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The second-generation high-temperature superconductor REBCO is one of the most valuable candidates that meet the requirements of future magnetic confinement fusion device. Quench behavior of an improved high-current REBCO composite conductor is simulated by a 1-D finite element method. Indium-tin alloy foil placed in the gap between stabilizer, jacket, and superconducting tape stacks can significantly reduce thermal contact resistance, which leads to an increase on the minimum quench energy of composite conductor. Short perturbations have a better response to this improvement than long perturbations, due to the low normal zone propagation velocity (NZPV) of REBCO conductor. For short perturbation (perturbation length ( $l_{p}$ = 0.1 m), the peak temperature in cable and helium drops 12.76 and 17.99%, respectively, after soldered. Due to the change of heat transfer in a composite conductor caused by helium wave propagation, the development and maximum length of normal zone significantly decrease from 13.06 to 4.78 m under long perturbation ( $l_{p}$ = 10 m) circumstance. High current REBCO composite conductor with proper indium-tin alloy soldered can enhance the stability and safety of composite conductors under severe working conditions, but it also slows down the NZPV and hinders the quench detection of high-temperature superconducting magnet.

This paper reports a pioneering demonstration platform of a cryogenic propulsion unit at liquid nitrogen temperature. A high temperature superconducting (HTS) machine is connected with a cryogenic power rectifier in a generator mode to prove the feasibility of a cryogenic propulsion unit for electric aircraft propulsion applications. Machine operation was carried out with a special focus on the total heat dissipation inside the HTS AC windings. Different types of 2G HTS tapes were tested to provided data for stator coils’ design. The transient operation was carried out to represent a short-circuit failure in one of the power electronic devices. The test shows AC loss performance of the HTS windings using calorimetric method during the short circuit event, indicating the importance of developing protection schemes for the cryogenic propulsion units to prevent damage to the HTS components. The paper provides initial insights into the interaction between superconducting machines and cryogenic power electronics within a cryogenic propulsion unit. It is an important first step to understand and further develop cryogenic propulsion technology for future electric aircraft.

High-temperature superconductor (HTS) machine is a promising candidate for the electrical aircraft propulsion due to its great advantage in high power density. However, the HTS machine always suffers the problem of low thermal stability during quench. In this article, we apply a no-insulation (NI) coil technique on the rotor windings of HTS machines to enhance the stability and safety of the electrical aircraft. The NI HTS rotor windings experience ripple magnetic fields, which leads to induced eddy currents through turn-to-turn contacts. This induced current and accompanying losses will considerably affect the practicality of this technique. To study this issue, an equivalent circuit network model is developed, and it is validated by experiments. Then, analysis using this model shows that most of induced current flows in the outermost turns of the NI HTS coil because of skin effect, and lower turn-to-turn resistivity leads to higher transport current induced and more significant accumulation of turn-to-turn loss. A grading turn-to-turn resistivity is proposed to reduce the transport current induced and ac loss accumulation and meanwhile keep the high thermal stability of the NI HTS coil. Optimization of turn-to-turn resistivity is required when the NI HTS coil is applied in the machines' environments.

How to charge high-temperature superconducting coils is a crucial problem for superconducting magnets. In recent years, flux pump is recommended as a promising method for current compensation since it can charge coils without electrical contact to current source. In this work, two types of charging methods, rotating magnet based flux pump and magnetically controlled persistent current switch, are discussed experimentally and numerically. Their output characteristics were tested and demonstrated. Charging performance on different types of loads including insulated HTS coil, no-insulation HTS coil and DC current source, is analyzed and compared. Results of this paper will prove the validity of those two charging methods in future superconducting magnet applications.

The high critical current density of second-generation high-temperature superconducting (2G-HTS) tapes is the result of the systematic optimisation of the pinning landscape for superconducting vortices through careful engineering of the size and density of defects and non-superconducting second phases. Here, we use scanning Hall probe microscopy to conduct a vortex-resolved study of commercial GdBaCuO tapes in low fields for the first time and complement this work with “local” magnetisation and transport measurements. Magnetic imaging reveals highly disordered vortex patterns reflecting the presence of strong pinning from a dense distribution of nanoscale Gd2O3 second-phase inclusions in the superconducting film. However, we find that the measured vortex profiles are unexpectedly broad, with full-width-half-maxima typically of 6 μm, and exhibit almost no temperature dependence in the range 10–85 K. Since the lateral displacements of pinned vortex cores are not expected to exceed the superconducting layer thickness, this suggests that the observed broadening is caused by the disruption of the circulating supercurrents due to the high density of nanoscale pinning sites. Deviations of our local magnetisation data from an accepted 2D Bean critical state model also indicate that critical state profiles relax quite rapidly by flux creep. Our measurements provide important information about the role second-phase defects play in enhancing the critical current in these tapes and demonstrate the power of magnetic imaging as a complementary tool in the optimisation of vortex pinning phenomena in 2G-HTS tapes.

It is of great industrial interest and academic importance to investigate current harmonics impacts on AC losses of superconductors, especially in large-scale power devices. However, only effect of amplitude of in-phase current harmonics on AC loss has been studied in the works of literature. We numerically characterized nonsinusoidal AC loss of superconducting tape carrying harmonic currents with orders below 20th versus phase angles. A drastic AC loss variation was found when phase angle was considered for harmonic components. We observed that different harmonic orders show different AC loss profile versus phase angle.

