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Performance of a resistive superconducting fault current limiter (SFCL) component is usually limited by temperature rise associated with energy input by fault current application during a fault. Therefore, it is expected that short application of the fault current may enhance the power ratings of the component. This can be accomplished by a combination of a HTS component and a mechanical switch. The fast switch (FS) developed recently enables the fault duration to be as short as 1/2 cycle after a fault. Various second-generation (2G) high temperature superconductors (HTS) and YBCO thin films have been tested. The relation between the rated voltage V and the fault duration time t was found to be V 2 ∼ t − 1 . Based upon the relation, we predict that when the FS break the fault current within 1/2 cycle after a fault, the amount of HTS components required to build an SFCL can be reduced by as much as about 60%, of that when breaking the fault current at three cycles.

We have constructed and completed the preparation for a long-term operation test of a superconducting fault current limiter (SFCL) in a Korea Electric Power Corporation (KEPCO) test grid. The SFCL with rating of 22.9 kV/630 A, 3-phases, has been connected to the 22.9 kV test grid equipped with reclosers and other protection devices in Gochang Power Testing Center of KEPCO. The main goals of the test are the verification of SFCL performance and protection coordination studies. A line-commutation type SFCL was fabricated and installed for this project, and the superconducting components were cooled by a cryo-cooler to 77 K in the sub-cooled liquid nitrogen pressurized by 3 bar of helium gas. The verification test includes un-manned – long-term operation with and without loads and fault tests. Since the test site is 170 km away from the laboratory, we will adopt the un-manned operation with real-time remote monitoring and controlling using high speed internet. For the fault tests, we will apply fault currents up to around 8 kArms to the SFCL using an artificial fault generator. The fault tests may allow us not only to confirm the current limiting capability of the SFCL, but also to adjust the SFCL – recloser coordination such as resetting over-current relay parameters. This paper describes the construction of the testing facilities and discusses the plans for the verification tests.

The stabilizer of a YBCO coated conductor (CC) plays a very important role in bypassing the over-current and transferring the heat generated at the moment of a fault. Therefore, one of big issues is to determine the best material for the stabilizer and its dimensions for high performance in a HTS power application system. In the case of a superconducting fault current limiter (SFCL), which requires it to react immediately to any fault, the characteristics of the stabilizer are decisive in limiting the fault current and recovering the superconducting properties during and after quenching. This study examined the electrical and thermal behavior of YBCO CCs with various stabilizer thicknesses in fault mode to determine the effects of the stabilizer thickness on the quench/recovery characteristics of YBCO CCs. In particular, the electric field intensity (E) of YBCO CCs in fault mode were calculated and compared with the experimental results.

In the first-peak non-limiting type superconductor-triggered fault current limiter (STFCL), which was developed by the collaborative work of KEPRI and LSIS, the first half cycle of fault currents is passed, unlimited. Due to these characteristics, information needed for the fault judgments can be provided to the protection relays, which is useful for the protective coordination of power system. In this fault current limiter, superconductors are used as a fault detector that does not generate resistive loss in normal operation state. Therefore, high J c and fast occurrence of large resistance are the essential properties of superconductors for the application to fault detectors. In this study, for the design of superconducting fault detector (SFD), we investigated the quench behavior of two kinds of superconductors, stabilizer-free coated conductors (SF-CC) and Au/YBCO/sapphire substrate, during the operation of the first-peak non-limiting type STFCL. The dimension of SF-CC was 12 mm wide × 1000 mm long and that of Au/YBCO/sapphire was 20 mm wide × 100 mm long. Their critical current ( I c ) values were 250 A and 200 A, respectively, and the critical temperature ( T c ) was both 90 K for both superconductors. When fault currents of over 3 kA were applied, the STFCL carried out the current limitation successfully. At the beginning of the quench, SF-CC generated a resistance of 150 mΩ, and 394 mΩ was generated in the Au/YBCO/sapphire substrate, when 150 V rms was applied. After the first half cycle, the resistance of the superconductors increased up to 263 mΩ, and 846 mΩ, respectively, and the temperature reached 132 K and 127 K. In addition, we also investigated the recovery characteristics of the SFDs. The SF-CC and Au/YBCO/sapphire substrate recovered the superconductivity by 353 ms and 357 ms, respectively. Based on the results of the analysis, we determined the required length of the superconductors for the SFD application.

