This paper deals with a transformerless hybrid active filter integrated into a medium-voltage three-phase diode rectifier used as the front end of an adjustable-speed motor drive. The hybrid filter is formed by a single LC filter tuned to the 7th-harmonic frequency per phase and a small-rated active filter based on a three-phase three-level diode-clamped PWM converter. A three-phase downscaled system rated at 400V and 15kW is designed, constructed, and tested to verify the filtering performance of the hybrid filter. Experimental results show that the current THD (total harmonic distortion) gets less than 5%, and that the voltage-balancing control proposed in this paper works properly even in transient states.
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