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6.36
SEL-421 Relay
Instruction Manual
Date Code 20171021
Protection Applications Examples
500 kV Parallel Transmission Lines With Mutual Coupling Example
Use the following formula to determine approximately how much primary fault
resistance coverage (R
F
) is provided by 51S1P on a radial basis:
Equation 6.18
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 =
1.68
.
51S1 Inverse-Time Overcurrent Time Dial (0.50–15.0)
Set the overcurrent element to emulate electromechanical reset.
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 (SEL
OGIC
Equation)
Zone/Level Direction
Zone 1 and Zone 2 distance element directions are fixed in the forward direction.
You can select the other zones independently as forward-looking (F), or reverse-
looking (R). Set Zone 3 distance elements reverse-looking, because these are
blocking elements for the POTT trip scheme.
DIR3 :=
R
.
Zone/Level 3 Directional Control (F, R)
Directional Control
The SEL-421 uses an array of directional elements to supervise the ground-
distance elements and residual directional overcurrent elements during ground
fault conditions. Internal logic automatically selects the best choice for the
ground directional element (32G) from among the negative-sequence voltage-
polarized directional element (32QG), zero-sequence voltage-polarized direc-
tional element (32V), and the zero-sequence current-polarized directional ele-
ment (32I).
The relay setting ORDER determines the order in which the relay selects direc-
tional elements to provide ground directional decisions. You can set ORDER
with any combination of Q, V, and I. The listed order of these directional ele-
R
F
PTR
CTR
------------
•
VNOMY
3
51S1P
-------------------------------------
=
4500
400
------------
•
111.11 V
3
0.50 A
-----------------------------------
=
1443
primary
=
Summary of Contents for SEL-421-4
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