Date Code 20010625
Trip and Target Logic
5-13
SEL-311L Instruction Manual
Note that SOTFE is asserted when the circuit breaker is open. This allows elements set in the
SEL
OGIC
control equation trip setting TRSOTF to operate if a fault occurs when the circuit
breaker is open (see Figure 5.4). In such a scenario (e.g., flashover inside the circuit breaker
tank), the tripping via setting TRSOTF cannot help in tripping the circuit breaker (the circuit
breaker is already open), but can initiate breaker failure protection, if a breaker failure scheme is
implemented in the SEL-311L Relay or externally.
When the circuit breaker is closed, the 3PO condition deasserts (3PO = logical 0) after the 3POD
dropout time (setting 3POD is usually set for no more than a cycle). The SOTF logic output,
SOTFE, continues to remain asserted at logical 1 for dropout time SOTFD time.
Close Bus Operated Switch-Onto-Fault Logic
Close bus operated switch-onto-fault logic is enabled by making time setting CLOEND
(CLOEND
x
OFF). Time setting CLOEND qualifies the deassertion of the load detection
element 50L (indicating that the circuit breaker is open).
Circuit breaker closure is detected by monitoring the dc close bus. This is accomplished by
wiring an optoisolated input on the SEL-311L Relay (e.g., IN105) to the dc close bus. When a
manual close or automatic reclosure occurs, optoisolated input IN105 is energized. SEL
OGIC
control equation setting CLMON (close bus monitor) monitors the optoisolated input IN105:
CLMON = IN105
When optoisolated input IN105 is energized, CLMON asserts to logical 1. At the instant that
optoisolated input IN105 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 instantaneous elements in SEL
OGIC
control equation
trip setting TRSOTF to trip after the circuit breaker closes (see Figure 5.4—middle of figure).
Time setting SOTFD is usually set around 30 cycles.
Switch-Onto-Fault Trip Logic Trip Setting (TRSOTF)
An instantaneous element is usually set to trip in the SEL
OGIC
control equation trip setting
TRSOTF (e.g., TRSOTF = M2P + Z2G + 50P1).
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 deenergized, 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
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