High Temperature Superconducting Coil Wound Research Articles

Fabrication and Test of HTS Magnet for Induction Heating Device in Aluminum Extrusion Processing

This paper reported the fabrication and the test results of the high temperature superconducting (HTS) magnet developed for the aluminum billet heater for aluminum extrusion processing. In this magnet, the HTS coils were combined with iron cores, because the required magnetic field in the working region was about 1T. Two HTS coils wound directly on the convex shape iron cores were fabricated. The HTS coils were made by the no-insulation winding of REBCO tape with the YOROI-coil technique. Each of them was separately installed in an individual cryostat. Two HTS coils with iron cores were disposed oppositely to each other and were magnetically connected by a C-shape yoke placed in room temperature. The HTS coil was cooled down to about 15 K by the GM cryocooler in the initial cooling. The magnetic field of 1.06 T at the center of the heating region was stably generated with the rated current of 200 A. It was also demonstrated that the HTS magnet could be charged up to 300 A without quench. According to the observation of the electromagnetic force direction from the outside of the cryostat, it was confirmed the validity of the force balance design of the magnet.

Low-resistance joint development for segment-fabrication of high-temperature superconducting fusion magnets

Two designs have been proposed for segment-fabrication of the high-temperature superconducting (HTS) helical coils in the FFHR-d1 helical fusion reactor, joint-winding of the HTS coils wound by connecting conductor segments, and the ‘remountable’ HTS magnet (here ‘remountable’ means being able to mount and demount repeatedly) assembled from coil segments with demountable joints. A bridge-type mechanical lap joint and mechanical edge joint are planned to be applied to those two designs, respectively. This paper presents the progress in electrical and mechanical performances of the mechanical joints of HTS conductors and methods to evaluate their quality for segment-fabrication of HTS helical coils. R&D of joint performance has progressed during this decade and it shows acceptable performance for the HTS helical coils. The contact-probing current transfer length method and x-ray computer tomography scanning are promising for quality assessment of the joints and HTS tapes.

Temperature Distribution Observation of No-Insulation HTS Coils Wound by Optical-fiber-encapsulated Tapes

Effective quench detection technologies are extremely important for protecting high temperature superconducting (HTS) applications. In recent years, an optical fiber encapsulated HTS tape (OF-tape) is proposed for quench detection by using distributed temperature sensing (DTS) technology. In this paper, an HTS coil (OF-coil) wound with a 39 m long OF-tape was prepared and energized by an AC overcurrent of 700 A to verify the feasibility of temperature observation during quench events. Furthermore, anti-stokes intensity was also measured to understand dependence of temporal resolutions on response times of coil quenches. The main results show that temperature variations of the OF-coil can be correctly measured after quench and the maximum temperature is shown at the middle turn of the OF-coil. Particularly, for the method of detecting anti-stokes intensity, quench response time of temporal resolution (TR.) with 30 ms is about 500 ms faster than that with 300 ms. However, when the TR. is 30 ms, more noise signal is observed. Accordingly, a trade-off should be considered between response time and noise signal for efficient quench detection.

AC losses and induced fields in HTS coil wound using two-ply coated conductors

AC losses and screening-current-induced fields (SCFs) in a high temperature superconducting (HTS) inserts for high field magnet are evaluated theoretically. The HTS inserts are composed of stacked pancake coils, which are wound using two-ply conductors, where the superconducting-layer sides of two rare-earth-based coated conductors are attached to each other without electrical insulation. The theoretical formulas of AC loss and SCF in the two-ply conductor are derived for the simultaneous applications of a transport current and an external magnetic field parallel to its broad face. By taking into account the magnetic-field profile in the HTS insert and the magnetic-field dependency of critical current density in the coated conductor, the AC losses and SCFs are estimated using the theoretical formulas for monotonical increase in a central field up to 25.5 T in 60 minutes in combination with low temperature superconducting outsert coils. It is guessed that the AC loss estimated for the HTS insert wound using the two-ply conductor could be cooled by using prepared cryocoolers. The magnitude of SCF in the HTS insert wound using the two-ply conductor becomes smaller than that for the single-tape conductor estimated previously.

