A Single-stage Single Phase Bidirectional AC-AC Converter for Solid State Transformer Application

This paper proposes a single-stage, single-phase acac converter based on the Dual Active Bridge converter. The converter is formed by two three-legged bridge circuits interlinked by a high-frequency transformer. The converter has a symmetrical structure, and the modulation strategy for both bridges are similar. The three-legged bridge act as a low-frequency ac to high-frequency ac converter. The high-frequency outputs of the primary and secondary side bridges are used for dual active bridge power flow through a high-frequency transformer. The converter has regulated dc buses at both bridges. By selecting the high-frequency transformer turns ratio appropriately, the converter can be designed to have a flattop DAB inductor current. The flattop DAB current has a low RMS value and improves the soft switching performance of the converter. Also, by employing the sinusoidal pulse width modulation, the converter draws sinusoidal current at unity power factor from the ac Input. The control strategy and modulation technique of the converter are explained. Experimental waveforms from a 1kW hardware prototype are provided for verification.