How to realize reliable identification and accurate location of DC side faults is an urgent problem to be solved in multi terminal flexible DC distribution network. In a mode of the low current grounding

the system polar voltage is biased and the steady-state fault current amplitude is small

so the fault position can not be directly identified by the steady-state differential current criterion under the single-pole fault. Therefore

a DC side fault identification and location method aiming at engineering application was proposed. For unipolar fault

the identification method based on full-fault currents was adopted. Firstly

the fault location was identified by the current-differential protection based on the transient direction. Then

the fault location was judged by the steady current differential protection based on the parallel small resistance switching

which cannot be accurately identified because of the change of the system operation mode or the large transition resistance. For pole-to-pole fault

a fault isolation recovery strategy based on fast reclosing of DC circuit breakers was adopted. If the fault was detected by fast protection device

the converter was immediately locked and the DC circuit breaker was switched off in order to isolate the fault quickly. At the same time

after the fault location information being obtained

the control device overlapped the DC circuit breaker which had been skipped in the non-fault area

and unlocked the converter in the new topology to realize the rapid recovery of the fault. Finally

a three-terminal MMC DC distribution network was built in PSCAD/EMTDC software. The simulation results show that the proposed scheme can be adopted to identify single-pole faults with transition resistances of more than 100 ohms

and realize ms-level isolation and S-level recovery; meanwhile

no phenonenon of overvoltage or overcurrent appears during the period.