Interleaved Buck-Integrated Modular Multilevel DC Transformer With Wide Voltage Range for DC Distribution Grids

In this article, an interleaved buck-integrated modular multilevel dc transformer (DCT) for dc distribution grids is proposed. The proposed DCT is derived from dual active bridge (DAB), where the medium-voltage (MV) side switches are replaced by series half-bridge submodules and the low-voltage (LV) side full bridge is integrated with the interleaved buck cell. The quasi-square-wave modulation is adopted for MV side and the pulsewidth modulation for LV side. The transferred power can be controlled by the phase-shift angle between MV and LV sides. Thanks to the interleaved buck cell, the voltage range can be wide and the voltage matching between MV and LV sides can be always realized under different MV-side voltages, which is desired for DAB. Moreover, the zero-voltage switching can be achieved for all switches within a wide voltage range. Based on the analysis of six conditions at steady state, the operation principles and the parameter design procedure are introduced in detail. Finally, a 2-kW prototype with input voltage from 300 to 500 V is built to verify the operation principles of the proposed DCT.