Performance of an inductive fault current limiter employing BSCCOsuperconducting cylinders

Inductive fault current limiters operating at high levels of short-circuit currents are plagued by appearance of overheated thermal domains in active superconducting elements. Excessive growth of thermal domains may lead to a fatal mechanical destruction of the superconducting element during a fault event. It has been determined that employment of superconductors with gradual dissipation onset controlled by flux relaxation processes can efficiently prevent local overheating. Operation of such elements, fabricated by melt cast technique, has been investigated experimentally in a small-scale open-core model of an inductive fault current limiter. The results of the experiments demonstrate the feasibility of application of superconducting cylinders having properties dominated by flux relaxation processes in inductive current limiters. The most important parameter of a superconducting element designated to operate in such devices is the rate of flux relaxation and its dependence on ac current amplitude. It has been found that ac losses associated with flux relaxation in the investigated cylinders allow for a reliable limiter operation at the nominal current level. Projection of the parameters of the investigated small-scale model to the full-scale device has been performed using the concept of physical modeling. The obtained results indicate that it is possible to build a full-scale device based on flux creep dissipation mechanisms for distribution networks     

An FDTD model for calculation of gradient-induced eddy currents in MRI system

In most magnetic resonance imaging (MRI) systems, pulsed magnetic gradient fields induce eddy currents in the conducting structures of the superconducting magnet. The eddy currents induced in structures within the cryostat are particularly problematic as they are characterized by long time constants by virtue of the low resistivity of the conductors. This paper presents a three-dimensional (3-D) finite-difference time-domain (FDTD) scheme in cylindrical coordinates for eddy-current calculation in conductors. This model is intended to be part of a complete FDTD model of an MRI system including all RF and low-frequency field generating units and electrical models of the patient. The singularity apparent in the governing equations is removed by using a series expansion method and the conductor-air boundary condition is handled using a variant of the surface impedance concept. The numerical difficulty due to the "asymmetry" of Maxwell equations for low-frequency eddy-current problems is circumvented by taking advantage of the known penetration behavior of the eddy-current fields. A perfectly matched layer absorbing boundary condition in 3-D cylindrical coordinates is also incorporated. The numerical method has been verified against analytical solutions for simple cases. Finally, the algorithm is illustrated by modeling a pulsed field gradient coil system within an MRI magnet system. The results demonstrate that the proposed FDTD scheme can be used to calculate large-scale eddy-current problems in materials with high conductivity at low frequencies.     

A 3-D finite element formulation for calculating Meissner currentsin superconductors

A three-dimensional (3-D) finite element formulation for calculating Meissner currents in superconductors is presented. The authors have chosen a magnetic vector potential formulation, which also enables them to simulate ferromagnetic shielding. The equations are written so that the problem can be solved by the use of a conjugate gradient algorithm without preconditioning. Numerical results on normal-superconductor junctions and on superconducting lines are compared with analytical solutions     

Calculation of per-unit-length resistance and internal inductance in 2-D skin-effect current driven problems

The study of two-dimensional (2-D) skin-effect transient problems by means of a formulation based on a magnetic vector potential A and on a scalar potential /spl phi/ is presented. It is shown that such 2-D problems in the cross-sectional plane of the conductors cannot be treated as voltage driven but only as current driven. As an application of the proposed method, the equivalent per-unit-length resistance and internal inductance of conductors carrying typical switch-mode power supplies nonsinusoidal current waveforms have been evaluated. The analysis clearly reveals that they depend both on switching frequency and duty cycle, and are rather different from dc and ac sinusoidal equivalent parameters. Time-varying instantaneous ohmic resistances and internal inductances can be also defined in sinusoidal steady-state as well as in generic operating conditions.     

Compute extremely low-frequency electromagnetic field exposure by 3-D impendance method

A 3-D impedance method has been introduced to compute the electric currents induced in a human body exposed to extremely low-frequency electromagnetic field.The 3-D impedance method has been deduced from Maxwell equations and is put into the computation and simulation effectively to the visible human body model, which has 196×114×626 cells and more than 40 types of tissues.As the result, two representative cases are investigated.One is exposure of the human body to 100 μT (1 000 mG), the limit recommended by the International Commission on Non-Ionizing Radiation Protection for the public and the other one is the exposure of human body to 0.4 μT (4 mG), the level at which a statistical link appears with a doubled risk of development of childhood leukaemia.The distribution of induced current density can be obtained and the maximum of induced current are found to be 16 mA/m2 and 0.07 mA/m2.     

