERCONDUCTING TRANSFORMER SHORT-CIRCUIT

High critical temperature superconductor power devices have unique characteristics that reduce energy losses during transmission and distribution of electricity. These superconductors allow for high current density, enabling the construction of smaller, lighter, and more transportable installations. A superconducting transformer can significantly reduce the dynamic effects of the short-circuit current during the initial milliseconds of a fault. High temperature superconductor windings, on the other hand, are relatively lightweight and have a low heat capacity. Despite being cryogenically cooled and managing already limited current, the windings made of second-generation high-temperature superconductor can be vulnerable to thermal damage, which may occur in a relatively short timeframe. A numerical study of a short circuit in a three-phase power system with a 10 MVA 115/16.5 kV superconducting transformer is presented in this paper. The superconducting tape's nonlinear electrical and thermal characteristics, as well as heat buildup in the transformer windings during a short circuit, are all taken into consideration by the transformer model.

Comparison of Commercial REBCO Tapes Through Flux Pinning Energy

This work presents a comparison of different commercial tapes belonging to the second-generation High-Temperature Superconductors (2G HTS) produced by SuNAM Co., Ltd., SuperOx, and Shanghai Superconductors Technology Co., Ltd. (SST) companies. The aim is to investigate pinning mechanisms responsible for best performances, looking at the anisotropy of the irreversibility field and of the flux pinning energy. The irreversibility line states the upper limit of current-carrying capacity, whereas the flux pinning energy explores the ability of material defects to act as weak collectively or strong single vortex pinning centers. All investigated samples have artificial pinning centers (APCs) included in the superconducting matrix: BHO-doped EuBCO for SST, Y2O3 in YBCO for SuperOx, and Gd2O3 particles trapped in GdBCO for SuNAM. Resistive transition curves were measured in high magnetic fields up to 16 T for magnetic field orientations parallel and perpendicular to the tape surface. We found that the anistropy of SST tape shows an overall independence both on temperature and magnetic field, while the other two samples show a more complex behavior. This leads to the conclusion that properly engineered APC optimization in coated conductors can further reduce anisotropy of superconducting properties.

Performance evaluation of a racetrack high temperature superconductor winding in an HTS levitation platform

Abstract High temperature superconducting (HTS) is a key technology for next generation high-speed railway systems, which guarantees high current and highly efficient traction power supplies. HTS devices can reduce system weight and thus improve loads of vehicles. In high-speed maglev trains, HTS windings wound by second-generation HTS tapes play an important role in suspension and traction. However, in complex ferromagnetic environments, the performance of windings is notably influenced by the varying magnetic fields. In this paper, a new maglev platform is designed and comprised of a winding of six racetrack HTS coils, an E-shaped iron core and a magnetic guide rail. The air core structure of the winding is modeled and tested and the electromagnetic characteristics are analyzed for three dimensional finite element analysis of the platform. An A formulation is used to calculate magnetic vectors A and infer magnetic flux densities and current densities for the structures. After the winding is installed on the iron core and the magnetic guide rail is suspended above the iron core, the air gap between the iron core and the rail is varied from 0 mm to 20 mm in models and experiments. The magnetic fields, critical currents and losses of the winding in the cases of the air core and the iron core conditions are used for analysis. The comparison between the air core and the iron core conditions confirms the reliability of the proposed models. This study provides a means for the analysis of complex superconducting maglev systems.

Preparation of double-layer REBa2Cu3O7−δ tapes for enhancing engineering current density by Ag-diffusion bonding

