DC Fault Analysis of Modular DC/DC Converter Employing a New Submodule With Damping-Resistor

Abstract

DC transformer is the key equipment to interconnect the high voltage dc systems with different voltage levels and build dc grid. Compared with the dc/ac/dc converter based on modular multilevel converter, the non-isolated modular dc/dc converter (MDCC) offers lower cost and higher efficiency, due to the elimination of the ac transformer and reduced submodules (SMs). However, the MDCC based on the half bridge SM (HBSM) can not block the dc short fault. Therefore, the MDCC with the hybrid configuration of the unipolar full-bridge SM (UFBSM) and HBSM, which achieves the dc fault blocking capability, is analysed in this paper, and a damping resistor is introduced in the UFBSM to reduce the ascending voltage of the SM capacitor under dc fault. The fault blocking mechanism of the converter is described, and the fast recovery strategy is presented to ride through the temporary fault. In addition, the design principle of the converter is discussed through a study case. Finally, the simulation performed in Matlab/Simulink verifies the theoretical analysis.

Keywords

• DC/dc power conversion
• high voltage direct current (HVDC) transmission
• modular multilevel converter (MMC)
• dc grid