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When a transformer is energized, inrush currents whose magnitudes may reach tens of its nominal current occur and may affect the power quality of the distribution system. When a large transformer owned by a customer is connected to a distribution system, the impact of possible inrush currents on the distribution system has to be assessed by the distribution system operator (DSO). To carry out detailed electromagnetic transient (EMT) simulations of inrush currents, the current−flux curve of a transformer is required. However, obtaining the current−flux curve of a customer-owned transformer is not an easy task for DSOs, since manufacturers provide current−time data of transformers only and do not provide their current−flux curves. Considering this point, the present paper proposes a method to estimate the current−flux curve of a transformer from its current time data so as to enable detailed EMT simulations for DSOs. As a preliminary study, the proposed method is validated using test results of a single-phase transformer in this paper.

Frequency Response Analysis (FRA) is capable of detecting troubles of transformers, such as deformation of winding, displacement of winding and deformation of iron core, by detecting change of transfer function. However, interpretation of the change with reference to the trouble is not clear. In this paper, an identification method of the troubles are proposed based on investigation using a model transformer. Furthermore, proposed method is applied to a real transformer to identify trouble caused by short-circuit tests. The identification agrees with the result of dismantling investigation.

We have proposed a flat type of the superconducting fault current limiter (FCL) which is a modified version of a magnetic shielding type of the fault current limiter. The flat type FCL basically consists of a pancake coil and doughnut-like form high Tc superconductor (HTS) disk, each of which works as a primary winding and a secondary one-turn winding, respectively. In order to clarify the static current-limiting performance of the flat type FCL, the magnetic field analyses were carried out for the small modules of the FCL. The inductance values of the FCL in the superconducting state and normal-conducting state of the HTS disk, L o and L limit , were calculated and their ratio L limit/L o was estimated. The high performance of the FCL must be brought about by the large inductance ratio. It was confirmed that lower L o was derived by sandwiching the pancake primary winding with two HTS disks and by using thin spacer between them, and larger L limit a is produced by connecting the built-up pancake primary windings in series. This suggests that a full-scale FCL may be designed and produced by building up many modules whose pancake winding is sandwiched with the HTS disks.

A high-temperature superconducting (HTS) power transmission cable is considered as one of the next-generation power transmission technologies. The introduction of HTS cables into the actual power grid requires a long-distance cooling tube, due to the location of the cooling stations at intervals of several kilometers along the cable length. Therefore, it is essential to understand the flow property of the liquid nitrogen as a coolant for the HTS cable, the current flow properties, electrical insulation characteristics, thermal insulation properties and mechanical behavior for the HTS cable taking into account its long length. This paper provides a brief summary of the HTS cable test system with a length of 500 m constructed in the Yokosuka area of CRIEPI.

A 500-m single-core high temperature superconducting (HTS) cable system has been constructed and tested in Yokosuka area, CRIEPI, by CRIEPI, The Furukawa Electric and Super-GM. In the actual power grid, the HTS cable distance extends sometimes several km. And has to be cooled down from about room temperature to liquid nitrogen temperature. As, the HTS cable is contracted and suffers mechanical stress. In the case of the long distance HTS cable, it remains a major development issue. The HTS cable constructed in this test system has an offset section for absorbing the thermal contraction. In this paper, we describe the validity of that offset section and the thermomechanical characteristics on the cooldown and warm-up test. The offset functioned successfully as designed.

We have proposed a flat type of the superconducting fault current limiter (FCL) which is a modified version of a magnetic shielding type of the fault current limiter. The new FCL basically consists of a doughnut-like form high Tc superconductor (HTS) plate and spiral coil, each of which works as a secondary one-turn winding and a primary winding, respectively. The FCL modules can be easily built up to make a large scale FCL. This simple construction may enable us to reduce the manufacturing costs. We made two small modules of the flat type FCL with Bi2223 bulk plate and YBCO thin film plate and investigated the generation of the limiting impedance due to an excess current. Higher ratio of the limiting impedance to normal one was produced by the YBCO thin film plate than by the Bi2223 bulk plate.

We have been developing an inductive type superconducting fault current limiter with a superconducting cylinder. For practical use, a high critical current density (Jc) is needed in a large diameter. To achieve these items, we have developed a Bi-2223 thick film on a MgO substrate, with a Bi-2212 buffer layer. This cylinder was 450 mm in diameter and 120 mm in length. We describe the superconductive characteristics of this cylinder, and the limiting properties of the fault current limiter using these cylinders.

Superconducting fault current limiter (SCFCL) is expected to be a first application of high-TC superconductors to electric power systems. We have been developing a magnetic shielding type of SCFCL. In this paper, we propose a design method for the magnetic shield SCFCL. The method is based on a newly developed computer program which can evaluate the dynamic thermal and electromagnetic behaviors of high-TC superconducting cylinder in fault current limiter operation. As an example, the trial design of a model system, which is 6.6 kV (rms) in rated voltage, 400 A(rms) in rated current, and 12.5 kA(rms) in maximum prospective short-circuit current, is shown. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 145(2): 50–57, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10197

Fault current limiters for electric power systems have been researched. We have studied a inductive type superconducting fault current limiter. The limiter has superconducting cylinder as active element. We developed a high Jc Bi-2223 thick film on a MgO cylinder. Jc was over 5800 A/cm/sup 2/ at 77 K. We made a inductive type superconducting fault current limiter with the Bi-2223 thick film cylinder and investigated limiting performances.

