Eriko Yoneda

6.6 kV/1.5 kA-class superconducting fault current limiter development

The authors have developed and tested a 6.6-kV/1.5-kA-class fault current limiter wound with a 42-strand AC superconducting wire having ultrafine NbTi filaments in a high-resistivity matrix. In experiments, voltages up to 7.2 kV were applied to the limiter with phase angles of 0, 45, and 90 degrees . The limiter was able to limit the fault current to 1.8 kA from the 55-kA short-circuit current that would flow in a circuit without a limiter. >

Design and test results of 6.6 kV high-Tc superconducting fault current limiter

A 6.6 kV single-phase fault current limiter (FCL) using a high-Tc superconducting coil as a limiting coil was developed. The development is a preliminary step to investigate the feasibility of the FCL application for high-voltage transmission lines. The FCL is of the rectifier type and is mainly comprised of a limiting coil, a sub-cooled nitrogen cryostat with a cryocooler, and a rectifier bridge. The limiting coil, wound as a solenoid by Ag/Mn sheathed Bi-2223 tapes, has an inductance of 30 mH. It is immersed in a liquid nitrogen bath in the cryostat. A Gifford-McMahon cryocooler cools the cryogen below 77.3 K. A pressure regulator keeps the cryogen at an atmospheric pressure. The coil has a critical current of 70 A at 64 K and endures a 50 Hz overvoltage of 22 kV against the ground. In a fault current limiting test with a short-circuit generator, a short-circuit current of 12.5 kA was limited to 1.2 kA.

Development of a large-capacity superconducting cable for 1000 kVA-class power transformers

A large-capacity superconducting cable for 1000-kVA-class power transformers has been designed and fabricated. The cable is a triply stacked multistrand (6*6*6) type. The elementary strand has 19050 NbTi filaments 0.63-mm thick in a CuNi matrix. The test cable is installed as the secondary winding in a superconducting transformer with iron core in a room-temperature space. The primary winding is the second-level subcable of the secondary one and the turn ratio is nearly 14. The designed capacity of the test cable is 4.545 kA at the secondary voltage of 220 V. The peak value of the current, 6.43 kA, is 78% of the critical current on the load line. The maximum current of the cable at 60-Hz operation was 3.78 kA (peak). The experimental results suggest that the degradation in maximum current of the test cable is related to current transfer between the cable and the copper terminal plate. >

Development of Superconducting AC Fault Current Limiter

The authors have developed a new superconducting fault current limiter whose impedance during normal operation is very small. During fault conditions, the limiter behaves as a superconducting reactor. The limiter consist of a superconducting limiting coil and a superconducting trigger coil. The former coil has a larger critical curent than the latter coil. These coils are wound non-inductively on coaxial cylindrical formers and are parallel connected to each other. They are wound with AC superconductor having ultra-fine NbTi filaments. The limiter has a very little impedance because both coils are in superconducting state during normal operation. On the other hand, in the case of fault conditions, the trigger coil quenches at a critical current. After the’ trigger coil quenching, the limiter becomes a superconducting reactor because current in the coil decreases very rapidly with rapidly developing resistive normal zone. The fault current is, therefore, limited by the superconducting limiting coil to a certain value determined by the coil inductance. In experiments, the authors have succeed in limiting a fault current level to 200A, with a limiter whose terminal voltage under limiting conditions was 54V.

Characteristics of a 40 kVA three phase superconducting transformer and its parallel operation with a conventional transformer

A 40 kVA three phase superconducting transformer has been developed and tested. From the test results, excellent voltage regulation of 0.3% with a pure resistive load was obtained. For application in a power system, parallel operation with a conventional power transformer using copper windings has been carried out. Although a superconducting transformer cannot continue to operate in case of quenching, the proposed parallel system can overcome the drawback and give an additional fault current limiting function.<<ETX>>

Quench current degradation in superconducting coil for 6.6 kV/2 kA fault current limiter

