Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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6.35
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
500 kV Parallel Transmission Lines With Mutual Coupling Example
Turn off CLOEND (CLSMON Delay) because this method is not used.
CLOEND := OFF .
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)
Phase Instantaneous/Definite-Time Overcurrent Elements
Use 50P1, Level 1 phase instantaneous overcurrent element, as a nondirectional
high-set phase overcurrent element for SOTF protection. If the local circuit
breaker closes into a close-in three-phase bolted fault with line-side potential
transformers, the polarizing voltage for the phase-distance elements is zero.
Therefore, the distance protection does not operate. In this case, the 50P1 ele-
ment quickly trips the circuit breaker because this overcurrent element does not
rely on the polarizing voltage.
To rapidly clear faults, set 50P1P equal to 50 percent of the fault current mea-
sured at the local terminal for a close-in three-phase fault; use weak source condi-
tions so that the relay operates for low-level fault current.
50P1P := 7.21 .
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; place no
conditions on torque control.
67P1TC := 1 .
Level 1 Torque Control (SEL
OGIC
Equation)
Selectable Operating Quantity Time Overcurrent Element 1
Use inverse-time overcurrent protection to provide backup protection for high-
resistance ground faults. The 51S1 element provides backup protection for unbal-
anced faults if both the communications-assisted and step distance protection fail
to operate.
NOTE:
Use your company practices
and philosophy when determining
these settings.
Select zero-sequence line current as the operating quantity.
51S1O := 3I0L .
51S1 Operate Quantity (Ia n , Ib n , Ic n , IMAX n , I1L, 3I2L, 3I0 n )
The n in the 51S1O setting is L for line, 1 for BK1, and 2 for BK2.
The fault current (3I
0
) measured by the relay for a bolted single phase-to-ground
fault at the remote station with both lines in-service is 2.25 A secondary. (A
remote end fault for the downstream relay gives a fault current of 1.22 A second-
ary.) Set the pickup to 30 to 50 percent of 3I
0
.
51S1P = 0.50 .
51S1 Overcurrent Pickup (0.25–16 A secondary)