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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.

Studies on second-generation (2G) high temperature superconductor (HTS) magnets cooled by a cryogenic refrigerator are actively underway due to the recent rapid advancement in 2G HTS tape technology. To reduce the initial cooling time, lower the operating temperature, and promptly extract the hot spot of the conduction cooled 2G HTS magnet during quenching, the magnet's equivalent thermal conductivity should be high. In general, conventional 2G HTS tapes use a copper stabilizer wrapped with a low thermal conductivity polyimide electrical insulation tape. Ultimately, the low thermal conductivity of these conventional HTS magnets lengthens the cooling time, causing the magnet to burn out during quenching. In this paper, by replacing the copper stabilizer with an aluminum stabilizer and anodizing it for electrical insulation instead of using polyimide tape, the 2G HTS tape's equivalent thermal conductivity was largely improved. Based on the results of thermal analysis and tests, the thermal characteristics of a conventional 2G HTS tape are compared with those of the 2G HTS tape presented in this paper.

The studies on the high temperature superconducting (HTS) current lead for superconducting magnets are actively underway with rapid advancement in HTS conductor technology recently. HTS leads reduce the heat load to superconducting magnets. HTS current lead consists of several HTS tapes in parallel which are soldered to the copper blocks. The quench occurs first in the HTS tape with lowest critical current among HTS tapes during charging of the transport current. Conventionally, HTS current leads use the equipotential voltage signal to detect quench but it is difficult to estimate the quench of an HTS current lead in fault because the quench voltage level of an HTS tape in a current lead is very low. In this paper, the quench detection of an HTS current lead using a hall probe was investigated. First, the magnetic field distributions of an octagonal shaped HTS current lead before and after quench in charging of transport current were computed using Comsol. Then, this octagonal-shaped HTS current lead was fabricated and tested in order to measure its central magnetic field. Based on the results, the application feasibility of the quench detection of HTS current lead using hall probe was confirmed.

This paper describes the design results, loss analysis, and experiment testing of the real manufactured toroid-type superconducting magnetic energy storage (SMES). We analysed heat load through the conduction and radiation of the cryostat and magnetization loss caused by the charging and discharging of the toroid-type SMES system. We also measured the characteristics of toroid-type SMES under three different types of operating conditions (steady state, one-cycle charging and discharging, series charging and discharging). The fundamental design specifications and the data obtained from the experiment will be applied to a large-scale toroid-type SMES system design.

Superconducting magnetic energy storage (SMES) can overcome fluctuations in frequency because of its fast response time in charging and discharging energy. To stabilize the fluctuations in frequency of wind power generation systems (WPGSs), HTS SMES systems should be connected to the terminal of the WPGSs. Ulleung Island's power network in Korea was modeled with a real-time digital simulator (RTDS) to demonstrate the effectiveness of SMES at stabilizing frequency. A toroid-type HTS SMES cooled by conduction cooling and a DC/DC chopper to charge and discharge current were fabricated for the experiment. The simulation results show the frequency stabilization effected by the HTS SMES system with its operational characteristics such as real time variation in current and temperature.

A superconducting property measurement system (SPMS) is used to acquire electrical and thermal properties of short HTS tape samples. The SPMS is composed of a specimen holder for mounting an HTS tape and magnet for applying a magnetic field externally. The magnet consists of two split racetrack windings and is designed to produce maximum 3 T of center field at 20 K. In this paper, an HTS magnet for the SPMS cooled by a cryocooler was designed. A HTS magnet consists of several pancakes winding wound from YBCO CC tape. We report out design for a conduction-cooled magnet using YBCO CC tape. For varying the number of double pancake coils (DPCs) and the bending radius, an electromagnetic field analysis is used. We subsequently find a suitable HTS magnet type for an SPMS. This paper describes the quench protection system for an HTS magnet of the SPMS. The detection of any normal zone must be preceded to protect the HTS magnet. We use simulated protection circuit to estimate the trigger conditions. The presented results in this paper will be verified using fabricated protection circuit.

