2.9 Backup Time Overcurrent Protection
113
7SD610 Manual
C53000-G1176-C145-4
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 by the binary
input „>O/C InstTRIP“ (No 7110) or by the operational automatic reclosure func-
tion. The binary input (if allocated) is applied to all stages of the time overcurrent pro-
tection. 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 protection 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 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 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
StagesI
ph
>, 3I
0
>,
I
ph
>ger, 3I
0
>ger for
Definite-time Over-
current Protection
For the setting of the current pickup value, Iph> (address 2620), or Iph> Dir. (ad-
dress 2681) the maximum operating current is most decisive. Pickup through over-
load 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 relatively 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 op-
tionally 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 stage3I0> (address2622) or 3I0> Dir. (address2684), should
be set to detect the smallest earth fault current to be expected.
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