4.5
Date Code 20080213 Instruction Manual SEL-351A Relay
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Load-Encroachment Logic (Not in SEL-351A-1)
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.
Load-Encroachment
Setting Example
Example system conditions:
The PTs are connected line-to-neutral.
Convert Maximum Loads to Equivalent Secondary Impedances
Start with maximum forward load:
Equation 4.1
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
0.10–128.00 Ω secondary (5 A nominal phase current inputs, IA, IB, IC)
0.50–640.00 Ω secondary (1 A nominal phase current inputs, IA, IB, IC)
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°)
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
800 MVA
1
3
---
⎝⎠
⎛⎞
• 267 MVA per phase=
Courtesy of NationalSwitchgear.com
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