Three-Port Bidirectional Operation Scheme of Modular-Multilevel DC–DC Converters Interconnecting MVDC and LVDC Grids

The modular-multilevel-based isolated dc–dc converter (MMDC) is an attractive solution for the interconnection of medium-voltage (MV) and low-voltage (LV) dc grids. With a modular-multilevel converter (MMC) on the MV side and full bridges (FBs) on the LV side, the MMDC can be flexibly scaled to different voltages and power ratings. However, the existing MMDC is limited to the two-port operation where the dc-link voltages and powers of FBs must be always kept identical. This letter proposes an operation scheme that enables fully independent three-port bidirectional operation of MMDC capable for the interconnection of MVDC and two isolated LVDC grids with different nominal voltages. A comprehensive operation strategy is proposed, which contains two operation modes with different modulation schemes for the MMC to optimize the transformer current in the full load range. Compared with the most popular three-port isolated dc–dc converter triple active bridge, the power flow among three ports follows a simple mathematical rule and can be regulated bidirectionally with small computational efforts as well as intrinsically eliminated circulating power. The experimental results validate the feasibility and effectiveness of the proposed operation scheme.