To improve the ecological protection of coal gangue slopes, this work developed a polymer curing agent using a pure acrylic emulsion and cement as the main raw materials. By testing the workability, waterproofing, durability, and adhesion of the curing agent with different polymer-to-cement ratios, the optimum mixing ratio of the polymer curing agent was obtained, and the protection mechanism was analysed based on the test results and by scanning electron microscopy (SEM). Then, simulated slope and practical engineering applications were carried out. The results show that pure acrylic emulsion curing agent with a polymer-to-cement ratio of 6 : 5 is most suitable for the ecological protection of coal gangue slopes. The polymer curing agent can form a network structure film layer with a good waterproofing effect on the coal gangue surface. Simultaneously, the network structure film layer can reduce the influence of weathering and leaching on the coal gangue, prevent the heavy metal elements in coal gangue from entering the surrounding soil, improve the growth environment of plants, and improve the effect of ecological restoration. This research can be applied to the ecological protection of coal gangue slopes.

To explore the effect of grass and shrub plant roots on the stability of soil slopes in rainy areas in the south, this article relies on the Longlang Expressway construction project. Cynodon dactylon and Magnolia multiflora were selected as research subjects. The plant distribution characteristics and mechanical properties are analyzed. This paper uses ABAQUS finite element software to construct a 3D model of the planted slope in the test section. The stress and strain on the root system and the soil were observed, and the variation law of slope stability before and after plant protection under different rainfall events was compared and analyzed. The test and simulation results show that the root content of Cynodon dactylon gradually decreases with increasing depth. Cynodon dactylon was mainly distributed in the 0–30 cm soil body, and its effect on improving the cohesion of the soil body reached 75%. Magnolia multiflora belongs to vertical roots and has a strong and longer main root with relatively developed lateral roots. Its root system passes through the sliding surface of the slope bottom, which reduces the maximum equivalent plastic stress generated inside the slope by 61%. When the total rainfall duration is unchanged, under the three rainfall intensities of small, medium, and large, herbaceous plants increase the safety factor of the soil by 1.33%, 2.08%, and 6.1%, respectively, and the roots of shrubs increase the safety factor of the soil by 3.29%, 4.08%, and 4.32%, respectively. When the rainfall intensity does not change, as the rainfall time increases, the effect of plants on the slope safety factor first gradually increases and eventually stabilizes. The research results provide a reliable theoretical basis for analyzing the effect of plant roots on soil consolidation and slope protection, and they also lay a technical foundation for the promotion and application of ecological slope protection technology.

Application of high-temperature superconducting devices become promising in power networks, and transportation, including ship, train, and electric aircraft propulsion systems, with the advantages of light weight, compact size, and high efficiency, compared to conventional devices. In reality, electric networks-either in grid or transportation propulsion system-are polluted with harmonics due to the widespread use of power electronic devices and nonlinear loads. It is essential to explore the dependency of harmonic ac losses of different coil configurations carrying nonsinusoidal current. We modeled and compared harmonic ac loss behaviors in three coil configurations, single pancake coil (SPC), double pancake coil (DPC), and solenoid coil (SNC), where SPC and SNC are wound by identical wire length and DPC has twice conductor number compared to SPC. The research work has been carried out by means of H-formulation finite element method in a 2-D axisymmetric modeling environment of COMSOL Multiphysics. We explored and reported ac losses in these three coil structures carrying nonsinusoidal current with the third and the fifth harmonic orders, respectively, under different total harmonic distortion (THD) and fundamental current levels. It has been concluded that ac loss in these coils first decreases with the increase of the third harmonic content, when THD of the third harmonic 0.2. AC loss in coils monotonically increases with the increase of the fifth harmonic, drastically. We found that ac loss in SPC carrying the third harmonic and the fifth harmonic at different THD are more than 3.8 times of that in DPC; ac loss in SPC carrying either third or fifth harmonics at different THD are around 4.5 times of that in SNC.

We report here a record 4.6 T trapped field generated by high temperature superconducting (HTS) persistent current loops using a HTS ring structure. By stacking 200 HTS rings into a compact magnet 90 mm in diameter, we performed a field cooling magnetisation at 25 K. The main advantage of the new magnet compared to existing trapped field HTS magnets is that the magnetic field is in the parallel direction to the ab plane of the HTS, leading to higher critical currents in the same magnetic field. Therefore, compact HTS magnets can be developed based on this principle to achieve high magnetic fields. Experimental results show that the final trapped field distribution depends on the ring geometry. We developed a new three dimensional model to simulate the magnetic field distribution within the HTS ring magnet and good agreement between experiments and simulation have been found. The temperature dependency and ramping rate dependency have been studied numerically as potential factors to influence the magnet field. The proposed HTS ring magnet will have promising applications in medical imaging devices, e.g. MRI, as well as electrical machines.

More efficient heat sinks are required due to the rapid increase of power density in microelectronic devices. In this study, a micro-array heat sink with stagger trapezoidal bumps was designed. Numerical simulations for the flow and heat transfer under various conditions were carried out to help us to fully understand the mechanisms of the heat transfer enhancement in such a heat sink. The effects of the structure of the heat sink, parameters of the bumps, and volume fraction of the nanofluid on the performance of heat sink were studied. The results show us that the bumps in the heat sink can result in chaotic convection, interrupting the thermal boundary layer and increasing the cooling area, subsequently improving the heat transfer performance. Furthermore, parametric investigations for trapezoidal bumps were conducted to obtain preferential values for parameters, such as the bump width, fore rake angle of the bump, bump height, and bump pitch.

