Xinzhou Dong, Yunshu Huang, Haozong Wang, Binshu Chen, Hao Wang, and Qihuan Dong
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In the development of power systems, the traditional AC power grids are gradually evolving into AC/DC hybrid grids. However, the safety of hybrid grids is threatened by cascading faults, which is a set of chain events caused by a single fault and will eventually lead to a blackout. Based on the blackout accident that happened in Brazil on March 21st, 2018, this paper analyzes the cause, mechanism, and development of cascading faults in AC/DC hybrid grids. Simulations based on PSCAD/EMTDC software have been carried out to illustrate the typical cascading fault scenario including HVDC commutation failure, DC blocking, power flow transferring, incorrect relay protection and the outage of the network. Both analysis and simulation results shows that the "strong DC weak AC" structure and unreasonable cooperation of relay protections and stability control devices are important factors of cascading faults. Finally, this paper prospects a series of protection and control technologies that are expected to solve the problem.