6.32
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
500 kV Parallel Transmission Lines With Mutual Coupling Example
Set TANGG to prevent the Zone 1 quadrilateral ground-distance reactance mea-
surement from overreaching for ground faults located at the remote bus.
Equation 6.15 (T
2
) from Quadrilateral Ground Polarizing Quantity was approxi-
mately zero. Therefore, set TANGG equal to zero.
TANGG := 0. Nonhomogeneous Correction Angle (–40.0 to +40.0 degrees)
Zero-Sequence Current Compensation Factors
Zero-sequence current compensation helps to keep the phase and ground-
distance elements at the same reach if you set the reach equal per zone (for exam-
ple, Z1MP = Z1MG). Ground-distance elements should measure fault impedance
in terms of positive-sequence impedance only.
The relay has three zero-sequence current compensation factors (k01, k0, and
k0R). The Zone 1 ground-distance element has a dedicated zero-sequence current
compensation factor (k01). Advanced Settings are enabled for this particular
example; set two independent zero-sequence current compensation factors, one
for forward-looking (k0) zones and one for reverse-looking (k0R) zones.
The SEL-421 ground-distance elements do not employ zero-sequence mutual
coupling compensation. Zero-sequence mutual coupling can cause under/over-
reaching problems on both the faulted line and the nonfaulted line relaying termi-
nals for parallel line applications employing ground-distance elements. Set the
residual current compensation factors k0 and k0R appropriately to compensate
for the effect of mutual coupling on parallel lines.
Apply the following expression for the Zone 1 zero-sequence current compensa-
tion factor.
Equation 6.16
k0M1 := 0.880. Zone 1 ZSC Factor Magnitude (AUTO, 0.000–10)
k0A1 := -7.60. Zone 1 ZSC Factor Angle (–180.0 to +180.0 degrees)
Figure 6.8 Nonhomogeneous Angle Setting
X
R
TANGG > 0
TANGG = 0
TANGG < 0
k01
Z
0L1
Z
1L1
–
3 • Z
1L1
------------------------------=
14.48 82.1 3.98 87.6–
3 • 3.98 87.6
-----------------------------------------------------------------------------------=
0.88 –7.6=
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