A Submodule Chosen Method to Reduce the Voltage Difference Between HBSMs and FBSMs of the Hybrid MMC During a DC Fault

Abstract

DC fault blocking is vital for modular multilevel converters (MMCs) based high-voltage direct-current (HVDC) transmission systems. Employing hybrid MMCs to deal with the dc fault is a good choice. After a dc fault occurs, both full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs) will discharge to dc lines. However, only FBSMs will absorb the energy stored in the dc side after submodules are blocked. Thus, the FBSM capacitor voltage will be high, and the HBSM capacitor voltage will be low after the dc fault is blocked. The voltage difference between FBSMs and HBSMs will slow down the restart of converters. The dc fault detecting time could be several milliseconds, during which the discharging of HBSMs and FBSMs can be controlled. In this paper, FBSMs will discharge preferentially once the dc fault is 20% larger than the rated value. The proposed submodule chosen method can decrease the FBSM capacitor voltage and increase the HBSM capacitor voltage during a dc fault. Thus, the restart of converters can be more straightforward.