Energy-Based Control of a DC Modular Multilevel Converter for HVDC Grids

DC-DC converters are expected to interconnect High Voltage Direct Current (HVDC) grids with different configurations or voltage levels, which will increase reliability and operational flexibility. The dc Modular Multilevel Converter (DC-MMC) is suggested as a potential non-isolated topology for HVDC applications. This paper presents an energy-based closed-loop control for the DC-MMC. Such control structure provides a full control of the current components, while ensuring energy balancing in normal and fault operation. First, the converter configuration is described and the fundamental equations are presented. Then, the normal operation of the converter is explained from the steady state conditions and the energy and current components. A closed-loop control is designed to ensure a proper dynamic response, balance the internal energies of the converter, and minimize the ac circulating current. Also, the control structure is modified to ensure dc fault ride through capability. Simulation results in MATLAB/Simulink are used to validate the converter control under normal operation and DC faults.