Generator Backup Overcurrent Protection

A concern that the characteristics and correct application of the generator backup relay are misunderstood is addressed in this report to the Power Systems Protection Committee. It is inherently a secure device, and rarely has the opportunity to operate in its intended capacity. So the question was asked, ``Do generator backup overcurrent relays really protect anything?'' In response a description of the function and operating characteristics of the backup relays, a discussion of generator fault current behavior, examples of relay settings for a typical application, and methods and criteria for determining that the relay both protects the generator and operates selectively with other protective devices are included.