DC-DC converter for HVDC grid application

There is an increasing interest in voltage source converter (VSC) based HVDC grids. The development of DC grids is likely to take place in phased manner with several voltage levels similar to that of the AC grids. These different high voltage DC levels will have to be interconnected with each other which will require DC-DC transformers. The paper presents a concept of DC-DC converter based on modular multilevel voltage source converters (MMC) and high frequency transformer. The MMC are connected as front ends on the DC side with interposed high frequency transformer. The design principle, selection of components and operation of the converter are discussed. The paper also presents open loop simulation results of a ±400/±400 kV, 2250 MVA DC-DC converter with the calculation of losses. The power transfer is controlled with simple phase shifting operation.