Shielding effectiveness for a three-phase transformer at various harmonic frequencies

A two-step optimisation procedure for the design of a passive shield aimed at decreasing the low-frequency magnetic field using a transformer is presented. After having introduced the relevant characteristics of the medium size three-phase transformer, the procedure is described in detail. It is based on the decomposition in the harmonic component of the radiated field, which is associated with the direct, inverse and homopolar sequences of the current of the transformer. The effect of the optimised shield is verified by measurements.