A Nb₃Sn supercouducting magnet to be stored 400 kJ was developed as a unit magnet for 2.4 MJ-SMES system used for stabilization studies of electrical power systems. The superconducting magnet consists of a cryostat and a Nb3Sn coil, the dimensions of the coil is 340 mm inner diameter, 700 mm outer diameter and 177 mm axial length. The pool cooled coil is a stacked up of twenty Nb₃Sn double pancakes, and the cooling channels are aligned between pancake coils. To reduce Joule loss in electrical power converters, the maximum operating current of the coil is designed to be 350 A which is one order less than the operating currents of other same scale coils for pulse use. Conductor is Nb, Sn monolithic conductor of which cross section is 1.50×2.38mm². For good superconducting stability and high dielectric strength of the coil, the Nb₃Sn double pancakes were wound by react-and-wind technique.
Operation of DC current to 105% (367.5 A) of the design operating current was achieved without quench. After the whole of the coil exposed out of liquid helium, the coil did not quench under 120 A current operation during over 2 hours. It was verified that the coil was stable for the SMES system.

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This paper deals with electrical insulation design of pool boiling helium cooled superconducting (s. c.) coils. Stressing voltage and insulation environment at normal operation and quenching state of s. c. coil are evaluated and a ratio of withstand voltage of the insulation system to the induced voltage for a studied s. c. coil is estimated. On the basis of these studies, main parameters predominating insulation space in the s. c. coil are found: delay time of quench protection switch, quench protection method, initial coil current, coil inductance and conductor geometry and size.

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A quench phenomenon is very important for superconducting power devices, such as SMES, superconducting generator, superconducting current limiter, and so on. Because the reliability of such devices depends on the stability against quench, in which superconducting state varies into normal state rapidly. We have produced two kinds of coils, and investigated experimentally the relation between stability and several parameters, for example, external magnetic field, electrical current, duration time of heating by a heater. A quench phenomenon has been simulated numerically with the same conditions as experiments, and the stability has been estimated quantitatively. As the result, it is found that the larger an external field or a coil current, the less stabilized for quench. Experimental results have been reproduced on our simulation. We improved our two-dimensional quench simulation code considering the delay time of heat transfer due to insulator and the thermal capacity of insulator and spacer. An expansion of quenched region has been recorded with high speed video camera, and the two-dimensional expansion of quenched region such as quench propagation between superconducting wires has been reviewed on our simulation.

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A national project of development on superconducting generator has been running from 1988 in Japan, because the generator has merits of stability improvement on transmission line, higher efficiency and compactness. The project has been carried out under the New Sunshine Project of AIST, MITI. 70 MW class superconducting generator is the goal of the project. Hitachi has been developing a rotor having superconducting field winding and a stator having air-gap armature winding. The filed winding is designed to be cryostable so as to recover from partial transition to normal conduction. The field winding was tested in non-rotating cryostat, and excitation performance and stability are confirmed to be enough for the 70MW class superconducting generator. A site test will be started in 1996. Now, the rotor and the stator are under construction and going to be completed in 1996.

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To realize the superconducting power apparatuses, it is very important subject to develop the fixing method of the conductor, especially for AC superconductor. For actual power apparatuses, the shapes of windings are very complex. In addition to the intricacy, the electro-magnetic force in superconducting winding is quite larger than normal-conducting winding, because of the high magnetic field and high current density of the winding. Moreover, stability of AC superconductor is lower than that of DC superconductor, because of high resistive matrix and continuous generation of AC losses. So, it is needed to fix the conductor rigidly for preventation of a disturbance like a conductor motion that has a possibility of cause for quenching.
In this paper, we developed a new winding method for AC superconductor and examined the attainment of this method. Although this method needed low AC loss superconductor, the coil wound by this method showed good stable performances.

