Abstract: Hidden failure of protection leads to protection misoperation when N-1 contingency occurs in power system,which contributes to the expansion of cascading failure.Based on investigation of hidden failure mechanism,protection hidden failure modes were analyzed and vulnerability zone of protections were identified by consideration of different protective device and different fault location of the first contingency.N-k contingency analysis method based on protection hidden failure modes analysis was proposed,which linked protection hidden failures with contingency screening and false tripping line set identification.A case study shows the effectiveness of the proposed method by assessing effects of relay hidden failures on system reliability.