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L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
GROUPED ELEMENTS
CHAPTER 5: SETTINGS
5
In general, a breaker failure scheme determines that a breaker signaled to trip has not cleared a fault within a definite
time, so further tripping action must be performed. Tripping from the breaker failure scheme should trip all breakers, both
local and remote, that can supply current to the faulted zone. Usually operation of a breaker failure element causes
clearing of a larger section of the power system than the initial trip. Because breaker failure can result in tripping a large
number of breakers and this affects system safety and stability, a very high level of security is required.
Two schemes are provided: one for three-pole tripping only (identified by the name “3BF”) and one for three pole plus
single-pole operation (identified by the name “1BF”). The philosophy used in these schemes is identical. The operation of a
breaker failure element includes three stages: initiation, determination of a breaker failure condition, and output.
Initiation stage
A FlexLogic operand representing the protection trip signal initially sent to the breaker must be selected to initiate the
scheme. The initiating signal is sealed-in if primary fault detection can reset before the breaker failure timers have finished
timing. The seal-in is supervised by current level, so it is reset when the fault is cleared. If desired, an incomplete sequence
seal-in reset can be implemented by using the initiating operand to also initiate a FlexLogic timer, set longer than any
breaker failure timer, whose output operand is selected to block the breaker failure scheme.
Schemes can be initiated either directly or with current level supervision. It is particularly important in any application to
decide if a current-supervised initiate is to be used. The use of a current-supervised initiate results in the breaker failure
element not being initiated for a breaker that has very little or no current flowing through it, which can be the case for
transformer faults. For those situations where it is required to maintain breaker fail coverage for fault levels below the
BF1
PH AMP SUPV PICKUP
or the
BF1 N AMP SUPV PICKUP
setting, do not use a current supervised initiate. Utilize this feature for
those situations where coordinating margins can be reduced when high speed reclosing is used. Thus, if this choice is
made, fault levels must always be above the supervision pickup levels for dependable operation of the breaker fail scheme.
This can also occur in breaker-and-a-half or ring bus configurations where the first breaker closes into a fault; the
protection trips and attempts to initiate breaker failure for the second breaker, which is in the process of closing, but does
not yet have current flowing through it.
When the scheme is initiated, it sends a trip signal after a pickup delay to the breaker initially signaled to trip (this feature is
usually described as re-trip). This reduces the possibility of widespread tripping that results from a declaration of a failed
breaker.
Determination of a breaker failure condition
The schemes determine a breaker failure condition via three paths. Each of these paths is equipped with a time delay, after
which a failed breaker is declared and trip signals are sent to all breakers required to clear the zone. The delayed paths are
associated with breaker failure timers 1, 2, and 3, which are intended to have delays increasing with increasing timer
numbers. These delayed paths are individually enabled to allow for maximum flexibility.
Timer 1 logic (early path) is supervised by a fast-operating breaker auxiliary contact. If the breaker is still closed (as
indicated by the auxiliary contact) and fault current is detected after the delay interval, an output is issued. Operation of
the breaker auxiliary switch indicates that the breaker has mechanically operated. The continued presence of current
indicates that the breaker has failed to interrupt the circuit.
Timer 2 logic (main path) is not supervised by a breaker auxiliary contact. If fault current is detected after the delay
interval, an output is issued. This path is intended to detect a breaker that opens mechanically but fails to interrupt fault
current; the logic therefore does not use a breaker auxiliary contact.
The timer 1 and 2 paths provide two levels of current supervision, high-set and low-set, that allow the supervision level to
change from a current which flows before a breaker inserts an opening resistor into the faulted circuit to a lower level after
resistor insertion. The high-set detector is enabled after timeout of timer 1 or 2, along with a timer that enables the low-set
detector after its delay interval. The delay interval between high-set and low-set is the expected breaker opening time.
Both current detectors provide a fast operating time for currents at small multiples of the pickup value. The overcurrent
detectors are required to operate after the breaker failure delay interval to eliminate the need for very fast resetting
overcurrent detectors.
For the L60 relay, the protection trip signal initially sent to the breaker is already programmed as a trip output. The
protection trip signal does not include other breaker commands that are not indicative of a fault in the protected
zone.
