R.1.94
SEL-421 Relay Reference Manual Date Code 20111215
Protection Functions
Permissive Overreaching Transfer Tripping Scheme
The weak-infeed logic sets the ECTT (Echo Conversion to Trip) element upon
satisfaction of the following:
➤ No reverse-looking elements have picked up (the reverse-
looking elements override operation of the weak-infeed and
echo logic for faults behind the relay location)
➤ LOP is deasserted when the setting ELOP equals Y1
➤ At least one phase-to-phase undervoltage element or the
residual overvoltage element operates
➤ The local circuit breaker(s) is closed
➤ A permissive trip signal is received for ETDPU time period
The EWFC setting enables the weak-infeed feature of the relay. When the
EWFC setting is Y, the ECTT (Echo Conversion to Trip) logic is enabled.
When the setting EWFC is SP, the relay can convert echo to a single-pole trip
at the local terminal. ECTT logic is disabled when the setting is N.
Three-Terminal Lines
If you apply the POTT scheme to a three-terminal line, program SELOGIC
control equation PT1 as follows:
PT1 := IN105 AND IN106 General Permissive Trip Received (SELOGIC
Equation)
Relay control inputs IN105 and IN106 assert when the relay receives a
permissive signal from each of the two other terminals. The relay cannot high-
speed trip until both inputs assert. These two control inputs were chosen for
this particular example. Use control inputs that are appropriate for your
application.
Cross-Country Faults
Refer to 500 kV Parallel Transmission Lines With Mutual Coupling Example
on page A.1.18 for a complete description of how to apply the SEL-421 using
M
IRRORED BITS communications. The SEL-421 POTT scheme logic
(ECOMM = POTT2 or POTT3) includes additional logic that ensures proper
single-pole tripping at both stations during cross-country faults. A cross-
country fault consists of simultaneous single phase-to-ground faults on both of
the parallel lines. If the simultaneous ground faults are beyond Zone 1 reach
with respect to the local station, unwanted three-pole tripping could occur.
Tabl e 1 . 66 POT T Set tin gs (Sheet 1 of 2)
Setting Description Range
Default
(5 A)
Z3RBD Zone 3 Reverse Block Time Delay
(cycles)
0.000–16000 5.000
EBLKD Echo Block Time Delay (cycles) 0.000–16000 10.000
ETDPU Echo Time Delay Pickup (cycles) 0.000–16000 2.000
EDURD Echo Duration Time Delay (cycles) 0.000–16000 4.000
EWFC Weak Infeed Trip Y, N, SP N
27PWI
a
Weak Infeed Phase Undervoltage
Pickup (V)
1.0–200 47.0
27PPW
b
Weak Infeed Undervoltage Pickup (V
) 0.1–300 80.0
59NW
b
Weak Infeed Zero-Sequence
Overvoltage Pickup (V)
0.1–200 5.0
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