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This paper represents a numerical calculation method that enables highly-accurate simulations on temperature analysis of superconducting magnets considering the heat flow between the magnet and liquid helium during a quench. A three-dimensional (3D) superconducting magnet space was divided into many cells and the finite-difference method (FDM) was adopted to calculate the superconducting magnet temperatures governed by the heat transfer and joule heating of the each cell during a quench. To enhance the accuracy of the temperature calculations during a quench, the heat flow between the superconducting magnet surface and liquid helium, which lowers the magnet temperatures, was considered in this work. The electrical equation coupled with the governing thermal equation was also applied to calculate the change of the decay of the magnet current related to the joule heating. The proposed FDM method for temperatures calculation of a superconducting magnet during a quench process achieved results that were in good agreement with those obtained from an experiment.

In this study, we introduce a highly accurate NMR magnetometer, which is capable of measuring the magnetic field for inhomogeneous HTS magnets. The magnetometer is based on the pulsed and Fourier-transformed NMR and 1H NMR signal analyses. The magnetometer system was fabricated according to these concepts and magnetic field measurements on a 3 T HTS magnet were carried out using various mapping paths. The test results demonstrate that the magnetometer can measure magnetic fields accurately in inhomogeneous conditions as high as approximately 864 ppm with a spherical volume of 10 mm in diameter nearly centered at the magnet center. The stronger and hence shorter RF pulse resulting from the high-power amplification enables measurements of magnetic fields in more inhomogeneous conditions. Because the permissible field homogeneity range is very wide, the magnetometer can be used for mapping fields at the preshim stage of the inhomogeneous magnets. To also verify the high-resolution performance of the magnetometer, the results are compared with measurement data obtained with a Hall magnetometer ordinarily used in such an inhomogeneity range. We expect that the proposed magnetometer will enable the precise measurements of the magnetic fields over a wide homogeneity range, and thereby bring about improvement in the shimming process of the inhomogeneous magnets.

This paper investigates an optimal design methodology for multiwidth (MW) high-temperature superconductor (HTS) magnets, where pancake coils wound with the narrowest tapes were placed at and near the magnet center and those with progressively wider tapes toward the top and bottom of the magnet. For a given design, target of field strength and winding bore, input parameters include tape width, number of grading tapes, and number of pancakes coils, while the main objective is to minimize the magnet volume, i.e., essentially the stored energy and thus the cost. A magnet with a single-width tape is also designed as a control sample for comparison. A case study on a design of 3-T 240-mm HTS magnet was carried out. In that case, MW magnets save 40% wire consumption of single-width magnets. The results are expected to be beneficial to determine the practical level of HTS tape grading and estimate a volume of an MW magnet.

A 3-T 100-mm winding bore conduction-cooled REBCO magnet has been designed, constructed, and tested. It consists of a stack of double pancake coils wound with metallic cladding (MC) REBCO tapes, for which 1–2 $\mu {\text{m}}$ stainless steel was clad in a hermetic way around the tapes. This approach, first proposed by the SuNAM Company Ltd. in 2015, is expected to substantially reduce the charging time delay of a no-insulation magnet, which in turn will be effective in the manipulation of screening currents in such a way as to improve the field homogeneity of magnets, particularly for nuclear magnetic resonance or MRI. This paper presents test results for our approach on the current sweep cycling, which is used to reduce the screening current-induced fields in a newly constructed 3-T 100-mm MC all-REBCO magnet

Flux diverters made of ferromagnetic materials can be used in high-temperature superconducting (HTS) magnets to change the magnetic field distribution and increase the critical current of the magnet. In this paper, the notch-shaped flux diverter increases the critical current of the HTS magnet by detouring the magnetic flux more effectively than the flat surface-shaped (simple) flux diverter. Numerical analyses are carried out to calculate the critical current using the simple-shaped and the notch-shaped flux diverters. Then, experiments are conducted to verify the results of the numerical analyses. As a result, the HTS coil with a notch-shaped flux diverter shows the increased critical current.

In this paper, we present the fabrication and experimental analysis of a 6.6 kV/100 A class superconducting fault current controller (SFCC) as a novel prototype fault current limiter. Recently, the integration of distributed generations into a grid has been issued due to increased power demand and their lower environmental impacts. The addition of distributed generators can increase the level of the fault current in the grid. To resolve the issue of increased fault current level, we propose a SFCC consisting of a superconducting coil, full-bridge thyristor rectifier, and control unit. The proposed SFCC can be described as a combination of a thyristor type solid-state breaker and a superconducting DC reactor coil. The inductance of the superconducting coil, which is about 10.4 mH, reduces the fault current amplitude. After the half cycle, through the control of the thyristors' phase angle, SFCC can control the level of the fault current to a proper level. In this paper, we present the design and fabrication of the superconducting coil, a rectifier, and a thyristor control unit. Based on the results of short circuit tests, which simulated the power grid, we expect that SFCC and its behavior can be helpful in protecting power grids from unpredictable fault currents.