The magnetism of substrate has significant influence on the transport loss in high temperature superconducting (HTS) coated conductors (CCs). Taking three types of substrates (i.e., nonmagnetic, weakly magnetic, and strongly magnetic) into account, a study on the frequency dependence (in the range of 10–1000 Hz) of transport loss in HTS CCs with these substrates is performed numerically and experimentally. Different loss components and magnetic field profile are calculated individually to clarify the effect of substrate magnetism. Results show that HTS CCs with magnetic substrate exhibit higher transport loss than CCs with nonmagnetic substrate. Both hysteresis loss in HTS layer and eddy current loss in conventional metal increase with the enhancement of substrate magnetism.

The magnetic AC losses of monofilament Sr 0.6 K 0.4 Fe 2 As 2 /Ag tapes are measured in the temperature range between 20 K and 30 K both in perpendicular and parallel field. The loss, measured by the standard magnetization technique, is determined from the area of the hysteresis loop using a vibrating sample magnetometer (VSM) in a cyclic field of amplitude up to 7 T. The results in perpendicular field are compared to that of the parallel-field loss and theoretical calculation of magnetization loss at various temperatures. There is a reasonable agreement between the theoretical model and the experimental results even in high field. The magnetic critical current density ( J c ) of the tape, obtained by the magnetic hysteresis measurements M ( H ), are investigated in two field directions and in the temperature range from 5 K to 30 K. The comparison between the magnetic J c in both field directions and the transport J c of the tape are also done at various temperatures and fields. The anisotropy of J c ( Γ = J c a b / J c c ) is very small.

AC losses in superconductors are essential for the design of cooling system for large scale power applications. Magnesium diboride (MgB 2 ) superconducting wires have been investigated and manufactured over the last decade due to cheap raw materials and flexibility for coil design. In addition, multifilamentary MgB 2 wires have been manufactured to reduce ac losses. In this paper, self-field ac losses of multifilamentary MgB 2 wires with magnetic barrier are investigated using both experimental and numerical methods. A short straight wire sample and a coil sample are tested under various temperatures and frequencies between 16-and 128-Hz. The test results show that the transportation loss is independent of the operating temperature. On basis of both theoretical and numerical study, it is found that hysteresis loss in superconductor accounts only for a small fraction of the transportation losses, ferromagnetic hysteresis loss in the magnetic barrier dominates when the transport current is low, whereas eddy current loss dominates when the transport current is close to the critical current.

ReBCO conductor-on-round-core (CORC) cable has become a promising candidate for high temperature superconducting (HTS) power applications, due to its great mechanical strength, high current carrying capacity, high flexibility, and low ac losses. However, ReBCO coated conductors are at risk of quenching, which significantly affects the thermal stability and reliability of the CORC cable. Three-dimensional (3-D) numerical study on the quench behavior of the CORC cable remains a challenge, for its complex geometry is difficult to cope with. In this paper, a 3-D time-dependent multi-physics quench model based on the T-A formulation has been developed. Three modules are coupled in this model; the T-A formulation model, a heat transfer model, and an equivalent circuit model. The quench behavior of a single-layer ReBCO CORC cable with non-uniform terminal contact resistances has been studied, when a hotspot is imposed on one of the tapes to induce a local quench. Results show that, the CORC cable has the highest MQE; in other words, it is the most stable situation, when the hotspot-induced quench occurs on the tape with the middle value of terminal contact resistance.

A Conductor on Round Core (CORC®) cable wound with a high temperature superconductor is an important cable concept for high current density applications. The design of a CORC cable makes understanding its electromagnetic performance—for example its AC losses—challenging. This paper presents a thorough study of CORC cables by combining experimental and numerical methods. In particular, it focuses on understanding how the cable structure influences the magnetization losses and on how these can be reduced. A novelty of this paper lies in the use of a new T-A formulation, which, for the first time, is employed for three-dimensional modelling of a CORC cable with real geometry. The use of the new T-A formulation in finite element software enables the study of how the winding direction and multiple-layer structure affect the magnetization losses of CORC cables. Moreover, influence of striation in CORC cables is studied as an effective way to reduce their losses. A CORC cable with striated tapes shows a significant magnetization loss reduction at high magnetic fields, in comparison to its counterpart without striated tapes. At low magnetic fields, tape striation leads to an increase in loss when the number of filaments is low, then the loss drops with a further increase in the number of filaments, but this loss reduction is much weaker than that at high fields. This paper provides an efficient tool for investigating the electromagnetic behaviour of CORC cables, which can provide valuable guidance in designing CORC cables with minimized losses for high energy physics and energy conversion applications.

One of the most promising applications for high temperature superconducting (HTS) material is magnet. HTS magnet is considered to be used in a wide range of applications including MRI, particle accelerator, electrical machine, and etc. HTS rotating machines are mainly utilized for cryo-electrification, i.e. application of superconductivity for modern electrification. One of the main aspects of cryo-electrification is electric aircraft application which is recently enabled by continuous progress in design development of superconducting magnets and HTS machines. This paper discusses the challenges facing a newly developed magnet type, i.e. HTS ring magnet, that being considered in superconducting rotating machine in future electric aircrafts. HTS ring magnet is compact, easy to develop, fault tolerant, and light in weight, and it recently reached to a high level of magnetic field.

