500kV IPT breaker failure protection: An application of dual timer scheme for short critical clearing time

Circuit breaker failure protection scheme is typically used in electric transmission system to provide backup protection against any fault occurring at any given time in case a circuit breaker fails to open following receiving a trip signal from system protection relays. An ideal breaker failure protection system should clear the fault by operating the least amount of contributing breakers, both locally and remotely, within a desired time frame that does not cause the system to become unstable. In general such scheme can be applied with proper timing coordination including safety margin before approaching the system critical clearing time. A system critical clearing time is defined as the time duration that a fault can remain on the system before system instability occurs. In Progress Energy Florida, a system planning study for a future scenario recommends its 500kV transmission system critical clearing times to meet 8 cycles for single-line-to-ground fault and 5.5 cycles for multi-phase faults. Challenged by such tight requirements, the Company's Transmission Protection and Controls (T-P&C) engineers have developed a 500kV breaker failure (BF) protection scheme that implements dual BF timer logics: • A single phase fault detector with a typical timer • A multi-phase (three-phase or phase-to-phase) fault detector with a shorter timer This paper presents how Progress Energy T-P&C engineers develop the dual-timer BF scheme by utilizing the associated state of the art protection system, equipment, and communication facility to meet short duration system critical clearing time while achieving operational speed, selectivity, and sensitivity.