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We have been developing optimal design methods for high-field multi-section superconducting magnets for MRI and NMR study using mathematical programing method. To enhance the central magnetic field homogeneity, notch coils are added outside of the main solenoid. In this design, there is difficulty that the distances between notches are continuous design variables while the number of turns and layers of the main solenoids and notch coils are discrete variables. So we tried to develop a method which can deal with such two kinds of design variables at the same time by applying a modified simulated annealing. However, its convergence is relatively slow. Therefore, we adopt the Genetic algorithm, which can obtaine an optimal solution quickly, and combine it with the modified simulated annealing. Furthermore, we developed those method so as to be applied to non-linear optimization problems with constraints, such as characteristics of superconductor (e. g. B-J characteristic), Lorentz force and so on. The details of the algorithm and several examples of its application to three-section superconducting magnets are shown.

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We investigated the area and volume effects on the breakdown strength in liquid helium and liquid nitrogen, and also carried out the statistical analysis using the Weibull distribution. From the experimental results, we quantified the area and volume effects in cryogenic liquids over a wide range of the stressed electrode area (SEA) from 10^-1 to 10⁵ mm² and the stressed liquid volume (SLV) from 10^-2 to 10⁵ mm³. Moreover, we examined the electrical insulation design of cryogenic liquids including the area effect and the volume effect and proposed the fundamental flow chart for the practical insulation of superconducting power apparatus. Finally, we pointed out that the stress level and the V-t characteristics should be estimated with considering the peculiar phenomena arising from the cryogenic environments such as the thermal bubble behavior and the quench-induced dynamic breakdown and so on.

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This paper describes the mechanism of quench-induced dynamic breakdown phenomena of liquid helium (LHe). The dynamic breakdown of LHe is induced under thermal bubble conditions due to quench of a superconducting wire at a considerably lower stress level than the static breakdown voltage without the quench. We measured partial discharge (PD) characteristics of LHe during the process from quench-onset to dynamic breakdown. The results revealed that PD were detected after quench and followed by the dynamic breakdown of LHe. This event corresponds to the process of bubble generation, growth and extinction in the vicinity of the superconducting wire with high electric field strength. There also existed the polarity difference of applied voltage in PD characteristics, i. e. PD were detected only in the negative polarity of superconducting wire. Consequently, PD associated with the thermal bubble behavior were recognized as pre-breakdown phenomena of the quench-induced dynamic breakdown of LHe.

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We studied pulse magnetic field using a small coil to magnetization of the high-T_c superconductor (HTSC), and assessed its pulse magnetization property. Because the HTSC bulk material has grains of various sizes, its magnetic properties are not uniform. Therefore, it is important for application of the HTSC to develop a nondestructive measurement method in order to evaluate characterization of magnetic properties of the superconducting bulk materials. We developed a magnetic field visualization system that enables to measure magnetic field using a Hall device probe and that produces visual images of the magnetic field on a computer display. The domains of large value of magnetization coincide with those of the grains. And we can see that the negative magnetic field is around the domain of the large magnetization value. Pulse magnetization method using the small coil can magnetize the HTSC without the field-cooling.

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As a usage of a superconducting transformer in a power system, parallel operation with a conventional transformer using copper windings is proposed and experiments using a 40kVA superconducting transformer have been carried out. In case of quenching of the superconducting transformer due to huge fault current in a power system, current commutation from the superconducting transformer to the conventional transformer occurred, and fault current was limited by impedance of the conventional transformer. After removing the fault, the superconducting transformer was successfully connected to the system without quenching, again. Although a superconducting transformer cannot continue to operate in case of quenching, the proposed parallel system can overcome the weakness and give us an additional value of the fault current limiting function.

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The AC superconducting power apparatus have been investigated actively in recent years. The air-core superconducting power transformer to which the authors are paying attention is one of such superconducting apparatus. The air-core superconducting transformer has no iron loss, magnetic noise and harmonics problem, which are caused by the iron-core, and it has more possibilities of reduction of loss, size and weight than the conventional and iron-core superconducting power transformers. However, since the air-core transformer has no specific path for the magnetic flux, the magnetic field pro-duced by its superconducting windings acts directly on the windings. Therefore, the investigations of the electromagnetic force, loss and stability of the superconducting windings are very important. In order to investigate these points, the quantitative analysis of the magnetic field acting on the superconducting windings is necessary.
In this paper, a calculation method of the magnetic field is presented. Then, by using this calculation method, the magnetic field and electromagnetic force acting on the superconducting wire are analyzed. From the results of these analyses, it is clarified that the distribution of the amplitude of the magnetic field and electromagnetic force acting on the wire are not uniform. It is also clarified that the magnetic field acting on wire under load has a rotating component in addition to the alternating component.

