Ho-Ik Du

Study on Maximum Operating Condition of Resistive Type SFCL Using YBCO Coated Conductor

The most important thing in developing a superconducting fault current limiter (SFCL) is to find the maximum operating condition for each current-limiting element to increase its SFCL capacity. The maximum operating condition can be defined using Vmax, Tr, Imax and Iq, which are detected after the quenching of the SFCL. In this study, YBCO coated conductor whose characteristics were proved as the superconducting fault current limiting element was used to test the operating characteristics of the unit current limiting element and to find the maximum operating condition using Vmax, Tr, Imax and Iq, which are detected after the quenching against the fault current according to the fault angle. YBCO coated conductors used in the test are one wire with stainless-steel stabilizing layer and one wire with no stabilizing layer. Critical current value is 70 Arms and the critical temperature is 90 K. The YBCO coated conductor with non stabilizing layer had a critical current of 80 Arms and a critical temperature of 90 K. Therefore, the two YBCO coated conductors in this study had different critical currents and an identical critical current of 90 K.

Study on Verification for the Realizing Possibility of the Fault-Current-Limiting-Type HTS Cable Using Resistance Relation With Cable Former and Superconducting Wire

In this study, we reduced the cable former (hereinafter, “copper wire”) diameter to verify the possibility of realizing the fault-current-limiting-type superconducting cable while maintaining the current-limiting ratio and the cable stability. Current application properties in the parallel connection of the copper stranded wire were analyzed with the superconducting wire with a different degree of non-resistance. Items that were verified included the change in the total current (the limited fault current), the voltage characteristics, and the results of the thermal stability evaluation through a resistance analysis. The verification targets included the combination of the superconducting wire without a stabilization layer and the copper wire (0.75 SQ and 1 SQ), the combination of the superconducting wire with a stainless stabilization layer and the copper wire (0.75 SQ and 1 SQ), and the combination of the superconducting wire with a copper stabilization layer and the copper wire (0.75 SQ and 1 SQ). Result showed that with the increasing degree of the superconducting wire non-resistance and the decreasing copper wire diameter, the limiting performance of the total current improved, but thermal stability was not stabilized. In addition, the superconducting wire volume was shown to have affected thermal stability.

Effect of the Resistance of Two Different Coated Conductor on the Current-Limiting Performance of Flux-Lock Type Superconducting Fault Current Limiters

The recent increase in power demand has been pressuring industries to continuously extend or expand power sources and transmission and transformer systems. On the other hand, the equivalent impedance of power systems is decreasing. Accordingly, the fault current magnitude in power systems is increasing. Because of such developments, and the rising need to counter this trend, current-limiting technology has been getting much attention as it can efficiently limit the short-circuit faults and improve power system reliability. For this purpose, studies are being carried out on the superconducting fault current limiter (SFCL). In particular, studies on SFCL where normal-conducting devices are combined, instead of the resistor-type SFCL that depends only on the superconductor, are being continued. The development of the element that is suitable for the superconducting fault current limiter combined with the normal-conducting device is also underway. In this study, YBCO thin-film wires that have and do not have a stainless steel stabilizer layer, which is recently studied as the superconducting current-limiting element, were used as superconducting elements of the flux-lock and transformer-shape SFCL consisting of normal-conducting core and coil. The effect of the difference between the resistance values of the two elements on the current-limiting performance of the flux-lock and trans former-shape current limiter was evaluated. For this purpose, the resistance values trend of the two superconducting elements was examined. The initial operational characteristics were compared in terms of quenching time (Iini, IImi, and Tr) and stability (Vmax).

Current Transport and Limitation Characteristics of YBCO Coated Conductor Having Different Stabilizer Connected in Series

YBCO coated conductors (CC) with high critical current density and index values are expected to be applied to superconducting power device. In particular, it is characterized by possibilities of selecting a stabilizing material surrounding YBCO CCs so that they could be applied to superconducting fault current limiters (SFCLs) and superconducting cables according to their physical properties. In this study, we conducted experiments by selecting YBCO CCs having different stabilizers of copper and stainless steel. First, we investigated the voltage-current variations with applied over-current beyond their critical currents and temperature variation generated by Joule heating. From the results, YBCO CC with copper stabilizer was more suitable for the current carrying characteristics and the CC with stainless steel showed successful current limiting properties. Finally, The 2 YBCO CCs were connected in series and electrical properties were investigated. From the results, we confirmed that the safe current carrying operation with fault current limitation could be carried out successfully.

Effect of Transport Current Properties on Connecting of YBCO Coated Conductor having Stabilizer Layer and BSCCO Tape

YBCO coated conductor called the second-generation superconducting tapes have resistance increase significantly more than BSCCO tapes in terms of the speed or amount when quench occurs, they may have different ranges of application. Such characteristics are thought to get different properties by selection of stabilizing materials in manufacturing superconducting tapes. It is important in selecting superconducting tapes which will be applied to power devices in the future. In this study, one kind of BSCCO tapes and two kinds of YBCO CC with different stabilizing materials and one kind of YBCO CC with non stabilizing materials were used to compare and examine transport characteristics in flux-flow state and quench state with each tape joint of HTS tapes.

