Multidimensional Design of DC-Link for Two-Stage Solid-State Transformer Cell Considering Second-Order Harmonic Current Distribution

In a two-stage solid-state transformer cell, the grid power pulsates at the double-line frequency, yielding second-order harmonic current (SHC) across the unit. SHC into the dc–dc stage causes extra losses in the semiconductors and magnetics, resulting in reduced efficiency and lifetime at the gain of lower dc-link requirement. This article investigates how the dc-link design affects the SHC distribution using a proposed impedance model, which in turn impacts losses, volume, and overall reliability. Particularly, the overall lifetime is evaluated considering the reliability interactions between components resulting from varied dc-link design. The newly introduced reliability dimension provides further insight into the dc-link design, in addition to the voltage ripple requirement and efficiency-power density tradeoff. The model and design concept are verified by the simulation and experiment.