Numerical and experimental development of multilayer magnetic shields

In this paper a detailed description of different passive shields for Extremely Low Frequency (ELF) applications is presented. The first part of the paper analyzes ferromagnetic and conductive materials by means of simulations and measurements. This step is mainly devoted to the identification of physical characteristics such as the relative permeability of the analyzed ferromagnetic materials. All the simulations are performed with a standard Finite Element (FE) code assuming linear behavior of the tested materials. The results are then used in the design of many shielding configurations and, finally, three different multilayer shield compositions are tested and presented. The second part of this paper outlines guidelines to assure a good shielding efficiency in the actual installation of a multilayer shield. The orientation of the shield as well as the possible decay of the shielding performance due to the discontinuity among the different slabs is investigated by means of experimental measurements. It is observed that the conductive part of the multilayer shield has to be faced to the source to have better performance. Moreover, it is important to assure the electrical conduction between the separate slabs. In this paper the conduction among different plates is conveniently obtained connecting them by straight bars of aluminum. In the third part of the paper the multilayer shield is tested under actual working conditions. A MV/LV substation was reproduced in the laboratory using a 630 kVA transformer working at its rated power. Several configurations were tested and the most significant results are presented. It is highlighted that, if the proper layout is not employed, the performance of the actual shield can be very different from the one of the material.