2nd generation high temperature superconducting (2G-HTS) tape consists of multi-layers such substrate, buffer layer, superconducting layer and reinforced lamination tapes. 2G HTS tape is a candidate of good materials for current lead of superconducting magnet system owing to its low thermal conductivity. However, joint resistance between 2G HTS tapes and terminal block can be a major problem because of high electrical resistivity of substrate, buffer layer and reinforced lamination tapes of 2G HTS. So specially considering joint resistance between 2G tapes and terminal block, 2G HTS current lead for 400 A was designed and fabricated. This current lead was consisting of two terminal blocks, a support bar or tube, protection tube and six 2G HTS tapes. Its total length was 300 mm and body diameter 18.3 mm. At liquid nitrogen temperature (77 K) critical current (Ic) of this HTS current lead was 600 A, about 1.5 times the operating current 400 A. Conductive heat loss of 2G HTS current lead between 60 K and 7 K was 50 mW.

A 2.5 MJ HTS SMES is being developed as a national program in Korea. This paper presents design results of modular HTS magnets for the 2.5 MJ toroidal SMES. We considered a choice of BSCCO-2223 or ReBCO tapes as the conductors for the HTS magnets of the SMES. Operating temperature was decided to be 20 K which could be accomplished by conduction cooling with a couple of GM cryo-coolers. Key design factors may include minimization of total conductor requirement and low AC loss.

A 600 kJ HTS SMES system has been developed for power system stabilization as a national project in Korea. Successful operating tests of the 600 kJ were recently completed. In this paper, a 2.5 MJ class SMES with HTS magnets of single solenoid, multiple solenoid and modular toroid type were optimized using a recently developed multi-modal optimization technique named multi-grouped particle swarm optimization (MGPSO). The objective of the optimization was to minimize the total length of HTS superconductor wires satisfying some equality and inequality constraints. The stored energy and constraints were calculated using 3-D magnetic field analysis techniques and an automatic tetrahedral mesh generator. Optimized results were verified by 3D finite element method (FEM).

We started a new project to develop the conduction-cooled measurement system for superconducting power apparatus. The measurement system is used to acquire electrical and thermal properties on short HTS tape samples and the system is composed of specimen holder for mounting HTS tape and a magnet for applying magnetic fields externally. The magnet consists of two split racetrack windings and is designed to produce maximum 3 T of center field at 20 K. The temperature of specimen holder can be varied in range from 10 K to 77 K. The field homogeneity, which is calculated the maximum magnetic field divided by the center field, is required less than 0.01 within 10 cm axially and 10 mm radially from origin. We have worked on the conceptual designs of the conduction-cooled magnet for the superconducting characteristic measurement system. The measurement system will be fabricated in near future based on the design parameters presented in this paper.

Superconducting magnetic energy storage (SMES) systems with High Temperature Superconducting (HTS) wires have been actively developed world-wide. A 600 kJ class SMES with Bi-2223 HTS wire has been in development as a national project since 2004 and is currently approaching the final testing stage of the first of three phases. In the second phase of the project, several MJ class HTS SMES will be developed. In this paper, designs of magnets for 5 MJ class SMES with DI-BSSCO and YBCO coated conductor are presented and compared.

In order to evaluate the stable operation of high temperature superconducting (HTS) power cable, the calculation of AC loss on HTS power cable is important factor. Because AC loss has a big effect on many superconducting equipments, which are fabricated using the HTS tape. In general, AC loss of the HTS tape can be calculated by using voltage variations of the tape with respect to the external magnetic field when AC current applied to the tape. In case of HTS power cable carrying AC current, however, because the magnetic field distribution in the cable is very complicated, it is hard to compute AC loss of the HTS cable directly using above method. In this paper, the stability evaluation of the HTS power cable was performed by the following procedures. Firstly, the voltage variations of the HTS tape carrying AC current in the external magnetic field with different field angle were measured and the magnetic field distribution of HTS power cable with AC current was analyzed. Then, AC loss of the HTS power cable was calculated by using the results of both the measurement and the analysis. The stable operation of the HTS power cable was evaluated on the basis of AC loss of the cable.

