6.63
Date Code 20171021
Instruction Manual
SEL-421 Relay
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
The SEL-421 at Station T measures the largest apparent fault impedance for
faults at Station R because the source at Station S is stronger than the source at
Station R. Therefore, Zone 2 at Station T must be set to 115.86
primary (plus a
safety margin) so that the relay can detect faults at Station R when the source at
Station S is in-service; this is the largest Zone 2 reach.
is an impedance diagram of the 345 kV tapped overhead transmis-
sion line; only the reactances (per unit) are shown.
To determine the greatest amount of overreach from a remote terminal during
reverse faults with respect to Station S, subtract the fault impedance from the cor-
responding apparent impedance measurement from
Calculate the overreach at Station R (SW-B open; SW-L and SW-R closed).
Equation 6.28
Calculate the overreach at Station T (SW-R open; SW-L and SW-B closed).
Equation 6.29
Table 6.18
Apparent Impedance Measurement for Remote Faults
Station
|ZAG|
|ZBC|
Relay at Station R, Fault at Station T 152.7
(0.128 per unit)
196.65
(0.165 per unit)
Relay at Station T, Fault at Station S
79.605
(0.418 per unit)
76.845
(0.404 per unit)
Relay at Station T, Fault at Station R 103.86
(0.545 per unit)
115.86
(0.608 per unit)
Figure 6.14
Impedance Diagram
0.0248
X1L1
X1L2
SW-L
SW-R
SW-B
X1L3
XH
XM
0.0372
0.108
-0.092
T
0.0248
S
R
Overreach
Z
APP
X
1L1
–
X
1L2
–
=
0.165 0.0248
–
0.0248
–
=
0.115 per-unit
=
Overreach
Z
APP
X
M
–
X
H
X
1L3
X
1L1
–
–
–
=
0.608
0.092
–
–
0.108 0.0372
–
0.0248
–
–
=
0.53 per-unit
=
Summary of Contents for SEL-421-4
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