The characteristic branch b (parameter Intersection 1 Irest and Slope 1 ) considers current-propor-
tional faults under normal operating conditions.
These faults can occur for the following reasons:
•
Transformation errors in the current transformer and the input transformer of the device
•
Adaptation deviations
The characteristic curve branch c (parameter Intersection 2 Irest and Slope 2 ) protects the differ-
ential protection against overfunction in case of high-current external faults and transformer saturation. In
this range, transformer saturation can arise due to high short-circuit currents, and because of this, a higher
differential current results.
In order to avoid overfunction in the event of external faults with current-transformer saturation, a logic is
implemented that uses the characteristic curve Additional stabilization of external faults. If the operating
point is located in the additional stabilization range and exceeds the operate curve from there, then the logic
becomes active and blocks the differential protection.
The additional stabilization range is derived from the Threshold add-on stabiliz. and Slope 1
parameters.
[dwdifaus-030912-01.tif, 1, en_US]
Figure 7-51
Pickup Characteristic of the Differential Protection
If the differential current multiplied by the dropout ratio of 0.7 does not reach the pickup characteristic, the
pickup drops out.
Detection of External Faults
To detect external faults with transformer saturation in a reliable way, the measurand curve is evaluated in the
Diff.-Rest level by means of a detector.
Capacitor Bank Protection
7.8 Differential Protection for Capacitor Banks
1050 SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
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