This paper proposes a Modular Solid-State-Transformer (SST) topology consisting of bidirectional Matrix-Based AC-DC Dual-Active-Bridge (DAB) submodules. Unlike the conventional SST topologies, the proposed topology offers a single stage Medium Voltage AC (MVAC) - Low Voltage DC (LVDC) conversion. The discussion is focused on the AC-DC DAB submodule of a 1 MVA SST for the Energy Control Centre (ECC) of a microgrid/nanogrid. The possible modulation modes along with the corresponding power transfer limits are presented and a suitable steady-state modulation scheme is identified for the required operating point. The modeling and small-signal analysis of the AC-DC submodule is comprehensively explained. Simulation results including the steady-state operation and the frequency response of the 17.6 kW submodule are presented. © 2020 IEEE.