Fault Analysis of Multi-terminal Flexible DC Distribution System With RES

The flexible DC distribution system benefits the construction of smart grids and the Energy Internet. Firstly, grounding modes and control strategies of the hand-in-hand two-terminal flexible dc distribution network were discussed. Subsequently, the paper theoretically analyzed the fault characteristic of pole-to-pole fault and pole-to-ground fault in medium voltage direct current (MV DC) system, as well as its effects on other sections and the impact of the grounding resistance on the fault characteristic. Unlike analytic methods used in VSC-HVDC projects, the blocking of IGBTs is not applied when the DC-side fault was analyzed, this is because that, in flexible distribution systems, it is supposed to use the external equipment, such as dc circuit breaker and current-limiting reactance, to isolate the fault instead of blocking the IGBT to maintain the power supply of healthy parts connected to the same DC bus. Finally, the simulation model was built in PSCAD/EMTDC. The simulation result proves the validity of the theoretical analysis.