In order to enhance both dependability and security which are two aspects of relaying system, self- checking and monitoring facilities are usually considered along with routine test. In practice back-up relay can have mal trip when not required and may fail to trip when required and can cause power system blackout due to cascading failure. Generally, this situation is not taken into account in the previous studies, so in this paper, a Markov model is proposed for reliability analysis when back-up relay is not perfectly reliable. Results show that optimum routine test and reliability indices decrease if back-up relay is not perfectly reliable. In the proposed Markov model, for simplification of the suggested Markov model, we suppose that there is only self-checking facility with routine test and monitoring test is not considered.

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