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ABSTRACT: Recently, high-temperature superconductor (HTS) cables have been widely studied because of their compactness and high power
capacity compared to conventional copper cables. In HTS cables, AC loss is an important issue since large losses reduce the
efficiency of the power line.
Among HTS cables, tri-axial cable is under intensive investigation recently, since it has a smaller amount of HTS tapes, small
leakage fields and small heat loss in leak when compared with the three single-phase cables. For realizing high current capacity,
more than one layer is required for each phase; therefore AC loss of the multi-layer tri-axial HTS cable should be carefully
examined.
In the tri-axial cable, different phase currents produce the out-of-phase magnetic fields on the other phase layers. In case
of multi-layer arrangement, net magnetic fields on layer surfaces may exceed the penetration field of the HTS tape. Therefore
in this paper, we analyze the AC loss of a tri-axial HTS cable which is composed of two layers per phase. Here, we treat the
tri-axial cable which consists of two different longitudinal segments and thus satisfies balanced phase and homogeneous current
distribution condition by controlling twist pitch and length of separate segments.
KeywordsAC loss–Balanced current distribution–HTS power cable applications–Transmission lines
Journal of Superconductivity and Novel Magnetism 04/2012; 24(1):975-980. · 0.65 Impact Factor
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D. Miyagi,
S. Teshima,
S. Nakazawa,
D. Arai,
M. Tsuda,
T. Hamajima, T. Yagai,
N. Koizumi,
Y. Nunoya,
K. Takahata,
T. Obana
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ABSTRACT: In order to investigate superconducting properties such as long decay time constants, current imbalances and critical current degradations, we need detailed information about all strand locations in the cable-in-conduit conductor (CICC), and hence we develop a new estimation method to obtain all strand locations. It is very difficult to estimate all strand positions because all strands in the real CICC are squeezed into the conduit and are not regularly arranged but displaced. In order to estimate these strand displacements due to the compression, we introduce mechanical potential energy among strands, and hence we search the minimum energy locations which should be realized in the conductor. In order to calculate this process, we perturb all strands from the original locations and continue these processes by using a genetic algorithm until the potential energy is minimized. This analytical method is very useful to simulate all strand positions and allows us to investigate all the electromagnetic phenomena in the CICCs.
IEEE Transactions on Applied Superconductivity - IEEE TRANS APPL SUPERCONDUCT. 01/2012; 22(3):4802104-4802104.
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ABSTRACT: An imbalanced current distribution is often observed in cable-in-conduit (CIC) superconductors which are composed of triplet type multi-staged sub-cables, and hence deteriorates the performance of the coils. Since it is very difficult to control homogeneous current distribution in the triplet type CIC, we propose a coaxial multi-layer type CIC to obtain the homogeneous current distribution. We use a circuit model to analyse the current distribution in the coaxial multi-layer CIC. After calculating inductances between adjacent layers in the coaxial multilayer cable, we can derive a generalized formula governing the current distribution as explicit functions of the superconducting cable construction parameters, such as twist pitch, twist direction, layer radius and SC and Cu strands number. We apply the formula to design the coaxial multi-layer CIC for SC magnet of Force Free Helical-type Fusion Reactor (FFHR). We can design the coaxial multi-layer CIC with the homogeneous current distribution, and investigate several SC strand arrangements in the CIC, and optimize the superconducting strand volume.
IEEE Transactions on Appiled Superconductivity 07/2010; · 1.04 Impact Factor
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ABSTRACT: Superconducting Fault Current Limiters (SCFCLs) have been intensively developed around the world these years, and the commercial SCFCL is expected to be available in near future. The main target of SCFCL include not only negligible small impedance under normal operation, but also fast and effective suppression of large fault current within the first current rise, and moreover repetitive operation with fast and automatic recovery. We designed a new type of three-phase SCFCL which is composed of a three-phase winding reactor type FCL and a magnetic shield type superconducting FCL. The proposed SCFCL is effective for symmetrical fault as well as unsymmetrical faults. In order to verify functions of the proposed SCFCL, we fabricate a small device and carry out the experiments. It is found from the test results that the proposed new type of three-phase SCFCL works to restrict the fault currents in all kinds of fault conditions. Moreover, the simulation results of EMTP have good agreements with the test results.
