Schweitzer Engineering Laboratories SEL-311L, Instruction Manual, Page: 152 / 734

4-6 Loss-of-Potential, CCVT Transient Detection, Date Code 20010625
Load-Encroachment, and Directional Element Logic
SEL-311L Instruction Manual
Settings Ranges
Refer to Figure 4.3.
Setting Description and Range
ZLF Forward Minimum Load Impedance—corresponding to maximum load flowing out
ZLR Reverse Minimum Load Impedance—corresponding to maximum load flowing in
0.05–64.00
8
secondary (5 A nominal phase current inputs, IA, IB, IC)
0.25–320.00
8
secondary (1 A nominal phase current inputs, IA, IB, IC)
PLAF Maximum Positive Load Angle Forward (-90
o
to +90
o
)
NLAF Maximum Negative Load Angle Forward (-90
o
to +90
o
)
PLAR Maximum Positive Load Angle Reverse (+90
o
to +270
o
)
NLAR Maximum Negative Load Angle Reverse (+90
o
to +270
o
)
Load-Encroachment Setting Example
Example system conditions:
Nominal Line-Line Voltage: 230 kV
Maximum Forward Load: 800 MVA
Maximum Reverse Load: 500 MVA
Power Factor (Forward Load): 0.90 lag to 0.95 lead
Power Factor (Reverse Load): 0.80 lag to 0.95 lead
CT ratio: 2000/5 = 400
PT ratio: 134000/67 = 2000
The PTs are connected line-to-neutral.
Convert Maximum Loads to Equivalent Secondary Impedances
Start with maximum forward load:
800 MVA • (1/3) = 267 MVA per phase
230 kV • (1/ ) = 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
8
secondary
This
8
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)]