Identification of Power Frequency Industrial Magnetic Field Sources for Shielding Purposes

In any industrial installation, power electric systems generate extremely low frequency magnetic fields. Field amplitude values must fulfill the limits related to health effects or to disturbances on electromagnetic devices, and a shielding system is sometimes necessary. The design of magnetic field shields requires the simulation of the sources and of the shields in order to evaluate the main parameters like geometry, material, and position with respect to sources. An alternative approach is proposed in this paper which requires one to measure a spatial distribution of the magnetic field amplitude. Actual field sources are simulated by means of suitable simplified sources handled by analytical relations. An identification process of measured magnetic fields is performed using a genetic algorithm applied to the simplified source system. The simplified system is a suitable combination of three-phase sources, and it seems able to create a realistic magnetic field distribution in both space and time. This paper presents the proposed method and its application to the design of a conductive shielding to reduce the leakage fluxes of a three-phase transformer without enclosure. The magnetic field to be shielded is evaluated with a simple 3-D model of a transformer constituted by three windings in air with current values deduced from the equivalent magnetomotive force which is considered the actual source. Unshielded fields on a plane region are used to identify the simplified source, and the effects of a conducting aluminum shielding are compared using actual and simplified sources. The influence of some parameters of the simplified sources on the results is discussed. Finally, the application for the design of a magnetic field shielding in real substation is presented.