IEEE Transactions on Appiled Superconductivity 07/2010; · 1.04 Impact Factor
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ABSTRACT: We have investigated levitation force characteristics of a magnetic levitation type superconducting seismic isolation device composed of HTS bulks and permanent magnets. Large levitation force at a small gap of less than 3 mm has been already obtained, while only small levitation force at a large gap of more than 5 mm. In a practical use of the superconducting device, the HTS bulks should be cooled by not LN<sub>2</sub> but a cryocooler. This means that large levitation force at a large gap of more than 5 mm is required due to a vacuum layer thickness in a cryostat. Therefore, we investigated the suitable size and arrangement method of the permanent magnet and the HTS bulk to improve the levitation force at a large gap of more than 5 mm effectively. By adopting a permanent magnet arrangement of "Halbach array", we could greatly improve the levitation force at a large gap of more than 5 mm. Based on the experimental and computed results of levitation force and magnetic field distribution, we could achieve further improvement of the levitation force by optimizing the size of the permanent magnet and the HTS bulk. The suitable width and thickness of the permanent magnet were 10 mm against a disk-shaped HTS bulk 32 mm in diameter and the suitable thickness of the HTS bulk was less than 2 mm at a gap of more than 5 mm.
IEEE Transactions on Appiled Superconductivity 07/2010; · 1.04 Impact Factor
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ABSTRACT: Non-uniform current distribution in a conductor composed of multiple HTS tapes is one of the important problems for increasing current carrying capacity of a large-scale coil system such as Superconducting Magnetic Energy Storage (SMES) and DC reactor. We focused on a manufacturing error in the HTS tape thickness and analytically investigated the relationship between the manufacturing error and the non-uniform current distribution. Even in a transposed conductor, the non-uniform current distribution is easily occurred by a small error in the tape thickness due to high coupling constant among the tapes. The non-uniformity of the current distribution reduces the current carrying capacity. However, the non-uniform current distribution can be improved by adopting a pancake coil with a large radius and small number of turns. An HTS tape with large thickness is also effective for reducing the non-uniformity of current distribution. Adopting parallel double pancake coils composed of single HTS tape is one of the promising candidates for increasing current carrying capacity effectively without any problems of the non-uniform current distribution. The suitable connecting method among the parallel double pancake coils for uniform current distribution, however, should be developed.
IEEE Transactions on Appiled Superconductivity 07/2010; · 1.04 Impact Factor
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ABSTRACT: Tri-axial cable has been rapidly developed recently because of the advantages such as more compact structure, small leakage field, and low heat and AC losses. One the other hand, it causes an inherent imbalanced three-phase currents distribution due to the different radii of concentric layers. In our previous research, it is demonstrated that a proposed tri-axial cable composed of two longitudinal sections with adjusted twist pitches can transmit balanced three-phase currents. However, before the cable can be installed in a real grid, the heat reliability of the cable in steady-state operation and fault condition should be ensured. The heat balance calculation of the tri-axial cable in steady state under the balanced current distribution allows us to estimate thermal parameters of the cable such as inlet and outlet temperatures, pressure drop and operation temperature. When a phase current suddenly changes under fault conditions, the remaining phase currents become large because of mutual inductances between fault and sound phases. Simulation of the typical fault suggests the copper stabilizer amount on the HTS tape for stability and safety. It is found that copper of 2 mm in thickness keeps the cable temperature under 110 K.
IEEE Transactions on Appiled Superconductivity 07/2010; · 1.04 Impact Factor
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ABSTRACT: High Temperature Superconducting (HTS) cables have been intensively developed because of low loss and compactness, compared with conventional copper cables. A tri-axial cable composed of three concentric phases has been studied, because it has advantages such as reduced amount of HTS tapes and low heat-in-leak, compared with the three single-phase cables. However, there is an inherent imbalance in the three-phase distribution in the tri-axial cable due to differences in radii of the three-phase layers. We proposed a theory to obtain the balanced three-phase distribution for the tri-axial cable by treating two longitudinal cable sections together and adjusting all twist pitches. We derived a generalized formula as functions of winding pitches satisfying the balanced distribution. We designed and fabricated a short HTS tri-axial cable composed of 1 layer/phase to verify the proposed theory. The test results demonstrated that the theory is right for an equivalent impedance circuit model. The theory should be applied to the unbalanced three phase distributions caused by fabrication errors and inherent imbalance of capacitances in the tri-axial cable. We calculate the unbalanced three phase currents and voltages in steady state, and resolve them into symmetrical components to evaluate an imbalance ratio, which is defined as zero-sequence or negative-sequence to positive-sequence component. It is found that the fabrication errors of twist pitch and radius cause the imbalance ratios less than 1%, and the unbalanced capacitances of the cable of 10 km in length cause imbalance ratios of about 1%.
