Optimized Modulation in Parallel Neutral-Point Clamped Inverters for Circulating Current Reduction: A Space Vector Analysis

Parallel Neutral-Point Clamped (NPC) inverters are commonly employed in medium voltage high power applications, such as wind energy system, smart transformer, and power conditioners. In literature, it is shown that the Phase Disposition modulation (PD) strategy provides better performance in terms of harmonic profile and current ripple for single NPC application. Therefore, the PD has been commonly employed as an optimal solution and used for optimized Space Vector Modulation (SVM). However, in case of interleaved operation and common dc-link, one of the main challenges is: the circulating current between inverters. With this consideration, comparisons between the PD and another carrier-based modulation - Alternative Phase Opposite Disposition (APOD) are carried out based on the space vector representation. The analysis shows that the APOD modulation offers much lower circulating current compared to the PD modulation, maintaining similar current ripple as well as spectrum quality of the total current. As a result, an APOD-based SVM technique is proposed in this paper, in order to reduce the circulating current and meanwhile maintain other superior characteristics. Full switching sequences as well as design criteria are presented for all sectors. Simulation and experimental results are provided to validate the effectiveness of the proposed modulation technique.