Control of MMC-based Isolated DC/DC converter for HVDC Tapping

With development of HVDC technology, HVDC tapping technology becomes more attractive as a solution to improve reliability and power quality as well as to replace fossil fuels generation of tiny-scale island power systems. This paper proposes a modular multilevel converter (MMC) based isolated DC/DC converter as a potential solution for HVDC tapping technology, which extracts a small amount of energy from the main HVDC link to a low-voltage DC grid, to support a local island power system. Firstly, a new mathematical model of the isolated DC/DC converter is developed by introducing the common-mode and the difference-mode voltage. The developed model illustrates that the AC current can only be affected by the difference-mode voltage, and the power is simultaneously affected by the AC current and the common-mode voltage. Secondly, based on the developed model, a control system consisting of the current and the voltage control loops is designed for an MMC-based isolated DC/DC converter. The proposed control method is more flexible, and the dynamic performance of the isolated DC/DC converter can be improved by using the proposed method. A simulation verifies the performance of the proposed control system.