Characteristics Measurements of HTS Tape with Parallel HTS Tapes

Many high temperature superconducting (HTS) tape manufactures make an effort to reduce the transport current loss of HTS tapes. The knowledge of critical current and self-field in an HTS tape is very useful to compute the transport current losses. The spatial distribution and magnitude of self-field are variable due to the neighboring materials. In this paper, the critical currents and the transport current losses of BSCCO and YBCO tapes with paralleled magnetic material (Ni tape) and/or diamagnetic material (BSCCO tape) are experimentally investigated to improve the AC loss properties. The critical currents of HTS tapes with paralleled Ni tape are slightly decreased and the transport current losses are markedly increased. However, the critical currents and transport current losses of HTS tapes with paralleled BSCCO tape have not current carrying are more improved than single HTS tape.     

Numerical Analysis of AC Loss Characteristics of Cable Conductor Assembled by HTS Tapes in Polygonal Arrangement

Numerical studies on AC loss characteristic of a assembled conductors of multiple HTS tapes with high aspect ratio simulating YBCO tape in polygonal arrangement were presented in this paper. We developed a numerical model to analyze the AC loss in the HTS tape in the polygonally assembled conductor based on our previously developed model. The relations between the geometrical configuration and the AC loss properties of the conductor were made clear by the numerical simulation. The numerical results give the important information for the design of the cable conductor assembled by the HTS tapes with high aspect ratio such as YBCO tapes.     

Geometry considerations for use of Bi-2223/Ag tapes and wires withdifferent models of Jc(B)

In typical power applications, Bi-2223 conductors carrying AC current will be subjected to external magnetic fields whose orientation and conductor's geometry are of major significance for the AC loss magnitude. This paper investigates the influence of the geometry and aspect ratio of nontwisted Bi-2223 conductors in reducing the AC loss for such applications. A numerical model of high-T_c materials has been used in finite-element-method (FEM) simulations. The model incorporates power-law E-J characteristics with J_c and n defined by both parallel and perpendicular local magnetic field components. It allows computations of field and current distributions with transport current and/or applied field of any orientation. Monofilamentary tapes of rectangular and elliptical geometry with anisotropic J_c(B), as well as square and round wires with isotropic J_c(B) have been used for simulations under various operating conditions. A comparison between AC losses, magnetic field, and current distributions in the tapes and wires is presented     

Numerical Evaluation of Total AC Loss in Parallel Tape Conductor With Transport Current in Oblique Magnetic Field

We numerically calculate AC losses in a three-strand parallel tape conductor with optimum transposition by means of the finite-element method that directly analyzes the magnetic field distribution. The parallel conductor carries an alternating transport current, whose amplitude is 70% of the critical current, in an in-phase external AC magnetic field that has an arbitrary angle to the flat face of Bi-2223 tapes. It is confirmed that the AC losses in the range of very small and large field amplitude are almost equal to those in cases of applying only a transport current to the parallel conductor and only an external magnetic field to a single Bi-2223 tape, respectively. However it is found that the AC losses are affected by the interaction among the transport current, external magnetic field, and strand number in the range around the full penetration field. Our proposed finite-element analysis helps us to evaluate the AC loss generated in such the middle range of field amplitude quantitatively.     

The Nitrogen Boil-Off Method for Measuring AC Losses in HTS Coils

We have developed a novel apparatus for applying a nitrogen boil-off method as a mean of making calorimetric measurements and allowing for simple measuring and evaluating of AC losses in HTS coils at liquid nitrogen temperature. With the ability to measure AC loss generated in superconductors directly, this method produces more reliable data than that obtained by general, electro-magnetic measurements. A sensitivity of about 0.1 W was achieved by improving sensitivity in measuring gas evaporation via the following three steps: (1) use of nonmetallic vessel, heat-insulated housing in which the sample is located; (2) optimization for sample vessel thickness and tube size; and (3) stabilization of evaporating gas emitted by the heater. It is widely expected that the proposed apparatus may be used for individual measurement of the magnetization loss and the transport current loss in HTS coils, as well as their total losses, in addition to the AC loss in irregularly shaped samples.     

Quench and recovery characteristics of HTS tapes under oscillating over-currents with different frequencies

Quench and recovery characteristics of Bi-2223/Ag high-temperature superconductor (HTS) tapes under oscillating over-currents with different frequencies have been observed. The experiment is carried out using an oscillating circuit consisted of capacitance and inductance, which produces oscillating currents with different frequencies. The HTS tapes are immersed in liquid nitrogen bath. The frequencies of the oscillating currents fed to HTS tapes are 180.8, 333.3, 434.8, 751.9, and 2016.1 Hz, respectively. In order to confirm the critical current of the tapes, dc current is fed to the tapes first. The magnitude of the oscillating currents is from a small current to 179 A, which is 2.5 times larger than the dc critical current of the tapes. The experimental result shows that the normal voltage generated by the over-current disappears fast after the end of the oscillating over-current, which shows that HTS tapes have good superconducting-to-normal conducting recovery characteristics under oscillating current with high frequency. The experimental results will encourage the possibility of the important applications of HTS tapes in pulsed power.     

