A Coordinated Strategy Providing Zero-Sequence Circulating Current Suppression and Neutral-Point Potential Balancing in Two Parallel Three-Level Converters

In two parallel three-level T-type converters (3LT2Cs) with the sharing dc bus and ac bus directly, the neutral-point (NP) potential balancing and the zero-sequence circulating currents (ZSCCs) have to be considered to ensure normal operation of parallel system. In the conventional control scheme, the distribution factor of small vectors with space vector pulse width modulation (SVPWM) strategies (or the injected zero-sequence voltage with SPWM strategies) has to carry out two tasks simultaneously: NP potential balancing and ZSCCs suppression. However, the common challenge is how to coordinately control ZSCCs and NP potential due to only one controllable degree of freedom for two different targets. In this paper, the conventional scheme is investigated comprehensively. Theoretical analysis shows that the control authority over ZSCCs and NP potential is limited. Furthermore, a coordinated control strategy is proposed as follows: the distribution factor of one converter is in charge of the common NP potential balancing with the auxiliary NP bus, while the distribution factor of the other converter is in charge of ZSCCs regardless of NP potential. Based on the strategy, the distribution factor of every individual converter only needs to control one target, achieving the independent control of NP potential and ZSCCs. Theoretical analysis shows that NP potential balancing and ZSCCs suppression can be achieved simultaneously in two parallel 3LT2Cs. Finally, the effectiveness of the proposed strategy is verified by experimental results.