Schweitzer Engineering Laboratories SEL-421-4 - Page 410

6.74
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
Calculate the transformer reactances.
Equation 6.43
Use these assumptions from Equation 6.42 to create a simplified form of the
downstream parallel impedance.
Equation 6.44
The secondary base impedance is calculated as follows:
Equation 6.45
Calculate the parallel impedance in secondary ohms.
Equation 6.46
To determine whether the 32QG element always operates correctly during
reverse unbalanced faults, check the following condition:
Z2F < |Z
1L1
| + |Z
2P
|
2  < 2  + 1.35 
2  < 3.35 
The condition is satisfied; the reverse negative-sequence voltage-polarized direc-
tional element decision is correct during reverse unbalanced faults.
X
H
0.5 X
HM
X
HL
X
ML
–+• =
0.5 0.016 0.6 0.4–+• =
0.108 per-unit=
X
M
0.5 X
HM
X
ML
X
HL
–+• =
0.5 0.016 0.4 0.6–+• =
0.092– per-unit=
X
L
0.5 X
HL
X
ML
X
HM
–+• =
0.5 0.6 0.4 0.016–+• =
0.492 per-unit=
Z
2P
jX
1L3
X
H
X
M
++X
1L2


=
j
X
1L3
X
H
X
M
++X
1L2
•
X
1L3
X
H
X
M
++X
IL2
+
-------------------------------------------------------------------



=
j
0.038 0.108 0.092–+0.025•
0.038 0.108 0.092–+0.025+
-----------------------------------------------------------------------------


=
j0.017 per-unit primary=
Z
base
CTR 345 kV
2
•
PTR 100 MVA•
---------------------------------------------=
200 345 kV
2
•
3000 100 MVA•
--------------------------------------------=
79.35 =
Z
2P ondarysec
Z
2P primary
Z
base
• =
J0.017 79.35 • =
1.35  secondary=