Schweitzer Engineering Laboratories SEL-700G Series, Instruction Manual

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4.130
SEL-700G Relay Instruction Manual Date Code 20170814
Protection and Logic Functions
Group Settings (SET Command)
unbalance), which circulates as a result of line asymmetries, CT saturation
during three-phase faults, etc.
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
:
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-polarized
directional elements to be enabled (|I
0
| > 0.1 • |I
1
|).
Z0F/Z0R—Forward/Reverse Directional Z0 Threshold.
Z0F and Z0R
are used to calculate the forward and reverse thresholds, respectively, for the
zero-sequence voltage-polarized directional elements (see Figure 4.78 and
Figure 4.80). If the setting ORDER does not contain V (no zero-sequence
voltage-polarized directional element is enabled), then there is no need to
make the settings Z0F and Z0R. The relay also does not display these settings.
If the setting ORDERX is set to U, then Z0FX and Z0RX are hidden and
forced to -0.10 and 0.10, respectively, (refer to Figure 4.80) for use with the zero-
sequence voltage-polarized directional element with IN as operate quantity.
When ORDER is set to contain V and if the enable setting EDIR := Y, you
calculate and enter settings Z0F and Z0R (zero-sequence impedance values),
but setting Z0R must be greater in value than setting Z0F by 0.1

secondary.
If enable setting EDIR = AUTO, the relay calculates the settings Z0F and Z0R
(zero-sequence impedance values) automatically, using the zero-sequence line
impedance magnitude setting Z0MAG as follows:
Z0F = Z0MAG/2 (

Secondary)
Z0R = Z0MAG/2 + z (

Secondary; z listed in Table 4.36)
Deriving Z0F and Z0R Settings.
Figure 4.91 shows the voltage and current
polarity for an SEL-700G in a zero-sequence impedance network (the same
approach can be instructive for negative-sequence impedance analysis, too).
For a forward fault, the SEL-700G effectively sees the sequence impedance
behind it as:
Z
M
= V
0
/(
–
I
0
) =
–
(V
0
/I
0
)
V
0
/I
0
=
–
Z
M
(what the relay sees for a forward fault)
For a reverse fault, the SEL-700G effectively sees the sequence impedance in
front of it:
Z
N
= V
0
/I
0
V
0
/I
0
= Z
N
(what the relay sees for a reverse fault)
I
0
= I
G
/3 3I
0
= I
G
= I
A
+ I
B
+ I
C
Table 4.36 z Constant for Z0R Setting
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.