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.     

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.     

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.     

Self-field and geometry effects in transport current applications of multifilamentary Bi-2223/Ag conductors

This paper describes quantitatively the influence of the self-field and the cross-sectional geometry on the effective critical current and the ac losses in transport current applications of nontwisted multifilamentary Bi-2223/Ag conductors. The results are obtained with finite-element method simulations. The numerical implementation includes an anisotropic model for the dependence of the critical current density J/sub c/ and the power index n on the local parallel and perpendicular magnetic field components. The relation is given between the intrinsic critical current density and the effective critical current for different multifilamentary conductors. Shown are examples of the current and magnetic flux density distributions in order to demonstrate their effect on the ac losses in self-field.     

Trapped field and related properties in a superconducting-disk magnetized by pulse field

The distributions of the magnetic field and temperature in a superconducting-disk magnetized by pulse field, and the levitation force between this disk and a permanent magnet are calculated. The calculation is based upon the current motion and the heat diffusion equations in the disk. The critical current density as a function of magnetic field and temperature is taken into account. The dissipation power in the superconducting or the normal state region is distinguished. The trapped field may reach 17 T at 29 K by pulsed field magnetization. The effects of the amplitude of the current pulse on the distributions of magnetic field in the disk and on the levitation force are discussed.     

Dependence of AC Loss With a Phase Difference Between Transport Current and Applied Magnetic Field

We experimentally and numerically investigated the characteristics of the AC losses in Bi2223/Ag sheathed tape by the phase difference between the transport current and external magnetic field perpendicular and parallel to the tape face. The characteristics can be explained by the time evolution of the current density distributions of the cross section of superconducting tape.     

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.     

Flux trapping in a ring-shaped YBCO bulk by pulsed fieldmagnetization

A pulsed magnetic field can be applied using small coils to generate a strong magnetic field for the magnetization of the high-Tc superconductors (HTS) to be used as quasi-permanent magnets in flywheels and motors. The dynamic electromagnetic behavior of two melt-processed ring-shaped Y-Ba-Cu-O bulks using the pulsed field magnetization (PFM) process has been experimentally investigated and analyzed. The flux trapped in the bulk by PFM process was compared to the flux trapped by field cooling process. Both cases then have been analyzed with a numerical model based on the finite-element method (FEM). The power-law model was utilized to relate the electric field to the current density inside the superconductor. The dependence of the critical current density on the magnetic field density was taken into account. Measured and calculated results are compared and discussed     

Magnetic-shield-type fault current limiter equivalent circuit

In order to investigate how a superconducting fault current limiter (SFCL) can enhance the performance of a power system, an accurate circuit model of the device needs to be introduced in power system simulators. In this paper, we present a finite-element numerical model to calculate the time evolution of the voltage across a magnetic-shield-type SFCL, when it is connected to an external circuit. The calculation of the voltage is carried out by using the energy conservation law, and requires the calculation, at any instant, of the current density distribution inside the superconducting tube and magnetization distribution inside the ferromagnetic core of the device. These distributions are determined by means of two coupled equivalent electric and magnetic circuits, whose topology and components are obtained through the spatial integration of quasi-static form of Maxwell equations. Comparisons between numerical and experimental results are shown.     

Mechanical Losses Due to Frictional Heat in Bi-2223 Tapes With Structural Materials of Various Thermal Expansion Properties

We have experimentally studied relation between mechanical losses and thermal expansion properties in superconducting coils. For the experiments, three kinds of structural materials (GFRP, DGFRP, and ZFRP) were used. A glass fiber reinforced plastic (GFRP) contracts with decreasing temperature; on the contrary, a Dyneema and grass fiber reinforced plastic (DGFRP) and a Zylon fiber reinforced plastic (ZFRP) expand with the decrease of temperature. We used sample holders made of those plastics and measured AC current losses of a Bi-2223 tape in liquid nitrogen. Even if the magnetic field applied to the tape and an AC transport current of the tape were same, the loss depended on the sample holder materials. When the sample holder is the GFRP, the loss was largest in the three kinds of plastics, and in case of the ZFRP, the loss was smallest. The ZFRP has the largest expansion property of the three materials, that is, the loss decreased with expansion of the sample holder's materials.     