Performance Study on the No-Insulation HTS Coil Wound With Narrow-Stacked Wire

As a novel high-temperature superconducting (HTS) conductor structure composed of the stacked 1-mm wide REBCO tapes, Narrow-Stacked (NS) wire has been demonstrated to have a significant reduction effect of AC loss and screening current induced field (SCIF) in the insulated HTS coils. Thus, NS wire is a promising HTS conductor design for practical HTS applications. Meanwhile, compared with traditional insulated HTS coil, no-insulation (NI) HTS coil has obvious advantages in engineering current density, thermal stability and self-protecting ability, and these features benefit for HTS magnets. Although NS wire was regarded as an improvement for insulated HTS coil, the NS wire effects on NI coil are not confirmed yet due to the special characteristics of the NI coil. In this paper, NI pancake coils were wound with NS wire and conventional HTS tape respectively. To evaluate the performance of NI coil wound with NS wire, its characteristic resistance and charging delay were analyzed at the liquid nitrogen temperature of 77 K. Besides, the over-current test results show that NI NS coil also has the satisfied self-protecting feature. This paper intends to provide a possible reference for NS wire application on NI HTS magnets.

The model coil manufacture and AC losses analysis of a 100-MJ HTS SMES

The second generation of high temperature superconducting (HTS) strip has the outstanding characteristics of high critical temperature, high load current density and high critical magnetic field, so the HTS coils wound by the strip have broad application prospects in the field of electric power, such as magnetic energy storage system, high speed maglev train and wind power generation system. In order to promote the further development of HTS magnetic energy storage system (SMES), this paper conducts the modelling and simulation research on the ac losses of a 100- MJ toroid-type HTS SMES. Firstly, the geometric structure of the system are introduced. Then, the ac loss models of HTS coils and the non-superconducting components in the energy storage system are established by H-formulation and A-formulation, respectively. Based on both the models, we calculate the ac losses of HTS coils and related components during the process of energy storage and release. Finally, the model coils are winded and impregnated to show the structure.

Fast Charging and Thermal Stability Improvement of a Conduction Cooled HTS Coil Wound by Graphene Oxide Coated HTS Tape

The equivalent thermal conductivity of the conduction cooled high-temperature superconducting (HTS) coil is one among important design parameters. It is possible to rapidly rise the hot spot temperature of HTS coil in quench, because the normal propagation velocity of HTS wire is lower than 100 times one of low-temperature superconducting wire such as NbTi or Nb3Sn alloy, and the conduction cooled HTS coil is operated on vacuum adiabatic condition. Generally, polyimide tapes for electrical insulation are inserted between layers of HTS coil but initial cool-down time due to low equivalent thermal conductivity of HTS coil get longer and the thermal runaway current in quench is decreased. Recently, the researches on HTS coil without insulation to improve the equivalent thermal conductivity have been performed. However, because HTS coil without insulation are short-circuit, the charging time to reach the designed magnet flux density of HTS coil is too long. In this paper, turn to turn insulation of HTS coil wound by HTS tapes with graphene-oxide coated Cu stabilizer is presented. The graphene oxide has relatively high resistivity and can be coated with a thickness of less than 10 μ m. To confirm the effect of the proposed method, two HTS coil samples that were insulated with polyimide tape and coated with graphene oxide were fabricated. Based on the results of thermal analysis and characteristics tests, the equivalent thermal conductivity, initial cool-down time, critical current, and thermal runaway current of HTS coils with different turn to turn insulation were compared.

Study on AC Loss Reduction in HTS Coil for Armature Winding of AC Rotating Machines

The fundamental study on the ac loss reduction in high-temperature superconducting (HTS) coils for armature windings of ac rotational machines is presented in this paper. In this paper, a simplified model that simulates a part of the distributed armature winding of ac rotational machines is used. For the ac loss reduction, we study a method to reduce the perpendicular magnetic field applied to the tape surface in the coil by arranging a magnetic material around the coil winding. The numerical magnetic field calculation shows that the perpendicular magnetic field clearly decreases by arranging the magnetic material around the coil winding in the slot of the iron core. To investigate the effect of the insertion of the magnetic material on the ac loss reduction, we fabricate the measurement setup using the test HTS coils wound by a Bi2223 tape and a YBCO tape, combined with the laminated iron core. The transport ac losses in the test coils are measured by changing the arrangement of the magnetic material. The measurement results demonstrate well the ac loss reduction by use of the magnetic material around the coil winding in the slot of the iron core.

