6.26
SEL-421 Relay
Instruction Manual
Date Code 20171021
Protection Applications Examples
500 kV Parallel Transmission Lines With Mutual Coupling Example
Phase-Distance Elements (21P)
Mho Phase-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 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, potential transformers,
modeled transmission line data, and fault study data do not permit setting of
Zone 1 for 100 percent of the transmission line. Unwanted tripping could occur
for faults just beyond the remote end of the line if you set Zone 1 for 100 percent
of the transmission line.
Set Zone 1 phase-distance protection equal to 80 percent of the transmission line
positive-sequence impedance.
Z1MP = 0.8 • Z1L1 = 3.18
Z1MP :=
3.18
.
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.
Set Zone 2 phase-distance reach to 120 percent of the positive-sequence imped-
ance of the transmission line. This setting provides high-speed tripping via the
communications channel for faults located in the last 20 percent of the line.
Z2MP = 1.2 • Z1L1 = 4.78
Z2MP :=
4.78
.
Zone 2 Reach (OFF, 0.05–64
secondary)
Zone 3 Phase-Distance Element Reach
Zone 3 phase-distance protection must have adequate reach to prevent unwanted
tripping during current reversals (this application example uses a permissive
overreaching transfer tripping (POTT) scheme). Set the Zone 3 reach equal to
Zone 2 and rely on the length of the protected transmission line for the safety
margin. This setting makes the Zone 3 fault coverage greater than the Zone 2
fault coverage at the remote terminal.
Z3MP = Z2MP = 4.78
Z3MP :=
4.78
.
Zone 3 Reach (OFF, 0.05–64
secondary)
Summary of Contents for SEL-421-4
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