Hints On Setting - Siemens SIPROTEC 7SS60 Manual

Functions
5.4.2

Hints on setting

Busbar protection
Pick-up value for
differential current
Delay of the TRIP
command
Restraint factor
5-18
The parameter %3VWDW () is used to activate (21) or deactivate (2))) the
busbar protection function. It can also be set to block the command and signal relays
while the protection function is active (%/2753).
For fault detection and tripping the protection only uses the normalized currents I
which exceed the threshold set in ,G! (). The fault detection is reset when
I
R
the value drops below the threshold again.
The threshold ,G! () has an influence on the sensitivity of the protection for
small fault currents. Where low supply conditions or the type of the starpoint earthing
create only earth fault currents in the load range, ,G! () would have to be set
below the nominal value to respond to them. It must be borne in mind, though, that
with a high load the failure of one current transformer might then cause a trip of the
busbar. In this case additional criteria must ensure the stability (e.g. additional release
via the displacement voltage by means of a feeder protection device).
A time delay is implemented between the protection algorithm initiating the tripping
and the output of the actual TRIP command. This time step causes the delayed
initialization of the TRIP command (if a tripping time other than zero is specified). The
TRIP command is only executed if during the entire time step the tripping is issued by
the protection algorithm. The time step will be reset prematurely if the protection
algorithm assumes the tripping conditions to be no longer fulfilled. The delay time of
the TRIP command can be specified as the parameter 10 TrpDly (1511). Due to the
system configuration the timer accuracy is ± 10 ms.
The restraint factor  . IDF () allows to adapt the stability of the protection to
service conditions.
Although a high setting for this factor improves the stability with regard to faults outside
the protected zone, it reduces the sensitivity for the detection of busbar faults.
 . IDF () should therefore be chosen as low as possible and as high as
necessary.
If the measuring system is to be used for zone-selective protection, which will be the
case in most applications, it is advisable to use the default setting of
 . IDF ().
Where a measuring system is used for protection of the so-called check zone, a lower
value for  . IDF is recommended to avoid that the load currents of the non-
faulted busbar sections cause an over-restraint. The recommended settings for  .
IDF () can be summarized as follows:
Condition
Transformer saturation times
≥ 3 ms (16.7 Hz, 50 Hz, 60 Hz)
Power system time constants
< 300 ms
Zone-selective protection
and
d
Setting of
10 K fac (1506)
0.6
7SS60 Manual
E50417-G1176-C132-A2