Measurements of current distributions in a multi-laminated HTS tape conductor for solenoid coils

A multi-laminated HTS tape conductor has recently been developed for large coils. If the HTS tapes are simply laminated to form the conductor, the current distribution in the laminated tape conductor of the coil is unbalanced because of the differences among all tape inductances. Transposition of the tape in the conductor is effective for homogeneous current distribution, but the tape may be damaged due to the lateral bending. In our previous paper, we proposed a new theory to analyze and control current distributions in the multi-laminated tape conductor for a solenoid coil with arbitrary layers. We applied the Maxwell integral equation to the region contoured by adjacent laminated tapes to analyze the current distributions of the tapes in the infinite solenoid coil, demonstrated that the flux across the region is conserved as long as the tapes are not saturated, and finally induced fundamental equations as functions of coil construction parameters, such as layer radius, laminated tape space, and winding pitch. In order to verify the theory, we designed two kinds of coils with homogeneous and inhomogeneous current distributions in the two-laminated tape conductor by adjusting the space between the tapes in the second layer, and fabricated them. In the case when the space between the tapes in the second layer is the same as that of the first layer, 0.31 mm in thickness, we measured the tape currents of 7:3 for the inner and outer tape of the first layer, respectively. We adjusted the space between the tapes of the second layer, 1.78 mm in thickness, while the space of the first layer remained unchanged, 0.31 mm in thickness. We obtained the homogeneous current distribution in the tape conductor. The experimental data were in good agreement with the theory.     

Study on cross section of high temperature superconducting coil

It is in particular of importance for HTS coils to secure a larger central magnetic field and/or a large stored energy with shorter length of HTS tapes. The critical current of an HTS tape depends on both the flux density and the flux angle against tapes. From this point, the performance improvement of HTS coils is taken into account with an analytical model. The minimum volume coil derived from the Fabry Factor constant curve is taken concerning the original coil shape, which is often employed in low temperature superconducting coils. The coil critical current was analyzed in consideration of the anisotropic properties of the tape.The electric field of HTS tapes in the coil was calculated at the coil critical current and the high electric field portion were cut out. The optimal coil cross section is obtained by iterating this calculation process. As a result, the critical current and the stored energy density of the coil were improved. The stored energy density increased about 17% and the central magnetic field was almost kept constant regardless of 19% reduction of HTS tapes, as compared with the original coil with the rectangular cross section.     

A Numerical Study on AC Losses of a Double Layer Polygonal BSCCO Conductor by Finite Element Method

High-temperature superconductor (HTS) cables are candidates for power transmission cables in the near future. A cylindrical arrangement of HTS tapes for the cable has proved able to reduce the AC loss. Many studies on AC loss characteristics of HTS cables have been done, but few numerical models of the cable were verified by experiments. In this paper, a numerical model of the double-layer polygonal bismuth strontium calcium copper oxide (BSCCO) conductor is developed. Current density and magnetic field intensity distribution in the inner and outer layers are also investigated. The numerical results of the AC loss for different layer current distributions are identical with the experimental ones. Accordingly, the reliability of the numerical model is verified. By using this model, the influence of distance between the inner and outer layers, gap between two neighboring wires, and layer current distribution on AC losses of different layers is evaluated. The results show that increasing distance between layers and narrowing gap between wires are effective to reduce AC loss, while the unbalance of layer current distribution increases the AC loss of the double-layer conductor.     

Recovery estimation for over-current test of non-inductive fault current limiters using numerical analysis

When an HTS coated conductor (CC) is used as a conductor of a superconducting fault current limiter (SFCL), the CC is expected to be exposed to the over-current and temperature of the CC is expected to be increased rapidly by electrical joule heating. Because the CC is a composite tape, thermal and electrical properties of composite materials could affects over-current limiting capacity and recovery time of SFCL. This paper presents experimental and numerical results of over-current test and recovery time measurement test on four bifilar wound SFCL modules. The temperature transitions of the samples were estimated from total electrical resistance of the coils. We fabricated one bifilar solenoid coil and three bifilar pancake coils whose cryogenic conditions were different from the other coils. An numerical model was also fabricated to simulate the temperature transition and the numerical results were compared with experimental results.     