Second-generation high-temperature superconducting (2G-HTS) tapes with high engineering critical current densities (Je) are widely used in engineering and technology applications. This study introduced a twisting device that successfully layered two REBa2Cu3O7−δ films onto a single substrate. Heating the twisting device to above 600 °C induced Ag-diffusion bonding at the interface between the two tapes. A thin film pressure sensor detected a silver layer pressure of about 6.5 MPa. After the oxygen annealing treatment, a current increase of 81.7 A was observed using the four-probe method, which was a 1.5-fold enhancement over the original tape current, and the Je was increased from 560 A/mm2 to 790 A/mm2 at 77 K. The use of reverse bending annealing effectively improved the flexibility of the DHOS conductors. Through microanalysis, it was observed that the bonding between MgO and Y2O3 in the buffer layer was weak, resulting in the detachment of LaMnO3 and MgO together with the superconducting layer. The exfoliated buffer layer prevented oxygen penetration, adversely affecting the performance of the double HTS layers on one substrate (DHOS). Moreover, the coverage of the buffer layer increases with the number of layers complicating the complete peeling of the superconducting layer. Further detailed studies point out the direction of improvement for future work in preparing multilayer conductors.

Improved prediction of critical currents for multi-pancake coil across variable temperature regions

With the increasing performance of the second-generation high-temperature superconducting (2G-HTS) tapes, the technology of all-REBa2Cu3O7-δ (REBCO, RE: rare earth) superconducting magnets has developed rapidly. However, the 2G-HTS magnet with hybrid cryocoolers constantly adjusts its operating temperature according to the actual demands, which involves the critical current when the magnet crosses variable temperature regions. In this paper, a modified ideal model based on the homogenization model is proposed, which can predict the critical current of 2G-HTS magnets more consistently. The in-field properties of REBCO tapes in variable temperature regions were tested by the physical property measurement system. Furthermore, the improved model was combined with the in-field properties of REBCO tapes to obtain the critical currents of a 2G-HTS multi-pancake (MP) coil at 20 K, 30 K, 65 K, and 77 K. The critical current of a multi-pancake coil at 77 K was experimentally verified. The modified ideal model uses the actual current density, which results are closer to the experimental results with an agreement of 97.3%. This new method proposed in this work is significant in quickly predicting critical currents for 2G-HTS magnets across variable temperature regions.

A novel low-resistance solder-free copper bonding joint using a warm pressure welding method for REBCO coated conductors

Abstract Constrained by the fabrication of second-generation high-temperature superconducting (2G HTS) tapes, connecting multiple pieces of tapes through joints is often necessary in large-scale applications. In the application of HTS magnets, joint technology is key for achieving closed-loop operation and reducing thermal loads. However, most soldered joints still cannot achieve the expected results. Thus, there is an urgent need to find a method for easily fabricating low-resistance joints. In this study, a low-resistance solder-free copper bonding joint for 2G HTS copper-plated tapes is proposed. The formation mechanism of the joint is presented, and the effects of the bonding temperature and pressure on the electrical and mechanical properties of the copper bonding joint are investigated. The results show that the copper bonding joint can be manufactured by pretreating the tape for 5 minutes and bonding it in the air for 3 minutes at 333 MPa at temperatures higher than (or equal to) 150 °C or at pressures greater than (or equal to) 250 MPa and 180 °C. The characteristic resistance of this joint is approximately 16.8 nΩ·cm2, which is approximately one-third lower than that of soldered joints, and it has mechanical properties similar to those of soldered joints under axial tension. We believe that the application of this type of copper bonding joint can significantly aid in the design and manufacturing of large HTS magnets.

A novel joint-less second-generation high-temperature superconducting toroidal coil: Promise for fabricating compact toroidal magnetic fields

This paper presents a novel joint-less toroidal magnet made of second-generation high-temperature superconducting (2G-HTS) tapes. This approach effectively resolves the closed-loop issue for 2G-HTS magnets and has the potential to provide higher and more stable magnetic fields. Compared with traditional 2G-HTS toroidal magnets, while the current loops of the joint-less magnets have no resistance, a decrease in magnetic field still occurs, especially in coils with insulation. Therefore, this work focuses on the decrease of the magnetic field of this novel magnet using experimental and microanalytical methods. For the first time, it was verified that, by winding two coil groups, insulated and no-insulated, parallel charging does not cause interference between them. Furthermore, the magnetic field area was expanded by finite element analysis, and simulations showed that the magnetic field converged with increasing number of coils. Besides, we found that the decrease of the magnetic field was related to the damage of the tape during slitting and winding, where insulated coils were more susceptible to damage during winding. The damage usually occurred at the starting point of the tape slitting, because the copper layer was separated due to adhesion during the winding of insulated coils, which further caused the superconducting layer to detach, resulting in a decrease in the critical current. From our perspective, benefiting from the high critical field of the 2G-HTS tapes, this novel toroidal coil structure has significant implications for the construction of compact toroidal magnets.