脉冲激光淀积高电流密度的YBCO超导带材

采用脉冲激光加辅助离子源的方法在长为６．０ｃｍ的ＮｉＣｒ合金基带上制备０．１３μｍ厚的Ｙ－ＺｒＯ２（ＹＳＺ）隔离层，再用脉冲激光在ＹＳＺ／ＮｉＣｒ带上制备１．５μｍ厚的ＹＢａ２Ｃｕ３Ｏ７－ｘ超导厚膜形成ＹＢＣＯ／ＹＳＺ／ＮｉＣｒ超导带材。实验测得在７７Ｋ，０Ｔｅｓｌａ下其临界电流密度为８．７５×１０４Ａ／ｃｍ２，超导转变温度为８８．６Ｋ。     

Hysteresis of superconducting cylinder in transverse magnetic field

A piecewise continuous distribution of critical currents is used to predict the hysteresis of the superconducting cylinder in the transverse magnetic field slowly oscillating in time. The proposed hysteresis model takes into account that the magnetic field has the peripheral and radial components due to the screening effect of critical currents. The model is used to predict the hysteresis loss in a round superconducting wire. An analytical formula for the hysteresis loss is given     

Superconductors as very high-speed system-level interconnects

The anticipated propagation characteristics at 77 K of superconducting transmission lines using the new high critical temperature superconductor Ba-Y-Cu-O are calculated and compared with propagation characteristics at 77 K and 300 K of normal aluminum interconnects. The major advantages of superconducting interconnections are: 1) packing density can be increased without incurring the penalties of high loss associated with very dense interconnects of normally conducting lines; 2) propagation time can be greatly reduced; and 3) dispersion is virtually nonexistent for frequencies up to several tens of gigahertz. The peak RF magnetic field (i.e., the linear surface critical current density) and the penetration depth are important material parameters in the operation of superconducting transmission lines.     

Finite-element method modeling of superconductors: from 2-D to 3-D

A three-dimensional (3-D) numerical modeling technique for solving problems involving superconducting materials is presented. The model is implemented in finite-element method software and is based on a recently developed 3-D formulation for general electromagnetic problems with solid conductors. It has been adapted for modeling of superconductors with nonlinear resistivity in 3-D, characterized by a power-law E-J relation. It has first been compared with an existing and verified two-dimensional (2-D) model: Compared are the current density distribution inside the conductors and the self-field ac losses for different applied transport currents. Second, the model has been tested for computing the current distribution with typical 3-D geometries, such as corner-shaped and twisted superconductors. Finally, it has been used with two superconducting filaments in the presence of external magnetic field for verifying the existence of coupling currents. This effect deals with the finite length of the conductors and cannot be taken into account by 2-D models.     

Self Field Effect Compensation in an HTS Tube

It is well known that the critical current density ${J}_{c}$ of a superconducting material depends on the magnetic flux density $B$ . There exists an electric method to measure the ${J}_{c}({B})$ deduced from the $U(I)$ measurements. The problem with this method is the self field effect because the magnetic flux density is always the sum of the applied magnetic flux density and the self magnetic flux density. This paper presents a special experimental arrangement, compensating fully or partially the self magnetic flux density in an HTS tube. It allows characterizing the true zero magnetic flux density behaviour of the superconducting material. The experimental results of the compensation are discussed. A theoretical analysis based on Bean's model is presented and gives results close to the experimental ones. The proposed compensation is not perfect but the experiments and the theoretical analysis allow validation of the compensation principle.      

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.     

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     

Improvement in Superconducting Properties of MgB_2 Superconductors by Nanoscale Carbon-Based Compound Doping

MgB2 is a relatively new superconductor; it has attracted great interest from superconductor researchers all over the world. Thorough investigations have been carried out to study the material fabrication, as well as to study the material and superconducting properties from a fundamental physics point of view. The University of Wollongong has played a very active role in this research and a leading role in the research on high critical current density and high critical magnetic fields. Our recent research on the improvement of critical current density and the upper critical magnetic field by carbon-based compound doping is reviewed in this paper.     

Analysis of AC Loss and Current Distribution Characteristics of Multi-Layer Triaxial HTS Cable for 3-Phase AC Power Transmission

This paper presented numerical studies on layer-to-layer current distributions and AC losses in triaxial multi-layer three-phase cable conductors. The simulations were performed for triaxial cables of 2-layer/phase configurations with several combinations of twist pitch to investigate the influence of the cable twist configuration on the layer-to-layer current distribution and the AC loss under 3-phase AC operation. From the numerical results, it was shown that the twist pitch combinations for uniform layer-to-layer current distributions could be found. The numerical results also showed, however, that the AC loss can not be reduced by equalizing the layer-to-layer current distribution. The reason is clarified from the numerical data of the current density profiles across the superconducting layers. For the cable design, it is necessary to select proper twist configuration considering not only the equalization of current distribution but also the minimization of AC loss.