Layer-current waveform of coaxial multi-layer HTS cable considering the flux flow state

It is important to control the layer current distributions of coaxial multi-layer HTS cables because homogeneous layer current distribution decreases AC loss and increases the largest operational current. In a previous paper, we proposed a theory that can control current distribution based on the concept of flux conservation between two adjacent layers, and demonstrated the theory is in good agreement with experiment results. The theory was effective for an operational current less than the critical current of the cable. It is important to investigate current distribution under the condition of operational current more than the critical current of the cable because the cable experiences fault currents. We have extended the theory to treat the operational current more than the critical current by considering V-I nonlinear characteristics of HTS tapes including flux flow resistance and contact resistance between the cable and terminals. In order to verify the extended theory, we have fabricated a two-layer cable with the same twisting layer pitch, and hence caused inhomogeneous current distribution. It was observed that almost all of operational current less than the critical current flowed on the outer layer because of its lower inductance. When the operational current increased above the critical current of the second layer, the flux flow resistance appeared and distorted the current waveform with phase deviations. Finally, in the case of operational current more than the critical currents of both layers, flux flow resistance strongly affected current waveforms, and thereby the currents of both layers were determined by flux flow resistance. The extended theory simulated the layer current distribution waveforms and demonstrated good agreement with the experimental results under all operational current regions.     

Numerical study on self-field losses of 30 m BSCCO HTS transmission cable

The self-field losses of the one phase of high-T_C superconducting (HTS) transmission cable are calculated by the electric circuit (EC) model. The one phase of HTS cable is constructed by the former of fine-strands copper rod, HTS conductor with four superconducting layers, the insulation made by polypropylene laminated paper, and HTS shielding with two superconducting layers, which was fabricated by Sumitomo Electric Industries (SEI). The length of the cable is 30 m. Each HTS layer comprises BSCCO tapes. The current-dependent resistance of HTS layers in EC model is estimated on the base of Norris expressions for ellipse. The calculated losses are compared with the experimental results measured by 4-terminal method by SEI. The calculation of alternating current (AC) losses, a summation of the self-field losses in HTS layers and the eddy-current losses in the former, is almost equal to the measurement at wide transport-current range below the lowest value of the layer critical current. This result indicates that the numerical calculation by EC model is quite reliable. The minimum AC loss is also calculated by obtaining the optimum helical-pitch lengths of HTS layers at transporting 1 kA_rms. The minimum loss is 36% lower than the loss of HTS cable designed by SEI at the transport current value. In HTS cable with the optimum helical-pitch lengths, the calculation of the layer currents are not uniform in HTS conductor but are almost uniform in HTS shielding, which is contradict to SEI’s one. It is considered that the numerical calculation by EC model is useful to obtain the optimum helical-pitch lengths in HTS cable with the minimum AC loss.     

Thermal characteristics of conduction cooled 600 kJ HTS SMES system

A 600 kJ HTS SMES is developed and tested in Korea. The HTS SMES consists of 22 double pancake coils wound on each aluminum alloy bobbin. It is cooled by two GM cryocoolers down to around 6 K and current is charged through HTS current leads up to 275 A. Beside the heat penetration from room temperature structures, heat generation in the HTS coil is inevitable because of the joint resistances and the intrinsic property of the HTS tape such as index loss. Moreover, during the charging and discharging operation, AC loss of the HTS conductor and eddy current loss in the coil bobbin and metallic structures are generated. Therefore, the heat generation should be effectively removed by the cryocooler to ensure the stable operation of the coil. In the HTS SMES, aluminum alloy conduction plates outside the each coil are used as thermal paths to the cryocoolers. This paper describes the thermal characteristics of the HTS SMES for the charging and discharge operation.     

Performance improvement and optimization of a high-temperature superconducting coil having different transport current portions by employing persistent current mode

To improve the performance of a high-temperature superconducting (HTS) coil, it is important to improve its transport current performance. In general, the critical current and n-value of an HTS (Bi-2223/Ag) tape depend on the applied magnetic fields and the angle between the magnetic field and the tape under a constant temperature. The critical currents in the coil edge of the tapes are particularly low because of the distribution of the magnetic fields. However, the critical currents in the central portion remain high. A large amount of current can be supplied to the central portion and the coil performance will improve by supplying different currents between these areas. In this study, I propose an HTS coil in which the coil edge and central portion are isolated for each excitation. Namely, I employ the characteristics of the persistent current mode. The analysis of varying regions of current separation confirmed an optimum current separation. This optimized coil improves the central magnetic field by 21% and the stored energy by 50% compared to those of a normal rectangular coil with an HTS tape of the same length.     

AC Loss of a Multi-Layer per Phase Tri-Axial HTS Cable with?Balanced Current Distribution

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.     

