Federico Moro

Concentrated Discharges and Dry Bands on Polluted Outdoor Insulators

This paper describes concentrated discharges on polluted insulators in the laboratory and in the field. The authors have observed the concentrated discharges at the high-voltage laboratories in Stuttgart, Zittau, and Mannheim, Germany, and Wroclaw, Poland. The concentrated discharges were also documented under natural conditions. These discharges are very dangerous especially for high-voltage apparatus, bushings, and polymer insulators. It has been shown that due to uneven voltage distribution at very low surface conductivity, the concentrated discharges can ignite even under uniformly polluted and uniformly wetted insulators

A Dynamic Circuit Model of a Small Direct Methanol Fuel Cell for Portable Electronic Devices

Direct methanol fuel cells (DMFCs) constitute nowadays a promising alternative to lithium ion batteries for powering portable devices. The effective design of power-management units for interfacing DMFCs requires accurate models able to account for variable-load conditions and fuel consumption. A dynamic nonlinear circuit model for passive methanol fuel cells is presented in this paper. The model takes into account mass transport, current generation, electronic and protonic conduction, methanol adsorption, and electrochemical kinetics. Adsorption and oxidation rates, which mostly affect the cell dynamics, are modeled by a detailed two-step reaction mechanism. The fully coupled multiphysics equivalent circuit is solved by assembling first-order differential equations into a nonlinear state-variable system in order to simulate the electrical evolution of the fuel cell from its initial conditions. The fuel-cell discharge and methanol consumption are computed by combining mass-transport and conservation equations. As a result, the runtime of a DMFC can be predicted from the current load and the initial methanol concentration.

Fast Analytical Computation of Power-Line Magnetic Fields by Complex Vector Method

The electromagnetic environment related to electric power installations is typically evaluated by numerical integration methods. Numerical techniques, although powerful, are not well suited for assessing the dependence of the field strength on electric and geometric parameters. In this paper, a fast procedure to analytically evaluate power-line magnetic fields, based on complex vectors, is proposed. The use of complex algebra greatly simplifies analytical calculations compared to other approaches proposed in literature, allowing also complex conductor arrangements to be taken into account. A general formula for the magnetic-field intensity of any multiphase single-circuit line configuration is obtained. An expression for practical three-phase line configurations is simply derived as a particular case of the general formula. The proposed approach is then extended successfully to double-circuit lines, taking the load differences between circuits into account. Approximate formulas are validated by comparing magnetic flux density values with those computed from the general expression.

An integral method for extremely low frequency magnetic shielding

A novel approach for analyzing conducting shields of extremely low frequency magnetic fields in linear media is presented. It consists of an integral formulation based on the cell method, expressed in terms of network-like loop currents and magnetic vector potential line integrals on the shield surface. This formulation leads to a considerable reduction of field problem variables, thus limiting the amount of allocated memory and speeding-up the numerical procedure compared to other differential and integral techniques. Eddy currents are computed first, then the magnetic vector potential and the magnetic flux density distributions are evaluated by applying the superimposition principle. A detailed comparison between this method and a three-dimensional finite element method code demonstrates the accuracy of the results and the advantages of the method.

Accurate Calculation of the Right-of-Waywidth for Power Line Magnetic Field Impact Assessment

In this work, approximate formulas are presented for computing the magnetic fleld intensity near electric power transmission lines. Original expressions are given for single circuit lines of any type of arrangement and double circuit lines in both super-bundle and low-reactance conductor phasing. These expressions can be used for assessing directly the Right-of-Way width of power lines related to maximum magnetic fleld exposure levels which may be e-ciently used in environmental impact assessment. The accuracy of approximate formulas is demonstrated by comparison with exact formulas for computing the rms fleld distribution.

Optimal Design of Micro Direct Methanol Fuel Cells for Low-Power Applications

Competitive costs, rapid recharge, and high-energy density make small direct methanol fuel cells promising power sources for electronic equipment. Cell performance depends on several parameters, including current density, methanol concentration, and catalyst loadings. In this paper, a 1-D analytical model able to simulate the cell performance is interfaced to a stochastic optimization procedure in order to maximize the battery duration while minimizing methanol crossover.

A Coupled Thermo-Electromagnetic Formulation Based on the Cell Method

Two discrete approaches for 3-D weakly coupled thermo-electromagnetic, magnetically linear, quasi-static problems in bounded domains are presented and compared. Both approaches are based, as far as the electromagnetic equations are concerned, on discrete potentials to model both conducting and nonconducting regions, whereas the thermal problem is solved by direct use of the temperature as unknown. The code implementing the formulations is validated by comparing results with those obtained by a commercial axisymmetric package with similar space and time discretizations.

A Cell Method Formulation of 3-D Electrothermomechanical Contact Problems With Mortar Discretization

A 3-D domain decomposition method for fully coupled electrothermomechanical contact problems is presented. The formulation is based on the cell method. Contacting domains are linked together by introducing a new reference frame (i.e., the mortar surface). Field discontinuities across contact interfaces are simulated by suitable constitutive operators. It is shown that the same coupling strategy can be adopted for the electrical, thermal, and mechanical contact problems. Compatibility constraints are imposed by means of dual Lagrange multipliers defined on the mortar surface. Coupled nonlinear algebraic equations are finally cast into a saddle-point problem, which is resolved by combining the Schur complement method with the Newton-Raphson method. The proposed mortar approach is validated with a commercial 3-D finite-element method multiphysics software package.

A Boundary Integral Formulation on Unstructured Dual Grids for Eddy Current Analysis in Thin Shields

A three-dimensional boundary integral method (3D BIM) capable of analyzing the magnetic field at extremely low frequency (ELF) next to thin conducting shields is presented. This novel approach is formulated in terms of global variables (loop currents), avoiding field discontinuities across the shield. Using non-orthogonal dual grids the unknowns can be defined on nodes, thus greatly reducing computing requirements compared to traditional Galerkins formulations. The procedure is validated using an axisymmetric problem. It is shown that analytical and numerical results are in good agreement even at few millimeters from the shield surface

A Boundary Integral Formulation for Eddy Current Problems Based on the Cell Method

A finite formulation for 3-D eddy current problems in unbounded domains is presented. This approach is based on the discrete magnetic vector potential to model conducting regions and on the magnetic scalar potential to model free space. In order to avoid meshing of the air region, integral boundary conditions coupling Dirichlet and Neumann data are used. A numerical strategy based on the Schur complement approach is used in order to improve the convergence properties of the iterative solver. Simulation results show that the proposed approach is both fast and accurate.