Date Code 20001006 Loss-of-Potential, Load Encroachment, and Directional Element Logic 4-5
SEL-351 Instruction Manual
Convert Maximum Loads to Equivalent Secondary Impedances
Start with maximum forward load:
800 MVA + (1/3) = 267 MVA per phase
230 kV + (1/√3) = 132.8 kV line-to-neutral
267 MVA + (1/132.8 kV) + (1000kV/MV) = 2010 A primary
2010 A primary + (1/CT ratio) = 2010 A primary + (1 A seconday/400 A primary)
= 5.03 A secondary
132.8 kV + (1000 V/kV) = 132800 V primary
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 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°
Courtesy of NationalSwitchgear.com
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