Considering momentary and sustained reliability indices in the design of distribution feeder overcurrent protection

In response to customer complaints about momentary interruptions, some electric utilities have removed fast tripping on distribution overcurrent protection devices. At the same time, on many systems, the sustained outage indices, SAIFI and SAIDI, have shown a trend towards increased frequency of interruptions per customer and longer accumulated outage durations. State regulatory agency emphasis on sustained outage indices is a cause for reconsidering overcurrent protection strategies. A scheme targeting reduced momentary interruptions (i.e. removing fast tripping) may not be the most effective strategy to satisfy both the regulating entities as well as the utility's customers. By prudently installing another layer of sectionalizing devices, including fast tripping, SAIFI, SAIDI and MAIFI can be decreased. The paper shows (by way of feeder application examples) that the overcurrent protection scheme can be designed to take full advantage of fast tripping, effectively reducing both sustained and momentary outages experienced by customers.