High-temperature Superconducting Field Research Articles

AC loss analysis on an HTS field winding of the generator with T-A formulation

In this study, we proposed a coupling model utilizing finite element software to estimate AC loss on the high-temperature superconducting (HTS) field winding of a 1 MW generator. The model comprises three distinct physical modules: rotating machinery and magnetic (RMM), electrical circuit (CE) and partial differential equation (PDE). These modules collaborate to resolve the generator's magnetic field distribution, rated output parameters and degree of current magnetization on the HTS field winding, employing Maxwell equations and T-A formulation. Given the placement of the HTS field winding on the rotor core, accurately calculating its loss in an electromagnetic field that varies both temporally and spatially presents a considerable challenge. To address this issue, we adjust the coordinate system within the PDE module to vary with the material rather than space. Our analysis reveals that the instantaneous AC loss of an HTS field winding comprises two primary components: the attenuation envelope and the oscillatory variation, resulting from the charging process and external harmonic magnetic fields. Additionally, we comprehensively discuss and analyze the influence of current excitation speed on instantaneous AC loss. Accurately estimating AC loss in HTS field windings is a critical and meaningful aspect of preliminary machine design. This study provides an essential reference for addressing this issue.

A design study on 20 kW class axial flux motor with HTS field winding for 100 MPa liquid hydrogen pump

This paper presents a design study of a low-speed, high torque 20 kW class motor with a high-temperature superconductor (HTS) field coil for a liquid hydrogen centrifugal pump. The motor is designed based on three key concepts: 1) employing an axial-flux motor topology that can increase torque density by reducing the axial length together with a centrifugal pump; 2) using liquid hydrogen in a pump as a coolant for HTS coil; 3) adopting no-insulation (NI) YBCO solenoid coils as field coils of the motor to improve coil protection as well as mechanical stability. In the design process, first, an electromagnetic design of the axial type motor is obtained considering the required performance of previous commercial pumps. Considering the current and temperature margin to secure the HTS coils' stable operation, two types of HTS coils (solenoid type and triangle type) are designed and their performances are compared. Second, to evaluate validity of the electromagnetic designs, mechanical stresses in the HTS coils are estimated. By comparing the solenoid type and the triangle type, it is confirmed that the solenoid type has a distinct stress-reducing effect in terms of magnetic stress as well as rotational stress. Our design study results show the potential superior performance of the axial flux motor with HTS coils for several tens of kW applications with low speed and high torque loads such as liquid hydrogen pumps.

A Study of the Dynamic Characteristics of an Electrical Rotating Machine Applying No-Insulation HTS Field Coils

This paper presents a dynamic characteristic analysis of an electrical rotating machine that applies no-insulation (NI) high-temperature superconductor (HTS) field coils. The HTS electrical rotating machine applying HTS field coils has high power density compared to conventional electrical rotating machines that utilize normal conductor field coils, meaning that it has great potential for next-generation mobility applications such as electric ships and aircraft. Recently, several studies have reported the possibility of solving the quench protection problem of the HTS field coil, which is one of the most pressing issues in HTS electrical rotating machines, and the key technology is the no-insulation (NI) winding technique. The NI winding technique enhances the electrical and thermal stability of the HTS field coil because the over-current can be automatically bypassed through the turn-to-turn contacts. Although there have been many studies of the electrical and thermal stability of NI HTS field coils, there are still issues related to NI HTS field coil applications to electrical rotating machines. In particular, there are few studies of the behavior of electrical rotating machines with a NI HTS field coil under dynamic conditions, such as active shortcircuit and speed drop conditions. In this study, we designed an electrical rotating machine that utilizes NI HTS field coils and analyzed the dynamic characteristics of the designed machine. The results demonstrate there is a possibility of larger torque fluctuations and armature current fluctuations in the NI HTS machine than in the insulated HTS machine.

