High Temperature Superconducting Wire Research Articles

Exfoliated YBCO filaments for second-generation superconducting cable

The second-generation high temperature superconductor (2G HTS) wire is the most promising conductor for high-field magnets such as accelerator dipoles and compact fusion devices. The key element of the wire is a thin Y1Ba2Cu3O7 (YBCO) layer deposited on a flexible metal substrate. The substrate, which becomes incorporated in the 2G conductor, reduces the electrical and mechanical performance of the wire. This is a process that exfoliates the YBCO layer from the substrate while retaining the critical current density of the superconductor. Ten-centimeter long coupons of exfoliated YBCO layers were manufactured, and detailed structural, electrical, and mechanical characterization were reported. After exfoliation, the YBCO layer was supported by a 75 μm thick stainless steel foil, which makes for a compact, mechanically stronger, and inexpensive conductor. The critical current density of the filaments was measured at both 77 K and 4.2 K. The exfoliated YBCO retained 90% of the original critical current. Similarly, tests in an external magnetic field at 4.2 K confirmed that the pinning strength of the YBCO layer was also retained following exfoliation.

2G HTS wires made on 30 μm thick Hastelloy substrate

REBCO (RE = rare earth) based high temperature superconducting (HTS) wires are now being utilized for the development of electric and electromagnetic devices for various industrial, scientific and medical applications. In the last several years, the increasing efforts in using the so-called second generation (2G) HTS wires for some of the applications require a further increase in their engineering current density (Je). The applications are those typically related to high magnetic fields where the higher Je of a REBCO wire, in addition to its higher irreversibility fields and higher mechanical strength, is already a major advantage over other superconducting wires. An effective way to increase the Je is to decrease the total thickness of a wire, for which using a thinner substrate becomes an obvious and attractive approach. By using our IBAD-MOCVD (ion beam assisted deposition-metal organic chemical vapor deposition) technology we have successfully made 2G HTS wires using a Hastelloy® C276 substrate that is only 30 μm in thickness. By using this thinner substrate instead of the typical 50 μm thick substrate and with a same critical current (Ic), the Je of a wire can be increased by 30% to 45% depending on the copper stabilizer thickness. In this paper, we report the fabrication and characterization of the 2G HTS wires made on the 30 μm thick Hastelloy® C276 substrate. It was shown that with the optimization in the processing protocol, the surface of the thinner Hastelloy® C276 substrate can be readily electropolished to the quality needed for the deposition of the buffer stack. Same in the architecture as that on the standard 50 μm thick substrate, the buffer stack made on the 30 μm thick substrate showed an in-plane texture with a Δϕ of around 6.7° in the LaMnO3 cap layer. Low-temperature in-field transport measurement results suggest that the wires on the thinner substrate had achieved equivalent superconducting performance, most importantly the Ic, as those on the 50 μm thick substrate. It is expected the 2G HTS wires made on the 30 μm thick Hastelloy® C276 substrate, the thinnest and with the highest Je to date, will greatly benefit such applications as high field magnets and high current cables.

Effect of Resistive Metal Cladding of HTS Tape on the Characteristic of No-Insulation Coil

This paper presents experimental and theoretical studies of the no-insulation (NI) winding method of second-generation high-temperature superconducting (HTS) wire. We compared two single pancake coils wound by two different HTS wires. One pancake coil is made of a normal HTS wire with an electroplated copper stabilizer. The other pancake coil is made of the same HTS wire with the only difference in an additional outermost layer made of stainless steel. We employed an equivalent circuit model to evaluate our experimental results. We tested both coils by the same simple operating procedure consisting of two steps: first, ramping up of current from zero to holding current (IH) and second, keeping the IH at minimum 500 s. We also tested the stability of the coil wound by an HTS wire with an additional layer of stainless steel by applying a current exceeding a critical current of the coil. We observed a charging time of the metal-cladding HTS coil reduced to a quarter of copper-electroplated HTS coil.

