Ok-Bae Hyun

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.

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.

Study on Protection Coordination Between Protective Devices in a Power Distribution System With an SFCL

As one of the countermeasures to solve the increase problem of the fault current in a power system, the superconducting fault current limiter (SFCL) has been noticed as the attractive device because it has no power loss in a normal time and can limit the fault current rapidly without additional device to detect the fault current. However, the introduction of the SFCL in a power distribution system affects the protection coordination between the existing protective devices such as the over-current relay (OCR) and the recloser, which causes the operation of the protective devices to get out of their original setting values. Therefore, the analysis on the effect of the SFCL introduction on the existing protective devices is required and the method to keep the protection coordination between the protective devices in a power system with the SFCL should be first considered. In this paper, the protection coordination experiments between the OCR and the recloser in the simulated power distribution system with the SFCL were performed. Based on the experimental results, the influence of the SFCL on the operation of these protective devices was analysed and the methods to keep the protection coordination between the OCR and the recloser in a power distribution system with the SFCL were discussed.

Shunt-assisted simultaneous quenches in series-connected resistive SFCL components

We investigated shunt-assisted quenches in resistive superconducting fault current limiter (SFCL) components based on YBCO thin films when they are connected in series. Slight differences in Ic between the components induces significant power imbalance, which causes uneven quenches between the components. The extremely fast superconductor-normal transition causes the uneven quenches. Therefore, an appropriate quench delay is needed for synchronized switching of all components. In addition to the currently practised ways, an alternative way was demonstrated for simultaneous activation of components (or units) connected in series, the shunt-assisted quench. The shunts of equal resistance across individual components are to increase the current over Ic to each of the superconducting components. This design successfully produced simultaneous quenches, resulting in equal voltages over all components, while the YBCO films were protected from excessive heating. The shunts are found to be somewhat insensitive to differences in Ic. This design provides a wide selection of shunt resistance and film uniformity, allowing practicality, particularly in engineering application.

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.

The Results of Installation and Preliminary Test of 22.9 kV, 50 MVA, 100 m Class HTS Power Cable System at KEPCO

As high temperature superconducting (HTS) power cables have some merits over conventional cables, several demonstration projects on the HTS cable system are presently under way around the world. Korea Electric Power Corporation (KEPCO) also initiated an HTS cable project in 2002 with the Korean governments support. A three phase 100 m HTS cable system with a capacity of 50 MVA has been installed at Gochang test yard, located in Chonnbuk province, Korea. The HTS cable system is composed of a 100 m-long cable, two terminations and a cooling system. The rated current is 1,250 Arms and the rated voltage is 22.9 kV considering compatibility with the conventional power distribution system in Korea. Main purposes of this project are to verify the performance of an HTS cable system and to evaluate the potential of the HTS cable system from the viewpoint of power utilities. The real grid application of the HTS cable system requires the demonstration of system reliability, accumulated operation experiences, and it has to meet the practical needs of the utilities. In such a meaning, this project provides various challenges for KEPCO, and the feedback will be delivered to cable manufacturers. This user initiative test will facilitate the introduction of HTS cable systems into a real grid network. The installation process of the HTS cable system and some results of the preliminary test were presented in this paper.

Resistance of superconducting fault current limiters based on YBa2Cu3O7 thin films after quench completion

Abstract We investigated the time dependence of the resistance of superconducting fault current limiters (SFCLs) based on YBa 2 Cu 3 O 7 thin films after quench completion. The 0.3 μm thick film was coated with a gold layer of 0.2 μm thickness and patterned into pairs of 1 mm wide and 26 cm long meander lines by photolithography. The SFCL was subjected to simulated AC fault currents of various peak values. The resistance of the SFCL consisted of an oscillatory component and a slow varying one. The slow varying component of the resistance increased exponentially with time after the initial rapid increase due to phase transition. The magnitude of the oscillatory component was small compared with the slow varying one. These results could be explained quantitatively with the concept of heat transfer within the film and to the surroundings.

Superconducting Fault Current Limiter Application for Reduction of the Transformer Inrush Current: A Decision Scheme of the Optimal Insertion Resistance

A conventional superconducting fault current limiter (SFCL) is usually only connected to a power system for fault current limitation. The study described in this paper, however, attempts to use the hybrid SFCL application to reduce the transformer inrush current. To accomplish this, this paper first suggests the concepts to expand the scope of the SFCL application in the power system. The power system operator should first determine the proper amount of current-limiting resistance (CLR) of the hybrid SFCL. Therefore, this paper suggests a decision scheme of the optimal insertion resistance in an SFCL application to reduce the transformer inrush current. This scheme and the SFCL model are implemented using the electromagnetic transient program (EMTP). We determine the optimal CLR by EMTP simulation, and this value is applied to model the SFCL by the EMTP. The simulation results show the validity and effectiveness of the suggested scheme and the ability of the SFCL to reduce the inrush current.

Initial quench development in uniform Au/Y-Ba-Cu-O thin films [superconducting fault current limiters]

The authors investigated the initial quench development process in resistive superconducting fault current limiters based on YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films of uniform quench current. The film was coated insitu with a gold layer and patterned into pairs of 1 mm wide and 26 cm long meander lines by photolithography. Voltage taps were mounted along the meander lines to detect quench development. Fabricated limiters were tested with simulated AC fault currents. Upon fault current passing quench current, all sections of the meander line made transitions into the pull flow regime simultaneously with similar flux flow resistivity. Transfer of the generated Joule heat, however, soon changed its distribution. At lower source voltages the center area of the meander line always had the highest resistivity and the edge area the lowest. At higher voltages quench started first and propagated fastest in the area close to the center electrode. But, once quench was completed, the resistivity became the highest in the central area of the meander line. This phenomenon was observed in all uniform samples on which measurements were taken and should be considered in design of fault current limiters made from uniform YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films. The heat transfer from limiter meander lines to surroundings explains the results.