Book excerpt: "This dissertation presents a comprehensive study of Dual Active Bridge (DAB) converters for Solid State Transformers (SSTs). The first contribution is to propose an ac-ac DAB converter as a single stage SST. The proposed converter topology consists of two active H-bridges and one high-frequency transformer. Output voltage can be regulated when input voltage changes by phase shift modulation. Power is transferred from the leading bridge to the lagging bridge. It analyzes the steady-state operation and the range of zero-voltage switching. It develops a switch commutation scheme for the ac-ac DAB converters. Simulation and experiment results of a scaled down prototype are provided to verify the theoretical analysis. The second contribution is to develop a full-order continuous-time average model for dc-dc DAB converters. The transformer current in DAB converter is purely ac, making continuous-time modeling difficult. Instead, the proposed approach uses the dc terms and 1st order terms of transformer current and capacitor voltage as state variables. Singular perturbation analysis is performed to find the sufficient conditions to separate the dynamics of transformer current and capacitor voltage. Experimental results confirm that the proposed model predicts the small-signal frequency response more accurately. The third contribution addresses the controller design of a dc-dc DAB converter when driving a single-phase dc-ac inverter. It studies the effect of 120 Hz current generated by the single-phase inverter. The limitation of PI-controller is investigated. Two methods are proposed to reduce the voltage ripple at the output voltage of DAB converter. The first method helps the feedback loop with feedforward from inverter, while the second one adds an additional resonance controller to the feedback loop. Theoretical analysis, simulation and experiment results are provided to verify the effectiveness of the proposed methods"--Abstract, leaf iii.