Development of a Quasi-Isotropic Strand Stacked by 2G Wires

This paper presents a concept of quasi-isotropic high-temperature superconducting (HTS) strands with round and square cross sections made from REBa2Cu3O7-x (RE=rare earth, REBCO) coated conductor, the so-called second-generation (2G) HTS wires under development at North China Electric Power University. Considering several key issues for fabricating the quasi-isotropic strands, for instance, in-plane bending performance of 2G wire, isotropic-strand bending performance, strand termination method, and critical current in liquid nitrogen (LN2 ) temperature and self-field, a feasible manufacturing method by using mature laser welding is suggested and investigated experimentally. After fabricating several short-length strand samples consisting of 72 REBCO wires with an outer diameter of 7.5 mm, critical currents in straight and bending and under direct current (DC) magnetic fields with different orientations were tested in LN2 temperature. The critical current in straight strand is in agreement with the simulated one; the anisotropy of the critical current is less than 8% in the orientation range of $0^{\circ}{-}90^{\circ}$ with a DC magnetic field of 0.5 T. After successfully finishing a 10-m-length strand, it was demonstrated that the laser welding technique is an attractive and feasible method for manufacturing practical long strands which have potential applications in large-scale high-field magnet and energy transmission.

A Study on Critical Current and AC Loss Characteristics of Novel 2G HTS Narrow Wires

In high-temperature superconducting (HTS) applications, the external magnetic field affects the performance of the critical current and the ac loss. In this study, to improve the performance, narrowing and twisting processes are first suggested for the second-generation (2G) HTS wires: the original wire with 4-12-mm width is incised into several narrow wires with the same width under 2 mm, and narrow wires are twisted around each other as tight as possible. Using the narrow wires, we prepared various types of novel wire samples with stacking and twisting structures. The main results show that the feasibility of the narrow wires with stack and twist structures is confirmed from both critical current and ac loss points of view.

Effects of Stabilizer Thickness of 2G HTS Wire on the Design of a 1.5-MW-Class HTS Synchronous Machine

In general, a metallic stabilization layer is overcoated on the outermost layer of a second-generation high-temperature superconducting (2G HTS) wire to stably transfer current against thermal and magnetic disturbances. Stabilizer thickness $T_{{s}}$ is one of the key issues in an HTS synchronous machine (HTSSM) application because it strongly affects the electrical output performance of the machine and the stable operation and reliable protection of the HTS field coil. In this paper, a design and characteristic analysis for manufacturing a 1.5-MW-class HTSSM was performed using a 2-D analytical design code and a 3-D finite-element method. Various $T_{{s}}$ values were considered in the HTS field-coil design to investigate their effects on the stability and protection of the HTS coil and the back-electromotive force of the machine. The design parameters are also discussed to determine suitable $T_{{s}}$ for the 2G HTS wire.

Shape Optimization of the Stacked HTS Double Pancake Coils for Compact NMR Relaxometry Operated in Persistent Current Mode

The low temperature superconducting (LTS) coils are used in NMR devices, and liquid helium is used for their cooling. It is possible to reduce the operating costs by manufacturing the superconducting magnet that is consisted of the high temperature superconducting (HTS) coil using liquid nitrogen cooling. We have been developing the NMR relaxometry devices using the HTS double pancake coils and operated in the persistent current mode at liquid nitrogen temperature. The required strength and homogeneity of the magnetic field of proposed NMR relaxometry devices are 1.5 T and 150 ppm/cm 3 , respectively. To achieve the target field strength of 1.5 T, it is necessary to increase the numbers of pancake coils and winding turns of each coil, and the large operating current is required as much as possible. In this paper, I c - B properties of HTS wire and superconducting joint are measured to determine the operating current (load line) for the proposed NMR relaxometry devices. Moreover, 3-D finite-element-model-based analysis was carried out to design the stacked HTS magnets for compact NMR relaxometry with downsizing and low cost.

Conceptual Design of a Resistive 154-kV/2-kA Superconducting Fault Current Limiter

The expansion of electric capacity is inevitable to satisfy the demands of rapid economic growth. An increase in electricity load causes a decrease in load impedance. As a result, the fault current would exceed the capacity limit for the short-circuit breaker installed in a power system. Fault current that exceeds the rating of a conventional circuit breaker will degrade the stability of the power grid system. Many types of high-temperature superconducting (HTS) fault current limiters (FCLs) have been developed to enhance the stability and reliability of a power grid system. In this paper, a resistive HTS FCL using 2G HTS wires is designed, taking into consideration the demand quantity of 2G HTS wires, the winding type of the coils, the number of series and parallel connections, electrical insulation, and mechanical stability in the fault state. The length of the superconducting coils and the number of series and parallel connections are calculated. A finite-element method is used to perform dielectric and mechanical design analysis.

