4.31
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
Internal Enables
Refer to Figure 4.20, Figure 4.21, Figure 4.23, Figure 4.24, and Figure 4.25.
Table 4.19 lists the internal enables and their correspondence to the ground
directional elements.
Note that Figure 4.23 has extra internal enable DIRQE, which is used in the
directional element logic that controls negative-sequence and phase overcur-
rent elements (see Figure 4.36).
Also, note that if a loss-of-potential condition occurs (Relay Word bit LOP
asserts), all the internal directional enables (except for DIRIE) are disabled
(see Figure 4.23, Figure 4.24, and Figure 4.25), unless VSCONN = 3V0. In
that case, the directional-element enables in Figure 4.24 and Figure 4.25 are
not affected by LOP.
The channel IN current-polarized directional element (with corresponding
internal enable DIRIE; Figure 4.24) does not use voltage in making direction
decisions, thus a loss-of-potential condition does not disable the element.
Refer to Figure 4.73 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 on page 4.53.
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 appre-
ciable 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.
The 0.2 A nominal neutral channel (IN) can measure up to 5 A secondary.
Under certain conditions, the logic in Figure 4.25 (and Figure 4.29 and
Figure 4.31) switches from monitoring neutral channel current I
N
to monitor-
ing residual ground current I
G
. Residual ground current I
G
is derived inter-
nally 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
. As shown in
Figure 4.22, the relay uses the settings CTR and CTRN, along with the magni-
tudes of I
G
and I
N
, to determine when current I
N
might exceed 5 amperes.
When such a condition is detected, the relay switches to I
G
. The switching
logic is designed such that the switch may occur when neutral current is less
than 5 amperes.
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