Digital relay reports verify power system models

Protective relays continuously monitor the power system to detect abnormal conditions, such as short circuits, that could be damaging to equipment or to the integrity of the system as a whole. They initiate corrective actions (often tripping circuit breakers) to remove the smallest possible portion of the electric system to isolate faulted equipment and allow the remaining part of the system to continue to generate and deliver power. The relay engineer uses "fault studies", or models of the power system operating under normal and short-circuit conditions, to determine what parameters should be measured and what relay settings should be used to discriminate reliably between acceptable and unacceptable system conditions. Like all models, the usefulness of fault study models depends on their validity and accuracy. This article encourages digital relay users to look more closely at their records of disturbances. In addition to the normal use of checking relay performance, these records can also be used to check the accuracy of the power system model.