Figure 271: Operation logic of the biased low stage
The high currents passing through a protected object can be caused by the short
circuits outside the protected area, the large currents fed by the transformer in
motor start-up or the transformer inrush situations. Therefore, the operation of the
differential protection is biased in respect to the load current. In biased differential
protection, the higher the differential current required for the protection to
operate, the higher the load current.
The operating characteristic of the biased low stage is determined by
Low operate
value
,
Slope section 2
and the setting of the second turning point of the operating
characteristic curve,
End section 2
(the first turning point is fixed). The settings are
the same for all the phases. When the differential current exceeds the operating
value determined by the operating characteristic, the differential function awakes.
If the differential current stays above the operating value continuously for a suitable
period, which is 1.1 times the fundamental cycle, the OPR_LS output is activated.
The OPERATE output is always activated when the OPR_LS output is activated .
The stage can be blocked internally by the second or fifth harmonic restraint, or by
special algorithms detecting inrush and current transformer saturation at external
Protection functions
1MRS757644 H
528 620 series
Technical Manual
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