This review paper describes the author's view of superconductor applications for electric power system, who was one of the planners of MITI/AIST Project of R & D on Power Applications of Superconductor Technology. In the near future, the power system in Japan will require critically new technologies to overcome the foreseenable difficult problems. Considering the system character, the first step to introduce superconductor technology in the power system should be to “superconductorize” the existing apparatuses in the system, with some analogy to biological evolution. According to a general way to commercialize new technology, the R & D of superconducting generators, which will increase not only generating efficiency but also system stability enormously, are under way, steadily step by step, by the Project team. These R & D works are believed to lead us in the future to “totally superconductorized power system, ” which has the highest efficiency, reliability, stability, availability and economy.
The application of superconducting technologies to electric power apparatuses is very important from the viewpoint of not only the promotion of energy and resources saving but also the improvement of power system stability. Super-GM was established in September, 1987 in order to proceed the research and development (R & D) on application of superconducting technologies to power apparatuses and Super-GM R & D project, which is sponsored by New Energy and Industrial Technology Development Organization (NEDO) under the Moonlight Project of Industrial Science and Technology, Ministry of International Trade and Industry, started from FY 1988 for a scheduled period of eight years. As to R & D on superconducting generator, the basic designs and analyses of 70MW class model machines and the research on element techniques concerning superconducting field winding, structural materials of multi-cylindrical rotor, warm damper, rotating cooling characteristics, helium transfer coupling and air gap armature winding of superconducting generator were conducted in FY 1988 and 1989, and many basic results were obtained. At present, R & D step is proceeding steadily from the research on element techniques to the examinations for manufacturing techniques using partial models such as field winding model, multi-cylindrical roter model, rotating cooling model, helium transfer coupling model and armature winding model for the manufactures and demonstration tests of 70MW class model machines. In this paper, the current situation of R & D on superconducting generator carried out by Super-GM is presented.
The application of a direct numerical analysis to the superfluid phenomena is attempted. The propagation of a second sound wave and the evolution of a finite amplitude thermal wave into a shock wave are simulated by applying the finite difference scheme to the two-fluid equation for superfluid helium (He II). Typically three types of thermal shock waves, that is to say, a front shock, a back shock and a double shock, are well simulated. The results, however, suggested necessity for introducing the effect of the interaction of a flow field with a tangled mass of quantized vortices to the governing equation in the breakdown state. The effect is taken into account on the basis of the Gorter-Mellink mutual friction formula modified to include an unsteady feature for the simulation of the propagation of a thermal shock, and of a steady heating.
An analysis of the thermal stability of forced-flow cooled superconducting composites has been carried out. Numerical analysis is made mainly of a cable-in-conduit conductor, taking account of the decrease in electric current owing to Joule heat, transient convective heat transfer, transient fluid motion, the temperature- and pressure-dependence of the physical properties of helium, and the dependence of the superconducting composite on temperature and magnetic flux density. The effects of the initial current and the duration of thermal disturbance on the temperature and stability of the superconucting composites are clarified. The conductors run away to the quench or recover into the superconducting state, according as the Joule heating rate in the conductors is greater or smaller than the cooling rate by helium after the termination of thermal disturbance.
Various boiling heat transfer characteristics have been experimentally investigated in a vertical channel opened at both ends to a pressurized (1 atm) He II bath. A wire heater is attached at the center of the rectangular cross-sectional channel. In the boiling states, the removed heat through the upper side of the channel increases with the increase of input power to the heater, while the removed heat through the lower side of the channel is kept constant. A temperature rise in the channel when the aspect ratio of the channel cross section is 1 is smaller than that when the aspect ratio of 7 after He II to He I transition occurs. The critical values of heat flux at which He II to He I and nucleate boiling to film boiling transitions occur in the vicinity of the heater surface in this channel are larger than those in a channel closed at the bottom, respectively.
Cu-Nb composite wires prepared in the first step of the in-situ process for the Nb3Sn superconducting wires are considered as composite materials for the conductor of water-cooled coils and for reinforcing stabilizer of high field superconductor. Mechanical and electrical properties are studied for several samples prepared by induction melting in a CaO crucible and drawing. These composites have large yield stress above 50kg/mm2 even after annealing at 750°C for 1 hour. Moreover annealed wires show magnetoresistance of about 0.25μΩcm at high field of 15T, comparable with that of Al2O3 dispersion strengthened copper (K. Noto et al.: Proc. MT-9 (1986) 700). Therefore, it is found that Cu-Nb composite wires are applicable not only to normal conductors in high field, but also to reinforcing stabilizers for large-scale and/or high-field superconducting wires.
Minimum quench energy (MQE) of superconducting wires, Cu/SC ratio 1 and 2 having a fixed SC area, was measured using a small ceramic resistive heater (1mm×1.5mm×0.5mm), which was cut from commercially-available large ceramic resistor tubes. The heater was soldered on the superconducting wire, thus minimizing thermal resistance inbetween. Current pulses generated by an FET switch, a pulse generator and a DC power supply were applied to the resistor via coaxial cable, thereby maintaining the transmitted pulse wavefrom. The transient thermal response of the locally heated superconductor was also measured using a thermocouple. The MQE of the Cu/SC ratio 2 wire was approximately twice as large as that of the Cu/SC ratio 1 wire, at a constant background magnetic field and a constant ratio of the operating current to its critical current.
Multi-core tapes are expected to improve Jc and stability performance for the future actual uses. In the present study silver sheathed Tl-Ba(Sr)-Ca-Cu-O tapes with single, 36 and 1, 332 cores were fabricated with various cross-sectional sizes by rolling or pressing method and their superconducting characteristics were investigated in various conditions. The highest Jc values 14, 500, 8, 600 and 1, 100A/cm2 were attained for the single-, 36- and 1, 332-core tapes with 0.1mm thickness, respectively, at 77K without external magnetic field. Observation of cross-section revealed a non-uniform area of superconducting filaments in multi-core tapes, which degraded the superconducting characteristics of the tapes. The Jc values of multi-core tapes were also measured under external magnetic fields less than 1T. The features of this Tl series oxide superconducting tapes showed that there were no indications of difference of the Jc values in any direction of the magnetic field to the tape surface. Bending tests were carried out with the multi-core tapes. The 36- and 1, 332-core tapes could endure higher strains than the single-core tapes.