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New double-shielded power cables generating low magnetic field levels

The design criteria of new medium voltage cables of a three-phase power line generating very low magnetic field levels are described. The double-shielded cables have the inner screen made of copper and the outer one realized by alloy-49 or steel-1010. The currents induced in the copper shields generate a magnetic field opposite to the one produced by the currents flowing in the inner conductors, thus reducing strongly the external magnetic field. The ferromagnetic shield has the function of confining the magnetic induction flux in order to increase the intensity of the circulating currents in the copper screens. The sizing of the cross-section of the copper screen, which is the crucial aspect of the project, is obtained by means of the thermal analysis of the cables and the computation of the generated magnetic field level. Moreover, the simulation of the three-phase line for the prediction of the power losses is carried out. Prototypes of the new cables are realized and the measurement of the 50 Hz magnetic field generated by the three-phase test line, 25 m long, is carried out in the laboratory. The very low levels of the measured magnetic induction prove the good performances of the new cables.