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×The quench current level of a superconducting power apparatus may be decreased by the self-magnetic field induced by its own current. As the self-magnetic field is spatially distributed in the apparatus, each section of the superconducting cable is exposed to the different magnetic field in the magnitude as well as in the direction. In this paper, the quench current level and quench initiation position are discussed for a 6kV-200A class superconducting fault current limiter(SC-FCL) which has been developed by us. We calculated spatial profile of strength and direction of self-magnetic field in the SC-FCL. As a result, the self magnetic field is applied to almost section of the superconducting cable in transverse direction (maximum value : 6.42×10^-4T/A), but the connection section between coils suffers the magnetic field with longitudinal component of 3.34×10^-4T/A.
On the other hand, quench current levels of a short sample of superconducting cable which was used for the SC-FCL were measured for the different magnetic field externally applied in the same, transverse and opposite direction to that of the transport current of the cable. The quench current level decreases at the rate of 100A par OAT in same direction magnetic field and 40A per OAT in transverse direction. However, in the case of opposite direction of magnetic field, the quench current level did not change with the magnitude of the magnetic field.
Taking these results into consideration, it can be pointed out that the quench may be initiated in the connection section between coils where the self-magnetic field is applied in the same direction. Furthermore, we discuss the configuration of the SC-FCL coils to suppress the decrease of the quench current level.

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We are developing high-temperature-superconductor (HTS) current leads for 1MW/lkWh module type SMES system. In module SMES, a pair of current lead would be installed in each module so that the heat load to low temperature region through leads must be minimized. Design study of the lead configuration including the safety lead and its thermal characteristics is presented herein. The safety lead composed of stainless steel is placed in parallel to the bulk HTS.
The safety lead experimental fixture has been developed and the simulated bulk HTS quench test was demonstrated. The maximum temperature of the safety lead reached 200K and the terminal voltage of the safety lead was only 1.2 V in case of 1000A quench test. This temperature rise was well below the designed value.

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The development of the superconducting generators (SCGs) by Super-GM, which is the biggest on-going SCG project in the world, will commence various tests on its 70 MW class machine for the purpose of verification of its operational performance and mechanical and thermal strength. So called pump-back configuration will be used in these tests.
In the pump-back configuration (or more simply MG method) the SCG will be connected to the synchronous motor (SM) electrically and mechanically, which enables the SM to drive mechanically the SCG using the electrical power from the SCG. Detailed study on the operational performance and the test condition of the MG method is very much required.
This study presents an excitation control scheme which properly regulates voltage and power. A compact model is proposed which represents the dynamic characteristics of the MG system. This model gives appropriate testing methods of voltage response and quench. It is clarified that a combinatorial test of no-load low-voltage and 180 degrees off-phase reconnection will give the disturbance equivalent to the case of three phase short circuit applied at the high voltage bus of the step-up transformer.

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Regarding a design problem of a closed cycle MHD generator as a shape optimization problem, the author attempts to design a generator by using the direct search method. It is shown that the generator design is possible by adopting sready state quasi-one-dimensional equations as an evaluation method, and by using genetic algorithms as a search method. As a result, the possibility of generator design by means of two- or three-dimentional calculations, which have been developed up to now, is suggested.

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Tower surge responses were computed by the numerical electromagnetic code. The code used in this paper is not the one based on the circuit theory such as the Electro-Magnetic Transients Program (EMTP), but the one that solves the electric field equations directly by the moment method. The accuracy of this method was shown to be satisfactory by comparison with experimental results that were carried out on simple structures. And then by using this method, the influences of tower elements such as slant elements, horizontal elements or crossarms on the tower surge characteristics were clarified.

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