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Functions
2.16 Backup Time Overcurrent Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
311
Overcurrent Stages
I
ph
>, 3
I
0
> in Definite-time Overcurrent Protection
For the setting of the current pickup value, Iph> (address 2620 ), the maximum operating current is most de-
cisive. Pickup due to overload should never occur, since the device in this operating mode operates as fault
protection with correspondingly short tripping times and not as overload protection. For this reason, a pickup
value of about 10 % above the expected peak load is recommended for line protection, and a setting of about
20 % above the expected peak load is recommended for transformers and motors.
When using a personal computer and DIGSI to apply the settings, these can be optionally entered as primary
or secondary values. For settings with secondary values the currents will be converted for the secondary side
of the current transformers.
Calculation Example:
110 kV overhead line 150 mm
2
maximum transmittable power
P
max
= 120 MVA
corresponding to
I
max
= 630 A
Current Transformer 600 A / 5 A
Safety factor 1.1
With settings in primary quantities the following setting value is calculated:
Set value
I
> = 1.1 · 630 A = 693 A
With settings in secondary quantities the following setting value is calculated:
The earth current stage
3I0>
(address
2622
) should be set to detect the smallest earth fault current to be ex-
pected.
The time delay
T Iph>
(address
2621
) results from the time grading schedule designed for the network. When
using the function as emergency overcurrent protection, shorter delay times make sense (one grading time
step above instantaneous tripping), since in this case the function is to work only if the main protection func-
tions, i.e. differential and/or distance protection, fail.
The time
T 3I0>
(address
2623
) can normally be set shorter, according to a separate time grading schedule
for earth currents.
The set times are mere additional delays for the independent stages, which do not include the inherent oper-
ating time of the protection. If only the phase currents are to be monitored, set the pickup value of the earth
fault stage to
∞
.
The parameter
I> Telep/BI
(address
2624
) defines whether the time delays
T Iph>
(address
2621
) and
T 3I0>
(address
2623
) can be bypassed by the binary input
„>O/C InstTRIP“
. The binary input (if allocat-
ed) is applied to all stages of the time-overcurrent protection. With
I> Telep/BI
=
YES
you define that the I >
stages trip without delay after pickup if the binary input was activated. For
I> Telep/BI
=
NO
the set delays
are always active.
Instantaneous tripping by the operational auto-reclosure function should only be chosen if the overcurrent pro-
tection is set to emergency function. Since the fast main protection function - differential protection and/or dis-
tance protection - guarantees a fast and selective tripping with or without auto-reclosure, the overcurrent pro-
tection as a backup protection may not perform a non-selective trip, even before auto-reclosure.