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.
The Operate delay to be set is derived from the time-grading schedule that has been prepared for the
system.
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]
The settings in primary and secondary values result in the following setting values which include a safety
margin of 10 %:
Protection and Automation Functions
6.5 Overcurrent Protection, Ground
SIPROTEC 5, Overcurrent Protection, Manual 385
C53000-G5040-C017-8, Edition 07.2017
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