An FBSM Capacitor Overvoltage Easing Method by Controlling the Arm Current Commutation of Hybrid MMC

Abstract

The hybrid modular multilevel converter (MMC) is a promising topology for voltage-source converter-based high-voltage direct current (VSC-HVDC) transmission systems. A serious full-bridge submodule (FBSM) overvoltage problem will occur during the dc fault blocking process. FBSMs need to absorb the enormous excess electromagnetic energy stored in the dc side during the dc fault blocking process. Owing to the current commutation process caused by ac voltages, FBSM capacitor voltages of only two arms will increase rapidly at the same time. The energy absorption is unbalanced between arms, which is the leading cause of the FBSM capacitor overvoltage problem. By reducing the reverse-biased voltage of some arms properly, the arm current commutation process is limited. So that the FBSM capacitor voltage increasing will be slowed down. By controlling the FBSM capacitor voltage increasing speed, the energy absorption between arms can be rebalanced.