Masatoyo Shibuya

Superconducting Fault Current Limiter Design Using Parallel-Connected YBCO Thin Films

Superconducting fault current limiters (SFCLs) are able to reduce fault currents to an acceptable value, reducing potential mechanical and thermal damage to power system apparatus and allowing more flexibility in power system design and operation. The device can also help avoid replacing circuit breakers whose capacity has been exceeded. Due to limitations in current YBCO thin film manufacturing processes, it is not easy to obtain one large thin film that satisfies the specifications for high voltage and large current applications. The combination of standardized thin films has merit to reduce costs and maintain device quality, and it is necessary to connect these thin films in different series and parallel configurations in order to meet these specifications. In this paper, the design of a resistive type SFCL using parallel-connected YBCO thin films is discussed, including the role of a parallel resistor and the influence of individual thin film characteristics, based on both theory and experimental results.

Inductive type fault current limiter with Bi-2223 thick film on a MgO cylinder

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.

Inductive type fault current limiter using Bi-2223 thick film on MgO cylinder with Bi-2212 buffer Layer

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.

Angular dependence of AC transport current losses in biaxially aligned Ag/YBCO-123/YSZ/Hastelloy coated conductor

AC transport losses in a biaxially aligned YBCO-123 tape (10 cm in length) were measured (at 77 K) as a function of frequency and amplitude of a sinusoidally varying transport current. In the frequency range of 30-320 and the transport current losses are hysteretic in nature. The measurements of the self-field losses caused by the AC transport current as a function of the magnitude and orientation of the applied low external DC magnetic field (up to 150 mT) are also reported. The external magnetic field was rotated in the range of +120 to -120 degrees with respect to the plane of the tapes, and was in all cases perpendicular to the transport current flow. Strong anomaly in angular critical current dependence on external magnetic field was observed.

Tests for Conceptual Design of 6.6 kV Class Superconducting Fault Current Limiter With YBCO Thin Film Elements

A superconducting fault current limiter (SFCL) for 6.6 kV and 400 A installed in a cubicle for a distribution network substation was conceptually designed. The SFCL consists of parallel- and series-connected superconducting YBCO elements and a limiting resistor. Before designing the SFCL, some tests were carried out. The width and length of each element used in the tests are 30 mm and 210 mm, respectively. The element consists of YBCO thin film of about 200 nm in thickness on cerium dioxide (CeO2) as a cap-layer on a sapphire substrate by metal-organic deposition with a protective metal coat. In the tests, characteristics of each element, such as over-current, withstand-voltage, and so on, were obtained. From these characteristics, series and parallel connections of the elements, called units, were considered. The characteristics of the units were obtained by tests. From the test results, a single phase prototype SFCL was manufactured and tested. Thus, an SFCL rated at 6.6 kV and 400 A can be designed.

Performance verification of a practical fault current limiter using YBCO thin film

Aiming at a practical use of superconducting fault current limiter, a newly designed device which consists of YBCO thin films and normal conducting films has been developed. The configuration allows increase of the resistance of the YBCO film without excess heating and partial overvoltage. To verify the current limiting performance and the reliability of the device, an analogue simulator test with a condition equivalent to a typical distribution system has been conducted. The developed device can limit a fault current to a target level within a half cycle and the increasing speed of the resistance is fast enough to mitigate the current overshoot after the fault. The series and parallel connection of the unit devices are feasible to attain large capacity. These test results prove that the proposed device has sufficient performance for applying to an actual power system.

Generating performance of limiting impedance in flat type of fault current limiter with high Tc superconducting plate

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.

Proving test on removal of PCB from pole transformer by vacuum heating separation method

In 1989, PCB was found in recycled insulation oils used in some pole transformers at a concentration of 50 mg per kg of insulation oil, prompting quick establishment of their safe separation method. One of the prospective methods for removing, for example, recycled insulation oils attached to and impregnated into the transformer coils, may be a vacuum heating separation method which heats and vaporizes PCB under vacuum conditions for separation. A proving test for the applicability of the specific vacuum heating separation method was carried out with actually used transformers, which confirmed that residual PCB on coils was minimized to less than the quantitative determination threshold of 0.05 mg per kg of insulation oil under the following standard applicable conditions: vacuum lower than 0.05 Torr; heating temperature 200 °C; and vacuum holding time 10 hours. Thus, the reduction of effects on the environment was verified.

Inductance Ratio of Flat Type Fault Current Limiter With High Tc Superconducting Disks

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.

Effect on transferred ac arc plasma stability of increasing ambient temperature and superimposing pulse at current zero point

The use of alternating current (ac) arc plasma involves a problem of instability of the arc plasma at current zero point. This paper describes the effects of ambient temperature and of superimposing a pulse current on stability of the ac argon arc plasma; the pulse current (10 A) is superimposed on an ac current only at each current zero point. The ac arc plasma is generated between a tungsten electrode of a plasma torch and a copper counter electrode in a chamber. The ac current is 100-500 Arms and the arc plasma length is 40 mm. As the ambient temperature increases from 300 K to 900 K, the stored energy in the arc plasma increases and the required supply voltage to stabilize the arc plasma decreases by about 20% (from 270 to 220 Vrms). Additionally, superimposing a pulse current on an ac current causes the required supply voltage to decrease by about further 20% (from 220 to 180 Vrms).