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Due to the existence of AC loss in superconducting materials when an alternating voltage is applied, high cryogenic costs are inevitable to operate superconducting devices in AC networks. Therefore applications of superconducting devices in DC electric power networks could be regarded as the optimum choice for superconducting devices, because superconductors show exactly zero resistance to a DC source. Recently, DC superconducting devices such as DC cables have received noticeable attention as DC power transmission lines. In order to develop DC superconducting devices, the DC insulation characteristics in cryogenic liquids should be clarified. However, up to now, limited research has been reported in this field. In this paper, to clarify the different breakdown characteristics of DC and AC applications, various kinds of cryogenic dielectric sheets including Kraft, Kapton (polyimide) and Nomex (polyamide) papers have been prepared. Furthermore, a penetrating breakdown test for three kinds of sheets and turn-to-turn breakdown tests have been performed in liquid nitrogen (LN2). Consequently, it was found that the prepared sheets have shown 1.7–2.7 times higher dielectric strength than those of AC. Moreover, the Nomex and Kraft sheets have shown a remarkable increase in their dielectric strength in liquid nitrogen compared to air. However, the dielectric strength of the Kapton sheet did not show a remarkable increase in liquid nitrogen. From the turn-to-turn breakdown test, it was proved that the dielectric strength has been linearly increased according to the wrapping number of the sheets and that the DC breakdown voltage was 1.1–2.5 times higher than AC. [ABSTRACT FROM PUBLISHER]

High temperature superconducting devices utilize liquid nitrogen (LN2) both for cryogenic liquids and insulating media. Regarding the insulating performance of LN2, it shows excellent characteristics comparable to insulating oil which is widely used for conventional electric equipment. But, detailed complex insulating structure in LN2 composed of cryogenic liquids, solid insulating material and conductor material has not been fully considered yet. In order to commercialize superconducting devices, insulating characteristics of LN2 should be analysed and confirmed to the level of insulating oil. In this work, we focused on the breakdown characteristics of LN2 especially for different electrode materials. Aluminum, Stainless steel, and Copper electrode systems were fabricated and tested in LN2 to verify uniform and non-uniform characteristics of LN2 according to the electrode materials. AC withstand voltage test and lightning impulse test has been performed. From the test results, it was revealed that the breakdown voltages were significantly changed according to the electrode materials. Especially for Stainless steel, it shows prominent enhancement of breakdown voltage compared to copper and aluminum electrode in LN2. Consequently, it was deduced that optimum choice of conductor materials in LN2 is another critical factor to determine complex insulating performance of superconducting devices. [ABSTRACT FROM PUBLISHER]

Superconducting fault current limiter (SFCL) is a promising novel electric equipment to reduce excessive fault current in electric power systems effectively. However, in practical application of this novel device into electric networks, SFCL could cause some unfavorable impacts on the electric power system. Especially, coordination between the SFCL and the recloser which has a conventional protection scheme is a major concern because it could cause severe reliability and stability problems in power systems. Several important characteristics of SFCL, which affects on a recloser operation scheme, are quenching phenomena, recovery from the quenching state, limiting impedance and its triggering time. Therefore, careful assessment of coordination issues with a recloser should be investigated to evaluate reliabilities of electric power system by installing SFCL into conventional power networks. This paper presents the impacts of SFCL on the recloser operation by considering various aspects of newly developed hybrid resistive SFCL into distribution systems in Korea. Consequently, it was deduced that conventional recloser scheme might be altered in order to meet the performance and requirements of hybrid SFCL into distribution electric power systems. [ABSTRACT FROM AUTHOR]

