Jungwook Sim

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.

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.

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.

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.

Development of a HTS squirrel cage induction motor with HTS rotor bars

Efficiency at full load and starting torque at starting are basic performance criteria of the induction motor. Conventionally deep bar or double cage bar have been used to increase the efficiency and starting torque of the induction motor. Effective resistance of the deep bar and double cage differs from starting to full load operation. To increase the efficiency and starting torque, HTS tapes can be used as the rotor bars. The resistance of HTS tapes develops at starting because of large starting current which exceeds the critical current of the HTS tape and it vanishes as the speed builds up. This paper presents the construction and test results of a HTS induction motor. End rings and short bars were made of HTS tapes. Stator of the conventional induction motor was used as the stator of the HTS motor. Rated capacity of the conventional motor was 0.75 kW. Performances of the HTS induction motor were compared with those of the conventional motor with same volume and specification. Test result showed that the speeds of the HTS induction motor were the same with synchronous speed up to 2.6 Nm and 1788 rpm at 9.7 Nm. It guarantees the high efficiency of the HTS motor. Starting torque of the HTS motor was more than twice of the conventional motor.

Test of an induction motor with HTS wire at end ring and bars

Motors with HTS wires or bulks have been developed recently. These are a large synchronous motor with HTS wires at the field winding in the rotor, hysteresis and reluctance motors with HTS bulk in the rotor. This paper presents the fabrication and test results of an HTS induction motor. Conventional end rings and short bars were replaced with HTS wires in the motor. Stator of the conventional induction motor was used as the stator of the HTS motor. Rated capacity and rpm at full rotor of the conventional motor were 0.75 kW and 1710 rpm. Two HTS wires are used in parallel to make the end rings and bars. The critical current of the BSCCO-2223 HTS wire which was used in the bars and end rings were 115 A. An electrodynamometer was coupled directly to the shaft of the rotor with HTS wires.

The application of fault current limiter at Icheon substation in Korea

A 25.8 kV / 630 A hybrid superconducting fault current limiter (SFCL) has successfully designed, tested and finally installed in a real power grid at Icheon substation for the purpose of pre-commercial demonstrations. The hybrid SFCL is composed of high temperature superconductor (HTS), fast switch (FS), and current limiting reactor (CLR) module. In this hybrid SFCL, the HTS module plays a role in sensing fault current and letting the direction of an electric current turn to the FS path. A reduction in amplitude of fault current is done by CLR module. The combination of HTS and FS module made of conventional electrical equipment enables the volume and the cost of total system to be reduced significantly, and the operating reliability to be improved. A short circuit test (12.5 kArms) was conducted and we were able to verify reliable fault current limiting capability of hybrid SFCL. After passing all the required tests, including the impedance test and the electrical insulation test, the hybrid SFCL was installed at Icheon substation in 2010 and now it is waiting for connection to Jang-pyong distribution line (D/L). Recently, a stable operation of HTS cryocooling system was clearly proven through applying load current modeled on the worst fluctuating load among real daily load profiles of Jang-pyong D/L.

Emergency Blackout Operation of Cryogenic System for Hybrid SFCL

Operation scheme of cryogenic system is investigated in preparation for temporary loss of cryocooler power in hybrid SFCL. Under normal operation, HTS trigger elements are cooled in liquid nitrogen pool at 77 K and 300 kPa by a GM cryocooler. A step-by-step procedure is devised to maintain the HTS elements at or below 78 K during blackout and recover period. On detecting the onset of temperature rise followed by power outage, a vent valve is opened to atmosphere so that thermal load can be covered by evaporation of liquid nitrogen at 77.4 K. When the cooling power is restored, liquid is pressurized by helium gas back to 300 kPa. The instrumentation and control parts needed for unmanned operation are designed and installed on a prototype of 22.9 kV/ 630 A SFCL. The emergency operation is successfully rehearsed on scheduled outage of 10 minutes, 1 hour, and 4 hours, respectively.

Two-Stage Cryocooling Design for Hybrid Superconducting Fault Current Limiter

New concept of two-stage cryogenic cooling system for full-scale hybrid superconducting fault current limiter (SFCL) is proposed and designed with an objective to significantly reduce the capital and operational costs. Even though we have developed and successfully tested a cryocooling system for 300 A level, it is not easy to scale up the same technology to 3¿/22.9 kV/2.5 kA level, mainly because of the heavy cost of GM or Stirling cycle coolers to cover the increased thermal load. It is proposed in this study to employ a large capacity of JT cooler for heat intercept at an intermediate location of current leads. The JT coolers with mixed refrigerant are very reliable and commercially available at an inexpensive price. Detailed cryogenic design of cryostat and heat intercept components is carried out in consideration of thermal characteristics of cryocoolers and electrical insulation. It may be concluded that the two-stage cooling with a JT cooler at 130 K and a GM cooler at 77 K is feasible in practice, and can significantly reduce the capital cost and power consumption.