Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

Page: 397 / 1518

6.61
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
Equation 6.22
Z1MP := 3.2 .
Zone 1 Reach (OFF, 0.05–64

secondary)
Zone 2 Phase-Distance Element Reach
Set Zone 2 phase-distance reach to include the tapped autotransformer.
Perform the following fault study to determine the apparent fault impedance the
SEL-421 distance elements measure for faults at the 138 kV terminals of the
autotransformer. Use these measurements to set the distance reach settings. Sta-
tion R should be in service to account for infeed. Place an AG and ABC fault at
the 138 kV terminals of the autotransformer and record the secondary voltage
and current the relay measures at Station S. Apply these quantities in
Equation 6.23 and Equation 6.26 , to determine the fault impedance the relay
measures for the two fault types shown in Table 6.17 . Use Equation 6.23 for an
A-Phase-to-ground fault and Equation 6.26 for the three-phase fault.
Equation 6.23
The relay uses the zero-sequence compensation factor to measure zero-sequence
quantities in terms of positive-sequence quantities.
Equation 6.24
The zero-sequence current is the sum of the phase currents.
Equation 6.25
The magnitude of the impedance for B-Phase-to-C-Phase, B-Phase-to-C-Phase-
to-ground, and three-phase faults is |Z
BC
|.
Equation 6.26
Z1MP 0.8 Z
1L1
Z
+
1L2
 
• =
0.8 2 2
+
  
•
 
=
3.2

=
where:
V
A
= A-Phase-to-neutral voltage
I
A
= A-Phase current
k
0
= zero-sequence compensation factor
3I
0
= zero-sequence current
where:
V
BC
= B-Phase-to-C-Phase voltage
I
BC
= B-Phase-to-C-Phase current
Z
AG
V
A
I
A
k
0
3I
0
• +
--------------------------------
=
k
0
Z
0L1
Z
1L1
–
3 Z
1L1
•
------------------------------ =
3I
0
I
A
I
B
I
C
+ + =
Z
BC
V
BC
I
BC
-----------
=