Schweitzer Engineering Laboratories SEL-311C - Figure 4.4 Migration of Apparent Positive-Sequence Impedance for a Fault Condition; Use SEL-321 Relay Application Guide for the SEL-311 C

4.8
SEL-311C Relay Instruction Manual Date Code 20060320
Loss-of-Potential, CCVT Transient Detection, Load-Encroachment, and Directional Element Logic
Load-Encroachment Logic
Figure 4.4 Migration of Apparent Positive-Sequence Impedance for a Fault
Condition
Refer to Figure 4.4. In a load condition, the apparent positive-sequence
impedance is within the ZLOUT area, resulting in:
ZLOAD = ZLOUT + ZLIN = logical 1 + ZLIN = logical 1
If a three-phase fault occurs, the apparent positive-sequence impedance moves
outside the ZLOUT area (and stays outside the ZLIN area, too), resulting in:
ZLOAD = ZLOUT + ZLIN = logical 0 + logical 0 = logical 0
Refer to Figure 3.22. To prevent phase time-overcurrent element 51PT from
operating for high load conditions, make the following SEL
OGIC control
equation torque control setting:
51PTC = !ZLOAD
For a load condition (ZLOAD = logical 1), phase time-overcurrent element
51PT cannot operate with this torque control setting (regardless of the phase
current level):
51PTC = !(logical 1) = NOT(logical 1) = logical 0
For a fault condition (ZLOAD = logical 0), phase time-overcurrent element
51PT can operate:
51PTC = !ZLOAD = !(logical 0) = NOT(logical 0) = logical 1
Use SEL-321 Relay
Application Guide
for the SEL-311C
The load-encroachment logic and settings in the SEL-311C are the same as
those in the SEL-321. Refer to Application Guide 93-10: SEL-321 Relay
Load-Encroachment Function Setting Guidelines for applying the load-
encroachment logic in the SEL-311C.
X
1
Line Angle
Fault
52
Line
3-Phase Fault
R
1
ZLOUT
(Load Out
Region)
ZLIN
(Load In
Region)
SEL-311C
*
Load