Schweitzer Engineering Laboratories SEL-351-5, Instruction Manual

Page: 196 / 706

4.52
SEL-351-5, -6, -7 Relay Instruction Manual
Date Code 20070117
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control Settings
Figure 4.28 shows the zero-sequence vector relationships described above for
Figure 4.27 (note: the zero-sequence currents I
0(1)
and I
0(2)
are what the relays
respectively “see,” per standard current transformer connections—see
Figure 2.19 ). The vectors shown in Figure 4.28 are perhaps somewhat
overdramatic as far as angle differences—they are primarily for illustrative
purposes.
There is always some resistance in a circuit and thus the V
0
and I
0
vector
relationship is not 90 degrees, as shown in Figure 4.28 . This system resistance
provides the “real power component” with which the wattmetric directional
element (
Figure 4.13 ) operates. Whether the zero-sequence network behind
Relay 1 appears net capacitive or net inductive, the wattmetric (real power)
portion for Relay 1/faulted Feeder 1 (labeled “WF”) is polar-opposite of the
wattmetric (real power) portion for Relay 2/unfaulted Feeder 2 (labeled
“WR”). The calculations for the 32WFP and 32WRP wattmetric pickups are
made as follows:
Real {3V
0
• conjugate (3I
0
)} = |3V
0
| • |3I
0
| • cos(
∠
3V
0
–
∠
3I
0
)
= |3V
0
| •|I
N
|•cos(
∠
3V
0
–
∠
I
N
)
The cosine part of the above calculation reveals forward or reverse fault
direction: forward faults produce negative calculation values and reverse faults
produce positive calculation values on Petersen Coil-grounded systems.
Calculate the 32WFP and 32WRP wattmetric pickup settings (in watts
secondary), with a margin of more sensitivity than the minimum detected
ground faults (forward and reverse, respectively). Enter wattmetric settings as
positive values.
Figure 4.28 Wattmetric Element Operation for Ground Fault on Feeder 1
The sum of settings 32WFP and 32WRP must be 0.1 watts secondary or
greater:
32WFP + 32WRP
≥
0.1 watts secondary
In Figure 4.28 , the calculated wattmetric value for a forward fault is a negative
value (shown as WF), while that for a reverse fault is a positive value (shown
as WR). Again, corresponding settings 32WFP and 32WRP are both entered
as positive values, with some margin of sensitivity. The above “0.1 watts
secondary” rule is effectively the minimum distance between settings 32WFP
and 32WRP in the wattmetric plane (setting 32WFP is put on the “negative”
side of the wattmetric plane: i.e., “–32WFP”; see
Figure 4.13 ).
Net capacitive behind
Relay 1 (under-
compensated system)
Relay 1 / Faulted Feeder 1
Calculated "negative"
wattmetric value
(forward fault)
Net inductive behind
Relay 1 (tuned or over-
compensated system)
0
V
0
V
0
V
I
0(1)
I
0(1)
Calculated "positive"
wattmetric value
(reverse fault)
Capacitive in
front of Relay 2
I
0(2)
Relay 2 / Unfaulted Feeder 2
WF
WR