This paper introduces a novel flux switching synchronous generator (FSSG) using high-temperature superconducting (HTS) field coils. Unlike conventional superconducting synchronous generators, the HTS field coils in the proposed FSSG are mounted in the stator and do not rotate during operation. Therefore, the design and fabrication of the cooling and excitation systems for HTS field coils are easier than those for the conventional superconducting synchronous generator. To verify the feasibility of the structure, a 15 MW-class FSSG was designed using a fundamental design methodology considering the mechanical stress of the rotor core. The design methodology uses mechanical and electromagnetic equations. It defines the size of the rotor first using mechanical equations for the rotor and designs the HTS field winding and armature winding using the electromagnetic equations and an FEM analysis. In addition, the operational characteristics of the FSSG were analyzed by a transient motion analysis using the FEM program. This paper could be utilized to develop superconducting synchronous generators.

In general, the stabilizer of a ReBCO-coated conductor (CC) plays a very important role in bypassing and transferring the excessive heat when quench occurs. For an optimal design, selecting the appropriate materials and stabilizer thickness is an important issue for enhancing the thermal/electrical stability of superconducting power devices. This study examined the electrical and thermal behavior of GdBCO CC tape in terms of the stabilizer thickness as well as the effects of variations of the Ag stabilizer thickness on the thermal/electrical stability through an over-current test. An electroplating technique was employed to obtain a Ag stabilizer with various thicknesses without serious degradation of the superconducting properties.

The power grid, especially distribution grid, has been more complicated due to distributed generations (DGs) with renewable energy sources and the smart grid. The complexity changes two things in terms of a fault; higher prospective fault current and the temporal variation of sources and loads. To correspond to those remarkable changes, we propose a fault current controller (FCC) named “smart FCC.” The smart FCC consists of a superconducting coil with a freewheeling diode if necessary, four thyristors, and a control unit. Smart FCC can not only limit but also control the current when a fault occurs. The smart grid technology can provide so enough information that it is possible to estimate which level of limited current should be the best in the real-time situation of the grid. Based on a real-time calculation of optimal fault current using the smart grid monitoring technologies, this new device is always ready to adjust the fault current. In this paper, we introduce a concept of the smart FCC and prove the concept. Various topologies have been proposed and simulated. Finally, a case study on a distribution class of 22.9 kV smart FCC has been conducted based on a conceptional system design.

High temperature superconductor (HTS) is a promising material as current leads for large current applications, because it has zero resistance. However, a single HTS tape has limitation on the capacity of current rating due to the critical current of the HTS tape. Therefore, stacking or parallel structure of the HTS tapes is inevitable in high temperature superconducting current leads (HTSCLs) for large current application. In this paper, several stacking methods with respect to the shape of the current terminal and the structure of the HTS tape are studied and the characteristics of the stacked samples are investigated by experimental analysis. The critical currents of the HTS modules were measured and the conductive heat leakage of a stabilizer layer was calculated. From this analysis, the characteristics of the current distribution and decrease in the critical current of HTS modules with respect to the shape of the current terminal were obtained. Based on the results, the required stacking number of the HTS tapes for certain operating current HTSCLs can be calculated.

A simultaneous quench has been one of challenging issues in a development of a superconducting fault current limiter (SFCL) as long as series-connected superconducting materials are not identical each other. This paper deals with a parameter determination of a condition for simultaneous quench in series-connected YBCO coated conductors (CC), as a preliminary step to develop its faculties for high voltage CC applications. An electrical and thermal analysis based on the finite element method (FEM) showed that decisive parameters were critical current and n-value. For verifying the analysis, two sets of series-connected samples from a batch of AMSC 344S, which is clad on stainless steel, were short-circuit tested. One set has quite different critical current, whereas the other similar ones. The analysis estimated the quench initiation characteristics among the samples in both cases. After the verification, the condition for simultaneous quench, i.e. allowable deviation of critical current and n-value, has been obtained.

