4-4 Loss-of-Potential, Load Encroachment, and Directional Element Logic Date Code 20001006
SEL-351 Instruction Manual
Note that a positive-sequence impedance calculation (Z
1
) is made in the load-encroachment logic in
Figure 4.2. Load is largely a balanced condition, so apparent positive-sequence impedance is a
good load measure. The load-encroachment logic only operates if the positive-sequence current (I
1
)
is greater than the Positive-Sequence Threshold defined in Figure 4.2. For a balanced load
condition, I
1
= phase current magnitude.
Forward load (load flowing out) lies within the hatched region labeled ZLOUT. Relay Word bit
ZLOUT asserts to logical 1 when the load lies within this hatched region.
Reverse load (load flowing in) lies within the hatched region labeled ZLIN. Relay Word bit ZLIN
asserts to logical 1 when the load lies within this hatched region.
Relay Word bit ZLOAD is the OR-combination of ZLOUT and ZLIN:
ZLOAD = ZLOUT + ZLIN
Settings Ranges
Refer to Figure 4.2.
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 W secondary (5 A nominal phase current inputs, IA, IB, IC) {150 V voltage
inputs}
0.10 - 128.00 W secondary (5 A nominal phase current inputs, IA, IB, IC) {300 V
voltage inputs}
0.25 - 320.00 W secondary (1 A nominal phase current inputs, IA, IB, IC) {150 V
voltage inputs}
0.50 - 640.00 W secondary (1 A nominal phase current inputs, IA, IB, IC) {300 V
voltage inputs}
PLAF Maximum Positive Load Angle Forward (-90° to +90°)
NLAF Maximum Negative Load Angle Forward (-90° to +90°)
PLAR Maximum Positive Load Angle Reverse (+90° to +270°)
NLAR Maximum Negative Load Angle Reverse (+90° to +270°)
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.
Courtesy of NationalSwitchgear.com
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