Thermal stability is a main issue in the development of 2G HTS coil applications, due to the difficulties of quench detection and of efficient protection system implementation. To avoid thermal gradients and possible coil damages with thermal cycling, the heat generated in all coil components should be removed efficiently towards the cool sink, being crucial the minimization of the coil - cooling system thermal impedances.
Those aspects become more relevant on conduction-cooled configurations using cryocoolers where the heat is removed directly from the coil components by cooper or aluminium plates thermally anchored to its cool head. Good thermal contacts and, simultaneously, good electrical insulation between the coil superconducting tapes and the refrigerating metallic plates are required. Moreover these thermal and electrical properties should remain, together with proper mechanical ones, after several low and high rate thermal cycles between 300 K and operating cryogenic temperatures.
In this work two different aspects are considered: First the mechanical resistance of cooper and aluminium plates glued with the common epoxies used in the fabrication of 2G HTS coils have been taken as reference. The influence of the metallic surface characteristics, such as roughness or types of grooves, on the mechanical properties of the metal/epoxy interfaces has been analysed, with special emphasis on the thermal cycling effects.

Secondly, we have explored improved thermal and electrical insulation contacts between superconducting coils and refrigerating plates by creating thin layers of alumina on the Cu plate. This has been achieved by combination of thermal spraying and laser treatments, which enables the creation and modification of alumina layers on the copper surface. It has been analysed how the processing conditions modify the electrical and thermal properties of these coatings. In particular, we have studied the possibility of controlling the aluminium oxide layer thickness, surface roughness and coating porosity and how these parameters affect the final coating properties.

This paper is subsequent research of “Static Characteristics of High Temperature Superconductor and Fluid Film Compound Bearings”, paper that submitted by the authors to IEEE Transactions on Applied Superconductivity and in revision right now. With the application prospect of new generation of liquid rocket turbopump and the status that the system safety and reliability are weaken by the friction and wear of rolling bearings, a kind of superconducting magnetic force and dynamic fluid film force compound bearing for axial balance is proposed in this paper. The most important feature of the compound bearing lies in a kind of “superconducting tilting pad” structure. Specifically, circular superconducting pad is installed on disc spring which can produce small elastic deformation, different from the fixed installation way in other superconducting bearings. The compound bearing consists of six superconducting tilting pads and thrust collar prepared by permanent magnet. During the operational process, compound bearing immersed in the cryogenic medium can not only possess superconducting magnetic force but also hydrodynamic pressure caused by the slight swing of pads. Dynamic characteristics of the compound bearing based on decoupling analytical method are studied. Stiffness and damping characteristics of compound bearing in the cooling and lubricating medium of liquid nitrogen are analyzed specially. The theoretical results show that this compound bearing can not only avoid the contacting friction in the stage of start and stop of liquid rocket turbopump, but also maintain a high stiffness in the working stage to resist impact load.

A superconducting linear magnetic bearing for a full scale MagLev vehicle was successfully demonstrated recently. This vehicle, named MagLev-Cobra, was designed to connect two buildings of the Federal University of Rio de Janeiro distant from 200 m. It is composed of four 1.5 m long wagons and is able to transport up to 24 passengers. The evolution of this work has been reported in previous editions of EUCAS. In order to achieve the superconducting levitation, it uses a permanent magnet guideway (PMG) and several cryostats containing YBCO superconducting bulks placed on the bottom of the vehicle. The interaction of the PMG static magnetic field with the superconductors produces a stable levitation force, in all degrees of freedom, except along the translational direction where the vehicle can move practically without friction losses. One economic issue of this technology is the cost of the raw materials involved, due to the large amount of rare earth permanent magnets (PM) used along the entire line. In this context, an optimization of the superconducting linear magnetic bearing (SLMB), focusing on the reduction of PM material used and the increase of the levitation force value, motivates the present work.
In the recent years, several models have been successfully developed by different research groups around the world to simulate superconducting magnetic bearings. While they work well, their use for optimization has been so far limited, because the problem is highly nonlinear and intrinsically time-dependent, leading to very large computing time in order to perform the numbers of simulations needed in the optimization process.
In this work, an H-formulation model with a power law E-J relationship is applied to simulate the superconducting material. The dependence of the superconductor critical current (Jc) on the magnetic flux density, the PMG real geometry and the iron nonlinearity are included. This allows us predicting accurately both the bearing levitation force and its lateral force. The model is validated by comparison with measurements performed on the MagLev-Cobra SLMB. Thanks to a novel modeling strategy presented here, the model is fast enough to perform optimization in a reasonable time. For such purpose, it is coupled with a multi-objective particle swarm optimization algorithm. The results demonstrate the possibility of substantially reducing the amount of PM, and consequently the cost, while keeping the same strength performances. This is of particular interest, mainly when considering the construction of lines of several kilometers. Overall, the analysis presented in this work further supports the idea of this promising transportation system.

