Internal Power Flow of a Modular Multilevel Converter With Distributed Energy Resources

This paper examines the internal power flow mechanisms that exist within a generalized modular multilevel converter (MMC) and examines alternatives for integration of distributed energy resources (DERs) within the MMC structure. Each phase leg of the MMC consists of two series connected strings of submodules, where each string of submodules is referred to as a phase arm. Based on analytically developed inter-arm power flow relations, a control methodology is proposed, which provides fully independent control of each arm´s real power flow, facilitating extreme levels of inter-arm power transfer. This eliminates the need for uniform integration of DER units across all submodules of the MMC. Among others, viable operating configurations are shown to include DERs only integrated in the upper (or lower) phase arms and DERs integrated only in the upper and lower phase arms of a single phase leg. Power balance is maintained internal to the MMC via dc and ac currents circulating within the converter without distorting input dc or output ac currents. To maximize conversion efficiency, a mechanism for minimizing the necessary circulating ac currents under any inter-arm power flow condition is also identified. Comprehensive simulation results validate both the developed model and controls.