This paper addresses adverse effects of dimensional uncertainties of an HTS insert assembled with double-pancake coils on spatial field homogeneity. Each DP coil was wound with Bi2223 tapes having dimensional tolerances larger than one order of magnitude of those accepted for LTS wires used in conventional NMR magnets. The paper presents: 1) dimensional variations measured in two LTS/HTS NMR magnets, 350 MHz (LH350) and 700 MHz (LH700), both built and operated at the Francis Bitter Magnet Laboratory; and 2) an analytical technique and its application to elucidate the field impurities measured with the two LTS/HTS magnets. Field impurities computed with the analytical model and those measured with the two LTS/HTS magnets agree quite well, demonstrating that this analytical technique is applicable to design a DP-assembled HTS insert with an improved field homogeneity for a high-field LTS/HTS NMR magnet.

This paper reports results, experimental and analytical, of the nonlinear behavior of a shim coil in the presence of an HTS coil assembled with double-pancake (DP) HTS-tape coils. The experimental results are from: 1) operation of a 700 MHz LTS/HTS NMR magnet (LH700) consisting of a 600 MHz LTS NMR magnet (L600) equipped with superconducting shim coils and a 100 MHz DP-assembled HTS insert (H100) and; 2) an experiment with a room-temperature (RT) Z1 shim coil coupled to a 50 MHz DP-as-sembled HTS insert (H50). A field mapping theory with a harmonic analysis is applied to interpret both results. Based on experimental results and analyses, we conclude that the screening-current-induced field (SCF) generated by a DP-assembled HTS insert is primarily responsible for the nonlinear behavior, including strength degradation, of a shim coil.

This paper presents experimental and simulation results of a screening current induced magnetic field (SCF) in a high temperature superconductor (HTS) insert that constitutes a low-/high-temperature superconductor (LTS/HTS) NMR magnet. In this experiment, the HTS insert, a stack of 50 double-pancake coils, each wound with Bi2223 tape, was operated at 77 K. A screening current was induced in the HTS insert by three magnetic field sources: 1) a self field from the HTS insert; 2) an external field from a 5-T background magnet; and 3) combinations of 1) and 2). For each field excitation, which induced an SCF, its axial field distribution and temporal variations were measured and compared with simulation results based on the critical state model. Agreement on field profile between experiment and simulation is satisfactory but more work is needed to make the simulation useful for designing shim coils that will cancel the SCF.

This work aimed to study the quench behavior of a test coil assembly consisting of pancake coils. Parameters such as the quench energy and the normal zone propagation velocity, which are useful in designing detection and protection systems for real-life HTS devices, have been measured. The goal is to define the limit of the applicability of commercial YBCO wire technologies, usually referred to as second generation (2G), used to wind superconducting devices. To go beyond the stability characterization, an active protection scheme has been designed, integrated to the test coil assembly and tested as well.

This paper deals with the design and test of a high temperature superconducting (HTS) power supply with YBCO coated conductor (CC) pancake load, also presents its operational characteristics through experiments. HTS power supply consists of two heaters, an electromagnet, a solenoid, and a pancake load of about 0.73 mH. The solenoid and pancake load were fabricated by using YBCO CC. The timing sequential control of two heaters and an electromagnet is an important factor to generate pumping-current in the pancake load. The heat transfer analysis of heater triggering parts of the YBCO CC solenoid according to the heater input was carried out. Based upon the analysis, the 3 A electromagnet current and the 0.55 A dc heater current were optimally derived, and 4 s, 8 s and 16 s for the pumping period were selected in this experiment. A region of the superconducting tape with a buried heating coil can be switched by means of its temperature change. In order to measure the pumping-current with respect to the magnet flux changes, a hall sensor was installed at the center of the YBCO CC pancake load. In the experiment, the maximum pumping current reached about 20 A.

This paper presents the results of a study of 2-dimensional normal zone propagation (NZP) in winding pack models comprised of 12 straight YBCO tapes stacked with wet, i.e., epoxied, insulating spacers. In each winding pack, the YBCO tapes were connected electrically in series in such a way that a transport current through the YBCO tapes zigzagged from one end of the winding pack to the other end of the winding pack. An insulated heater strip placed at the middle of the stack was energized to trigger a quench in YBCO tapes with a combination of heater taps which permits the length of the heated section to be varied for 1- or 2-dimensional NZP. Voltage taps of various lengths were placed within the winding packs to measure the transverse and longitudinal NZP speeds. Measurements with wet-spacers in which a large contact thermal resistance at each YBCO tape and spacer interface is chiefly responsible for very slow transverse NZP velocities, effectively eliminate the interface thermal resistance, resulting in faster transverse NZP speeds. Numerical simulation that neglects interface thermal resistance agrees quite well with experiment.

