Harmonic reduction in DC link current of a PWM induction motor drive by active filtering

Substitution for DC motors by pulse-width modulation (PWM) inverter-fed induction motors in railways supplied by DC voltage leads to high harmonic levels in the overhead line current. This results in interference in the signaling systems. In order to satisfy the requirements of Belgian railway operators, the problem of the harmonic reduction in overhead line current has been addressed. An active filtering system seemed the best solution to this problem. Consequently, a feedback filtering system was developed. It includes a power amplifier feeding a transformer, which reinjects a voltage signal in the DC line to reduce robustness harmonics; the signal is proportional to the overhead line current itself. The experimental results on 100-kW equipment proved the performance of the system: the harmonic reduction is effective in the required frequency range in both steady-state and transient conditions