Test and Analysis of Electromagnetic and Mechanical Properties of HTS Coil During Quench State

This paper deals with the analysis of the electromagnetic and mechanical properties of a high-temperature superconducting (HTS) coil, as well as its characteristic as determined through tests using strain gauges. When an operating current is applied to an HTS coil, strain is generated by electromagnetic force. However, additional strain effects in the HTS coil are formed by the Joule heating during quench. Strain was measured with a full-bridge strain gauge mounted directly over the turns. We fabricated an HTS coil wound in a single pancake type to investigate strain characteristics. We also investigated structural analysis by a finite-element method to calculate the strain of the HTS coil. In tests, the strain in the HTS coil was measured during excitation. Test results of the HTS coil have been compared with the theoretical results. Finally, we performed quench propagation tests and could verify the strain effect during quench. The reaction times of the strain gauge at 30 and 80 A are 31.8 and 4.3 s, respectively.

DC characterization and 3D modelling of a triangular, epoxy-impregnated high temperature superconducting coil

The direct current (dc) characterization of high temperature superconducting (HTS) coils is important for applications, such as electric machines, superconducting magnetic energy storage and transformers. In this paper, the dc characterization of a triangular-shaped, epoxy-impregnated HTS coil wound with YBCO coated conductor intended for use in an axial-flux HTS motor is presented. Voltage was measured at several points along the coil to provide detailed information of its dc characteristics. The coil is modelled based on the H-formulation using a new three-dimensional (3D) technique that utilizes the real superconducting layer thickness, and this model allows simulation of the actual geometrical layout of the HTS coil structure. Detailed information on the critical current density’s dependence on the magnitude and orientation of the magnetic flux density, Jc(B,θ), determined from experimental measurement of a short sample of the coated conductor comprising the coil is included directly in the numerical model by a two-variable direct interpolation to avoid developing complicated equations for data fitting and greatly improve the computational speed. Issues related to meshing the finite elements of the real thickness 3D model are also discussed in detail. Based on a comparison of the measurement and simulation results, it is found that non-uniformity along the length exists in the coil, which implies imperfect superconducting properties in the coated conductor, and hence, coil. By evaluating the current–voltage (I–V) curves using the experimental data, and after taking into account a more practical n value and critical current for the non-uniform region, the modelling results show good agreement with the experimental results, validating this model as an appropriate tool to estimate the dc I–V relationship of a superconducting coil. This work provides a further step towards effective and efficient 3D modelling of superconducting devices for large-scale applications.

Development of the Current Bypassing Methods into the Transverse Direction in Non-insulation HTS Coils

In the case of motors and generators, the benefits of using high temperature superconducting (HTS) coils can be represented by the reduction of 50% in both losses and sizes compared to conventional machines. However, it is hard to establish quench detection and protection devices for the HTS coils applied to the rotors of motors and generators. So, the stability of the coils is lower than for the quiescent coils applied to NMR, MRI and so on. Therefore, it is important to improve the self-protection ability of HTS coils. We have studied the methods to improve the self-protection ability of HTS coils by removing the layer-to-layer insulation and inserting metal tape instead of the electrical insulation. The operating current in the non-insulated HTS coil was bypassed into the transverse direction by the generated normal region because of their electrical contact among the winding. In this study, we examined the method to control the current bypassing on layer-to-layer for controlling the inductance of the non-insulated HTS coil. The current bypassing properties on non-insulated HTS coil wound with 2G wires will be discussed.

Measurement and Numerical Analysis of AC Loss in High Temperature Superconducting Coil

The paper reports on experimental and numerical studies of AC loss in a high temperature superconducting coil wound with a Bi2223/Ag tape. A measurement method of the AC loss in the HTS coil by using a power meter is proposed. The AC loss is measured on the test HTS coil made by stacking pancake coils wound with the Bi2223/Ag tape. The AC loss is also calculated using the numerical method developed in our previous work. The measured AC loss is compared with the calculated one. The agreement of measured and calculated data is reasonable and this shows the validity of the proposed measurement method.

Thermal Stability of Conduction-Cooled YBCO Pancake Coil

When a high-temperature superconducting (HTS) coil is operated using a conduction-cooling technique, the coil should be impregnated with epoxy resin because the coil is not sufficiently cooled without it. On the other hand, it is important that the coil should have no damaged area in the winding because the damaged area would generate heat locally and the coil would eventually quench. However, whenever we fabricated an impregnated HTS coil wound with YBCO tapes and evaluated its V-I characteristics, the n-value of the coil was much lower than we expected, indicating that the critical currents of some areas in the winding drastically decreased. Therefore, we have started to improve the performance of an impregnated HTS coil wound with YBCO tapes. In this paper, we investigated the cause of the degradation and found that the degradation did not occur when we decreased the radial tensile stress in the windings. Then we fabricated four single-pancake coils, stacked them, and tested them in a conduction-cooled condition. The measured V-I curves were in good agreement with the calculated ones, suggesting that we successfully developed a technique of fabricating an impregnated HTS coil wound with YBCO tapes with no degradation. We also measured thermal runaway currents of a conduction-cooled HTS coil composed of two single-pancake coils wound with YBCO tapes and numerically simulated the thermal properties by using a three-dimensional heat conduction equation in order to study the thermal stability of the YBCO coil. The measured thermal runaway currents were in good agreement with the calculated ones.