According to recent studies, economical feasibility of resistive superconducting fault current limiters (FCLs) in transmission and distribution power line is confirmed. Since the SFCL is operated under alternating current (AC), it causes power loss, so-called AC loss, during normal operation condition even though it is superconductor. Bifilar pancake winding with coated conductor (CC) is expected to reduce AC loss of the SFCL by canceling magnetic fields efficiently as compared with other type SFCLs using CC in the same length. AC loss characteristic of the bifilar pancake type coil for SFCL application was investigated in this paper. Two bifilar pancake coils were wound using CC with facing on high temperature superconductor (HTS) sides each other and substrate (Ni-W) sides each other, respectively. AC losses of the coils were measured at 77 K and theoretical analysis was performed by finite element method (FEM). The experimental results are compared with the Norris equations and numerical values using FEM.

This paper presents the development status of joint method between the superconducting YBCO coated conductors (YBCO CC). In general, the YBCO-CC tape consists of an upper stabilizer, substrate, buffer, YBCO and a bottom stabilizer. It is suggested that the stabilizer between the two YBCO-CC tapes need to be removed, in order to improve the characteristics of the joints. The removing process was conducted by using heat and partial etching. The resistance and critical current of the joints were measured from the curve. The optical micrographs were analyzed for microstructures between the joint areas. After several times of over current test, the critical currents of the various types of joint samples were examined for applying to the high temperature superconducting magnet system.

Recent development of CC, usually called second generation (2G) HTS, is actively in progress. Because of its higher critical current density as well as higher n -value, 2G HTS is feasible for the applications such as superconducting fault current limiter and superconducting cable. For operating the HTS equipment stably, it needs to investigate the characteristics of normal zone propagation occurred by quench. Investigations on the fundamental characteristics can be one of the indispensable foundations for research and development of power equipments. In this paper, normal zone propagation (NZP) characteristics according to various insulation materials are researched. By heating with NiCr heater and insulating with epoxy, we applied the operating current with respect to the critical current for calculation of minimum quench energy (MQE) and measurement of NZP.

Recently, the resistive superconducting fault current limiter (SFCL) using YBCO coated conductor (CC) has been developed in many countries. Since CC has many advantages such as high n-value and high Jc compared with BSCCO tape, CC is useful to develop the resistive SFCL. The important thing for the operation of the SFCL is to reduce the AC loss like transport current loss and magnetization loss. Although the bifilarly wound magnet is a non-inductive coil, the transport current loss will certainly be produced by self-field when the AC current flows through the CC tape. A measurement of AC loss should be essential for the commercialization of the SFCL. In this paper, the transport current loss of the bililar magnet using CC is measured by the electric method and the calorimetric method and compared with Norris equation because inductance of the bifilarly wound magnet is very small.

As a part of the 21st Century Frontier R&D Program in Korea being performed from 2004, a non-inductive winding type superconducting fault current limiter (SFCL) is being developed. The target of the second year in phase II of the program is to develop a 220 V/300 A class non-inductive winding type SFCL as a prototype for a 13.2 kV/630 A class, the final goal of phase II. This SFCL has three solenoid type non-inductively wound coils in series using a 2G high temperature superconducting (HTS) wire and it was tested in sub-cooled nitrogen of 65 K, 1 atm. A coil which is composed of four parallel windings in a bobbin and winding directions are opposite to have non-inductive characteristics. Three coils were connected in series and the total length of 108 m of 2G HTS wire was used. Short-circuit tests were performed at applied voltage of 220 V and the SFCL limited the fault current to a few kA extents at the tests. Recovery time of the SFCL was measured after short-circuit tests.

This paper describes the characteristics analysis of a high-Tc superconducting (HTSC) power supply considering flux creep effect, and also presents its operational characteristics through experiments. The power supply can be employed to charge an HTSC magnet. An HTSC power supply consists of two heaters, an electromagnet, a Bi-2223 solenoid, and a series-connected Bi-2223 pancake load. The pancake load was fabricated by connecting four double pancake coils in series. In the experiments, two cases of the pumping period, which correspond to 8.5 and 17 s, were used with an electromagnet of 331 mH and a dc heater current of 0.8 A. A region of the superconducting tape with a buried heating coil can be switched by means of its temperature change. In order to measure the pumping-current with respect to the magnet flux changes, a hall sensor was installed at the center of the Bi-2223 pancake load. The experimental observations have been compared with the theoretical predictions. In this experiment, the pumping-current has reached about 22.9 A

