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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).

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

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.