5-8
Trip and Target Logic
Date Code 20020215
SEL-351R Instruction Manual
Circuit breaker closure is detected by monitoring the dc close bus. This is accomplished by
wiring an optoisolated input on the SEL-351R (e.g., IN104) to the dc close bus. When a manual
close or automatic reclosure occurs, optoisolated input IN104 is energized. SEL
OGIC
control
equation setting CLMON (close bus monitor) monitors the optoisolated input IN104:
CLMON = IN104
When optoisolated input IN104 is energized, CLMON asserts to logical 1. At the instant that
optoisolated input IN104 is energized (close bus is energized), the circuit breaker is still open so
the output of the CLOEND timer continues to be asserted to logical 1. Thus, the ANDed
combination of these conditions latches in the SOTFD timer. The SOTFD timer outputs a
logical 1 for a time duration of “SOTFD” cycles any time it sees a rising edge on its input
(logical 0 to logical 1 transition), if it is not already timing. The SOTF logic output, SOTFE,
asserts to logical 1 for SOTFD time.
Switch-Onto-Fault Logic Output (SOTFE)
Relay Word bit SOTFE is the output of the circuit breaker operated SOTF logic or the close bus
operated SOTF logic described previously. Time setting SOTFD in each of these logic paths
provides the effective time window for the overcurrent elements in SEL
OGIC
control equation
trip setting TRSOTF to trip after the circuit breaker closes (see Figure 5.1—middle of figure).
Time setting SOTFD is usually set around 30 cycles.
Switch-Onto-Fault Trip Logic Trip Setting (TRSOTF)
An instantaneous overcurrent element is usually set to trip in the SEL
OGIC
control equation trip
setting TRSOTF (e.g., TRSOTF = 50P2).
If the voltage potential for the relay is from the line-side of the circuit breaker, the instantaneous
overcurrent element in the SEL
OGIC
control equation trip setting TRSOTF should be
nondirectional. When the circuit breaker is open and the line is reenergized, the relay sees zero
voltage. If a close-in three-phase fault condition exists on the line (e.g., safety grounds
accidentally left attached to the line after a clearance) and then the circuit breaker is closed, the
relay continues to see zero voltage. The directional elements have no voltage for reference and
cannot operate. In this case, the instantaneous overcurrent element in the SOTF trip logic should
be nondirectional.
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