GE D90 Plus Instruction Manual page 243

CHAPTER 7: PROTECTION
Determination of a breaker failure condition
Breaker failure scheme outputs
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
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 may be the case for
transformer faults. For those situations where it is required to maintain breaker fail
coverage for fault levels below the
setting, a current supervised initiate should not be used. This feature should be
Pickup
utilized for those situations where coordinating margins may 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 immediately sends a trip signal 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.
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 will
enable 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.
Timer 3 logic (slow path) is supervised by a breaker auxiliary contact and a control switch
contact used to indicate that the breaker is in/out of service, disabling this path when the
breaker is out of service for maintenance. There is no current level check in this logic as it is
intended to detect low magnitude faults and it is therefore the slowest to operate.
The outputs from the breaker failure scheme are:
• FlexLogic™ operands that report on the operation of portions of the scheme.
• A FlexLogic™ operand used to re-trip the protected breaker.
GROUPED PROTECTION ELEMENTS
Phase Current Supv Pickup or the Neutral Current Supv
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