Schweitzer Engineering Laboratories SEL-421-4 - Table 6.26 XAG Measurement for Remote AG Fault

6.101
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
The results in Tab le 6.26 show that the XAG calculation overreaches by 6.2 per-
cent (i.e., 100% – 93.8% = 6.2%) if the sheath is the return path for the ground
fault; therefore, you should not set k01 based on the sheath and ground (parallel)
return path.
Sheath Return Path
Table 6.27 lists the corresponding XAG (reactance of the phase-to-ground fault)
calculations for the remote single phase-to-ground fault for each of the three pos-
sible zero-sequence cable impedances when the k01 calculation is based on the
sheath return path.
The results in Table 6.27 show that there is no Zone 1 ground-distance overreach.
Therefore, set k01 based on Z
0L1(sheath)
.
k0M1 := 0.385. Zone 1 ZSC Factor Magnitude (AUTO, 0.000–10)
k0A1 := –46.7. Zone 1 ZSC Factor Angle (–180.0 to +180.0 degrees)
k0 and k0R
Set the forward (k0) and reverse (k0R) zero-sequence current compensation fac-
tors so that the overreaching zones of ground-distance protection do not under-
reach for any internal ground fault. Put both parallel cables in service. Table 6.28
lists the corresponding XAG calculations for a remote (Station R) ground fault
for each of the three possible zero-sequence cable impedances when the k0 calcu-
lation is based on Z
0L1(ground)
. (Replace k01 with k0 or replace k01 with k0R in
Equation 6.61 and Equation 6.63.)
The results of Table 6.28 show that the XAG calculation does not underreach
when you set k0 based on the ground return path. Set k0 and k0R based on
Z
0L1(ground)
.
Table 6.26 XAG Measurement for Remote AG Fault (k01 = 0.374 –39.2°,
Sheath and Ground Return Path)
Calculation Z0L1(sheath) Z0L1(ground) Z0L1(sheath and ground)
XAG (secondary ohms) 0.45  3.04  0.48 
Overreach/Underreach
a
a
O indicates overreach, U indicates underreach.
93.8% (O) 633% (U) 100%
Table 6.27 XAG Measurement for Remote AG Fault (k01 = 0.385 –46.7°,
Sheath Return Path)
Calculation Z0L1(sheath) Z0L1(ground) Z0L1(sheath and ground)
XAG (secondary ohms) 0.48  3.17  0.51 
Overreach/Underreach
a
a
O indicates overreach, U indicates underreach.
100% 660% (U) 106% (U)
Table 6.28 XAG Measurement for Remote AG Fault (k0 = 6.105 44.5°, Ground
Return Path)
Calculation Z0L1(sheath) Z0L1(ground) Z0L1(sheath and ground)
XAG (secondary ohms) 0.04  0.48  0.05 
Overreach/Underreach
a
a
O indicates overreach, U indicates underreach
8.33% (O) 100% 10.4% (O)