Characteristics and Control & Protection Strategy of a Multi-line CFC Integrated M²TDC Grid Under Non-instantaneous Open-circuit Faults

With the increasing complexity of the Meshed Multi-terminal HVDC (M²TDC), a transmission line in the meshed grid is disconnected due to a fault, which will lead to a decrease in the degree of freedom (DF) of control. This paper analyzes the fault characteristics of an M²TDC including a multi-line DC current flow controller (CFC) under non-instantaneous disconnected faults and has got three contributions: 1) When a fault occurs at the near end, a switching control strategy of the CFC is proposed to quickly remove the sub-module (SM) on the faulted line, quitting its running; 2) When a fault occurs at the near-end line where the SM is responsible for the CFC capacitor voltage control, a SM switching control for the capacitor voltage stability is proposed to ensure the normal operations of the CFC; 3) When a fault occurs at the remote end, the fault characteristics and the necessity of the CFC control and protection actions are analyzed. An active SM bypassing control is proposed to match the CFC control degree with the DF of the M²TDC. Finally, a five-terminal HVDC (MT₅DC) network with a four-line CFC is established on the PSCAD/ EMTDC. When the non-transient disconnected faults occur at different locations, the impacts of different SMs quitting operations on the system performance are evaluated, which verifies that the proposed strategies can greatly improve the control flexibility and protection ability of the CFC in case of the near/remote-end faults. These strategies may ensure the stable operations of the CFC and the control ability of the power flow of the M²TDC grid under non-transient disconnected fault conditions.