IEEE Transactions on Appiled Superconductivity 07/2009; · 1.04 Impact Factor
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ABSTRACT: Coupling current losses in large scale cable-in-conduit conductors (CICCs) for fusion apparatuses are sometimes annoyance because nobody can estimate how large the coupling loss is before fabricating hundred-meters of long conductor. Our approach for analyzing and estimating the loss based on real strand traces is unique compared with that of other research groups. In process of this approach, we have already measured the traces in two types of CICCs, one has circular cross section and the other has rectangular one. The cabling pattern of the former is 3<sup>4</sup> = 81 the latter is 3<sup>4</sup> times 6 = 486. The flux linkage area in a coupling current loop which consists of two contacting strands is evaluated as an indicator of driving force of the coupling current, which is proportional to the square root of AC loss per unit time. The flux linkage areas showed that the losses of rectangular-shape CICC would be larger than that of circular-shape CICC.
IEEE Transactions on Appiled Superconductivity 07/2009; · 1.04 Impact Factor
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ABSTRACT: Using a model levitation system composed of an HTS bulk and permanent magnet rows, we investigated the dynamic characteristics of vibration transmission against a vertical vibration as functions of the weight of a levitating object, vibration amplitude, initial and actual gaps between the bulk and the permanent magnet rows. The bulk vibrated in substantially synchronism with the permanent magnet rows and the waveform of relative displacement between the bulk and the permanent magnet rows was sinusoidal. The vibration transmissibility measured in the frequency range below 5 Hz was between 1.00 and 1.08. Using the experimental results of spring and damping constants, we theoretically evaluated the natural frequency and vibration transmissibility of the model system in the frequency range of 0 Hz to 100 Hz. The natural frequency decreased with the weight of the levitating object at a constant actual gap. This means that the vibration removal performance is improved by increasing the initial gap. The larger actual gap at a constant weight of the levitating object was effective for improving the vibration transmissibility in the vibration frequency range above the natural frequency, while the smaller actual gap was effective for improving the damping effect. Therefore, it is important to choose the most suitable field-cooling condition of the bulk by considering the trade-off relationship between the vibration transmissibility and the damping effect according to the weight of the levitating object.
IEEE Transactions on Appiled Superconductivity 07/2009; · 1.04 Impact Factor
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ABSTRACT: For future power system, a micro power grid system, which is mainly composed of several power modules, such as superconducting (SC) cable, superconducting magnetic energy storage (SMES) system, hydrogen system, fuel cell (FC) system, renewable energy modules, and power converter modules, is expected. In the grid system, hydrogen mainly produced by renewable energy is liquefied for cooling down of the SC cable and SMES, and is stored in a tank for generation of the electric power through the FC. Since the SMES has quick response to power fluctuation and the fuel cell with the hydrogen can supply constant electric power for longer time, the combination of the SMES and the FC can generate highly qualified electric power. The cable can simultaneously transfer both electric power and hydrogen fuel with refrigerant energy. We investigate functions of various power modules and simulate the power balance of the micro grid to estimate the energy recovery rate. It is found that the proposed micro grid can reduce the energy transfer loss.
