Schweitzer Engineering Laboratories SEL-421-4 - Page 440

6.104
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Select the close bus option for this application and set the close enable delay
(CLOEND) shorter than the shortest reclose open interval.
CLOEND := 10.000. CLSMON or Single-Pole Open Delay (OFF, 0.000–16000
cycles)
SOTF Duration
Setting SOTFD determines the longest period the SOTF logic can assert after the
circuit breaker closes.
SOTFD := 10.000. Switch-Onto-Fault Enable Duration (0.500–16000 cycles)
Close Signal Monitor
Assign the Relay Word bit CLSMON to a control input, so the relay can detect
execution of the close command.
CLSMON := IN102. Close Signal Monitor (SELOGIC Equation)
Phase Instantaneous/Definite-Time Overcurrent Elements
Use Level 1 instantaneous phase overcurrent element (50P1) as a nondirectional
high-set phase overcurrent element for SOTF protection. To rapidly clear faults,
set pickup threshold 50P1P equal to 50 percent of the fault current measured at
the local terminal for a close-in three-phase fault; use weak source conditions so
that the relay operates for low-level fault current.
50P1P := 9.57. Level 1 Pickup (OFF, 0.25–100 A secondary)
This application uses 50P1 as an instantaneous overcurrent element; you do not
need time delay.
67P1D := 0.000. Level 1 Time Delay (0.000–16000 cycles)
This application uses 50P1 as a nondirectional overcurrent element; you do not
need torque control.
67P1TC := 1. Level 1 Torque Control (SELOGIC Equation)
Negative-Sequence Instantaneous/Definite-Time Overcurrent
Elements
Negative-sequence directional overcurrent protection is an excellent choice for
underground cable. The cable zero-sequence impedance depends on the current
return paths, but the cable negative-sequence impedance does not. Negative-
sequence directional overcurrent protection provides reliable and sensitive pro-
tection for cables against all unbalanced faults. Be sure to set negative-sequence
overcurrent elements above system unbalances.
Negative-Sequence Overcurrent Elements
Enable three levels of negative-sequence overcurrent elements.
E50Q := 3. Negative-Sequence Instantaneous/Definite-Time Overcurrent Ele-
ments (N, 1–4)