Electrodeposited Cu-Stabilization Layer for High-Temperature Superconducting Coated Conductors

We have developed a nonaqueous-based process for electrodepositing a Cu-stabilization layer on a YBCO superconductor tape. Conventional approaches to electroplating Cu layers use a cyanide-based solution to prevent uncontrolled hydrogen evolution from the aqueous-based solution; these are very reactive with the superconductor layer, and thus destroy its critical-current capability when plated directly onto high-temperature superconductor (HTS) tape. It has been found that a capping layer at least 1 micron thick is needed between the superconductor and stabilizer layers to avoid such a reaction and the subsequent reduction in the critical-current capability of the superconductor layer. In contrast, the nonaqueous electroplating solution is nonreactive to the HTS layer, allowing the Ag capping layer to be thinner. We demonstrated that direct Cu plating on YBCO tapes using a nonaqueous solvent does not destroy the superconducting YBCO layer. The superconducting current capabilities of these tapes were measured by noncontact magnetic measurements. Contact current?Cvoltage (I?CV) measurements required a 0.1-micron-thick Ag capping layer on YBCO tapes, which is sufficient for subsequent Cu plating from the non-aqueous solvent.     

Low resistance splices for HTS devices and applications

This paper discusses the preparation methodology and performance evaluation of low resistance splices made of the second generation (2G) high-temperature superconductor (HTS). These splices are required in a broad spectrum of HTS devices including a large aperture, high-field solenoid built in the laboratory to demonstrate a superconducting magnetic energy storage (SMES) device. Several pancake coils are assembled in the form of a nested solenoid, and each coil requires a hundred meters or more of 2G (RE)BCO tape. However, commercial availability of this superconductor with a very uniform physical properties is currently limited to shorter piece lengths. This necessitates us having splices to inter-connect the tape pieces within a pancake coil, between adjacent pancake coils, and to attach HTS current leads to the magnet assembly. As a part of the optimization and qualification of splicing process, a systematic study was undertaken to analyze the electrical performance of splices in two different configurations suitable for this magnet assembly: lap joint and spiral joint. The electrical performance is quantified in terms of the resistance of splices estimated from the current-voltage characteristics. It has been demonstrated that a careful application of this splicing technique can generate lap joints with resistance less than 1 nΩ at 77 K.     

Development of Bi-2223 HTS tape and its application to coil and current leads

We are developing Bi-2223/Ag tapes with a high engineering critical current density by optimizing the powder-in-tube process and are studying its application to coil and current leads. We have fabricated 250 m-long tape and investigated optimized processing conditions to enhance engineering critical current density. More bubbling was found when the tape was heat-treated with a higher heating rate. Different kinds of superconducting joints were fabricated with multi-filamentary Bi-2223/Ag tapes, and 58% of retained I_c was achieved using the insertion of Bi-2223 core between two exposed tapes. Current decay property of the persistent mode HTS coil was investigated. Rapid current decay was observed when the operating current is in a flux-flow range. We could successfully fabricate a low heat leak type HTS current lead with Bi-2223/Ag–Au tapes by employing a stepped geometry. Using this lead, safe operation of 2 kA current transport was confirmed.     

Current Distribution in YBCO Coated Conductors of Toroidal Coil Composed of Multiple Double Pancake Coils

We should prevent non-uniform current distribution due to manufacturing errors of tape size, coil size, and conductor deformation in the toroidal coil composed of multiple YBCO double pancake coils. This is because the non-uniform current distribution causes the increase of AC losses and the decrease of critical current in the YBCO tapes. Therefore, we analytically investigated the relationship between the manufacturing error and the current distribution in the toroidal coils composed of the eight series-connected double pancake coils composed of the transposed conductor with two parallel-connected YBCO tapes and the eight sets of two parallel-connected double pancake coils composed of single YBOC tape conductor connected in series. The parallel-connected multiple double pancake coils composed of single tape conductor is much more effective for preventing the non-uniform current distribution than the double pancake coil composed of the transposed conductor with parallel-connected multiple tapes. The non-uniformity of current distribution can be improved by increasing the length of parallel-connected current path by increasing the number of series-connected double pancake coils in each current path.     

Characteristic and magnetic field analysis of a high temperature superconductor axial-flux coreless induction maglev motor

A new high temperature superconductor axial-flux coreless maglev motor (HTS AFIM) is proposed, of which the primary windings are made of HTS tapes and the secondary is a non-magnetic conductor. The main works of this paper are the magnetic-field computation and characteristics analysis of HTS AFIM. For the first one, the reduction of magnetic fields near outer and inner radius of the HTS AFIM is solved by introducing the sub-loop electro-magnetic model along the radial position. For the second one, the AC losses of HTS coils are calculated. The relationships between the device's characteristics and device parameters are presented, and the results indicate that under certain frequency and current levitation device can output enough lift force. The conclusions are verified by finite element calculations.     

