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Functions
2.3 Overcurrent protection (optional)
High-current elements I
The I>> stages Iph>> (address 2610) and 3I0>> PICKUP (address 2612) together with the I> stages or the
I
stages result in a two-stage characteristic. Of course, all three stages can be combined as well. If one stage
p
is not required, the pickup value has to be set to ∞. The I>> stages always operates with a defined delay time.
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
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 reactances (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 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 Transformer 600 A / 5 A
From that the line impedance Z
/s = √0.19
Z
1
= 0.46 Ω/km · 60 km = 27.66 Ω
Z
L
The three-phase fault current at the line end is I
With a safety factor of 10 %, the following primary setting value is calculated:
Set value I>> = 1.1 · 2150 A = 2365 A
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 immediately be cleared by the time overcurrent pro-
tection.
74
>>, 3I
>>
ph
0
2
:
= 60 km
= 0.19 Ω/km
= 0.42 Ω/km
= 2.5 GVA
and the source impedance Z
L
2
2
Ω/km = 0.46 Ω/km
+ 0.42
stages (see below). In this case only the different delay times are
p
are calculated:
S
:
sc end
C53000-G1176-C159-3, Release date 05.2009
SIPROTEC, 7VK61, Manual
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