5.127
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Functions
Permissive Overreaching Transfer Tripping Scheme
arc (i.e., the fault will restrike following autoreclose at the strong terminal).
Because the strong terminal is beyond the Zone 1 reach, it cannot trip for end-
zone faults.
The faulted phase voltage(s) is depressed at the weak-infeed terminal, a condition
that generates significant residual voltage during ground faults. The SEL-421
uses phase-to-phase undervoltage level detectors and a residual overvoltage level
detector to qualify a weak-infeed condition. If setting EWFC equals Y, the relay
enables the weak-infeed logic and settings 27PPW and 59NW are active. For sin-
gle-pole tripping applications, set EWFC to SP and setting 27PWI is active.
The weak-infeed logic sets the Echo Conversion to Trip (ECTT) 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 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 permis-
sive 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 6.18 for a complete description of how to apply the SEL-421 using M
IR-
RORED BITS communications. The SEL-421 POTT scheme logic (ECOMM =
POTT2 or POTT3) includes additional logic that ensures proper single-pole trip-
ping 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 sta-
tion, unwanted three-pole tripping could occur.
Table 5.72 POTT Settings (Sheet 1 of 2)
Setting Prompt 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
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