— In this work, a new contribution was given, to calculate the interaction forces between permanent magnet (PM) and hard typt-II superconductor (SC) used as classical levitation system. The forces and also mutual inductances are developed via Green’s functions using the amperian model assuming a cuboidal thin coil for (PM) and a superposition of closed loop of rectangular cross section for the (SC). For the superconductor, the value of critical current is known but the variable penetration thickness of current is given from external magnetic fields by respectively Bean’s, Kim’s and the exponential models [1-2]. All major or minor cycles of magnetization process are obtained with an interesting presentation of different lateral thicknesses of critical current densities. The most important part was the new analytical developments of vertical force between (PM) and (SC), realized after several integral's calculations. some results was be discussed for vertical displacement of a permanent magnet proving the accuracy of our analytical model compared with other realized by Flux3D FEM software.

Magnetic shielding represents an important issue in space applications where intense magnetic fields could be used to protect the crew from cosmic radiation. It is also relevant for confining the exposed region in MRI imaging and more generally everywhere sensitive electronic equipment has to be preserved.
We report on the fabrication of MgB₂ bulks for magnetic shielding via reactive liquid infiltration (RLI). Test samples with a disk shape have been produced and numerically modeled by the COMSOL Multiphysics software. The magnetic field distribution as a function of the distance from the top surface of the disk has been measured at T=20 K in order to validate the different models. It turns out that implementation of the Helmholtz equation can be justified by the minimum amount of calculation time (< 1 min) only when fields as low as 21 mT are involved and accuracies not better than 13% are required, while for fields of the order of 1T the error increases up to 60%. On the other hand, implementation of the H-formulation along with the J_c(B) curve extracted from magnetization hysteresis loops measurements enables to obtain a good agreement with the experimental profile within the experimental error up to the field of 0.2 T, even though at the expense of a much longer calculation time. However, fields as high as 1T still show discrepancies up to 45% of the induction field between the predicted and measured values. Correspondingly, magnetic relaxation measurements show that flux creep can be observed on the time scale of 10³ s only for fields larger than 0.2 T, where other traces of magnetic inhomogeneity can also be observed. The validated model has been used to predict the magnetic shielding of an MgB₂ tube produced by RLI with 2:1 length to diameter ratio, resulting in maximum shielding factors of about 10⁷ for fields lower than 0.2 T. For comparison, the shielding factor of an equivalent tube of mumetal has also been calculated, obtaining values between 8 and 50 for analogous fields. The magnetic characterization of the MgB₂ tube is currently under way.

The high current compact power cables are an interesting practical application of superconductors. To design and minimize the cost of these systems, the AC losses must be calculated as accurate as possible, and this task is made easier during the design process if one can predict the losses by means of numerical simulations. The physics of AC losses can be well described with Maxwell equations with appropriate constitutive laws for the materials. In the literature, the problem is most frequently expressed in terms of so-called the H-formulation using finite element method (FEM). Because of the superconductors’ peculiarities including the high nonlinearity of the E-J law, the modeling problem remains an interesting challenge and different methods and formulations are investigated in order to improve accuracy, convergence, and particularly computational speed.

In this context, Volume Integral Method (VIM) presents an interesting alternative to FEM. It is well suited for the modeling of devices with a large volume free space (air regions) compared to actives regions (magnetic and conductive parts). Indeed, the use of the VIM in these cases offers good accuracy and convergence to the solution.
In this paper, we propose to develop 3D volume integral formulation based on a generalization of the PEEC method (Partial Element Equivalent Circuit) for the computation of AC losses in the superconducting materials. In this formulation, the degrees of freedom are associated with the normal component of current density on facets of the mesh. The divergence-free current density condition is achieved by using the circuit approach. Several strengths appear with this approach. The method which is both light (we don’t require to mesh air region) and precise, ensures strongly the conservation of current density and therefore leads to good performance in solving highly nonlinear problems.
In order to illustrate the interest of the method and its reliability, some comparisons have been made with a FEM method using H and T-Ω formulations in the case of an externally applied magnetic field.