Transport AC Losses in YBCO Coated Conductors

Modeling and design of superconducting power devices (e.g. fault current limiters), based on second generation HTS tapes, requires accurate evaluation and prediction of AC losses. Transport AC losses measurements have been performed on samples of YBCO coated conductors at 77 K, as a function of current. The results have been compared with the classical analytical model for self field AC losses calculation, taking into account the role of the magnetic substrate.     

Analysis of magnetic field and circulating current for HTS transformer windings

In a high-temperature superconducting (HTS) transformer, the leakage magnetic field decreases the critical current and increases the ac loss in the tapes. Moreover, because of nearly zero resistance of HTS tapes, a slight unbalance of the branch inductances of the windings might result in heavy circulating current. So, the numerical analysis of the leakage magnetic field and circulating current is especially necessary for an HTS transformer design. In this paper, the influence of the winding configurations on the stray field and circulating current is studied. That is, the magnetic field distribution is analyzed by finite-element method and then, based on the inductance matrix obtained after a magnetic field analysis, the circulating current is calculated by circuit analysis. Some measures for improving the leakage field and circulating current distribution are also proposed to make HTS transformers more efficient.     

Losses in a high-temperature superconductor exposed to AC and DCtransport currents and magnetic fields

Summary form only given, as follows. The development of high-temperature superconductors (HTSs) is for the moment intensive and it is likely that HTSs will be used in different electric devices in a few years. In some of these devices that conductor will be exposed to both AC and DC transport currents and magnetic fields. In the design of electric devices, the power loss is indeed one of the most important parameters. Therefore there is a need of models that predict the power loss in an HTS under application-like conditions. In this paper, the authors present a semi-empirical model including a transport current and an applied magnetic field with both AC and DC components. The semi-empirical model is mainly based on the critical state theory. The results of the model are compared and found to be in good agreement with experimental results obtained with a calorimetric experimental set-up. The power loss was measured on a multi-filamentary Bi-2223 tape as function of transport current and applied magnetic field at a fixed temperature and frequency     

Mechanical and AC loss properties in mono-and multi-filamentary Bi-223 Ag-sheathed tapes

The evaluation of the bending strain tolerance and AC loss properties for monoand multi-filamentary Bi-2223 Ag-sheathed tape were carried out at liquid nitrogen temperature. For tapes with a filament number of over 19, the critical current (I_c) was maintained at the same values up to the bending strain of 0.3%, although the I_c of the mono-filamentary tape at the condition of 0.2% strain degraded to 90% of the value for the no-strain condition. The AC loss of the monofilamentary tape was the hysteresis type. On the contrary, the AC loss of the multi-filamentary tape was substantially dominated by the eddy current loss in the Ag matrix.     

AC losses in HTS tapes in applied longitudinal magnetic fields at variable temperatures

In most power devices, the conductor is carrying an ac transport current while it is exposed to an ac magnetic field transverse to the current path. In certain applications, such as power cables or a control winding in a controllable reactor, the conductors are exposed to a magnetic field component longitudinal to the tape axis that is parallel to the current path. To create an improved base for the design of such power devices it is of interest to study the losses in high-temperature superconductor tapes due to longitudinal field in detail. We have investigated the losses at several temperatures of a nontwisted multifilamentary Bi-2223 tape when it was exposed to a longitudinal magnetic field. The losses were measured with a calorimetric method and the results were compared with the critical state hysteresis loss model. The hysteresis losses are dominating at power frequencies (50, 60 Hz) in the investigated field range 2-200 mT and are accurately described by the critical state hysteresis model.     

AC Loss Characteristics of YBCO Coated Conductors With Varying Magnitude of Critical Current

Three YBCO coated conductors with various critical currents, up to 189 A, were prepared. The magnetization losses without transport current and the total AC losses of the conductors carrying various transport currents in transverse magnetic fields with various orientations were measured electromagnetically to study the AC loss characteristics systematically. The measured total AC losses were compared with the numerical values using a one-dimensional FEM model.     

Numerical Study on AC Loss Characteristics of HTS Coils With Various Cross Sections and Methods of AC Loss Reduction

AC loss characteristics of high temperature superconducting coils wound by Bi2223/Ag tape with various cross sections are numerically investigated. The numerical results show that the smaller coil aspect ratio, the smaller AC losses in the HTS coil. It was considered from the numerical results that the coil aspect ratio giving the minimum coil volume is the better shape judging from the many aspects such as the required length of tape, the stored energy, the central magnetic field and the critical current together with the AC loss. It was also shown by the numerical study that the optimal coil design giving the reduced AC loss can be done by employing the stepped cross section coil and the turn number graded coil.     

