Schweitzer Engineering Laboratories SEL-421-4 - Page 404

6.68
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
The transformer MVA rating is the maximum load. Assume that Station S can
supply the total load the autotransformer draws. Set load encroachment accord-
ing to maximum load for the protected line (4.2 A secondary). The bus voltage at
Station S is 65.7 V line-to-neutral during maximum load.
V
LN
= 65.7 V
I

= 4.2 A
Therefore, the minimum load impedance the relay measures is as follows:
Equation 6.33
Note that this load impedance is well inside the Zone 3 mho ground-distance ele-
ment reach, Z3MG, or 50.5 .
Multiply Z
load
by a safety factor of 80 percent to account for overload conditions.
Equation 6.34
Set the forward and reverse load impedance thresholds (ZLF and ZLR, respec-
tively) according to the minimum load impedance.
ZLF := 12.5. Forward Load Impedance (0.05–64  secondary)
ZLR := 12.5. Reverse Load Impedance (0.05–64  secondary)
To be conservative, assume a load angle range of ±45°. Assume both forward
(export) and reverse (import) load ranges to be the same.
PLAF := 45.0. Forward Load Positive Angle (–90.0 to +90.0 degrees)
NLAF := –45.0. Forward Load Negative Angle (–90.0 to +90.0 degrees)
PLAR := 135.0. Reverse Load Positive Angle (+90.0 to +270.0 degrees)
NLAR := 225.0. Reverse Load Negative Angle (+90.0 to +270.0 degrees)
Phase Instantaneous/Definite-Time Overcurrent Elements
Use 50P1, Level 1 phase instantaneous overcurrent element, as a nondirectional
high-set phase overcurrent element for SOTF protection. If the local circuit
breaker closes into a close-in three-phase bolted fault with line-side potential
transformers, the polarizing voltage for the phase-distance elements is zero.
Therefore, the distance protection does not operate. In this case, the 50P1 ele-
ment quickly trips the circuit breaker because this overcurrent element does not
rely on the polarizing voltage.
To rapidly clear faults, set 50P1P equal to 50 percent of the fault current mea-
sured at the local terminal for a close-in three-phase fault; use weak source condi-
tions to ensure the relay operates for low-level fault current.
50P1P := 49.80. Level 1 Pickup (OFF, 0.25–100 A secondary)
Z
load
V
LN
I

-----------=
65.7 V
4.2 A
-----------------=
15.6 =
Z
load
0.8 15.6 • =
12.5 =