Y.K. Kwon

A "permanent" high-temperature superconducting magnet operated in thermal communication with a mass of solid nitrogen

Abstract A new design for a portable “permanent” superconducting magnet system is explored. The design involves a persistent-mode high-temperature superconducting (HTS) magnet that is cooled by a solid heat capacitor. The system is an alternative to permanent low-temperature superconducting (LTS) magnet systems where the magnet is cooled by a bath of liquid helium. An apparatus was constructed to demonstrate stable operation of a permanent magnet wound with Bi2223/Ag conductor while in thermal communication with a mass of solid nitrogen. The apparatus includes a room-temperature bore and can function while it stands alone, detached from its cooling source, power supply, and vacuum pump. The magnet is operated in the 20–40 K temperature range. This apparatus is the first to demonstrate the operation of a superconducting magnet with a permissible temperature variation exceeding a few degrees kelvin. Models are developed to predict the experimental systems warming trend and magnetic field decay. The models are validated with a good agreement between simulations based on these models and experimental results. The results indicate that present HTS conductor critical current and index are not yet sufficient to provide field strengths and field decay time constants that are required for typical persistent-mode applications.

Joint resistances between two parallel high Tc superconducting tapes

Eight different types of joining between two parallel high Tc superconducting (HTS) tapes were prepared and current-voltage (I-V) characteristic curves were investigated at 77 K, liquid nitrogen temperature. Contact resistances of joint parts with 25 mm in length were estimated from I-V curves. Lowest contact resistance among eight samples having different types of joining was 0.497 /spl mu//spl Omega/. The best joining method is to do crossly joining with HTS tapes on two parallel HTS tapes after filling solder paste into their gap. In this case, lowest joint resistance was 0.15 /spl mu//spl Omega/ at 77 K and 0.05 /spl mu//spl Omega/ at 28 K. Joule heat of a joint sample generated at 30 K and at 100 A was calculated 0.492 mW. Also, in I-V curve of samples crossly joined with HTS tapes, it was found that joint resistance does not depend on variation of magnetic fields (0/spl sim/7000 G) at 77 K. From consideration of Joule heat generation, such joining method is available for fabricating the field winding of superconducting rotating machine.

Design analysis of a solid nitrogen cooled “permanent” high-temperature superconducting magnet system

Abstract Potential performance advantages of a solid nitrogen cooled “permanent” high-temperature superconducting (SN2/HTS) magnet system over a liquid helium cooled low-temperature superconducting (LHe/LTS) system are explored. The SN2/HTS system design includes a second solid heat capacitor that cools a radiation shield. Recooling of the heat capacitors is performed with a demountable cryocooler. The SN2/HTS system offers both enhanced stability and improved portability over a LHe/LTS system. Design codes are constructed to compare the SN2/HTS system design with a LHe/LTS design for a general permanent superconducting magnet system employing a room temperature bore. The codes predict the system volume and mass that should be expected for a given set of design requirements, i.e. field strength and bore size, and a given set of conductor properties. The results indicate that present HTS conductor critical current and index are not yet sufficient for producing SN2/HTS systems of a size that is comparable with that expected for a LHe/LTS system; however, the conductor properties of Bi2223/Ag have been consistently improving, and new HTS conductors are expected to be developed in the near future. The codes are used to determine the minimum Bi2223/Ag conductor performance required for a SN2/HTS system to be competitive with a LHe/LTS system.

Development of a 100 hp synchronous motor with HTS field coils

A 3-phase, 100 hp, 4 pole, 1800 rpm superconducting synchronous motor built and tested in Korea Electrotechnology Research Institute(KERI). This machine consists of HTS rotor and air-core stator. The HTS field windings are composed of the double-pancake coils wound with AMSCs stainless steel-reinforced Bi-2223 tape conductor. These were assembled on the support structure and fixed by a bandage of glass-fiber composite. The cooling system is based on the heat transfer mechanism of the thermosyphon by using GM cryocooler as cooling source. The cold head is in contact with the condenser of a Ne-filled thermosyphon. The rotor assembly was tested independently at the stationary state and combined with stator. The open circuit, no-load, and short circuit characteristic were obtained. Also, load tests in motor mode driven by inverter and generator mode connected to resister load bank were conducted. This paper will present design, construction, and experimental test results of the 100 hp HTS machine.

