6.94
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Set Zone 1 phase-distance protection to 80 percent of the cable positive-sequence
impedance.
Z1MP = 0.8 • Z
1L1
= 0.8 • 0.48 = 0.38
Z1MP := 0.38. 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
cable. Set Zone 2 phase-distance protection to 120 percent of the cable positive-
sequence impedance. With this reach, high-speed tripping occurs via the commu-
nications channel for faults located in the last 20 percent of the cable.
Z2MP = 1.2 • Z
1L1
= 1.2 • 0.48 = 0.58
Z2MP := 0.58. 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 when the parallel line is in service because this
example uses a POTT scheme. So that Zone 3 has greater fault coverage than
Zone 2 at the remote terminal, set the reach to remote Zone 2 and rely upon the
length of the protected cable as the safety margin.
Z3MP = Z2P = 0.58
Z3MP := 0.58. Zone 3 Reach (OFF, 0.05–64 secondary)
Ground-Distance Elements (21XG)
Quadrilateral Ground-Distance Element Reach
The main advantage of ground-distance protection is that Zone 1 provides instan-
taneous protection independent of the communications channel. Typically cable
faults have little fault resistance; it is advantageous to conservatively set the resis-
tance reach for quadrilateral ground-distance protection. Supplement quadrilat-
eral ground-distance protection with directional negative-sequence overcurrent
elements. The directional negative-sequence overcurrent elements employed in
the communications-assisted tripping scheme provide excellent resistive cover-
age for high-resistance ground faults (e.g., a contaminated pothead flashes over).
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 6.21).
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