Date Code 20020215
Loss-of-Potential, Load Encroachment, and Directional Element Logic
4-5
SEL-351R Instruction Manual
This
Ω
secondary value can be calculated more expediently with the following equation:
[(line-line voltage in kV)
2
• (CT ratio)]/[(3-phase load in MVA) • (PT ratio)]
Again, for the maximum forward load:
[(230)
2
• (400)]/[(800) • (2000)] = 13.2
Ω
secondary
To provide a margin for setting ZLF, multiply by a factor of 0.9:
ZLF = 13.2
Ω
secondary • 0.9 = 11.90
Ω
secondary
For the maximum reverse load:
[(230)
2
• (400)]/[(500) • (2000)] = 21.1
Ω
secondary
Again, to provide a margin for setting ZLR:
ZLR = 21.1
Ω
secondary • 0.9 = 19.00
Ω
secondary
Convert Power Factors to Equivalent Load Angles
The power factor (forward load) can vary from 0.90 lag to 0.95 lead.
Setting PLAF = cos
-1
(0.90) = 26°
Setting NLAF = cos
-1
(0.95) = -18°
The power factor (reverse load) can vary from 0.80 lag to 0.95 lead.
Setting PLAR = 180° – cos
-1
(0.80) = 180° – 37° = 143°
Setting NLAR = 180° + cos
-1
(0.95) = 180° + 18° = 198°
Apply Load-Encroachment Logic to a Phase Time-Overcurrent
ZLOAD = ZLOUT + ZLIN
Refer to Figure 4.3. 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 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
Summary of Contents for SEL-351R
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