Experimental study on the an-isotropic critical current of REBCO tape.

Superconducting magnets are widely used in nuclear fusion reactors, high-energy particle accelerators, steady-state high magnetic fields, etc. Higher magnetic fields and higher operating temperatures are two application trends. High temperature superconducting (HTS) materials are the only choice for high temperature and high field magnets in the future. The first- and second-generation HTS materials have a typical tape structure; their critical performance is magnetic field angle and temperature dependent. A new test facility is developed for an experimental study on the an-isotropic critical current. The field angle can be changed from 0° to 360° with a resolution of 1°. The rotation deviation angle is measured to be 0.2° when the upper part rotates 90°. The temperature can be changed from 4.2 to 80K. The temperature errors are ±50, ±80, and ±135 mK for 4.2-20, 20-40, and 40-80K, respectively. The angle dependence of critical current (Ic) of the tested rare-earth barium copper oxide tape within 0°-30° is strong. From 30° to 90°, the sample Ic almost does not change with the magnetic field angle. The implementation of the project will not only promote the structural optimization of HTS tapes but also promote the miniaturization and economical application of HTS magnets.

Fracture behavior of superconductor REBCO tapes with multiple edge cracks under electromagnetic force

It is often necessary to narrow wide tapes through a slitting process in applications of the second-generation high-temperature superconductor REBa2Cu3O7-x (REBCO, RE = rare earth) tapes. However, this results in some random length edge cracks appearing on the slit edge and extending at certain angles towards the center of the tape. In this paper, we proposed a randomly distributed multiple-edge crack model to investigate the fracture mechanism of the superconducting tape under electromagnetic force due to high magnetic fields and current density (∼10 T and ∼ 1010A/m2). The electromagnetic body force distributions within the superconducting tape, incorporating multiple edge cracks, are simulated based on the H-formulation. Then a fracture analysis is conducted under the combined loading of tensile and electromagnetic forces, with the numerical calculation of stress intensity factors (SIFs) using the J-integral method. To validate the model's accuracy, we compare the body force distribution caused by an edge crack and the SIFs of edge cracks under uniform tension with existing results. Detailed analyses are undertaken to explore the impacts of electromagnetic body force non-uniformity, interactions between neighboring cracks, and the stochastic nature of geometrical parameters (length, angle, and spacing) on the fracture behavior under an alternating magnetic field. Numerical results indicate that non-uniform electromagnetic body forces arising from defects are a critical factor in crack propagation. Furthermore, the randomness of the geometrical parameters amplifies the interactions between some of the neighboring cracks, causing the tendency of the tape containing random edge cracks to fracture. Therefore, it is advisable to avoid random cracks with substantial length, angle, and spacing during the mechanical slitting process.

Design and investigation of losses of a low-current HTS AC cable for a promising cryogenic generation system

Currently, there are a large number of projects dedicated to electric machines aimed at use in the field of transport and equipped with windings made of high-temperature superconductors (HTS). These machines in their majority have rated currents significantly less than 1kA, which is a too low value for AC HTS cables in the power industry, where current values can reach up to 10kA. On the other hand, connecting, for example, an electric generator with cryogenic cooling and its load (which, most likely, will also have cryogenic cooling) with copper wires on board of a vehicle does not look like an effective solution. This article is devoted to the development of the concept of an alternating current transmission system for machines with a rated current of less than 1 kA through HTS cable. In order to achieve a compact design, it was decided to use a cable consisting of stacks of second-generation (2G) HTS tapes. A series of experiments was carried out with different numbers of tapes and directions of current in them for stacks of 2, 4 and 6 the HTS tapes. The results were compared with finite element modeling. The article shows that when current flows in adjacent cable HTS tapes in opposite directions, like as in bifilar windings, it is possible to achieve a significantly lower level of losses per tape than when current flows in the cable tapes in one direction.

