Routine test interval decision for protective systems based on probabilistic approach

An appropriate routine test interval is necessary for each protective scheme to obtain the maximum availability in a protective system. The main purpose of this paper is to estimate the optimum routine test interval, and to determine the reliability indices for several configurations of overcurrent relay protection. In this paper, a Markov model is employed to represent each protective device, which is then combined to form a particular protective scheme. Each scheme together with its protected component are combined to form a complete system to be inspected. A mathematical model of an inspected system is defined to evaluate the abnormal unavailability and optimum test interval. The results obtained from the analysis shows that this method can distinguish the differences of several configurations of overcurrent relay scheme. In addition, abnormal unavailability and optimum test interval depend directly on several parameters, e.g. failure rate, repair rate, self-test effectiveness etc.