Analysis of DC transmission line disconnection fault characteristics based on MMC-HVDC system under symmetrical monopole connection mode

DC transmission line disconnection fault is one of the common faults in DC side of MMC-HVDC system. Under the symmetrical monopole connection, the transmission line disconnection fault leads to the interruption of the power transmission of the flexible HVDC transmission system. During the period of transmission line disconnection fault, the submodule capacitor charges or discharges due to the function of the outer loop controller. The DC voltage of the faulty pole changes much comparing with the DC voltage of the healthy pole. The voltage of the two terminal AC side grounding reference point is no longer consistent, and the fault current with small amplitude but gradually increasing on the healthy pole DC line. There is a gradually increasing DC voltage offset component in the AC voltage of converter valve side. Finally, the validity and correctness of the transmission line disconnection fault analysis was verified by a two-terminal MMC-HVDC system which was established in PSCAD/EMTDC.