Siemens 6MD85, Manual

Page: 641 / 1256

Parameter:
Threshold
,
Operate delay
•
Default setting
(_:751:3) Threshold
=
1.20 A
(for the first stage)
•
Default setting
(_:751:6) Operate delay
=
0.300 s
(for the first stage)
Set the
Threshold
and
Operate delay
parameters for the specific application.
The following details apply to a 2-stage characteristic curve (1st stage = definite-time overcurrent protection
stage and 2nd stage = high-current stage).
1st stage (overcurrent stage):
The setting depends on the minimal occurring ground-fault current. This must be determined.
For very small ground-fault currents, Siemens recommends using the Ground-fault protection against high-
resistance ground faults in grounded systems function.
The
Operate delay
to be set is derived from the time-grading schedule that has been prepared for the
system. Where overcurrent protection is used in emergency mode, shorter time delays might be reasonable
(one grading time above fast tripping), since the emergency mode only operates if the main protection func-
tion fails.
2nd stage (high-current stage):
This tripping stage can also be used for current grading. This applies in the case of very long lines with low
source impedance or ahead of high reactances (for example, transformers, shunt reactors). Set the
Threshold
parameter to ensure that the stage does not pick up in case of a short-circuit at the end of the
line.
Set the
Operate delay
parameter to
0
or to a low value.
Siemens recommends that the threshold values be determined with a system analysis. The following example
illustrates the principle of grading with a current threshold on a long line.
EXAMPLE
High-current stage: 110-kV overhead line, 150 mm 2 cross-section
s (length) = 60 km
Z
L
/s = 0.46 Ω/km
Ratio of zero-sequence impedance and positive-sequence impedance of the line: Z
L0
/Z
L1
= 4
Short-circuit power at the beginning of the line:
S
sc
' = 2.5 GVA
Ratio of zero-sequence impedance and positive-sequence impedance of the source impedance at the
beginning of the line: Z
P0
/Z
P1
= 2
Current transformer = 600 A/5 A
Resulting in the following values for the line impedance Z
L
and the source impedance Z
P
:
[fo_OCP002_030311, 1, en_US]
[fo_OCP003_030311, 1, en_US]
The 1-pole short-circuit current at the end of the line is I
scG end
:
[fo_ocp005_030311, 1, en_US]
Protection and Automation Functions
7.5 Overcurrent Protection, Ground
SIPROTEC 5, High-Voltage Bay Controller, Manual 641
C53000-G5040-C015-9, Edition 11.2017