Modified Isolated Modular Multilevel Converter for Bipolar DC Distribution System

The Bipolar Low-voltage DC distribution system has the advantages of flexible load connection and high operational reliability. The Isolated Modular Multilevel DC/DC Converter (IMMDC) is an attractive solution for interconnecting medium voltage DC (MVDC) grids with low voltage DC (LVDC) grids. With a modular multilevel converter (MMC) on the MVDC side and a full-bridge (FB) or a half-bridge (HB) on the LVDC side, the IMMDC can be flexibly extended to different voltages and power ratings. However, existing IMMDC topologies are limited to two-port operation where the full-bridge DC voltage and power must always remain the same. This paper presents a modified IMMDC for interconnecting MVDC grids and bipolar LVDC grids. The mathematical model and equivalent circuit of the proposed modified IMMDC are derived. Then the power flow under balanced/unbalanced power transfer and the operation of a single pole short circuit fault on the bipolar LVDC grids are considered. At the same time, a control scheme for this converter under different operating conditions is proposed. Finally, MATLAB/ Simulink simulation results are presented to verify the feasibility and effectiveness of the proposed modified IMMDC.