Conemaugh Project New Concepts for 500-kV System Protection

The Conemaugh 500-kV system stability studies indicated that stability was achieved for all faults that were cleared in 4.5 cycles. If a breaker fails to clear a 3-phase fault, it is necessary to resort to a breaker failure timing scheme of 7.5 cycles and to down grade the fault to a single phase-to-ground fault by independent pole operation of the circuit breaker in order to maintain stability. To cover abnormal conditions when instability may still occur, out-of-step relays on generators are being applied to trip the unit when the first unstable swing appears in the unit transformer.