Functions
2.14 Instantaneous High-Current Switch-onto-Fault Protection (SOTF)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
293
I
>>>> Stage
The
I
>>>> stage (address
2405
) works regardless of the circuit breaker position. Since it trips extremely fast
it must be set high enough not to pickup on a load current flowing through the protected object. This means
that it can be used only if the protected object allows current grading, as is the case with transformers, series
reactors or long lines with small source impedance. In other cases it is set to
∞
(default setting). This parameter
can only be altered with DIGSI under
Additional Settings
.
When using a PC and DIGSI for the parameterisation, the values can be optionally entered as primary or sec-
ondary quantities. For settings with secondary quantities the currents will be converted to the secondary side
of the current transformers.
Exemplary calculation:
110 kV overhead line 150 mm
2
with the data:
s (length)
= 60km
R
1
/s =
0.19
Ω
/km
X
1
/s =
0.42
Ω
/km
Short-circuit power at the feeding end:
S
k
" = 3.5 GVA (subtransient, since the
I
>>>>stage can respond to the first peak value)
Current transformer 600 A/5 A
From that the line impedance Z
L
and the source impedance Z
S
are calculated:
Z
1
/s =
√
0 .19
2
+ 0.42
2
Ω
/km = 0.46
Ω
/km
Z
L
=
0.46
Ω
/km · 60 km = 27.66
Ω
The three-phase short-circuit current at line end is
I
"
sc end
(with source voltage 1.1· U
N
):
With a safety factor of 10 %, the following primary setting value results:
Setting value
I>>>>
= 1.1 · 2245 A = 2470 A
or the secondary settings value:
i.e. in case of fault currents exceeding 2470 A (primary) or 20.6 A (secondary) you can be sure that a short-
circuit has occurred on the protected line. This line can be disconnected immediately.
Note:
The calculation was carried out with absolute values, which is sufficiently precise for overhead lines. A
complex calculation is only needed if the angles of the source impedance and the line impedance vary consid-
erably.
