Impact of the DC-DC Stage on Grid-Connection Stability in Solid-State Transformer

This paper addresses the impact of the dc-dc stage on the grid-connection stability of a three-phase Solid-State Transformer based on the Cascaded H-Bridge and Dual Active Bridge topologies. In particular, the analysis aimed to reveal and discuss the impact of the isolated bidirectional dc-dc stage on the input dq-frame impedance matrix properties in terms of its passivity. For this purpose, the d-axis input impedance has been mainly addressed in this work. Its low-frequency simplified expression has been derived, by means of which its real part and the negative-resistance region upper limit can be analytically deduced, enabling a passivity-oriented design procedure. It is demonstrated that the dc-dc converter lowers that limit thus enhancing the grid-connected Solid-State Transformer passivity. The analysis performed in this work can be applied to a generic ac-dc-dc topology.