屏蔽电缆转移阻抗时域测量方法研究

提出了屏蔽电缆转移阻抗时域测量的一种新方法。建立的时域测量系统由高压脉冲源、三同轴装置、阻抗匹配网络、数字存储示波器以及同轴衰减器组成,能够通过时域测量获得电缆表面转移阻抗曲线,同时可直接评估电缆芯线的电磁脉冲响应。与频域测量获得的转移阻抗比较表明,时域测量是准确的;建立的参数化模型能够准确估计屏蔽电缆的电磁脉冲响应,多组验证数据表明,模型使用效果良好。     

Stability criterion for radial wave equation in finite-differencetime-domain method

A relaxed time-step stability criterion is derived for the radial wave equation in Schelkunoff form approximated by a finite-difference time-domain (FDTD) method. The criterion is established by comparing with the Cartesian Laplacian operator. This is the first step in developing stability analysis for a spherical wave implementation in FDTD     

Implementation of a surface impedance formalism at obliqueincidence in FDTD method

The authors point out that modeling of interfaces between two media, using time-domain surface impedances, permits one to reduce the discretization volume in the finite-difference-time-domain (FDTD) technique. The method presented here is based on an exact formulation of surface impedances, starting from Fresnel reflection coefficients for oblique incidence of the incident wave. The concept, valid for homogeneous and frequency-independent media, is then introduced into an FDTD algorithm where it is converted into a surface-impedance boundary condition (SIBC) for vertical or horizontal polarizations of locally plane waves. Two- and three-dimensional results are compared to those computed with classical FDTD or Fresnel reflection coefficients involving a Fourier transform     

Detection of buried dielectric cavities using the finite-differencetime-domain method in conjunction with signal processing techniques

We address the problem of detecting low-dielectric contrast cavities buried deep in a lossy ground by using the finite-difference time-domain (FDTD) method in conjunction with signal processing techniques for extrapolation and object identification. It is well known that very low frequency probing is needed for deep penetration into the lossy ground, owing to a rapid decay of electromagnetic (EM) waves at higher frequencies. It is also recognized that numerical modeling using the FDTD method becomes very difficult, if not impossible, when the operating frequency becomes as low as 1 Hz. To circumvent this difficulty, we propose a hybrid approach in this paper that combines the FDTD method with signal processing techniques, e.g., rational function approximation and neural networks (NNs). Apart from the forward problem of modeling buried cavities, we also study the inverse scattering problem-that of estimating the depth of a buried object from the measured field values at the surface of the Earth or above. Numerical results for a buried prism are given to illustrate the application of the proposed technique     

An efficient implementation of surface impedance boundaryconditions for the finite-difference time-domain method

An efficient way to implement the surface impedance boundary conditions (SIBC) for the finite-difference time-domain (FDTD) method is presented in this paper. Surface impedance boundary conditions are first formulated for a lossy dielectric half-space in the frequency domain. The impedance function of a lossy medium is approximated with a series of first-order rational functions. Then, the resulting time-domain convolution integrals are computed using recursive formulas which are obtained by assuming that the fields are piecewise linear in time. Thus, the recursive formulas derived here are second-order accurate. Unlike a previously published method [7] which requires preprocessing to compute the exponential approximation prior to the FDTD simulation, the preprocessing time is eliminated by performing a rational approximation on the normalized frequency-domain impedance. This approximation is independent of material properties, and the results are tabulated for reference. The implementation of the SIBC for a PEC-backed lossy dielectric shell is also introduced     

The efficient implementation of the surface impedance boundarycondition in general curvilinear coordinates

This paper discusses the efficient incorporation of skin-effect losses into the nonorthogonal finite-difference time-domain technique. A survey of previous work is presented and it is shown that the exponential approximations used by previous methods may lead to considerable error when good conductors are modeled using a fine time discretization. Subsequently, an improved exponential approximation is given and applied to various curved conducting waveguide surfaces using the nonorthogonal finite-difference time-domain technique     

Design of cavity-backed slot antennas using the finite-differencetime-domain technique

A cavity-backed slot antenna is thought to be one of the most suitable elements for the wireless transmission of microwave energy. A design technique is developed for the cavity-backed slot antenna using the finite-difference time-domain (FDTD) method. The technique is effective in characterizing antenna performance such as the input impedance and the far-field pattern since it takes into account the geometry of the feeder as well as the cavity. We present a method that overcomes difficulties when the FDTD method is used to design the antenna. Moreover, we discuss how to determine the calculation parameters used in the FDTD analysis. Several numerical results are presented, along with measured data, which demonstrate the validity, efficiency, and capability of the techniques. The paper proposes a new prediction method for the frequency characteristics of the cavity-backed slot antenna, which applies computational windows to time-sequence data. It is emphasized that windowing the slow decaying signal enables the extraction of accurate antenna characteristics. We also discuss how to estimate the antenna patterns when we use a sinusoidal voltage excitation     

