A high step-up ratio soft-switching 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 dc-dc converters with high step-up ratio to interconnect medium voltage (MV) dc distribution grids and high voltage (HV) dc transmission grids. This paper presents an isolated soft-switching dc-dc converter combining two-level converters in parallel on MV side and a modular multilevel converter (MMC) on HV side. Moreover, a comprehensive control method of the proposed converter is presented. By the proposed method, a certain reactive current is injected into the MV side by the MMC to ensure soft-switching on MV side. The proposed converter presents low semiconductor losses over a wide power range at variable input/output voltages. Simulations of a 50 kV/400 kV, 400 MW system are conducted to evaluate semiconductor losses and verify the validity of this work.