Performance analysis of a triple-active bridge converter for interconnection of future dc-grids

Dc-dc converters are a promising technology for interconnection of future dc grids. Besides the relatively low volume and space requirements, dc-dc converters provide good controllability of the power flow. This is particularly important with regard to a more decentralized energy generation, where a fully bidirectional power flow — even between grids of equal voltage levels — is desired to increase overall grid efficiency and stability. This paper analyzes the performance of a three-phase triple-active bridge converter (3ph-TAB) which interconnects a 5 kV medium-voltage dc grid and two low-voltage dc grids with nominal voltages of 380 V and 760 V, respectively. First, the modulation strategy of the converter is described. The different cases of operation are analyzed and a method is developed which significantly simplifies the theoretical analysis of the converter. The design specifications for the leakage inductances of the transformer and the dc-link capacitors are derived and analyzed. Furthermore, the soft-switching boundaries are derived analytically. The theoretical assessment is supported by a semiconductor loss simulation for the whole operating range.