Modeling of 3-D Surface Roughness Effects With Application to μ-Coaxial Lines

Effects of 3-D surface roughness on the propagation constant of transverse electromagnetic transmission lines are calculated using finite-element method software by solving for the fields inside conductors. The modeling is validated by comparison with available literature results for the special case of 2-D surface roughness and by simulations using the finite-integration technique. Results for cubical, semiellipsoidal, and pyramidal indentations, as well as rectangular, semicircular, and triangular grooves in conductor surfaces are presented. A developed surface roughness model is applied to rectangular mu-coaxial lines. It is shown that roughness contributes up to 9.2% to their overall loss for frequencies below 40 GHz     

Electromagnetic modeling of waveguides involving finite-sizedielectric regions

A moment method for handling arbitrarily shaped 2-D and 3-D waveguides that involve either conductors, finite-size dielectric regions, or both is presented. A procedure for modeling the dielectric allows 2-D rooftop functions to represent both the 3-D polarization current in the dielectric and the surface current on the conductors, and precludes the presence of fictitious charge within the dielectric. Examples include coaxial, microstrip, and dielectric waveguides. Numerical convergence, consistency with physical principles, and agreement with the literature are demonstrated     

Effectiveness of 2-D and 2.5-D FDTD Ground-Penetrating Radar Modeling for Bridge-Deck Deterioration Evaluated by 3-D FDTD

Computational modeling effectively analyzes the wave propagation and associated interaction within heterogeneous reinforced concrete bridge decks, providing valuable information for sensor selection and placement. It provides a good basis for the implementation of the inverse problem in defect detection and the reconstruction of subsurface properties, which is beneficial for defect diagnosis. The objective of this study is to evaluate the effectiveness of lower order models in the evaluation of bridge-deck subsurfaces modeled as layered media. The two lower order models considered are a 2-D model and a 2.5-D model that uses the 2-D geometry with a compressed coordinate system to capture wave behavior outside the cross-sectional plane. Both the 2- and 2.5-D models are compared to the results obtained from a full 3-D model. A filter that maps the 3-D excitation signal appropriately for 2- and 2.5-D simulations is presented. The 2.5-D model differs from the 2-D model in that it is capable of capturing 3-D wave behavior interacting with a 2-D geometry. The 2.5-D matches results from the corresponding 3-D model when there is no variation in the third dimension. Computational models for air-launched ground-penetrating radar with 1-GHz central frequency and bandwidth for the detection of bridge-deck delamination are implemented in 2-, 2.5-, and 3-D using FDTD simulations. In all cases, the defect is identifiable in the results. Thus, it is found that in layered media (such as bridge decks) 2- and 2.5-D models are good approximations for modeling bridge-deck deterioration, each with an order of magnitude reduction in computational time.      

Superconducting cylinder in a static transverse magnetic field

A piecewise continuous distribution of the critical currents is proposed to make compatible the Maxwell equations and constitutive equations of superconducting media. A magnetic moment of the infinite superconducting cylinder is calculated on the base of the proposed shell model. Some generalizations of the calculation method are given for superconductors of more complicated shapes     

An efficient multiregion model for electromagnetic scattering and radiation by PEC targets

An efficient multiregion model has been proposed for the fast implementation of the electromagnetic scattering by perfectly electrical conducting (PEC) targets and the radiation of point sources or wire antennas near PEC targets. In the multiregion model, the PEC target under consideration is divided by multiple regions depending on the position of point source/antenna or the incident direction of plane waves. Then the method of moments (MoM) is used on the first region, which is close to the source or is the illuminated region, to obtain the accurate electric current. The mutual coupling between different regions are considered approximately based on the magnetic-field integral equation, from which closed-form approximations for electric currents on other regions are derived. Because MoM is only performed on the first region, the number of unknowns in the new model is much fewer than that in the full MoM analysis, making the new model much more efficient. Compared with the published hybrid methods, the multiregion model gives a more reasonable physical explanation, and provides a better accuracy in both currents and scattered fields. Numerical simulations for two-dimensional (2-D) problems (transverse-magnetic/transverse-electric) and 3-D problems are given to test the validity and efficiency of the proposed modeling.     