Quench Protection of Bi-2223/Ag Tape Using SFCL Made of YBCO Thin Film

As a fundamental study for the safe operation of high temperature superconducting (HTS) machines, AC over-current characteristics of HTS tapes and the method for the protection using superconducting fault current limiters (SFCLs) were investigated. Firstly, the AC over-current characteristics of a Bi-2223/Ag tape were investigated. Various amounts of AC over-currents were applied and the quench behavior was analysed, measuring resistance increase with applying time. Secondly, based on the over-current characteristics of the HTS tapes, SFCLs made of YBCO thin films was applied to the protection of the HTS tapes for the safe operation. At this time, the quench characteristics of the HTS tape and the SFCL were considered simultaneously and each electrical operation was compared. At the transition range from super- to normal state in the HTS tape, quench behavior of both materials was investigated in detail. From the experiments, the safe operation state of HTS tapes was achieved and the optimal design factors for the SFCL applications were acquired.

Study on Voltage Rating Improvement of the YBCO Thin-Film Superconducting Wire its Application for the Power Apparatus

A superconducting fault current limiter plays the role of quickly limiting the fault current when a failure occurs in an electric power system. The SF12050 YBCO thin-film superconducting wire is used as the current-limiting element of superconducting current limiters due to its high voltage rate and fast superconducting to normal conduction phase transition speed. However, SF12050 has a thin (2000-nm-thick) stabilizing layer. Therefore, it may be damaged by thermal quenches while carrying a fault current. The stabilizing layer is the part that is responsible for the thermal burdens on YBCO thin-film wires. If such thermal burdens could be reduced by improving the structure of the stabilizing layer, superconducting current limiting elements with enhanced performance could be manufactured. Thus, in this study, the thickness of the stabilizing layer of SF12050, an Ag layer, was increased to 500 nm to improve its structure using the thermal deposition method, and to verify the increases in the thermal stability and voltage rate (increased electric capacity).

Evaluation on Current-Limiting Performance With the Fault Type of the 3-Phase Resistive Type SFCLs

Ground fault accounts for 70 to 80% of accidents in power systems. Failure to prevent ground fault at an early stage could lead to damage on other power equipment through the generation of abnormal voltages in the nominal lines. Therefore, it is very important to prevent ground fault at an early stage. This study performed the application tests of the fault currents for ground faults caused by the single-line-to-ground, double-line-to-ground, or triple-line-to-ground according to the fault types to develop the superconducting current-limiting elements and to evaluate the current-limiting capabilities. From the results, the current-limiting characteristics by the ground fault types were determined by the difference between the level of the fault current, which was applied at the fault point of R-phase 0° defined as the fault standard in the test, and the time of the quenching initiation of each phase.

Analysis of Fault Current Limiting Characteristics According to the Fault Angle in an Integrated Three-Phase Flux-Lock Type Superconducting Fault Current Limiter

The integrated three-phase flux-lock type superconducting fault current limiter (SFCL) consists of three-phase flux- lock reactor wound on an iron core with the same turns ratio between the primary coils and the secondary coils for each phase. When a SFCL is operating under normal condition, and assuming that the leakage flux is neglected, the magnetic flux generated between the primary and the secondary coils of each phase is canceled out perfectly. Therefore, the impedance of the SFCL is zero and the power system can operate normally without any loss. However, when a fault occurs, quench happens in high-TC superconducting (HTSC) elements and the magnetic flux generates in an iron core. Therefore, the impedance of the SFCL is large enough to limit fault currents. In this paper, we investigated the fault current limiting characteristics according to the fault angle in SFCL in fault types such as the single-line-to-ground fault, the double-line-to-ground fault and the triple-line-to-ground fault. Experiment results show that the fault current limiting characteristics of the SFCL are dependant on the quench characteristics of HTSC elements in each phase.

Study on Initial Current-Limiting Performance of the Resistive-Type SFCL Element According to the Electrical Coupling Condition With Cores and Coils

This paper was conducted to widen the use of the YBCO-coated conductors superconducting current-limiting element under the electrical coupling condition with cores and coils. Using cores and coils, diverse winding directions and turn ratios were available, and the initial operating performance of the superconducting current-limiting element could be improved. In this paper, three kinds of YBCO-coated conductors with stabilization layers that had different specific resistivity values were used to fabricate superconducting current-limiting elements, and an electrical coupling condition, which did not produce resistance in the normal operating condition when it was combined with the superconducting current-limiting elements, was presented. After the superconducting current-limiting elements were combined with the electrical coupling condition, an overcurrent was applied, and the initial operation performance was examined. The initial operation performance indicators were the current-limiting rate, response time, and steep slope of the initial current-limiting curve. The current-limiting rate was excellent when the specific resistivity of the superconducting current-limiting element was high and when it was combined with the electrical coupling condition to increase the equivalent impedance. However, when the specific resistivity of the YBCO-coated conductors to the superconducting current-limiting elements was too low, the electrical coupling condition did not work properly. The response time and steep slope of the current-limiting curve characteristics showed that they did not depend on the specific resistivity of the superconducting current-limiting elements when the equivalent impedance of the electrical coupling condition was higher than the resistance of the superconducting current-limiting elements.