The purpose of this study is to develop a superconducting magnet energy storage system (SMES), which protects sensitive loads on the power system, when an interruption or voltage sag occurs. Industries have many sensitive machines, and keeping the power in a good condition is very important for nonmilitary machines also. Korea Electrotechnology Research Institute (KERI) has developed a 3 MJ/750 kVA SMES system to improve power quality in sensitive electric loads. It consists of an IGBT based power converter, NbTi mixed matrix Rutherford cable superconducting magnet, and a cryostat with HTS current leads. The operating current of the 3 MJ SMES magnet was 1000 A. The SMES system is tested under short time power interrupt to verify the feasibility of the SMES system as a 750 kVA power converter

During the operation of HTS power cable, large fault current can be introduced to a HTS power cable due to several accidents. In this case, a circuit breaker limits the fault current to protect the HTS power cable just as conventional power cables. However, heat is necessarily generated until the circuit breaker operates and severe performance degradation or even burn-out can occur at HTS tapes. To ensure the safety against the fault current, thermal characteristic of the HTS power cable should be verified under the fault current. Several experiments with a simple cable are performed using an AC pulse power supply. During the experiment, the increase of temperature and current redistribution are measured for the various fault current conditions. Through the experiments, safety margin of Korean HTS power cable is verified and the allowable peak current is suggested

To obtain realistic data on HTS power cable, single-phase 30 m long, 22.9 kV class HTS power transmission cable system have been developed by Korea Electrotechnology Research Institute (KERI) and LG cable Ltd. that is one of 21st century frontier project in Korea. The HTS cable consists of Ag/Bi-2223 tapes, high voltage insulation paper which is impregnated by LN/sub 2/. The cable is rated at 22.9 kV, 50 MVA, 60 Hz and is cooled with pressured liquid nitrogen at temperature from 70 to 80 K. This paper describes the results of design, fabrication and evaluation of the single-phase, 30 m HTS power cable system.

The design of a superconducting coil and cryostat for 3MJ class SMES was completed in 2002. Before assembling the full system, some important parts were fabricated in order to find out whether their operating properties were fit for our 3MJ SMES or not. The assembling process will be performed in 2003. The 3MJ SMES will be operated at 1000 amps in the cryogenic environment of 4.2 K. So, it is essential to minimize the heat load from room temperature and to optimize its ohmic heating. A bundle of brass sheets with optimized length/area ratio was used for the metal part of the current lead and it is connected to HTS current lead in series. Two GM cryocooler sinks out the heat load and also the ohmic heating of the current lead which is located in vacuum chamber of the cryostat. Cold head of the cryocooler which is extended to the HTS lead with layered OFHC sheets keeps the temperature under the critical temperature of the HTS lead. We performed some experiments to examine the thermal and electrical properties of the current lead. In this paper, the fabrication process and test results of the current lead will be introduced in detail.

Several companies in the world are marketing superconducting products, such as wires, films, and bulk, and so on. High-T/sub c/ superconducting (HTS) systems have begun to be commercialized with these HTS products, and the demand for them is increasing. A database on the electrical degradation of high-T/sub c/ superconductors is an essential requirement to commercialize HTS systems. The electrical degradation of Bi-2223 wire has been investigated in this paper. To simulate the conditions of real systems, two types of specimens were prepared. One was named Ring Specimen with Bi-2223 wire on a bobbin of 400 mm diameter and the other was named Double-Pancake Specimen with Bi-2223 wire coiled on a bobbin of 100 mm diameter. The Bi-2223 wire of the Double -Pancake Specimen was coiled with several winding tensions. The continuous current transport method was used for the measurement of the electrical degradation. Various levels of transport current were applied to each conductor. The levels are 90, 95, 98, and 110% of the critical current (I/sub c/) in Ring Specimens and 95 and 150% in Double-Pancake Specimens. Although a difference of the I/sub c/ in the long Bi-2223 wire was found, there was not any severe degradation in the Ring Specimens. When the level of transport current was below the critical current, degradation did not appear in the Double-Pancake Specimens either. However, they degraded very quickly when the level of transport current was 150% of I/sub c/.

