6.7
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
230 kV Overhead Distribution Line Example
Phase-Distance Elements (21P)
Mho Phase-Distance Element Reach
Employ each zone of mho phase-distance protection as follows:
➤ Zone 1—Instantaneous underreaching tripping
➤ Zone 2—Time-delayed overreaching backup tripping
Zone 1 Phase-Distance Element Reach
Zone 1 phase-distance protection provides instantaneous protection for phase-to-
phase, phase-to-phase-to-ground, and three-phase faults in the first 80 percent of
the transmission line. Errors in the current transformers and potential transform-
ers, modeled transmission line data, and fault study data do not permit setting
Zone 1 for 100 percent of the transmission line. If you set Zone 1 for 100 percent
of the transmission line, unwanted tripping could occur for faults just beyond the
remote end of the line.
Set Zone 1 phase-distance protection equal to 80 percent of the transmission line
positive-sequence impedance.
Z1MP = 0.8 • Z1L = 1.56
Z1MP := 1.56. Zone 1 Reach (OFF, 0.05-64 secondary)
Zone 2 Phase-Distance Element Reach
Zone 2 phase-distance protection must have adequate reach to detect all phase-to-
phase, phase-to-phase-to-ground, and three-phase faults along the protected line
to make certain delayed tripping occurs for faults located in the last 20 percent of
the line. Set Zone 2 phase-distance reach equal to 120 percent of the positive-
sequence impedance of the transmission line.
Z2MP = 1.2 • Z1L = 2.34
Z2MP := 2.34. Zone 2 Reach (OFF, 0.05–64 secondary)
Ground-Distance Elements (21MG)
Mho Ground-Distance Element Reach
Employ each zone of mho ground-distance protection as follows:
➤ Zone 1—Instantaneous underreaching tripping
➤ Zone 2—Time-delayed overreaching backup tripping
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; i.e., the reach setting can be no greater
than 80 percent of the line.
Z1MG = 0.8 • Z1L = 1.56
Z1MG := 1.56. Zone 1 (OFF, 0.05–64 secondary)
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