Schweitzer Engineering Laboratories SEL-421-4 - Page 436

6.100
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Equation 6.62
Equation 6.63
k01
You can set k01 based on three values for the zero-sequence cable impedance:
Z
0L1(sheath)
, Z
0L1(ground)
, or Z
0L1(sheath and ground)
. Select the zero-sequence cable
impedance that prevents Zone 1 ground-distance element overreach.
To determine the best setting for k01, place an A-Phase-to-ground fault at
Station R with the parallel cable out of service. Find the ground-distance reac-
tance measurement XAG that does not overreach for this fault. Perform this eval-
uation using Z
0L1(sheath and ground)
and Z
0L1(sheath)
for the zero-sequence cable
impedance. There is no need to determine the XAG measurement for ground
faults at the remote terminal when k01 is set based on Z
0L1(ground)
because severe
overreach occurs in all cases for the ground-only path.
Sheath and Ground Return Path
First apply Equation 6.61 with k01 based on Z
0L1(sheath and ground)
(k01 equal to
0.374 –39.2°). This is the most common ground fault return path. Set TANGG
equal to zero and assume that IPOL is equal to negative-sequence current (i.e.,
XGPOL is equal to I2).
Table 6.26 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
parallel return path. Use Equation 6.64 to determine the amount of overreach/
underreach:
Equation 6.64
where:
V
A
= A-Phase-to-ground voltage measured at Station S
I
A
= A-Phase current measured through Cable 1 at Station S
3I
0
= zero-sequence current measured through Cable 1 at Station S
I
POL
= negative-sequence or zero-sequence current measured through
Cable 1 at Station S (based on the XGPOL setting, see Quadri-
lateral Ground Polarizing Quantity on page 6.29)
TANGG = nonhomogeneous correction angle
Im[ ] = imaginary part
* = complex conjugate
where:
Z
1
= Cable 1 positive-sequence impedance
z1
Z
1
Z
1
---------=
k01
Z
0L1
Z
1L1
–
3Z
1L1
•
------------------------------=
Overreach/Underreach
XAG
Z
1L1
---------------
100% • =