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
°
Summary of Contents for SEL-351
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