A superconducting solenoid with large inner diameter has much lower current capacity than expected from short wire sample tests, and a solenoid of multi-strand cable has worse results. It is known that this degradation is observed when the inner diameter of the solenoid is bigger than a certain value (30 cm usually). However the reason is not clarified yet and some people called this just the "size effect." To obtain some clues to understand the size effect and to develop big solenoids with higher operating current, we prepared and tested two sample solenoids made of Rutherford type cables with and without soldering. Quench currents and AC losses of the two solenoid coils are measured and compared. The test results show that the soldering on the Rutherford cable improves the current capacity, but makes the AC loss much higher.

Internal tin processed Nb/sub 3/Sn wires of different diameters were locally heated before reaction heat treatment. Local heating at an intermediate stage of drawing process decreased the superconducting properties and workability. When the local heating temperatures were lower than the melting point of Sn, non-Cu J/sub c/'s decreased a little compared to non-locally heated wire. But when the local heating temperatures were higher than melting point of Sn, non-Cu J/sub c/'s decreased significantly. This study also suggested it is important that the Sn reservoir be arranged uniformly and is dispersed.

This paper deals with the initial magnetic field decay for a large scale superconducting magnet e.g. NMR/MRI magnet. The high resolution image can not be obtained during the periods of the initial field decay. It is known that all superconducting materials have the property of diamagnetism. This diamagnetism is usually explained with the concept of screening current. We assumed that the existence of the screening current makes the current distribution in the superconducting wire non-uniform. The current distribution in the energizing state is different from the distribution in the steady current state in view of its pattern. The initial magnetic field decay is caused by the change of the current distribution between energizing state and persistent current mode. In this paper the theoretical analysis for the current distributions has been carried out for each state. The experiments have been carried out for various transport currents in order to verify the theoretical analysis. Through experiments, It was found that the initial field decay is small at the higher transport current. At the end of paper a method able to eliminate effectively the initial magnetic field decay is suggested.

This paper describes a series of experiments to investigate the operational characteristics of a superconducting fluxpump with a superconducting excitation coil. In this experiment, the idea of excitation in the fluxpump is introduced to control the pole-flux in the air gap of the machine. The operating current of the fluxpump is designed to have the value of 300 amps. for the rotational speed of 600 rpm. Sensors installed on the Nb sheet yield the information on the spatial and temporal behaviors of the magnetic field in spot and on the characteristics of the fluxpump. The experimental observations are compared with the theoretical predictions.

The study on the minimum quench energy of YBCO coated conductor (CC) samples with different copper stabilizer thicknesses at various operating temperatures is performed. In this paper, we analyzed the thermal properties of YBCO CC in the adiabatic condition. Also, we measured the minimum quench energy of the three kinds of YBCO CC samples with different copper stabilizer thicknesses fabricated by Superpower Incorporated at various temperatures and currents. The results through this research will be utilized to determine the operating current of the conduction-cooled HTS magnet and to design the external protection circuit to keep the HTS magnet from being damage by a thermal quench.

A 2.5 MJ Superconducting Magnetic Energy Storage System (SMES) is being developed as a national R&D project in Korea. It is in 2nd phase of total 3 phases program. In phase 1, we developed a 600 kJ SMES which had the maximum storage over 1 MJ. Its magnet was an HTS solenoid, whereas the HTS magnet for phase 2 is a toroid which is composed of 28 HTS double pancake coils. The final target of phase 3 is to develop a 5 MJ SMES, which will start from next year. The operating temperature of the HTS magnet will be below 20 K by conduction cooling. In this paper, we present and compare conceptual designs of the magnet in the shape of toroid and the configuration of serial connection of two 2.5 MJ toroids. The most important design criterion is to minimize the total length of HTS conductors.