Resistive superconducting fault current limiters (SFCL's) are assumed to be a prospective solution for commercialization. But in spite of excellent current limiting performances realized by resistive SFCL's, commercialization and installation of SFCL's have been delayed due to difficulties in overcoming several technical problems such as a coordination with conventional relays. Resistive SFCL's 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.

We have designed and tested a superconducting fault detector (SFD) for a 22.9 kV superconductor triggered fault current limiters (STFCLs) using Au/YBCO thin films. The SFD is to detect a fault and commutate the current from the primary path to the secondary path of the STFCL. First, quench characteristics of the Au/YBCO thin films were investigated for various faults having different fault duration. The rated voltage of the Au/YBCO thin films was determined from the results, considering the stability of the Au/YBCO elements. Second, the recovery time to superconductivity after quench was measured in each fault case. In addition, the dependence of the recovery characteristics on numbers and dimension of Au/YBCO elements were investigated. Based on the results, a SFD was designed, fabricated and tested. The SFD successfully detected a fault current and carried out the line commutation. Its recovery time was confirmed to be less than 0.5 s, satisfying the reclosing scheme in the Korea Electric Power Corporation (KEPCO)’s power grid.

Although, a superconducting fault current limiter (SFCL) guarantees the fast limiting operation, it usually needs a considerably long time to recover to superconducting state after the quench. Considering the reclosing time in the protection coordination of power systems, the time required for the recovery should be investigated clearly. In this study, the quench and recovery characteristics of Au/YBCO thin films designed as an SFCL element with a bi-spiral pattern were investigated. The quench development of the SFCL was measured by two kinds of methods. Firstly, after applying the fault current of 5.5 cycles, we measured the resistance of the YBCO by a small current flowing through the pattern of Au/YBCO thin film. The temperature variation above the critical temperature, 85 K, was investigated indirectly from the resistance variation. Secondly, in order to measure the temperature from 85 K to 77 K, a meander line shape of Au thin film was evaporated on the back side and used as a temperature detecting sensor. The temperature variations detected by both methods were compared and analyzed. For the investigation of the recovery characteristics, the required time for the recovery of the superconductivity was measured for various magnitude and duration of the applied voltages. In addition, for the purpose of examining the dependence of the line impedance on the recovery time, resistors of various resistances were inserted in the fault current testing circuit and the recovery time was measured and analyzed.

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 government's 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.

We have been developing a magnetic field measurement system with two-axis hall-probes to study the critical current density distribution or magnetic properties of high- T c superconductors. In this study, the current distributions at corners of the meander type resistive fault current limiters were measured by hall probe with high spatial resolution. The both of axial and radial magnetic field components ( B x and B z ) according to the direct (DC) and alternating (AC) transport currents were measured by hall probe to estimate the current density distribution with high accuracy. And, it is confirmed that the measured radial components are very useful to estimate the current distribution.

Based on the electrical behavior of Bi-2223/Ag tape in transient state, the possibility of SFCL application was investigated as a fundamental study for the protection of HTS power apparatus. For the experiments, the Bi-2223/Ag tape having the critical current of 56 A and the resistive type SFCL fabricated using three elements of YBCO thin films were examined. When the applied current exceeds the rated current of the SFCL, the current was limited to sufficiently low level, however, higher than 40 Vrms was not allowed considering power density of SFCL. On the other hand, as a method to increase the power density, a shunt resister across the SFCL was adopted to increase the allowed power density. In this case, the results showed that the over current was limited below the safe range for HTS tape and the SFCL was safe, although 100 Vrms was applied to the circuit. In conclusion, the switching power capacity of a SFCL could be increased by using a shunt resistor, and HTS tape could be protected safely within the quench margin

