(Hebei Key Laboratory of Distributed Energy Storage and Microgrid(North China Electric Power University), Baoding 071003, China)

A new protection scheme for the medium voltage DC line in the “hand-in-hand” multi-terminal flexible DC distribution network is designed based on the communication system and DC circuit breakers. Firstly, the configuration of the flexible DC distribution system and the fault characteristics of the medium voltage DC system are described. Then, a quick-acting main protection scheme is designed for pole-to-pole faults and pole-to-ground faults with low fault resistance. The current information and current state matrix of each relay is used to identify the fault type and fault area to isolate the fault area within 5 ms. Subsequently, a backup protection scheme is designed for possible pole-to-ground faults with high fault resistance. In such cases, the unbalanced voltage is taken as the startup criterion, with the fault area isolated according to the state of the unbalanced current of each relay. Besides, a first-step and a second-step remote backup protection each are designed in case of failures of DC circuit breakers and communication devices. Additionally, the cooperation between the protection system and the control system is taken into consideration to make sure the healthy area is supplied normally. Finally, simulation results in PSCAD/EMTDC verify the effectiveness of the proposed protection scheme.