Schweitzer Engineering Laboratories SEL-421-4 - Page 363

6.27
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
500 kV Parallel Transmission Lines With Mutual Coupling Example
Ground-Distance Elements (21MG and 21XG)
Mho Ground-Distance Element Reach
Employ each zone of distance protection as follows:
➤ Zone 1—Instantaneous underreaching direct tripping
➤ Zone 2—Forward-looking tripping elements for the POTT scheme
and backup tripping
➤ Zone 3—Current reversal guard for the POTT scheme, echo tripping,
and weak infeed logic
Zone 1 Mho Ground-Distance Element Reach
Zone 1 mho ground-distance reach must meet the same requirement as that for
Zone 1 mho phase-distance protection; the reach setting should be no greater than
80 percent of the line.
Z1MG = 0.8 • Z
1L1
= 3.18 
Z1MG := 3.18. Zone 1 (OFF, 0.05–64  secondary)
Zone 2 Mho Ground-Distance Element Reach
Zone 2 mho and ground-distance reach must meet the same requirement as that
for Zone 2 mho phase-distance protection; the reach setting is 120 percent of the
line.
Z2MG = 1.2 • Z1L1 = 4.78 
Z2MG := 4.78. Zone 2 (OFF, 0.05–64  secondary)
Zone 3 Mho Ground-Distance Element Reach
Zone 3 mho ground-distance reach must meet the same requirement as that for
Zone 3 mho phase-distance protection; it equals the Zone 2 reach.
Z3MG = Z2MG = 4.78 
Z3MG := 4.78. Zone 3 (OFF, 0.05–64  secondary)
Quadrilateral Ground-Distance Element Reach
The reactive reach for each zone of quadrilateral ground-distance protection lies
on the relay characteristic angle (Z1ANG), rather than on the ordinate (reactance)
of the impedance plane (see Figure 6.5).