Based on the conditions checked from the through-fault module, the DC (component)
detection module and the CT saturation-based blocking modules, the biased low-stage
module decides whether the differential current is due to the internal faults or some false
reason. In case of detection of the TF, DC or CT saturation, the internal differential
blocking signal is generated, which in turn blocks the trip signal. In case of internal faults,
the operation of the differential protection is affected by the bias current.
The
Low trip value
setting for the stabilized stage of the function block is determined with
the equation:
Lowtrip value
I
I
d
n
=
⋅
1
100%
GUID-22A3726A-4B8C-418F-B8C3-D6A5AACD165F-ANSI V1 EN
(Equation 107)
The
Slope section 2
setting is determined correspondingly:
Slope section
I
I
d
b
2
100
2
2
=
⋅
%
GUID-6707BA3B-6446-4B54-B09B-7D8D1F0DDF32 V1 EN
(Equation 108)
Slope section
I
I
d
b
3
100
3
3
=
⋅
%
GUID-ADB22AAC-F1CD-4B6F-B103-E86DBCC8069D V1 EN
(Equation 109)
The end of the first section
End section 1
can be set at a desired point within the range of
0 to 100 percent (or % I
n
). Accordingly, the end of the second section
End section 2
can
be set within the range of 100 percent to 300 percent (or % I
n
).
The slope of the operating characteristic for the function block varies in different parts of
the range.
In section 1, where 0.0 < I
b
/I
n
<
End section 1
, the differential current required for tripping
is constant. The value of the differential current is the same as the
Low trip value
setting
selected for the function block. The
Low trip value
setting allows for small inaccuracies
of the current transformers but it can also be used to influence the overall level of the
operating characteristic.
Section 2, where
End section 1
< I
b
/I
n
<
End section 2
, is called the influence area of the
setting
Slope section 2
. In this section, variations in
End section 2
affect the slope of the
characteristic, that is, how big the change in the differential current required for tripping
is in comparison to the change in the load current. The
End section 2
setting allows for CT
errors.
In section 3, where I
b
/I
n
>
End section 2
, the slope of the characteristic can be set by
Slope
section 3
that defines the increase in the differential current to the corresponding increase
in the biasing current.
Section 4
1MAC059074-MB A
Protection functions
572
615 series ANSI
Technical Manual
Summary of Contents for Relion 615 series
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