Il-Sung Oh

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.

Introduction of a Hybrid SFCL in KEPCO Grid and Local Points at Issue

This report presents efforts for introducing a hybrid superconducting fault current limiter (SFCL) associated with the substation upgrade in the Korea Electric Power Corporation (KEPCO) grid and points at issue on the utility demands. The substation upgrade includes replacement of 154 kV/22.9 kV main transformers and applying 22.9 kV/3 kA SFCLs to protect them. The SFCL is expected to meet not only the general requirements, but also such local conditions as (1) small size to be installed in an in-house substation, (2) sustainable current limitation without the power line breaking by circuit breakers (CB) for maximum 2 seconds. Optionally, recommended are (3) the reclosing capability and semi-active function. Different types of currently developed SFCLs have been examined associated with the conditions. We have proposed a hybrid SFCL of first peak non-limiting type, which potentially fulfills all the local conditions. As an intermediate step, a 22.9 kV/630 A hybrid SFCL was built. This SFCL meets the field requirements of the size limit, 1.5 seconds sustainability of current limitation, 0.3 s reclosing capability and semi-active function. The upgraded hybrid SFCL will be installed in the KEPCOs test line for operation tests. There are two major test targets; long term operation test of the SFCL and protection coordination test using artificial faults.

Practical Application Issues of Superconducting Fault Current Limiters for Electric Power Systems

The discovery of high-temperature superconductors were the great opportunity for developing fault current limiters using quenching phenomena. Superconducting fault current limiters is highly effective solutions to reduce and control excessive fault current which is inevitable to electric power systems. So, many companies and national research institutes were trying to develop this novel electric equipment under the assistance of national government. Due to their efforts, various types of superconducting fault current limiters were designed and tested. But, unfortunately, the practical applications and commercialization of superconducting fault current limiters are pending due to many unsolved problems. This paper discusses the current issues and commercialization problems of superconducting fault current limiters considering various aspects such as coordination with conventional relay, high voltage and high current issues, performance, cost, size, and life and maintenance issues. Then, emerging solutions for fault current limiters using hybrid method were introduced and analyzed. Finally, the viable method and our newly developed hybrid superconducting fault current limiters in order to solve the practical problems of conventional superconducting fault current limiters was briefly introduced.

Reliability Enhancement of the Fast Switch in a Hybrid Superconducting Fault Current Limiter by Using Power Electronic Switches

We have investigated reliability enhancement of the fast switch (FS) by using power electronic switches such as integrated gate commutated thyristors (IGCT) in the line commutation type hybrid superconducting fault current limiter (SFCL). The FS utilizes a vacuum interrupter (VI) to open and close the primary power line. The operation of the FS highly relies upon the complete line breaking by the VI. Since the primary line resistance including the arc resistance may not be extremely high after the VI opens, there may be non-zero arc current in the VI, causing a failure in the line communication. The IGCTs are to completely remove the remanent current in the VI, guaranteeing the arc extinction and enhancing reliability in operation. We fabricated and successfully tested the SFCL which was equipped with the IGCT-assisted FS.

Hybrid Superconducting Fault Current Limiter of the First Half Cycle Non-Limiting Type

Resistive superconducting fault current limiters (SFCLs) are assumed to be a prospective solution for commercialization. But in spite of excellent current limiting performances realized by resistive SFCLs, commercialization and installation of SFCLs have been delayed due to difficulties in overcoming several technical problems such as a coordination with conventional relays. Resistive SFCLs may have difficulty in controlling an initiation of current limiting operation that sometimes is very important for coordination with relays, because the superconductor in the SFCL will quench immediately after the fault current reaches its critical current. In order to solve these problems, a novel hybrid SFCL of the first half cycle non-limiting type was developed. This paper presents the characteristics of this hybrid SFCL, its configuration and test results.

