Pole-to-ground fault ride through strategy for half-/full-bridge hybrid MMC-based radial multi-terminal HVDC systems with low-impedance grounded

Pole-to-ground (PTG) fault is one of the most common DC-side faults in multi-terminal high-voltage direct-current (HVDC) systems. During the PTG fault, the loss of system transmission power is a challenging issue. This study proposes a PTG fault ride through strategy for low-impedance grounded radial multi-terminal HVDC systems. It is realised by the coordinated operation between half-/full-bridge hybrid modular multilevel converters and mechanical DC switches. The mechanical DC switch is utilised to isolate the fault and a ground return path is formed to transmit the active power during PTG fault. If the fault is located at the sending terminal, half of the rated transmission power can be kept at the entire fault duration. If the fault is located at the receiving terminal, the successfully restarted active power regulator stations can further return to normal operation after the isolation of the PTG fault. With the proposed strategy, the loss of transmission power during PTG fault is minimised, which can improve the transient angle stability of the AC grid. PSCAD/EMTDC simulation is performed to verify the proposed strategy.