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Functions
2.16 Backup Time Overcurrent Protection
If the I> stage, when switching the line onto a fault, is to retrip without delay or with a short delay SOTF Time
DELAY (address 2602, see above under side title „General"), set parameter I> SOTF (address 2625) to YES.
We recommend, however, not to choose the sensitive setting for the fast tripping as switching onto a fault typ-
ically causes a solid short circuit. It is important to avoid that the selected stage picks up due to transients during
line energization.
Overcurrent Stages I
, 3I
P
In the case of the inverse time overcurrent stages, various characteristics can be selected, depending on the
ordering version of the device and the configuration (address 126). With IEC characteristics (address 126
Back-Up O/C = TOC IEC) the following options are available in address 2660 IEC Curve:
Normal Inverse (inverse, type A according to IEC 60255-3),
Very Inverse (very inverse, type B according to IEC 60255-3),
Extremely Inv. (extremely inverse, type C according to IEC 60255-3), and
LongTimeInverse (longtime, type B according to IEC 60255-3).
The characteristics and equations they are based on are listed in the „Technical Data".
For the setting of the current thresholds Ip> (address 2640) and 3I0p PICKUP (address 2650) the same
considerations as for the overcurrent stages of the definite time protection (see above) apply. In this case, it
must be noted that a safety margin between the pickup threshold and the set value has already been incorpo-
rated. Pickup only occurs at a current which is approximately 10 % above the set value.
The above example shows that the maximum expected operating current may directly be applied as setting
here.
Primary: Set value IP = 630 A,
Secondary: Set value IP = 5.25 A, i.e. (630 A/600 A) X 5 A.
The time multiplier T Ip Time Dial (address 2642) derives from the time grading schedule set for the net-
work. For use as emergency overcurrent protection shorter delay times make sense (one grading time step
above instantaneous tripping), since this function is to work only if the main protection functions, i.e. differential
and/or distance protection, fail.
The time multiplier setting T 3I0p TimeDial (address 2652) can usually be set smaller according to a sep-
arate earth fault grading plan. If only the phase currents are to be monitored, set the pickup value of the earth
fault stage to ∞.
In addition to the current-dependent delays, a time fixed delay can be set, if necessary. The settings T Ip Add
(address 2646 for phase currents) and T 3I0p Add (address 2656 for earth currents) are in addition to the
time delays resulting from the set curves.
The parameter I(3I0)p Tele/BI (address 2670) defines whether the time delays T Ip Time Dial (ad-
dress 2642), including the additional delay T Ip Add (address 2646), and T 3I0p TimeDial (address
2652), including the additional delay T 3I0p Add (address 2656), can be bypassed by the binary input „>O/C
InstTRIP" (No. 7110). The binary input (if allocated) is applied to all stages of the time-overcurrent protection.
With I(3I0)p Tele/BI = YES you define that the IP stages trip without delay after pickup if the binary input
was activated. For I(3I0)p Tele/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.
312
for Inverse-time Overcurrent Protection with IEC Characteristics
0P
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
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