Busbar Protection
109
7SS52 V4 Manual
C53000-G1176-C182-3
The CT with the highest burden factor K
b
must be the basis for selecting the
factor k.
The parameter
) is used to match the busbar and breaker
failure protection to the system conditions and to the treatment of the starpoint. An ex-
planation of the standard characteristic and of the earth fault characteristic is given on
page 95.
With the setting
blocked
always the standard characteristic is used.
With the setting
the measuring system switches to the more sensitive char-
acteristic in the presence of a 1-pole fault. This characteristic has a lower diff-current
threshold, which is set with the parameters
(
) or
6111A/CU
), and a stabilizing current threshold which
) or
CU
).
The setting
enables also the marshalling of the binary input annunciation
for characteristic switchover "
“ (
).
) is used to set the threshold for the bus zone-se-
lective protection in the presence of small fault current.
For pickup and tripping of the protection, only those normalized currents I
d
are consid-
ered which exceed the diff-current threshold set in the parameter
).
A current below this threshold causes the pickup to drop off.
To calculate the highest permissible setting, determine first the smallest fault current
to be expected in case of a busbar short-circuit. The set value should have an extra
response margin of about 20%.
The lowest permitted setting is I
max feeder
+ 30%.
1.3 I
max feeder
<
< 0.8 I
scc min
The parameter
) is used to set the diff-current threshold I
s
/
I
no
for the bus zone-selective protection in the presence of a 1-pole earth fault.
This parameter is only displayed if the parameter
CU
) is
Where due to low infeed conditions or to the type of starpoint earthing earth fault cur-
rents do not exceed the normal load range, the threshold must be set to less than nom-
inal value. A separate characteristic is provided for such cases. It should be noted,
however, that with this setting the failure of one current transformer in combination
with a high load can lead to a trip of the busbar. Additional criteria are required in such
cases to ensure sufficient stability. Such a criterion may be, for instance, additional re-
lease by a feeder protection device through detection of the offset voltage.
To calculate the highest permissible setting, determine first the smallest fault current
to be expected in case of a busbar short-circuit. The set value should have an extra
response margin of about 20%.
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