Study on Optimal Design of High Temperature Superconducting Coil Based on AC Loss Minimization

This paper presents an optimal design method of winding cross section of high temperature superconducting coils wound by Bi2223/Ag tape based on AC loss minimization. We try to optimize the winding cross section starting from the conventional HTS coil with the rectangular cross section to minimize the AC loss. An example of the optimal cross section coil is presented. The result shows that the AC loss in the optimized coil is significantly reduced and the critical current of the optimized coil is increased compared with the rectangle cross section coil. It is verified that our optimal design method is effective to reduce the AC loss in the HTS coil. The disadvantageous points of the optimized coil are also discussed.

Numerical Study on AC Loss Characteristics of HTS Coils With Various Cross Sections and Methods of AC Loss Reduction

AC loss characteristics of high temperature superconducting coils wound by Bi2223/Ag tape with various cross sections are numerically investigated. The numerical results show that the smaller coil aspect ratio, the smaller AC losses in the HTS coil. It was considered from the numerical results that the coil aspect ratio giving the minimum coil volume is the better shape judging from the many aspects such as the required length of tape, the stored energy, the central magnetic field and the critical current together with the AC loss. It was also shown by the numerical study that the optimal coil design giving the reduced AC loss can be done by employing the stepped cross section coil and the turn number graded coil

Analysis of AC loss characteristics of HTS coils with various cross-sections

An analytical study of AC loss characteristics of a high temperature superconducting coil wound by a Bi2223/Ag tape is presented in this paper. The AC loss in the high temperature superconducting coil wound by Bi2223/Ag tape was numerically calculated by using the measured DC data of the tape. The calculated AC loss was compared with the measured AC loss in the Bi2223 HTS coil obtained in the previous study. For the comparison of the measured and calculated AC losses, the mechanical loss was subtracted from the measured total loss data. The calculated AC losses were compared with the measured AC losses in the coil, producing rather good agreements. The relation between the AC loss characteristics in the HTS coil and shape of coil cross-section was numerically studied. A possible way to reduce the AC loss in the HTS coil was also presented and discussed.

Analysis of AC Loss Characteristics of High Temperature Superconducting Coil

This paper reports an analytical study on AC loss characteristics of a high temperature superconducting coil wound with a Bi2223/Ag tape. The electromagnetic AC loss in the high temperature superconducting coil made by stacking pancake coils wound with the Bi2223/Ag tape was numerically calculated by using the measured DC data of the HTS tape. The calculated AC loss was compared with measured AC losses in Bi2223 HTS coils obtained in a previous study. It had been found that the measured AC losses in the coils contained large mechanical loss. For the comparison of the measured and calculated losses, the mechanical loss was subtracted from the measured total loss data to estimate the electromagnetic loss. The calculated AC losses were compared with the measured electromagnetic losses in the coils, producing rather good agreement. A possible way to reduce the AC loss in the HTS coil was also presented and discussed.

Transient Thermal Characteristics of Cryocooler-Cooled HTS Coil for SMES

Thermal behavior of high-temperature super-conducting (HTS) coil is exceedingly different from that of low-temperature superconducting (LTS) coil. Therefore for the sake of the application of HTS coil to practical use, establishment of a new stability criterion for coil design becomes indispensable. In this study, a small-sized cryocooler-cooled HTS coil wound with Bi-2223/Ag tape was constructed and tested to evaluate the electromagnetic and thermal characteristics in the cryocooler-cooled HTS coil. We carried out experiments subjected to pulse current operation supposing SMES for power system stabilization. The simulation by a newly developed computer program based on the finite element method (FEM) agreed well with experimental results. In the computer program, short-sample data concerning the E-J characteristics, which depend on the orientation of applied magnetic field to the wide face of Bi-2223Ag tape, was adopted. By using the computer program, we also evaluate the thermal behavior of HTS coil graded according to the magnetic field distribution.