Common-mode currents in multi-cell Solid-State Transformers

Solid-State Transformers (SSTs) are considered important building blocks of a future Smart Grid, where they will replace conventional transformers and provide enhanced functionality and controllability. To deal with the medium-voltage levels at the input of an SST for distribution applications, commonly cascaded cells converter topologies are proposed. This paper presents a detailed analysis of the common-mode currents appearing in cascaded cells converter systems as a consequence of steep changes of the cells´ potentials caused by the individual cells´ switching actions. Common-mode chokes placed at the AC terminals of each individual cell are identified as the most feasible way of mitigating these common-mode current disturbances. Detailed, analytic design guidelines are provided and applied to a 1MVA, 10 kV/400V SST.