5-18 Applications Date Code 20000421
SEL-251, -2, -3 Instruction Manual
Breaker Failure Implementation Notes
The "close TRIP output contacts" condition drives the trip failure timer. This condition ends if
all trip conditions vanish and all overcurrent element pickups in Relay Word row R1 drop out.
For more information, see Figure 2.21.
Even if a circuit breaker trips successfully and all overcurrent element pickups in Relay Word
row R1 drop out, a trip condition can still be present.
For example, TR(1246) = KT+..., from the previous underfrequency or undervoltage load-
shedding applications.
close TRIP output contacts = TR(1246) +...
In this example, a false breaker failure operation can occur if KT remains asserted long enough
for the trip failure timer to expire. This problem can be overcome if trip condition KT is
qualified by the presence of load current or limited in duration.
Qualify Trip Condition With Presence of Load Current
Phase instantaneous overcurrent element 50C is not used in the reclosing relay reset inhibit
function and can be set below load levels. 50C can assert with the presence of load current or
deassert in its absence. With this low setting 50C can indicate whether the circuit breaker is
closed or open. There is no 52A dependence.
Using SELOGIC Control Equations to qualify KT:
A(12) = 50C
K(1234) = IN5 or 27 (underfrequency or undervoltage application)
V(56) = A*KT
TR(1246) = V+... (TR is the programmable trip condition)
close TRIP output contacts = TR + ...
Limit Trip Condition Time Duration
Limiting the KT trip condition duration to a time shorter than TFT prevents false breaker failure
operation.
Using SELOGIC Control Equations to effectively limit the duration of KT:
S(123) = IN5 or 27 (underfrequency or undervoltage application)
L(1234) = ST
K(1234) = IN5 or 27 (underfrequency or undervoltage application)
V(56) = KT*!L (!L limits the duration of KT)
TR(1246) = V+... (TR is the programmable trip condition)
close TRIP output contacts = TR + ...
To Next Page