The fuel cell and the superconductor coil are electrical devices that give us a big number of expectations in the applied research domain. We know that the fuel cell is used as an alternative power supply that produces electrical energy from hydrogen and oxygen redox reaction. This is a clean power source that produces only water as waste. Whereas the fuel cell can produce important currents of a few hundred Amps in a dedicated working zone, the voltage produced is too small and needs to be conditioned by power electronics to make it suitable for use. That increases their cost and limits his mass production. In addition, the superconducting coils are used to produce strong magnetic fields, with less energy, at cryogenic temperature, e.g. the temperature of liquid Helium. Their main application is in the Magnetic Resonance Imaging (MRI) devices or in embedded systems. In order to create these magnetic fields, the superconducting coils require special kinds of power supply capable to provide high current at low voltage. Usually, these power supplies generate a non-negligible harmonic. Hence we have found a solution that can give a response to these problems. We are working on a system that uses a fuel cell as autonomous power supply for superconducting systems. The fuel cell can replace a high-current power supply and it fits perfectly as an alternative power supply for these systems, with the benefit of zero harmonic pollution. In this paper, we are working on a model for a set of a fuel cell and a superconducting coil. We also propose solutions to connect these two devices and to ensure the proper energy management.

The LTS superconducting coils are designed to produce strong magnetic fields at the working temperature of liquid Helium. Some kinds of them have been created to produce strong and homogeneous magnetic fields (MRI). Furthermore, the critical and mechanical parameters of superconducting material limit the performances of the coils. Most of the time, the LTS coils are also build to produce a homogeneous magnetic flux density in one single direction |B|=|Bx|. Consequently, it is necessary to rotate the subject or the sample, if we want to make a study depending of the magnetic field orientation. This cannot be suitable for special kinds of experiments involving HTS systems or biomedical applications where the subject would need to be rotated. To find an answer to this problem we have designed a new coils setup that will satisfy these requirements. Hence, we are working on orientation along 3 axes of a strong magnetic flux density. The design has been made using a 3-D electromagnetic software in order to calculate the performances of the coils system, to optimize the homogeneity and to find a cost effective solution. The first results reported are for a three Helmholtz coils system where each axis is placed following the Cartesian coordinates: x, y, z. This system is designed for 3 T of magnetic flux density at the center of the coils and a peak value on the coils around 7 T, for an operating current of 160 A in each coil.

When applying superconducting bulk to various industrial appliances is considered, it is the problem of shape processing to be set to one of the big problems. It is necessary to customize shape for superconducting bulk according to the characteristics of an appliance. At this time, it is very important to suppress the variation in a magnetic properties as much as possible.
For example, when making the rotating machine using bulk, by the variation in a magnetic properties, eddy current loss will be generated and a rotation loss will become large. Therefore, when applying bulk to an appliance until now, it was almost the case to use cylinder bulk in a form as it is. When processing shape, there is much use by the shape of a hollow cylinder type which hollowed the central part of bulk. However, bulk with a big diameter is needed, and since manufacture is very difficult for hollow cylinder type bulk, a magnitude has a limit. And, it is also considered that a manufacturing cost becomes very high.
Then, we examined the segmentation processing method for the ability to suppress the variation in a magnetic properties as much as possible, combining small bulk multiple. This considered as important the magnetic properties at the time of combining each bulk only regardless of the magnetic properties of a bulk simple substance, and examined construction suitable for an appliance application.

Ring spinning is currently the most commonly used technology for the production of short staple yarn. The primary limiting factor in the productivity of the ring spinning process is the frictional heat between ring and traveler, which increases with higher spindle speed. It causes wear of the ring-traveler twisting element and can lead to melting of synthetic yarns. Concepts to increase the productivity thus include the replacement of the ring-traveler system by a rotating ring that is supported by e.g. rolling, air or an active magnetic bearing.
Recently, a superconducting magnetic bearing (SMB) has been proposed as a twisting element in ring spinning with the prospect to eliminate the problem of frictional heat in the existing system. [1, 2] This passive SMB-system permits the levitation and rotation of a permanent magnet (PM) in a stable position above a cooled superconductor, without the need for an elaborated positioning system. Each rotation of the yarn driven PM-ring imparts the required twist in the yarn during spinning. [3] The presented SMB twisting element has the great potential to spin yarn at much higher speed compared to the use of the existing ring-traveler system in the ring spinning machine.
The analysis of the static and dynamic behaviour of the rotating PM-ring in particular at higher speed is especially important as it influences the yarn tension as well as the stability of spinning process. Therefore, the dynamics of the rotating PM-ring will be discussed considering the acting forces of the yarn on the PM-ring and its vibration modes.

