A Three-Level Modular DC/DC Converter Applied in High Voltage DC Grid

Modular dc/dc converter (MDCC) based on cascaded submodules (SMs) has been an attractive converter topology for the interconnection of high voltage dc (HVDC) grids for its low cost and high efficiency; however, the quite large filter inductor implemented at lower voltage side significantly increases the system volume. In this paper, a buck three-level type MDCC (Buck-TL-MDCC) is derived from the classic buck three-level converter (Buck-TLC) by replacing the switches and blocking capacitor with cascaded SMs for HVDC application. Just like the major merit of the Buck-TLC, the filter inductance of the Buck-TL-MDCC can be reduced, and the high voltage blocking capacitor is eliminated by the cascaded SMs. Instead of the sine-wave modulation employed by prior arts of MDCCs, the stepped two-level modulation is adopted; it offers the following merits: 1) smaller SM capacitance requirement; 2) smaller ac circulating current; 3) the switching frequency is equal to the fundamental frequency, which avoids extra switching actions; 4) the computation burden is dramatically reduced. Finally, the simulation and experimental results verify the effectiveness of the proposed Buck-TL-MDCC.