6.29
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
500 kV Parallel Transmission Lines With Mutual Coupling Example
Rearrange Equation 6.9 as follows to calculate RG1:
Equation 6.11
SEL recommends that you apply a safety margin of 50 percent because single
pole tripping is enabled for this particular application.
RG1 = 0.5 • 15.9 = 7.96
RG1 := 7.96. Zone 1 Resistance (0.05–50 secondary)
Zone 2 Reactance
Zone 2 quadrilateral 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.
XG2 = 1.2 • Z
1L1
= 1.2 • 3.98 = 4.78
XG2 := 4.78. Zone 2 Reactance (OFF, 0.05–64 secondary)
Zone 2 Resistance
Set Zone 2 quadrilateral resistive reach as follows:
Equation 6.12
RG2 := 12.00. Zone 2 Resistance (0.05–50 secondary)
Zone 3 Reactance
Zone 3 quadrilateral ground-distance reach must meet the same requirement as
that for Zone 3 mho phase-distance protection; it equals Zone 2 reach.
XG3 = XG2 = 4.78
XG3 := 4.78. Zone 3 Reactance (OFF, 0.05–64 secondary)
Zone 3 Resistance
The Zone 3 quadrilateral resistive reach is also scaled by an additional factor of
125 percent to ensure that it has greater coverage than the remote Zone 2 during
external resistive ground faults behind the local terminal.
RG3 = 1.25 • RG2 = 1.25 • 12.00 = 15
RG3 := 15.00. Zone 3 Resistance (0.05–50 secondary)
Quadrilateral Ground Polarizing Quantity
You must enter two final settings for quadrilateral ground-distance protection
because Advanced Settings are enabled. These settings are XGPOL and TANGG.
RG1 1 m– • 20 • X
1L1
=
10.8– • 20 • 3.977 =
15.9 =
RG2 XG2 •
RG1
XG1
------------=
4.78 •
7.96
3.18
-------------------=
11.97 =
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