Design of field coil for 100 hp class HTS motor considering operating current

The value of critical current (I/sub c/) in High Temperature Superconductor (HTS) tape is greatly influenced by perpendicular magnetic flux density B/spl perp/. Therefore, I/sub c/ of HTS magnet is determined by not only operating temperature but also the B/spl perp/. In this paper, flux distribution and operating current are calculated according to the field coil change in 100 hp class HTS motor. The magnetic flux density of field coil is calculated by changing the outer radius and inner width of field coil. Biot-Savart equation is used as analytic method for the characteristic analysis of magnet. 3D FEA (Three Dimensional Finite Element Analysis) is used for the magnetic field distribution in the HTS motor. The operating current of 100 hp class HTS motor is calculated by using I/sub c/-B/spl perp/ curve.

Quench and recovery characteristics of a racetrack double pancake coil wound with YBCO-coated conductor

The reliability of high temperature superconducting (HTS) racetrack coils is one of the most important factors for the development of large-scale rotating machines. However, it is necessary to investigate the stability and normal zone propagation characteristics of racetrack coils for large-scale applications such as ship propulsion motors and power generators. In this study, the quench/recovery characteristics of a racetrack-type, double pancake (DP) coil, which could be applied to HTS rotating machines, were investigated using the voltage and temperature profiles in a cryogenic conduction cooling system. The minimum quench heating flux and quench propagation velocity of the racetrack DP coil are also discussed.

Design of Damper to Protect the Field Coil of an HTS Synchronous Motor

High Temperature Superconducting (HTS) synchronous motors consist mainly of rotational (HTS coil) and stationery (copper coil) parts. The HTS coil running at a synchronous speed is not influenced by the magnetic field generated by the stator coil but is affected considerably at the initial or transient operation. Therefore, computational analysis to determine the optimal design of the damper is an important factor in protecting the field coil of a HTS synchronous motor. This paper presents the optimal design parameters of the damper for a 5 MW HTS synchronous motor using the computational analysis of the transient characteristics of the stator current, which is proportional to the alternating magnetic field of the stator coil.

Status of HTS Motor Development for Industrial Applications at KERI & DOOSAN

This work is development of HTS motor at DOOSAN heavy industry and Korea Electrotechnology Research Institute in Korea, and is sponsored by DAPAS program which is supported by Korean government. The final aim of the project is realization of HTS motor in the field of industry such as large driving pumps, fans and compressors for utility and industrial environments. In the first phase (2001-2004), 100 hp HTS motor was developed in order to implement the preliminary technology for the large applicable HTS motor. All of the performance characteristics are well met to the designed ones. In second phase (2004-2007), 1 MW HTS motor is developed for the purpose to fully represent the design and manufacturing issues for the larger capacity machine. The machine is 2 pole and 3600 rpm, and all of the components are completely manufactured. This machine is now under assembly. This paper summarizes the status of 1 MW HTS motor development, such as design, construction, and experimental test results.

A study on the application effects of HTS power cable in Seoul

In this study, we performed the long-term expansion planning for the conceptual design of HTS power cable in Seoul area. In Korea, underground power cable has been required gradually with increasing demand of electric power transmission density and low loss characteristics in the comparison with a conventional power cables, so we assumed that the HTS power cable is applied between the downtown area and the outskirts of the city for the large power transmission capacity. This paper is to show the effects of HTS power cables in Seoul based on the power system analysis.

Fabrication of Bi-2223 HTS magnet with a superconducting switch

A test pancake magnet, equipped with persistent superconducting switch, was fabricated with Bi-2223/Ag-Mg tape. The magnet was operated in persistent mode at 20 and 77 K. For large operating currents that forces the magnet to operate in the flux flow range, the field decays is exponentially with time. A simple circuit model is used to compute field decay time constants. It was also found that Pb-Sn solder joints are unsuitable for persistent mode operation.