Simulation and analysis of cascading faults in hybrid AC/DC power grids

With the increasing use of DC transmission in existing AC networks, the possibility of AC and DC circuits running parallel to each other and sharing the same right-of-way or even the same tower is increasing. Cascading fault is a newly emerging fault type in hybrid AC/DC grids which severely threatens the power quality and safe operation of the network. Cascading fault is defined as an event which initiated from a fault in the inverter AC system or an abnormal operation of the DC system which leads to the multiple fault incidents in hybrid AC/DC power grids. In this paper, first, the mechanism and process of cascading fault in hybrid AC/DC grids are described. Subsequently, based on the analysis of several fault cases which may result in cascading faults in AC power grids, the typical characteristics of hybrid AC/DC power grids are demonstrated. The results from the simulation case studies in PSCAD/EMTDC indicate that the cascading fault is initiated from an inverter AC fault which results in a long-lasting communication failure, leading to a DC system block. Afterward, the power flow transfers from the blocked HVDC line to the paralleled AC lines which results in cascading trips in AC lines. Finally, some feasible solutions to prevent the cascading faults in hybrid AC/DC grids are pointed out.