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Date Code 20011205
Loss-of-Potential, CCVT Transient Detection,
4-7
Load-Encroachment, and Directional Element Logic
SEL-311B Instruction Manual
Now, calculate the equivalent secondary impedance:
66.4 V secondary / 5.03 A secondary = 13.2
W
secondary
This
W
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
W
secondary
To provide a margin for setting ZLF, multiply by a factor of 0.9:
ZLF = 13.2
W
secondary • 0.9 = 11.90
W
secondary
For the maximum reverse load:
[(230)
2
• (400)] / [(500) • (2000)] = 21.1
W
secondary
Again, to provide a margin for setting ZLR:
ZLR = 21.1
W
secondary • 0.9 = 19.00
W
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 4.3:
ZLOAD = ZLOUT + ZLIN
Содержание SEL-311B
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