Siemens siprotec 7SD5 User Manual

2.14 Backup Time Overcurrent Protection
277
7SD5 Manual
C53000-G1176-C169-1
protection function - differential protection and/or distance protection - guarantees a
fast and selective tripping with or without auto-reclosure, the overcurrent protection as
a back-up protection
may not perform a non-selective trip, even before auto-reclosure.
If the I>> stage, when switching the line on to a fault, is to trip without delay or with a
short delay, SOTF Time DELAY (address 2680, see above under margin heading
„General“), the parameter I>> SOTF (address 2615) is set to YES. Any other stage
can be selected as well for this instantaneous tripping.
Overcurrent Stages
I
ph
>, 3I
0
> in Defi-
nite-time Overcur-
r e n t P r o t e c t i o n
For the setting of the current pickup value, Iph> (address 2620), the maximum oper-
ating current is most decisive. 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 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.
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 expected.
The time delay T Iph> (address 2621) results from the time grading schedule de-
signed for the network. For the use as emergency overcurrent protection shorter delay
times make sense (one grading time step above instantaneous tripping), since this
function is to work only in case of a failure of the main protection functions, i.e. differ-
ential and/or distance protection.
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 operating 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 allocated) is applied to all stages of the
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Siemens siprotec 7SD5 Specifications

General

Brand	Siemens
Model	siprotec 7SD5
Category	Relays
Language	English