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 trip, the higher the load current.
The operating characteristic of the biased low stage is determined by
Low trip 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
TRIP
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 faults. When the
operation of the biased low stage is blocked by the second harmonic blocking
functionality, the
BLKD2H
output is activated.
When operation of the biased low stage is blocked by the fifth harmonic blocking
functionality, the
BLKD5H
output is activated. Correspondingly, when the operation of
the biased low stage is blocked by the waveform blocking functionality, the
BLKDWAV
output is activated according to the phase information.
When required, the trip outputs of the biased low stage can be blocked by the
BLK_OPR_LS
or
BLOCK
external control signals.
Section 4
1MAC059074-MB A
Protection functions
510
615 series ANSI
Technical Manual
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