Abstract: When the bipolar short-circuit fault occurs at the low-voltage DC side of the multi-branch flexible DC distribution network which takes the power electronic transformer with cascaded H-bridge（CHB-PET） as the main station, the fault current rises rapidly and the peak value is high. However, power electronic devices lock up soon after the fault, resulting in the loss of fault information, which brings challenges to the reliable operation of DC distribution network line protection. To solve this problem, a block-free fault ride-through scheme is proposed based on the cooperation between the input stage and the isolation stage of CHBPET. By setting the inward shift ratio of the dual active bridge high-voltage switching tube in the isolation stage, the DC capacitor discharge of the input stage is controlled, so that it does not overcharge or overdischarge. And the overcurrent of the power electronic devices in the CHB-PET is eliminated, so as to ensure the sustainable stable current output of the CHB-PET after the fault. On this basis, the Fourier transform is used to extract the high frequency component of the low-voltage DC-side current of CHB-PET with the fault ride-through strategy, and the protection criterion is constructed by the ratio of the integral value of the high-frequency current amplitude to the integral value of the full current. Then a single terminal protection scheme is designed. A multi-branch DC distribution network model including CHB-PET is built on the PSCAD/EMTDC simulation platform. The simulation analysis shows that the proposed method can identify and remove faulty lines quickly and reliably.