5.20
SEL-387-0, -5, -6 Relay
Instruction Manual
Date Code 20170601
Metering and Monitoring
Through-Fault Event Monitor
shows the time progression of a through fault, such as that in
. Typically, SEL
OGIC
setting THRU would be set to some
instantaneous overcurrent element (e.g., THRU = 50P23). When THRU
asserts at the outset of the through fault, event duration timing begins. When
THRU de-asserts (e.g., the distribution feeder breaker interrupts the fault),
through-fault event duration timing ends and the duration time is recorded for
that event.
If SEL
OGIC
setting THRU is asserted (by whatever means), then maximum
currents are recorded for the monitored current inputs.
shows the
current jumping up to a short-term maximum before being interrupted
(perhaps the feeder fault “burned through” and became more “bolted”). This
short-term maximum current, which occurred within the duration timing, is
what gets recorded for the particular monitored phase.
Figure 5.7
Through-Fault Triggering, Duration, and Maximum Current
Through-Fault
Calculation
The through-fault event monitor uses the recorded duration time value and
maximum currents to perform simple I
2
t calculations and cumulatively store
results of these calculations for each monitored phase. For example, if a
through fault is 6000 A primary (maximum) and lasts 0.067 sec., the monitor
would calculate I
2
t for that event as follows:
Equation 5.8
If the above calculation were for the example in
, it would be a
conservative calculation (i.e., the calculation would indicate more I
2
t stress/
wear than actually occurred). This is because the current peaked momentarily.
Through-Fault Alarm
shows an I
2
t alarm, with Through-Fault I
2
t Alarm Threshold
setting ISQT. When the cumulative I
2
t for any monitored phase exceeds
setting threshold ISQT, Relay Word bit ISQTAL (I-squared-t alarm) asserts.
Setting threshold ISQT would usually be set to alarm for excessive,
cumulative transformer bank stress, as such stress corresponds to a certain
level of cumulative I
2
t. Output Relay Word bit ISQTAL can be assigned to an
output for annunciation or perhaps also be used to modify distribution feeder
auto-reclosing (e.g., reduce the number of reclosures from 3 to 2).
Through-fault
event duration
(SEL
OGIC
setting THRU asserted)
trigger
max.
current
I
t
6 kA
2
0.067 sec.
•
2.412 (kA)
2
seconds
=
Содержание SEL-387-0
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