A Transformerless Bipolar Modular Multilevel DC-DC Converter With Wide Voltage Ratios

This paper presents a transformerless bipolar dc-dc converter based on series-connected submodules. It is intended for transforming dc voltage and managing power flow for medium/high-voltage dc grids. The dc-dc converter is composed of positive-pole and negative-pole subsystems, each of which consists of six arms/branches. Every two arms and one branch are constructed into a T-type circuit with the outer terminals interface to primary-and secondary-side dc buses. The two T-type circuits in one subsystem are connected in parallel to transmit power between primary and secondary sides. The transformerless structure (none bulky coupled inductors as well) solves the challengeable insulation and cooling issues of the extremely high-power magnetics in prior arts, while not sacrificing the power efficiency and system reliability. A +/- 10-kV 2-MW simulation model performed in MATLAB/Simulink verifies the feasibility of the bipolar dc-dc converter. Experimental results obtained from a laboratory setup also confirm the validation of the proposal.