Determination of the optimum routine test and self-checking intervals in protective relaying using a reliability model

Relaying reliability is generally separated into the two different aspects of dependability and security. The reliability of a protection relay can be improved by carrying out routine maintenance or by including built-in monitoring and self-checking facilities during the design stages. A Markov model is described in this paper which can be used to examine these features. The model also recognizes common-cause failures, temporary and permanent faults and the associated clearing times, operation of back-up protection, and relay mal-trips. Studies have been conducted to illustrate the expected reliability benefits by inclusion of monitoring and self-checking facilities within the relay.