AC Loss Characteristics of HTS Wires Carrying Currents With Different Waveforms

When applying high-temperature superconductor (HTS) devices in power systems, ac loss is a significant obstacle from an economic perspective. Many studies have been performed to investigate this issue, but the ac losses for different current waveforms have rarely been reported. However, in power systems, many nonlinear devices can distort current waveforms. For example, currents of converters have ripples, no-load currents of transformers are peaked waveforms, and currents of motors are normally nonsinusoidal. To investigate the influence of current waveforms on ac losses, three types of currents with different waveforms, namely, sine waves, peaked waves, and flat-topped waves, are studied using three types of HTS wires, namely, Bi2223 wire, YBCO wire with nonmagnetic substrate (Hastelloy C276), and YBCO wire whose substrate is ferromagnetic (Ni-5at.%W). The experimental results show that ac losses of HTS wires are independent of frequency when carrying sine currents and that they are dependent on frequency when carrying peaked and flat-topped wave currents. Moreover, for all types of measured wires, particularly YBCO wire with ferromagnetic substrate, measured ac losses are highly dependent on current waveforms. In the end, the authors provide a discussion for the experiment phenomenon from the point of magnetic flux creep.

Critical Performance Analysis of HTS Magnet Wires Using an Induced Current-Based Measurement System

Studies on wide and stacked high-temperature superconducting (HTS) wires aim to increase the critical current of the wires. This paper describes the design and fabrication of an induced current-based performance measurement system and a critical performance analysis of HTS magnet wires using this system. In the experiment, the induced current, critical current, and joint resistance of stacked HTS wires under charge and discharge conditions were investigated. The results obtained were compared with those achieved using the four-terminal method, which is the conventional measurement method. The measurement system and the results of the critical performance analysis of the HTS wire are discussed in detail.

Progress in American Superconductor’s HTS wire and optimization for fault current limiting systems

American Superconductor has developed composite coated conductor tape-shaped wires using high temperature superconductor (HTS) on a flexible substrate with laminated metal stabilizer. Such wires enable many applications, each requiring specific optimization. For example, coils for HTS rotating machinery require increased current density J at 25–50K. A collaboration with Argonne, Brookhaven and Los Alamos National Laboratories and several universities has increased J using an optimized combination of precipitates and ion irradiation defects in the HTS. Major commercial opportunities also exist to enhance electric power grid resiliency by linking substations with distribution-voltage HTS power cables [10]. Such links provide alternative power sources if one substation's transmission-voltage power is compromised. But they must also limit fault currents which would otherwise be increased by such distribution-level links. This can be done in an HTS cable, exploiting the superconductor-to-resistive transition when current exceeds the wires’ critical J. A key insight is that such transitions are usually nonuniform; so the wire must be designed to prevent localized hot spots from damaging the wire or even generating gas bubbles in the cable causing dielectric breakdown. Analysis shows that local heating can be minimized by increasing the composite tape's total thickness, decreasing its total resistance in the normal state and decreasing its critical J. This conflicts with other desirable wire characteristics. Optimization of these conflicting requirements is discussed.

Hysteresis Losses Analysis in 2G HTS Cables

The Russian Scientific R&D Cable Institute is involved in many projects on R&D high-temperature superconducting (HTS) power cables including those made of modern second generation (2G) HTS wires. Evaluation of ac losses and methods for the reduction of the ac losses are important in the design of the cables. A fine experimental technique has been developed and implemented to measure ac losses in representative cable models with a length of ~5 m. However, only calculation can provide ac loss prediction in cases when it could be difficult to make measurements. Thus, we need a comprehensive simulation model to calculate ac losses. In HTS cables made of 2G wires, the main ac loss is a hysteresis loss, which only numerical simulation can predict for full-size 2G cables. We develop a finite-element method model to solve the electromagnetic problem for the calculation of ac losses. The model takes into account the strong nonlinear resistivity of the HTS materials, the hysteresis behavior of the magnetic substrate, the nonuniformity of critical current density across the tapes of the cable, and the possible ferromagnetism of a substrate. The model also takes into account any geometrical aspect of the 2G HTS cables. The details of the model and comparisons of calculations with our measurements of the ac losses are presented.

