Common-mode EMI solutions for modular back-to-back converter systems

This paper presents solutions to common-mode (CM) electromagnetic interference (EMI) problems in back-to-back converter systems with parallel- and/or series-connection of three-phase voltage-source converters (VSC). The main objective is to reduce the CM EMI emission at both the input and output terminals. Additionally, attention will also be given to the CM current through the converters and the circulating currents among parallel modules in an effort to reduce power losses and to improve control performance. A new CM filter arrangement with CM capacitors connected between the input and output terminals of cascaded VSC is proposed as a way to reduce the CM currents at both input and output terminals. Other system-level CM EMI reduction techniques, such as asymmetric interleaving, the use of inter-module chokes, optimization of dc bus grounding capacitance, and balancing of CM impedance to contain CM emission locally, are also presented and demonstrated.