Siemens 6MD85, Manual

Page: 642 / 1256

The settings in primary and secondary values result in the following setting values which include a safety
margin of 10 %:
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In case of short-circuit currents exceeding 1246 A (primary) or 10.39 A (secondary) there is a short-circuit on
the line to be protected. The overcurrent protection can cut off this short circuit immediately.
Note: The amounts in the calculation example are accurate enough for overhead lines. If the source impe-
dance, line impedance and zero-sequence impedance have very different angles, you have use complex
numbers to calculate the
Threshold
.
Parameter:
Dropout delay
•
Recommended setting value
(_:751:101) Dropout delay
=
0
This parameter is not visible in the basic stage.
Siemens recommends using the default setting
0
since the dropout of a protection stage must be done as fast
as possible.
You can use the
Dropout delay
parameter ≠
0
to obtain a uniform dropout behavior if you use it together
with an electromechanical relay. This is required for time grading. The dropout time of the electromechanical
relay must be known for this purpose. Subtract the dropout time of your own device (see Technical Data) and
set the result.
Parameter:
Dropout ratio
•
Recommended setting value
(_:751:4) Dropout ratio
=
0.95
This parameter is not visible in the basic stage.
The recommended set value of
0.95
is appropriate for most applications.
To achieve high-precision measurements, the setting value of the parameter
Dropout ratio
can be
reduced, for example, to
0.98
. If you expect highly fluctuating measurands at the response threshold, you
can increase the setting value of the parameter
Dropout ratio
. This avoids chattering of the tripping stage.
Settings
Addr. Parameter C Setting Options Default Setting
General
_:2311:101 General:Emergency
mode
•
no
•
caused by main prot.
•
caused by binary input
no
_:2311:9 General:Measured value
•
3I0 calculated
•
IN measured
IN measured
General
_:751:1 Definite-T 1:Mode
•
off
•
on
•
test
off
_:751:2 Definite-T 1:Operate &
flt.rec. blocked
•
no
•
yes
no
_:751:26 Definite-T 1:Dynamic
settings
•
no
•
yes
no
_:751:27 Definite-T 1:Blk. w.
inrush curr. detect.
•
no
•
yes
no
7.5.4.3
Protection and Automation Functions
7.5 Overcurrent Protection, Ground
642 SIPROTEC 5, High-Voltage Bay Controller, Manual
C53000-G5040-C015-9, Edition 11.2017