MMC based SST

The Modular Multilevel Converter (MMC) is a type of DC-AC electronic power converter suitable for HVDC applications, thanks to its modularity and to the symmetrical design of the cells. Cells in conventional MMCs use a capacitor in the DC link, meaning that the net power balance of the cell needs to be equal to zero. It is possible however to modify the cells of the MMC to provide the capability to transfer (inject or drain) power. The use of such cells opens several new functionalities and uses for the MMC. Cells with power transfer capability can be used to connect the medium/high voltage DC and AC ports intrinsic to the MMC, with low voltage DC/AC ports, the power transfer among ports being realized at the cell level. This results in multiport power converters, potential applications including solid state transformers (SST). This paper analyzes the design and control of multiport power converters based on MMC topologies, including their use as a SST. Topologies, control strategies and implementation issues will be covered.