Abstract: An active booster transformer system with a booster transformer core (S, 10, 17) of the type which is applied around an electrical cable (K, 1, 13) for reducing stray currents and in which the booster transformer core is a body built up of a magnetizable material provided with a continuous channel intended to accommodate the cable. The system includes a current sensor (G, 2-3, 14-15) which senses the net current in the cable (K, 1, 13), an amplifier unit (F, 4-7), the input signal of which is controlled by the current (Iv) sensed by the current sensor, and a magnetizing winding (L, 3, 15) situated on the booster transformer core (S, 10, 17). The output of the amplifier unit is arranged to drive the magnetizing winding and to create a magnetic flow in the booster transformer core, the flow, in turn, inducing a longitudinal voltage in the cable which counteracts the stray current.