Protection of MV Converters in the Grid: The Case of MV/LV Solid-State Transformers

Solid-state transformers (SSTs) are a promising technology as they provide new functionalities and services enabling future smart grids. An SST establishes the interface between an MV ac grid and an LV (ac or dc) grid or load. The SST must provide high reliability even in the event of certain grid faults, where the SST is subject to exceptional electrical stresses. In addition, the MV and LV grids are exposed to failures of the SSTs. Spurred by such challenging requirements, this paper defines the different relevant stresses, identifies the corresponding protection needs, and proposes adequate protection circuitries and devices. The protection mechanisms that are currently used for low-frequency transformers are analyzed and an adapted version is proposed for SSTs. MV short circuits and overvoltages are identified as the most critical situations and are analyzed in detail. From the presented in-depth investigation, guidelines are extracted for designing robust SSTs and are applied to a 1 MVA, 10 kV SST.