Siemens SIPROTEC 7SD610 Manual page 116

Functions
2.9 Backup Time Overcurrent Protection
Note: the calculation was carried out with absolute values, which is sufficiently precise for overhead lines. If the
angles of the source impedance and the line impedance vary considerably, a complex calculation must be
carried out.
A similar calculation must be carried out for earth faults, with the maximum earth current occurring at the line
end during a short-circuit being decisive.
The set time delays are pure additional delays, which do not include the operating time (measuring time).
The parameter I>> Telep/BI (address 2614) defines whether the time delays T Iph>> (address 2611) and
T 3I0>> (address 2613) can be bypassed via the binary input „>O/C InstTRIP" (No. 7110) or by the op-
erational automatic reclosure function. The binary input (if allocated) is applied to all stages of the time-over-
current 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 differential protection guarantees a fast and selective tripping
with or without auto-reclosure, the overcurrent protection as a backup protection may not perform a non-selec-
tive trip, even before auto-reclosure.
If the I>> stage, when switching the line onto a fault, is to re-trip without delay or with a short delay, SOTF Time
DELAY (address 2602, 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.
Time Overcurrent Stages I
For the setting of the current pickup value Iph> (address 2620), or Iph> Dir. (address 2681), the maximum
operating current is most decisive. Pickup through overload must be excluded, since the device works in this
operating mode as a short-circuit protection, not as an overload protection, and the command times are rela-
tively short. A pickup value setting equal to 10% is recommended for line protection, and a setting equal to 20
% of 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
maximum transmittable power
P
max
corresponding to
I
max
Current Transformer
Safety factor
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) or 3I0> Dir. (address 2684), should be set to detect the small-
est earth fault current to be expected.
116
>, I
>, 3I >Dir, 3I >Dir for Definite-time Overcurrent Protection
ph 0 ph 0
2
= 120 MVA
= 630 A
600 A / 5 A
1.1
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011