Trapezoidal operation of modular multilevel DC-DC converter for HVDC interconnections

High Voltage Direct Current (HVDC) grids are becoming a preferred option for the expansion of the traditional electrical networks as a result of their potential economic, technical and environmental benefits. Developing HVDC grids includes the linking of existing HVDC buses which may be implemented with different voltage levels, different HVDC technology and various grounding schemes. Interconnecting these HVDC systems will require DC-DC converters to provide voltage matching, isolation and a means for splitting the HVDC grid into sub systems. This paper presents a modified trapezoidal operation of MMC-DC-DC converter, MTrMMC-DC-DC, with high semiconductor device utilization and lower device current rating in addition to an increase of converter's energy density. Better utilization of the semiconductor devices results from conducting the current through the arms of the TrMMC-DC-DC converter continuously. The reduction of the energy storage requirements is achieved through careful design of the submodule (SM) capacitors and arm inductors to achieve resonance between the effective equivalent capacitor of each arm and the arm inductor at a frequency below twice the fundamental operating frequency. The circuit parameter design is validated for an 800MW, 400kV HVDC-based system through PLECS simulations.