Quench behavior of YBaCuO films for fault current limiters under magnetic field

The resistive superconducting fault current limiters (SFCLs) are very attractive devices for the electric power network. But they have some serious problems when the YBCO thin films were used for the current limiting materials due to the inhomogeneities caused by manufacturing process. When the YBCO films have some inhomogeneities, simultaneous quenches are difficult to achieve when the fault current limiting units are connected in series for increasing operating voltage ratings. In order to solve these problems, vertical magnetic fields varied from 0 to 130 mT were applied to the YBCO elements. Then, extensive electric field-current (E-I) and quench characteristics were investigated for all elements by using both electrical measuring method and observations of bubble behaviors. The experimental results were compared with the quench properties of YBCO elements, which were connected in series. From the experiment works, it was revealed that applied magnetic fields generated by surrounding coils could induce uniform quench distribution for all strips and simultaneous quenches were realized in all YBCO elements. Finally, by applying vertical magnetic fields perpendicular to the limiting devices, 1.2 kV/sub rms/ rated resistive fault current limiter were realized using five YBCO films in series.

Conduction-Cooled Brass Current Leads for a Resistive Superconducting Fault Current Limiter (SFCL) System

This paper presents the design and performance results of a pair of conduction-cooled brass current leads for a resistive superconducting fault current limiter (SFCL) system. The 24 kV class SFCL, which has been recently developed by the KEPRI-LSIS collaboration group in Korea, requires three pairs of conduction-cooled brass current leads operated continuously at 630 A. When the SFCL system is in the fault-mode, the current leads have experienced a 60-Hz fault current of ~10 kA at 24 kV for 3-5 cycles. In this paper, we present the performance results of the conduction-cooled brass (commercial brass, 10% Zn) leads having a rated current of 667 A operated in a bath of liquid nitrogen.

Optimized current path pattern of YBCO films for resistive uperconducting fault current limiters

High critical current density, high n value and fast recovery characteristics of YBCO thin films are attractive characteristics for developing resistive type fault current limiters (FCLs). In order to get desirable resistance during quench state, it is important to design optimum current path etched on the surface of YBCO films, Therefore it is needed to determine the optimum current path patterns of YBCO thin films in order to prevent the distortion caused by both localized heat caused by excessive fault currents and breakdown occurred from intensification of electric field during quench period. In this study, to clarify the optimum current path design of YBCO films, electromagnetic analysis of various current paths including meander pattern, spiral pattern, and bi-spiral pattern were considered and to verify the analysis results, experimental tests including quench test, and insulation tests were performed. From the analytical and the experimental results, it was concluded that bi-spiral pattern of YBCO thin film was rather effective for quench and insulation than the other patterns. Finally, this bi-spiral current path pattern was adopted for YBCO thin film design for developing 6.6 kV/200 A 3 phase resistive fault current limiters, and it was possible to get satisfactory results.

A Compact Cryocooling System With Subcooled Liquid Nitrogen for Small HTS Magnets

A compact cryogenic cooling system is developed for small HTS magnets to be continuously refrigerated in subcooled liquid nitrogen at temperatures below 77 K by a cryocooler. This thermal design is particularly useful when electrical insulation and compactness are significant at the same time. HTS elements are immersed in a cylindrical liquid-nitrogen vessel, and a copper band is brazed around the exterior sidewall of the cylinder at vertical location just under the liquid level in order to achieve peripherally uniform cooling with a GM cryocooler. The vapor space above liquid nitrogen in the vessel provides a fully open room for current leads and mechanical supports to the magnets. Heat is removed from the HTS magnets by natural convection of subcooled liquid nitrogen to the cold sidewall. The key components are designed based on heat transfer analysis. A prototype is fabricated, and it is successfully demonstrated that the thermal load is effectively removed and temperature is spatially uniform.

Theoretical Prediction of the Quench Behavior of a SFCL Module Having a BSCCO-2212 Bulk Coil and a Shunt Coil

This paper deals with theoretical prediction of the quench behavior for a superconducting fault current limiter (SFCL) module. The SFCL module consists of a monofllar type BSCCO-2212 bulk and a shunt coil made of copper or brass. The BSCCO-2212 bulk coil is placed inside the shunt coil, and they are connected in parallel. In order to analyse the quench behavior of the module, the equivalent circuit equation was first derived from the experimental circuit structure and then the partial differential equation was solved. The inductance values of the BSCCO bulk coil and impedance of the shunt coil are calculated by Bio-Savart and Ohms formula, respectively. We computed the voltage and current behavior during quenches using those values, and compared the results with experimental short-circuit data for the SFCL module. Both computation and test results agreed well. Therefore, we conclude that the analytic result can be applied effectively to design of a SFCL module.