Heesun Kim

Installation and Testing of SFCLs

A 22.9 kV/630 A-class superconducting fault current limiter (SFCL) was installed on a distribution line in Icheon Substation for real-grid operation. The substation is located in a semi-urban area with moderate loads. The SFCL is of hybrid type. After installation it was subjected to a series of on-site tests. Test procedures were determined by following convention in testing both superconductivity-related and not-related specifications of the SFCL. Tests performed were minimum limiting current test, temperature test, dielectric test, and impedance measurement. After successfully passing the tests, the cooling system of the SFCL was operated for more than 5 months under various load conditions to optimize the operation condition. During that period, temperatures, liquid nitrogen level, and internal pressure remained within ±0.1 K, ±0.5 cm, and ±0.5 bar range, proving stability in cooling superconducting elements. The SFCL was then energized and went into real-load operation successfully.

Demonstration of a Superconducting Fault Current Limiter in a Real Grid

A 22.9 kV superconducting fault current limiter (SFCL) has been in operation in a real grid in Korea. The SFCL is of hybrid type, in which the fault current is detected by a superconductor and bypassed by a high-speed switch to a reactor in a parallel circuit for current limitation. It has a current rating of 630 A, and has been operated in a distribution line in Icheon Substation, which is located in a suburban area with moderate loads. The SFCL has been operated very stably with no degradation in performance. Temperatures and level of the liquid nitrogen cooling the superconductors have been maintained within 0.1 K and 0.3 cm, respectively, under large daily load variation of about 100 A. Performance of the SFCL was tested by measuring minimum limitation current and impedance of the SFCL, and was proved to maintain the initial level of performance. There was a single line-to-ground fault event in the distribution line where the SFCL has been operated, and the SFCL limited the fault current successfully. Data analysis showed that the SFCL limited the current as designed. These results proved reliability and performance of the SFCL.

Dynamic Characteristics of a 22.9 kV Hybrid SFCL for Short-Circuit Test Considering a Simple Coordination of Protection System in Distribution Networks

This paper presents the results of short-circuit tests with a 22.9 kV hybrid superconducting fault current limiter (SFCL) on the KEPCO test grid. The hybrid SFCL we used for the test had suffered from lots of field tests including a long term operation and it is still in working order. We have already proved that the SFCL showed high reliability as well as feasibility through a long term operation and short-circuit tests performed before. In this paper, we tried to study a dynamic characteristics of the same SFCL on the distribution power grid through short-circuit tests with a circuit breaker and a reclosing relay. An artificial fault generator (AFG) was used to generate fault currents in the KEPCO power test center. The two objectives of the tests are (1) verifying a precise reaction of the SFCL to reclosing operation and (2) a study for protection coordination with SFCL. The test results showed that the SFCL worked precisely with a fast recovery of the superconducting elements when we applied a one-time reclosing operation with the interval of 0.6 second. We also suggested a proper manipulation of the coordination of the protection system in distribution networks with the SFCL in the paper.

Development and Grid Operation of Superconducting Fault Current Limiters in KEPCO

Development and grid operation of superconducting fault current limiters (SFCLs) have been carried out in Korea Electric Power Corporation (KEPCO), as a possible measure to handle the increasing fault current in Korea. A 22.9 kV SFCL has been successfully operated unmanned on a distribution line of Icheon Substation. It has been very stable throughout the operation of more than 1.5 year. Temperatures and level of liquid nitrogen that cools the superconducting element have been maintained constant. Performance of the SFCL maintained the initial level. The SFCL was modified so that it can limit the fault current within the first half cycle. A short-circuit test on the modified SFCL showed it started limiting the current within 2 ms. In parallel, a 154 kV SFCL has been also developed. A superconducting element was designed and fabricated. A short-circuit test was performed on a superconducting unit module, and showed that the module limited the current effectively. The element is planned to be integrated into a single-phase 154 kV SFCL together with the cooling system and other components, and tested soon.

