Relay operation from bushing potential devices

Abstract

CONSTRUCTION of the Osage hydroelectric plant and its interconnection with the existing system of the Union ElectricLight & Power Company of St. Louis, Mo., made desirable a study of the relaying schemes as applied for the 66-kv. and 132-kv. trans mission lines.Stability investigations and operating experience definitely indi cated the desirability of high speed relays and high speed oil circuit breakers. On the existing system it was possible in a number of cases to use the old oil circuit breakers, rebuilding them for high speed operation.In the selection of relays for phase-to-phase and three-phase faults, it appeared that during parallel line operation balanced percentage differential relays of the high speed type would offer the simplest and most efficient solution. An analysis of the individual circuits, how ever, showed that the application of this type of relay was rather limited, and distance relays of the impedance directional type or reactance directional type were required on some of the lines. These two latter types also were used for protection against phase-tophase and three-phase faults on single lines. Time delay overcurrent relays were used throughout for back-up protection.For protection against line-to-ground faults, balanced ground cur rent relays were installed in the majority of cases for parallel lines, instantaneous directional ground overcurrent relays being required in two cases for parallel lines. For protection against line-to-ground faults on single lines, either time delay or instantaneous ground relays were used, most of them being of the directional type. Special means were developed for determining the calibration of the reactance and impedance relays.To date the performance of the high speed relays as applied to this system has been questionable. In favor of these relays it may be said that the relays and the associated high speed circuit breakers have cleared a large number of faults rapidly and correctly, while in their disfavor several cases of operating trouble have occurred. These indicate that the present types of high speed directional relays probably are not applicable to certain types of systems. Further analysis indicates that there are fundamental defects in the design of certain types of high speed relays. Modifications of this particular type of relay, however, are being made. (A.