In order to realize bulk power transmission with high reliability and high efficiency, electric power companies have made efforts to connect several electric power systems together. In this case, fault current may exceed the rated current of such power devices as a circuit breaker and a power transformer in the case of contingency and they have to be replaced with the devices with the higher rated value. Fault current limiters (FCL) have been expected to be installed in an electric power system to suppress fault current and to prevent the replacement. This paper first reviews the necessity of FCL in electric power system, and R & D status of various types of FCL in the world. The review is focused on superconducting FCL made of AC superconducting wires. Second, the experimental evaluation of the new superconducting FCL which has been developed by Tokyo Electric Power Co. and Toshiba Corporation is described. The success of the current suppression up to 1, 800A is emphasized.
Structural metallic materials in cryogenic engineering relating with superconducting magnets are generally used under high magnetic field and stress. However, only a few researches on cryogenic structural metallic materials have taken into consideration the effects of magnetic field on mechanical and other properties. The reason for this fact is mainly due to the lack of experimental data about the effects. It is very important to make clear the effects in order to develop widely and soundly cryogenic and superconducting technologies. In the present article, several subjects relating with the effects of magnetic field on mechanical properties, martensitic transformation and other properties mainly at cryogenic temperatures are reviewed.
High cycle fatigue tests at cryogenic temperatures were performed for titanium alloys and austenitic steels. The S-N curves shifted to higher stress or longer life side with a decrease in test temperature with some exceptions in forged Ti-6Al-4V alloys. There was little change in fatigue strength between 77K and 4K for the forged Ti-6Al-4V alloys. Fatigue strength at 106 cycles increased in proportion to a gain in strength by temperature decrease. However, a ratio of the farigue strength to tensile or yield strength depended on material and its processing. From the viewpoint of combination of yield strength and fatigue strength, austenitic steels had a higher fatigue strength than titanium alloys at a given yield strength. Among the titanium alloys, the rolled ones exceeded the forged ones.
In cryogenic high cycle fatigue for titanium alloys and nitrogen-strengthened austenitic steels, subsurface crack initiation apparently occurs without the existence of any defects like inclusion and pore. Microstructural origin of subsurface crack initiation site and deformation behavior for those alloys were investigated, and the mechanism of subsurface crack initiation was discussed. Subsurface crack initiation site is consisted of facets which are assigned their microstructure. Subsurface crack initiation is caused by a microcracking related with microstructural inhomogeneity. A principal slip system is dominant deformation mode and dislocation motion is planar. Even at lower stress level, piled-up dislocations of short range order are blocked or sharply localized at grain boundaries. Namely, the stress concentration at grain boundary locally occurs. Then, the stress concentration might produce the microcracking. On the other hand, the size of subsurface crack initiation site depends on the maximum stress range. Therefore, subsurface crack initiation mechanism is considered that a microcracking due to localized deformation grows or coalesces until a critical crack size and is chosen for the main crack.
Influence of the thermal disturbances of various temporal and spatial profiles on the stability limit of a cable-in-conduit superconducting coil, forced-flow cooled by supercritical helium, has been numerically analyzed. The maximum deviation of the stability limits for any temporal profiles from those for rectangular temporal profile is 45 percent. In the case where the length of thermal disturbance is ninety hydraulic diameter and less, the effect of the axial-heat conduction in superconductor on the stabiliry limit is not negligible. The transient valiation of the maximum conductor temperature is almost similar in both the quasi-rectangular and quasi-parabolic spatial profiles of the thermal disturbances.
The superconducting YBa2Cu3O7-x (YBCO) thin films on MgO (100) substrates were successfully fabricated by a reactive evaporation method under the oxygen plasma condition. In this paper, we described the details of the reactive evaporation method and the optimum preparation conditions. The superconducting transition temperature, surface morphology and crystal quality were examined as a function of the substrate temperature (Tsub) and the oxygen partial pressure (PO2). The oxygen was introduced into the film by the injection nozzle. After deposition, the films were allowed to cool with the same pressure as in the deposition under the reactive O2 plasma. The as-grown films showed Tczero=83K with Tconset=88K under the preparation conditions of Tsub=620°C and PO2=1.4×10-3Torr. The XRD analysis showed preferentially c-axis oriented superconducting phases perpendicular to the film surface. The crystal quality and degree of preferred orientation of the films were improved as the substrate temperature increased. The as-grown films were post-annealed at a various temperature and the changes of Tczero and lattice parameter c were examined. As a result, the values of Tczero were not affected by the post-annealing, although the lattice parameter c was shortened at the annealing of above 750°C. This results may be related to the deficiency of oxygen in the CuO2 plane of the YBCO perovskite structure.
Bi-based oxide superconductors have been formed by a simple process where a paste layer containing oxide powders of component elements is painted on either Cu or MgO substrate and irradiated using a YAG laser. This process requires no substrate heating, post annealing and slow cooling. The starting composition of mixed oxide powders corresponds to that for the so-called 2223 phase. Irradiation is carried out at a laser beam energy of 50-70W and a sample moving speed of 0.025mm/s. Although the major Bi-oxide superconductor formed is the so-called 2212 phase without c-axis oriented texture, a small amount of the high Tc 2223 phase is sometimes found in samples with the MgO substrate. The maximum Tc (onset) and Tc (zero resistivity) obtained were 70K and 57K, respectively. The maximum Jc obtained was 1, 300A/cm2 at 4.2K.
A 14T superconducting split solenoid magnet was developed and tested in KEK. The magnet has a hybrid construction of Nb3Sn and Nb-Ti. Excitation experiments were made using a computer data acquisition system. A new method to analyze quench data is discussed in this work. The quench propagation through coils was clarified from the data analysis. The magnet was operated upto almost 100% of Ic of the Nb3Sn conductor on the load-line.
Pinned type II superconductors can repel magnetic fields due to flux pinning. The recently developed Melt-Powder-Melt-Growth process enabled us to introduce fine non-superconducting particles into YBa2Cu3Ox matrix and to achieve large flux pinning force. It was found that an addition of silver was effective to reduce the amount of cracking and to increase the repulsive force of the superconductors against magnets. A heavy object could be levitated above the MPMG grown YBaCuO-Ag cooled with liquid nitrogen. We have also succeeded in levitating a person.
Heat flux in liquid helium in a rotating superconducting magnet was measured under the rotational speeds of 500 to 3, 600rpm in order to help analyzing the stability of field winding of superconducting turbine generators. Maximum centrifugal acceleration at the location of heat flux measurement was 2, 045G. Measured maximum heat flux in the nucleate boiling region was increased up to 2W/cm2 at the rotational speed of 500rpm; it changed little with the increase of rotational speed above this level. In the film boiling region, both heat flux and minimum heat flux were increased with the increase of the rotational speed up to 3, 600rpm.