A transformerless hybrid active filter using a three-level diode-clamped PWM converter

This paper discusses a transformerless hybrid active filter integrated into a 6.6-kV, 1-MW adjustable-speed motor drive system. 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 both of which are directly connected in series. 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 based on superimposing a 6th-harmonic voltage on the active-filter voltage reference is introduced to the three-level converter. Experimental waveforms obtained from a 400-V 15-kW down-scaled experimental system verify the viability and effectiveness of the proposed transformerless three-level shunt hybrid filter, keeping two dc-capacitor voltages balanced well.