M. Takeo

Preliminary tests of a 500 kVA-class oxide superconducting transformer cooled by subcooled nitrogen

The authors have designed and fabricated a 500 kVA-class oxide superconducting power transformer operated in sub-cooled nitrogen. The primary and secondary windings are three- and six-strand parallel conductors of Bi-2223 Ag-sheathed multifilamentary tapes, respectively. In the parallel conductors, the strands are transposed several times for uniform current distribution among them. A transformer, cooled by liquid nitrogen of 77 K, was steadily operated with a 500 kVA secondary inductive load. The efficiency in full operation at 77 K was 99.1 %, even with the refrigeration penalty of liquid nitrogen, 20, for the thermal load to the coolant. They installed the transformer in a continuous flow system of sub-cooled nitrogen as a fundamental step for compact superconducting transformers operating in sub-cooled nitrogen with a single-stage refrigerator. Short-circuit tests of the transformer were also performed in a region of temperature below 70 K. The transformer was operated with no quenching up to a level of critical current at 66 K, that is equivalent to 800 kVA. The efficiency estimated was improved to 99.3 % in the sub-cooled nitrogen.

Critical current properties in HTS tapes

Abstract The author’s recent studies on the critical current properties in Bi-2223 multifilamentary tapes as well as YBCO coated IBAD tapes have been reviewed. Extended electric field vs. current density ( E – J ) characteristics were studied over wide range of temperature, T , magnetic field, B , and angle, θ . Based on the analysis of statistic J c distribution and scaling, we derived an analytical expression for J as a function of E , T , B , and θ . This method allows us to describe and extrapolate the nonlinear E – J characteristics even in extremely low E region and/or high B region based on rather simple measurements.

Estimation of E–J characteristics in a YBCO coated conductor at low temperature and very high magnetic field

Abstract E – J characteristics in a YBCO coated conductor at low temperature and very high magnetic field up to 27 T have been estimated using experimental E – J characteristics below 12 T along with a scaling analysis based on the percolation model. 1 mm thick YBCO films was deposited on a biaxially aligned Y 2 O 3 /YSZ/Hastelloy substrate fabricated by the IBAD technique. The critical current I c for the 1 cm wide tape was 85 A at 77 K in self-field. We measured E – J characteristics in perpendicular field up to 12 T using 100 μm wide and 1 mm long bridge patterned in the tape. Statistic J c distribution and pinning properties were analyzed from the E – J characteristics. Using the pinning parameters determined by the scaling analysis, we can calculate the critical current properties as a function of temperature and magnetic field. E – J characteristics at low temperature and very high magnetic field up to 27 T were estimated analytically and compared with the experimental data. Estimated value of J c and n -value, and their error analysis were also discussed.

Percolative transition and scaling of transport E-J characteristics in YBCO coated IBAD tape

We have investigated extended electric field-vs.-current density E-J characteristics in YBCO coated IBAD tapes. The results of a Monte-Carlo study on the depinning in a random pin medium were compared with measurements. Using a low temperature scanning laser microscope, we examined the percolative behavior of the local resistive transition in the YBCO tape. It was also shown that the depinning probability, which is proportional to the dynamic resistance of the E-J curves, is scaled as a function of reduced current density with the aid of a power index. Consequently, the E-J characteristics in the tapes can be described by the combination of the two kinds of scaling laws: one is the scaling law of the depinning probability predicted in a random network and the other is the scaling law for the flux pinning force. These properties agree well with the percolation model of depinned clusters.

Stability and quench development study in small HTSC magnet

Stability and quench development in a HTSC magnet have been experimentally studied with the transport current in the magnet being below or above the “thermal quench current” level. The magnet was tested at both cryocooler cooling and liquid nitrogen cooling, with and without background magnetic field (up to 4 T). The temperature and electrical voltages in different sections of the magnet were measured using 20 thermocouples and 24 potential taps embedded in the winding. In this paper, the experimental procedure and the results are described. The results are compared with those obtained earlier in the experiments with the smaller HTSC specimens and are analysed by using the scaling theory of the thermal quench.

Thermal quench study in HTSC pancake coil

Abstract In spite of rather high general stability of high temperature superconducting (HTSC) Bi-based magnets, catastrophic thermal quench (TQ) may appear in them under certain circumstances. It happens because of non-linearity of voltage–current characteristics in HTSC superconductors. Starting with small samples in our previous works, we continue to study the TQ with large samples. We prepared a highly instrumented HTSC pancake coil. It is wound using the Bi-2223-based tape. We attached many potential taps to the tape and installed in the winding 10 cryogenic thermocouples (TC) and two heaters. Quench development in the coil was measured under different temperatures, different magnetic fields and different cooling conditions. In this paper, the experimental details and the results obtained are presented. The results are discussed from the point of view of scaling theory for quenching in HTSC devices.

Results of tests on components and the system of 1 kWh/1 MW module-type SMES

A full system of 1 kWh/1 MW superconducting magnetic energy storage (SMES) has been completed early this year. This SMES is the first step to the realization of practical SMES system for power line stabilization. Main points in its design are two module type arrangement of six coils having three coils and one converter as one unit of module, modified-D-shaped coils with mechanical supports, liquid helium vessel type cooling of coils, and high-temperature superconducting current leads. The first test experiment was carried out on the site recently. The above design points were examined. A preliminary test for power line control was also made in the distribution line at the site. Satisfactory results were obtained.

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. >

Instability in kiloamp class a.c. superconducting cables

Abstract In kiloamp (kA) class superconducting cables for a.c. use, where each strand does not have an insulation layer, the quench current in 60 Hz operation was found to be no more than 50% of the d.c. critical current, even on the load line. The degradation in the 60 Hz quench current of the kA class cables is discussed with respect to: 1, an a.c. instability in the multifilamentary wire; 2, magnetic coupling among strands in concentric multistrand cables; 3, the influence of local disorder in cabling on current distribution; and 4, the temperature rise due to steady a.c. losses. While points 2 and 3 induce non-uniform current distribution, points 1 and 4 were found to be factors in the reduction of quench current in the strands with higher current density. Some local overheatings may also contribute to the intense degradation.

Quench characteristics in HTSC devices

Quench dynamics in a YBCO HTSC film and a Bi-based small HTSC coil have been studied. While the stability margin of HTSC against a local disturbance was very large, quench current was limited by a catastrophic temperature rise originated from the nonlinear characteristic of Joule heating in HTSC. The crucial parameter for the quench becomes the nonlinear resistance in HTSC as a function of temperature and transport current. It has been shown that the dynamic characteristics of the quench in both the film and the coil can be described quantitatively by the simplified one-dimensional heat balance equation even though the time scales are different by more than six orders, i.e., several hundreds micro seconds for the film and several hundreds seconds for the tape coil.