IEEE Transactions on Appiled Superconductivity 07/2009; · 1.04 Impact Factor
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ABSTRACT: Coupling loss with long time constants has been found a troublesome phenomenon for large size magnet application of superconductor because it would not be simply estimated from AC loss measurement of short sample conductor. In order to investigate the mechanism of the loss, we measured trajectories of strands of sample CIC (3<sup>4</sup> = 81) conductor. The measured length is 1 m along the conductor axis. By analyzing those trajectories, two important facts are cleared. One is that contact periods between two strands are calculated by the function of twisting pitches, not always by the Least Common Multiplier of twisting pitches. The other is that contact probabilities of two strands at each contactable point of sub cables depend on the difference of rotation angles of sub cables. To confirm the validity of this method, we calculated lengths of coupling current loops within 1 m in length by using experimentally obtained contact probabilities. The results are in very good agreement with lengths obtained from experimental results of strand trajectories. Then we computed the loop length in the long conductor (<100 m), it was obtained that the average loop length would reached about 3 m.
IEEE Transactions on Appiled Superconductivity 07/2008; · 1.04 Impact Factor
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ABSTRACT: We have developed a magnetic levitation type seismic isolation device. The device could remove any horizontal vibrations very effectively. Three-layer structure was adopted in the device. The bottom layer was composed of permanent magnets. The middle layer comprised HTS bulks and permanent magnets. The top layer consisted of HTS bulks. The horizontal vibration characteristics in the middle layer depended on the air gap between the bottom and middle layers; the air gap was closely related to the load weight distribution in the top and middle layers. Inhomogeneous load weight distribution caused the different air gap in each bulk. Horizontal levitation against the inhomogeneous load weight distribution, however, could be obtained by adjusting the initial offset angle of each bulk in field-cooling process. The smaller initial air gap was effective in realizing horizontal levitation; the difficulty of horizontal levitation increased with the initial air gap. Independent of the load weight distribution, almost the same characteristics of horizontal vibration were obtained in the small initial air gap.
IEEE Transactions on Appiled Superconductivity 07/2008; · 1.04 Impact Factor
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ABSTRACT: High temperature superconducting (HTS) cables have been studied because of low loss and compactness, compared with conventional copper cables. Three-phase cables are usually composed of three single-phase concentric cables. Recently, a tri-axial cable, composed of three concentric phases, has been intensively developed, because it has advantages such as reduced amount of HTS tapes, low leakage fields, low heat leak and compactness, compared with the three single-phase cables. We analysed the three-phase current distributions in the tri-axial cable as functions of winding pitches of three concentric phase layers, and showed the balanced three-phase current distributions in the tri-axial cable. The each layer supplies a transport current under external magnetic field with the same frequency and different phase. We formulate the general form of AC loss of the transport current in combination with the external field with different phase, and analyse the AC loss of the tri-axial cable.
IEEE Transactions on Appiled Superconductivity 07/2007; · 1.04 Impact Factor
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ABSTRACT: An HTS bulk field-cooled by a permanent magnet can levitate stably without any other control systems. The stable levitation can be realized by a specific characteristic of the HTS bulk that the HTS bulk returns to its original position by restoring force against horizontal displacement. We devised a magnetic levitation type seismic isolation device taking advantage of the specific characteristic of the HTS bulk. This device is quite different from conventional one composed of bearing and damper, because this device can completely remove horizontal vibration. We made a model device composed of YBCO bulks and permanent magnets to verify the seismic isolation effect of our device. Levitation force and vibration stability strongly depend on magnetic stiffness between HTS bulk and permanent magnet. Therefore, we prepared an YBCO bulk and a permanent magnet unit arranged on a row and investigated the characteristics of levitation force and horizontal vibration as functions of vibration amplitude, vibration frequency, and initial air gap between the bulk and the permanent magnet unit. Large initial air gap is desirable for levitation force, while small air gap under operation is effective for securing vibration stability.
IEEE Transactions on Appiled Superconductivity 07/2007; · 1.04 Impact Factor
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ABSTRACT: AC losses with long time constants can not be simply estimated from a short sample conductor because there are many irregular loops formed by strands strongly displaced from their original positions. In our previous work, we measured trajectories of 81 strands of NbTi conductor and it was proved that strongly displaced strands produced many line contacts with other strands, and thereby caused low contact resistance and long time constants. Long loops due to the displacement of strands should also produce large AC loss because the time constant of the loss is proportional to the inductance, i.e., the length of coupling current loops. In order to investigate the long loops in practical conductors, we developed a method to estimate the strand positions over the entire length. In this method, only one cross section of the conductor is required to calculate gravities of each sub-cable. The strand trajectories are obtained in a manner that the same order sub-cables rotate around the gravity to form one order higher sub-cable. The estimated trajectories are in good agreement with the measured ones, with errors of 1 mm.
