In this paper, a numerical model for the analysis of electromagnetic transients in a power system, based on the finite element technique, was developed. The simplicity of the finite element technique ...is manifested in the fact that the problem of solving any mathematically described phenomena in an area is reduced to solving those same phenomena in a small part of that area, i.e., finite elements. Based on appropriate mathematical models, numerical models of the synchronous generator and other parts of the power system have been developed. System of differential equations of each power system element have been included in the numerical model in such a way that we employ numerical integration of the differential equations using the generalized trapezoidal rule (ϑ-method) and reduce it to a system of algebraic equations in each time step. In a practical sense, this method enables a very simple solution to the problem of a real power system, i.e., a system with many generators, transformers, transmission lines and other elements. The accuracy of the developed numerical model for the analysis of electromagnetic transients in a power system has been confirmed by comparing the calculated results with the results obtained using the EMTP software (version 3.3) package.
The aim of the study was to investigate the association between some lifestyle-attributable risk factors of atherosclerosis, such as body mass index (BMI), oral contraceptives, hormone replacement ...therapy, smoking and alcohol consumption with breast arterial calcification (BAC) and its intensity on mammograms, and to assess the impact of these lifestyle risk factors on mammography findings of BAC. This prospective study included 300 women aged 47-69, i.e. a group of 149 women with BAC on mammograms and control group of 151 women without BAC. Self-reported BMI, use of oral contraceptives, hormone replacement therapy, smoking and alcohol consumption were recorded by medical interview. The presence of BAC and its intensity on mammography was compared according to the presence of high BMI and use of hormone therapy, smoking and alcohol consumption. The results showed the highest proportion of smokers (28.9%) in the group with mild BAC as compared with the groups without calcification (14.6%) and with intense calcification (12.1%). Women taking oral contraceptives had a higher level of calcified breast arteries but no significant between-group difference was found for high BMI, hormone therapy and alcohol consumption. Thus, study results showed the mammographic finding of BAC to be inadequate to identify women with some lifestyle-attributable risk factors such as BMI, hormone replacement therapy, smoking and alcohol consumption.
This article deals with an infinitesimal current source (ICS) in a multilayer conducting medium that consists of air and multilayer soil. Closed-form kernel functions of the spectral-domain Green's ...function for scalar electric and vector magnetic potentials (Lorentz potentials) were obtained for every layer of the multilayered medium as analytic solutions of the systems of linear equations. Validation of the proposed closed-form expressions is performed by numerical solving of the system of linear equations and by numerical comparison with expressions derived from existing closed-form expressions developed for the source layer of the multilayer dielectric medium. For an arbitrary-oriented ICS, scalar electric potential of its horizontal component is chosen as a unique scalar electric potential. Accordingly, dyadic Green's function of vector magnetic potential is corrected.
doi: 10.1111/j.1741‐2358.2011.00552.x
Study of temporomandibular joint disorder in older patients by magnetic resonance imaging (MRI)
Objectives: To compare characteristics in older patients in a ...sample of the general population of those with temporomandibular joint disorder (TMJD).
Materials and methods: A prospective study was carried out between 2001 and 2008 in patients with TMJD. The whole sample consisted of 141 patients divided in two groups: 31 patients aged over 60 (median age 67.9, ranging from 60 to 82) and the remaining 110 patients (median age 36.3, ranging from 12 to 59) who were seeking treatment. Clinical diagnostics was confirmed by MRI. Pain intensity was rated on a visual analogue scale (VAS 0‐10).
Results: There was no statistical difference between average pain in older patients (6.2) and patients aged up to 59 (5.7) evaluated by VAS. There was a statistically significant difference (p = 0.002) in pain duration: older patients reported shorter duration of experienced pain (7.8 months) than patients aged up to 59 (12.2 months).
Conclusion: In this study, it was found that 22% were older patients with TMJD. A higher level of anxiety was shown in both patients’ groups, regardless of shorter pain experience in the older patients.
•Computation of per-unit-length self and mutual impedances of overhead power lines is presented.•High-accurate numerical algorithm based on exponential approximation of the integral’s kernel function ...is elaborated.•Approximate formulas used for computation of per-unit-length self and mutual impedance corrections are presented.•Results obtained by proposed algorithm and approximation methods are shown and compared to the 7‐digit accurate results.
In this paper, analytical expressions for a highly accurate computation of Carson formulas are developed. Highly accurate analytical expressions for per-unit-length (pul) self and mutual impedance corrections are now reduced to two linear combinations of 50 functions. Coefficients of these linear combinations are computed using exponential approximation of integrals’ kernel function. Proposed algorithm has simplified computation in comparison with other highly accurate numerical algorithms. Results computed by proposed algorithm and by two approximation methods are compared with 7‐digit accurate results computed by piecewise quadratic approximation for large frequency range.
In this manuscript, a novel method for computation of per-unit-length
internal impedance of a cylindrical multilayer conductor with conductive and
dielectric layers is presented in detail. In ...addition to this, formulas for
computation of electric and magnetic field distribution throughout the
entire multilayer conductor (including dielectric layers) have been derived.
The presented formulas for electric and magnetic field in conductive layers
have been directly derived from Maxwell equations using modified Bessel
functions. However, electric and magnetic field in dielectric layers has
been computed indirectly from the electric and magnetic fields in contiguous
conductive layers which reduces the total number of unknowns in the system
of equations. Displacement currents have been disregarded in both conductive
and dielectric layers. This is justifiable if the conductive layers are good
conductors. The validity of introducing these approximations is tested in
the paper versus a model that takes into account displacement currents in
all types of layers.
SUMMARY
This paper presents improved algorithm for electromagnetic transient calculations on multiconductor transmission line (MTL) achieved by improvement of the time integration when forming a ...local system of equations for a finite element. Improvement of accuracy was obtained by using Heun's method. Previously developed local system of equations of MTL has been obtained using the generalized trapezoidal rule (ϑ‐method). The use of the generalized trapezoidal rule for the time integration sometimes causes numerical oscillations or numerical diffusion of numerical solutions. Numerical solutions obtained using Heun's method and using the generalized trapezoidal rule for different values of a time integration parameter are compared with analytical solution. It has been shown that Heun's method yields the results with much higher accuracy comparing to results obtained by generalized trapezoidal rule.
Improvement of the finite element method algorithm for electromagnetic transient analysis on multiconductor transmission line has be achieved by improving the time integration when forming a local system of equations for a finite element. Improvement of accuracy was obtained by using Heun's method. It has been shown that Heun's method yields the results with much higher accuracy comparing to results obtained by generalized trapezoidal rule (ϑ‐method).
Novel approach for the high-accurate computation of Carson formulas is presented. Carson formulas are used for computation of per-unit length (pul) self and mutual impedances of infinitely long ...parallel conductors. Numerical algorithm described in this paper uses a piecewise approximation of the kernel function which appears in the Carson formula corrections. Approximated kernel function is multiplied by the rest of the integrands in the impedance correction expressions and analytically integrated. By using the proposed algorithm, highaccurate results with the desired computed n-digit accuracy can easily be obtained. Results computed by the proposed algorithm are compared with the two most commonly used approximation methods for large frequency range.
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