State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (North China Electric Power University), Beijing 102206, China

The structure and operation modes of flexible DC grid are diverse, and there are strong coupling relationships among various power electronic devices. The fault characteristics are complex, coordination control is difficult, and active control of fault current still faces great challenges. Based on the development mechanism of fault current in flexible DC grid, this paper compares and summarizes the fault characteristics of AC and DC grids, and illustrates the necessity and possibility of active control of fault current in flexible DC grid. The basic scheme of active control scheme for fault current of flexible DC grid is determined. Domestic and foreign research results are summarized, and active control schemes are classified based on selectivity principle and suppression principle. Taking several typical schemes as examples, the characteristics of different types of active control schemes are analyzed and simulated in PSCAD/EMTDC, and the characteristics of different active control schemes are compared and summarized. The results show that the active control of fault current coordinated by the equipments on both source side and grid side can isolate fault quickly, save the cost of line investment and recover from fault rapidly.