Kijun Park

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.

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.

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.

A study of wireless power transfer topologies for 3.3 kW and 6.6 kW electric vehicle charging infrastructure

Inductive wireless power transfer (WPT) systems for electric vehicle (EV) charging applications are designed at 3.3 and 6.6 kW power classes. A comparison of compensation circuits between SS (series - series) and LCL-LCL topologies is carried out. Measurement results of 3.3 kW WPT system show that the SS topology has higher transfer efficiency than the LCL-LCL one with a peak transfer efficiency of 93.1 % and 89.5 % for SS and LCL-LCL topologies, respectively, at a transfer distance of 100 mm. The LCL-LCL topology is more robust than the SS topology in terms of power factor when a misalignment between coils occurs or when the frequency varies. Measurement results of 3.3 kW WPT system are consistent to the circuit simulation. The WPT systems operate at a frequency of 85 kHz.

Time–Frequency-Based Insulation Diagnostic Technique of High-Temperature Superconducting Cable Systems

For the electrical insulation of a high-temperature superconducting (HTS) cable, wrapped polypropylene laminated paper (PPLP) tape is typically used. Unfortunately, it is possible that unexpected faults at insulation layers will be present in the cables as a result of either a problematic manufacturing process or an incomplete installation procedure. In order to protect against operational failures of grid-connected HTS cable systems, this paper proposes a nondestructive diagnostic technique, i.e., time-frequency domain reflectometry (TFDR), and focuses on the characteristic of HTS cable that caused the local insulation defects . To verify the performance of the proposed method, detection and localization of local insulation failure via TFDR are compared with traditional time-domain reflectometry. The experiments are conducted at room temperature and under liquid nitrogen in order to check the efficacy of the proposed method in varieties of HTS cables conditions. In addition, to improve the accuracy of detection and localization, a methodology to analyze incident signals, which are composed of upchirp and downchirp signals, is presented.

Monitoring Electrical and Thermal Characteristics of HTS Cable Systems via Time–Frequency Domain Reflectometry

A high-temperature superconducting (HTS) cable system with the 22.9 kV, 50 MVA, and 410 m length is installed and operated at 154 kV Icheon substation of Korea Electric Power Corporation (KEPCO). Unfortunately, it is a difficult task to diagnose and monitor electrical and thermal characteristics of the HTS cable system in a real-time manner. In order to protect operational failures of grid-connected HTS cable systems, this paper proposes time–frequency domain reflectometry (TFDR) and analysis techniques, i.e., time-frequency cross correlation and instantaneous frequency estimation. To verify the performance of the proposed method, the temperature is changed via the cryogenic refrigeration system and the status of the grid-connected HTS cable is monitored via TFDR in a real-time manner.

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.

Analysis of Wave Propagation of HTS Cables for Compensation of Thermal Loss on Connectors

High temperature superconducting (HTS) cable operates under relatively low temperature and its unique operating condition brings new challenges in the area of maintenance and diagnostics of the cable. As an example, a temperature difference occurs on connection systems between instruments on room temperature and HTS cables which eventually leads to thermal energy loss. The thermal energy loss affects frequency characteristics of the cable which are critical aspects for simulation and maintenance of the HTS cable systems. In this paper, an attempt to reduce thermal loss is introduced with an improved connector applying a Peltier module. The performance of the designed connector is verified with analysis of electromagnetic wave propagation properties of HTS tapes and a cable. The investigation of S -parameters and the proposed connection method is expected to be further applied to various HTS systems in future.

Detection of Local Temperature Change on HTS Cables via Time-Frequency Domain Reflectometry

High temperature superconducting (HTS) cables are drawing attention as transmission and distribution cables in future grid, and related researches on HTS cables have been conducted actively. As HTS cables have come to the demonstration stage, failures of cooling systems inducing quench phenomenon of the HTS cables have become significant. Several diagnosis of the HTS cables have been developed but there are still some limitations of the experimental setup. In this paper, a non-destructive diagnostic technique for the detection of the local temperature change point is proposed. Also, a simulation model of HTS cables with a local temperature change point is suggested to verify the proposed diagnosis. The performance of the diagnosis is checked by comparative analysis between the proposed simulation results and experiment results of a real-world HTS cable. It is expected that the suggested simulation model and diagnosis will contribute to the commercialization of HTS cables in the power grid.