A modular bidirectional buck/boost dc/dc converter suitable for interconnecting HVDC grids

HVDC system has gained an advantageous development over traditional HVAC system in the market of long-distance bulk-power transmission. Although most of the existing HVDC transmissions are two-terminal, HVDC grids will likely be the choice for future power transfer. Dc/dc converters are key components in HVDC grids, facilitating the voltage stepping and energy exchange. This paper proposes a modular bidirectional buck/boost (MBBB) dc/dc converter which is suited to connect two individual HVDC grids. The half-bridge submodule (HBSM) concept derived from modular multilevel converter (MMC) is utilized, contributing to staircase switching sequences. By doing so, the dv/dt of the inductor voltage can be reduced. A sorting-based control strategy is applied to balance the submodule capacitor voltages, and the pulse-widths of the chain-link terminal voltages are controlled in closed loop to obtain the desired output voltage or current. Compared to the modular multilevel dc/dc converter, the MBBB dc/dc converter uses much smaller submodule capacitors, which will significantly reduce the system volume and weight. Simulation results validate the feasibility of the proposed converter.