Schweitzer Engineering Laboratories SEL-351A - Page 140

4.14
SEL-351A Relay Instruction Manual Date Code 20080213
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control for Neutral-Ground and Residual-Ground Overcurrent Elements (Not in SEL-351A-1)
Internal Enables
Refer to Figure 4.4, Figure 4.5, Figure 4.6, Figure 4.7, and Figure 4.8.
Table 4.1 lists the internal enables and their correspondence to the ground
directional elements.
Note that Figure 4.6 has extra internal enable 32QE, which is used in the
directional element logic that controls negative-sequence and phase
overcurrent elements (see Figure 4.19).
Additionally, note that if enable setting ELOP = Y or Y1 and a loss-of-
potential condition occurs (Relay Word bit LOP asserts), all the internal
enables (except for 32IE) are disabled (see Figure 4.6, Figure 4.7, and
Figure 4.8). There is an exception when global setting VSCONN = 3V0,
which causes Relay Word bit 3V0 to be asserted. In that case, the directional
element enables in Figure 4.7 and Figure 4.8 are not affected by LOP. This is
explained in Loss-of-Potential Logic (Not in the SEL-351A-1) on page 4.1.
The channel IN current-polarized directional element (with corresponding
internal enabled 32IE; Figure 4.7) does not use voltage in making direction
decisions, thus a loss-of-potential condition does not disable the element.
Refer to Figure 4.1 and accompanying text for more information on loss-of-
potential.
The settings involved with the internal enables (e.g., settings a2, k2, a0, a0N)
are explained in Directional Control Settings (Not in SEL-351A-1) on
page 4.38.
Switch Between I
N
and I
G
for Low-Impedance Grounded and Ungrounded/
High-Impedance Grounded Systems
If an ungrounded or high-impedance grounded system (setting ORDER = U)
has appreciable circuit length, the capacitance levels can be such that
appreciable current flows for a ground fault. A low-impedance grounded
system (setting ORDER contains S) can also have appreciable current flow for
a ground fault.
Refer to Figure 4.8. The 0.2 A nominal neutral channel (IN) can discriminate
up to 5 A secondary. If neutral channel current I
N
exceeds 5 A secondary, then
the logic in Figure 4.8 (and Figure 4.12 and Figure 4.14) switches from
monitoring neutral channel current I
N
to monitoring residual-ground current
I
G
(residual-ground current I
G
is derived internally from phase current
channels IA, IB, and IC; I
G
is effectively 3I
0
and has a much higher upper range
than neutral channel current I
N
). Of course, this switching of currents (from I
N
to I
G
) requires the following 50NFP/50NRP settings (based on current I
N
) in
the Figure 4.8 logic to be effectively changed to the new I
G
base (done
internally with CT ratio settings):
50NFP • CTRN/CTR (I
G
base)
50NRP • CTRN/CTR (I
G
base)
If the logic in Figure 4.8 (and Figure 4.12, and Figure 4.14) operates on
neutral current I
N
, then settings 50NFP and 50NRP are not adjusted, and just
operate as:
50NFP (I
N
base)
50NRP (I
N
base)
This transition is “seamless” if the lower detection threshold of the residual-
ground current I
G
(0.05 A secondary for 5 A nominal phase; 0.01 A secondary
for 1 A nominal) effectively overlaps with the upper detection threshold of
neutral channel current I
N
(5 A secondary):
Courtesy of NationalSwitchgear.com