Aging effect on copper stabilizers in second-generation high-temperature superconducting tapes

Copper stabilizers play an essential role in second-generation high-temperature superconducting (2G-HTS) tapes. Corrosion or aging of the copper stabilizer is unavoidable and affects the tape performance. In this study, aging tests were performed on fresh copper-plated 2G-HTS tapes under steady-state damp heat conditions, in accordance with the IEC 60068–2-78:2012 regulations. To accelerate the aging, the conditions were set as 40 °C, a relative humidity of 93%, and test durations of up to 48 h. The surface contact resistivity (ρc) in liquid nitrogen was measured for fresh and aged specimens under normal pressure of up to 160 MPa. The results indicated that ρc increased with the test duration and tended to become saturated. After 48 h aging tests, the ρc of the specimens was three times higher that of the fresh 2G-HTS tapes. Moreover, the microstructure of the copper stabilizer was characterized. Height fluctuations were observed on all the specimen surfaces, which might be the reason for the ρc scattering. X-ray photoelectron spectroscopy elemental analysis revealed higher amounts of carbon and oxygen on the surfaces of the specimens after aging, which significantly reduced the purity of the copper stabilizer. It was also found that Cu2O was the main product on the tape surface during the reaction with oxygen and, water vapor and so on. This study provides a reference for the design of non-insulated coils and the preservation of 2G-HTS tapes.

Electromagnetic-mechanical coupling analysis of high-temperature superconducting racetrack coil

The second-generation high-temperature superconducting (HTS) coated conductor has been recognized as one of the most promising materialsfor high field magnets due to its superior electromagnetic and mechanical performances. HTS racetrack coils wound with the coated conductors are an extensively used configuration in engineering applications, such as HTS machines and high-speed maglevs. In this paper, in order to analyze the electromagnetic and mechanical behaviors of HTS racetrack coils, a 3D coupled electromagnetic-mechanical model is used to consider the effect of coil deformation and the strain dependence of critical current. The effectiveness of the coupled model is validated by comparing the numerical results with experimental data in the literature. A numerical simulation of a 3D HTS racetrack coil subjected to an external electromagnetic field is carried out using coupled and uncoupled models. The results indicate that the structure deformation can reduce the penetration depth of the screening current, and the hoop stress and strain are mainly concentrated on the circular part of the racetrack coil. Afterwards, the influences of various parameters on the electromagnetic and mechanical responses of the HTS racetrack coil are also investigated.

Enhancement mechanism of uniaxial tensile electro-mechanical behaviors in second-generation high-temperature superconducting tapes

The second-generation high-temperature superconducting (2 G-HTS) tapes are subjected to complex stress/strain in practical applications. For the complex operation conditions, lamination techniques are widely used to improve the robustness of 2 G-HTS tapes. However, lamination makes the 2 G-HTS tape structure more complicated, which can significantly affect their electro-mechanical behaviors. In this work, uniaxial monotonic tensile tests were performed to obtain the stress-strain relationship and the strain dependence of critical current (Ic(ε)) in liquid nitrogen conditions. Two types of laminated 2 G-HTS tapes (copper-laminated and stainless steel-laminated) were compared with copper-plated 2 G-HTS tapes. The results showed that the irreversible tensile strains of Cu-laminated and SS-laminated tapes were much higher than that of Cu-plated tapes, which increased by ∼35 % and ∼60 %, respectively. High consistency between the experimental and theoretical Ic values in 2 G-HTS tapes under tensile strain was verified by the Ic(ε) model. Based on the residual strain (εres) calculation of composite multilayer structures, we confirmed that it was mainly due to the increase of the compressive residual strain of superconducting layers. Further, tensile fatigue tests also confirmed the significant improvement of the tensile fatigue electro-mechanical behaviors in the laminated 2 G-HTS tapes. Finally, 2D X-ray and SEM cooperative analysis were used to characterize the tape microstructure before and after fatigue, and the correlation between micro-defects and non-destructive imaging features was established. It was found that the internal solder layer and its interface were seriously damaged to release the stress. This work provides a strategy to improve the electro-mechanical behaviors of 2 G-HTS tapes by adjusting the composite structure to change residual strain.

