Siemens SIPROTEC 7SS523 V3.2 Manual page 124

Functions
Id> BZ
108
K = I / I
b scc max sat
The saturation current I
fective factor of the symmetrical rated short-circuit current K'
called operational overcurrent factor n')
I = I * K'
sat pn ssc
I = primary rated current
pn
The factor K' can be calculated from the data and the actual burden of the current
ssc
transformers:
K' = (R +R ) / (R
ssc ct b
with R' = R + R
b l relay
R = secondary winding resistance (75 °C)
ct
R = resistive rated burden
b
R' = connected burden
b
R = line resistance
l
R = protection burden
relay
K' = factor of the symmetrical rated short-circuit current
ssc
For burden factors K
k-factor is:
K
b
with K
-----------------------
k
>
4 K – 1
b
The setting value k thus obtained comprises 20 % safety.
The basis for this fomula is summarized in the Chapter A.2, page 333.
The CT with the highest burden factor K
factor k.
The parameter Id> BZ (6102/CU) 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
ered which exceed the diff-current threshold set in the parameter Id> BZ (6102/CU).
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
< Id> BZ < 0.8 I
1.3 I
max load
can be calculated from the rated current I
sat
+R' ) * K
ct b ssc
< 2, a k-factor of 0.6 is sufficient. For K
b
> 2
b
must be the basis for selecting the
b
scc min
and the ef-
pn
(which used to be
ssc
> 2, the minimum
b
are consid-
d
+ 30%.
max load
7SS52 V4 Manual
C53000-G1176-C182-1