Schweitzer Engineering Laboratories SEL-421-4 - Page 400

6.64
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
Station T has the greatest overreach. Use Equation 6.30 to set Zone 3 phase-
distance element reach.
Equation 6.30
where:
Equation 6.31
Z3MP := 50.50. Zone 3 Reach (OFF, 0.05–64  secondary)
Ground-Distance Elements (21MG and 21XG)
Mho Ground-Distance Element Reach
Employ each zone of mho ground-distance protection as follows:
➤ Zone 1—Instantaneous underreaching tripping
➤ Zone 2—DCB scheme tripping
➤ Zone 3—DCB scheme blocking
Zone 1 Mho Ground-Distance Element Reach
Zone 1 mho ground-distance element reach must meet the same requirement as
that for Zone 1 mho phase-distance protection; the reach setting can be no greater
than 80 percent of the protected line.
Z1MG = Z1MP = 3.2 
Z1MG := 3.20. Zone 1 (OFF, 0.05–64  secondary)
Zone 2 Mho Ground-Distance Element Reach
Set Zone 2 ground-distance element reach equal to Zone 2 phase-distance ele-
ment reach; this ensures that Zone 2 ground-distance elements can see faults
internal to the tapped autotransformer. Zone 2 phase-distance element reach was
set to see the largest apparent fault impedance for faults at the 138 kV terminal of
the tapped autotransformer.
Z2MG = Z2MP = 11 
Z2MG := 11.00. Zone 2 (OFF, 0.05–64  secondary)
Zone 3 Mho Ground-Distance Element Reach
Set Zone 3 ground-distance element reach equal to Zone 3 phase-distance ele-
ment reach; this ensures that Zone 3 ground-distance elements coordinate with
the remote Zone 2 ground-distance elements at Station R and Station T for out-
Z3MP
CTR
PTR
------------=% Zper-unitZ
base
120%• •
200
3000
------------ 0.53 1190.25 1.2• • • =
50.5 =
Z
base
345kV
2
100 MVA
-------------------------=
1190.25 =