One of the most important topics regarding the application of superconducting fault current limiters (SFCL) for upcoming smart grid is related to its possible effect on the reduction of abnormal fault current and the suitable location in the micro grids. Due to the grid connection of the micro grids with the current power grids, excessive fault current is a serious problem to be solved for successful implementation of micro grids. However, a shortage of research concerning the location of SFCL in micro grid is felt. In this work, a resistive type SFCL model was implemented by integrating Simulink and SimPowerSystem blocks in Matlab. The designed SFCL model could be easily utilized for determining an impedance level of SFCL according to the fault-current-limitation requirements of various kinds of the smart grid system. In addition, typical smart grid model including generation, transmission and distribution network with dispersed energy resource was modeled to determine the location and the performance of the SFCL. As for a dispersed energy resource, 10 MVA wind farm was considered for the simulation. Three phase faults have been simulated at different locations in smart grid and the effect of the SFCL and its location on the wind farm fault current was evaluated. Consequently, the optimum arrangement of the SFCL location in Smart Grid with renewable resources has been proposed and its remarkable performance has been suggested. [ABSTRACT FROM AUTHOR]

Cryogenic dielectric insulation skills play a significant role in the development of superconducting electric equipment for transmission and distribution electric network. Nowadays, newly developed nano-composites have shown enhanced electrical, thermal, and mechanical properties of polymer insulation materials. And the application of nano-composites in high voltage power systems could be implemented in the near future. Among the various nano-composites, epoxy nano-composites have been paid much attention as a new insulating material for high voltage insulation. In this paper, we presented experimental results of epoxy nanocomposites in liquid nitrogen and determined the possible applications of nano-composites as insulating material for superconducting equipment. In order to determine their dielectric breakdown properties in liquid nitrogen, various kinds of epoxy based nano-composites have been made by mixing \SiO2, \Al2\O3, \TiO2 fillers, respectively. AC withstand voltage test and partial discharge (PD) inception voltage tests have been performed to verify the insulation breakdown characteristics of nano-composites in cryogenic environment. Consequently, it was deduced that the breakdown strength of epoxy nano-composites have been improved compared to epoxy with micro-fillers in cryogenic environment. And epoxy nano-composites immersed in liquid nitrogen showed a similar breakdown voltage characteristic when compared with that of insulating oil. In addition, the effects of nano-composites have been varied according to the concentration of nano-fillers and their content of fillers. [ABSTRACT FROM AUTHOR]

Enlarging the electric power supply capacity of 22.9 kV distribution line by upgrading the main transformer from 60 MVA to 100 MVA has been under consideration in order to cope with increasing electric power demands of distribution line in Korea which aims to replace some 154 kV supply demands by 22.9 kV large-capacity distribution systems. In this case, the prospective fault current of 22.9 kV distribution line is much bigger than the rated withstand fault current level of conventional electric equipments. As a major feasible solution to reduce excessive fault current, superconducting fault current limiters (SFCLs) are considered to be installed at 154 kV/22.9 kV substation. But until now, certified specifications or test procedures were not available considering utilities' requirements. And the performances of various types of SFCLs were not deeply investigated regarding the reclosing scheme of conventional networks. In this paper, in order to determine the suitable type of SFCL for newly installed 22.9 kV large distribution line in Korea, requirements and specifications of SFCLs were proposed considering the installation spots of large-capacity distribution line. Four types of SFCLs were modeled and analyzed using EMTP and their performances focusing on the coordination with reclosers were investigated to verify the suitability into the large-capacity distribution line. Consequently, detailed specifications and expected installation sites were proposed and suitable types of SFCLs for 22.9 kV large-capacity distribution line were suggested. [ABSTRACT FROM AUTHOR]

We have proposed continuously transposed coated conductor (CTCC) in striation for power devices reducing AC loss. It consists of a number of coated conductors with filaments made by striation. The transport of current may distribute non-uniformity in the filament particularly while CTCC conducts AC current. The array of Hall sensors is capable of measuring the current distribution in the filaments and in conductors. However, the search coil array may be applied as ersatz for Hall array in case of existing AC current. The filamentary CTCCs are prepared and the current distribution is then measured by the array of search coils. [ABSTRACT FROM AUTHOR]

