ABB RELION REX640 Technical Manual page 725

1MRS759142 F
Figure 410: 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 high currents fed by the transformer in
a motor startup or transformer inrush situations. Therefore, the operation of
the differential protection is biased with respect to the load current. In biased
differential protection, the higher the differential current required for the protection
to trip, the higher the load current.
The operation characteristic of the biased low stage is determined by
value , Slope section 2 and the second turning point setting of the operating
characteristic curve,
part of the characteristic are fixed). The settings are the same for each phase. When
the differential current exceeds the operating value determined by the operating
characteristic, the differential function is activated. If the differential current stays
above the operating value continuously for a suitable period, the OPR_LS output is
activated. The activation of the OPR_LS output activates the OPERATE output.
The stage can be blocked internally by the second or the fifth harmonic restraint,
or by special algorithms detecting the inrush and current transformer saturation at
external faults. When the operation of the biased low stage is blocked by the second
harmonic blocking functionality, the BLKD2H output is activated.
The BLKD5H output is activated when the biased low stage is blocked by the fifth
harmonic blocking functionality. Correspondingly, when the operation of the biased
REX640
Technical Manual
End section 2 (the first turning point and the slope of the last
Protection functions
Low operate
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