Solid-state transformer stability and control considerations

This paper addresses the stability issues and some control solutions in solid-state transformers (SST). An SST is built using three cascaded stages, including an ac-dc rectifier, a dual active bridge (DAB) converter, and a dc-ac inverter. Multistage cascaded converters are highly prone to instabilities due to the interaction between stages. Hence, the control system must be designed in a way that solves this detrimental interaction effect. Additionally, the stability issue can become even more complicated when a distributed energy storage device (DESD) and a distributed renewable energy resource (DRER) are added to the SST. In this paper, the stability of the SST is studied by applying the Middlebrook´s criterion. To do so, the converters´ small-signal models will be derived, and the frequency domain analyses is carried out to predict the time domain behavior of the SST. An average model of SST will be simulated in PSCAD to validate the stability analyses predictions.