A superconducting magnet which is operated in persistent current mode in SMES, NMR, MRI and MAGLEV has many advantages such as high uniformity of magnetic field and reduced thermal loss. A high temperature superconducting (HTS) persistent current switch (PCS) system was designed and tested in this research. The HTS PCS was optimally designed using two different HTS tapes, second generation coated conductor (CC) HTS tape and Bi-2223 HTS tape by the finite element method (FEM) in thermal quench characteristic view. The CC tape is more prospective applicable wire in these days for its high n value and critical current independency from external magnetic field than Bi-2223 tape. Also a prototype PCS system using Bi-2223 tape was manufactured and tested. The PCS system consists of a PCS part, a heater which induces the PCS to quench, and a superconducting magnet. The test was performed in various conditions of transport current. An initial current decay appeared when the superconducting magnet was energized in a PCS system was analyzed. This paper would be foundation of HTS PCS researches.

In recent years, a development of Coated Conductor (CC) that is a called the second generation superconductor tape is opened out. Therefore a commercialization of superconducting power equipments will be realized presently. To realize a commercialization, it is necessary to develop a stable superconducting magnet. A superconducting magnet has to keep thermal stability as well as electrical stability. In this paper, thermal conductivity of impregnating materials, epoxy compounds, was measured at 65K, 77K, 100K and 200K. Dielectric Strength of superconducting magnet modeled electrode system with impregnating materials was also analyzed. Stycast ® blue/catalyst 23LV is good materials to apply to the superconducting magnets.

This paper deals with the development of a 6.6 kV/200A HTS magnet for inductive superconducting fault current limiter (SFCL) that uses a sub-cooled nitrogen cooling system which is expected to be very useful to the applied HTS machines. The several characteristics of the HTS magnet such as critical current, electrical insulation and heat transfer can be enhanced in the sub-cooled nitrogen cooling system rather than in saturated liquid nitrogen (LN2) cooling system. To obtain sub-cooled nitrogen, LN2 was cooled down to 64 K with GM-cryocooler and its pressure was kept at 101 kPa with gaseous helium. In this investigation, the HTS magnet for a 6.6 kV/200 A SFCL was developed and its long-run operation characteristics were measured and analyzed. The characteristics of the sub-cooled nitrogen system like temperature, level of LN2 and pressure of cryogenic system were not degraded in spite of the several over current tests, electrical insulation tests, and even long-run operation tests with rated transport current

With the successful commercialization of Bi-2223 powder-in-tube tape and the improving quality & length-scale of YBCO coated tape, HTS magnets for high magnetic field and electric power applications have been developed. Although the operating temperature of HTS magnets must be kept in the designed level, the magnetic energy and mechanical disturbances can cause a thermal run-away. Especially, heat energy generated in the inner HTS winding is apt to accumulate, so a normal region appears in the HTS winding. This paper deals with the quench initiation and propagation characteristics of Bi-2223 wires and YBCO coated conductor, cooled by liquid nitrogen in adiabatic condition. A NiCr heater, mounted onto the surface of the HTS wire was used to provide the artificial heat that initiated quench. The quench characteristics of HTS wires were mainly affected by the material type and the superconductor to matrix ratio.

With the successful commercialization of Bi-2223 powder-in-tube wire, various attempts in the R&D of the high- T c superconducting (HTS) magnets for high magnetic field applications are being implemented actively. Operating temperature of HTS magnet has to be maintained at the designed level but the magnetic energy and mechanical disturbance can cause unstable operational temperature of HTS magnet. Especially, the generated heat energy of inner HTS winding is apt to be accumulated, so the normal region appears in HTS winding. This paper deals with the quenching characteristics of three kinds of selected Bi-2223 wires: the high current density wire (HC-A) and the high strength wire (HS-A) made by AMSC and HTS wire (HW-I) made by Innost. The Innost wire has the highest minimum quench energy (MQE). The high current density wire has the highest normal zone propagation velocity (NZPV).

In general case of DC reactor type superconducting fault current limiter (SFCL), a fault current gradually increases during the fault. It takes above 5 cycles to cut off the fault in the existing power system installed the conventional circuit breakers (CBs). Therefore, the fault current increases during the fault even if the SFCL is installed. This paper proposes a technique for decaying the fault current with the function of the fault detection and control of power converter of the SFCL. Using the proposed method, the fault current can decay after 1-2 cycles when the fault occurs. To analyze this technique, three-phase 6.6 kV/200 A SFCL was fabricated. The SFCL has just one DC reactor, an AC to DC power converter which has thyristors as the rectifying device, and a three-phase transformer, which is called magnetic core reactor (MCR). The short-circuit tests of this SFCL were performed successfully. Comparing the result using the proposed technique with the typical result, the fault current is decreased effectively by the proposed technique. This result shows that this SFCL using the fault detection and control of power converter can be applied to the existing power system which has conventional CBs.

