| This paper reviews research on evaluating indications and models of protection reliability. The basic concept of reliability and the contents of condition-based maintenance are elaborated in this paper.This paper analyzes the composition of the digital protection and introduces familiar life distribution by researching the Bathtub Curve of protection. It explains a way of confirming life distribution:the histogram, and performs a statistical analysis of failures in secondary system. The histograms of failure frequency distribution and cumulative frequency distribution are provided for protective devices from a certain manufacturer to illustrate some characteristics of failure rate of the secondary system. A method is proposed that condition based maintenance should combine statistics with reliability assessment.The condition assessment of protection systems is introduced considering the self-checking function of protective relays in this paper. Once condition assessment recognizes that the protection system is at a fault state, maintenance is proceeded. Furthermore, it analyzes the protective system according to the Markov Model and establishes the condition models of protective systems and the components being protected. The relationship between condition assessment, the reliability of devices and the optimum interval of routine test is discussed using Markov state space method. The influence of condition assessment on the reliability of devices and the interval of routine test is also analyzed.When the failure rate of the loops is random, this paper proposes a calculation method for reliability of the protection system combining routine test with operation. When the repair time of the protection system does not comply with the exponential distribution, this paper also proposes a calculation method for availability when the protection is non-Markov model, which provides a new approach for reliability assessment of condition based maintenance of the protection system.
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