Korea's superconductivity technology in the power sector has progressed rapidly since the launch of the 21st Century Frontier R&D Program for superconductivity. Under this program, large-scale active research has been conducted. The program supports HTS cable, motors, transformers, fault current limiters, and basic materials for 2G wire development. Current technology is somewhat less advanced compared to developed countries such as the USA, Japan and Europe. It is expected that the efforts and strong motivation of the Koreans will soon make the country competitive with advanced nations. In addition, the power industry has also begun to support the research and development of superconducting power technology via ETEP. Including such support, we review the recent progress of applied superconductivity research and development in Korea, mainly in the power sector.

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.

We investigated the quench characteristics of resistive superconducting fault current limiters (SFCLs) based on YBa2Cu3O7 (YBCO) films. The SFCL consists of meander-type YBCO stripes covered with 200 nm Au layer grown in situ for current shunt and heat dispersion at hot spots. The SFCL quenched in a 100 Vrms circuit to produce resistance of 29 Ω at 55 ms. This resistance indicates the gold film temperature of about 200 K. The SFCL showed reproducible characteristics during more than a hundred times of repeated experiments. Two nearly identical SFCLs were connected in series to investigate simultaneous quench. A slight difference in the rate of voltage increase between two SFCL units produced significantly imbalanced power dissipation between units. The imbalance of power was matched by a shunt resistance connected in parallel. The appropriate values of shunt resistance here were 80 Ω at 75 Vrms, 100 Ω at 100 Vrms and 110 Ω at 120 Vrms, respectively. Increased power input at high voltages also tends to balance the power dissipation, but with increasing in film temperature to higher than 200 K.

High-J/sub c/ YBa/sub 2/Cu/sub 3/O/sub y/ thick films were prepared by the liquid phase epitaxy (LPE) method. The growth rate was typically about 2 /spl mu/m/min, which was 10-10/sup 2/ times larger than that by conventional vapor growth techniques. The T/sub c/ of the best film exceeded 88 K after oxygen annealing, and the transport zero-field J/sub c/ was over 10/sup 5/ A/cm/sup 2/ at 77 K. The peak effect in J/sub c/-B characteristics was observed at several tesla by magnetic and transport measurements. Based on the micro-structure, the relevant peak effect is considered to be caused by stacking faults which act as a field induced pinning center. >

The superconducting free-energy surface of grain-aligned Nd{sub 1.85}Ce{sub 0.15}CuO{sub 4{minus}{ital y}} was measured to determine whether these electron-carrier materials have thermodynamic variables similar to the other high-temperature superconductors. There is a wide area in the {ital H}-{ital T} plane near {ital T}{sub {ital c}}, where the magnetization is reversible, just as was found for the Y, Bi, and Tl cuprates. The magnetic-field dependence of the specific heat derived from these data, {ital C}{sub 0}{minus}{ital C}{sub {ital H}}, shows a peak in the fluctuation region near {Tc} of about 1.2 mJ/cm{sup 3} K for {ital H}=2 T and {bold H}{parallel}{bold c}. At lower temperatures, below 16 K, the magnetically derived specific heat is nearly independent of field just as was found for the other high-{Tc} materials. A study of the transformation from the reversible to the irreversible regime using flux-creep and critical-current studies shows that the pinning potential to thermal energy ratio, {ital U}{sub eff}/{ital kT}, gradually rises from 2 to 20 at roughly the same value of reduced temperature, {ital T}/{ital T}{sub {ital c}}, as the Bi and Tl compounds.

