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The development of a 1MJ conduction-cooled low temperature superconducting (LTS) pulse coil for uninterruptible power supply (UPS) superconducting magnetic energy storage (SMES) is discussed. Results suggest that the total cooling capacities of crycoolers are sufficient for the main steady heat loads from 1 kA high temperature superconducting (HTS) current leads.

Two-dimensional finite elementary method is used for examining the stability margin of a conduction-cooled low temperature superconducting (LTS) pulse coil with stored energy of 1 MJ. Results suggest that the 1 MJ coil has a large stability margin, which is eight times the rated ac loss.

A conduction cooled LTS pulse coil of 100 kJ class is developed by using a new winding method, in order to provide a principle actual proof of UPS-SMES of MW class as protection from momentary voltage drops.

Electrical loss measurements are carried out in liquid nitrogen for long Bi-2223 tapes wound into a solenoidal coil under various external conditions of ac transport currents and ac magnetic fields. A new measuring-system, which is an extension of our original measuring system applicable to short and straight HTS tapes, is developed.

Bi-Sr-Ca-Cu-O wires with transposed filaments were studied to determine reduction of transport current losses.

The UPS-SMES is developed as a protection from momentary voltage drop and power failure. The conclusion states that it is difficult for the conventional LTS coils to fulfill maintenance-free operation since the cooling methods are either pool boiling with liquid helium or forced flow of supercritical helium.

Highly efficient liquid hydrogen storage system is studied with magnetic levitation using high-temperature superconducting (HTS) coils. The system also has high safety in case of emergency, such as an earthquake, with a seismic isolation to absorb vibrations provided by HTS levitation coils setup on the ground side. In such an emergency case, it is considered that a large amount of ac losses are generated in HTS coils, and the winding temperature may rise to lead to a coil quench. In this study, the self-oscillation-type heat pipe (OHP), whose thermal transport property is much greater than that of solid thermal conduction, is used to cool the coil windings. As a result, an HTS coil equipped with an OHP cooling system can be realized, supporting both low heat loads in the usual operation and high heat loads in an emergency. [ABSTRACT FROM PUBLISHER]

A new quench monitoring system by using the Poynting Vector method was proposed. This new system can indirectly detect normal transitions which are suddenly induced during an operation of superconducting coils. In this system, the superconducting coil is surrounded by the thin metallic cylinder composed of several pieces, where the cylinder is electrically insulated from the coil; potential leads to measure local electric fields are directly attached to this cylinder surface and pickup coils to measure local magnetic fields are put on the surface. The Poynting vector distribution around the coil can be obtained by using many sets of both the potential lead and the pickup coil. Since sudden changes of energy flow profiles around the superconducting coil are induced in the event of local normal transitions of the coil winding, such an event of coils can be detected by the change of the Poynting vector. This system has no risk of the destructive voltage breakdown which may happen in case of the existing balance voltage method, because there is no need of direct contacts on the coil windings. In addition, this method is expected to be higher reliability and sensitivity than the acoustic emission method. A demonstration test was carried out of monitoring the quench for the Bi-2223 coil partially immersed in liquid nitrogen, and a sign of normal transitions before the quench was successfully detected by this method. [ABSTRACT FROM AUTHOR]

In order to evaluate the transport current loss characteristics in HTS conductors with large current capacity, we developed a new compact measurement system. This system enables us to measure the transport current losses electrically in the straight HTS conductors with the current capacity of 1 kA class by using the small capacity AC power supply of about 50 A. In this system, an HTS current transformer is used to feed current to the sample conductor for the measurement. The secondary current of this current transformer can be induced at about 1 kA at 77 K when the primary current is 40 A. The current transformer is made very compactly, and the externals size is 70mm in diameter, and 110mm in length. The sample conductor is connected with the secondary coil of this current transformer. The loss measurement is done by the Poynting vector method. The distribution of Poynting vectors around the sample conductor can be obtained by using many sets of both a potential lead and a pickup coil. To confirm the validity of this measurement system, we measured the transport current losses in an HTS conductor. As a sample conductor, we used a parallel conductor composed of 16 Ag-sheathed Bi-2223 multifilamentary tapes. The obtained results showed that this compact system was useful for measuring AC loss characteristics of large scale HTS conductors. [ABSTRACT FROM AUTHOR]