Influence of mechanical stress on electron transport properties of second-generation high-temperature superconducting tapes

Second-generation high-temperature superconductor (2G HTS) tapes have demonstrated the ability to generate high magnetic fields and critical currents at a wide operating temperature range. In this paper, we study the mechanical properties of 2G HTS tape measured simultaneously with its critical current. The lattice deformations in the tape’s substrate caused by applied mechanical stress were measured by neutron diffraction. In our experiments, the 2G HTS tape was exposed to uniaxial tensile force ranging from 250 to 1100 N at temperature 77 K. The current through the tape was scanned in the range from 22 to 42 A. The experimental results have been obtained in a cryogenic testing chamber for neutron scattering measurements of internal stresses under load with the incorporated HTS current leads. Critical stress is a vital parameter required for the modeling and designing of advanced superconducting magnets and also a variety of different superconducting applications based on 2G HTS tapes.

Characteristics of Non-Insulated High-Temperature Superconductor Coils in Time-Varying Magnetic Fields

Non-insulated coils made of second-generation high-temperature superconductors (HTSs) have been developed to prevent the burning of superconducting wires. Although these coils perform reasonably well in a constant magnetic field, the induced current in the coil is a serious concern in a time-varying magnetic field. Hence, we aimed to determine the characteristics of non-insulated HTS coils in a time-varying magnetic field. We manufactured two HTS coils: 1) non-insulated coil and 2) metal-insulated coil with a higher turn-to-turn resistance. Experiments were performed under a time-varying field generated using the stator of a 1 MW rotating machine. The HTS coil in the rotating machine was severely affected by an asynchronous or transient magnetic field. In addition, we designed a special structure to simulate severe operating conditions. As a result, the magnetic field of the HTS coil decreased in the time-varying field. The metal-insulated coil with a higher turn-to-turn resistance exhibited a lower field reduction than the non-insulated coil. We also performed circuit analyses on the experimental results. Our findings suggest the feasibility of applying HTS coils without insulation in rotating machines.

Evaluation of Frequency Loss Induced Quench Protection Prototype at 77 K Using HTS Coils

A prototype design of the Frequency Loss Induced Quench (FLIQ) protection system for high temperature superconducting magnets was validated on a small Second-Generation High-Temperature Superconducting (2G HTS) coil. Calorimetric measurements, based on liquid nitrogen boil-off rate, were performed to quantify the heat energy introduced by FLIQ system in the coil. The technique was previously used for steady state AC loss measurements and adopted for the transient losses generated in FLIQ operation. A series of tests of the FLIQ system on 2G HTS pancake coils were performed to study the dynamics of the quench and the energies involved in the quenching process. The results suggest the suitability of the FLIQ technique for quench protection of HTS coils. A measurement protocol of extended repetitive high-frequency current pulses established in the study provided a way to estimate the energy deposited by the AC losses generated in the coil by the FLIQ system. The measurement method described is a practical way to validate the design parameters of the FLIQ system and to evaluate the sensitives of the various design parameters of the FLIQ. The paper reports an experimental technique used to study the sensitivities of various FLIQ design parameters by establishing and measuring quasi-equilibrium heat load and boil-off rate as a result of extended repetitive high-frequency current pulses.

