A transformerless hybrid active filter for integration into a medium-voltage motor drive with a passive front end

This paper discusses a transformerless hybrid active filter integrated into the 6.6-kV, 1-MW adjustable-speed motor drive having a three-phase diode rectifier at the front end. The hybrid filter consists of an active filter using a three-level diode-clamped PWM converter rated at 60 kVA, and a 250-kVA passive filter tuned to the 7th-harmonic frequency. They are directly connected in series without transformer. This circuit configuration enables to use 1.2-kV IGBTs because the dc voltage of the three-level converter is 1.32 kV (20% of 6.6 kV). Voltage-balancing control characterized by superimposing a 6th-harmonic zero-sequence voltage on the active-filter voltage reference in each phase is introduced to the three-level converter with triangle-carrier modulation. Experimental waveforms obtained from a 400-V 15-kW down-scaled system verify the viability and effectiveness of the proposed hybrid filter, keeping the two dc-capacitor voltages balanced well.