The development study of a 1 MJ conduction-cooled low temperature superconducting (LTS) pulse coil used for a 1 MW, 1 s UPS-SMES is summarized. We have developed a conduction-cooled LTS pulse coil as a key technology for the UPS-SMES. The AC loss reduction and the high stability are required for the SC conductor for a LTS pulse coil because of a limited cooling capacity of 4 K cryocooler. The conductor of a NbTi/Cu compacted strand cable extruded with an aluminum was designed to have the anisotropic AC loss properties to minimize the coupling loss. The coil was wound, utilizing a specially developed automatic winding machine which enables an innovative twist-winding method. The Dyneema FRP (DFRP) spacers and the Litz wires (braided wires of insulated copper strands) were inserted in each layer in order to enhance the heat transfer in the coil windings. The coil was installed in the test cryostat and was connected to three GM cryocoolers, which have a total cooling capacity of 4.5 W at 4 K and 240 W at 50 K. The coil was cooled conductively without liquid helium by attaching the end of the Litz wires directly to the cold heads of the cryocoolers. The cooling and excitation test of the 1 MJ coil has been done successfully. The test results validated the high performance of the conduction-cooled LTS pulse coil, because the high thermal diffusivity resulted in the rapid temperature stabilization in the coil. [ABSTRACT FROM AUTHOR]

We have been developing a 1 MW, 1 sec UPS-SMES for the protection of production lines of an industrial plant or large-scale experimental devices such as a fusion device from a momentary voltage drop and an instant power failure. A conduction cooled prototype LTS pulse coil of 100 kJ class was developed as a key component of the UPS.SMES. The prototype coil has demonstrated excellent thermal characteristics during cooling and exciting tests. In this paper, measurements of the temperature in the coil during experiments and thermal analysis by using two-dimensional finite element methods were compared to clarify the high heat transfer properties of this prototype coil. This coil was wound with a NbTi/Cu Rutherford cable, which is extruded with aluminum. In order to realize the conduction cooled LTS pulse coil, FRP with polyethylene fibers (Dyneema FRP) and Litz wires were used as spacers. Dyneema FRP improves the heat transfer from layer to layer in the windings. Litz wires increase the heat transfer from turn to turn in the windings and enable conduction cooling of the coil by attaching the end of the Litz wires directly to the cold heads of the cryocoolers. It was clarified that these spacers were very effective and the coil has a large stability margin in terms of the design values. [ABSTRACT FROM AUTHOR]

A conduction-cooled low temperature superconducting (LTS) pulse coil has been developed as a key technology for UPS-SMES. We have been developing a 1 MW, 1 s UPS-SMES for a protection from a momentary voltage drop and an instant power failure. A conduction-cooled LTS pulse coil has excellent characteristics, which arc adequate for a short-time uninterruptible power supply (UPS). The LTS coil has better cost performance over the HTS coil at present and the conduction cooling has higher reliability and easier operation than the conventional cooling schemes such as pool boiling with liquid helium or forced flow of supercritical helium. To demonstrate the high performances of the LTS pulse coil, we have fabricated a prototype coil with stored energy of 100 kJ and have conducted cooling and excitation tests. The successful performance test results including current shut-off test with a time constant of 1.3 s and repeated excitation of a triangular waveform with high ramp rate are reported. [ABSTRACT FROM AUTHOR]

Inter-strand coupling loss measurements were carried out in liquid helium for Bi-2212 Rutherford cables composed of 30 non-twisted strands. Short, straight samples composed of three stuck cables were set in a uniform field space inside the NbTi magnet of race-track and split type. Frequency characteristics of coupling losses were observed for transverse ac magnetic field, both perpendicular and parallel to the flat face of cables. An additional measurement of ac losses was carried out in order to clarify properties of hysteresis losses and intra-strand coupling losses. The obtained effective filament diameter describing the hysteresis loss property indicated that almost all filaments in strands were strongly connected to each other. From both this result and the frequency characteristics of coupling losses for the perpendicular field case, the inter-strand coupling time-constant was given in the order of 10 m. Moreover, the effective contact resistance between crossover strands in the cable was also given in the order of 10-10 Ωm²,which was the same order as that for the NbTi Rutherford cable composed of Cu matrix strands. [ABSTRACT FROM AUTHOR]