Development of metal-organic deposition-derived second-generation high-temperature superconductor tapes and artificial flux pinning

Development of metal-organic deposition-derived second-generation high-temperature superconductor tapes and artificial flux pinning

Enhancement of electro-mechanical behaviors in RE–Ba–Cu–O composite superconducting tapes with laser slit edges

The slit technique is an essential process for narrowing the second-generation high-temperature superconducting (2G-HTS) tapes. It is still very challenging to eliminate edge defects introduced during slitting. In this work, we developed a set of reel-to-reel laser slit (LS) equipment for narrowing 2G-HTS tapes using a femtosecond infrared laser. In comparison with the commonly used mechanical slit (MS) technique, the LS technique produced no cracks at the tape edges. Electro-mechanical behaviors of 2G-HTS tapes with MS and LS edges were compared in liquid nitrogen conditions. The results of the uniaxial tensile tests showed only a minor difference in irreversible tensile stress/strain of the two kinds of tapes. However, uniaxial tensile fatigue tests showed that the critical current (I c) retention of the LS tapes was about 14% higher than that of the MS tapes after 104 cycles (@ 77 K, σ max = 0.8σ y, and a stress ratio of 0.1). Moreover, U-Spring compressive tests revealed that no irreversible I c degradation occurred on the LS tapes under compressive strain up to −0.8%, with high reproducibility. On the contrary, the irreversible compressive strains of the MS tapes were rather scattered, ranging from −0.75% to −0.2%. Enhancement of electro-mechanical behaviors is strongly linked to fewer edge defects in the LS tapes. This research demonstrates significant advantages of the LS technique over the conventional MS technique, showing its promising prospects for demanding applications.

Computation of AC loss of HTS coils wound by coated conductors with different widths

The second-generation high-temperature superconductor (HTS) tape is a promising material for the HTS application. However, the extremely large aspect ratio of the HTS coated tape leads to high power dissipation in the environment of a time-varying magnetic field. Reducing the width of HTS coated tape is a feasible way to decrease the alternating current (AC) loss of devices composed of HTS tapes. In this study, a numerical model of the HTS coil group composed of six sub-coils based on the T-A formulation is presented in which each HTS sub-coil is wound with original or narrow width Y-Ba-Cu-O (YBCO) tape. The 1/3 and 1/4 narrow width tapes are used to represent the tapes, which are divided into three and four equal parts, respectively. Then, we calculate the AC losses in 1/3 and 1/4 narrow width HTS coil groups in different cases. The estimated results of AC loss are compared with those of the HTS coil group wound by HTS tapes with original width. The numerical results show that AC transport losses of 1/3 and 1/4 narrow width HTS coil groups are smaller than that of the original HTS coil group for the applied high current. Furthermore, with the decrease in tape width, the reduction of AC loss becomes more significant. In contrast with the original width HTS coil group, the magnetization losses of the 1/3 and 1/4 narrow width HTS coil groups will decrease at the high magnetic field. In addition, the influences of harmonic components on AC loss are also considered.

Critical current and n-value prediction of second-generation high temperature superconducting conductors considering the temperature-field dependence based on the back propagation neural network with encoder

The qualities of superconducting conductors are usually characterized by their critical current and n-value. In this paper, the critical current and the n-value of the second-generation (2G) high temperature superconducting (HTS) conductors considering the temperature-field dependence are predicted by a back propagation (BP) neural network as J c(B, θ, T) and n(B,θ,T). A correlation exists between the critical current and the n-value, thus in our BP neural network, the tasks of estimating the critical current and the n-value can be carried out in one network. The outputs used to predict the critical current and the n-value share the same hidden layers of the network, and therefore the critical current and the n-value can be calculated simultaneously. The critical current and the n-value of HTS conductors vary for different manufacturers and even for the same manufacturer but different production batches. In our BP network, we use an encoder structure to encode different manufacturers and production batches to ensure that the network can be adapted to the HTS conductors so produced. The predictions on three different samples of HTS conductors are carried out and the results are compared with those obtained from linear interpolation at different temperature-field conditions. In addition to the high accuracy, when compared with the linear interpolation, the proposed network can suppress the fluctuations existing in the experimental data and ensure the prediction results to be more reasonable. The time required for the proposed network to obtain the prediction results of the critical current and the n-value is also discussed, which is within 5 s. Therefore, the proposed network has the potential to be applied to the optimization and analysis of the superconducting related equipment.