2.3 Overcurrent Protection
79
7ST6 Manual
E50417-G1176-C251-A3
Switching onto a
Fault
The internal line energization detection can be used to achieve quick tripping of the
circuit breaker in case of an earth fault. The time overcurrent protection then issues an
instantaneous trip command.
Pickup Logic and
Tripping Logic
The pickup signals of the individual stages are
OR
-combined and cause the indication
„O/C PICKUP“
to be output when one stage picks up.
Likewise, the trip signals of the individual stages are
OR
-combined. Tripping of one
stage causes the indication
„O/C TRIP“
to be output.
2.3.2
Setting Notes
General
The time overcurrent protection is only effective and accessible if address
126
Back-
Up O/C
has been set to
Enabled
during the configuration of the functional scope. If
the function is not required,
Disabled
is to be set.
In order to be effective, the time overcurrent protection function must also be switched
ON
in address
2601
OverCurrent
. The time overcurrent protection operates inde-
pendently of the other protection functions, i.e. as a backup time overcurrent protec-
tion. In address
2601
the time overcurrent protection can also be switched
OFF
.
The
I>>>
stage (address
2605
), the
I>>
stage (address
2608
) and the
I>
stage (ad-
dress
2614
) together create a three-stage characteristic.
The very high-set current stage (
I>>>
stage) and the high-set current stage (
I>>
stage) operate always as a definite-time overcurrent protection.
The overcurrent stage (
I>
stage) may operate either as a definite-time overcurrent
protection (address
2602
CURVE O/C
=
Definite Time
) or as an time overcurrent
protection with current-dependent characteristic (address
2602
CURVE O/C
=
TOC
IEC
).
If some stages are not needed, those not needed can be deactivated by setting the
pickup value to
∞
. If you set one assigned time stage to
∞
, the delay time cannot
expire. As a result, the trip indications are suppressed but not the pickup indications.
In case of switching onto a fault, the protection should trip instantaneously. To do so,
set in address
2603
O/C SOTF Trip
=
YES
. If the pickup threshold is exceeded when
switching onto a fault, the delay time set for the function is bypassed, and the time
overcurrent protection issues an instantaneous trip command.
The time overcurrent protection is blocked by the inrush restraint function if address
2302
Inrush O/C
is set to
YES
.
Definite Time Very
High-set Current
Stage I>>>
With the parameter
I>>>
(address
2605
) the pickup threshold of the very high set
current stage is specified.
When using a personal computer and DIGSI
®
to apply the settings, these can be op-
tionally entered as primary or secondary values. If secondary quantities are used, all
currents must be converted to the secondary side of the current transformers.
The delay time of the very high set current stage
T I>>>
is set in address
2606
.
The delay set is in addition to the pickup time which does not include the operating
time (measuring time).
The dropout to pickup ratio of the stage is 0.95.
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