For the first time, a high temperature superconducting (HTS) demonstration cable system has been installed in a utility network supplying electricity to consumers. The cable is a 30-m long, 30kVrms, 2, 000Arms cable, installed in the network of Copenhagen Energy at a substation supplying approximately 50, 000 customers. The purpose of the cable installation is to obtain installation and operating experience with this type of cable. The cable system has been in operation since May 28th 2001, and the first operating experiences of the cable have been satisfactory. The cable has been in operation for more than one year, and the cable has during the first year supplied 101, 000MWh of energy to the customers.
We fabricated MgB2 tapes and wires by a powder-in-tube (PIT) method, using MgB2 powder and several sheath materials such as Cu, Cu-Ni, Ni, pure Fe, carbon steel (Fe-C), and stainless steel. Jc of the as-cold rolled (un-sintered) tape significantly increased with the increase of total cross-sectional area reduction by the cold working. Hard sheath materials such as Fe-C and stainless steel are effective to enhance the packing density of MgB2 core and, therefore, to increase Jc values. Non heat treated MgB2 tapes with Fe-C and stainless steel sheaths showed extrapolated Jc values of-3.0-4.5×109A/m2 at 4.2K and zero field. Heat treatment after the cold rolling is effective to enhance Jc values. An order of magnitude higher Jc values were obtained for Fe, Fe-C, and stainless steel sheathed tapes after the heat treatment. Extrapolated Jc values well above 1010A/m2 at 4.2K and a zero magnetic field were obtained for stainless steel and Fe-C sheathed MgB2 tapes. Jc values of PIT-processed MgB2 tapes are less sensitive to strain than those of PIT-processed Bi-based oxide tapes. It is reported that the addition of soft metal powder such as In or Sn to the MgB2 powder is effective to enhance Jc values. We made -10m-long tapes with Ni sheaths, and fabricated small solenoid coil with this tape without sintering. The Ic of the coil was about 80% of the Ic of the short tape, indicating that the homogeneity of Ic in the 10m tape is fairly good. The coil generated 1.3kGauss at 4.2K.
Heat transfer property of Bi-2223/Ag multifilamentary tape impregnated with solid nitrogen was investigated by measuring the voltage and temperature traces on the tape. Solid nitrogen was produced by two different methods: i.e., one by conduction cooling with GM cryocooler, and another by evacuation with rotary vacuum pump. The heat transfer coefficient between the tape and the solid nitrogen was estimated based upon the numerical simulation. It was shown that the equivalent heat transfer coefficient of the tape impregnated with solid nitrogen made by conduction cooling at a very slow cooling rate was more than six times larger than that made by evacuation.
Experiments of Peltier current lead (PCL) were performed by the way of half-wave-rectified current (HWRC) for an evaluation of the PCL system in the drive with the large-rippled current. The current ripple of the HWRC is large, and we discussed the cooling capability of the current ripple. The experimental results revealed that the temperature difference of the thermoelectric-element (TE) increased with the magnitude of the current in the PCL system, despite the large current ripple. Calorimetric measurements revealed that the PCL reduced the heat leak of 60% for the peak current 90A. We compared the PCL systems of the direct current (dc) mode and the HWRC mode. The results showed that the current dependence of the temperature difference in the HWRC mode did not match that of the dc mode, but those of the heat leak matched well. The performance of the Peltier cooling in the HWRC mode is reduced to be 2/πtime of the Seebeck coefficient for the dc mode by using the time-average method.
After extensive research of superconducting fault current limiters for electric power systems, we report that we were successful in developing a new one. We developed a superconducting fault current limiter. It has a superconducting cylinder as the active element, and we chose Bi-2212 thick film for the cylinder as a superconductor. However, Bi-2212 thick film has low critical current density (Jc). It is necessary for the practical use of the superconducting fault current limiter that the superconductor has high Jc. So to get high Jc, we tried Bi-2223 as superconductor material, and prepared Bi-2223 thick film. The raw paste of the Bi system was sprayed on an MgO substrate, then sintered in the air with intermediate pressing by a cold isostatic press method (CIP). The sintered Bi-2223 thick film had good orientation of the polycrystalline. The Jc of the 270μm thick film was 7, 000A/cm2 (Ic=94.5A) at 77K. The Jc changes depended on the thickness of the film, and dropped drastically when the thickness of the film become thinner than 150μm. As a result of our investigation by XRD, ICP and EPMA make it clear that a decrease of Pb affects Jc.
The boiling heat transfer characteristics of saturated and subcooled nitrogen were investigated to clarify the effect of subcooling. The objective of this study is to supply information on the cooling design of high-Tc superconducting apparatuses such as the high-Tc super-conducting transformer. A thin wire of platinum-30% rodium with a diameter of 0.05mm was simultaneously used for the heater and the thermometer. The effect of space restrictions around wire on heat transfer characteristics was also examined by using transparent acrylic tubes covering thin wires, which simulated the complicated structure of a cooling system for high-Tc superconducting apparatuses. The heat transfer characteristics of subcooled nitrogen were compared with those of saturated nitrogen, and it was found that heat transfer was remarkably enhanced with the degree of subcooling.