In order to maximize the availability of protection system and minimize the risk of mal-operation, usually protective relay test carry out in power electrical industry. Determination of precise optimum routine test and self-checking intervals plays a vital role in decrease of maintenance and reparation cost of protective system. Self-checking and monitoring facilities increase the reliability and optimum routine test interval of a protection system. Modern digital relays are equipped by self-checking and monitoring tests in addition to routine test however, electromagnetic and static relays are just equipped by routine test. In the previous studies, the back-up protection system is assumed to be perfectly reliable for reliability analysis. This assumption is not correct in the operation because back-up relay has the same structure on the primary relay, so back-up relay can also has mal-trip when not required and may fail to trip when required which may cause power system blackout due to cascading failure. Therefore, In this paper, an extend Markov model is proposed for reliability analysis when back-up relay is not perfectly reliable considering self-checking and monitoring facilities in the protective system. Results show that, the optimum routine and self-checking tests interval of protection system increase significantly by increasing of monitoring and self-checking tests effectiveness of back-up protection system.

Keywords