The HVDC transmission based on modular multilevel converter shows the
weak feed characteristic of limited fault current amplitude when
connected to the traditional AC power grid, which leads to the risk of
sensitivity reduction or even failure of the traditional pilot
protection. In this paper, a MMC-HVDC AC side transmission line
protection scheme based on positive sequence fault component current
ratio is proposed. The positive sequence fault component current of the
MMC-HVDC side is calculated by using the positive sequence fault
component current of the power grid side, and the fault is identified
through the significant difference between the ratio of the positive
sequence fault component current of the MMC-HVDC side and the actual
positive sequence fault component current of the MMC-HVDC side in the
case of internal and external faults. This method has the advantages of
simple principle, low requirement for synchronous data, not affected by
capacitive current and strong resistance to high resistance. The
simulation results show that the algorithm can correctly partition
internal and external faults, and is not affected by fault location,
fault type and operation mode of converter station, and has high
sensitivity and reliability.