Transformerless Hybrid Active Filter for Medium Voltage Motor Drives

This paper introduces a transformer less hybrid active filter designed for integration into medium-voltage motor drives, aimed at improving energy efficiency. The proposed filter effectively mitigates harmonic currents generated by the threephase diode rectifier commonly used in motor drive front ends. The system consists of a passive filter tuned to the seventh harmonic frequency, directly connected in series with an active filter based on a three-level pulse-width modulated (PWM) converter. A key focus of this study is the voltage-balancing control strategy for the two split DC capacitors within the active filter. To validate the proposed approach, a 400-V, 15-kW motor drive system—representing a scaled-down version of a medium-voltage drive without regenerative braking—was designed, built, and tested. Experimental results confirm that the hybrid filter effectively reduces harmonic distortion and maintains stable voltage balancing across various load conditions, demonstrating its reliability and practical feasibility.