This paper investigates how the current distribution and ac losses of second-generation high-temperature superconducting (2G HTS) pancake coils are affected by the material anisotropy. The 2G HTS tape has a strong anisotropy, which means that its critical current densities are different at different magnetic field orientations. 2G HTS pancake coils with different anisotropies have different ac losses. We studies this phenomenon by both experimental and numerical methods. Three 2G HTS coils with different turns are fabricated and tested in liquid nitrogen. AC losses with different frequencies are measured and compared with simulation. We use the critical-state model to calculate the current distribution and ac losses. Our modeling results are well validated by experimental measurements. We apply this numerical model to pancake coils with different anisotropies and calculate the current distribution, magnetic field, and ac loss. We find that the anisotropy of 2G HTS affects the ac loss and current distribution. It is possible to decrease the ac loss by an advanced pinning technique.

Growing electricity demand from a range of sources with higher loads and the industry structural changes open the possibility of more frequent and larger fault currents producible in liver power grids. Traditional solutions to the fault have difficulties in satisfying the requirement of the new power grid requirement due to many factors, such as high cost and additional impact to power grid in normal loading condition, which leads to the research for an efficient alternative solution of interest to both academia and industry: Superconducting fault current limiter (SFCL). Critical current Ic is used to describe the maximum current that a superconductor can transport and is one of the most important parameters to be considered while designing a SFCL. Guaranteeing the homogeneous distribution of the critical current density flowing in a superconductor is not possible due to manufacturing process limitations. In this paper, the impact of critical current inhomogeneity of coated high temperature superconductors (HTSs) during the quench process is studied experimentally. The results show that both the amount of Ic degradation and the size of Ic degraded segments have a great impact on the maximum temperature generated in the quench process of HTSs when the prospective fault current is low ( 1.13Ic to 1.51Ic. Higher amount of localized Ic degradation allows higher maximum temperatures under low voltage fault. Additionally, smaller Ic degraded segment allows higher maximum temperatures.

This paper focuses on the 2G HTS armature winding design for fully HTS machines. The main challenge for HTS armature windings is the ac loss calculation. This paper proposes an effective way to estimate 2G HTS armature efficiency. The originality of ac loss calculation is effectively coupling a machine model with a HTS model using boundary conditions to simulate machine environment while considering material anisotropy. The paper then uses H formulation to estimation the total armature ac loss for a 1-MW propulsion motor. The ac loss is calculated for different electrical loadings and magnetic loadings. A new double-layer armature winding is proposed to reduce the spatial harmonics of HTS armature.

Fully superconducting machines provide the high power density required for future electric aircraft propulsion. However, superconducting windings generate AC losses in AC electrical machine environments. These AC losses are difficult to remove at low temperatures and they add an extra burden to the aircraft cooling system. Due to heavy cooling penalty, AC losses in the HTS stator, is one of the key topics in HTS machine design. In order to evaluate the AC loss of superconducting stator windings in a rotational machine environment, we designed and built a novel axial-flux high temperature superconducting (HTS) machine platform. The AC loss measurement is based on calorimetrically boiling-off liquid nitrogen. Both total AC loss and magnetisation loss in HTS stator are measured in a rotational magnetic field condition. This platform is essential to study ways to minimise AC losses in HTS stator, in order to maximum the efficiency of fully HTS machines., Comment: 17 pages, 15 figures

Superconducting fault current limiter (SFCL) has been extensively applied for many application in electrical grids. In this paper, the overcurrent and overvoltage characteristics of a pancake-structured SFCL module are tested and evaluated. First, a single superconducting (SC) tape is tested for critical current and room temperature resistance. Then, a pancake coil is developed using the considered tape with a proper length to exhibit a resistance of 3 Ω. This resistance value was found effective by many researchers with either AC or DC systems. After winding the coil, its critical current is reevaluated and its inductance is measured at cryogenic temperature. The overcurrent test is done at different cases of supply voltage until the prospective current attained about four times of coil critical current. The prospective current and limited was measured for each case. Based on the results, it could be discriminated between inductive limit and resistive limit of the coil. The overvoltage test is done on a SC tape sample to assess the surface flashover withstand voltage and compare it to the maximum allowed electric field along the coil length. The flashover results are analyzed statistically using Weibull analysis.

High-temperature superconducting (HTS) triaxial conductor on round core (CORC) cable is a potential candidate for use in the transmission line of hybrid-electric aircraft because of its advantages in compactness, high power density, and reduced usage of HTS tapes. The harmonic currents generated by ac/dc converters, generators, and motors in the aircraft affect the ac loss of the transmission cable, thus influencing the efficiency of the cable. This paper analyzes the influence of harmonic current on the magnetization loss of a triaxial CORC HTS cable (10 MW, 3 kV/2 kA), which is designed for hybrid-electric aircraft. A finite element method model based on the T-A formulation has been developed for the HTS triaxial CORC cable, which calculates the losses induced both with and without applied harmonic currents. The results show that the magnetization loss has a significant nonuniform distribution among three phases even under a three-phase balanced rated load. The results also show that a small harmonic current (less than 10% of the rated load current) can lead to a considerable increase in the magnetization loss of the cable (up to 40%). In the triaxial CORC cable, harmonic currents lead to the highest increasing rate of magnetization loss with respect to frequency in the innermost phase.

The AC superconducting power transmission cable projects are being carried out around the world, while the DC superconducting cable is also proven to be very competitive for power transmission in electrical grid due to the negligible AC losses. The critical current and current distributions are two important factors to improve the performance of DC HTS cable for large transmission capacity. In this paper, we proposed an experimental method to investigate the behavior of critical current affected by various gap distances between the individual HTS tape and transverse tape width. The results show that the critical current of the HTS tape in the cable field can be increased by about 10% if the gap distance is less than 1 mm. Finite element method (FEM) is used to calculate the magnetic field distribution to verify the experimental results. A short HTS cable has been constructed to investigate the influence of contact resistance on the current distribution among the HTS tapes. The results show that the contact resistances have significant influence on the current sharing of each HTS wire in the cable structure.

