After the bipolar short circuit fault occurs on the low-voltage DC side of the low-voltage DC distribution network with a power electronic transformer, the fault current amplitude is high and the rise speed is fast, which leads to the rapid locking of power electronic devices, and the loss of fault information of line protection. Based on the multi-branch DC distribution network with CHB-type PET, this paper proposes a fault current limiting strategy. PET starts current limiting measures after detecting the DC side fault, which can provide stable and continuous fault transient information for line protection while providing a small amount of short circuit current. On this basis, using the fault current at both ends of the line, longitudinal polarity protection without strict communication synchronization is proposed. The simulation analysis based on PSCAD/EMTDC software shows that the scheme can realize fast and reliable fault line selection and has strong resistance to transition resistance.