Overvoltage under the Valve-side Single-phase-to-ground Fault in Bipolar MMC-HVDC Systems

For bipolar modular multilevel converter high voltage direct current (MMC-HVDC) systems, though the occurrence of the valve-side single-phase-to-ground fault at the transformer secondary side is rare, however, this will generate a serious overvoltage on both the AC side and DC side. This paper studies the generation mechanism and the characteristics of the overvoltage under such a fault. Firstly, during the time interval between the fault occurrence and MMC is blocked, the charging/discharging circuits for the sub-module (SM) capacitors and the DC lines are formed. Then the generation mechanism of the arm overvoltage and the healthy pole overvoltage is investigated. Based on Xiamen two-terminal HVDC system, the fault characteristics under the single-phase- to-ground fault are further illustrated. It can be concluded from the results that there is an overvoltage on the SM capacitors after the blocking because of the discharge of the DC line distributed capacitors. The overvoltage of the healthy pole occurs prior to blocking, which results from the discharge of the sub-module capacitors and the charge of the AC voltage.