A study of critical currents, flux creep and vortex line energy was undertaken for a variety of materials to determine some of the factors that control the intragranular critical current. Creep measurements for Nd 1.85 Ce 0.15 CuO 4 and TI 2 Ba 2 Ca 2 Cu 3 O 10 , plotted as a function of reduced temperature, t = T / T c , show essentially the same behaviour. This implies that the Nd compound, which has a much longer coherence distance and may have many electron carriers, has the same effective pinning potential to kT ratio at the same reduced temperature. Hence these two factors do not seem to be dominant in controlling J c . A change in the vortex line energy, however, seems to be very important. With the addition of small amounts of Fe, the line energy and J c both fall exponentially with Fe content in Y 1 Ba 2 (Cu 1− x , Fe x ) 3 O 7−σ . Hence the free energy per unit length of vortex is a very important factor controlling the intragranular critical currents.

Experimental aspects of flux expulsion in ${\mathrm{Nb}}_{3}$Sn and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ type-II superconductors are presented. There is a clear distinction in magnetization between field-cooled measured-upon-cooling (FCC) and field-cooled measured-upon-warming (FCW) results. This thermal hysteresis, predicted in the temperature-dependent critical-state model at low fields by Clem and Hao, was observed for measuring fields up to about 0.5 T. The model explains the observation of increases in diamagnetism after field cooling and thermal cycling. The thermal hysteresis, together with weak links, accounts for the occurrence of a negative peak in FCW magnetization. The FCC-FCW bifurcation observed for a 0.1 mT field down to 5 K might imply that flux lines are not completely frozen below ${\mathit{T}}_{\mathit{c}1}$, the temperature at which the lower critical field is equal to the measuring field, but are expelled from the sample even in the Meissner state.

Linearly polarized synchrotron radiation is used to study the structure of magneticallyaligned layered copper oxide high temperature superconductors (HTSC). We report polarizedextended x-ray absorption fine structure (EXAFS) measurements of oriented powders ofYBa2Cu3O69 at temperatures ranging from 20-300 K. Measurements made at the Cu K-edge with the x-ray polarization vector oriented perpendicular and parallel to the c axis can be used toeffectively probe the structure around the Cu-atoms in the ab (horizontal layer) plane as well as inthe c (vertical layer) plane. 1. INTRODUCTION Since the discovery of layered copper oxide as HTSC's [1] the structure and compositionof La2CuO4, Nd2CuO4, YBa2Cu3O69 and have been extensively studied via XRD [2], neutron diffraction[3] and EXAFS [4,5]. The first two techniques give more accurate results for long range order, while the EXAFS probes the local structure around the absorbing atom. Moreover due to the linearly polarized nature of the synchrotron radiation, the EXAFS will show anorientation dependence for an anisotropic sample, thereby yielding a determination of the

In general, conventional high-Tc superconducting fault current limiters (SFCLs) are operated by cooling systems with a liquid cryogen, such as liquid nitrogen (LN2). However, in the fault mode, LN2 evaporates because of joule heating in the SFCL module, so the SFCL system experiences an enormous increase in nitrogen gas volume. In this case, the thermal stability and protection of the system become the primary concerns for the design of the SFCL cooling system. In order to enhance the thermal stability and safety of the system, an SFCL cooled by solid nitrogen (SN2) as a large heat capacitor has been proposed as an alternative. In this paper we report the quench/recovery characteristics of a YBCO-coated conductor (CC) evaluated in an SN2 cooling system for the SFCL. A feasibility study is also conducted on the reference design codes and thermal requirements for the optimal design of the SN2 cooling system. The results demonstrate that the improved thermal contact obtained between the SN2 cooling system and the SFCL module renders the proposed system a suitable cryogen for the SFCL module. Detailed experimental results for the LN2 and SN2 cooling systems are presented and discussed.

Procedures have been developed to make Nd1.85Ce0.15CuO4 powders which have thec-axis of all the grains aligned in one direction. By rotating a powder sample mixed with liquid epoxy about an axis perpendicular to a strong applied magnetic field, thec-axis of the grains aligns with the axis of rotation. The procedure works for any material having the maximum room-temperature susceptibility perpendicular to thec-axis. Using this technique, we measured the magnetic anisotropy of this electron-carrier high-temperature superconductor.