Design Aspects of a Transformerless Solid-State Transformer Concept Considering Lightning Impulse Behavior

A huge variety of different concepts for solid-state transformers (SSTs) has been proposed in recent years. In this paper, a modular transformerless SST concept based on capacitively coupled switching cells in input-series, output-parallel configuration is analyzed in detail with special focus on lightning impulse behavior. Requirements and design constraints are addressed, referring to an exemplary 400kW system (12kV dc input, 800V dc output), and relevant design aspects ensuring proper lightning impulse withstand capability of the converter are discussed. The analysis shows that a surge arrester network at the converter input is crucial to reduce voltage stress and ensure proper voltage sharing among the individual converter components. Finally, the concept is comparatively evaluated with a conventional magnetically coupled modular SST concept, where it is shown that the transformerless structure is a serious alternative due to its increased efficiency.