Fault current limiters for electric power systems have been researched. We have been studying on a inductive type superconducting fault current limiter. The limiter has superconducting cylinder as active element. It is necessary for practical use of the superconducting fault current limiter that superconductor has high critical current density (Jc). So as to get high Jc, we tried Bi-2223 as a material for the superconductor, and prepared Bi-2223 thick film. Raw paste of Bi system was coated on a polycrystalline MgO substrate and then was sintered in the air with intermediate pressing by a cold-isostatic-pressing (CIP) technique. The sintered Bi-2223 thick film had good crystal orientation. The Jc of the 500 /spl mu/m thick film was over 6000 A/cm/sup 2/ at 77 K.

Superconducting fault current limiters have been researched to protect electric power systems. We have developed a superconducting fault current limiter. As the active element, we choose Bi-2212 thick film for the superconductor cylinder. In order to attain practical use of the superconducting fault current limiter, larger scale superconducting cylinders must be developed. Processing energy increases as the size of the limiter is increased. If there are local defects in the superconducting cylinder and the processing energy is concentrated on them, there is a possibility that the cylinder will be damaged. We therefore investigated the thermal runaway propagation of the Bi-2212 thick film from a local heat point and how local defects affect the current-carrying capacity. The result is that the velocity of the thermal runaway propagation in the thick film from the local heat point is very slow. Thermal runaway doesn't propagate within a few milliseconds. If there are local defects such as cracks in the superconducting cylinder, the current will concentrate at the tip of the defect and the Joule temperature rises. The cracks then grow as the result of thermal stress. When a crack runs from end to end on the cylinder, superconductor on the cylinder melts due to arc discharge.

Superconducting fault current limiter (SCFCL) is expected to be the first application of a high-temperature superconductor (HTS) to electric power systems. The authors have been developing a magnetic shielding type of SCFCL that uses a cylindrical Bi-2223 HTS bulk. Short-circuit fault tests in a small SCFCL model were performed experimentally. A computer program based on the finite element method (FEM) taking the voltage-current (E-J) characteristics of the bulk material into account was developed to analyze the performance in the short-circuit fault tests and to investigate the dynamic electromagnetic behavior within a bulk superconductor. Because the E-J characteristic of HTS bulk depends on temperature and magnetic field, they investigated experimentally the E-J characteristics of a bulk superconductor in various operating temperatures and magnetic fields. The computer program considering the measured E-J characteristics simulated the electromagnetic behaviors in an SCFCL test model successfully (BiPb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/.

The superconducting fault current limiter is expected to be the first application of high-Tc superconductors (HTSs) in power systems. To develop a magnetic shielding-type superconductor fault current limiter, we have carried out some fundamental experiments on the magnetic shielding characteristics of an HTS bulk cylinder. In this paper, the experimental results of magnetic flux penetration into Bi-2223 cylinders are shown. An ac magnetic field is applied to the sample by a primary winding (copper coil) excited by ac triangular- and sinusoidal-waveform currents in the frequency range from 0.1 Hz to 100 Hz. We also developed a finite element method computer program for evaluation of the dynamic electromagnetic behavior of the HTS cylinder in a time-varying external magnetic field. The results of computer simulations considering the voltage–current (E–J) characteristic are compared with experiments. Next we carried out a current-limiting test with a small limiter model, and the developed finite element method computer program successfully simulated the electromagnetic behavior in current-limiting operation. © 2000 Scripta Technica, Electr Eng Jpn, 134(1): 17–27, 2001

Sr/sub 1-x/Nd/sub x/CuO/sub y/ thin films were grown by both RF magnetron sputtering and laser ablation on SrTiO/sub 3/

Frequency response analysis (FRA) is a diagnosis method to detect troubles of a transformer, which are deformation and displacement of winging and so, by detecting change of a transfer function compared with that measured when the transformer is normal. However, identification of the troubles detected by FRA is not clear. In this paper, firstly, identification method of the troubles is investigated using a model transformer. Secondly, the proposed identification method is applied to identify a trouble of a real transformer caused by short-circuit tests.

A high-Tc superconducting (HTS) power cable is considered to be a major candidate for the bulk power transmission device in the future. The HTS power cable with a cold dielectric type electrical insulation system can make the outer diameter of the HTS cable compact in order to replace the conventional XLPE cable laid in underground ducts. Therefore, establishment of the electrical insulation design, HV testing and evaluation methods is the most important in the realization of the HTS power cable. From these viewpoints, the electrical insulation design method and determination of testing voltages for a 500-m HTS power cable constructed for verification tests are discussed, and the result of the various tests are described in this paper.

A high-Tc superconducting (HTS) power cable is one of the major candidates for reinforcing power transmission grids dynamically. For operation of the HTS cable, a quench phenomenon would occur due to over-current operation, degradation of the HTS materials, and so forth. In that case, local heating would occur and local evaporation of liquid nitrogen would occur, which may lead reduction of the electrical insulation capability and generate partial discharge (PD) under operational voltage application. This paper deals PD inception characteristics with local heating simulating local quench of HTS material. As a result, it is revealed that there is a time delay from the start of heating to PD generation, and the delay depends on the pressure of liquid nitrogen.

In recent years, interest in the application of superconductivity to the electric power apparatuses has grown rapidly. In ac power apparatuses, loss induced by alternating current is a impeding problem which has to be reduced. A superconducting power cable is one of the promising ways for transmitting huge electric power efficiently in the future. We have proposed the new ac superconducting cable design with a similar electrical insulation structure to the conventional extruded cross-linked polyethylene cable (CV cable). The feature of this cable is to exploit the excellent electrical insulation capability of polymers at cryogenic temperature by separating helium coolant from the electrical insulation. The conductor of the superconducting cable consists of niobium clad over a copper flat wire and niobium tape, which are wound helically in the opposite direction each other on a copper pipe. In order to clarify the ac loss mechanism in this cable design, measurements and analysis of the ac loss of the superconductor have been carried out at the liquid helium temperature.