Improving Distribution Reliability (the “N 9 Problem”) by the Addition of Primary Feeders

Some power system loads require hyper reliability– this is the level of reliability that is characterized by a level of 0.999… availability of service. In this paper, the concept of high and hyper reliability is revisited (the “ 9 problem” referring to reliability as 0.999… showing nines in the reliability figure). A model is proposed for the reliability of multiple distribution feeders, and that model appears to behave like actual distribution engineering experience. The model is termed the discounted model because as the number of primary feeds increases the effectiveness of those feeds on the ultimate load service reliability is discounted.