Generalized Average Modeling of DC Subsystem in Solid State Transformers

Solid state transformers (SSTs) include dc subsystems to enable plug-and-play support of dc loads, generation sources, and energy storage. Dual active bridge (DAB) converters are a suitable topology for both the primary energy conversion and load interface applications in an SST. This study considers an SST that uses DAB converters for both applications, and describes a method of generating a full model of the dc subsystem. The system-level model is constructed from generalized average models of the individual converters and therefore retains information related to both the ac and dc stages of the DABs. The size of the resulting model is limited by preserving the decoupling of key state equations in the model combination process, thereby avoiding the scalability issues involved in generalized average modeling techniques. The accuracy of the model is verified through comparisons to results from a hardware testbed. The primary application of the model is to provide a framework for small-signal stability assessment that is applicable regardless of the flow of power in the SST. While the methods used in this study are motivated by the SST application, they are also suitable to the more general case of a dc distribution system.