Secondary currents filtering by means of a properly sized winding in a multicoil traction transformer for high power applications to limit psophometric Currents

In high power railway traction applications the modulation of traction converters, at low switching carriers, produces harmonic currents in the range of audio frequencies. To ensure correct operation modes of the signalling circuits, the interference magnitudes levels have to be reduced. Psophometric current (p.c.) gives an index of the interference level produced by traction equipments: it takes into account the harmonic content of the primary current waveform in the multiwinding transformer. To limit the p.c. in the feeding network, a method for filtering harmonic currents produced by the modulation of interlaced PWM rectifiers (V.S.R.) is described. A properly sized additional winding is considered on the multicoil transformer used to supply the traction equipment. In the paper the system is analysed by means of the mathematical model and some design guidelines are proposed.