Concept design and optimization for a high-temperature superconducting (HTS) rotor of a 100-kW generator is studied in this paper. The rotor has eight HTS magnets constructed by air-cored racetrack coils. The HTS magnets with a unique circular cross-sectional shape can produce a gap field over 2.0 T. The present design is based on a new graphical method developed to estimate the maximum operating current of an HTS magnet that takes both I c -B characteristics and angular dependence into consideration. The optimized stepped shape design saved more than 8% of the HTS material compared with the rectangular one without sacrificing the gap field.

Using a high-temperature superconducting (HTS) coil as the transmitting coil is an optional way to increase the efficiency and the distance of a wireless power transfer (WPT) system. In practical cases, receiving coils might be not only one, or not convenient to get a low-temperature environment, or not in the same sizes with the transmitting coil. Thus, we constructed a WPT system from one HTS coil to two copper coils, and one of the copper receiving coils had a different size from the HTS transmitting coil. In addition, we placed one HTS relaying coil between the transmitting coil and the receiving coils to increase the efficiency and the distance. Based on this system, we conducted several experiments and analyzed the results. We found that a denser coil could obtain more power in an HTS WPT system and that WPT from one HTS coil to two copper coils was more efficient than WPT from the same HTS coil to one of the two copper coils. We also demonstrated that an appropriate layout of the HTS relaying coil can increase the HTS WPT efficiency.

The second generation high temperature superconductor, specifically REBCO, has become a new research focus in the development of a new generation of high-field (>25 T) magnets. One of the main challenges in the application of the magnets is the current screening problem. Previous research shows that for magnetized superconducting stacks and bulks the application of an AC field in plane with the circulating current will lead to demagnetization due to vortex shaking, which provides a possible solution to remove the shielding current. This paper provides an in-depth study, both experimentally and numerically, to unveil the vortex shaking mechanism of REBCO stacks. A new experiment was carried out to measure the demagnetization rate of REBCO stacks exposed to an in-plane AC magnetic field. Meanwhile, 2D finite element models, based on the E–J power law, are developed for simulating the vortex shaking effect of the AC magnetic field. Qualitative agreement was obtained between the experimental and the simulation results. Our results show that the applied in-plane magnetic field leads to a sudden decay of trapped magnetic field in the first half shaking cycle, which is caused by the magnetic field dependence of critical current. Furthermore, the decline of demagnetization rate with the increase of tape number is mainly due to the cross-magnetic field being screened by the top and bottom stacks during the shaking process, which leads to lower demagnetization rate of inner layers. We also demonstrate that the frequency of the applied AC magnetic field has little impact on the demagnetization process. Our modeling tool and findings perfect the vortex shaking theory and provide helpful guidance for eliminating screening current in the new generation REBCO magnets.

In this paper, the properties of current capacity and ac transport loss of striated high temperature superconductor (HTS) coil is compared with ordinary HTS coil by using both experimental and simulation methods. The measurements were carried out by transporting a sinusoidal varying current at 77 K, with an amplitude range of 10-50 A in frequency from 70 up to 300 Hz. Measurement facilities and methods are explained in more detail in the paper. The critical current of the 4-mm width multifilament coil made with four filaments at a spacing 0.03 mm was found to be lower than that of the nonstriated coil. The frequency dependent characteristics agreed well in both experimental and simulated results. Reducing ac loss of HTS is one enabling factor for widespread adoption of the technology, and therefore, understanding its characteristics is important and discussed in this paper. Future plans based on this preliminary work are the testing of multifilament tapes in an axial flux motor field environment.

An MJ-class superconducting magnetic energy storage (SMES) system has a wide range of potential applications in electric power systems. The composite high-temperature superconducting HTS conductor, which has the advantages of carrying large critical currents and withstanding high magnetic fields, is suitable for winding an MJ-class magnet coil. However, the Lorentz force of an HTS wire is so large that its induced mechanical stresses should be examined to ensure that the magnet is in good condition. By means of the equivalent material properties method and the sequential coupling method, this paper studies the mechanical properties of a three MJ toroidal SMES magnet wound by a composite HTS conductor. Based on the electromagnetic-structural coupling analysis, the Von-Mises stress, the radial stress, and the hoop stress of a magnet coil are calculated and employed to validate the stability of the MJ-class toroidal SMES magnet.

A single pancake coil without turn-to-turn insulation was tested in this paper to investigate the transient responses under different situations. We performed charging and discharging test, AC current test, and regional quench emulation test on the non-insulated (NI) coil. The experimental test results show a significant time delay for charging and discharging characteristics of NI coil and can be validated by a simple proposed equivalent electrical circuit. Under the AC operating current, the NI coil can bypass nearly all the AC current from the coil spiral path to the radial path such that it is not possible for NI coil to store or be affected by the AC magnet field. Additionally, while carrying AC current, the AC loss dissipation of NI coil is inversely proportional to the frequency of the AC operating current. When a regional quench occurs, the NI coil can bypass the current in the regional quench zone to avoid further temperature accumulated and protect the NI coil itself.

