Sel SEL-421 - Page 67

R.1.25
Date Code 20111215 Reference Manual SEL-421 Relay
Protection Functions
Loss-of-Potential Logic
The following text gives additional description of the steps shown in
Figure 1.16:
(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 percent |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 of Figure 1.17.
(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 1.17.
(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 1.17. If |I
1
| is less than five percent
nominal current (I
nom
), this angle check does not block LOP.
(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 earlier. If this difference is greater than six percent nominal
current, the condition measured is not an LOP, even if all other
conditions are met. This input is labeled as |I
0
| > 6% in Figure 1.17.
(5) Zero-sequence current angle is not changing. Measure zero-sequence
current 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 conditions are met. This input is labeled as I
0
> 5° in
Figure 1.17. For security, this declaration requires that |I
0
| be greater
than five percent of nominal current 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 10 percent (see
point (1) above), and
2. the magnitude of V
1
is larger than 85 percent of Vnom, and
3. the magnitude of |V
0
| is not larger than 10 percent of magnitude
|V
1
|