Computation of 2-D Current Distribution in Superconductors of Arbitrary Shapes Using a New Semi-Analytical Method

This paper presents an original semi-analytical method (SAM) for computing the 2D current distribution in conductors and superconductors of arbitrary shape, discretized in triangular elements. The method is a generalization of the one introduced by Brandt in 1996, and relies on new and compact analytical relationships between the current density (J_x), the vector potential (A_x), and the magnetic flux density (B_x,By), for a linear variation of J over 2D triangular elements. The derivation of these new formulas, which is also presented in this paper, is based on the analytic solution of the 2D potential integral. The results obtained with the SAM were validated successfully using COMSOL Multiphysics, a commercial package based on the finite-element method. Very good agreement was found between the two methods. The new formulas are also expected to be of great interest in the resolution of inverse problems.     

Novel SPICE Compatible Partial-Element Equivalent-Circuit Model for 3-D Structures

This paper presents a novel SPICE compatible partial-element equivalent-circuit (PEEC) model for general linear medium. In this new model, the magnetized current in conductive magnet and fictitious magnetized current through the magnetic interface are considered, as well as conduction current in conductor and polarization currents in a dielectric, to model the magnetization and conduction loss effect of a conductive magnet. Corresponding equivalent circuits are derived. The magnetic field couplings between inductive cells are taken into account as current controlled current sources to avoid the time-consuming calculation of time derivatives. The new model was applied in 3-D magnetically enhanced inductor structure analysis and antenna modeling. Obtained results are compared with those obtained from commercial numerical electromagnetic simulation software and show good agreement.      

Reconstruction of 3-D geometry using 2-D profiles and a geometric prior model

A method has been developed to reconstruct three-dimensional (3-D) surfaces from two-dimensional (2-D) projection data. It is used to produce individualized boundary element models, consisting of thorax and lung surfaces, for electro- and magnetocardiographic inverse problems. Two orthogonal projections are utilized. A geometrical prior model, built using segmented magnetic resonance images, is deformed according to profiles segmented from projection images. In the authors' method, virtual X-ray images of the prior model are first constructed by simulating real X-ray imaging. The 2-D profiles of the model are segmented from the projections and elastically matched with the profiles segmented from patient data. The displacement vectors produced by the elastic 2-D matching are back projected onto the 3-D surface of the prior model. Finally, the model is deformed, using the back-projected vectors. Two different deformation methods are proposed. The accuracy of the method is validated by a simulation. The average reconstruction error of a thorax and lungs was 1.22 voxels, corresponding to about 5 mm     

Control of a new active power filter using 3-D vector control

The growth in distorting loads has given an interest in the design of three-wire active power filters, switching circuits which can be installed in an installation and generate harmonic currents to neutralize the loads' distorting current. With no neutral, the currents have two degrees of freedom and can be represented by two-dimensional (2-D) vectors. This approach leads to a control strategy which can compensate for balanced three-phase distorting loads. However, there has been a growth of single-phase distorting loads which give large neutral currents at triplen frequencies and which cannot be dealt with by the three-wire active power filter control. This paper reports the development of a three-dimensional (3-D) vector treatment of unbalanced three-phase circuits which can be used instead for the development of a control strategy. Simulation studies show that the control is effective, not only for the removal of harmonics, but also for the compensation of fundamental unbalance currents     

Effect of Domain Shape Modeling and Measurement Errors on the 2-D D-Bar Method for EIT

The D-bar algorithm based on Nachman's 2-D global uniqueness proof for the inverse conductivity problem (Nachman, 1996) is implemented on a chest-shaped domain. The scattering transform is computed on this chest-shaped domain using trigonometric and adjacent current patterns and the complete electrode model for the forward problem is computed with the finite element method in order to obtain simulated voltage measurements. The robustness and effectiveness of the method is demonstrated on a simulated chest with errors in input currents, output voltages, electrode placement, and domain modeling.     

3-D ICs: a novel chip design for improving deep-submicrometerinterconnect performance and systems-on-chip integration

