ABB RELION 620 Series Technical Manual page 523

1MRS757644 F
620 series
Technical Manual
GUID-0E927DF9-5641-4CAE-B808-0B75EA09EA95 V3 EN
Figure 261: 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.
Section 4
Protection functions
517