Siemens 7SR18 Solkor - Section 3: Protection Functions; Differential Protection; Overall Biased Differential (87 L-N); Figure 3.1.1-1 Biased Differential Characteristic (Offset Disabled)

7SR18 Description Of Operation
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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.