System protection is laid between the defenses for power system protective relaying and the emergency control. Under the premise of ensuring the safety of electrical equipment, it strives to ensure the safety of the system, block the chain of occurrence and growth of cascading faults, and effectively avoid the occurrence of large-scale blackout catastrophes. This book systematically elaborates on the dealing technology of a special type of fault, the "cascading fault", in the AC-DC hybrid large-scale power grid. The main contents include immunization distance protection for accident overload; distance protection that is immune to oscillation; inverter control technology to prevent long-term or continuous commutation failure; DC participation emergency power flow control technology used to share the accident transfer overload caused by inverter lockout; and overhead transmission line adaptive overload protection. The basis of English translation of this book from its Chinese original manuscript was done with the help of artificial intelligence (machine translation by the service provider DeepL.com). A subsequent human revision of the content was done by the author.