Compensation of NI Behaviors in Synchronous Motors With NI HTS Field Winding Using Harmonic Current Injection

Focusing on the improved operation reliability of no-insulation (NI) high-temperature superconductor (HTS) coils, various attempts are being conducted to replace conventional superconductor coils with NI HTS coils for industrial and energy conversion purposed applications. When applying NI techniques, unique behaviors of NI HTS coils due to the removal of turn-to-turn insulation should be considered with given operating conditions, because it could affect the overall characteristics of the machines. In particular, in the case of synchronous motors to which NI HTS field winding is adopted, it has been reported that periodic turn-to-turn leak currents were identified even in steady-state operation due to harmonic components, which may result in torque and loss characteristics changes. Here we report a method to offset these additional NI behaviors that may appear when NI HTS field winding is applied to synchronous motors. A brief mechanism of the proposed method was presented, and the proposed method was applied to an example model through finite element analysis. As a result, it was shown that turn-to-turn contact loss could be reduced to 2.18% without adverse effect on torque characteristics.

A Double Rotor Flux Switching Machine With HTS Field Coils for All Electric Aircraft Applications

The increasing demand for high-power density motors in electric transport industries opens a new research opportunity to develop motor topologies with less weight and high efficiency. In particular, all-electric aircraft applications require very high power density motors. This paper shows the design of a new 1 MW 20-pole/15-slot double rotor flux switching machine with high-temperature superconducting field coils and thermal management system. The proposed motor features an air-core stator. Aluminum Litz wire is used for the armature conductors, and the YBCO- high-temperature superconducting material is used for the field coils. The armature winding and field coils operate at 95K and 65K, respectively. The active part power density including rotors, armature windings and field coils obtained with the proposed design is 29.3kW/kg. The specific power density of proposed motor considering mechanical, support structure and TMS is about 18.5 kW/kg. The efficiency can be higher than 98.7% which satisfies the requirement of electric aviation.

Design and Characteristic Analysis of an Axial Flux High-Temperature Superconducting Motor for Aircraft Propulsion.

In line with global environmental regulations, the demand for eco-friendly and highly efficient aircraft propulsion systems is increasing. The combination of axial flux motors and superconductors could be a key technology used to address these needs. In this paper, an axial flux high temperature superconducting (HTS) motor for aircraft propulsion was designed and its characteristics were analyzed. A 2G HTS wire with high magnetic flux characteristic was used for the field winding of the 120 kW axial flux HTS motor, and the rotational speed and rated voltage of the motor were 2000 rpm and 220 V, respectively. The axial flux HTS motor implements a revolving armature type for solid cooling of the HTS field coil. The electromagnetic and thermal features of the motor were analyzed and designed utilizing a 3D finite element method program. The HTS coil was maintained at the target temperature by effectively designing the current lead and cooling system to minimize heat loss. These results can be effectively used in the design of propulsion systems for large commercial aircraft in the future as well as for the design of small aircraft with less than 4 seats.

Reluctance-Based Circuit for High-Temperature Superconductor Generator Lumped-Parameter Model

The present work addresses the ongoing development of a mesh-based magnetic equivalent circuit (MEC) to estimate the magnetic flux distribution in a superconducting hybrid rotating synchronous machine, superconducting (SC) rotor, and conventional stator. The MEC is to be integrated in an existing thermoelectric lumped-parameter machine model for the transient simulation of High Temperature Superconductor (HTS) field coil synchronous generator (SG) in the dq0 frame of reference. It will allow the inclusion of the magnetic flux density inside the definition of the non-linear resistance of the superconductor in addition to the already-present temperature and current. The objective is to gain further accuracy on the coupled thermal and electromagnetic behavior of superconducting hybrid machines during partial or total loss of the superconducting state. In the present work, the MEC is benchmarked against a Finite Element Model (FEM). It is shown that the MEC can provide proper magnitude of the magnetic flux density over the HTS winding achieving a good estimation of the magnetic response of the field winding by taking into account the penetration of current within the superconductor. The case study is a 100-MVA hydroelectric HTS field coil SG considering a field current ramp.

