Schweitzer Engineering Laboratories SEL-421-4 - Page 408

6.72
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
For 32V, if the apparent zero-sequence impedance measured by the relay (z0) is
less than Z0F, the unbalanced fault is declared forward. If z0 is greater than Z0R,
the unbalanced fault is declared reverse.
The SEL-421 automatically calculates these four settings as follows when
Advanced Settings are disabled and setting E32 is AUTO:
Equation 6.37
Equation 6.38
Equation 6.39
Equation 6.40
Perform the following two checks to make sure the ground directional element
does not incorrectly make a forward decision during a reverse unbalanced fault.
32QG Reverse Directional Check
You set Z1MAG equal to Z
1L1
plus Z
1L2
so the fault locator provides correct
results for internal faults not located on the tap (i.e., source T is extremely weak
and provides practically no infeed).
Figure 6.16 is the negative-sequence network for the 345 kV tapped overhead
transmission line. Assume that the negative-sequence impedances are equal to
the positive-sequence impedances.
Z2F 0.5 Z1MAG• =
0.5 4.00 • =
2.00 =
Z2R Z2F
0.5
I
NOM
--------------+=
2.00  0.10 +=
2.10 =
Z0F 0.5 Z0MAG• =
0.5 12.88 • =
6.44 =
Z0R Z0F
0.5
I
NOM
--------------+=
6.44  0.10 +=
6.54 =