Tae Kuk Ko

Bipolar resistive switching behavior in Ti/MnO2/Pt structure for nonvolatile memory devices

This study examined the electrical properties of Ti/MnO2/Pt devices with stable and reproducible bipolar resistive switching behavior. The dependency of the memory behavior on the cell area and operating temperature suggest that the conducting mechanism in the low resistance states is due to the locally conducting filaments formed. X-ray photoelectron spectroscopy showed that nonlattice oxygen ions form at the MnO2 surface. The mechanism of resistance switching in the system examined involves the generation and recovery of oxygen vacancies with the nonlattice oxygen ions.

Study on a Series Resistive SFCL to Improve Power System Transient Stability: Modeling, Simulation, and Experimental Verification

This paper presents a study to determine the optimal resistive value of a superconducting fault-current limiter (SFCL) for enhancing the transient stability of a power system more effectively. A resistive type of SFCL, which provides quick system protection, is modeled. Then, the optimal resistive value of the SFCL connected in series with a transmission line during a short-circuit fault is systematically determined by applying the equal-area criterion based on the power-angle curves. To verify the effectiveness of the optimal value of the proposed SFCL for reducing the value of fault current, several case studies are carried out by both simulation and experimental tests, particularly including the 220-V/300-A-scale laboratory and 13.2-kV/630-A-scale distribution system hardware tests. The results show that the optimal resistive value of the SFCL determined by the proposed method improves effectively the transient stability and damping performances during a fault over the other values determined by an ad hoc approach.

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.

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.

Analysis of a Joint Method Between Superconducting YBCO Coated Conductors

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.

Study on Optimal Location of a Resistive SFCL Applied to an Electric Power Grid

This paper describes a study on the optimal location of a resistive superconducting fault current limiter (SFCL) applied to an electric power grid. The resistive SFCL, which is designed to provide the quick system protection in the event of a fault, can have different effects on the planning and operation of a power system depending on its location. To select the optimal location of the SFCL, the sensitivity analysis of power changes and/or power losses in the system with respect to its resistive value occurred in series with a transmission line during a fault is introduced. Moreover, the optimal location determined by the proposed method is coordinated with the corresponding optimal resistive value of the SFCL to improve low-frequency oscillation damping performance of the system. The IEEE benchmarked four-machine, two-area test system is used to evaluate the effectiveness of the proposed method with the case studies based on time-domain simulation.

The effects of co-wound Kapton, stainless steel and copper, in comparison with no insulation, on the time constant and stability of GdBCO pancake coils

Recently, studies of partially insulated, high-temperature superconducting (HTS) coils have shown application in the design and construction of compact, stable and self-protecting HTS coils. This article presents the electrical characteristics of HTS coils based on the turn-to-turn inserted materials and conditions. Three partially co-wound pancake coils were fabricated and tested. Each coil was partially co-wound with Kapton, stainless steel and copper tape at every fourth turn of the winding. Tested coils were co-wound on every turn with Kapton, stainless steel and copper tape, and coils without turn-to-turn insulation were the control group. Charge–discharge, sudden-discharge and over-current tests were performed to evaluate the performance of the fabricated coils. The experimental results show that the properties of materials inserted into the coil can control the time constant (τ). Therefore, HTS coils can be designed for specific purposes according to the time constant control. The experimental results of the study could be useful in designing HTS coil applications.

Development of a 13.2 kV/630 A (8.3 MVA) High Temperature Superconducting Fault Current Limiter

This paper deals with fabrication and development of a high temperature superconducting (HTS) fault current limiter (FCL) based on YBCO coated conductor (CC) wire for distribution systems. The capacity of the developed HTS FCL is 8.3 MVA and its rated voltage is 13.2 kV which corresponds to a three-phase power equipment voltage class of 22.9 kV. Tests of the developed prototype HTS FCL were conducted at Korea Electrotechnology Research Institute (KERI) accredited as a testing laboratory by the Korea Laboratory Accreditation Scheme (KOLAS). A short-circuit test and an AC dielectric withstand voltage test for the HTS FCL were conducted under sub-cooled liquid nitrogen (LN2 ) conditions of 3 bar and 65 K. The magnitude of an asymmetric short- circuit current without FCL reached 30 kApeak in a short-circuit test. The superconducting coil quenched instantaneously after the fault, and the magnitude of the fault current was limited to 3.6 kApeak within quarter cycle by the developed resistance of the superconducting coil. An AC dielectric withstand voltage test was performed, and the HTS FCL successfully withstood 143 kV for 1 minute. Also, it was found that there was no electrical or mechanical damage on the superconducting coil after the tests.

Stability analysis of a power system with superconducting fault current limiter installed

As a process of developing high temperature superconducting fault current limiter (SFCL), the stability of a power system in which SFCLs were installed was analyzed. For the investigation into the effect of SFCLs to a power system, we have proposed a simple model power system that had SFCL circuits. The modeling parameters of SFCL are obtained by experiment of a prototype SFCL, which is 440 V class and a shielding type model. This electric circuit was solved for transient performance by numerical methods. In case the SFCLs are installed in a power system, it can effectively protect synchronization both in a symmetrical three-phase fault and a single-phase line to ground fault by maintaining synchronism of the synchronous machines for a long time. By this analysis, we found a quantitative effect of SFCLs to a power system. Limiting fault currents means not only an improvement of circuit breaker abilities but also a protection of synchronism. So its synchronism protection property must be considered for a design of superconducting fault current limiters.

Characteristics of Contactless Power Transfer for HTS Coil Based on Electromagnetic Resonance Coupling

This paper describes a operating characteristics of contactless power transfer (CPT) system from normal conducting coil to HTS coil based on the electromagnet resonance coupling. The basic principle is that two separate coils with same resonance frequency are possible to form a resonant system based on high frequency magnetic coupling and exchange energy in a high efficiency. The CPT technique with the electromagnetic resonance coupling has been expected as a useful option for contactless charge and storage devices. Since the CPT technology using normal conducting coils is sensitive to tune impedance matching, the transfer power efficiency is limited. From this reason, we proposed the combination CPT technology with superconducting receiver coils, it is called as, superconducting contactless power transfer (SUCPT). The SUCPT system can reduce joint loss of superconducting connection and increase the portability of the superconducting system. In this study, as a fundamental step, the optimal power transfer conditions needed to generate inductive power transfer between normal conducting coils and superconducting coil were experimentally examined. The power transfer profile for coupled resonance coils with high frequency power was investigated in order to minimize operating power.