A heat-shrink tube insulation for second-generation high-temperature superconducting (2G HTS) coils is studied experimentally in this paper. The insulation can be used to prevent the delamination of 2G HTS due to epoxy impregnation. According to the experiment results, the insulated coil shows no degradation before and after the epoxy impregnation. To validate the performance of 2G HTS coils with the heat-shrink insulation, we measured the transport loss, magnetization loss and total loss of a 2G HTS coil. The study demonstrates the feasibility of the heat-shrink tube insulation method for electrical applications.

The high-temperature superconductor (HTS) (RE)Ba 2Cu 3O x (REBCO) conductor on round core (CORC) cable has great advantages with its high current capacity and power density. In REBCO CORC cables, current is redistributed among tapes through terminal contact resistance (TCR) when a local quench occurs. Therefore, its quench behavior is different from the single tape situation. To better understand the underlying physical process of local quenches in CORC cables, a new 3D multi-physics modeling tool for CORC cables is developed and presented in this paper. In this model, the REBCO tape is treated as a thin shell without thickness, and four models are coupled: a T-formulation model, an A-formulation model, a heat transfer model, and an equivalent circuit model. The T-formulation model is applied to the conductor shell only to calculate current distribution, which will be input into the A-formulation model; the A-formulation model is applied to the whole 3D domain to calculate the magnetic field, which is then fed back to the T-formulation model. The hot spot-induced quenches of CORC cables are analyzed. The results show that the thermal stability of the CORC cable can be considerably improved by reducing the TCR. The minimum quench energy (MQE) increases rapidly with the reduction of TCR when the resistance is in a middle range, which is about in this study. When the TCR is too low () or too high (), the MQE shows no obvious variation with TCR. With a low TCR, a hot spot in one tape may induce an overcurrent quench on other tapes. This will not happen in a cable with high TCR. In this case, the tape with a hot spot will quench and burn out before inducing a quench on other tapes. The developed modeling tool can be used to design CORC cables with improved thermal stability.

Most high-temperature superconducting (HTS) electric devices have been built by employing a superconducting composite cable. These HTS devices generally have large current-carrying capacities. Therefore, the electromagnetic characteristic of a composite superconducting cable with high current density becomes a key point for the application of these HTS devices. A helical superconducting cable and a Roebel cable consisted of YBCO-coated conductors with a critical current of 500 A @ 77 K in the self-field are both proposed in this paper. The helical superconducting cable that has two conductor layers and the structure parameters of the helical cable are given. The Roebel cable has four layers with a critical current of 500 A. Two 3-D models are built for a helical cable and a Roebel cable with variable geometries. Their magnetic flux distributions along the axis are presented based on the finite-element analysis method, and their electromagnetic characteristics are discussed as well. The analysis model can be further applied for the large-scale HTS electric device design with high current-carrying ability.

More and more high temperature superconducting (HTS) devices have been built by employing the high current-carrying superconducting cable which is made by a few coated conductors in parallel. These HTS devices generally have a large capacity because of the high performance of the current carrying. Therefore, a cable with high current density becomes a key point for the HTS devices application. There are two kinds of superconducting cable, a helical cable and a Roebel cable, are proposed in this paper. Their optimal structure and the electromagnetic characteristics are presented based on finite element analysis method. Two 3D models are built for a helical cable and Roebel cable with variable geometry. The magnetic flux distributions along the axis are discussed considering the different geometry parameters. Our model can be further applied for the cable design in high current application under magnetic field.

As we known, the high temperature (77 K) superconducting (HTS) motor is considered as a competitive electrical machine by more and more people. There have been various of designs for HTS motor in the world. However, most of them focus on HTS tapes rather than bulks. Therefore, in order to investigate possibility of HTS bulks on motor application, a HTS magnet synchronous motor which has 75 pieces of YBCO bulks surface mounted on the rotor has been designed and developed in Cambridge University. After pulsed field magnetization (PFM) process, the rotor can trap a 4 poles magnetic field of 375 mT. The magnetized rotor can provide a maximum torque of 49.5 Nm and a maximum power of 7.8 kW at 1500 rpm.

High-temperature superconductors have created the opportunity for a step change in the technology of power applications. Racetrack superconducting coils made from YBCO coated conductors have been used in several engineering applications including SMES, rotor or stator windings of electric machines. AC loss is one of the most important factors that determine the design and performance of superconducting devices. In this paper, a numerical model is developed to calculate the AC losses in superconducting racetrack coils in different magnetic conditions. This paper first discusses the AC losses of the coils in self-field or external field only. It then goes to investigate the AC losses of the coils being exposed to AC ripple field and a DC background field. Finally, the AC losses of the coils carrying DC current and being exposed to AC field are calculated. These two scenarios correspond to using superconducting coils as the rotor field winding of an electric machine.

YBaCuO-coated conductors offer great potential in terms of performance and cost-saving for superconducting fault current limiter (SFCL). A resistive SFCL based on coated conductors can be made from several tapes connected in parallel or in series. Ideally, the current and voltage are shared uniformly by the tapes when quench occurs. However, due to the non-uniformity of property of the tapes and the relative positions of the tapes, the currents and the voltages of the tapes are different. In this paper, a numerical model is developed to investigate the current and voltage sharing problem for the resistive SFCL. This model is able to simulate the dynamic response of YBCO tapes in normal and quench conditions. Firstly, four tapes with different Jc's and n values in E-J power law are connected in parallel to carry the fault current. The model demonstrates how the currents are distributed among the four tapes. These four tapes are then connected in series to withstand the line voltage. In this case, the model investigates the voltage sharing between the tapes. Several factors that would affect the process of quenches are discussed including the field dependency of Jc, the magnetic coupling between the tapes and the relative positions of the tapes.

