Schweitzer Engineering Laboratories SEL-751 - Figure 4.29 Zero-Sequence Voltage-Polarized Directional Element (Low-Impedance Grounded Systems)

4.42
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
q From Figure 4.30 and Figure 4.31; w to Figure 4.32 and Figure 4.33;
e from Figure 4.25.
Figure 4.29 Zero-Sequence Voltage-Polarized Directional Element (Low-
Impedance Grounded Systems)
The 3V0 input to Figure 4.29 may be either a calculated value (when
VSCONN := VS and DELTA_Y := WYE) or a measured value (when
VSCONN := 3V0). See Zero-Sequence Voltage Sources on page 4.33.
I
N
3V
0
If Z0F Setting > 0, Forward Threshold = 1.25 • Z0F — 0.25 •
I
N
3V
0
If Z0F Setting ≤ 0, Forward Threshold = 0.75 • Z0F — 0.25 •
Forward Threshold:
I
N
3V
0
If Z0R Setting < 0, Reverse Threshold = 1.25 • Z0R + 0.25 •
I
N
3V
0
If Z0R Setting ≥ 0, Reverse Threshold = 0.75 • Z0R + 0.25 •
Reverse Threshold:
Direction Element Characteristics
R0
X0
Forward Threshold
Reverse Threshold
Z0 PLANE
RDIRN
FDIRN
Forward
Reverse
"S" listed in
setting ORDER
50NF
Enable
Forward
Threshold
DIRNE
Reverse
Threshold
50NR
Re[3V
0
•(I
N
•1∠Z0MTA)*]
|I
N
|
2
Z0 =
I
N
3V
0
Relay
Word
Bits
Relay
Word
Bit
Relay
Word
Bit
Relay
Word
Bit
"Forward"
Outputs
"Reverse"
Outputs
1
1
2
3
3
3
NOTE: Residual ground current I
G
is
used in place of neutral current I
N
under certain conditions. See Switch
Between I
N
and I
G
for Low-Impedance
Grounded and Ungrounded/High-
Impedance Grounded Systems on
page 4.31