Dynamic performance and fault-tolerant capability of a TLC-MMC hybrid DC-DC converter for interconnection of MVDC and HVDC grids

DC grid technology is regarded as a promising solution for future electric networks integrating a great amount of renewable energies. It calls for high-efficiency bidirectional dc-dc converters with a high step-up ratio to interconnect mediumvoltage dc (MVDC) distribution grids and high-voltage dc (HVDC) transmission grids. This paper focuses on dynamic performance and fault-tolerant capability of a highly-efficient isolated bidirectional dc-dc converter for this application which combines two-level converters (TLCs) on the MV side and a modular multilevel converter (MMC) on the HV side. A comprehensive current and capacitor-voltage control strategy is proposed. Based on the proposed control strategy, a fault-tolerant operation method in case of multiple device failures is developed to significantly increase the availability of the converter. Dynamic performance of the TLC-MMC hybrid dc-dc converter interconnecting MVDC and HVDC grids is evaluated by PLECS simulation. The simulation results present satisfying dynamic performance of control and seamless transition into the fault-tolerant operation mode.