Modeling of thin dielectric structures using the finite-differencetime-domain technique

The finite-difference time-domain (FDTD) technique is applied to scattering problems involving thin dielectric sheets, conductor-backed dielectric sheets, and conductor-backed dielectric sheets containing cracks in the dielectric material. A smart cell technique is developed that enables these geometries to be modeled with a spatial grid that is much larger than the dielectric slab and crack widths. This technique is computationally more efficient than the `brute force' (or ordinary) FDTD approach, which must use cells small enough to resolve the dielectric sheets. Numerical results are presented which show that this technique yields accurate scattering results at a large savings in computational resources     

Calculation of electromagnetic waves scattering by non-homogeneous surface impedance using moment method

In this paper, using the method of moments to calculate the current density and mono-static and bi-static radar cross section of an unlimited strip by a non-homogeneous impedance. Here, incident wave is a plane wave. To authenticate the method, using the iterative method to solve the integral equations is engaged. Simulation results show that the surface with non-homogeneous impedance has reduced or increase in potential for bi- and mono-static radar cross section in certain azimuth.     

The surface impedance for a vertically striated half-space

A special anisotropic model for the Earth subsurface is adopted in which the horizontally directed conductivities may be different from one another. Allowance is also made for contrasts in the magnetic permeability in the horizontal directions. It is shown that the surface impedance will depend on the lateral variations of the exciting field. It is also found that the dependence is more complicated than for a laterally isotropic half-space     

Modeling the nonlinear surface impedance of high-Tc thin films

To explore the idea that weak links are responsible for power dependence, we have measured and modeled the microwave impedance of fabricated Josephson-junctions, in which we have observed evidence for Josephson-vortex creation and annihilation by the microwave currents. We believe that we have observed long-junction effects in fabricated high-T_c Josephson-junctions and that these are the first observation of microwave-frequency vortex creation. To explain the nonlinear surface impedance of thin films of the high-T_c materials, an extended-coupled-grain model is introduced and discussed. The model incorporates a series array of Josephson-junction defects with a distribution of I_cR_n products. Comparison with experiments shows good qualitative agreement.     

Radiation from a slot in an impedance surface

The effects of a lossy-ground plane on the radiation from a slot are analyzed by using a Green's function of an impedance surface. Numerical results of the radiation field, radiated power, and loss power are presented for various lossy ground planes. In particular, the radiation field and the radiated power from a short slot are given by simple closed expressions. The analytical model of a slot in an impedance surface is validated by radiation field measurements     

Measurement of surface impedance at microwave frequencies

A technique for measuring the surface impedance of a plane stratified media at microwave frequencies is given, and preliminary tests are made for a variety of situations ranging from capacitive impedances to surfaces, which display a purely inductive reactance. The technique utilises a modulated scatterer to measure the elliptically polarised electric field of a TM wave propagation over the surface.     

阻抗圆动作特性的保护校验方法讨论

随着我国基础设施建设步伐的加快,电力工程建设规模在不断的扩大,保障电力系统的正常运转对于保持地方经济发展和提升群众的生活质量有着重要意义。阻抗电器广泛应用于超高压线路的保护,对于保证电力系统的正常工作和电能质量发挥着重大作用,本文通过多年来实际现场校验的经验和方法,对阻抗圆动作特性的保护校验方法进行了分析探讨,以供从事继电保护技术人员参考。     

The boundary element method in the forward and inverse problem of electrical impedance tomography

