2 Functions
158
7ST6 Manual
E50417-G1176-C251-A3
terval, the current-time integral measurement is started. Then the last zero crossing of
the current is determined.
Using the TRIP
Command
In this case, the starting time of the current-time integral determination is computed
with the following formula:
where:
t
TRIP
Instant at which the TRIP command is issued by one of the protection
functions in the device,
TS
Output delay time (semiconductor switch) approx. 2 ms to 3 ms,
T00
CB response time (time between the TRIP command and the contact
movement, can be set in address
4102
).
Calculation of t
End
The current-time integral measurement ends with the quenching in a current zero
crossing.
This current zero crossing is the last one if no new zero crossing occurs during the
following cycle T (power system frequency). tend is the time of the last current zero
crossing plus one cycle T.
Current-Time Inte-
gral Measurement
When the function is configured in address
141
FCT It-Calc.
, one of the above
methods for determining the time
t
Start
is selected.
The entire measuring time range starts with
t
start
and is limited to the duration 2T +
parameter
T CB BREAKTIME
.
If the current is not interrupted within the test time, e.g. because of a breaker failure,
there is no calculation, and the indication
„It Overflow“
(FNo. 13849) is generat-
ed. The sums of
I dt
(indication
„
Σ
It=“
; FNo. 13851) and
I
2
dt
(indication
„
Σ
I2t=“
;
FNo. 13852) are not changed in that case.
If the current is successfully interrupted, the values of the current-time integrals of the
current
I dt
and of the squared current
I
2
dt
are generated for the current trip. The ac-
cumulated sums of these values for all trips since the device startup or since the last
reset are output in the indications
„
Σ
It=“
and
„
Σ
I2t=“
. The accumulators can
be individually set and cleared.
The cumulated fault current is also output in the indication
„
Σ
I=“
(FNo. 13927).
The values of the last individual trip are output in the indications
„Last It=“
(current-
time integral over absolute value, FNo. 13853) and
„Last I2t=“
(current-time inte-
gral squared current, FNo. 13854).
If the limit of the current-time integral set in parameter
It ALARM
(address
4104
) is
exceeded by the last trip, the indication
„It Alarm“
(FNo. 13847) is output.
Neither does the use of auto-transformer systems affect the measured value acquisi-
tion as described above.
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