5.9
Date Code 20080213 Instruction Manual SEL-351A Relay
Trip and Target Logic
Switch-Onto-Fault (SOTF) Trip Logic
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 ≠ 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-351A (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 Enable (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.
A SOTF trip illuminates the SOTF front-panel LED.
Switch-Onto-Fault
Trip Logic Trip Setting
(TRSOTF)
An instantaneous overcurrent element is usually set to trip in the SELOGIC
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 de-energized, 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.
Courtesy of NationalSwitchgear.com
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