Comparison of Sub-Modules for Cascaded Multi-Level MVAC to DC Solid State Transformer

High-power DC applications require medium voltage grid integration to reduce transmission losses as it requires lower current for the same power. Conventionally the DC port is interfaced to the medium voltage grid using low voltage power electronic converter and line frequency transformers (LFT) to step up low voltage AC (LVAC) to medium voltage AC (MVAC). Line frequency transformer-less MV grid integration is an attractive solution as it gets rid of heavy LFT, improving the power density, configurability, and efficiency of the overall system. Cascaded modular multi-level converters have become one of the most preferred choices with input series output parallel (ISOP) structure per phase using low voltage devices and modules. There are different kinds of sub-modules used for the input series output parallel architecture. Though two-stage and single-stage sub-modules are used in this ISOP structure, a detailed comparison between them in terms of size and efficiency is missing in the literature. A comprehensive comparison is made between the two-stage and single-stage sub-modules within ISOP solutions, and it is presented in this paper. Three central matrices, semiconductor VA rating, efficiency and power density/volume, are the basis for the comparison. The results are presented for a theoretical design of 400 kW, 11 kV/50 Hz MVAC to 800V LVDC system using SiC devices, and the conclusions are drawn.