Schweitzer Engineering Laboratories SEL-421-4 - Page 412

6.76
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
Calculate the effect of transformer low side and transformer tertiary impedances.
Equation 6.49
Use these assumptions to create a simplified form of the downstream parallel
impedance (from Equation 6.48).
Equation 6.50
Calculate the parallel impedance using Z
base
from Equation 6.45.
Equation 6.51
To determine whether the zero-sequence voltage-polarized 32V element always
operates correctly during reverse unbalanced faults, check the following condition:
Z0F < |Z
0L1
| + |Z
0P
|
6.44  < 6.44  + 3.8 
6.44  < 10.24 
The condition is satisfied; the reverse zero-sequence voltage-polarized direc-
tional element decision is correct during reverse unbalanced faults.
Pole-Open Detection
The setting EPO offers two options for deciding what conditions signify an open
pole, as listed in Table 6.19.
Z
0PP
Z
M
Z
0T
+Z
L

=
j
2
X
M
X
L
•
jX
M
X
L
+•
-----------------------------------=
j– 1– 0.092– 0.492• • •
0.092– 0.492+
-------------------------------------------------------------=
X
0PP
j0.113 per-unit, X
0PP
–=
Z
0P
X
0L3
X
H
X
0PP
++X
0L2


=
j
X
0L3
X
H
X
0PP
++X
0L2
•
X
0L3
X
H
X
0PP
++X
0L2
+
-----------------------------------------------------------------------



=
j
0.122 0.108 0.113–+0.081•
0.122 0.108 0.113–+0.081+
-----------------------------------------------------------------------------


=
j0.048 per unit=
Z
0P
Z
0P
Z
base
• =
j 0.048 79.35 • =
3.8  secondary=