A new type of cable-in-conduit conductor composed of HTS tapes was proposed as a winding for high-performance HTS coils with high current capacities and low AC losses. In the fabrication of the conductor, the twist of stacked tapes around their axes was made before inserting them inside conduits. The twist angle should be changed continuously along the axis to reduce face-on oriented magnetic fields applied to the tape in the winding conductor during coil operation. In order to confirm the high current capacity of this type conductor, two single-layered solenoidal coils wound with sample conductors composed of five stacked Bi-2223 tapes with bias angles of 20 and 0° were fabricated and tested in liquid nitrogen. A copper magnet system was used in this experiment to generate the spread magnetic field with a spread angle of 20° from the coil axis to the radial direction, which simulates the profile of magnetic fields near the edge windings of practical coils. A large improvement on critical-current degradation affected by spread magnetic fields was successfully observed for the test coil wound with the 20° bias conductor. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 153(4): 12–19, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20218 [ABSTRACT FROM AUTHOR]

We are planning to develop a 1 MW, 1 sec UPS-SMES for a protection from a momentary voltage drop and an instant power failure. As the first step, we have been developing a 100 U class prototype UPS-SMES, using a low temperature superconducting coil because of its better cost and performance over the high temperature superconducting coil. However, the difficulty to utilize a pool-boiling LTS pulse coil is the reliability of operation. To solve this problem, a conduction-cooled LTS pulse coil has been designed and fabricated as a key component of the UPS-SMES. The reduction of AC loss and high stability are required for the SC conductor for the conduction-cooled coil because of a limited cooling capacity. The SC conductor of a NbTiICu compacted strand cable extruded with an aluminum is designed to have the anisotropic AC loss properties to minimize the coupling loss under the specified orientation of the time varying magnetic field. The coil was wound with a new twist-winding method in which the variation of twist angle of the conductor was controlled with the winding machine designed specifically for this purpose. The fabrication technique and performance of a conduction-cooled prototype LTS pulse coil are described. [ABSTRACT FROM AUTHOR]

The stability of a prototype conduction-cooled LTS pulse coil for UPS-SMES of 100 U was evaluated. This coil has been developed as a first step of a project to develop a 1 MW, 1 UPS-SMES to protect semiconductor chip production equipment and nuclear fusion experimental devices, etc, from momentary voltage drop and power failure. The winding conductor is an NbTi/Cu Rutherford cable, which is extruded with aluminum. This conductor has both low AC losses and high stability under specified orientation of changing transverse magnetic fields. The 100 U-coil are wound by the new winding method. In order to improve the heat conduction properties in the coil, Dyneema FRP and Litz wires are used as spacers. Litz wires were connected with the cryocooler as cooling paths. On the pulse operation, the operating current is reduced from 1000 A to 707 A in 1 s. In this paper, the thermal properties of the 100 kJ-coil are calculated by finite element method under pulse operation. In order to estimate the stability, a calibration experiment was carried out. Results indicated that our prototype LTS pulse coil has high stability to enable to allow over 10 times as large heat as AC losses. [ABSTRACT FROM AUTHOR]

We have been developing the UPS-SMES as a protection from momentary voltage drop and power failure. The superconducting system is suitable as electric power storage for large energy extraction in a short time. The most important feature of superconducting coil system for the UPS-SMES is easy handling and maintenance-free operation. We have selected Low Temperature Superconducting (LTS) coils instead of High Temperature Superconducting (HTS) coils from the viewpoint of cost and performance. However, it is difficult for the conventional LTS coils to fulfill maintenance-free operation since the cooling methods are either pool boiling with liquid helium or forced flow of supercritical helium. Thus, a conduction cooled LTS pulse coil has been designed as a key component of the UPS-SMES. The development program of 1 MW, 1 sec UPS-SMES is explained. [ABSTRACT FROM AUTHOR]