Functions
2.16 Backup Time Overcurrent Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
303
2.16.2
Functional Description
Measured values
The phase currents are fed to the device via the input transformers. The earth current 3·
I
0
is either measured
directly or calculated.
If
I
4
is connected to the starpoint of the current transformer set (address
220
I4 transformer
=
In prot.
line
, see section2.1.2 of
P.System Data 1
), the earth current will be directly available as measured value.
It is used considering the
I4/Iph CT
factor (address
221
).
If the device has a sensitive earth current transformer (MLFB pos. 7 = 2 or 6), only the calculated earth current
is used in the time overcurrent protection function, even if the earth current is connected to the fourth current
input
I
4
. If the earth current of the own line is not connected to the fourth current input
I
4
(address
220
I4
transformer
not set to
In prot. line
), the device will calculate the earth current from the phase currents.
Of course, all three phase currents of three star-connected current transformers must be available and con-
nected.
Definite Time High-set Current Stage
I
>>
Each phase current is compared with the setting value
Iph>>
after numerical filtering; the earth current is com-
pared with
3I0>> PICKUP
. After pickup of a stage and expiry of the associated delay times
T Iph>>
or
T
3I0>>
a trip command is issued. The dropout value is approximately 7% below the pickup value, but at least
1.8% of the nominal current.
Figure 2-153 shows the logic diagram of the
I
>> stages. The stages can be blocked via binary input
„>BLOCK
O/C I>>“
. In addition, the earth current stage can be blocked separately via the binary input
„>BLOCK O/C
Ie>>“
. During a 1-pole dead time, the earth current stage is always blocked to avoid spurious pickup.
The binary input
„>O/C InstTRIP“
and the evaluation of the indication „switch“ (onto fault) are common to
all stages and described below. They may, however, separately affect the phase and/or earth current stages.
This can be achieved with two parameters:
•
I>> Telep/BI
(address
2614
), determines whether a non-delayed trip of this stage via binary input
„>O/C
InstTRIP“
is possible (
YES
) or impossible (
NO
). This parameter is also used for instantaneous tripping
before automatic reclosure.
•
I>> SOTF
(address
2615
), determines whether this stage shall issue non-delayed tripping (
YES
) or not (
NO
)
in case of switching-onto-fault conditions.
