Schweitzer Engineering SEL-351P - Convert Power Factors to Equivalent Load Angles; Apply Load-Encroachment Logic to a Phase Time-Overcurrent

Date Code 990430 Loss-of-Potential, Load Encroachment, and Directional Element Logic 4-5
SEL-351P Manual Técnico
132800 V primary • (1/PT ratio) = 132800 V primary • (1 V secondary/2000 V
primary)
= 66.4 V secondary
Now, calculate the equivalent secondary impedance:
66.4 V secondary/5.03 A secondary = 13.2 Ω secondary
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
Again, from
Figure SECTION 4: .2:
ZLOAD = ZLOUT + ZLIN
Refer to
Figure SECTION 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