As a part of the 21st Century Frontier R&D Program in Korea being performed from 2001, a DC reactor type superconducting fault current limiter (SFCL) is being developed. The target of the first phase is to develop a 6.6 kV/200 A class 3-phase SFCL as a prototype for a 154 kV class. This prototype SFCL has a magnet wound with a high-T/sub c/ superconducting wire (Bi-2223) operating at 65 K. The magnet uses solenoid bobbins with grooves for the tape wire. Its inductance is about 84 mH. An AC-DC power converter was made with light trigger thyristor (LTT) having a phase controller. The maximum voltage is 7 kV. The DC reactor, which is cooled by sub-cooled nitrogen, uses a three phase transformer with dual tap for series connection with a power line. Through several experiments this SFCL shows current limiting performance about 50% of short circuit current at short run tests.

The high-temperature superconducting (HTS) electric equipment using Bi-2223 wire successfully commercialized 1st generation HTS wire, has been developing by many research groups. HTS coil is one of the most important parts in HTS electric equipment. To enlarge the critical current, the operating temperature of the equipment tends to go down and number of HTS wire tends to increase. Thus, it is very important to determine the whole critical current of HTS coil for the stable operation of HTS equipment. The whole critical current of multi-stacked wire of HTS solenoid coil is not equal to the sum of the critical current of each stacked wire because of nonuniform current sharing occurs in multi-stacked HTS wires and each HTS path does not has the same critical current. In this paper, current sharing in two types of 5-stacked HTS solenoid coils, made up of 1 copper layer which is a current buffer layer of HTS coil and 4 HTS wire layers, was analyzed. One was wound with Bi-2223 wire insulated with polyimide tape and the other was wound with noninsulated Bi-2223 wire. All of the Bi-2223 wires were reinforced with stainless steel. The simulation results of current sharing in multi-stacked HTS coils coincided well with the experimental results. Based on these results, the current sharing ratio of large scale HTS coil was also expected. The copper layer acted as a good current path when all the HTS wires were quenched.

The conduction-cooled cryogenic cooling system for 1.2 kV/80 A inductive superconducting fault current limiter (SFCL) was fabricated and tested for its cooling characteristics in 2002. The observed thermal stability of the conduction-cooled system was very unreliable and precarious for the applied superconducting equipment with very large variation of currents like SFCL. Therefore, we replaced the conduction-cooled system with the sub-cooled nitrogen system for the 6.6 kV/200 A SFCL. In this paper, the design techniques and test results of cooling characteristics were introduced. The conditions for achieving the sub-cooled nitrogen state were 1 atm and 64 K. First, the temperature of 64 K was achieved by using the rotary pump and then the pressure of 1 atm was achieved by GHe. The characteristics of liquid nitrogen were strongly enhanced in these conditions. This fabricated cryogenic system merely for the short run operation test SFCL. Finally, the cryogenic system for the long run operation test SFCL was introduced.

This paper presents determination of heater triggering parameters of high-T/sub c/ superconducting (HTS) power supply with series-connected double-pancake load, as well as analysis of its characteristics as determined through experiments. A HTS power supply consists of two heaters, an electromagnet, a Bi-2223 solenoid, and a Bi-2223 series-connected double-pancake load of 6.0 mH. The timing sequential control of two heaters and an electromagnet is an important factor to generate pumping-current in the pancake load. The thermal analysis of switching parts of the Bi-2223 solenoid according to the heater input was carried out. Based upon the analysis, the 8 A electromagnet current and the 1.5 A dc heater current were optimally derived, and 8 s and 20 s for the pumping period were selected in this experiment. In the experiment, the maximum pumping current reached about 24 A.

The high- T c superconducting (HTS) magnet is an important element for developing HTS power equipments such as the dc reactor of the inductive type superconducting fault current limiter (SFCL). In order to use the HTS magnet for the large-scale power system, its critical current needs to be high enough. Generally, the double pancake HTS magnet has the severe decrease in the critical current because of magnetic field perpendicular to the tape surface. To fabricate a high critical current magnet, we wound a solenoid with the stacked tape. In this paper, the characteristics of the critical current of the HTS solenoid wound with the stacked tape were investigated. The results of this research can be used as the background data for the design of the large-scale HTS magnet.

