Integrated Transformer, Feeder, and Breaker Protection: An Economic and Reliable Solution for Distribution Substations

New digital technology allows us to integrate transformer, feeder, and breaker protection in a single multifunction relay. This relay can also provide breaker reclosing, breaker control, and monitoring functions plus high resolution fault recording capabilities that simplify fault analysis. The new system has fewer components and offers a more efficient, economical, and reliable approach to substation protection, monitoring, and control. This paper describes the application of a single microprocessor-based relay to protect a distribution substation consisting of a distribution transformer and two associated feeders at Northern States Power Company (NSP). The economic advantages of using a single relay allow users to afford a second relay for redundant protection. INTRODUCTION Traditionally, protection packages for distribution substations consisted of several discrete relays, each performing a single protective function, such as transformer, bus, and feeder protection. With the development of microprocessor-based relay designs, users could combine some of these discrete relays to provide individual power apparatus protection. Now, with further technologies, we can provide complete protection of all equipment in a distribution substation using a single multifunction relay. The diagram in Figure 1 illustrates this approach. In this paper, we discuss the current practice of using individual devices for separate protection functions, then describe a new multifunction digital relay that provides complete protection for a distribution substation.