Siemens 6MD85, Manual

Page: 769 / 1256

The stage is released only if the binary input signal
>release
is active.
Method of Measurement, Threshold Value
The stage works with 2 different methods of measurement.
•
Measurement of the fundamental component:
This method of measurement processes the sampled current values and filters out the fundamental
component numerically. A DC component is thus eliminated. The RMS value of the fundamental compo-
nent is compared with the set threshold.
•
Evaluation of the unfiltered measurand:
If the current exceeds a preset threshold value by
current ≥ 2·√2·threshold value
this stage will use unfiltered measurands in addition. Thus, very short operate times are possible.
Application and Setting Notes
Parameter: Activation
•
Default setting
(_:3901:101) Activation
=
on CB closure
Using the parameter
Activation
, you define the conditions under which the stage is released.
Parameter Value Description
on CB closure
Select this setting to activate the stage only when the circuit breaker is closed.
always active
Select this setting to release the stage statically.
only with binary
signal
Select this setting to release the stage via an external signal.
Parameter:
Threshold
•
Default setting
(_:3901:3) Threshold
=
10.0 A
for I
rated
= 1 A or
50.0 A
for I
rated
= 5 A
The stage works independently of the position of the remote circuit breakers. For this reason, set the
Threshold
so that the fault current flowing through does not trigger the stage. Thus, use this stage only if
current grading over the protected object is possible, that is, for transformers, shunt reactors or long lines with
low source impedance. In other cases, deactivate the stage.
EXAMPLE
Calculation example for current grading of a 110 kV overhead line measuring 150 mm 2
s (length) = 100 km;
R
1
/s = 0.21 Ω/km;
X
1
/s = 0.43 Ω/km
Since the stage is non-directional, the calculation must consider the maximum short-circuit power at the start
of the line or at the opposite end:
Ssc" = 3.5 GVA (subtransient, because the function can respond to the 1st peak value)
Current transformer: 600 A/5 A
The line impedance Z
L
and the minimum source impedance Z
S
are calculated on this basis:
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7.11.4
Protection and Automation Functions
7.11 Instantaneous High-Current Tripping
SIPROTEC 5, High-Voltage Bay Controller, Manual 769
C53000-G5040-C015-9, Edition 11.2017