5.8
SEL-351A Relay Instruction Manual Date Code 20080213
Trip and Target Logic
Switch-Onto-Fault (SOTF) Trip Logic
Three-Pole Open
Logic
Three-pole open (3PO) logic is the top half of Figure 5.3. It is not affected by
enable setting ESOTF (see Other Enable Settings on page SET.2).
The open circuit breaker condition is determined from the combination of:
➤ Circuit breaker status (52A)
➤ Load current condition (50L)
If the circuit breaker is open (52A = logical 0) and current is below phase
pickup 50LP (50L = logical 0), then the three-pole open (3PO) condition is
true:
3PO = logical 1 (circuit breaker open)
The 3POD dropout time qualifies circuit breaker closure, whether detected by
circuit breaker status (52A) or load current level (50L). When the circuit
breaker is closed:
3PO = logical 0 (circuit breaker closed)
Determining Three-Pole Open Condition Without Circuit Breaker Auxiliary
Contact
If a circuit breaker auxiliary contact is not connected to the SEL-351A,
SEL
OGIC control equation setting 52A is set:
52A = 0 (numeral 0)
With SEL
OGIC control equation setting 52A continually at logical 0, 3PO
logic is controlled solely by load detection element 50L. Phase pickup 50LP is
set below load current levels.
When the circuit breaker is open, Relay Word bit 50L drops out (= logical 0)
and the 3PO condition asserts:
3PO = logical 1 (circuit breaker open)
When the circuit breaker is closed, Relay Word bit 50L picks up (= logical 0;
current above phase pickup 50LP) and the 3PO condition deasserts after the
3POD dropout time:
3PO = logical 0 (circuit breaker closed)
Note that the 3PO condition is also routed to the loss-of-potential (LOP) logic
(see Figure 4.1).
Circuit Breaker
Operated Switch-
Onto-Fault Logic
Circuit breaker operated switch-onto-fault logic is enabled by making time
setting 52AEND (52AEND ≠ OFF). Time setting 52AEND qualifies the
three-pole open (3PO) condition and then asserts Relay Word bit SOTFE:
SOTFE = logical 1
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 5.1). 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-351A (see output contact OUT103 example
in Output Contacts on page 7.31) or externally.
Courtesy of NationalSwitchgear.com
To Next Page
Loading...