Design and Simulation of HTS Coils With a New Structure for an Inductive Pulsed Power Generator

Inductive pulsed power supplies (PPS) attract attention from several research institutions and scholars since the energy density of inductors is approximately one order of magnitude higher than that of capacitors. To effectively improve the output current of high-temperature superconductor (HTS) PPS and lower requirements for the switch, research studies have been done on STRETCH meat grinder circuit with HTS coils as an inductive module of XRAM. This paper mainly discusses the possibility of HTS coils applying to the STRETCH meat grinder as one of the fundamental research for PPS. With the characteristics of strong mechanical properties, high Ic, and low loss, HTS wire has become sufficiently available to build prototypes and practical instruments. This paper proposes a new coil winding structure that makes HTS coils applicable to the STRETCH meat grinder. Simulation results show that the transposition method can obtain a relative even current distribution in HTS parallel coils. By using the HTS magnet in the STRETCH meat grinder, maximum pulse current amplitude of 625 A can be obtained with a current multiplication factor of 5.2. Compared with the simulation results, experiments will be taken in the near future to verify the feasibility of the proposed coil structure and will be described in detail in further research work.

Modeling and Comparison of In-Field Critical Current Density Anisotropy in High-Temperature Superconducting (HTS) Coated Conductors

The development of high-temperature superconducting (HTS) wires is now at a stage where long lengths of high quality are commercially available, and of these, (Re)BCO coated conductors show the most promise for practical applications. One of the most crucial aspects of coil and device modeling is providing accurate data for the anisotropy of the critical current density J c (B, θ) of the superconductor. In this paper, the in-field critical current density characteristics J c (B, θ) of two commercial HTS coated conductor samples are experimentally measured, and based on these data, an engineering formula is introduced to represent this electromagnetic behavior as the input data for numerical modeling. However, due to the complex nature of this behavior and the large number of variables involved, the computational speed of the model can be extremely slow. Therefore, a two-variable direct interpolation method is introduced, which completely avoids any complex data fitting for J c (B, θ) and expresses the anisotropic behavior in the model directly and accurately with a significant improvement in computational speed. The two techniques are validated and compared using numerical models based on the H-formulation by calculating the self-field and in-field dc critical currents and the ac loss for a single coated conductor.

Electrical Characteristic Analysis According to Contact Resistance Between Turns of HTS Coil

This paper presents our analysis of the output characteristics of a high-temperature superconductor (HTS) coil depending on the turn-to-turn contact resistance. Important design factors were investigated by taking into account the turn-to-turn contact resistance based on the analysis results. Analyses were performed on five self-fabricated coil models with different turn-to-turn contact resistances. To consider only the turn-to-turn contact resistance, we used a first-generation HTS BSCCO wire with simpler architecture and mechanical properties than the second-generation HTS wire. From this study, it was confirmed by experiment that as the turn-to-turn contact resistance decreases, the electrical stability increases but the output efficiency decreases compared with the input.

Study of HTS Machine System Cooling With a Closed-Loop Thermosyphon: Stability of Unsteady Heat Load and Transient Conduction

Thermosyphoning (TS) is a heat exchange mechanism that uses a circulating refrigerant based on natural convection with no need for a mechanical pump. This cooling technique has been employed to circulate liquid coolant and gaseous coolant for high-temperature superconductor (HTS) rotating machine systems. These machines operate with an HTS field pole in the temperature range from 25 to 40 K, which are suitable temperatures for the survivability of HTS wires. The cryomechanical components are simple; there are a condenser, an evaporator, and adiabatic tubes located between the condenser and the evaporator. For applications to ship propulsion motors or generators of the radial gap type, the liquid/gas coolant flow along the adiabatic tube inside the rotating shaft may be considerably affected by the ship's rolling and/or pitching motion. In this work, we studied the temperature stability and heat load survivability of cryogenic TS, by constructing a 100 W @ 30 K-grade model. We investigated the temperature, thermal resistance, and heat flux, with a maximum load up to 60 W, of ground and onboard tests. The observed TS behavior indicates that there is sufficient survivability to assemble megawatt-class ship propulsion systems.