This paper begins with introducing the winding techniques of two superconducting double-pancake coils wound using 2G coated conductors. These winding techniques are able to guarantee a good performance for the superconducting coils. Then the coil critical currents were measured and compared with a simulation model. The results were consistent. Finally the coil AC losses were measured using an experimental circuit including a compensation coil. The simulation results are close to the experiment results.

AC loss can be a significant problem for any applications that utilize or produce an AC current or magnetic field, such as an electric machine. The authors are currently investigating the electromagnetic properties of high temperature superconductors with a particular focus on the AC loss in coils made from YBCO superconductors. In this paper, a 2D finite element model based on the H formulation is introduced. The model is then used to calculate the trans port AC loss using both a bulk approximation and modeling the individual turns in a racetrack-shaped coil. The coil model is based on the superconducting stator coils used in the University of Cam bridge EPEC Superconductivity Group's superconducting permanent magnet synchronous motor design. The transport AC loss of a stator coil is measured using an electrical method based on inductive compensation using a variable mutual inductance. The simulated results are compared with the experimental results, verifying the validity of the model, and ways to improve the accuracy of the model are discussed.

In order to design a High Temperature Superconducting (HTS) machine that is able to operate safely and reliably, studies on the characterization of Second Generation (2G) HTS tapes are of paramount importance. This paper presents an experimental setup to measure critical current of 2G HTS tapes in high DC magnetic fields (up to 5 Tesla) with an AC current ripple superimposed, as well as various temperatures ranging from 25 K to 77 K. The 2G tape measured is the SGS12050 coated conductor made by SuperPower. The critical current is measured by a flux vector with reference to the widest sample face from 0 to 90 degrees in 10 degree steps. Smaller steps are required close to 0°. A Variable Temperature Insert (VTI) is utilized to control temperature change.

High temperature superconductors, such as melt-processed YBCO bulks, have great advantages on trapping strong magnetic fields in liquid nitrogen. To enable them to function well, there are some traditional ways of magnetizing them, in which the YBCO bulks are magnetized instantly under a very strong source of magnetic field. These ways would consume great amounts of power to make the superconductors trap as much field as possible. Thermally Actuated Magnetization (TAM) Flux pump has been proved a perfect substitution for these expensive methods by using a relatively small magnet as the source. In this way, the field is developed gradually over many pulses. Unlike conventional flux pumping ways, the TAM does not drive the superconductor normal during the process of magnetization. In former experiments for the flux pump, some fundamental tests were done. In this paper, the experiment system is advanced to a new level with better temperature control to the thermal waves moving in the Gadolinium and with less air gap for the flux lines sweeping through the superconductor. This experiment system leads to a stronger accumulation of the magnetic field trapped in the YBCO bulk. We also tried different ways of sending the thermal waves and found out that the pumping effect is closely related to the power of the heaters and the on and off time.

This paper investigates the electromagnetic properties of high-temperature superconductors with a particular focus on the AC loss in coils made from YBCO superconductors. The numerical analysis and finite element modelling of the YBCO superconductors used in Cambridge's superconducting permanent magnet synchronous motor currently in development is described. The stack of tapes in the superconducting coils is modelled using the direct H formulation, a B-dependent critical current density and a bulk approximation. Magnetic boundary conditions for this model are derived from a 2D finite element method (FEM) motor model. The combination of these models allows the total AC loss (combined transport and magnetisation losses) in the HTS coils used in an all-superconducting machine design to be estimated. The raw AC loss figures are compared to the output power of the motor for two test cases, and it is found that the AC loss contributes significantly to the total loss and therefore efficiency. An experimental rig is also described, which has been built in order to test the electromagnetic properties and performance of the motor. It is explained how this rig will be used to investigate the magnetisation of the rotor and carry out AC loss measurements on the stator coils.

This paper presents an SMES coil which has been designed and tested by University of Cambridge. The design gives the maximum stored energy in the coil which has been wound by a certain length of second-generation high-temperature superconductors (2G HTS). A numerical model has been developed to analyse the current density and magnetic field distribution and calculate the AC losses during the charge and discharge process of the coil. A cryostat has been designed and a test of the I-V curve measurement of the coil has been accomplished. In addition, the power electronics control of the SMES coil has been simulated.

It is particularly attractive for High Temperature Superconductors (HTS), such as the melt-processed YBCO single-grain bulks, to trap strong magnetic fields at cryogenic temperatures. With the flux density orders of magnitude much greater than the rare earth magnets, the YBCO bulks can be formed as small and as compact as them. As a result, YBCO bulks are used as magnets in magnetic bearings, MRI scanners and motors, etc. The trapped field by the YBCO magnet is decided by the magnetization, which normally includes three different ways. However, the traditional ways to magnetize the YBCO will always need the applied field to be as high as the expected field on the superconductor or much higher than it. In this paper, we will describe a technique which facilitates the creation of the high magnetic field and utilize a normal permanent magnet in stead. By using this rare earth permanent magnet which is not as strong, this technique involves the superconducting flux pump concept, which means that a much larger field being trapped by the superconductor bulk is caused by a small field repeatedly applied to it. To achieve such pumping effect, an intermedia material is necessary and in the related experiments, Gadolinium is used as such an important thermally actuated material. Traveling magnetic field through it will magnetize the YBCO in such a way that the flux density will be accumulated step by step on the surface on the bulk.

