Overview of Efficient and Reliable Control Strategies for Three-Level NPC-Based DAB Converters

Dual-active-bridge (DAB) converter has been widely employed in isolated DC-DC power transmission applications due to its advantages of high power density, inherent soft-switching capability, galvanic isolation, high step-up ratio, and bidirectional power transfer. With the advancement of medium-voltage DC (MVDC) systems, some multilevel DC-DC converters, evolved from two-level DAB converter, have emerged to meet the high-voltage and high-power demands. Among various multilevel variants of DAB converter, the neutral-point-clamping (NPC)-based DAB converters have gained increasing attention in recent years due to simplified control structure, relatively low costs, and widespread commercialization. The overall performance of MVDC systems depends highly on the efficiency and reliability of DAB converters. Therefore, this paper presents a comprehensive overview of the state-of-the-art control strategies aimed at enhancing efficiency and reliability in NPC-based DAB converters. The review covers modulation schemes designed to optimize converter efficiency, categorized by degrees of freedom (DoFs), solution types, optimization objectives, and algorithms. Furthermore, control strategies to improve converter reliability, including neutral-point voltage balancing, fault diagnosis and tolerance, transient-state DC bias suppression, and power loss balancing, are exhibited in detail. Finally, the challenges remaining in prior efficiency- and reliability-oriented design are discussed, and corresponding potential research directions are identified to further advance the performance of multilevel DAB converters.