Schweitzer Engineering Laboratories SEL-351A, Manual

Page: 174 / 666

4.48
SEL-351A Relay Instruction Manual
Date Code 20080213
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control Settings (Not in SEL-351A-1)
50NFP—Forward Directional Neutral-Ground Current Pickup
50NRP—Reverse Directional Neutral-Ground Current Pickup
Setting Range:
0.005–5.00 A secondary (0.2 A nominal neutral channel input, IN )
If preceding 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.
The 50NFP setting (I
N
current value) is the pickup for the forward fault
detector 50NF of the zero-sequence voltage-polarized directional elements:
low-impedance or ungrounded/high-impedance grounded (see
Figure 4.8 ).
Ideally, 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
detector 50NR of the zero-sequence voltage-polarized directional elements:
low-impedance or ungrounded/high-impedance grounded (see
Figure 4.8 ).
Ideally, 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
Setting Range:
0.001–0.500 (unitless)
If preceding setting ORDER does not contain S or U (zero-sequence voltage-
polarized directional elements: low-impedance grounded or ungrounded/high-
impedance 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 4.8 . The following comparison is made as part of internal
enable 32NE (for low-impedance grounded and ungrounded/high-impedance
grounded systems):
Equation 4.16
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
system 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 2.20 ,
Figure 2.22 , and Figure 2.23 ).
If a core-balance current transformer is connected to neutral channel IN , it
most likely has a different ratio, compared to the current transformers
connected to the phase current channels
IA
,
IB
, and
IC
(CT ratio settings CTRN
and CTR, respectively).
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 4.30 , settings 50NFP and
50NRP are applied to Figure 4.8 ,
Figure 4.12 , and Figure 4.14 , 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).
I
N
a0N I
1
•
>
Courtesy of NationalSwitchgear.com