Haigun Lee

Design and Experiments of Novel Hybrid Type Superconducting Fault Current Limiters

In order to apply resistive superconducting fault current limiters into electric power systems, the urgent issues to be settled are as follows, such as initial installation price of SFCL, operation and maintenance cost due to ac loss of superconductor and the life of cryostat, and high voltage and high current problems. The ac loss and high cost of superconductor and cryostat system are main bottlenecks for real application. Furthermore in order to increase voltage and current ratings of SFCL, a lot of superconductor components should be connected in series and parallel which resulted in extreme high cost. In addition, the method to quench all components at the same instant needs very sophisticated skill and careful operation. Due to these problems, the practical applications of SFCL were pending. Therefore, in order to make practical SFCL, the price of SFCL should be lowered and should meet the demand of utilities. LSIS and KEPRI designed novel hybrid SFCL which combines superconductor and conventional electric equipment including vacuum interrupter, power fuse and current limiting reactor. The main purpose of hybrid SFCL is to drastically reduce total usage of superconductor by adopting current commutation method by use of superconductor and ultra fast switch. Consequently, it was possible to get the satisfactory test results using this method, and further works for field tests are in the process.

Investigation of HTS Racetrack Coil Without Turn-to-Turn Insulation for Superconducting Rotating Machines

This paper reports a study of two HTS racetrack coils wound with GdBCO coated conductors, one without turn-to-turn insulation and the other with Nomex insulation. The coils were characterized by charge-discharge, over-current, and sudden discharge tests. Thermal and electrical characteristics of the GdBCO racetrack coil without insulation outperformed the insulated one. The test results confirmed that although the coil without insulation has a slow charging/discharging time constant, the no-insulation winding technique may enable a compact and protection-free racetrack-typed coil with enhanced mechanical integrity as well as better thermal and electrical stabilities.

Partial insulation of GdBCO single pancake coils for protection-free HTS power applications

The partial insulation winding was examined to ameliorate the slow charge–discharge shown by coils wound without insulation. Single pancake coils of GdBCO coated conductor were wound without insulation, with kapton tape every five turns, and with the full use of kapton tape. They were characterized by charge–discharge, sudden discharge, and over-current testing. The improved charging and discharging and high thermal and electrical stabilities of the partially insulated coil demonstrate its potential for use in HTS power applications.

No-Insulation Coil Under Time-Varying Condition: Magnetic Coupling With External Coil

This paper presents experimental and analytical studies on the time-varying behavior of an NI (no-insulation) high-temperature superconductor pancake coil, alone or magnetically coupled to an external coil. An NI coil and another insulated coil (as an external), both of identical winding i.d. and number of turns, were fabricated. Another external coil used in this study was a 300-mm/5-T low-temperature superconductor magnet. An equivalent circuit model is proposed to simulate the NI coil, and the external coil, under time-varying conditions. Good agreement between experiment and simulation shows that the proposed equivalent circuit model is valid to characterize the time-varying electromagnetic behavior of an NI coil, alone or magnetically coupled to an external coil.

The effects of partial insulation winding on the charge–discharge rate and magnetic field loss phenomena of GdBCO coated conductor coils

GdBCO single-pancake coils wound with Kapton insulation every 3, 6, 9 turns, and without insulation, were characterized by charge–discharge and over-current tests. The magnetic field saturation at higher currents than the critical current was affected by the portion of the insulated turns that restricted the surplus current flow away from the azimuthal current path. The charge–discharge delay decreased with the increase of the portion of the insulated turns, which suggested that partial insulation winding may be effective for use in HTS power applications. Magnetic field loss of the coils with reduced insulation at excessive high currents was mainly affected by the decrease in ampere-turns due to the electrically shorted-circuits between the non-insulated turns, which resulted in the over-current flow bypass through the turn-to-turn contacts.