Investigation on Time-Varying Behavior of NI HTS Field Coil for Synchronous Motors Considering Armature Reaction and Slotting Effect

This paper presents calculation results of time-varying behavior of no-insulation (NI) high temperature superconductor (HTS) field coil for synchronous motor considering armature reaction and slotting effect. We selected four NI HTS motor models having different stator topologies for confirmation of armature reaction and slotting effect. Also, before NI behavior analysis, we proposed a wound field synchronous motor (WFSM) dq-equivalent circuit which can be considered as a circuit model of insulated (INS) HTS motor. The circuit model used in this paper contains parameters varying with the position of rotor and dq-axes cross-coupling inductance for describing the armature reaction and slotting effect of practical motor. Next, to investigate time-varying behavior of NI field coil during steady-state operation, simulations of a new circuit model combining proposed dq-equivalent circuit and NI lumped circuit were implemented. Turn to turn radial leak current values of selected models were calculated at different armature current density. Also, torque ripple values of the NI and the INS case were obtained by circuit simulations. It can be confirmed that peak to peak radial current occurs by largest value at the topology having the greatest flux linkage harmonics and torque ripple value of NI motor is smaller than that of INS HTS motor.

Performance evaluation of a superconducting flywheel energy storage system incorporating an AC homopolar motor layout

In this paper, a novel high-temperature superconducting flywheel energy storage system (SFESS) is proposed. The SFESS adopts both a superconducting magnetic bearing and a superconducting alternating current (AC) homopolar motor. The superconducting AC homopolar motor has structural advantages in high-speed operation, however performance of the high-temperature superconducting (HTS) field coil is easily affected by the external magnetic field generated by armature windings in the process of flywheel charge and discharge, and should be paid attention. We established an electromagnetic-thermal co-calculation model of the HTS coil to analyze the coil characteristics in this application. Simulation results demonstrate that the magnetic flux density is small near the central region of the coil and is the largest at the edges of the coil. The temperature rise caused by the change of magnetic field distribution reduces the critical current density of the tapes and increases AC loss. Furthermore, we studied the grid connection control strategy of the SFESS, which adopts double closed-loop control and direct power control, and verified the feasibility of the control strategy by multi software real-time and dynamic co-simulation. Works in this paper give some guidance to further development of a 50 kW·h SFESS prototype. All the configurations of the SFESS, the method of calculating AC loss of HTS field coil, and the control strategy of the SFESS system could be extended to large commercial units.

Studying magnetic-polaron cooper-pair-breaking effects in Bi2Sr2CaCu2O8+δ by femtosecond photoexcitation

The relation between magnetism and superconductivity, whether magnetism is beneficial to superconductivity or they permit co-existence and competition, has been a long debate in the high-temperature superconducting field. Here, to explore this odd issue, we study the relaxation dynamics of photoinduced magnetic-polaron and superconductivity quasiparticles by an ultrafast time-resolved optical reflectivity technology in Bi2Sr2CaCu2O8+δ . When the pump fluence is less than 7 mW, time-resolved reflection spectrum ΔR(t)/R 0 is dominated by the superconducting component. It is noteworthy that a large of photoinduced magnetic-polaron component can cause the inhibition of the superconducting component when the variable pump fluence is more than the value 7 mW. It can be attributed to the strong photoinduced magnetic-polaron Cooper-pair-breaking mechanism. Further studies have shown that the photoinduced magnetic-polaron Cooper-pair-breaking mechanism is similar to the pair-breaking effect caused by the magnetic impurity Fe in the Bi2Sr2CaCu2O8+δ superconductor, although the forms of their action are different.

Electromagnetic characteristic analysis of a REBCO magnet with a current bypass/distribution winding technique under an asynchronous rotating magnetic field

Generally, high-temperature superconducting rotating machines (HTSRMs) are considered synchronous machines. If the output of the HTSRM fluctuates based on frequent changes in the electrical or mechanical loads, there is the concern that an asynchronous rotating magnetic field (RMF) is applied from the stationary copper armature winding to the high-temperature superconducting (HTS) field winding in the rotary. This may act as a magnetic disturbance to the HTS field magnet, resulting in permanent damage. To enhance the reliability of HTS magnets in wind power and electric propulsion applications, winding methods with current bypass/distribution characteristics, such as no-insulation (NI) and metal-insulation (MI), have attracted scholarly attention because of their high thermal and electrical stabilities, resulting in their self-stabilizing and protective performances. To verify the feasibility of the NI and MI winding techniques for wind power generators, the basic characteristics under a time-varying magnetic field must be studied, contrary to HTS magnet applications under a time-static magnetic field. Therefore, the electromagnetic characteristics of rare-earth barium copper oxide (REBCO) magnets applied with NI and MI winding technologies were compared and analyzed in this study, considering the magnetically transient situations in which an asynchronous RMF is applied to REBCO magnets. In addition, we developed a characteristic evaluation device similar to a synchronous rotating machine to generate the unsynchronized RMF. Moreover, various basic tests were performed to target the small racetrack-type field windings. The critical current, n-value, terminal voltage, and center magnetic field are investigated under various values of the frequency and current amplitude of the three-phase armature winding, and their behaviors are discussed in detail based on the characteristic resistances of the two test magnets.

