Considerations in applying ratio differential relays for bus protection

THE PROBLEM of bus differential protection is receiving increased attention by relay engineers because of the serious consequences of a bus fault. Several cases of bus faults have recently occurred indicating that regardless of care used in the design of structures, some unexpected failure or accident may cause a disastrous outage. In spite of the many improvements that have been made in the protection of transmission lines in recent years, little attention has been given to the protection of station busses. This has been due partly to the impression that they were not particularly subject to faults and partly to the feeling that the protective relays would do more harm than good through incorrect tripping. Recent experiences, however, indicate that a bus fault, though of infrequent occurrence, can cause much greater damage and a longer outage period than many transmission line faults. Also, high-grade performance has been obtained when a bus protective scheme is properly installed with due consideration given to the many factors which might lead to faulty operation. The purpose of this paper is to discuss some of the important factors to be considered in bus differential protection and the operating characteristics and results of a field test on a new bus differential relay.