Circulating Current Analysis and Suppression for Configured Three-Limb Inductors in Paralleled Three-Level T-Type Converters with Space-Vector Modulation

In high-power applications, paralleling three-level T-type converters (3LT²Cs) with common ac and dc sides represents a modular solution for overcoming limitations of active switches. However, when independent space-vector pulse-width modulation (SVPWM) is employed, zero-sequence circulating currents (ZSCCs) will emerge. Especially, the use of three-phase three-limb inductors (3P3LIs) without additional passive components to reduce cost and improve density of overall parallel system, will worsen ZSCC problem. In this paper, detailed analysis of ZSCCs generated by SVPWM based on a derived average model taking into account magnetic circuit characteristics of 3P3LIs is proposed. Furthermore, four inter-related methods are sequentially proposed: 1) synchronizing sector numbers based on modified vector diagram partition for eliminating periodical ZSCC jumps caused by asynchronous change of starting small vector; 2) feeding forward grid voltages for eliminating starting current jumps due to computation delay; 3) connecting neural points to eliminating constant dc ZSCC caused by neural-point voltage deviation; and 4) adjusting relative duration of negative and positive small vectors in pairs to suppress remaining low-frequency ZSCCs. Consequently, 3LT²Cs operated in parallel can separately regulate output powers with a good circulating current suppression performance. The validity of theoretical analysis and proposed methods is verified by experiment with three-parallel 175-kw 3LT²Cs.