Date Code 20011205
Loss-of-Potential, CCVT Transient Detection,
4-7
Load-Encroachment, and Directional Element Logic
SEL-311A Instruction Manual
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 3.14 in
Section 3: Distance and Overcurrent Elements
. 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
Содержание SEL-311A
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