Schweitzer Engineering Laboratories SEL-421-4 - Figure 6.16 345 Kv Tapped Line Negative-Sequence Network

6.73
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
Zn is an approximation of the impedance calculation for the n-sequence voltage-
polarized directional statement for a reverse fault.
If z2 is less than Z2F during a reverse unbalanced fault, 32QG incorrectly
declares that the fault is forward with respect to the relay location (CT shown in
Figure 6.16). The relay automatically sets Z2F equal to one-half Z1MAG.
Equation 6.41 is the apparent negative-sequence impedance z2 measured by the
32QG element during a reverse unbalanced fault.
Equation 6.41
The downstream parallel impedance, Z
2P
, is the Line 3 impedance, the trans-
former reactances, and the Bus R impedance.
Equation 6.42
Use the following two assumptions to simplify the calculations:
1. Assume the power system is purely reactive
2. Ignore source impedances Z
1R
and Z
1T
(a conservative assumption)
Figure 6.16 345 kV Tapped Line Negative-Sequence Network
where:
Z
2P
= parallel combination of the Line 3 impedance, transformer
reactances (neglect resistance), and the Bus T impedance with
Line 2 and the Bus R impedance
X
H
= transformer high-side winding reactance
X
M
= transformer low-side winding reactance
X
L
= transformer tertiary winding reactance
Z
1T
Z
M
Z
H
S
Z
1L3
Z
1L2
Z
1L1
I
2
I
2
Z
1R
Z
1S
R
T
SEL-421
z
0
Z
0L1
Z
0P
+=
Z
2P
Z
1L3
jX
H
jX
M
jX
1T
++ +=
Z
1L2
Z
1R
+
