The reliability of protection and control systems for transmission feeders

This paper introduces a technique for assessing the reliability of protection and control systems using reliability models and event tree analysis. This technique allows a quantitative reliability assessment to be made, which can then be used to determine the effect of integrating different protection and control functions into a single unit. The term reliability when applied to protection system failure can be considered in terms of dependability and security. Dependability is defined as the probability that the protection operates satisfactorily when required, i.e. it does not fail to trip. Security is defined as the probability that the protection does not operate when not required to do so, i.e. it does not produce unwanted trips. Dependability results are given in this paper for feeder protection schemes based on differential protection, distance protection and combined differential and distance protection. The effect on the dependability of these schemes of including a separate and an integral intertrip unit is also considered. The former is housed in its own hardware, and the latter is integrated into the differential protection. Both intertrip units can be activated by either protection