Schweitzer Engineering Laboratories SEL-421-4 - Page 191

5.31
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Functions
Loss-of-Potential Logic
(4) Zero-sequence current magnitude is not changing. Measure zero-sequence
current magnitude (|I
0k
|) and compare it to |I
0(k–1 cycle)
| from one cycle ear-
lier. If this difference is greater than six percent nominal current, the condi-
tion measured is not an LOP, even if all other conditions are met. This input
is labeled as |I
0
| > 6% in Figure 5.19.
(5) Zero-sequence current angle is not changing. Measure zero-sequence cur-
rent angle (I
0k
) and compare it to I
0(k–1 cycle)
. If this difference is greater
than 5 degrees, the condition measured is not an LOP even if all other condi-
tions are met. This input is labeled as I
0
> 5° in Figure 5.19. For security,
this declaration requires that |I
0
| be greater than five percent of nominal cur-
rent to override an LOP declaration.
If the criteria identified in all five steps listed above are met, the LOP logic
declares an LOP condition.
The relay resets LOP logic when the following conditions are true for 30 cycles:
1. A decreasing delta change in V
1
is less than 1
0
percent (see point (1)
above).
2. The magnitude of V
1
is larger than 85 percent of V
NOM
.
3. The magnitude of |V
0
| is not larger than 1
0
percent of magnitude |V
1
|.