Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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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
Z1T
ZM
ZH
S
Z1L3
Z1L2 Z1L1
I2
I2
Z1R
Z1S
R
T
SEL-421
z
0
Z
0L1
Z
0P
+ =
Z
2P
Z
1L3
jX
H
jX
M
jX
1T
+ + +
 
=
Z
1L2
Z
1R
+
 
