In a solid state transformer (SST), one of the most important roles is played by the dc-dc converter interfacing the high voltage port and the low voltage port while providing galvanic isolation. This paper proposes a quad-active-bridge (QAB) converter as the basic cell of a Modular Multilevel Converter (MMC) based solid state transformer for the integration of distributed energy resources (DER) and distributed energy storage systems (DESS) into the grid. This study comprises power transfer analysis, modeling and control of the QAB for this application. Furthermore, this paper shows an alternative method based on the star-mesh transformation to built the multiwinding tranformer equivalent circuit, that relates any two ports of the multiple-active-bridge (MAB) converter through a link inductance, which allows to describe power flow. Results are obtained by simulations in PLECS.