A 6.6 kV/2 kA class fault current limiter that consists of noninductive superconducting windings was developed and tested. The limiter can deliver continuous power at 2 kArms and limit fault current of more than 20 kA to 4 kA in 6 kV. The limiter recovers to the superconducting state within a few seconds and can perform a fault current limiting operation in 60 seconds after the last limiting operation. After high-voltage operation, however, the quench current of the limiter dropped significantly. This quench current degradation is related to the coil bobbin strain due to helium pressure raised by large ohmic heating in quenching. The degradation is a fatal problem for a fault current limited if the quench current falls below the operating current because the limiter cannot revert to the normal operation. High Youngs modulus bobbin is effective to remove the coil degradation. The authors have obtained a fault current limiter without any degradation after repeated limiting operations.

The current status of superconducting fault current limiter development

Abstract Demand for electric power in Japan has been steadily increasing in recent years.To meet this demand, power networks have been more robustly connected with a view to improving their reliability. However, this has been accompanied by a trend toward a greater incidence of fault currents in these power networks. If a fault current limiter is used to connect these systems, electric power utilities will be able to accommodate electric power through the superconducting limiter from a system with excess capacity to another system in which the available power is insufficient, while the limiter protects systems from the influence of faults in a system. The authors have developed a 6.6 kV/2.0 kA class superconducting fault current limiter used for power distribution substations, as the preliminary stage of development of current for trunk power systems. In order to reduce impedance under normal operating conditions and quickly switch to high impedance in a fault condition, the limiter consists of a pair of double-layer non-inductive superconducting windings connected in series. Each winding is wound with a 36-stranded AC superconducting wire having ultra-fine NbTi filaments in a high-resistivity matrix. The device successfully limited a 6.9 kA short-circuit current to 3.4 kA and transmitted continuous power at 2.0 kArms.

Superconducting Current Lead Development of BiPbSrCaCuO

Cylindrical superconducting rods composed of Pb doped Bi-Sr-Ca-Cu-O, 10∼ 15 cm long and 10∼ 20 mm in diameter, were fabricated by a cold isostatic pressing (CIP) method. To evaluate their electric properties as current leads, critical current (Ic) and contact resistance to the ceramic superconductor were investigated. As a result of improvements, transport Ic value of 480 A at 77 K, that of more than 1000 A at 4.2 K, and contact resistance of less than 1.7×10−9 Ω (contact resistivity of less than 1.3×10−8 Ωcm2) were achieved.

Tests on a 30 kVA class superconducting transformer

Abstract To demonstrate the applicability of superconductors to electric power machines, the present authors made and tested a 30 kVA class single-phase superconducting transformer. The aim of the study was to determine the superconducting transformer properties. Therefore the superconducting transformer has a simple structure, i.e. the primary to secondary voltage ratio is 1:1 and the iron core is immersed in liquid helium. The core loss, evaluated from no-load tests, was 13 W and leakage impedance, obtained by short circuit tests, was 0.02 Ω in accordance with a calculated value. The superconducting transformer showed the limitation effect of fault currents. The authors succeeded in continuous operation with a 0.5 Ω load resistance. These results suggest that efficiency can be 98.5%, if the iron core is located outside the cryostat and if high Tc superconductors are used as current leads. Superconducting windings exhibit training quenches in general. The authors also developed a superconducting transformer quench detector with a third winding around the iron core. The quench detector revealed that the secondary winding quenches before the primary winding.

Design and Operating Characteristics of Superconducting Transformer with Simple Configuration

A 40kVA three phase superconducting transformer has been developed and tested. In order to simplify its configuration, whole the transformer including windings and iron core was immersed in liquid helium. Reducing iron losses is the key to design such type of superconducting transformer. Iron loss characteristics of three types of core materials were measured under liquid nitrogen temperature. Among them, ultra fine crystallized metal (FINEMET) showed excellent low iron loss characteristics. Superconducting transformer made of such core and high Tc superconducting ac wire, whose whole the structure is cooled by liquid nitrogen, is very promising alternative.