Analysis of a Solid-State Transformer Employing Capacitively Isolated Series-Stacked Converter Cells and a Single Medium-Frequency Transformer

Solid-state transformers (SST) provide voltage scaling and galvanic isolation between medium-voltage (MV) and low-voltage (LV) dc busses. Most SST topologies arrange several isolated dc-dc converter cells in an input-series, output-parallel (ISOP) configuration, whereby each cell’s medium-frequency transformer (MFT) must withstand the very high lightning impulse (LI) surge test voltage resulting in a significant volume overhead from bushings and clearance distances. This paper focuses on an alternative structure that employs capacitive isolation of the individual cells (CC-SST) and thus requires only a single output-side MFT with LI withstand capability, whose dry-type isolation, advantageously, is not stressed with high dc voltages during normal conditions, which ensures a long lifetime of the insulation material. The operating principle of the CC-SST is thoroughly explained using an exemplary system (12kV dc input, 800V dc output, 400 kW), and design challenges, in particular circulating currents among the converter cells, are highlighted. Finally, a comparative evaluation against a similar concept using ISOP-connected MFTs indicates that the capacitive coupling approach could achieve an efficiency improvement of up to 0.5 percentage points without an increase in size.