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Parameter: Threshold, Operate delay
•
Default setting (_:661:3) Threshold = 1.500 A (for the 1st stage)
•
Default setting (_:661:6) Operate delay = 0.30 s (for the 1st 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 maximum occurring operating current. Pickup by overload must be excluded since
overcurrent protection operates with short tripping times as short-circuit protection and not as overload
protection. Therefore, set the Threshold parameter for lines to approx. 10 %, for transformers and motors to
approx. 20 % above the maximum load that is expected.
EXAMPLE
Overcurrent-protection stage: 110-kV overhead line, 150 mm
Maximum transmittable power
P
max
Correspondingly
I
max
Current transformer
Safety factor
Settings in primary and secondary values result in the setting values:
[foocpph1-030311-01.tif, 2, en_US]
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
s (length)
Z
/s
L
Ratio of zero-sequence impedance and positive-sequence impedance of the line: Z
Short-circuit power at the beginning of the line:
S
'
sc
SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
= 120 MVA
= 630 A
= 600 A/5 A
= 1.1
2
cross-section
= 60 km
= 0.46 Ω/km
= 2.5 GVA
Protection and Automation Functions
6.3 Overcurrent Protection, Phases
2
cross-section
/Z
= 4
L0
L1
333
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