With the construction of the “3-level, 3-vertical-line, and 1-circle” power backbone in China, it’s stricter and stricter on relay protection system and security and stability control system (SSCS) for reliable power transmission. Lots of blackouts in the world had happened, one main reason for which is the hidden failures of relay protection system or SSCS. Much work had been done about the hidden failure of relay protection, including classification, probability model, analysis methods of effects on power grid, and monitoring measures, which was summarized in the paper. The operation experiences of SSCS indicated that there might be hidden failures in five links of the security and stability control device (SSCD), e.g. measuring, control strategy, setting, communication and voting pattern. In addition, the coordination hidden failure among relay protection system, SSCS, and power plant’s parameters related to the power grid was pointed out for more attention. In the future, amounts of work will be expected to be conducted on hidden failure: model building, assessment methods, application of research achievements, operation management of secondary equipment, and coordination problem between the relay protection system and the SSCS.