Abstract－In the past few years, there has been a significant rise in short-circuit current levels in transmission and distribution networks. This is due to increasing power demands on the network combined with, the addition of many sources of distributed generations (DGs) as renewables become more prevalent. Based on relatively simple SFCL models which largely on the basis of the insertion of a step resistance, several researches regarding the performance and optimal locations of the resistive type SFCL in power grid models have been done. These have successfully proved the effectiveness of a resistive type SFCL in power systems in terms of improving system stability. The work described here expands these models to consider the real behavior of a SFCL and includes electric field-current density (E-J) characteristics and thermal properties of High-Temperature Superconductors (HTSs). We present a step resistance model as well as a dynamic temperature-dependent model for resistive type SFCL. Both models were integrated into a power system model built based on the UK standard where their impact on the electricity network was studied. As a representative renewable energy source, a 90 MVA wind farm was considered for the simulation. Three fault conditions have been simulated and the reduction of fault current contributed by two SFCL models in several locations has been compared. Consequently, it has been found that the E-J characteristics and thermal properties are crucial for performance simulation of the resistive type SFCL in power grid models. This information will be valuable to SFCL manufacturers and system operators in terms of resistive type SFCL design and installation.

Index Terms－High-temperature superconductors, fault current, wind farm, power system modelling, superconducting fault current limiters.

This paper presents the design and tests of the superconducting fault current limiter (SFCL) for medium voltage as a device allowing for a more effective use of the existing power network infrastructure. The limitation of short-circuit currents by the SFCL to safe levels will result in the network elements being susceptible to smaller electrodynamic and thermal overloads. The application of a SFCL leads to an increase of the allowable short-circuit power at the point of connection of new power generating sources, which is determined by the short-circuit parameters of the power network. This, in turn, will result in an increase of the capability of the power network for connecting distributed generation energy sources based on renewable energy sources. The light and compact construction of coreless superconducting fault current limiter consists of three magnetically coupled windings cooled in liquid nitrogen: primary and secondary windings made of HTS 2G tape and parallel connected primary copper winding. The high magnetic coupling between superconducting primary and secondary windings gives a low voltage drop on the limiter at nominal current. The presented solution reduces the size and the weight of the device. Tests performed at high power test facility shows the limiting performance of the coreless inductive SFCL.

With the development of the high temperature superconducting (HTS) materials and the practical need, studies on the application of the superconducting fault current limiter (SFCL) in dc distribution system have been proposed. SFCL could limit the fault current and decrease the requirement of action speed and interrupting capacity for the dc circuit breaker when a dc fault happens, without producing additional power loss during normal operation. However, the current limiting effects of different SFCLs have not been analyzed, this paper analyzes the limiting effects of different SFCLs (mainly reactance type SFCL and resistive type SFCL) based on the transient characteristics of dc faults, getting the conclusion that the resistive SFCL (R-SFCL) has a better limiting effect for the whole system than that of the reactance type SFCL. The simulation experiment also verifies the correctness of the conclusion above.

Next, this paper discusses the application of the R-SFCL based on the E-J characteristic, which has a great significance on the current limiting behavior of the R-SFCL. According to the E-J characteristic, the R-SFCL would develop a resistance when the current density exceeds the critical value, and this is the reason why the R-SFCL could limit the current after a fault happens. However, to achieve the target of satisfied current limiting for the whole system including the dc side, ac side and the converter, there is a requirement of the quenching characteristics for the R-SFCL after a dc fault, which is also analyzed in this paper combining with the theoretical analysis of the dc fault transient characteristics.

Since that the quenching characteristics, including the equivalent resistance value and the quenching speed, are heavily dependent on the design parameters of the R-SFCL and the fault characteristics, a large number of simulation experiments distinguished by the different design parameters and different faults are analyzed in this paper. Finally, this paper presents the influences of design parameters and fault types on the quenching characteristics, and then discusses the influences on the fault current limiting effect of the R-SFCL, which provides the theoretical reference for the design and manufacture of the R-SFCL.

