The outline of following activities in the area of VAMAS TWA-6 will be reported. (1) Establishment of reliable measurement techniques for superconducting wires. 1.1 Critical current measurement in multifilamentary Nb3Sn wires. 1.2 AC loss measurement in Nb-Ti wires. (2) Establishment of reliable measurement techniques for cryogenic structural materials. 2.1 Tensile measurement at 4.2K in SUS 316 LN, YUS 170 and Ti-Al-Sn alloy. 2.2 Fracture toughness measurement at 4.2K in the same steels and alloy. 2.3 Electrical strain gauge calibration at cryogenic temperatures. Future possible cooperation programs of this area will be also described.
The SMES researches have been performed mainly aiming at large scale SMES for nighttime energy storage and daytime power supply. Recently, however, the usefulness of medium or small scale SMES have been recognized and its studies have been started. In this paper, SMES technology is reviewed, both of large scale and small one, and the two projects now in progress are explained, the one in Japan for 100kWh SMES and the other also in the United States for 20MWh one. The former is a prototype of the small scale SMES aiming the multipurpose usage in power systems such as compensation for impulsive loads, and system stabilization for disturbances and heavy loads etc. The latter is considered as an intermediate step for the development of the ultimate large scale SMES. The hopeful role expected for the energy problems under the global environmental conditions will drive us to continue the SMES R and D enthusiastically.
In recent years, SQUID is used in various fields. This paper presents the basic information for the application of SQUID to various measurements. The principle of SQUID, the configuration of pick up system and the way to decrease noise are described.
The responses of superconducting transformers to lightning surge are discussed theoretically. For this purpose, a 300/3MVA, 275/√3kV superconducting transformer is conceptually designed, where the dielectric strength of supercritical He is supposed to be 35kV/mm. Equivalent circuits and basic equations which can describe the quench and evaluate the ac loss of superconducting windings are proposed. The responses of superconducting transfomers to the full wave with 1, 050kV in peak value of voltage and 1.2μs in front time are numerically simulated. In order to elucidate the design problems of superconducting transformers, the responses of four types of typical windings, i.e. ordinary circular-disk type, high-series-capacitance type, condenser-coupling-shield type and multi-layer type windings, are respectively examined numerically. It is shown that multi-layer type windings have excellent properties both for initial voltage distribution and current oscillation. Whereas high-series-capacitance type and condenser-coupling-shield type windings have been commonly used for conventional transformers in place of ordinary circular-disk type windings for improving the initial voltage distributions, ac losses due to current oscillation amount to 104 times as much as under rated operation and quenches are shown to be induced by temperature increment. The study on the insulation design of these windings by taking account of the degradation of dielectric strength of supercritical He due to temperature increment shows that dimensions of these windings should considerably be enlarged in comparison with ordinary circular-disk type windings. The present results indicate that the conventional analysis of only voltage response to lightning surge is insufficient for the insulation design of superconducting transformers.
Current and magnetic-field oscillation forms induced by lightning surge in superconducting transfomers were examined in detail by the numerical simulation method proposed in the previous paper for ordinary circular disk type, high-series-capacitance type and multi-layer type windings. Ac losses of the superconducting cable due to the major components of magnetic-field oscillation were evaluated quantitatively for each type of winding and compared with proposed theoretical expressions. Reduction method of the ac loss was also discussed by comparing the current responses to lightning surge of several kinds of winding structures for ordinary circular disk and high-series-capacitance type windings with each other.
Induced current by lightning surge increases monotonically with time, while small oscillating components are superposed on it as shown in the previous papers. In this paper, it is shown that a quench occurs when the induced current exceeds the critical current of superconducting cable, even if amplitude of current oscillation is too small to induce quench by ac losses. Firstly a simplified equivalent circuit and basic equations which can describe the quench due to excess current are proposed. Its applicability is confirmed by the fact that obtained analytic results show good agreement with numerically calculated results by full equivalent circuit and basic equations shown in the previous paper. The dependence of peak value of induced current on wave tail duration of lightning surge is estimated for various voltage-level superconducting transformers by taking account of line surge impedance at secondary side and for the number of simultaneous penetration phase. It is shown that the current peak becomes large in case of underground line at the secondary side and a small % impedance. With the aid of the obtained results, methods to evade the quench are discussed in connection to lightning impulse withstand level and the ratio of transportable current of superconducting cable to the rated current.
Boiling heat transport characteristics of pressurized (1atm) He II in a vertically placed rectangular cross-sectional channel have been experimentally and analytically investigated. A wire heater is attached at the midpoint of the channel. The temperature distribution of helium in the bottom closed channel is almost the same as that in the bottom opened channel when the boiling occurs. It is considered from the present results that almost all of the input heat is removed by a natural convection when the boiling occurs and low temperature helium is supplied mainly from the top of the channel. When the heat generation ceases, the temperature of the heater in the bottom opened channel recovers to the He II bath temperature faster than that in the bottom closed channel. The time for the recovery does not depend on He II bath temperature for the case of the bottom opened channel.
To increase the COP of small cryogenic refrigerators which have a reciprocating expander, it is needed to reduce shuttle loss. The shuttle loss consists of the following two factors; shuttle heat transfer and gas pressure cycling. In this report only the shuttle heat transfer was investigated, and new analyses for the shuttle heat transfer were presented by taking into account various phase differences and effective heat-transfer area. Based on results of the analyses and numerical solution, a new method to reduce the shuttle heat transfer was proposed. To discuss totally shuttle loss in the refrigerators, the effect of gas pressure cycling must be further incorporated into the analyses.