An Effect of HTS Wire Configuration on Quench Recovery Time in a Resistive SFCL

We experimentally investigated the correlation between the high-temperature superconducting (HTS) wires configuration in an HTS element and recovery time after quench in a resistive superconducting fault current limiter. The variables of the configuration are horizontal and vertical gap distances between HTS tapes in an element. Eight samples were made with different gap distances and tested. A SUS-stabilized YBCO tape with 4.4 mm width had been used in the experiment. It was cooled by LN2 in a cryostat under the pressure of 1 bar, saturated state. In the short-circuit test, the temperature of the wires surface was measured. Recovery time of the HTS sample increased with decreasing horizontal and vertical gap distance due to stagnation of bubbles. When the gap distance was larger than a size of a bubble, the effect of gap distance was ignorable. Considering a volume and recovery time, the sample that has narrower gap distance was favorable.

Design of Post Metal Shields Through Electric Field Distribution Analysis for a 154-kV SFCL

Korea Electric Power Corporation has developed a 154-kV superconducting fault current limiter (SFCL). This report is a part of the design process of the SFCL, particularly for fixation of posts supporting the superconducting element on the cryostat wall side. For supporting the superconducting element, the use of a post insulator is inevitable; however, the post insulator and cryostat with liquid nitrogen (L-N2) during operation of the SFCL form three junction points where electric field is intensified. In this study, we aim to design the metal shield in order to relax the electric field intensity at triple points (TPs) through numerical analysis of electric field distribution. For the electric field distribution analysis, a commercial software based on the finite-element method was employed. Each design for the metal shield was checked whether it makes the electric field intensity at the TP sufficiently lower than dielectric strength in L-N2 for 750-kV input and whether there is any electrically weak point on the metal. The designs of the metal shields were improved through four critical steps where thermal contraction, manufacture tolerance, and insulation distance in L-N2 were considered. It was experimentally verified that there was no electric breakdown in L-N2 between the metal shield and the fiber-reinforced plastic post insulator for the lightning impulse test and the ac breakdown voltage test according to the IEC 60137 standard.

Development and Test of a Cooling System for a 154 kV Superconducting Fault Current Limiter

The superconducting fault current limiter (SFCL) is an electric power device that limits the fault current immediately in a power grid. Korea Electric Power Corporation (KEPCO) has been developing a 154 kV, 2 kA SFCL since 2011 to protect power grids from increasing fault current and improve the stability and quality of electric power. This SFCL adopts 2G YBCO wires and operates at 71 K and 5 bars. In this paper, a cooling system for the 154 kV SFCL and its cooling test results are reported. This cooling system uses a Stirling-type cooler to make sub-cooled liquid nitrogen (LN2), which cools the superconductor modules of the SFCL. The LN2 is circulated between the cooler and the cryostat that contains superconductor modules. The LN2 also plays the role of a high voltage insulator between the modules and the cryostat, so the pressure was maintained at 5 bars for high insulation performance. After installation in a test site, the cooling characteristics of the system were tested. In this operation test, some important data were measured such as temperature distribution in LN2, pressure change, performance of the heat exchanger, and cooling capacity of the total system. Consequently, the results indicate that the cooling system operates well as designed.

Performance test of the cryogenic cooling system for the superconducting fault current limiter

A Superconducting Fault Current Limiter is an electric power device which limits the fault current immediately in a power grid. The SFCL must be cooled to below the critical temperature of high temperature superconductor modules. In general, they are submerged in sub-cooled liquid nitrogen for their stable thermal characteristics. To cool and maintain the target temperature and pressure of the sub-cooled liquid nitrogen, the cryogenic cooling system should be designed well with a cryocooler and coolant circulation devices. The pressure of the cryostat for the SFCL should be pressurized to suppress the generation of nitrogen bubbles in quench mode of the SFCL. In this study, we tested the performance of the cooling system for the prototype 154 kV SFCL, which consist of a Stirling cryocooler, a subcooling cryostat, a pressure builder and a main cryostat for the SFCL module, to verify the design of the cooling system and the electric performance of the SFCL. The normal operation condition of the main cryostat is 71 K and 500 kPa. This paper presents tests results of the overall cooling system.