Protection of MV/LV solid-state transformers in the distribution grid

Solid-State Transformers (SSTs) are a promising technology since they combine a high efficiency with the integration of new functionalities and services in the grid. A SST establishes the interface between a MV AC three-phase grid and a LV AC or DC grid. For this reason a SST is subject to electrical stresses due to faults occurring in the grid. The MV and LV grids are in return exposed to failures of the SSTs. Therefore, a suitable design of the SST and its corresponding protection devices has to be found. This paper identifies the different stresses which are relevant for a SST, analyzes the protection mechanisms which are currently used for low-frequency transformers and proposes adapted protection schemes for SSTs. MV short circuits and overvoltages are identified as the most critical stresses and are analyzed in detail. Finally, some guidelines are extracted for designing a robust SST.