Startup Detection
The startup detection is particularly useful in the case of motors. In contrast to transformers, the inrush
current with motors is a flowing current. Differential currents can arise, however, by the current transformers
having different residual magnetization and being supplied with current at different operating points of their
hysteresis. These differential currents are low in fact, but they can lead to overfunction with sensitive setting
of the differential protection.
As additional security against overfunction when switching in a previously protected object that carries no
current, you can use the pickup-value increase at startup. If the restraint current falls below a settable value
Thresh. startup detection in all phases, the pickup-value increase is activated. The pickup value
Threshold and Slope 1 are raised by the set value in accordance with parameter Factor increasing
char.. As a rule here, Slope 1 cannot exceed a value of 0.85.
If the parameter Thresh. startup detection is exceeded, dropout occurs and a timing element (param-
eter: Max. perm. Start. time) is activated. Once this time has elapsed, the characteristic curve that was
increased in all phases is reset to its original value. Figure 7-54 shows the logic diagram of the startup detec-
tion and Figure 7-55 the increased characteristic curve.
[loanlauf-170712-01.tif, 2, en_US]
Figure 7-54 Logic Startup Detection
[dwanlken-201112-01.tif, 1, en_US]
Figure 7-55 Characteristic-Curve Increase at Startup
Capacitor Bank Protection
7.8 Differential Protection for Capacitor Banks
SIPROTEC 5, Overcurrent Protection, Manual 1053
C53000-G5040-C017-8, Edition 07.2017
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