Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

Page: 190 / 1518

5.30
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Loss-of-Potential Logic
The following text gives additional description of the steps shown in Figure 5.18 :
NOTE:
When an enabled breaker is
set to single-pole open mode, and a
single-pole open condition (SPO)
occurs, the open-pole voltages are
replaced with 0 in the positive-
sequence voltage calculation.
(1) Magnitude of positive-sequence voltage is decreasing.
Measure positive-
sequence voltage magnitude (called |V
1(k)
|, where k represents the present
processing interval result) and compare it to |V
1
| from one power system
cycle earlier (called |V
1(k–1 cycle)
|). If |V
1(k)
| is less than or equal to 90 per-
cent |V
1(k-1 cycle)
|, assert LOP if all of the conditions in the next four steps
are satisfied. This is the decreasing delta change in V
1
(–

|V
1
| > 10%) shown
as an input in the logic diagram in Figure 5.19 .
(2) Positive-sequence current magnitude not changing.
Measure positive-
sequence current magnitude (|I
1(k)
|) and compare it to |I
1(k–1 cycle)
| from one
cycle earlier. If this difference is greater than two percent nominal current,
the condition measured is not an LOP, even if all other conditions are met.
This input is labeled as

|I
1
| > 2% in Figure 5.19 .
(3) Positive-sequence current angle is not changing.
Measure positive-
sequence current angle (

I
1k
) and compare it to

I
1(k–1 cycle)
from one cycle
earlier. If this difference is greater than 5 degrees, the condition measured is
not an LOP, even if all other conditions are met. This input is labeled as

I
1
> 5° in Figure 5.19 . If |I
1
| is less than five percent nominal current
(I
NOM
), this angle check does not block LOP.
Figure 5.18 LOP Logic Process Overview
START
|V1|
decreasing?
∠
I1
changing?
|I1|
changing?
|3I0|
changing?
END
Declare
LOP
2
1
3
4
Y
Y
Y Y
N
N
N
N
5
Y
N
∠
3I0
changing?