Bidirectional Isolated Ripple Cancel Dual Active Bridge Modular Multile vel DC-DC Converter

Medium-/long-distance power transmission in future DC power distribution network systems requires medium voltage (1 kV~20 kV) to reduce the transmission losses. Power semiconductor devices of the conventional full-bridge configuration, however, face high voltage stress issues. Several topologies were investigated to solve this issue, including multilevel converters and solid-state transformers. In this study, a bidirectional isolated ripple cancel dual active bridge modular multilevel DC-DC converter was investigated for medium-voltage DC applications, such as the 6.6 k VDC power transmission between two local 800 VDC distribution power network systems. The proposed modular multilevel circuit was developed based on prior research on a bidirectional ripple cancel dual active bridge converter. The proposed converter inherits the latter’s features, including ripple cancel characteristic on DC-link capacitors, soft switching (ZVS), transformer DC bias current elimination, and automatic voltage balancing on series-connected semiconductors. Thus, the modular multilevel DC-DC converter can be expected to have high reliability. This paper discusses the circuit operation, ripple cancel, DC bias current elimination, and voltage balancing features of the proposed circuit. The prototype of the proposed circuit was fabricated at a scaled down version, 900 V/ 600 V/ 900 W, and its basic operation was tested to demonstrate its feasibility.