4.70
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
50NFP—Forward Directional Neutral Ground Current Pickup
50NRP—Reverse Directional Neutral Ground Current Pickup
If setting ORDER does not contain S or U (zero-sequence voltage-polarized
directional elements: low-impedance or ungrounded/high-impedance
grounded, are not enabled) or the model does not have a 0.2 A nominal neutral
channel (IN), then settings 50NFP and 50NRP are not made or displayed.
NOTE: 50NFP and 50NRP (A
secondary) are set in terms of the
neutral current I
N
, from neutral
current channel IN. However, as
discussed in Internal Enables on
page 4.31, settings 50NFP and 50NRP
are applied to Figure 4.25,
Figure 4.29, and Figure 4.31, where
residual current I
G
(derived from
phase current channels IA, IB, and IC)
can be applied, depending on current
magnitudes. Settings 50NFP and
50NRP are adjusted internally to
operate on this residual current I
G
base, when needed (effectively,
50NFP • CTRN/CTR and 50NRP •
CTRN/CTR).
The 50NFP setting (I
N
current value) is the pickup for the forward fault detec-
tor 50NF of the zero-sequence voltage-polarized directional elements: low-
impedance or ungrounded/high-impedance grounded (see Figure 4.25). Ide-
ally, this setting is above normal load unbalance and below the lowest
expected zero-sequence current magnitude for unbalanced forward faults.
The 50NRP setting (I
N
current value) is the pickup for the reverse fault detec-
tor 50NR of the zero-sequence voltage-polarized directional elements: low-
impedance or ungrounded/high-impedance grounded (see Figure 4.25). Ide-
ally, this setting is above normal load/system unbalance and below the lowest
expected zero-sequence current magnitude for unbalanced reverse faults.
a0N—Positive-Sequence Current Restraint Factor, I
N
/I
1
If setting ORDER does not contain S or U (zero-sequence voltage-polarized
directional elements: low-impedance grounded or ungrounded/high-imped-
ance grounded, are not enabled) or the model does not have a 0.2 A nominal
neutral channel (IN), then setting a0N is not made or displayed.
Refer to Figure 4.25. The following comparison is made as part of internal
enable DIRNE (for low-impedance grounded and ungrounded/high-imped-
ance grounded systems):
|I
N
| > a0N • |I
1
|
I
N
is the secondary current measured by neutral channel IN. I
1
is the positive-
sequence secondary current derived from the phase current channels IA, IB, and
IC. Presumably, channel IN is connected in such a manner that it sees the sys-
tem zero-sequence current (e.g., channel IN is connected to a core-balance CT
through which the three phase conductors pass; in such a connection, channel
IN sees 3I
0
zero-sequence current, I
N
= 3I
0
; see Figure 2.29, Figure 2.31, and
Figure 2.32).
If a core-balance current transformer is connected to neutral channel IN, it
most likely has a different ratio, compared to the current transformers con-
nected to the phase current channels IA, IB, and IC (CT ratio settings CTRN and
CTR, respectively).
From a primary system study, load profile values, or metering values, derive
a0N as follows:
a0N = (3I
0
pri./I
1
pri.) • (CTR/CTRN)
3I
0
pri. = standing system unbalance current (zero-sequence; A primary)
I
1
pri. = maximum load current (positive-sequence; A primary)
Adjust the final setting value of a0N from the above derived value of a0N,
depending on your security philosophy, etc.
The a0N factor increases the security of the zero-sequence voltage-polarized
directional elements: low-impedance grounded or ungrounded/high-imped-
ance grounded. It keeps the elements from operating for zero-sequence current
(system unbalance), which circulates because of line asymmetries, etc.
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