Modular multilevel converter with high-frequency transformers for interfacing hybrid DC and AC microgrid systems

Recently, concepts of both ac and dc microgrids have been proposed for future energy systems. A novel converter topology which has three ports: 12 kV ac, 22 kV dc and 400 V dc for future hybrid ac and dc microgrid systems is presented in this paper. Each port has the capability of bi-directional power flow which is required in future hybrid ac and dc microgrids. The 400 V dc bus, which is connected to distributed energy resources, injects/draws power to/from both the ac grid and the dc grid through solid-state transformers. The topology avoids the use of line-frequency transformers which are considered to be expensive and bulky. Because the hybrid converter is connected to both the ac grid and the dc grid, enhanced reliability is achieved. A modular design of the sub-module of the modular multilevel converter is proposed for reducing the sub-module failure probability. Extensive time-domain simulations using Matlab/Simulink™ are presented demonstrating the feasibility of the proposed topology.