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Sampling frequency
The sampling frequency and the synchronism between the ADCs (analog-to-digital converters) is continuously
monitored. If deviations that may occur cannot be corrected by another synchronization, the device sets itself
out of operation and the red LED „ERROR" lights up. The device-ready relay drops off and signals the malfunc-
tion by its „life contact".
Measurement value acquisition - currents
Up to four input currents are measured by the device. If the three phase currents and the earth current from the
current transformer starpoint or a separated earth 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 recognised if
I
= |I
+ I
F
L1
L2
(address 221 I4/Iph CT) takes into account a possible different ratio of a separate I
Factor k
I
(e.g. cable core balance current transformer). ΣI THRESHOLD and ΣI FACTOR. are setting parameters.
The component ΣI FACTOR Σ | I | takes into account permissible current proportional ratio errors of the input
transformers which are particularly prevalent during large fault currents (Figure 2-202). Σ | I | is the sum of all
currents:
Σ | I | = |I
| + |I
L1
As soon as the current summation supervision detects a fault in the current transformer circuits, the differential
protection is blocked. This supervision is signalled as „Failure Σi" (No. 289). In order to avoid a blocking
due to transformation errors (saturation) in case of high fault currents, this monitoring function is not effective
during a system fault.
Note
Current sum supervision can operate properly only when the ground current of the protected line is connected
to the fourth current measuring input (I
transformer (address 220) as In prot. line. Furthermore, the fourth current measuring must be de-
signed as normal I
Figure 2-202
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
| > ΣI THRESHOLD + ΣI FACTOR·Σ | I |
+ I
+ k
·I
L3
E
I
| + |I
| + |k
·I
|
L2
L3
I
E
). The I
4
transformer. With a sensitive transformer type, the current sum supervision is not active.
4
Current sum monitoring
transformer must have been configured with parameter I4
4
Functions
2.24 Monitoring Functions
transformer
E
419
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