Schweitzer Engineering Laboratories SEL-751 - Page 186

4.60
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
For setting a2 = 0.1, the negative-sequence current (I
2
) magnitude has to be
greater than 1/10 of the positive-sequence current (I
1
) magnitude in order for
the negative-sequence voltage-polarized directional elements to be enabled
(|I
2
| > 0.1 • |I
1
|).
k2—Zero-Sequence Current Restraint Factor, I
2
/I
0
Note the internal enable logic outputs in Figure 4.23:
➤ DIRQE—internal enable for the negative-sequence voltage-
polarized directional element that controls the negative-
sequence and phase overcurrent elements
➤ DIRQGE—internal enable for the negative-sequence voltage-
polarized directional element that controls the neutral ground
and residual ground overcurrent elements
The k2 factor is applied to internal enable DIRQGE. The negative-sequence
current (I
2
) magnitude has to be greater than the zero-sequence current (I
0
)
magnitude multiplied by k2 in order for the DIRQGE internal enable (and fol-
lowing negative-sequence voltage-polarized directional element in
Figure 4.26) to be enabled:
Equation 4.2
This check ensures that the relay uses the most robust analog quantities in
making directional decisions for the neutral-ground and residual-ground over-
current elements.
The zero-sequence current (I
0
), referred to in the above application of the k2
factor, is from the residual current (I
G
), which is derived from phase currents
I
A
, I
B
, and I
C
:
Equation 4.3
If both of the internal enables:
➤ DIRVE—internal enable for the zero-sequence voltage-
polarized directional element that controls the neutral-ground
and residual-ground overcurrent elements
➤ DIRIE—internal enable for the channel IN current-polarized
directional element that controls the neutral-ground and
residual-ground overcurrent elements
are deasserted, then factor k2 is ignored as a logic enable for the DIRQGE
internal enable. This effectively puts less restrictions on the operation of the
negative-sequence voltage-polarized directional element.
k2 Set Automatically. If enable setting EDIR := AUTO, setting k2 is set
automatically at:
k2 = 0.2
For setting k2 := 0.2, the negative-sequence current (I
2
) magnitude has to be
greater than 1/5 of the zero-sequence current (I
0
) magnitude in order for the
negative-sequence voltage-polarized directional elements to be enabled (|I
2
| >
0.2 • |I
0
|). Again, this presumes at least one of the internal enables DIRVE or
DIRIE is asserted.
I
2
k2 I
0
• 
I
0
I
G
3
-----=
3I
0
I
G
I
A
I
B
I
C
++==