Hyun Chul Jo

A Study on the No Insulation Winding Method of the HTS Coil

This paper reports a study on BSCCO and ReBCO HTS (high temperature superconducting) test coils, layer-wound and double-pancake, with and without turn-to-turn insulation. Over current tests were performed in a bath of liquid nitrogen to compare stabilities of the test coils at 77 K. Saturation of magnetic fields from the NI (no insulation) coils, both BSCCO and ReBCO, was observed owing to current bypassing through turn-to-turn contacts in local quenches at operating currents higher than their critical currents. In the NI ReBCO coils, global quenches occurred at operating currents higher than the coil critical currents and quench recoveries were observed during discharge of the coils. The experimental results, obtained to date, demonstrate that the NI winding method enables remarkable improvement of thermal stability of the HTS coils.

Experimental Analysis of a Splice Method Between YBCO Coated Conductors on Various Bending Diameters

In recent years, the YBCO coated conductor (CC) has been developed as a suitable material for commercial superconducting apparatuses. In general, YBCO CC in superconducting apparatuses is wound into a coil. In order to fabricate a superconducting magnet of a large capacity, the splice between the CC tapes is inevitable because of the length and the shape of the CC tapes. In the previous researches, we have developed a splice method for short YBCO CC tape. However, the splice in a coil or between coils has certain bending diameters. Thus, we have developed a soldering machine capable of soldering on various bending diameters in this research. The splices varying with pressures, a temperature profile, a solder and bending diameters between YBCO CC tapes were conducted. The characteristics of each splice method were measured from V - I curve and a suitable splice method for superconducting magnets was developed.

Numerical Analysis and Design of Damper Layer for MW-Class HTS Synchronous Wind Turbine Generator

This paper presents a numerical analysis on MW-class high-temperature superconducting (HTS) synchronous generator, especially focusing on damper layer design. Our research program to develop a large-scale HTS wind turbine generator (WTG) core technology, a two-dimensional electromagnetic FEM model of the direct-drive HTS WTG, comprised of 24 air-core type HTS race track coils (24 poles), has been built. The damper is used for stable operation of HTS field coil against varying magnetic field by change of rotation speed. This model is designed to obtain basic operating parameters for HTS WTG, including magnetic field distribution and induced electromotive force, in static and transient condition. First, the parameters in static condition, which means that the rotating speed is constant, are compared with our design parameters to confirm the feasibility of our numerical analysis. Second, in transient condition, we focused on the reaction between rotating magnetic field and its stator components, damper, and armature winding. By changing damper material having different electrical conductivity and magnetic permeability, we concluded our damper design to be applicable to our MW-class HTS synchronous generator model.

Evaluations on AC Electrical Characteristics of No-Insulation Coil for Power Devices

This paper deals with the ac electrical characteristics of high temperature superconducting pancake coils without turn-to-turn insulation (no-insulation coils). Several research groups have done studies of the electrical characteristics of no-insulation coils. However, most of the studies have been done with respect to the dc electrical characteristics, and studies with respect to the ac electrical characteristics are rare. In this paper, we evaluated the ac electrical characteristics, such as the fault current, recovery time, and ac loss of the no-insulation coil. The results can be expected to suggest useful data for the application of no-insulation coils in the area of power devices.

An Experimental Study on the Joint Methods Between Double Pancake Coils Using YBCO Coated Conductors

In these days, some researches on the HTS insert coil/LTS background coil (hybrid coil) for nuclear magnetic resonance (NMR) system have been conducted. Magnetic field homogeneity is very important in NMR system. Because the direction of the joint part between a stack of double pancake coils (DPCs) can cause magnetic field homogeneity problem, the joint methods between the DPCs are important. YBCO coated conductor tapes are manufactured in various widths.They can be used as an auxiliary superconducting tape at the joint part between the DPCs for enhancing magnetic field homogeneity. The joint characteristics between the DPCs can vary with the conditions of the auxiliary tape such as numbers, widths, and directions. In this work, we conducted joint experiments under various conditions of the auxiliary tape and analysed the joint characteristics. The joint methods in this work are expected to be used to fabricate an insert coil in NMR system.