Test Coils Made of Tl-1223 Tapes

Silver sheathed Tl-1223 tapes were prepared by a powder-in-tube process. The critical current density of short samples was 18 kA/cm² at 77 K. Longer tapes up to 1.2 m, prepared by sequential pressing, had a critical current density of 12 kA/cm². From these tapes we have wound two coils. A solenoid coil with 5 windings was made of 8 tapes with a total length of 4.5 m. At 77 K the critical current of the coil was 23 A in the self generated magnetic field (18 Gauss at the centre of the coil). Using an iron yoke the critical current remained at 22 A while the generated magnetic field increased to 120 Gauss. A pancake coil with 15 windings, made of 5 tapes with a total length of 5 m, generated a magnetic field of 149 Gauss at the critical current of 12 A. From measurements of the critical current density of our tapes in applied magnetic fields, we conclude that coils made of Tl-1223 tapes can be used to generate higher magnetic fields at 77 K.     

Instabilities above critical current region in Bi-2223/Ag superconducting coils cooled by liquid nitrogen

A sudden drop of the coil voltage and a hysteresis of I-V curve were observed in measurement of one-layer Bi-2223/Ag coils cooled by liquid nitrogen at currents well above critical current region. Their temporal behavior indicates, that the improvement of the cooling and corresponding decrease of temperature after the jump takes place. To study this phenomenon we measured I-V curves of two Bi-2223/Ag coils made from tapes with various degree of critical current homogeneity and analogical curves of two non-superconducting coils made from thin Cu tapes having various widths. In Cu coils we really observed a sudden drop of the temperature, measured in parallel with Cu resistance drop, after reaching heat flux of about 0.4 W cm⁻² during current ramping up. In spite of non-superconducting character of the tape, the hysteresis, i.e. difference between increasing branch and decreasing branch of I-V curves, was observed too! Approximately the same value of heat flux, at current corresponding to the jump, was found also in superconducting coil on segment with least value of local critical current. We conclude that observed voltage drop of the Bi-2223/Ag does not bear upon superconducting nature of the coil and, as that for Cu coil, can be explained by dynamics of heat transfer to liquid nitrogen and its history.     

The electric characteristics of HTS double-pancake coil with AC pulse over currents

A double pancake coil was designed and manufactured with a 36-m long Bi-2223/Ag tape. The tape was insulated by 25 μm thick Kapton tapes, which can stand a voltage of 400 V_rms in liquid nitrogen. The whole double pancake was impregnated with epoxy resin. AC over-current experiments of the coil were performed by applying constant AC voltages to the two terminals of the coil and lasted for 3 s. The experiment began first at a lower voltage of 33.6 V_rms, and then the voltage stepped up till the coil was burned out at the pulse voltage of 202.7 V_rms. All of the experiments were carried out with the coil dipped in liquid nitrogen. The current waveforms were measured. The impedance and resistance characters of the HTS coil with its over pulse currents were analyzed from the experiment results. At the end of this paper, some conclusions derived from the experiment results and their analyses are given, which are helpful for the safety operating of the HTS coils in power applications.     

Investigation on AC loss of a high temperature superconducting tri-axial cable depending on twist pitches

High temperature superconductor (HTS) cables have been intensively studied because they are more compact compared with conventional copper cables. Since it is strongly expected that the HTS cables replace conventional power lines, some HTS cables are designed, manufactured, installed in power grids and tested to demonstrate full time operation. Recently, a tri-axial cable composed of three concentric phases has been developed, because of its reduced amount of HTS tapes, small leakage field and low heat loss when compared with single phase and co-axial HTS cables. The layers inside the tri-axial cable are subject to azimuthal fields applied from inner layers and axial fields applied from outer layers with different phase from their transport currents. These out-of-phase magnetic fields should be calculated under the condition of the three phase-balanced distribution of the tri-axial cable, and thereby AC losses should be evaluated. In this paper, the AC loss in the tri-axial HTS cable consisting of one layer per phase is theoretically treated for simplicity. The AC losses in the cable are calculated as functions of the twist pitches of HTS tapes. It is found that the AC losses rapidly decrease with increasing twist pitch.     

舰船电力系统用1 MJ高温超导储能磁体设计研究

介绍了舰船电力系统用的1MJ螺管型高温超导储能磁体的设计优化步骤，给出了用Bi-2223超导带进行1 MJ磁体线圈的设计和优化结果，分析了高温超导体的各向异性对磁体临界电流的影响，讨论了储能容量一定的多螺管磁体系统在漏磁、储能密度和所需超导线材方面的变化。