Design Considerations of Dual-Active Bridge DC Grid-Forming Converter for DC Buildings

The global focus on renewables and energy efficiency makes dc distribution inside buildings an attractive concept, which allows easier integration of generation and loads. When connected to a conventional ac grid, such buildings require a bidirectional ac-dc converter for dc grid forming and energy exchange. Until recently, the works in this direction had a variety of realizations and were mostly not arranged in terms of voltage levels and converter requirements. Newly introduced and actively developing standards open the possibility to make the research of such converters more focused. For example, with a requirement of galvanic isolation, the solution with two stage conversion, where a nonisolated power factor correction rectifier is followed by an isolated dc-dc converter, can be considered as the default approach. In this case, the dual active bridge converter (DAB) for the dc-dc stage is one of the most common industrial solutions. At the same time, this converter has natural limitations in terms of soft-switching range, circulating current, and losses. This article address for the first time the design considerations for DAB topology operating as the dc grid-forming converter with droop control. By including the droop control characteristic in the converter's design, it is possible to simultaneously obtain a wide soft-switching range and low circulating current. The proposed analysis was used to design a 5-kW prototype, which was implemented and tested, showing a flat efficiency curve with a peak value of 97.8%.