Schweitzer Engineering Laboratories SEL-421-4 - Page 432

6.96
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Rearrange Equation 6.55 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
• • =
10.8–20 0.323 • • =
1.29 =
RG2 XG2
RG1
XG1
------------
• =
0.58 
1.29 
0.38 
-----------------•


=
0.58  3.4• =
1.97 =