ABB RELION REX640 Technical Manual page 644

Section 4
Protection functions
638
GUID-E0C8B330-8C8B-4A05-8419-DE3552107FB2 V1 EN
Figure 351: 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 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 Low operate
value, Slope section 2 and the second turning point setting of the operating
characteristic curve, End section 2 (the first turning point and the slope of the last 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.
1MRS759142 C
REX640
Technical Manual