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7SR18 Description Of Operation
Unrestricted Page 28 of 78
©2018 Siemens Protection Devices Limited
Section 3: Protection Functions
3.1
Differential Protection
There are two biased differential elements
87L-1
&
87L-2.
The fundamental frequency current is measured with the line CT inputs. These line currents are both multiplied
and corrected before being applied to the current differential elements.
3.1.1
Overall Biased Differential (87L-n)
Figure 3.1.1-1 Biased Differential Characteristic (Offset Disabled)
Figure 3.1.1-1 illustrates the biased differential characteristic. The biased differential elements calculate the
operate current for each phase from the vector sum of local and remote currents i.e.
I
operate
=
I
L
+
I
R
.
The bias (or restraint) current is calculated from the total current of I
L
and I
R
currents i.e.
I
restrain =
.
Within the relay the fundamental frequency RMS line currents are modified by the
CT Multiplier
settings before
being applied to the biased differential elements. The
CT Multiplier
setting is applied to the line currents – the CT
secondary currents. The multiplier is used to correct any CT ratio mismatch so that ideally nominal current is
applied to the biased differential algorithm.
The
87L Initial
setting defines the minimum differential current required to operate the relay.
The
87L 1
st
Bias Slope
setting is used to ensure protection stability in the presence of steady state errors e.g. the
effects of cable capacitive current etc.
The
87L 1
st
Bias Slope Limit
setting defines the border between the 1
st
and 2
nd
bias slopes.
The
87L 2
nd
Bias Slope
setting is used to modify the sensitivity of the differential algorithm at higher current
levels.
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