Recent Progress in Compact, Robust, and Superior Field-Tolerant QMG Current Leads Using RE–Ba–Cu–O Bulk Superconductors

High-temperature superconducting (HTS) bulk materials of RE–Ba–Cu–O (RE: Y or rare-earth elements) exhibit excellent $J_{c}\!-\!B$ properties, even at 77 K, leading to current leads with large current capacity and superior magnetic field tolerance. The thermal conductivity of HTS bulk materials is two orders of magnitude smaller than that of the conventional current lead material of copper, and Dy–Ba–Cu–O has particularly low thermal conductivity in the RE–Ba–Cu–O family. Unlike HTS wires, HTS bulks have no metal sheath such as silver or silver alloys with high thermal conductivity, resulting in the reduction of heat leak through the current lead, even in the compact size. Although they are a kind of brittle ceramic or oxide, HTS bulks can be transformed into robust current leads by reinforcing them with the support cover made of glass-fiber-reinforced plastics with low thermal conductivity and high strength. In addition to excellent mechanical stress tolerance, HTS bulk current leads also have a robust structure that minimizes thermal stress, which was experimentally demonstrated by a rapid cooling test to liquid nitrogen over 100 cycles. Due to distinctive advantages such as compactness, robustness, and superior field tolerance, more than 500 pieces of HTS bulk current leads have been successfully provided so far.

Development and Characterization of a 2G HTS Roebel Cable for Aircraft Power Systems

Lightweight megawatt-range power cables in electric aircraft propulsion systems will allow reducing fuel consumption by more than a half. Taking into account the low voltage limitations for onboard use, superconductivity providing high current-carrying capacity becomes an enabling technology for this application. In the frame of a preliminary study of an onboard superconducting power distribution grid, we fabricated and tested a high-temperature superconducting (HTS) Roebel cable. The work was done by a consortium of institutions and supported by Airbus Group Innovations. SuperOx provided a 400 A-class 12-mm-wide 2G HTS wire. The team at the Karlsruhe Institute of Technology prepared 5.5-mm-wide Roebel strands with a transposition length of 226 mm and assembled the strands into a 6-m-long Roebel cable. The team at the Russian Scientific Research and Development Cable Institute performed characterization of the cable. Short samples of the cable were tested to determine their critical currents, mechanical properties, and stability during thermal cycling. For a 4-m-long section of the cable, we measured the critical current, as well as the transport ac losses at frequencies in the range from 50 to 400 Hz. This paper presents the cable design and test results. The feasibility of using 2G HTS Roebel cables in electric aircraft systems is discussed.

Design of a 12-MW HTS Wind Power Generator Including a Flux Pump Exciter

A flux pump (FP) exciter injects dc current into the high-temperature superconducting (HTS) field coils of an HTS rotating machine without a slip ring and current leads. When designing a large-scale HTS generator with integrated FP exciter, the coil inductance, field current, and time constant need to be optimized for better performance of the machine. In this paper, a 12-MW HTS wind power generator with integrated FP exciter was designed. The essential parameters of a 12-MW HTS generator were optimized using the Taguchi method, targeting the minimization of weight and volume of the generator, the length of HTS wire, and the inductance. In particular, the FP exciter was adopted for supplying dc current to the HTS field coils without the power supply and the slip ring. The magnetic field distribution was analyzed using the 3-D finite-element method. The induced dc current and charging and discharging times of the FP exciter were compared with the metal current leads, for confirmation of the effectiveness of the FP exciter. The detailed results of the HTS generator design were discussed in detail.

A study on the required performance of a 2G HTS wire for HTS wind power generators

YBCO or REBCO coated conductor (2G) materials are developed for their superior performance at high magnetic field and temperature. Power system applications based on high temperature superconducting (HTS) 2G wire technology are attracting attention, including large-scale wind power generators. In particular, to solve problems associated with the foundations and mechanical structure of offshore wind turbines, due to the large diameter and heavy weight of the generator, an HTS generator is suggested as one of the key technologies. Many researchers have tried to develop feasible large-scale HTS wind power generator technologies. In this paper, a study on the required performance of a 2G HTS wire for large-scale wind power generators is discussed. A 12 MW class large-scale wind turbine and an HTS generator are designed using 2G HTS wire. The total length of the 2G HTS wire for the 12 MW HTS generator is estimated, and the essential prerequisites of the 2G HTS wire based generator are described. The magnetic field distributions of a pole module are illustrated, and the mechanical stress and strain of the pole module are analysed. Finally, a reasonable price for 2G HTS wire for commercialization of the HTS generator is suggested, reflecting the results of electromagnetic and mechanical analyses of the generator.