An overview of multilevel converter topologies for grid connected applications

This paper provides an overview of multilevel converter topologies for grid connected applications. Different converter topologies such as Diode Clamp Converter (DCC), Capacitor Clamp Converter (CCC) and Chain Link Converter (CLC) are considered and compared from different aspects such as component sizing, complexity, efficiency and reliability. Thanks to its modular structure, the CLC appears as the most attractive topology for high power and high voltage applications. Single-Star Bridge-Cell (SSBC), Single-Delta Bridge-cell (SDBC), Double-Star Bridge-Cell (DSBC) and Double-Star Chopper-Cell (DSCC) are four basic subset topologies of CLC. A comprehensive comparison of these subset topologies for the CLC is provided for the particular application of grid connected converters. The problem of operation under unbalance grid conditions and the ability of this converter topology to inject negative sequence into the grid are discussed. It will be shown that due to the lack of a common DC link between phases, this converter topology presents limited negative sequence injection capability when the phase legs are connected in delta, while in case of star connection voltage balancing between the different cells of the converter becomes problematic due to the uneven current distribution among phases. This represents the main limitation of this converter topology, especially when used for load balancing and active filter applications.