In this paper, a new formulation of the reconstruction problem of electrical impedance tomography (EIT) is proposed. Instead of reconstructing a complete two-dimensional picture, a parameter representation of the gross anatomy is formulated, of which the optimal parameters are determined by minimizing a cost function. The two great advantages of this method are that the number of unknown parameters of the inverse problem is drastically reduced and that quantitative information of interest (e.g., lung volume) is estimated directly from the data, without image segmentation steps. The forward problem of EIT is to compute the potentials at the voltage measuring electrodes, for a given set of current injection electrodes and a given conductivity geometry. In this paper, it is proposed to use an improved boundary element method (BEM) technique to solve the forward problem, in which flat boundary elements are replaced by polygonal ones. From a comparison with the analytical solution of the concentric circle model, it appears that the use of polygonal elements greatly improves the accuracy of the BEM, without increasing the computation time. In this formulation, the inverse problem is a nonlinear parameter estimation problem with a limited number of parameters. Variants of Powell's and the simplex method are used to minimize the cost function. The applicability of this solution of the EIT problem was tested in a series of simulation studies. In these studies, EIT data were simulated using a standard conductor geometry and it was attempted to find back this geometry from random starting values. In the inverse algorithm, different current injection and voltage measurement schemes and different cost functions were compared. In a simulation study, it was demonstrated that a systematic error in the assumed lung conductivity results in a proportional error in the lung cross sectional area. It appears that our parametric formulation of the inverse problem leads to a stable minimization problem, with a high reliability, provided that the signal-to-noise ratio is about ten or higher.     

Time-harmonic impedance tomography using the T-matrix method

A time-harmonic formulation for the electrical impedance tomography (EIT) inverse problem accounting for electrodynamic effects is derived. This work abandons the standard electrostatic impedance model for a full-wave T-matrix model. The advantage of this method is an accurate physical model that includes finite frequency effects, such as diffusion phenomena, and electrode contact impedance effects. This model offers the potential for increased resolution and larger invertible contrast objects than other methods when used on experimental data, because it may represent a more realistic physical model. Also, an accurate gradient matrix is used in the Newton iterative method so the image reconstruction converges in a few iterations. These advantages are realized with no increase in the computational complexity of this algorithm, compared to the static finite element model. A calibration technique is suggested for measurement systems, to test the validity of a theoretical model that includes electrode contact impedance effects.     

Finite-element method for the impedance analysis of traveling-wavemodulators

A low-frequency finite-element method (FEM) developed to calculate the impedance of multi-strip-line electrodes on traveling-wave modulators is discussed. In this method, the divergence theorem is used to evaluate the electrode capacitance from the node potential values of discrete elements. This method is used to calculate the impedance of electrodes on anisotropic-inhomogeneous dielectric media. The effect of nonzero electrode thickness and a groove excavated at the electrode gap is analyzed     

Temporal resolution of the skin impedance measurement in frequency-domain method

The temporal-resolution of the frequency-domain method for the identification of the impedance locus depends on the basis frequency used in the current waveform construction, i.e., the higher basis frequency provides the better temporal resolution. The impedance locus can be characterized by the impedance parameters Z0, beta, and taum. The frequency distribution of limited number of data in the impedance locus would affect the accuracy of the estimated impedance parameters. Therefore, we investigated the relationship between the estimation accuracy of the impedance parameters and the frequency coverage of the impedance locus in relatively low to high impedance conditions (dc impedance Z0= 51 k ohms - 45 M ohms). As the basis frequency, 100 Hz was enough for the usual impedance with Z0 less than 203 k ohms. On the other hand, 10 Hz and 1 Hz were required for the medium-level (Z0 = 517 k ohms), and high-level (Z0 = 45 M ohms) impedance, respectively. The required basis frequency, accordingly the temporal resolution, depended much on the central relaxation time taum which affects the frequency distribution on the impedance locus. The results of this study are expected to serve as the reference of the frequency selection in the frequency-domain analysis of the skin impedance.     

The use of the SPSA method in ECG analysis

The classification, monitoring, and compression of electrocardiogram (ECG) signals recorded of a single patient over a relatively long period of time is considered. The particular application we have in mind is high-resolution ECG analysis, such as late potential analysis, morphology changes in QRS during arrythmias, T-wave alternants, or the study of drug effects on ventricular activation. We propose to apply a modification of a classical method of cluster analysis or vector quantization. The novelty of our approach is that we use a new distortion measure to quantify the distance of two ECG cycles, and the class-distortion measure is defined using a min-max criterion. The new class-distortion-measure is much more sensitive to outliers than the usual distortion measures using average-distance. The price of this practical advantage is that computational complexity is significantly increased. The resulting nonsmooth optimization problem is solved by an adapted version of the simultaneous perturbation stochastic approximation (SPSA) method of. The main idea is to generate a smooth approximation by a randomization procedure. The viability of the method is demonstrated on both simulated and real data. An experimental comparison with the widely used correlation method is given on real data.     

Fdtd simulation for the EMI properties of impedance surface enclosures

An enclosure with impedance interior surfaces is studied using the FDTD method. The FDTD simulation shows a wonderful depression of the resonance-related EMI signals in the impedance surface enclosure.