6.96
SEL-421 Relay
Instruction Manual
Date Code 20171021
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Rearrange
to calculate RG1:
Equation 6.56
RG1 :=
1.29
.
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 cable.
XG2 = 1.2 • |Z
1L1
| = 1.2 • 0.48 = 0.58
XG2 :=
0.58
.
Zone 2 Reactance (OFF, 0.05–64
secondary)
Zone 2 Resistance
Use the following formula to set RG2:
Equation 6.57
RG2 :=
1.97
.
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 = 0.58
XG3 :=
0.58
.
Zone 3 Reactance (OFF, 0.05–64
secondary)
Zone 3 Resistance
Set the Zone 3 resistance reach equal to Zone 2 resistance reach and multiply the
reach by 125 percent for a safety margin to account for external resistive ground
faults.
RG3 = 1.25 • RG2 = 1.25 • 1.97 = 2.46
RG3 :=
2.46
.
Zone 3 Resistance (0.05–50
secondary)
Z
1L1
= R
1L1
+ jX
1L1
= 0.48
42.5°
= 0.354 + j0.323
RG1
1 m
–
20 X
1L1
• •
=
1 0.8
–
20 0.323
• •
=
1.29
=
RG2
XG2
RG1
XG1
------------
•
=
0.58
1.29
0.38
-----------------
•
=
0.58
3.4
•
=
1.97
=
Summary of Contents for SEL-421-4
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