A Solid Nitrogen Cooled

A 700-mm bore superconducting magnet was built and operated in our laboratory to demonstrate the feasibility of newly developed superconductor wire for fabricating MRI magnets. The magnet, an assembly of 10 coils each wound with a reacted and s-glass insulated wire 1-km long, was immersed in solid nitrogen rather than in a bath of liquid cryogen. This magnet was designed to operate in the temperature range 10-15 K, maintained by a cryocooler. A combination of this ldquowiderdquo temperature range and immersion of the winding in solid nitrogen enables this magnet to operate under conditions not possible with a low temperature superconductor (LTS) counterpart. Tested individually at 13 K, each coil could carry current up to 100 A. When assembled into the magnet, some coils, however, became resistive, causing the magnet to prematurely quench at currents ranging from 79 A to 88 A, at which point the magnet generated a center field of 0.54 T. Despite the presence of a large volume (50 liters) of solid nitrogen in the cold body, cooldown from 77 K to 10 K went smoothly.

{\hbox{MgB}}_{2}

This paper deals with an analysis on the optimum lap joint method of the YBCO coated conductor (CC) wire. Effective low resistance joint is important for the development of persistent current mode system using the second generation high temperature superconducting (2G HTS) wire, YBCO CC. CC tape is stacked with copper or stainless steel stabilizer, silver layer, YBCO layer, buffer and substrate. The joint between CCs was performed with low temperature InBi solder. YBCO samples with various contact conditions were fabricated and tested. Characteristics of jointed CC tape were evaluated from the V-I curve and contact resistance was derived from experimental results. Characteristics of jointed CC were investigated by electrical analysis and optical micrograph. The BSCCO wires were jointed at different conditions, and the results were compared with the joint between the YBCO CCs.

“Demonstration” Coil for MRI Applications

A high temperature superconducting (HTS) coil without turn-to-turn insulation is proposed for the field coil of a wind turbine. In the case of the field coil, epoxy impregnation is generally necessary to protect the coil from mechanical disturbances by time-varying magnetic fields and rotational vibrations to provide high mechanical integrity. This study examined the thermal and electrical stabilities of non-insulated GdBCO racetrack pancake coils impregnated with CTD-521, Stycast 2850 FT, and paraffin through cool down, over-current, and repetitive cooling tests. Among the three epoxy impregnated coils, the Stycast 2850 FT-impregnated coil exhibited the best thermal and electrical stabilities during over-current testing. In repetitive cooling conditions, the CTD-521-impregnated coil exhibited less degradation of its superconducting property due to the well-matching of the thermal contraction between the GdBCO racetrack pancake coil and the epoxy.

Analysis of a Joint Method Between Superconducting YBCO Coated Conductors

Abstract We present results of a comprehensive study, both theoretical and experimental, of an ‘electromaglev’ system, in which a high-temperature superconducting bulk sample, e.g. YBa 2 Cu 3 O 7 − δ (YBCO), is levitated stably in a DC magnetic field generated by electromagnets placed underneath the floating object. Results of the zeroth-order theory agree quite well with experimental results on lift-to-weight ratio and suspension stiffness for three bulk samples: (1) a solid YBCO disc; (2) a YBCO annulus; and (3) a YBCO annulus with a neodymium-iron-boron (Nd-Fe-B) permanent magnet disc (PMD) filling the centre. The experiment has also verified the need to satisfy two requirements to achieve stable levitation with a DC magnetic field only: (1) the spatial flow of the supercurrent in the sample must have at least two degrees of freedom; and (2) the electromagnets must generate a magnetic field profile that satisfies spatial requirements for lateral and pitch stability. A permanent magnet disc has only one degree of freedom for its Amperian current, thus it cannot be levitated stably in this system; the experiment has also demonstrated that an HTS solenoid (wound with silver-sheathed BSCCO-2223 tape) cannot be levitated stably, because the solenoid supercurrent flow is also restricted to the azimuthal direction only. The zeroth-order theory together with the Bean model shows that the supercurrent induced in a YBCO sample is independent of the critical current density, J c , of the material but is directly proportional to the axial component of the field and that the lift of the sample is directly proportional to the product of the axial and radial components of the magnetic field generated by the electromagnets.

Effects of Impregnating Materials on Thermal and Electrical Stabilities of the HTS Racetrack Pancake Coils Without Turn-to-Turn Insulation

This paper presents quench/recovery results, both experimental and simulation, of a 10-cm long YBCO test sample subjected to an over-current pulse. The sample was cooled by liquid nitrogen, either in a bath or forced through a narrow channel. Bath provides a better cooling than flow, at least for velocities of 3 and 5 cm/s used in the experiment.