Functions
6-1717SA6 Manual
C53000-G1176-C156-2
Definite Time
Overcurrent Stages
Iph>, 3I0>
For the setting of the current pick-up threshold Iph> (address 2620), the maximum
operating current that can occur is decisive. Pick-up due to overload must be excluded
as the device operates as short-circuit protection with correspondingly short tripping
times and not as overload protection. The setting is therefore: on overhead lines ap-
proximately 10 %, on transformers and motors approximately 20 % above the maxi-
mum expected (over-)load current.
When using a personal computer and DIGSI
®
4toapplythesettings,thesecanbeop-
tionally entered as primary or secondary values. When applying the setting parame-
ters as secondary values, the primary currents must be converted to the secondary
side of the current transformer.
Calculation example:
110 kV overhead line 150 mm
2
as in the example in Sub-section 6.2.4.2:
maximum transmittable power
P
max
= 120 MVA corresponds to
I
max
=630A
current transformers 600 A/5 A
security margin 1.1
When applying settings with primary values, the following setting results:
Set value I> = 1.1 · 630 A = 693 A
When applying settings with secondary values, the following setting results:
The earth current stage
3I0> (address 2622),muststillbeabletodetectthesmallest
earth fault current that may be present. For very small earth currents the earth fault
protection is most suited (refer to Section 6.7).
Thetimedelay
T Iph> (address 2621) which has to be set is derived from the grading
plan of the system. If implemented as emergency overcurrent protection, shorter trip-
ping time delays (one grading time margin longer than the fast tripping stage) are ad-
visable, as this function is only activated when the
local measured voltage fails.
The time
T 3I0> (address 2623) can usually be set with a smaller time delay accord-
ing to a separate earth fault grading plan.
The set times of the definite time stages are pure additional time delays which do not
include the operating (measuring) time of the protection. If only the phase currents of
a particular stage should be monitored, the pickup value of the earth current stage is
set to
∞.
The setting parameter
I> Telep/BI (address 2624) determines if it is possible to
use the binary input “
>O/C InstTRIP” to bypass the trip delay times T Iph> (ad-
dress
2621)andT 3I0> (address 2623). The binary input (if assigned) is common
to all stages of the overcurrent protection. With
I> Telep/BI = Yes it is therefore
determined that the I>–stages trip without time delay following pick-up, if an operate
signal is present at the binary input; if the setting is
I> Telep/BI = No the set trip
time delays always come into force.
IftheI>-stageistotripwhenswitchingthelineontoafaultwithorwithoutashortde-
lay,
SOTF Time DELAY (address 2680, see above and refer to Sub-section “Gener-
al”), the parameter
I> SOTF (address 2625)issettoYes. We recommend, however,
not to choose the sensitive stage for the switch on to fault function as energizing of the
Setting value I> 1.1=
630 A
600 A
----------------
5A⋅⋅5.8 A=
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