Precharge Strategies for Isolated Modular DC-DC Converters Under Two Different Start-up Conditions

The isolated modular DC-DC converter (IMDCC) is a new topology designed to connect high-voltage direct current (HVDC) lines with different voltage levels, which ties two DC grids by using two modular multilevel converters (MMCs) via a medium-frequency transformer. Due to the large value of capacitance in the IMDCC, proper precharge strategies before the start-up are significant for the safety and reliability of the whole system. This paper presents two closed-loop precharge control strategies to fully charge the sub-module (SM) capacitors of the IMDCC, considering two different start-up operating conditions: 1) both sides of the IMDCC are connected to active sources, and 2) only one side is connected to an active source while another side is linked with passive loads. These proposed methods are aimed at regulating a controlled charging current and eliminating the inrush currents. Moreover, a capacitor balancing control is also embedded into the controller to equally charge the SM capacitors. Finally, the effectiveness of the proposed methods is verified experimentally on a downscale platform.