Soft-switching operation strategy for three-phase multiport-active bridge DC-DC converters

Three-phase multiport-active bridge (3ph-MAB) dc-dc converters are a promising technology for the interconnection of multiple loads and power sources. They can handle different voltage and power levels, provide a bidirectional and highly dynamic power flow and require relatively small dc-link capacitors. However, 3ph-MAB converters suffer from poor low-load efficiencies due to hard-switching operation. This paper introduces a new soft-switching operation strategy for 3ph-MAB converters, the so-called parallel-phase operation (PPO), to overcome this disadvantage. Furthermore, a novel multiport duty-cycle (MDC) operation is introduced for 1ph-MAB converters which enables soft switching of all ports. The advantages of PPO and MDC are exemplarily demonstrated for a three-phase triple-active bridge (3ph-TAB) converter, i.e., a 3ph-MAB converter with three ports. The 150 kW converter prototype interconnects a 5 kV medium-voltage dc-demonstrator grid using 10 kV SiC MOSFETs with two low-voltage dc grids with nominal voltages of 760 V and 380 V. Simulations show efficiency gains of more than 25% for low-load operation. The impact of PPO and MDC on the design of the semiconductors, the dc-link capacitors and the transformer is analyzed. Based on this analysis, design challenges of the converter as well as further improvements of MDC operation are discussed.