IEEE Transactions on Appiled Superconductivity 07/2007; · 1.04 Impact Factor
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ABSTRACT: AC losses consist of both regular losses that are proportional to cable twisting pitch squared and irregular losses that could not be estimated from short conductor sample test results. It was explained from our previous works that irregular loops in conductor which are caused by asymmetric strand positions as a result of low void fraction of CIC conductor, produce the losses with long time constants up to several hundred seconds. The observed long time constant indicates that the typical loop length should be about LCM (Least Common Multiplier) of all sub-staged cable pitches, and that contact conditions between the two strands forming the loop should be line contact. In order to investigate the contact conditions in detail, we traced 81(=3times3times3times3) strands every 11 mm of CIC sample conductor with 1 m in length whose strands are NbTi/Cu without any surface coating. The measured traces of 81 strands show that asymmetric strand positions, in other words, large displacements of strands from their original positions due to compressing the conductor provide many line contacts. It is found that the averaged line contact length reaches about 10 mm that is three order of magnitude larger than the 10<sup>-2 </sup> mm of point contact length
IEEE Transactions on Appiled Superconductivity 07/2006; · 1.04 Impact Factor
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ABSTRACT: High Temperature Superconducting (HTS) cables have been studied because of low loss and compactness, compared with conventional copper cables. Three-phase cables are usually composed of three single-phase coaxial cables. Recently, a tri-axial cable, composed of three concentric phases, has been intensively developed, because it has advantages such as reduced amount of HTS tapes, small leakage fields and small heat loss in leak, compared with the three single-phase cables. However, there is an inherent imbalance in the three-phase currents in the tri-axial cable due to the differences in radii of the three-phase current layers. The imbalance of the currents causes additional loss and large leakage field in the cable, and deteriorates the electric power quality. Therefore, we introduce more variables to obtain the solutions of the balanced three-phase currents and homogeneous current distribution in each phase of the tri-axial cable. We propose a tri-axial cable composed of two longitudinal sections with different twist pitches, and hence the number of variables increases. We derive general equations satisfying both the balanced three-phase currents and homogeneous current distribution, as functions of winding pitches. Finally we apply the general equations to the simplest cable and find out reasonable twist pitches
IEEE Transactions on Appiled Superconductivity 07/2006; · 1.04 Impact Factor
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ABSTRACT: We have investigated the relationship between current distribution and transport AC losses in coaxial multi-layer HTS transmission cables and clarified that the same layer current in conducting layers can reduce the transport AC losses effectively. It can be, however, considered that magnetization losses are much larger than the transport AC losses in a large transport current. Therefore, we analytically investigated effective current and magnetic field distributions for reducing total AC losses in an HTS transmission cable with a large transport current. The total AC losses in the case of the same applied magnetic field in the conducting layers were much smaller than those of the same layer current in the conducting layers. This result means that inhomogeneous current distribution is more effective for reducing the total AC losses than homogeneous current distribution. The total AC losses decreased with the electrical insulation thickness between the conducting and shielding layers; the electrical insulation thickness is one of the important parameters in terms of reducing the total AC losses
IEEE Transactions on Appiled Superconductivity 07/2006; · 1.04 Impact Factor
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ABSTRACT: High Temperature Superconducting (HTS) cables have been recently studied because of low loss and compactness, compared with conventional copper cables. We, so far, analyzed current distributions in a single-phase coaxial cable with and without shield current. Three-phase cables are usually composed of the three single-phase cables. However, a tri-axial cable, composed of concentric three phases, has more advantages compared with the three coaxial cables, because of reduced HTS tapes and leakage field. We derive equations governing layer current and current density distributions in the tri-axial cable considering other phases. The current and current density distributions are described as explicit functions of cable construction parameters, such as radius, twist pitch and twist direction. We calculate the homogeneous current and current density distributions by choosing adequate cable construction parameters.
IEEE Transactions on Appiled Superconductivity 07/2005; · 1.04 Impact Factor