Losses in a BSCCO/Ag tape carrying AC transport currents in ACmagnetic fields applied in different orientations

In most prospective electric power applications of high-temperature superconductors (HTSs), the conductor is wound in a coil configuration. For a coil of finite length, the magnetic field orientation is axial in the middle of the coil, while the field contains a substantial radial component at the coil ends. In a superconducting BSCCO/Ag tape, the AC losses depend strongly on the orientation of the magnetic field. In this study, we present experimental results of the AC losses in a multifilamentary silver-sheathed Bi-2223 HTSs tape, carrying alternating transport currents in externally applied alternating magnetic fields at different orientations, with respect to the face of the tape and perpendicular to the transport current. The AC losses were measured calorimetrically, at fixed temperature and frequency. The results are compared to semi-empirical models of the AC losses in HTSs tape. We present a more general model of the angular dependence of the AC losses     

A high-temperature superconducting Butler matrix

This paper presents a novel configuration of a beamforming 16-port Butler matrix centered on a frequency of 2 GHz. The structure is implemented using high-temperature superconductors (HTS). In communication and remote sensing systems, multibeam antenna systems are gradually replacing single-beam systems. Microwave beamformer circuits for these applications require a large number of couplers and phase shifters, which result in a large circuit size. By using microstrip structures on high permittivity substrates, the circuits can be miniaturized. However, the insertion loss of the beamformer increases due to the conductor loss. The use of HTS allows reduction in the size of the circuit while maintaining low insertion loss, due to the low conductor loss compared to conventional conductors. The Butler matrix described here uses a two-layer configuration, which removes any microstrip line crossovers; it can be constructed by traditional photolithographic methods. In this paper, the design of the matrix is discussed, together with the experimental and simulated results.     

Superconducting control for surge currents

Systems designed to use superconductors to limit fault currents in power grids are undergoing testing. The authors describe superconducting fault current limiters (SCFCL) which may be categorised into resistive or shielded core types. The features and operation of each type of device are outlined. Both the shielded-core and resistive types of SCFCL use the same amount of superconductor material to achieve a given limitation behavior. This is because the rated power per volume of conductor is determined by the product of fault-induced field and critical current, which is the same for both devices, assuming the same type of superconducting material is employed. The shielded-core limiter works only with AC currents and is much larger and heavier than the resistive SCFCL. While there is only one large program left in the low-temperature type of SCFCL, more than 10 major projects are under way worldwide on the high-temperature type of device. The main reason is the lower HTS cooling cost     

Model of the flux flow losses in a high-temperature superconducting tape exposed to both AC and DC transport currents and magnetic fields

Recent progress in the high-temperature superconductor technology will probably be of importance for future power applications. In these applications, the power loss is a very important quantity, which has to be described in a relevant manner to obtain an optimum utilization of the conductor. In most power applications the magnitude of the transport current will be quite close to the critical current. In addition to the hysteresis losses, for such high currents, the flux flow losses have to be taken into account. We present a semiempirical model of the flux flow losses in a tape shaped conductor, based on measurements and reasonable physical assumptions. The model gives the flux flow loss as function of temperature, a transport current consisting of both ac and dc components and an applied homogeneous magnetic field with an amplitude proportional to the current.     

AC Losses in YBCO Coated Conductors Subjected to Tensile Stress

Stress dependencies of AC transport current losses, magnetization losses and total losses were investigated applying tensile stresses to an YBCO (IBAD/PLD) conductor. Measured transport current losses showed slight stress dependence in the range of 0$sim$1 GPa. That can be explained by stress dependence of the critical current of the conductor. The magnetization and total losses are less sensitive to the stress than the transport current losses.     

Temperature Dependence of Total AC Loss in High-Temperature Superconducting Tapes

A versatile experimental facility was designed and set up to measure transport ac losses, magnetization ac losses, and total ac losses in high-temperature superconductors at variable temperatures. Several sets of measurements were carried out in the temperature range of 35 K to 100 K. Sample temperature during the measurements could be controlled within ${pm}$0.5 K of set temperature. Temperature dependence of transport losses reflects variation of critical current density of the tapes with temperature. Temperature dependence of magnetization losses exhibits an interesting behavior with a peak, whose position shifts to lower temperatures as the magnetic field is increased. Experimental data of ac losses at various temperatures are compared with those calculated using numerical methods. Generally, the simulated results reproduce well the experimental data.      

High Tc Superconducting Materials for Strong Current Applications： Approach at the First Stage

Strong current and large-scale application is the most important prospect of high Te superconductors （HTS）. Practical HTS samples in various forms have been produced with high critical currents operated at economic cryogenic temperatures. Engineering applications of those HTS materials have been studied with various HTS prototype devices. The applicable HTS materials produced in different forms are verified in this paper with regard to their strong current characterizations, and the HTS applications are summarized along with the HTS prototypes made.