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ABSTRACT: It has been widely established that gaseous helium (GHe) can be injected into a sub-cooled liquid nitrogen (LN<sub>2</sub>) cooling system to control the pressure of such a system because of its non-condensing characteristics. A sub-cooled LN<sub>2</sub> cooling system is known as one of the most promising method to develop a large scale superconducting machine. Unfortunately, the electrical breakdown voltage of GHe is not good enough for developing a high voltage electric machine such as a superconducting fault current limiter (SFCL). To enhance the dielectric characteristics of current leads placed in GHe, the use of solid barriers is regarded as an efficient method. In this study, experiments are made on the dielectric characteristics of GHe and the barrier effects and results are analyzed in order to improve the dielectric performance of a high voltage SFCL using a sub-cooled LN<sub>2</sub> cooling system. The dielectric experiments of the energized sphere and the grounded plane in GHe are examined according to various pressures with a solid insulating barrier. The dielectric experiments are performed by installing barriers made of glass fiber reinforced plastics (GFRP) between two electrodes and the electrical breakdown voltage according to the pressure of GHe and the position of a barrier is observed. As a result, it is found that the position of a solid barrier and the pressure of GHe are important in enhancing the dielectric characteristics of a sphere-to-plane electrode system with quasi-uniform field.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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ABSTRACT: Sub-cooled liquid nitrogen (LN<sub>2</sub>) condition is a very useful method to increase the current rating capacity of YBCO tapes. Thus, the lightning impulse dielectric characteristics of sub-cooled LN<sub>2</sub> condition should be investigated for designing high voltage superconducting machines. The sphere-plane electrode systems were used to simulate the internal structure of high voltage superconducting machines such as superconducting fault current limiters. The lightning impulse dielectric tests on saturated and sub-cooled LN<sub>2</sub> condition were carried out by using the sphere-plane electrode systems. In addition, a quasi-uniform electric field distribution between the sphere-plane electrodes was calculated by the finite element method (FEM) simulation tool. The field utilization factor was calculated by the FEM simulation results. The lightning impulse breakdown voltage of various geometrical configurations between the sphere-plane electrodes in sub-cooled LN<sub>2</sub> conditions was compared with that of in saturated LN<sub>2</sub> condition. Finally, the lightning impulse breakdown voltage with 50% probability and its relations with the utilization factor have been described in this paper.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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ABSTRACT: YBCO coated conductor (CC) is useful for application to the insert coil for the NMR because of its great property on critical current under high magnetic field. For the safe operation of CC magnets, design of the stabilizer is imperative issue. The minimum quench energy, minimum propagation zone (MPZ), and quench propagation velocity are obviously affected by both properties and thickness of the stabilizer. In this paper, stacks of CCs were used to be partly substituted for a double pancake magnet for simplicity in experiments, and the CCs in a stack were pressed from top and bottom to enhance thermal and electrical contact. The heater made of Hastelloy C-276 was attached to initiate quench. The stacks which comprise only CCs and pairs of CC and additional copper tape were fabricated, and tests of longitudinal and transverse quench propagation in the stack were carried out in both liquid nitrogen (LN<sub>2</sub>) of 77 K and liquid helium (LHe) of 4.2 K. Copper tapes were attached on each CC without soldering in the stack for the tests to see effects of the thickness of stabilizer and the contact between CC and copper tape. Numerical analysis was performed by using finite element method (FEM). Comparisons of experiment and simulation results were reported and discussed from the point of view of magnet stability and protection.
IEEE Transactions on Appiled Superconductivity 07/2010; · 1.04 Impact Factor
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ABSTRACT: A Hybrid fault current limiter (FCL) proposed by our group previously is composed of a superconducting coil, a fast switch and a bypass reactor. The superconducting coil wound with two kinds of HTS wire has zero impedance when normal current flows in the coil. However, different quench characteristics of the HTS wire generate magnetic flux in the coil when fault current flows in the coil. As a result, the fast switch was opened by the repulsive force applied to the aluminum plate above the coil. In previous studies, our group verified operating characteristics and feasibility of the fast switch. In this paper, comparison of pancake type and solenoid type non-inductive coil wound with two kinds of the HTS wire was performed by using short-circuit test and finite element method. From these results, short-circuit characteristic of a coil can be acquired and magnitude of the repulsive force and magnetic field can be analysed.
