Chapter 1) 7SR242 Duobias Description Of Operation
©2015 Siemens Protection Devices Limited
Chapter 1 Page 24 of 60
3.1.2 Overall Biased Differential (87BD)
2
I
I
W2
W1
W2
W1
I
I
Figure 3-1 Biased Differential Characteristic
Figure 3.1-1 illustrates the biased differential characteristic. Within the relay the fundamental frequency RMS line
currents are modified by the
ICT Multiplier
and
ICT Connection
settings (see 3.1.1) before being applied to the
biased differential elements. The biased differential elements calculate the operate current for each phase from
the vector sum of winding 1 and winding 2 currents i.e. I
OPERATE
= I
W1
+ I
W2
. The bias (or restraint) current is
calculated from the total current of winding 1 and winding 2 currents i.e.
2
I
I
I
W2
W1
RESTRAIN
.
The
87BD Initial
setting defines the minimum differential current required to operate the relay.
The
87BD 1
st
Bias Slope
setting is used to ensure protection stability in the presence of steady state errors e.g.
the effects of an on-load tap changer.
The
87BD 1
st
Bias Slope Limit
setting defines the border between the 1
st
and 2
nd
bias slopes.
87BD 2
nd
Bias Slope Type
setting allows the user to select the preferred characteristic shape i.e.
Line
or
Curve
.
The
87BD 2
nd
Bias Slope
setting is applied when
87BD 2
nd
Bias Slope Type = Line.
This setting is used to
modify the sensitivity of the differential algorithm at higher current levels.
The output of
87BD Delay
can be mapped to relay outputs.
Operation of the biased differential elements can be inhibited from:
Inhibit 87BD
A binary or virtual input.
87BD Inrush Action: Inhibit
Operation of the inrush current detector
87BD Overfluxing Action: Inhibit
Operation of the overfluxing detector
User Inhibit
Reylogic (graphical logic).
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