High-current high temperature superconducting (HTS) cables have been developed for use in HTS power devices. This paper presented the structures of HTS cables, including Conductor on Round Core (CORC) cable, Twisted Stacked-tape Conductor (TSTC) cable, and Double coaxial cable. Subsequently, three-dimensional finite element method numerical models were built to analyze the electromagnetic characteristics of the cables, and the critical current of the cables is about 380 Ampere 77 K, self-field. Using the T-A formulation, the numerical model assumed a sheet approximation for conductors, which shortened computational time. The T-A formulation were verified by experiments on a superconducting tape. Then HTS cables with different configurations were made, as functions of different transport current and background magnetic field, and different pitches of Double Coaxial Cable inner conducting layer. According to the results, the ac losses of Double coaxial cable and CORC cable decreased 40% than the TSTC cable with different transport current, and the Double coaxial cable ac loss decreased 20% than the CORC cable when background magnetic field was in the range of 20-60 mT. Conclusions obtained from this study will be helpful for understanding the ac loss properties of HTS cables and useful in design of HTS power devices (such as HTS transformer), using HTS cables.

Application of the second generation High Temperature Superconducting (HTS) YBCO tapes has been increasingly popular since the low-cost superconducting materials were discovered. This paper mainly presents the properties of two types of 2G YBCO tapes from American Superconductor Corporation (AMSC) and SuperPower, Inc, respectively. All superconducting materials are cooled by liquid nitrogen (77 K). At first, we will introduce the basic design strategy for synchronous motor and characteristics of superconducting materials. Then, the basic principle and algorithm for calculation and measurement of Ic and AC transport loss will be explained. Secondly, the novel measurement system using a high-precision digital lock-in amplifier is presented. Finally, we present and analyze simulation and experimental results for two types of YBCO tapes in bending condition.

This paper describes in detail the magnetization process of a synchronous motor, constructed with High Temperature Superconducting (HTS) elements on the rotor. The Pulse Field Magnetization (PFM) method is considered to magnetize the HTS rotor as a four-pole trapped flux magnet. A 2D model is built for the magnetization process. During the machine operation, intolerable torque ripples occur, which are mainly caused by the irregular field distribution from the magnetized HTS rotor. In order to suppress the large torque ripples, an iterative learning control method is used in addition to the traditional field-oriented control method.

Due to the increased fault-level currents, superconducting fault current limiter (SFCL) is more likely to penetrate into a low voltage and medium voltage transmission network to improve their stability and lower the electric devices capacity. Therefore it is important to model a SFCL in power system to analyze its performance and study its characteristics. In this paper, a simulation model for a resistive type SFCL consisted of YBCO tapes is developed using Matlab/Simulink software. This model will take into account SFCL's internal electromagnetic behavior by coupling its internal resistance and the current density characteristics based on the E-J power law. Finally, the SFCL simulation model is applied in a transmission and a wind farm power grid, respectively. Different fault limiting scenarios are investigated and the results show that the SFCL is effective in limiting fault currents with a maximum of 50% in transmission lines, particularly for wind farm networks.

This paper presents a new trapped-field magnet made of second-generation high-temperature superconducting (2G HTS) rings. This so-called ring-shape 2G HTS magnet has the potential to provide much stronger magnetic fields relative to existing permanent magnets. Compared to existing 2G HTS trapped- field magnets, e.g. 2G HTS bulks and stacks, this new ring-shape 2G HTS magnet is more flexible in size and can be made into magnets with large dimensions for industrial applications. Effective magnetization is the key to being able to use trapped-field magnets. Therefore, this paper focuses on the magnetization mechanism of this new magnet using both experimental and numerical methods. Unique features have been identified and quantified for this new type of HTS magnet in the field cooling and zero field cooling process. The magnetization mechanism can be understood by the interaction between shielding currents and the penetration of external magnetic fields. An accumulation in the trapped field was observed by using multiple pulse field cooling. Three types of demagnetization were studied to measure the trapped-field decay for practical applications. Our results show that this new ring-shape HTS magnet is very promising in the trapping of a high magnetic field. As a super-permanent magnet, it will have a significant impact on large-scale industrial applications, e.g. the development of HTS machines with a very high power density and HTS magnetic resonance imaging devices.

This paper is to study ramping turn-to-turn loss and magnetization loss of a no-insulation (NI) hightemperature superconductor (HTS) pancake coil wound with (RE)Ba2Cu3Ox (REBCO) conduc-tors. For insulated (INS) HTS coils, a magnetization loss occurs on superconducting layers during aramping operation. For the NI HTS coil, additional loss is generated by the “bypassing” current onthe turn-to-turn metallic contacts, which is called “turn-to-turn loss” in this study. Therefore, theNI coil’s ramping loss is much different from that of the INS coil, but few studies have beenreported on this aspect. To analyze the ramping losses of NI coils, a numerical method is developedby coupling an equivalent circuit network model and a H-formulation finite element method model.The former model is to calculate NI coil’s current distribution and turn-to-turn loss, and the lattermodel is to calculate the magnetization loss. A test NI pancake coil is wound with REBCO tapesand the reliability of this model is validated by experiments. Then the characteristics of the NIcoil’s ramping losses are studied using this coupling model. Results show that the turn-to-turn lossis much higher than the magnetization loss. The NI coil’s total ramping loss is much higher thanthat of its insulated counterpart, which has to be considered carefully in the design and operation ofNI applications. This paper also discusses the possibility to reduce NI coil’s ramping loss bydecreasing the ramping rate of power supply or increasing the coil’s turn-to-turn resistivity.