A modular multilevel DC-DC converter with self-energy equalization for DC grids

Abstract

Medium-/High Voltage DC grids are interesting for the integration of renewable energy sources. DC-DC conversion systems are highly needed for the development of DC grids. Recently, Modular Multilevel Converter (MMC) is the most promising technology for medium-/high-voltage applications, but employing the conventional MMC with DC output voltage leads to diversion in the Submodule (SM) capacitor voltages, that is energy drift. This paper proposes a modified modular multilevel DC-DC converter with self-energy equalization, which ensures successful DC-DC conversion with balanced capacitors voltages. In the modified topology, clamping Insulated Gate Bipolar Transistors (IGBTs) are employed in each arm to enable parallel-connection of the capacitors in the same arm. During the operation (equalization period), the parallel-connected upper capacitors are connected to the parallel-connected lower capacitors in each leg through a small limiting inductor to transfer energy between the arms to ensure balanced capacitor voltages. The proposed configuration, along with the operational concepts, mathematical analysis, and design, are presented. Finally, simulation and experimental results are presented for validation.