Dependability and security-based failure considerations in protection system reliability studies

The reliability of protection systems has a considerable effect on the reliability of supply, and hence appropriate protection system reliability models must be incorporated in power system reliability studies. These studies assume increasing prominence, especially in the wake of influx of smart grid technologies, making it imperative to handle the accompanying failure dependencies in detail. This paper presents the results of one such related investigation carried out to incorporate the impact of transmission line failure modes on account of various protection system response scenarios on supply reliability indices. In addition to the basic frequency and duration indices, indices such as annual power interrupted, annual energy not supplied, and annual interruption costs are computed, built on a minimal cut set based approach. The approach presented circumvents the need for complex Markov models to include protection system reliability considerations. An illustrative case study is employed to draw attention to the impact of identified comprehensive failure scenarios in protection and control equipment on power system reliability. Comparisons are made with an existing simplified method, and also with a case where protection system reliability is assumed to be perfect. The results bring forward the emphasis to be placed on initiatives to include the study of impact of reliability of protection systems on reliability of supply.