A process is disclosed for regulating a four-quadrant regulator (1) that works as a mains power converter. The output of the four-quadrant regulator (1) feeds three-phase current drives through a direct voltage intermediate circuit (5) and a pulse-controlled inverter (6) and its input is connected to an alternating voltage network (4), preferably through a multiple winding transformer (2). The alternating voltage network (4) has secondary windings (8, 9) for the four-quadrant regulator (1) and additional consumers, for example auxiliary frequency converters (10) and a noise limiter set (3). This process is particularly suitable for catenary-powered three-phase locomotives. In order to actively and adequately limit with a simple process both 100 Hz parasitic currents and other low-frequency parasitic currents in the signalling frequency range, in particular 42 Hz parasitic currents, the current regulation carried out by the four-quadrant regulator (1) is supplemented by a parasitic current regulation (16) tuned to a current harmonic to be reduced in the power line current (if). The current harmonic is periodically sensed and appropriately decomposed into active and reactive components, for example into two orthogonal components or into components with different intensities and angles, that are in turn supplied to two separate parasitic current regulating circuits (16) that are set to maintain defined set values (wa3f, wb3f), preferably equal to zero, and whose output supplies set values (wa3, wb3) for the current harmonic components to be regulated by the four-quadrant regulator (1).