Performance of deep-submicrometer very large scale integrated (VLSI) circuits is being increasingly dominated by the interconnects due to decreasing wire pitch and increasing die size. Additionally, heterogeneous integration of different technologies in one single chip is becoming increasingly desirable, for which planar (two-dimensional) ICs may not be suitable. This paper analyzes the limitations of the existing interconnect technologies and design methodologies and presents a novel three-dimensional (3-D) chip design strategy that exploits the vertical dimension to alleviate the interconnect related problems and to facilitate heterogeneous integration of technologies to realize a system-on-a-chip (SoC) design. A comprehensive analytical treatment of these 3-D ICs has been presented and it has been shown that by simply dividing a planar chip into separate blocks, each occurring a separate physical level interconnected by short and vertical interlayer interconnects (VILICs), significant improvement in performance and reduction in wire-limited chip area can be achieved, without the aid of any other circuit or design innovations. A scheme to optimize the interconnect distribution among different interconnect tiers is presented and the effect of transferring the repeaters to upper Si layers has been quantified in this analysis for a two-layer 3-D chip. Furthermore, one of the major concerns in 3-D ICs arising due to power dissipation problems has been analyzed and an analytical model has been presented to estimate the temperatures of the different active layers. It is demonstrated that advancement in heat sinking technology will be necessary in order to extract maximum performance from these chips. Implications of 3-D device architecture on several design issues have also been discussed with special attention to SoC design strategies. Finally some of the promising technologies for manufacturing 3-D ICs have been outlined     

A comprehensive 2-D inductance modeling approach for VLSI interconnects: frequency-dependent extraction and compact circuit model synthesis

Although three-dimensional (3-D) partial inductance modeling costs have decreased with stable, sparse approximations of the inductance matrix and its inverse, 3-D models are still intractable when applied to full chip timing or crosstalk analysis. The 3-D partial inductance matrix (or its inverse) is too large to be extracted or simulated when power-grid cross-sections are made wide to capture proximity effect and wires are discretized finely to capture skin effect. Fortunately, 3-D inductance models are unnecessary in VLSI interconnect analysis. Because return currents follow interconnect wires, long interconnect wires can be accurately modeled as two-dimensional (2-D) transmission lines and frequency-dependent loop impedances extracted using 2-D methods . Furthermore, this frequency dependence can be approximated with compact circuit models for both uncoupled and coupled lines. Three-dimensional inductance models are only necessary to handle worst case effects such as simultaneous switching in the end regions. This paper begins by explaining and defending the 2-D modeling approach. It then extends the extraction algorithm to efficiently include distant return paths. Finally, a novel synthesis technique is described that approximates the frequency-dependent series impedance of VLSI interconnects with compact circuit models suitable for timing and noise analysis.     

Accurate and computationally efficient analytical 1-D and 2-D ionimplantation models based on Legendre polynomials

Computationally efficient ion implantation modeling has become the essential tool for efficient and accurate CMOS design as aggressive scaling of devices continues. Specifically, computationally efficient two-dimensional (2-D) analytical models are often more attractive than physically-based Monte Carlo simulations since the latter are expensive in terms of computational time. Here we present new computational-efficient analytical models to simulate one-dimensional (1-D) and 2-D impurity and damage profiles. Legendre polynomials are used as basis functions in view of their orthogonality and good interpolation property. Conventional superposition approaches for 2-D implant modeling are explained and the shortcomings are analyzed. A dose splitting approach is incorporated in the new 2-D model to account for the nonlinear dc-channeling effect as implantation-induced damage accumulates. Good agreement with a physically-based and experimentally verified Monte Carlo simulator (UT-MAR-LOWE with TOMCAT) has been obtained for both impurity and damage profiles with a 50× reduction of computational time for medium-energy implants     

Superconducting materials for large scale applications

Since the 1960s, Nb-Ti (superconducting transition temperature T/sub c/=9 K) and Nb/sub 3/Sn (T/sub c/=18 K) have been the materials of choice for virtually all superconducting magnets. However, the prospects for the future changed dramatically in 1987 with the discovery of layered cuprate superconductors with T/sub c/ values that now extend up to about 135 K. Fabrication of useful conductors out of the cuprates has been difficult, but a first generation of silver-sheathed composite conductors based on (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ (T/sub c//spl sim/110 K) has already been commercialized. Recent progress on a second generation of biaxially aligned coated conductors using the less anisotropic YBa/sub 2/Cu/sub 3/O/sub 7/ structure has been rapid, suggesting that it too might enter service in the near future. The discovery of superconductivity in MgB/sub 2/ below 39 K in 2001 has brought yet another candidate material to the large-scale applications mix. Two distinct markets for superconductor wires exist-the more classical low-temperature magnet applications such as particle accelerators, nuclear magnetic resonance and magnetic resonance imaging magnets, and plasma-containment magnets for fusion power, and the newer and potentially much larger market for electric power equipment, such as motors, generators, synchronous condensers, power transmission cables, transformers, and fault-current limiters for the electric utility grid. We review key properties and recent progress in these materials and assess their prospects for further development and application.