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6.106
SEL-421 Relay
Instruction Manual
Date Code 20171021
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Therefore, if the reverse-looking directional element at the local station is not
more sensitive than the forward-looking directional element at the remote station,
unwanted tripping can occur during external ground faults; the local 67Q3 ele-
ment can fail to detect a reverse unbalanced fault that the remote 67Q2 element
sees.
Use a short-circuit study to determine I
2S
for a close-in reverse single phase-to-
ground fault with respect to Station S; make sure to perform the fault calculations
for the parallel cable both in service and out of service. The results of the study
for this particular application show that the maximum difference between I
2S
and
I
2R
for any close-in reverse unbalanced fault at Station S is 8.5 mA secondary.
Therefore, the existing settings provided for 50Q2P and 50Q3P maintain coordi-
nation for external unbalanced faults.
There is no need to add any intentional time delay on pickup for Level 2 or 3 neg-
ative-sequence overcurrent elements.
67Q2D :=
0.000
.
Level 2 Time Delay (0.000–16000 cycles)
67Q3D :=
0.000
.
Level 3 Time Delay (0.000–16000 cycles)
Set the Level 2 torque control equation to the forward decision from the ground
directional element.
67Q2TC :=
32GF
.
Level 2 Torque Control (SEL
OGIC
Equation)
Set the Level 3 torque control equation to the reverse decision from the ground
directional element.
67Q3TC :=
32GR
.
Level 3 Torque Control (SEL
OGIC
Equation)
Selectable Operating Quantity Time Overcurrent Element 1
Use inverse-time overcurrent protection to provide backup protection for unbal-
anced faults including high-resistance ground faults. Selectable Operating Quan-
tity Time Overcurrent Element 1 51S1 provides backup protection for
unbalanced faults if both the communications-assisted and step distance protec-
tion fail to operate.
NOTE:
Use your company practices
and philosophy when determining
these settings.
Select negative-sequence line current 3I2L as the operating quantity rather than
3I0L because ground current return paths vary.
51S1O :=
3I2L
.
51S1 Operating Quantity
(IA
n
, IB
n
, IC
n
, IMAX
n
, I1L, 3I2L, 3I0
n
)
The fault current (3I
2
) that the relay measures for a bolted single phase-to-ground
fault at the end of the longest line from the remote station with the parallel cable
in service at minimum generation is 3.0 A secondary (this is the minimum of
minimums). Set the pickup between 30 to 50 percent of this current.
51S1P = 1/3 • 3I
2FAULT
= 1/3 • 3.00 A = 1.00 A
51S1P =
1.00
.
51S1 Overcurrent Pickup (0.25–16 A secondary)
Содержание SEL-421-4
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