V. Maló Machado

Series-impedance of underground transmission systems

A general solution for the magnetic field in the soil is developed that satisfies arbitrary boundary conditions on an underground cylindrical surface at a finite depth below an earth/air plane surface. A quasi-steady state hypothesis is assumed. The series impedance of an underground cable with a perfectly conducting sheath is evaluated taking into account skin effect in the soil. The numerical results are compared with F. Pollaczeks (1926) classical solution. >

Series-impedance of underground cable systems

A general solution for the magnetic field in the soil is developed that satisfies arbitrary conditions on a set of underground cylindrical surfaces at a finite depth below the earth/air surface. The solution takes into account singularities of all orders located in the axis of each circular boundary, allowing boundary conditions on the earth/air plane to be satisfied and imposing regularity at infinity. The method is used to evaluate the series-impedance matrix of a system comprising three underground cables in a flat configuration. Numerical results are presented for the contribution of the Earth return path to the cable self-impedances as well as for the mutual impedances between cables. The results are compared with Pollaczeks classical solution. >

Eddy current and hysteresis losses in ferromagnetic media

This paper is a contribution to the evaluation of losses in ferromagnetic media, due to eddy currents and magnetic hysteresis, the field source being the time harmonic induced e.m.f. The fundamental field equations are solved numerically by using a finite element formulation, corresponding to the space discretization, and a second order trapezoidal (Crank-Nickolson) method for the time integration. The method is applied to a soft ferromagnetic sample, being the magnetic hysteresis simulated by using a classical Preisach model.

Ground return effect on wave propagation parameters of overhead power cables

It is noted that the propagation properties of overhead three-phase cables are usually analyzed assuming that the pipe conductor establishes a perfect shielding between the inner conductor set and any outer conductor, i.e. the power cable is assumed to be an isolated system. The influence of a lossy ground plane in the neighborhood of the cable is examined in the present work. The propagation parameters for both approaches are compared, and significant differences are found to exist, in the zero mode, at low working frequencies. Relative errors up to 60% have been determined in the attenuation, velocity, and surge impedance. A physical interpretation of these results is presented. It is based on the imperfect screening action of the pipe which allows the magnetic field to spread outside the cable and penetrate the imperfect soil, giving rise to a higher series inductance and a smaller series resistance. >

Transients computation in electrical circuits in the presence of ferromagnetic hysteresis

This article is a contribution to the evaluation of transients in nonlinear inductive circuits containing a soft ferromagnetic sample in the magnetic circuit where hysteresis and eddy currents are taken into account. A hybrid finite element method (FEM)/boundary method is used to evaluate the instantaneous magnetic field. The FEM formulation is adequate to describe the field inside the ferromagnetic sample and, on the other hand, the boundary method is used to impose the adequate relation between the field and its normal derivative values on the boundary. This relationship is obtained from the exterior circuit equation. The dynamic field is evaluated in the time domain for which an implicit integration approach is used based on a second order trapezoidal (Crank–Nicolson) method. A classical Preisach model is used to simulate the magnetic hysteresis.

Analysis of the Mutual Inductance in Current Transformers Taking Primary Conductors Eccentricity Into Account

In this work a current transformer (CT) with toroidal core is analyzed from the viewpoint of the eccentricity of the primary circuit where one conductor goes through the cores window and the other is outside the core. The research is focused on the influence of the position of the primary circuit conductors on the evaluation of the mutual inductance between the primary circuit and the secondary winding-an influential factor on the CTs amplitude error. Purely analytical results and closed-form expressions for the mutual inductance are obtained when the core is made of a nonmagnetic material. A finite-element method is also utilized for the numerical computation of the mutual inductance, for nonmagnetic and ferromagnetic cores. Results are cross-validated using both methods. Results are presented and discussed taking into account the system geometry and the number of turns of the secondary winding. This novel contribution shows that the response of nonmagnetic-core current transformers can be significantly affected by the positional eccentricity of the primary circuit, especially when the secondary winding does not possess a large number of turns.