6.70
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
Set Level 3 torque control equation to the reverse decision from the ground direc-
tional element, 32GR.
67G3TC := 32GR. Level 3 Torque Control (SELOGIC 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 Operating Quantity (IAn, IBn, ICn, IMAXn, I1L, 3I2L, 3I0n)
The n in the 51S1O setting is L for line, 1 for BK1, and 2 for BK2.
The relay measures 4.8 A secondary of 3I
0
for a bolted single-phase-to-ground
fault at the 345 kV terminals of the autotransformer. Set the pickup to 20 percent
of 3I
0
.
51S1P := 0.96. 51S1 Overcurrent Pickup (0.25–16 A secondary)
Use the following formula to determine approximately how much primary fault
resistance coverage (R
F
) is provided by 51S1P on a radial basis:
Equation 6.36
Use the following as a guide to set the curve and time dial; for secure backup pro-
tection, perform a coordination study.
Set the local overcurrent element to coordinate with the downstream overcurrent
element such that there is an 18-cycle (60 Hz nominal) safety margin for ground
faults in front of the first downstream overcurrent element. Assume the operating
time of the downstream overcurrent element is 12 cycles for a close-in ground
fault. Therefore, set the local time-overcurrent element to operate approximately
30 cycles for ground faults in front of the first downstream overcurrent element.
51S1C := U3. 51S1 Inverse-Time Overcurrent Curve (U1–U5)
51S1TD := 2.0. 51S1 Inverse-Time Overcurrent Time Dial (0.50–15.0)
Set the overcurrent element to emulate electromechanical reset, so the overcur-
rent element coordinates properly with electromechanical overcurrent relays.
51S1RS := Y. 51S1 Inverse-Time Overcurrent Electromechanical Reset (Y, N)
Torque control the overcurrent element with the forward decision from the
ground directional element.
51S1TC := 32GF. 51S1 Torque Control (SELOGIC Equation)
R
F
PTR
CTR
------------
VNOMY 3
51S1P
-----------------------------------• =
3000
200
------------
115V 3
0.96A
------------------------•
=
1037 primary=
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