Siemens SIPROTEC 4 7VK61 Manual page 76

Functions
2.3 Overcurrent protection (optional)
short delay can be useful in case of long cables for which high inrush currents can be expected, or for trans-
formers. This delay depends on the intensity and the duration of the transient overcurrents as well as on
which stages were selected for the fast switch onto fault clearance.
High current stages Ιph>>, 3Ι0>>
The Ι>>-stages Iph>> (address 2610) and 3I0>> PICKUP (address 2612) together with the Ι>-stages or the
Ιp stages from a two0stage characteristic curve. Of course, all three stages can be combined as well. If one
stage is not required, the pickup value has to be set to ∞. The Ι>> stages always operate with a defined delay
time.
If the Ι>> stages are used for instantaneous tripping before the automatic reclosure (via CFC interconnection),
the current setting corresponds to the Ι> or Ιp stages (see below). In this case, only the different delay times
are of interest. The times T Iph>> (address 2611) and T 3I0>> (address 2613) can then be set to 0 s or a
very low value, as the fast clearance of the fault takes priority over the selectivity before the automatic reclo-
sure is initiated. These stages have to be blocked before final trip in order to achieve the selectivity.
For very long lines with a small source impedance or on applications with large reactances (e.g. transformers,
series reactors), theΙ>> stages can also be used for current grading. In this case, they must be set in such a
way that they do not pick up in case of a fault at the end of the line. The times can then be set to 0s or to a
small value.
When using a personal computer and DIGSI to apply the settings, these can be optionally entered as primary
or secondary values. For settings with secondary values the currents will be converted for the secondary side
of the current transformers.
Calculation Example:
110 kV overhead line 150 mm
s (length)
R /s
1
X /s
1
Short-circuit power at the beginning of the line:
S '
k
Current Trans-
former
From that the line impedance Z
[fo_7sa6_fkt-UMZ_bsp, 1, en_GB]
Z = 0.46 Ω/km · 60 km = 27.66 Ω
L
[formel-ueberstromzeit-hochstrom-1-oz-010802, 1, en_GB]
The 3-phase short-circuit current at the end of the line is Ι
[formel-ueberstromzeit-hochstrom-2-oz-010802, 1, en_GB]
With a safety factor of 10%, the following primary setting value is calculated:
Setting value Ι>> = 1.1 · 2150 A = 2365 A
or the secondary setting value:
76
2 :
= 60 km
= 0.19 Ω/km
= 0.42 Ω/km
= 2.5 GVA
600 A/5 A
and the source impedance Z
L
are calculated:
S
:
k end
C53000-G1176-C159-5, Edition 05.2018
SIPROTEC 4, 7VK61, Manual