Schweitzer Engineering Laboratories SEL-351-5, Instruction Manual

Page: 150 / 706

4.6
SEL-351-5, -6, -7 Relay Instruction Manual
Date Code 20070117
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Load-Encroachment Logic
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) • (1000 kV/MV) = 2010 A primary
2010 A primary • (1/CT ratio) = 2010 A primary • (1 A secondary/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
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
(0.95) = 180° – 18° = 162°
Setting NLAR = 180° + cos
-1
(0.80) = 180° + 37° = 217°
Apply Load-Encroachment Logic to a Nondirectional Phase Time-
Overcurrent
Again, from Figure 4.2 :
ZLOAD =
ZLOUT + ZLIN