G. Molinari

Multiobjective optimization in magnetostatics: a proposal for benchmark problems

A proposal for benchmark problems to test electromagnetic optimization methods, relevant to multiobjective optimization of a solenoidal superconducting magnetic energy storage with active and passive shielding is presented. The system has been optimized by means of different optimization procedures based on the global search algorithm, evolution strategies, simulated annealing and the conjugate gradient method, all coupled to integral or finite element codes. A comparison of results is performed and the features of the problem as a test of optimization procedures are discussed.

A multiquadrics-based algorithm for the acceleration of simulated annealing optimization procedures

A procedure to significantly reduce the computational cost of the simulated annealing optimization algorithm coupled with analysis methods is proposed for EM devices. The chosen approach relies on the application of the simulated annealing method to an analytical approximation of the true objective function, expressed in the form of a multiquadric expansion. The algorithm is fully described, its potential advantages are pointed out and some test cases showing the effectiveness of the implemented strategy are reported and discussed. As an example the superconducting magnet energy storage device is mentioned.

A user oriented modular package for the solution of general field problems under time varying conditions

In this paper a modular package, named COMPELL, for the solution of two-variable time-varying problems of the elliptic type is presented. The package allows the solution of standard magnetic problems, also under quasi-stationary or rapidly time-varying conditions, in the cases with nonlinear, anisotropic and nonhomogeneous materials with eddy currents, but can also face different problems, taking into account for instance the presence of moving bodies or the reciprocal interaction between magnetic fields, thermal fields and so on. The procedures used for field solution are generalized finite differences, least square finite differences, first presented in this paper, or finite elements. In the paper examples of magnetic field solutions are given, in particular by showing the field in a Tokamak structure accounting for the Hall effect. The data deck and the obtained results are displayed and the main features of the package are discussed.

Field-Enhanced Motion of Impurity Particles in Fluid Dielectrics under Linear Conditions

It has been pointed out by several authors that the influence of impurity particles on dielectric phenomena in non-solid dielectrics can be significant in some cases, especially for technical liquids. In order to get information on such impurity influence, a computer-aided study of the motion of a charged spherical particle under general transient conditions, and with unrestricted conductivity and permittivity of both the medium and the particle has been carried out. To verify the model consistency, a set of tests in n-hexane with sphere-plane electrode geometry have been carried out using different experimental techniques, including high-speed-photography, and satisfactory agreement has been observed. On this basis, a model of the low-field conduction process by impurity particles has been developed assuming negligeable interactions between particles. A computer-aided procedure for the evaluation of impurity convection currents under different conditions has been implemented. The results obtained are compared with tests carried out on n-hexane with controlled pollution. Finally, a detailed discussion of the results obtained is given.

Effect of streamer shape and dimensions on local electric field conditions

The knowledge of the local electrical field in the immediate vicinity of the electrode surface is very desirable if one were to understand the charge injection process at an electrode. Many attempts have been made to determine this local field experimentally.[1,3] None of these attempts has so far succeeded in estimating the field in front of the primary streamer which is formed as a result of the charge injection process.

A package for computer aided design for power electrical engineering

A package developed for both research and industrial application for computer aided design of electromagnetic devices is presented and the main features are discussed. From the research point of view the main feature of the package is to allow the definition of the differential equations of the problem to be solved in their analytical form by means of input data for both finite differences and finite elements. From the point of view of practical application, the main features are based on an input processor allowing the input of data in a parametric form. Besides, procedures for optimization of integral and differential quantities and for profile optimization are implemented in the package. For both purposes, automatic grid generation procedures controlled by the user are at disposal and grid optimization is at an advanced stage of development.

Some data on the AC breakdown strength of liquid helium in the millimetre gap range

Abstract In this paper the first results of an investigation aimed to obtain additional data on breakdown strength of liquid helium are presented; the work is performed under as controlled as possible conditions not too far removed from those relevant to practical engineering applications, by means of a purpose-built helium cryostat whose main features are briefly described. To get results suitable for direct engineering use, a first group of tests was performed with IEC normalized spherical cap electrodes, investigating the statistical behaviour of breakdown strength as a function of gap setting and test conditions. A second group of tests was performed with cylindrical electrodes to get data on the amount of the area effect on the statistical features of breakdown, also as function of test conditions. Finally, a statistical analysis of the obtained data is performed and results are discussed.

Analysis of the charge exchange mechanism between impurities and electrodes in a dielectric liquid

Abstract On the basis of a more accurate experimental study of the charge exchange mechanism between particles and electrodes, some improvements are introduced in the charge exchange models previously developed by the authors. This allows the application range of a model of particle motion to be extended to sizes of greater practical interest, down to some microns. New identification procedures of the model parameters are proposed, whose extension to smaller sizes seems to be more practical. To show the potentiality of the model, experimental data obtained by high speed movie are compared with model simulations and results are discussed.

Finite difference and finite element grid optimization by the grid iteration method

A grid optimization iterative procedure is presented in the paper. It represents the first step of a two step algorithm and is named grid iteration method. Together with the second step, named metric iteration method, it has been developed previously by the authors for simple examples and has been proven to be potentially very efficient. In this paper the grid iteration method is further developed. It is extended to problems with various unknowns; more efficient techniques are developed in order to apply the method to more complex geometries, to improve the convergence tests and to better solve magnetic problems.

Preliminary results of computer simulation of D.C. conduction due to impurity particles in a dielectric liquid and comparison with experimental data

Abstract On the basis of models, previously developed by the authors, of the motion of impurity particles in a dielectric liquid and of the mechanism of charge exchange at the electrodes, a model of conduction current due to charge convection by impurity particles in a dielectric liquid is described. The model can be applied to three-dimensional axisymmetrical geometries with conducting or dielectric impurity particles. Experimental tests performed, under d.c. conditions, with different impurity particles, down to a radius of a few microns, and with different electrode geometries, are reported. A comparison between experimental data and simulation results indicates satisfactory agreement.