High Temperature Superconducting (HTS) materials of various compositions, constructions, form-factors and arrangements are increasingly finding application across modern electric power transmission and distribution schemes, worldwide. The HTS fault current limiter have been presented as a convenient avenue for alleviating the deleterious effects which may arise, whenever short circuit fault currents result from any malfunctions, or undesired electric power system incidents.
Studies of the Resistive-type superconducting fault current limiter configuration as applied to HVDC networks are widespread. However, a key disadvantage plaguing this device is an inherent lack of galvanic isolation. Some scope therefore exists to explore any potential influence, the inductive-type superconducting fault current limiter shall present toward the inception of fault current propagation, under a range of system fault scenarios.
This work presents a new inductive-type HTS fault current limiter for HVDC systems. A key aspect of this fault current limiter operation involves the exploitation of intrinsic HTS material properties, in a superconducting direct current coil wound on the secondary side of a transformer. Relevant details of applied design parameters used for this inductive-type superconducting fault current limiter shall be given. Software-based equivalent circuit models of the inductive-type superconducting fault current limiter shall be implemented in the MATLAB/Simulink package. Simulation studies shall highlight the performance of this scheme under a range of system fault scenarios, guiding avenues toward feasible HVDC system deployment. In addition, these studies shall indicate the nature and impacts arising from deployment of an inductive-type high temperature superconducting fault current limiter to system planners, designers, operators and other stakeholders when applied in a simple VSC-HVDC network. Preliminary investigations suggest that during any DC current transitions from a nominal system status toward a fault state, an inductive–type scheme may offer a means to manage the observed magnitudes of such fault current transitions within desirable bounds.

Nowadays the concepts of development of an air transportation for the period till 2050 is developed. The concepts differ in terms and details of realization, but all of them are directed on gradual refusal of hydrocarbonic fuels and transition to electric thrust. Such approach is supposed to be realized in two stages: the first is creation (by 2025 in Europe) a hybrid medium-haul airliner, the second is creation of (by 2050 in Europe, by 2030 in Japan) completely electric aircraft. This technological trend indirectly is confirmed by development in other area of transport. One more trend influencing development of electric motion in aircraft is tight control of ecological requirements, by reduction of emissions of carbon dioxide.
The development of the aviation industry poses new challenges to developers of electrical equipment.
The development of all-electric aircraft, requires the electrical machines with specific power from 15 to 50 kW/kg. Whereas for conventional electrical machines, this figure reaches the value of 4 kW/kg, due to the presence in the electrical machine the ferromagnetic core. Thus, increasing the power of electrical machine of conventional design will lead to an increase in its weight.
The increasing of specific power of the electric machine is possible with abandoning of iron core. However, the iron-free electric machines, until recently, could not compete with traditional machines due to low values of magnetic induction in the core and large dimensions. The situation was changed with the development of high-temperature superconducting tapes of the second generation (HTS 2G), which allows to obtain high magnetic fields at liquid nitrogen temperatures. The implementations HTS 2G materials in electric machines can increase significantly the specific power and achieve the required performance in the 15-50 kW/kg.

The design schematics of iron-free HTS electric machines are discussed. The simulation results of fully superconducting HTS machines with high specific output power density are presented.

The different application of iron-free HTS electric machines are described.

As a newly emerged PM synchronous machine (PMSM), PM vernier machine (PMVM) with the merit of large output torque at low speed operation owing to the specific flux modulation effect is signifcantly suitable for direct-drive applications, such as vechiles and vessel propulsion [1]. By introducing the high-temperature superconductor (HTS) winding windings or HTS bulks into PMVM, one HTS-PMVM has been proposed and designed [2], however, the eletromagnetic performance of which is still unsatisfary, suffering from lower torque density and power factor.
In this paper, one modular HTS-PMVM with spoke-array PM rotor is proposed and designed, the stator of which adopts multi-tooth split poles (the split poles generally called flux modulation poles, namely FMPs) structure to produce the flux modulation effect. Three-phase concentrated windings are used to shorten the winding ends of the proposed machine so as to reduce the copper loss, and each phase windings are serially connected together with the purposes of insulation among three phases and modular design. Meanwhile, the stator of the proposed machine consists of nine modules, each of which contains stator teeth, FMPs and dove-groove yokes. Additionally, the HTS bulks are embedded into the slots between adjacent FMPs, which can bring less flux leakage around the split-FMPs. The performance of the proposed machine under different gear ratios is further investigated with the different FMPs numbers. The electromagnetic performance of the proposed machine with different NFMPs are calculated and analyzed by using the finite element method, which show that the torque and power densities of the proposed modular HTS-PMVM can be effectively improved compared with its traditional counterparts. In addition, the gear ratio of proposed machine has an extremely influence on its torque characteristics, such as the ripple torque and cogging torque.
The detailed design and analysis of the proposed modular HTS-PMVM will be given in full paper.
[1] A. Toba and T. A. Lipo, “Generic torque-maximizing design methodology of surface permanent-magnet vernier machine,” IEEE Trans. Ind. Appl., vol. 36, no. 6, pp. 1539-1546, Nov. 2000.
[2] J. G. Li and K. T. Chau, “A Novel HTS PM Vernier Motor for Direct-Drive Propulsion,” IEEE Trans. Appl. Supercond., vol. 21, no. 3, pp.1175-1179, Jun. 2011.