Functions
2.16 Backup Time Overcurrent Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
313
If the
I
P 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(3I0)p SOTF
(address
2671
)
to
YES
. We recommend, however, not to choose the sensitive setting for the fast tripping as switching onto a
fault typically causes a solid short circuit. It is important to avoid that the selected stage picks up due to tran-
sients during line energization.
Overcurrent Stages
I
P
, 3
I
0P
for Inverse-time Overcurrent Protection with ANSI Characteristics
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 ANSI characteristics (address
126
Back-Up O/C
=
TOC ANSI
) the following options are available in address
2661
ANSI Curve
:
Inverse
,
Short Inverse
,
Long Inverse
,
Moderately Inv.
,
Very Inverse
,
Extremely Inv.
and
Definite Inv.
.
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
I
P = 630 A,
Secondary: Setting value
I
P = 5.25 A, i.e. (630 A/600 A) X 5 A.
The time multiplier
Time Dial TD Ip
(address
2643
) derives from the time grading schedule set for the
network. 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
TimeDial TD3I0p
(address
2653
) 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 inverse-time delays, a delay of constant length can be set, if necessary. The settings
T Ip
Add
(address
2646
for phase currents) and
T 3I0p Add
(address
2656
for ground current) are added to the
times of the set characteristic curves.
The parameter
I(3I0)p Tele/BI
(address
2670
) defines whether the time delays
Time Dial TD Ip
(ad-
dress
2643
), including the additional delay
T Ip Add
(address
2646
), and
TimeDial TD3I0p
(address
2653
), 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
I
P 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.