This paper deals with determination of equivalent circuit parameters of the Low-T/sub c/ Superconducting (LTS) power supply for charging of the LTS magnet, as well as analysis of its characteristics as determined through experiments. The power supply consists of two exciters, a rotor, a stator, and an LTS magnet. In this experiment, a 25.8 mH LTS magnet was used, and rotor revolutions from 30 to 300 rpm were used. The magnetic flux distribution analysis of the system was carried out. Based upon this analysis, maximum 150 A DC excitation current was optimally derived. In order to measure the pumping-current with respect to the magnet flux changes, a Hall sensor was installed at the center of the LTS magnet. The experimental observations have been compared with the theoretical predictions. In this experiment, the pumping-current has reached about 372 A.

To develop a DC reactor type Superconducting Fault Current Limiter (SFCL), the most important elements are superconducting magnets or inductors. Recently large size magnets have been developed, according with the improvement of high temperature superconducting wire. In the DC reactor type SFCL, the purpose of a magnet is to store the generated electric energy of the power system immediately after a fault. Therefore most inductors are designed with an air core since the magnetic core inductor is too easy saturated to absorb the energy. Therefore the inductor consumes so much superconducting wire to make a large inductance and these expensive coils are a weak point of the DC reactor type SFCL. To solve this problem, the Reverse Magnetization Bias (RMB) method is introduced. The energy capacity of magnetic core is expanded to several times. With a shorter length of HTC superconducting wire, a much improved effect was obtained in the 40 V prototype SFCL.

A superconducting DC reactor protects a power system by limiting the amplitude of fault current with its inductance. Therefore, it is very important to design and simulate the DC reactor precisely for making the power system stable and effective. In this paper, we designed the superconducting DC reactor of an inductive superconducting fault current limiter conceptually and acquired the optimal design parameters by using Finite Element Method (FEM). We manufactured the superconducting DC reactor and tested its characteristics at cryocooler-cooled 20 K temperature. Moreover, we compared experimental characteristics with simulation results and analyzed them. We introduced the design method of the superconducting DC reactor and the fabrication method of a 1.2 kV/80 A class DC reactor for an inductive superconducting fault current limiter. Finally, we performed the short circuit test and discussed the results.

This paper is describing an operational characteristics of low-Tc superconducting (LTS) power supplies considering the connections of superconducting sheets (niobium). An LTS power supply has four rotating poles, which produce rotational magnetic flux. This flux penetrates superconducting sheets and causes currents which charge an LTS load. While LTS power supplies have used only parallel-connected LTS sheets up to now, this paper deals with series-connected sheets type power supply. Two power supplies were manufactured in order to verify and analyze the operational characteristics using a 1.81 mH LTS load. The only difference between the two power supplies is hereby the connection method of the superconducting sheets. Experimental results show that the current pumping rate of the series-connected type is higher than that of a parallel-connected type.

This paper presents the design and fabrication of a high-Tc superconducting (HTS) power supply. Its characteristics have been analyzed through experiments. A HTS power supply consists of two heaters, an electromagnet, and a Bi-2223 solenoid and a Bi-2223 pancake load. The timing sequential control of two heaters and an electromagnet is an important factor to generate pumping-current in the Bi-2223 load. The thermal analysis of switching parts of the Bi-2223 solenoid according to the heater input was carried out. Based upon the analysis, the 331 mH electromagnet and the 0.8 A DC heater current were optimally derived, and 8.5 sec and 17 sec for the pumping period were selected in this experiment. In the experiment, the maximum pumping current reached 1.7 A.

This paper deals with comparison of characteristics of continuous-sheet type low-Tc superconducting (LTS) power supply and discrete-sheet type LTS power supply. These characteristics have been analyzed through experiments. These power supplies consist of two exciters, a rotor, a stator, and LTS Load. A continuous-sheet type has a single continuous niobium (Nb) sheet attached to the inner surface of on the stator. In the case of discrete-sheet type, four separated Nb sheets are used. This experiment uses a 1.81 mH LTS magnet load and maximum 30 A DC exciter current. A discrete-sheet type is expected to produce much better pumping rate than a continuous-sheet type. The experimental observations have been compared with the theoretical predictions. In this experiment, the maximum pumping-current has reached about 926 A.

