4.63
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
The a0 factor increases the security of the zero-sequence voltage-polarized
and channel IN current-polarized directional elements. This factor keeps the
elements from operating for zero-sequence current (system unbalance), which
circulates because of line asymmetries, CT saturation during three-phase
faults, etc. Refer to Figure 4.24.
The zero-sequence current (I
0
), referred to in the application of the a0 factor,
is from the residual current (I
G
), which is derived from phase currents I
A
, I
B
,
and I
C
:
Equation 4.4
a0 Set Automatically. If enable setting EDIR := AUTO, setting a0 is set
automatically at:
a0 := 0.1 For setting a0 := 0.1, the zero-sequence current (I
0
) magnitude
has to be greater than 1/10 of the positive-sequence current (I
1
) magnitude in
order for the zero-sequence voltage-polarized and channel IN current-polar-
ized directional elements to be enabled (|I
0
| > 0.1 • |I
1
|).
Z0F—Forward Directional Z0 Threshold
Z0R—Reverse Directional Z0 Threshold
If setting ORDER does not contain V or S (no zero-sequence voltage-polar-
ized directional element is enabled), then settings Z0F and Z0R are not made
by the user or displayed.
Z0F and Z0R are used to calculate the Forward and Reverse Thresholds,
respectively, for the zero-sequence voltage-polarized directional elements (see
Figure 4.27 and Figure 4.29).
If enable setting EDIR := Y, settings Z0F and Z0R (zero-sequence impedance
values) are calculated by the user and entered by the user, but setting Z0R
must be greater in value than setting Z0F by 0.1 secondary.
Z0F and Z0R Set Automatically. If enable setting EDIR := AUTO, set-
tings Z0F and Z0R (zero-sequence impedance values) are calculated automat-
ically, using the zero-sequence line impedance magnitude setting Z0MAG as
follows:
Z0F = Z0MAG/2 ( secondary)
Z0R = Z0MAG/2 + z ( secondary; “z” listed in table below)
If setting ORDER := U (ungrounded or high-impedance grounded system; see
Figure 4.31), the following settings are made internally and hidden:
Z0F = -0.10 secondary
Z0R = 0.10 secondary
I
0
I
G
3
-----=
3I
0
I
G
I
A
I
B
I
C
++==
Relay Configuration z ( secondary)
5 A nominal current 0.2
1 A nominal current 1.0
NOTE: If Z0F or Z0R exceeds the
setting range, the quantity is set to
the upper limit of the setting range.
NOTE: Z0F and Z0R ( secondary)
are set in reference to the phase
current channels IA, IB, and IC, as are
settings Z2F and Z2R. However,
settings Z0F and Z0R are applied to
Figure 4.29, and Figure 4.31, where
neutral current I
N
, from neutral
current channel IN, is also applied.
Settings Z0F and Z0R are adjusted
internally (with CT ratio settings) to
operate on this I
N
current base, when
needed (effectively, Z0F • CTRN/CTR
and Z0R • CTRN/CTR). See Internal
Enables on page 4.31.
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