Functions
2.14 Monitoring Functions
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
164
Measured-Value Acquisition – Currents
Up to four input currents are measured by the device. If the three phase currents and the ground fault current
from the current transformer neutral or a separated ground current transformer of the line to be protected are
connected to the device, their
digitized
sum must be zero. Faults in the current circuit are
recognized
if
I
F
= |
I
A
+
I
B
+
I
C
+ k
I
·
I
E
| >
Σ
I THRESHOLD
+
Σ
I FACTOR
·
Σ
|
I
|
Factor k
I
(address
221
I4/Iph CT
) takes into account a possible different ratio of a separate
I
E
transformer
(e.g. cable core balance current transformer).
Σ
I THRESHOLD
and
Σ
I FACTOR
are setting parameters.
The component
Σ
I FACTOR
Σ
|
I
| takes into account the permitted current-proportional ratio errors of the
input transducers which are particularly prevalent during large short-circuit currents (Figure 2-64).
Σ
|
I
| is the
sum of all currents:
Σ
|
I
| = |
I
A
| + |
I
B
| + |
I
C
| + |k
I
·
I
E
|
Once a summation current fault is detected outside the context of a system disturbance, the differential protec-
tion is blocked. This fault is signaled as
„Failure
Σ
i“
(No. 289). In order to avoid a blocking due to trans-
formation errors (saturation) in case of high fault currents, this monitoring function is not effective during a
system fault.
Note
Current sum monitoring can operate properly only when the ground current of the protected line is fed to the
fourth current measuring input (
I
4
) of the device. The
I
4
transformer must have been configured with parameter
I4 transformer
(address
220
) as
In prot. line
.
Figure 2-64
Current sum monitoring