The discovery of high-temperature superconductors were the great opportunity for developing fault current limiters using quenching phenomena. Superconducting fault current limiters is highly effective solutions to reduce and control excessive fault current which is inevitable to electric power systems. So, many companies and national research institutes were trying to develop this novel electric equipment under the assistance of national government. Due to their efforts, various types of superconducting fault current limiters were designed and tested. But, unfortunately, the practical applications and commercialization of superconducting fault current limiters are pending due to many unsolved problems. This paper discusses the current issues and commercialization problems of superconducting fault current limiters considering various aspects such as coordination with conventional relay, high voltage and high current issues, performance, cost, size, and life and maintenance issues. Then, emerging solutions for fault current limiters using hybrid method were introduced and analyzed. Finally, the viable method and our newly developed hybrid superconducting fault current limiters in order to solve the practical problems of conventional superconducting fault current limiters was briefly introduced. [ABSTRACT FROM AUTHOR]

In order to apply resistive superconducting fault current limiters into electric power systems, the urgent issues to be settled are as follows, such as initial installation price of SFCL, operation and maintenance cost due to ac loss of superconductor and the life of cryostat, and high voltage and high current problems. The ac loss and high cost of superconductor and cryostat system are main bottlenecks for real application. Furthermore in order to increase voltage and current ratings of SFCL, a lot of superconductor components should be connected in series and parallel which resulted in extreme high cost. In addition, the method to quench all components at the same instant needs very sophisticated skill and careful operation. Due to these problems, the practical applications of SFCL were pending. Therefore, in order to make practical SFCL, the price of SFCL should be lowered and should meet the demand of utilities. LSIS and KEPRI designed novel hybrid SFCL which combines superconductor and conventional electric equipment including vacuum interrupter, power fuse and current limiting reactor. The main purpose of hybrid SFCL is to drastically reduce total usage of superconductor by adopting current commutation method by use of superconductor and ultra fast switch. Consequently, it was possible to get the satisfactory test results using this method, and further works for field tests are in the process. [ABSTRACT FROM AUTHOR]

This paper presents the design and performance results of a pair of conduction-cooled brass current leads for a resistive superconducting fault current limiter (SFCL) system. The 24 kV class SFCL, which has been recently developed by the KEPRILSIS collaboration group in Korea, requires three pairs of conduction-cooled brass current leads operated continuously at 630 A. When the SFCL system is in the fault-mode, the current leads have experienced a 60-Hz fault current of ∼10 kA at 24 kV for 3–5 cycles. In this paper, we present the performance results of the conduction-cooled brass (commercial brass, 10% Zn) leads having a rated current of 667 A operated in a bath of liquid nitrogen. [ABSTRACT FROM AUTHOR]

Upper critical magnetic fields Hc2(T) have been measured resistively for the high-temperature (Tc>=90 K) superconductors TmBa2Cu3O9-δ and YBa2Cu3O9-δ in applied magnetic fields H up to 9 T. If H at the resistive midpoint is defined as Hc2, then the upper critical field slopes (dHc2/dT)Tc at the superconducting transition temperatures Tc are respectively -2.8 and -1.1 T/K. The Tm-specimen initial slope is apparently the highest yet reported for 90 K superconductors. Extrapolations from this slope, based on standard, three-dimensional, dirty-limit Werthamer, Helfand, Hohenberg, and Maki theory, yield (1) zero-temperature upper critical fields Hc2(T=0)=99–175 T, where the range is determined by the degree of paramagnetic limitation; (2) Hc2(T=77 K)≊36 T, the upper critical field at the boiling point of liquid nitrogen. [ABSTRACT FROM AUTHOR]

Presents the abstract of the study 'EPR, Magnetization, and Resistivity Studies in Doped (4-f or 3-d ions) and Undoped R barium[sub2] copper[sub3] oxygen[suby] High T[subc] Superconductors (R=Yttrium, Praseodymium, Neodymium, Europium, Gadolinium, Holmium, Erbium, or Ytterbium).'