Hybrid modular multilevel converter based multi-terminal DC/DC converter with minimised full-bridge submodules ratio considering DC fault isolation

An isolated high-power multi-terminal DC/DC converter is studied in this paper, based on hybrid modular multilevel converter configuration consisting of full-bridge submodules (FBSMs) and half-bridge submodules. To decrease the investment and power losses, a reduced arm FBSMs ratio (<0.5) scheme is adopted. A detailed analysis on the relationship of the DC/DC converter inner AC voltage and the arm FBSMs ratio under reduced DC voltage is presented. Based on this, a control strategy during DC fault is proposed which continues operating the converter connected to the faulty DC side with reactive current absorption. Under the same arm FBSMs ratio, compared to the conventional strategy of blocking the faulty side converter during a DC fault, the proposed unblocking method with reactive current injection can not only achieve greater DC fault current declining rate, but also ensure maximum power transfer between the interconnected healthy DC grids by maintaining a higher inner AC voltage in the DC/DC converter. The two strategies are compared and validated by simulations using PSCAD/EMTDC under different arm FBSMs ratio.