Structure and stability of possible new L i-Y-H ternary hydrides

The research on the superconductivity of hydrogen-rich compounds has become a hot research topic in the field of high-temperature superconductors in recent years and yttrium hydride YH<sub>9+x</sub> has been experimentally confirmed to have high temperature superconductivity (near room temperature (Tc = 262 K)), following behind the research of H<sub>3</sub>S (Tc = 200 K) and LaH<sub>10</sub> (Tc = 260 K). The theoretical study of binary hydrogen-rich systems is relatively mature, while the structural characteristics and superconductivity of ternary or quaternary hydrogen-rich compounds are still under exploration. In this paper, nLiH + YH<sub>3</sub>→Li<sub>n</sub>YH<sub>n+3</sub> (<i>n</i> = 1–3) is the synthesis way to explore the stable configuration of ternary hydride LinYH<sub>n+3</sub> in a pressure range of 0–300 GPa. The crystal structure, electronic structure, thermodynamic and kinetic stability of LiYH<sub>4</sub>, Li<sub>2</sub>YH<sub>5</sub> and Li<sub>3</sub>YH<sub>6</sub> in the pressure range of 0–300 GPa are studied based on the structure prediction by particle swarm optimization algorithm and first-principles calculation. The CALYPSO method is used to search for 1–4 times molecular formula structures for Li-Y-H ternary systems with different stoichiometric ratios in the pressure range of 0–300 GPa in steps of 50 GPa. The results show that LiYH<sub>4</sub>-<i>P</i>4/<i>nmm</i>, Li<sub>2</sub>YH<sub>5</sub>-<i>I</i>4/<i>mmm</i>, and Li<sub>3</sub>YH<sub>6</sub>-P4/nmm can be respectively synthesized with a certain ratio between LiH and YH<sub>3</sub> respectively in a pressure range of 169–221 GPa, 141–300 GPa and 166–300 GPa. The Li<sub>2</sub>YH<sub>5</sub> has the lowest stable pressure and widest range which can be the possible choice in experiment. The results can provide the data support for the superconductivity research and experimental synthesis of hydrides in Li-Y-H ternary system.

Temperature Distribution in the Field Coil of a 500-kW HTS AC Homopolar Motor

AC homopolar motors with high-temperature superconductors (HTSs) are considered to be a promising technology for high-power applications. With superconducting components being placed exclusively in nonrotating parts, a motor can reach high rotational speeds. However, the dynamic behavior of the system creates a nonnegligible magnetic ripple field, which generates heat and makes thermal management of the cryogenic system more challenging. In this article, we present a case study for the thermal behavior of a conduction-cooled high-temperature superconducting field coil assembly in a 500-kW asynchronous homopolar motor. The analysis focuses on the investigation of the magnetic ripple field and its impact on heat generation due to ac loss in the HTS field coil and eddy current loss in copper parts present in the assembly. Using electromagnetic and thermal finite-element models, we construct a detailed description of the thermal bus design, which considers different cooling scenarios and sizing of the conductive elements. Depending on the geometry of the thermal bus, we observe heat loads from ac losses of up to 0.4 W and eddy current losses in the copper elements of up to 1 W. These heat loads translate to temperature increases as high as 10 K. Techniques to reduce the temperature rise based on thermal bus redesign are presented, along with recommendations for the most promising cooling scenarios and optimal thermal bus geometry.