IEEE Transactions on Appiled Superconductivity 07/2010; · 1.04 Impact Factor
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ABSTRACT: Development of superconducting power equipments is closely related to that of superconducting wires. Recently, continuing from the first generation (1G) BSCCO wire, commercialization of the second generation (2G) wire, coated conductor (CC), is near at hand. Superpower Inc., has already commercialized 2G wire of 600 m piece length. With the development of CC, superconducting power equipments are actively developed and some research groups reported successful demonstration results of the power devices such as cable, fault current limiter, and so on. Since critical current of commercialized coated conductor (CC) of 1.2 mm width is about 250 A at 77 K, methods like stacking or parallel connection that increase operating current are needed. This paper describes fabrication of large current limiting coil and presents electrical test results. The limiting coil was fabricated by assembling 2 modules in parallel; each module was wound by stacking 4 CC wires on a non-inductive pancake bobbin.
IEEE Transactions on Appiled Superconductivity 07/2010; · 1.04 Impact Factor
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ABSTRACT: The electrical breakdown characteristics of liquid nitrogen should be investigated for developing of a high voltage superconducting machine. This paper deals with the experimental study for the verification of dielectric characteristics of sub-cooled in accordance with utilization factors. AC dielectric experiments were carried out by using sphere-plane electrode systems. The utilization factors of simulated electrode systems were controlled by gap distance between two electrodes and diameter of a sphere electrode. Also, dielectric experiments according to various pressures, 1.25, 1.5 and 2 bar, by gaseous nitrogen were carried out. In this paper, the electrical breakdown criteria of sub-cooled conditions are derived with various sphere diameters. In addition, the sub-cooled condition and saturated condition were compared with AC dielectric characteristics. Research results could be applicable to designing of high voltage superconducting applications such as superconducting fault limiters (SFCLs), cables and large-capacity transformers.
IEEE Transactions on Appiled Superconductivity 07/2010; · 1.04 Impact Factor
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ABSTRACT: In recent years, the YBCO coated conductor (CC) has been developed as a suitable material for power apparatuses. To fabricate a superconducting apparatus of a large capacity, the joint between the superconducting tapes are inevitable because of the need to have a long length conductor. Consequently, the joint characteristic is very important for the efficiency of the superconducting part in the apparatuses. Besides, maintaining the characteristic of the joint between the superconducting tapes even after repetitive over-current is also important for stability of power apparatuses. Therefore the joints between CC tapes were developed in differing joint length and the characteristics of the joints were compared. In this paper, we present the experimental results of several over-current experiments with the jointed CC tapes. The results of the over-current experiments and the trend of the joint characteristics are compared in point of stability of the joints. Resistance and critical current of the joints were measured from the V - I curve before and after several over-current experiments for observation of the trend of the joint characteristics. This study of repetitive over-current characteristics of the joints may be useful to the development of superconducting apparatuses in forms of magnets.
IEEE Transactions on Appiled Superconductivity 07/2009; · 1.04 Impact Factor
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ABSTRACT: YBCO coated conductors (CC) have actively been researched and manufactured these days. Recently, stabilizer-free (SF) CC with Hastelloy substrate has been developed for superconducting fault current limiter (SFCL). The SF CC has higher resistivity than other existing CCs with thick metallic stabilizer. Higher voltage per unit length applicable can be increased. In this research, the bifilar pancake coil using SF CC was manufactured and characteristic tests of the coil for SFCL were performed. Though the bifilar coil is supposed to have no inductance during normal operation, the transport current loss is be produced due to AC current flowing through the SF CC. AC loss was measured and short-circuit test was performed in liquid nitrogen. In order to develop large scale SFCL, it is indispensable to consider the dielectric characteristics of pancake module. Dielectric breakdown tests were carried out to find applicable maximum voltage in one pancake module. These results provided to suitable design parameters and showed technical feasibility of pancake type module as an SFCL.
IEEE Transactions on Appiled Superconductivity 07/2009; · 1.04 Impact Factor
