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2 Functions
If the I>> stages are used for instantaneous tripping before the automatic reclosure
(via CFC interconnection), the current setting corresponds to the I> or I
below). In this case only the different delay times are of interest. The times T
Iph>>(address 2611) and T 3I0>> (address 2613) can than be set to 0 or a very
low value, as the fast clearance of the fault takes priority over the selectivity before the
automatic reclosure 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 reac-
tances (e.g. transformers, series reactors), the I>> 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 0 or to a small value.
When using a personal computer and DIGSI to apply the settings, these can be op-
tionally 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
Short-circuit power at the beginning of the line:
Current Transformer 600 A / 5 A
From that the line impedance Z
The three-phase fault current at the line end is I
With a safety factor of 10 %, the following primary setting value is calculated:
or the secondary setting value:
i.e. in case of fault currents exceeding 2365 A (primary) or 19.7A (secondary) you can
be sure that a short-circuit has occurred on the protected line. This fault can immedi-
ately be cleared by the time overcurrent protection.
Note: the calculation was carried out with absolute values, which is sufficiently precise
for overhead lines. If the angles of the source impedance and the line impedance vary
considerably, a complex calculation must be carried out.
112
s (length)
= 60 km
= 0.19 Ω/km
R
/s
1
= 0.42 Ω/km
X
/s
1
S
'
= 2.5 GVA
k
/s = √0.19
Ω/km = 0.46 Ω/km
2
2
Z
+ 0.42
1
= 0.46 Ω/km · 60 km = 27.66 Ω
Z
L
Set value I>> = 1.1 · 2150 A = 2365 A
2
:
and the source impedance Z
L
:
sc end
stages (see
p
are calculated:
S
7SD610 Manual
C53000-G1176-C145-4
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