Kwon-Bae Park

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.

Switching properties of a hybrid type superconducting fault current limiter using YBCO stripes

Presents investigations of a hybrid type superconducting fault current limiter (SFCL), which consists of transformers and resistive superconducting switches. The secondary windings of the transformer were separated into several electrically isolated circuits and linked inductively with each other by mutual flux, each of which has a superconducting current limiting unit of YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) stripes as a switch. Simple connection in series of the SFCL switches tends to produce ill-timed switching because of power dissipation imbalance between SFCL units. Both electrical isolation and mutual flux linkage of the switches provides a solution to power dissipation imbalance, inducing simultaneous switching by independent quenches and current redistribution of the YBCO films. This design enables increase of voltage ratings of SFCL with given YBCO stripes.

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.

Long-Term Operation and Fault Tests of a 22.9 kV Hybrid SFCL in the KEPCO Test Grid

This paper reports on the operation test of a 22.9 kV hybrid superconducting fault current limiter (SFCL) in the KEPCO test grid. The SFCL works at a rated voltage and current of 22.9 kV and 630 A, respectively. There are two major objectives of the operation test: (1) long-term operation and (2) fault tests for protection coordination study. The operation lasted for more than a year. We experienced several times of cryostat suspension due to blackouts and false alarms due to sensor failures during the operation. We also carried out short circuit tests in the test grid, equipped with circuit breakers and a recloser. An artificial fault generator was used to generate fault currents. The tests showed that the SFCL, together with the recloser, functioned reliably under repeated faults. We also confirmed the reclosing capability of the SFCL. We have presented herein the operation details, cost of operation, short-circuit test results, and our plans for further tests for protection coordination.

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.