Effects of Control Strategies and DC-Link Mid-Point Connection for Voltage Balancing and Switching Losses in Dual-Three-Phase NPC Inverters

Dual-three-phase multilevel inverters have gained attention due to their numerous features, including high-quality voltage and current waveforms, increased power ratings, and enhanced fault tolerance. This study presents a comparative analysis of two distinct control strategies for dual three-phase, three-level neutral point clamped inverters, and considers the interconnection of the dc-link mid-points. The objective is to analyze the effects of the control strategy that aims to ensure balanced voltage distribution across the dc-link capacitors under all operating conditions, suppress low-frequency voltage oscillations, and reduce switching losses through the adoption of a conventional single-step modulation strategy. The boundary limit of the operating region in which the voltage oscillations of the dc-link capacitors can be eliminated is identified and compared for two different control strategies and the traditional space vector pulse width modulation. Experimental tests are performed with an asymmetrical dual-three-phase induction motor fed by two three-phase, three-level NPC inverters.