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 (address221).
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.
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