For the highly reliable protective relay

the software failures

human errors and other failure factors cannot be ignored. Meanwhile

these failure rates are just time-varying in the whole lifecycle. To solve these problems

a 5-state Markov model for single protective relay considering multi-factors and a 14-state Markov model for dual protective relays are developed

respectively. Based on the time-varying failure rate and maintenance factors and by referring to the current maintenance strategies of the State Grid

the time-varying unavailability of partial maintenance is distinguished from that of complete maintenance under different maintenance strategies

with the optimal maintenance strategy given. This work is supported by National High Technology Research and Development Program of China (863 Program) (No.2012AA050208)

National Natural Science Foundation of China (No.51222703) and Fundamental Research Funds for the Central Universities (No.12MS30).