Three-phase Multilevel NPC Rectifier with High-Frequency Pulsating DC-bus, Reduced Number of Switches and Galvanic Isolation with Transformer with Center Tap

This paper proposes a three-phase three-level AC/DC converter with reduced number of controlled switches and high-frequency DC-bus based on NPC (Neutral Point Clamped) topology, for unidirectional power flow applications. Galvanic isolation is reached by using a center tap transformer, which requires only a full-bridge at it primary side. No bulky electrolytic capacitors are needed on the DC-bus, in contrast to conventional multilevel topologies. A PWM strategy with carriers arranged in alternating phase opposition (PWM-APOD) is used to operate the proposed converter. The applied PWM technique is able to operate the converter stage on the AC side with only two switches commuting per switching period. The switches of the isolated pulsating DC/DC converter are commutated with half the switching frequency of the converter on the AC side. A dual-loop control strategy with PI voltage and PI resonant current controllers is adopted to ensure reference load voltage and almost sinusoidal grid currents. Simulation and experimental results are presented to validate the proposed converter operation.