Considering failure probability for back-up relay in determination of the optimum routine test interval in protective system using Markov model

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 has mal trip when not required and may fail to trip when required which may cause power system blackout due to cascading failure. Generally, this situation is not taken into account in the previous studies. Therefore in this paper, a Markov model is proposed for reliability analysis when back-up relay is not perfectly reliable. Simulation results demonstrate that if back-up relay is not perfectly reliable, the optimum routine test interval and reliability indices decrease. In the proposed Markov model, it is assumed that there is only self-checking facility with routine test and monitoring test is not considered for simplification.