Laser-Assisted Field Evaporation of (R = Gd, Sm) High-Temperature Superconducting Coated Conductors

Abstract In the field of high-temperature superconductors, atom probe tomography is a relatively new instrument, with the ability to provide a new perspective on the 3D nanoscale microstructure. However, field evaporation of nonmetallic materials is fraught with unique challenges that matter little in the world of metallic evaporation. In this study, we review the laser absorption, correlated evaporation, molecular dissociation, and the crystallographic effects on the field evaporation of 800-m (R = Gd, Sm) coated conductor tapes deposited by Reactive Co-Evaporation Cyclic Deposition and Reaction (RCE-CDR). Ultraviolet 355 nm laser pulsing was found to have a substantial beneficial effect on minimizing the fracture probability compared with 532 nm illumination, especially when evaporating insulating oxide precipitates. This, in turn, allows for the 3D compositional analysis of defects such as flux pinning centers introduced by precipitation and doping. As a result, evidence for the precipitation of nanoscale is discussed. The effect of crystallographic orientation is studied, where [001] aligned evaporation is found to develop compositional aberrations.

Design and Characteristic Analysis of a Homopolar Synchronous Machine Using a NI HTS Field Coil

This paper deals with a homopolar synchronous machine (HSM) applying high-temperature superconducting (HTS) field coils. Superconductors, especially high-temperature superconductors, have high potential as advanced materials for next-generation electrical machines due to their high critical current density and excellent mechanical strength. However, coils made with high-temperature superconductors have a high risk of being damaged in the event of a quench due to the intrinsic low normal zone propagation velocity (NZPV). Therefore, the coil protection issue has been regarded as one of the most important research fields in HTS coil applications. Currently, the most actively studied method for quench protection of the HTS coils is the no-insulation (NI) winding technique. The NI winding technique is a method of winding an HTS coil without inserting an insulating material between turns. This method can automatically bypass the current to the adjacent turn when a local quench occurs inside the HTS coil, greatly improving the operating stability of the HTS coils. Accordingly, many institutions are conducting research to develop advanced electrical machines using NI HTS coils. However, the NI HTS coil has its intrinsic charge/discharge delay problem, which makes it difficult to successfully develop electrical machines using the NI HTS coil. In this study, we investigated how this charging/discharging problem appeared when the NI HTS coil was used in an HTS homopolar synchronous machine (HSM) which is one of the electrical machines with a high possibility of applying the HTS coil in the future because it has a stationary field coil structure. For this, the characteristic resistances of HTS coils were experimentally obtained and applied to the simulation model.

Design and test results of a novel quench protection circuit for a HTS ship propulsion motor

This paper describes a novel configuration of a quench protection circuit for a high-temperature superconducting (HTS) ship propulsion motor that can protect the HTS coils from an unexpected outage in its cryogenic cooling system. The quench protection circuit consists of conductor parts placed adjacent to the HTS coils and dump resistors and diodes placed in the cryogenic-temperature and normal-temperature areas, respectively. The conductor parts are shaped and arranged such that the magnetic coupling with the HTS coil is extremely large. Thereby, the current of the HTS coils can be instantaneously attenuated immediately after the quench protection is activated. With this configuration, the electromagnetic energy of the HTS coils can be absorbed in the dispersed parts. This can make the dump resistors smaller and make it easier to install them on a HTS ship propulsion motor. By analysing the equivalent electric circuit model of the protection circuit, we derived the conditions under which instantaneous current attenuation of the HTS coil occurs. In accordance with the conditions, we have fabricated a quench protection circuit that protects one HTS field pole of the 3 MW HTS motor developed by Kawasaki Heavy Industries. The fabricated circuit was empirically evaluated and proved to be highly effective in quench protection. The present quench protection circuit technology contributes to the practical application of HTS ship propulsion motors.

Development and Testing of 2G High Temperature Superconducting (HTS) Field Coils for HTS Synchronous Machines

High temperature superconducting (HTS) synchronous machines are compact and light weight machines with higher torque to weight ratio, lower noise and better dynamic response for strategic and critical industrial applications. The development of HTS field coil is one of the main technological challenges in realizing the HTS synchronous machines, as it involves the selection of HTS tape, winding technology, handling of the HTS tape while winding on a winding machine and testing of the developed HTS coil at desired cryogenic temperatures. With advancements in HTS tape technology, recent 2G HTS tapes are offered with higher engineering current densities with respect to 1G HTS tapes even at higher magnetic fields. Hence, the replacement of 1G HTS field coils of previously designed 200 kVA HTS synchronous machine with 2G HTS field coils at the same operating cryo-temperature shows a potential to enhance its capacity to 350 kVA (more than 75% increase in machine output). In the present work the details of design, development and testing of a double pancake type 2G HTS field coil are presented for capacity enhancement of earlier 1G based 200kVA HTS synchronous machine.