Design, Fabrication, and Operating Test of the Prototype HTS Electromagnet for EMS-Based Maglev

The prototype high-Tc superconducting electromagnet (HTS-EM) for high-speed EMS Maglev was successfully demonstrated in EMS system up to the vehicle running speed of 500 km/h. The EM was designed to interface with the existing propulsion system used in the German high-speed Maglev. The rating levitation force of the EM was designed to correspond to the weight of the EM at a 20 mm levitation gap. Electromagnetic forces were simulated and compared with the test results in order to verify the design feasibility of the HTS-EM. Operating tests revealed that the HTS-EM was fully operational in zero-power control mode. However, the operating current of the HTS coil was affected by the control current when the guideway disturbance occurred. The design feasibility and operability of the HTS-EM was verified in the study.

Characteristic Comparison for the Various Winding Methods of HTS Magnets

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.

Operating Characteristics of Oval-Shaped Resistive Superconducting Fault Current Limiter

In order to limit fault current from an accident in power grid, many research groups have developed resistive superconducting fault current limiters (SFCLs) with a bifilar solenoid and a bifilar pancake types. Comparing with a bifilar solenoid type, a bifilar pancake type has the advantage of its volume, but it has problems such as recovery and insulation of SFCLs. To eliminate the drawback of these two types, in this paper, a new type of coil that is an oval-shaped SFCL, have proposed. This type consists of inner and outer layers which are connected in parallel. A small-scale sample coil was fabricated and experiments for both a short circuit test and recovery characteristic test were performed in liquid nitrogen. Experimental results were compared with two types of coils; the bifilar solenoid and bifilar pancake coil. Consequently, we confirmed the feasibility of an oval-shaped type.

A Study on the Non-Inductive Coils for Hybrid Fault Current Limiter Using Experiment and Numerical Analysis

A Hybrid fault current limiter (FCL) proposed by our group previously is composed of a superconducting coil, a fast switch and a bypass reactor. The superconducting coil wound with two kinds of HTS wire has zero impedance when normal current flows in the coil. However, different quench characteristics of the HTS wire generate magnetic flux in the coil when fault current flows in the coil. As a result, the fast switch was opened by the repulsive force applied to the aluminum plate above the coil. In previous studies, our group verified operating characteristics and feasibility of the fast switch. In this paper, comparison of pancake type and solenoid type non-inductive coil wound with two kinds of the HTS wire was performed by using short-circuit test and finite element method. From these results, short-circuit characteristic of a coil can be acquired and magnitude of the repulsive force and magnetic field can be analysed.

A Study on the Optimization of an HTS Quadrupole Magnet System for a Heavy Ion Accelerator Through Evolution Strategy

This paper attempted to use a high-temperature superconductor (HTS) quadrupole magnet in a heavy ion accelerator. In such application, HTS quadrupole magnets are superior to conventional superconducting magnets because their heat loads can be removed more efficiently. Generally, the magnetic field quality of quadrupole magnets is assessed through harmonic analysis. In quadrupole magnets, the second (four-pole) component is the main magnetic field, and the field inhomogeneity is composed of the sixth (12-pole) component, the tenth (20-pole) component, and so on because of its symmetry. Therefore, the harmonic contents in a good field region can be defined to express the field quality of a quadrupole magnet by using higher order components (order > 2) with respect to the second component. In conventional analysis, multipole components can be obtained by a Fourier series of the radial field component. This paper analyzed the difference between radial and azimuthal multipole components. The HTS quadrupole magnet is wound with a rectangular tape and requires specific structures for refrigeration, which increases the harmonics. In this paper, the effects on the harmonic contents in relation to the HTS coil position and the shape of the iron pole were analyzed based on the HTS quadrupole magnet of the RAON system for the Rare Isotope Science Project in Korea. Several parameters were selected in designing the HTS quadrupole magnet system by using the evolution strategy method. Finally, all harmonic contents were optimized, and an HTS quadrupole magnet design was suggested.