Gérard Meunier

Influence of skull anisotropy for the forward and inverse problem in EEG : Simulation studies using FEM on realistic head models

For the sake of realism in the description of conduction from primary neural currents to scalp potentials, we investigated the influence of skull anisotropy on the forward and inverse problems in brain functional imaging with EEG. At present, all methods available for cortical imaging assume a spherical geometry, or when using realistic head shapes do not consider the anisotropy of head tissues. However, to our knowledge, no study relates the implication of this simplifying hypothesis on the spatial resolution of EEG for source imaging.

Coupled field-circuit problems: trends and accomplishments

The formulations and numerical methods for two-dimensional eddy-current problems in which conductors with eddy currents are connected by external circuits are reviewed. A general formulation valid for arbitrary circuit connections is obtained using the loop-current method. >

Comparison of numerical methods for modeling of superconductors

Different finite-element method (FEM) formulations have been developed in order to model the electromagnetic behavior of type-II superconductors. This paper presents a comparison between simulations with A-V formulation models implemented in two FEM software packages (FLUX2D and FLUX3D) and a numerical method based on analytical model for superconductors in applied magnetic field. These models can be used for superconductors with complex geometry and power-law current-voltage characteristics. Simulated is a 37-filamentary tape with applied transport current in self-field and alternating current (ac) magnetic field parallel to the wide side of the tape. A good agreement is found between the ac-loss and current distributions obtained with the different models.

Calculating the impedance of a grounding system

Following up previous research concerning low frequency electromagnetic emission from a grounding system [Nekhoul et al., 1995], the authors suggest a methodology which allows a complete electromagnetic study by finite element method (FEM) of a grounding system in the significant frequential spectrum of a stroke of lightning or a short circuit (f/spl les/1 MHz). In this study as well as calculating the electromagnetic field at any point in space (ground and air), we pay particular attention to variation with frequency of the impedance of a grounding system. Suggested modelling takes into account eddy currents in buried conductors and ground and non-linear phenomena (ionization of the ground) around the buried conductors when high currents flow via the earth. It also deals with displacement currents when their effect becomes noticeable in the earth for high frequency. The interest of this work lies in data needed to implement an optimal grounding system in non-homogeneous media. All the results of calculations are compared with measurements made by the French Electricity Board (E.D.F., France).

A shell element for computing 3D eddy currents-application to transformers

A skin depth-independent shell element to model thin conducting sheets is described in a finite element context. This element takes into account the field variation through depth due to skin effect. The finite element formulation is first described, then boundary conditions at the edge of conducting shells and the possibility of describing non conducting line gaps and holes are discussed. Finally, a computation of an earthing transformer model with an aluminium shield modelled with shell elements is presented. >

A finite element method for calculating the electromagnetic fields generated by substation grounding systems

This paper develops a basic idea which allows low frequency electromagnetic fields generated by cylindrical conductors, with very small radii compared with their length, to be calculated. The proposed method can be applied to both power overhead conductors and those buried in a conducting environment. The paper presents a direct application for this concept which consists in calculating the electromagnetic fields created by a substation grounding system following an accidental short circuit or a lightning stroke. The finite element method (FEM) with several formulations is used. Currents in the cylindrical conductors which make up the earth network grid are taken into account without volume meshing; simple line discretization of these is carried out. Electromagnetic fields generated by leakage currents and currents induced underground are taken into account. Furthermore, open boundaries of both air and earth environments are processed by introducing a spatial transformation. This application for an earth network enables to show the advantages of FEM processing compared with the method traditionally used in antenna theory for this kind of problem. >

A 3-D finite-element computation of eddy currents and losses in laminated iron cores allowing for electric and magnetic anisotropy

A 3-D scheme based on the finite element method, which takes electric and magnetic anisotropy into consideration, has been developed for computing eddy-current losses caused by stray magnetic fields in laminated iron cores of large transformers and generators. The model is applied to some laminated iron-core samples and compared with equivalent solid-iron cases. >

Surface impedance for 3D nonlinear eddy current problems-application to loss computation in transformers

A method based on surface impedance and the limit theory using a rectangular B(H) curve has been developed. The method, as well as the finite element formulation is first described. A computation on a three-phase three-limb 100 MVA transformer is then presented.

Different formulations to model superconductors

This paper presents a comparison between three different formulations using A, T/spl Phi/ or E as the state variable in order to model bulk superconductors using the Newton Raphson time stepping method. Then results concerning AC losses in superconducting strands are reported.

Computation of 2D and 3D eddy currents in moving conductors of electromagnetic retarders

The AV-A formulation is applied to problems where motion is the source of eddy currents. The Lorentz gauge is used to ensure uniqueness of the solution and leads to a two-step computation procedure: A is computed first, followed by V on moving regions only. A stable and accurate solution of A is obtained through implementation of a Petrov-Galerkin method. Numerical results of 2D and 3D problems are included. >