the basis. The currents are defined positively to the node. The restraint current is the maximum current of all
measuring points.
[fomome27-170712-01.tif, 1, en_US]
where
x Variable (1, 2, 3) for the phase currents (A, B, C)
m Variable (1, 2, ..., N) for the measuring points
N Number of measuring points
The evaluation of the restraint current is used to recognize saturated, high-current external faults.
Figure 7-57 shows the operate curve. The parameter Threshold is set here such that it lies over the
maximum fault current flowing. The slope of the characteristic curve is fixed at 0.8.
In the algorithm, the following conditions are processed:
•
Condition 1: The increase of the restraint current di
rest
(t)/dt is determined continuously and compared
with an internal threshold value.
•
Condition 2: After that, a comparison determines whether the differential current is larger than the
restraint current (characteristic curve Slope in Figure 7-57): i
diff
(t) > 0.8 · i
rest
(t).
•
Condition 3: After that, a comparison determines whether the differential current exceeds the parameter
Threshold.
When all conditions are met, tripping occurs immediately. If the 1st condition is recognized and not the 2nd
condition, then the function is blocked for 300 ms.
[dwidfast-300114-01.tif, 1, en_US]
Figure 7-57 I-DIFF Fast Characteristic Curve
The following figures show the behavior during an internal and an external fault. It is typical for the internal
fault (Figure 7-58) that the differential current rises more quickly than the restraint current. After a few
sampled values, the trip signal is issued. In case of an external fault, a differential current arises only after
transformer saturation occurs. You can always assume, however, that the current is transferred a few millisec-
onds after occurrence of the fault. In accordance with Figure 7-60, the restraint current is larger than the
Capacitor Bank Protection
7.8 Differential Protection for Capacitor Banks
SIPROTEC 5, Overcurrent Protection, Manual 1055
C53000-G5040-C017-8, Edition 07.2017
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