Stability model of bulk HTS field pole of a synchronous rotating machine under load conditions

High-temperature superconducting (HTS) bulks act as magnets with trapped magnetic flux and potentially exhibit high magnetic field performance, superior to that of permanent magnets. In the fourth industrial revolution, HTS magnets could potentially facilitate the development of highly efficient and lightweight motors and generators for propulsion. To evaluate and verify the durability of HTS rotating machines, we developed a radial-gap-type rotating machine whose field-pole modules comprised HTS bulks. We examined the stability of the trapped magnetic flux of the field poles during the machine’s operation. The rotating machine was operated with different loads, and the magnetic field associated with the trapped flux of the HTS bulks was analyzed. The analysis was performed for different loads and revolution speeds, and the long-term operation for a constant load was investigated. The trapped flux of the bulk field poles and the output power were observed to be almost constant for over 360 h, and the HTS bulk magnet’s surface temperature hardly changed below 40 K during the machine’s operation. Furthermore, the magnetic flux decay determined from magnetic flux measurements before and after a load test was less than 1%. An analysis of the magnetic field distribution around the HTS bulk revealed that the transverse magnetic flux flowed through the magnetic guide of the rotor core. These results indicate the potential synchronous machine application of the high trapped magnetic flux of the field-pole modules accumulated by the appropriate positioning the HTS bulks three-dimensionally.

Investigation on Key Parameters of NI HTS Field Coils for High Power Density Synchronous Motors

As global environment regulations are gradually strengthened, active research has been conducted on electric propulsion applications. High power density is one of the major research interests, as well as a machine's efficiency for a high-performance propulsion system. Among various electric propulsion technologies under development, a motor incorporating the no-insulation (NI) high-temperature superconducting (HTS) winding has the advantages of high current density and robustness to external disturbances, which are suitable for high specific power. However, still, there is little research on the motor with NI HTS coil. Therefore, as basic research, we investigated the potential application of the NI HTS motor, then derived required machine parameters and NI HTS coil designs in this paper. For that, first, basic parameters of the existing propulsion motors were investigated, then the target application and performance of the NI HTS motor were determined. Based on the target performance, conceptual designs of a motor with NI HTS coil were obtained by changing key design parameters including rated power, main dimensions, operating temperature, and width of air-gap. Calculated parameters were numerically compared, and expected design requirements and challenges were suggested.

Effects of stainless steel thickness and winding tension on electrical and thermal characteristics of metal insulation racetrack coils for 10-MW-class HTS wind generator

Experimental results on the electrical and thermal characteristics of GdBCO racetrack coils co-wound with stainless steel (SS) tapes of various thicknesses and winding tensions are presented in this paper. The SS thickness and winding tension are identified as the factors affecting the charging time and thermal stability of metal insulation (MI) coils. To verify these characteristics, three types of MI-SS racetrack coils were fabricated with various SS thicknesses and winding tensions: 100-μm SS thickness and 10-kgf winding tension; 100-μm SS thickness and 5-kgf winding tension; and 50-μm SS thickness and 10-kgf winding tension. Further, three MI-SS racetrack coils were characterized by sudden discharging, charging, and overcurrent tests. The sudden discharging and charging tests were performed in a steady state to investigate the decay time constant of the center magnetic field discharging the coils and the delay time of the magnetic field charging the test coils, respectively. To estimate the thermal stability of the three MI-SS coils, overcurrent tests were conducted in a transient state at 1.1 times the critical current (Ic) and 1.05 Ic. Based on the experimental results of three small-scale test coils, the total contact resistance between the layers of the test coils was calculated and applied to design a field winding using a second-generation high-temperature superconducting (HTS) tape for a 10-MW-class HTS generator used in an offshore wind turbine environment. Subsequently, an equivalent electrical circuit model was proposed to analyze the electromagnetic response and thermal stability characteristics of three HTS field coils of a 10-MW-class HTS generator with the key parameters of the HTS field coils, which were obtained using electromagnetic finite-element analysis. Finally, the charging delay time and thermal stability of the three HTS field coils were compared and analyzed in detail.