Circulating Currents Suppression and Neutral-Point Potential Balancing Strategy for Parallel Three-Level T-type DC-AC Converters

Parallel operation of three-level T-type converters is a promising way to provide higher power capacity, higher reliability and lower output waveform harmonics. However, zero-sequence circulating currents (ZSCCs) are generated which distort output currents and reduce the converter efficiency. In addition, the three-level converters have an inherited problem of the neutral-point (NP) voltage unbalancing because of the spilt dc-link capacitors, which degrades the output waveform quality. Compared with the space vector modulation (SVM) schemes which are widely used to address these two issues, the carrier-based discontinuous pulse-width modulation (CB-DPWM) can improve the efficiency and can be easily realized in practical applications. In this paper, a CB-DPWM method, which injects the calculated zero-sequence voltages to original modulation signals, is proposed to achieve NP voltage balancing and low-frequency ZSCCs suppression for parallel three-level T-type converters. Validity and utility of the proposed modulation method are also demonstrated.