This paper presents the design and fabrication of a high-Tc superconducting (HTSC) power supply with the Bi-2223 pancake load, as well as its characteristics as determined through experiments. HTSC power supply consists of two heaters, an electromagnet, a Bi-2223 solenoid, and a Bi-2223 pancake load. In this experiment, a 331-mH electromagnet and 0.8-A dc heater current were used, and 8.5 and 17 s were used for pumping periods, respectively. Mechanism of the superconducting switch is used for heater-trigger. In order to measure the pumping-current with respect to the magnet flux changes, a hall sensor was installed at the center of the Bi-2223 pancake load. The experimental observations have been compared with the theoretical predictions. In this experiment, the pumping-current has reached about 0.6 A with 0.01-V electromagnet voltage.

This paper presents experimental and analytical results of trapped field characteristics of a stack of square YBCO thin film plates for compact NMR magnets. Each YBCO plate, 40 mm × 40 mm × 0.08 mm, has a 25-mm diameter hole at its center. A total of 500 stacked plates were used to build a 40-mm long magnet. Its trapped field, in a bath of liquid nitrogen, was measured for spatial field distribution and temporal stability. Comparison of measured and analytical results is presented: the effects on trapped field characteristics of the unsaturated nickel substrate and the non-uniform current distribution in the YBCO plate are discussed.

This paper examines the effects of alternating fields on high-temperature superconducting (HTS) coils according to the winding insulation condition. Alternating fields can occur in synchronous machines (armature reaction, faults) and other devices. In superconducting synchronous machines, alternating fields affect the operational characteristics of the machine and the superconducting field coil. Therefore, a method of reducing the effects of alternating fields is necessary in superconducting synchronous design. In this study, the effects of alternating fields on the HTS field coil according to the winding insulation condition were experimentally evaluated. The experimental results show that HTS coils made using the no-insulation technique can be a solution for reducing the effects of the alternating field. These results are expected to suggest useful data for applications of HTS field coils in superconducting synchronous machines.

For an effective fault current management in a smart grid, a fault current controller (FCC) has been proposed and actively developed. A previous attempt to reduce the superconducting wire consumption in a high-temperature superconducting coil for the FCC was to use a reversely magnetized core (RMC) using permanent magnets. Despite the previous study being effective to shorten the wire, there were some potential drawbacks: a demagnetization of the permanent magnets and uncontrollability. In the smart grid with renewable sources, a nominal line current can be time-varying, so a reverse bias point should not be fixed. Therefore, an actively controllable RMC is required. In this paper, an electromagnet was applied for a source of RMC. A reverse-bias point was actively adjusted by taking account of time-varying line current. This proposed bias method could reduce wire consumption. With optimal reverse bias current, the effective inductance was 4.3 times larger than that of the air core case. If we want to make the same inductance without RMC, wire consumption should be more than 2.07 times. This proposed method was so effective to apply to the distribution class FCC, which is our final target.

Recently various high temperature superconducting (HTS) conductors have been commercialized in the power applications. While a BSCCO becomes tape or wire, others coated conductors (CC) are tapes. An HTS magnet utilized with these conductors can be fabricated as a form of pancake winding or layer winding. In the case of HTS magnet using a thin and rectangular tape, the pancake winding method is satisfied in the practical cases because of its suitable features. However, one of the disadvantages of this winding method is increasing brands and splices. From this point of view, we propose a layer winding method using an HTS tape and fabricated HTS layer coils. To verify the feasibility of the proposed winding method, critical current and over-current tests are investigated in liquid nitrogen and the results are compared with those of pancake coils. In addition, operating characteristics on the condition of various winding tensions are analyzed.

Since 2000, a three-phase program with a final goal to complete a 1 GHz high-resolution low-/high-temperature superconductor (LTS/HTS) nuclear magnetic resonance (NMR) magnet has been conducted at the Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology (MIT). In a LTS/HTS magnet assembly, a HTS insert is placed in the cold bore of a LTS background magnet. To date, two LTS/HTS magnets have been designed, constructed, and tested: a 350 MHz (LH350) in phase 1 and a 700 MHz (LH700) in phase 2. The program’s target has recently been upgraded from the original goal of 1 GHz to a new goal of 1.3 GHz. In this paper, we present extensive performance analyses of the two LTS/HTS NMR magnets. Spatial homogeneity and temporal stability of LH350 and LH700, examined with harmonic analysis, and four key issues that became evident in the operation of these two magnets are discussed: (1) field constant reduction, (2) “large” residual Z1 gradient and its temporal decay, (3) large one-periodic tesseral fi...