Research on zero-sequence circulating currents in parallel three-level grid-tied photovoltaic inverters

Abstract

This paper proposes a parallel-connected system where two three-phase three-level T-Type photovoltaic inverters with common ac and dc sides to improve the power rating. However, zero-sequence circulating currents (ZSCCs) will occur, which will distort currents, decrease efficiency, and increase loss. In this paper, the fundamental mechanism of ZSCCs based on the derived model of ZSCCs is analyzed in details including the dead time effects. It is can be seen that the ZSCCs generated in three-level case are more complex than parallel two-level inverters due to existing more space vectors. Then, a synchronizing starting small vector control method with PI controller and feed-forward control method is proposed to suppress the ZSCCs. As a result, a good circulating current suppression performance can be obtained. The effectiveness of the analysis and proposed method is validated by experimental results.