Siemens SIPROTEC 7UT613 series Manual page 203

Sensitivity Consid-
erations for High-
Impedance
Protection
7UT613/63x Manual
C53000-G1176-C160-2
The voltage across R is then
= I
U · ( 2R + R )
R 1 a2 i2
Furthermore, it is assumed that the pickup value of the 7UT613/63x corresponds to
half the knee-point voltage of the current transformers. The extreme case is thus
U = U / 2
R S
This results in a stability limit I
the scheme remains stable:
Calculation example:
For the 5 A CT as above with U
longest CT connection lead 22 m with 4 mm
that is 15 × rated current or 12 kA primary.
For the 1-A CT as above with U
longest CT connection lead 107 m with 2,5 mm
that is 27 × rated current or 21.6 kA primary.
As before-mentioned, high-impedance protection is to pick up with approximately half
the knee-point voltage of the current transformers. Resistance R can be calculated
from it.
Since the device measures the current flowing through the resistor, resistor and mea-
suring input of the device are to be connected in series. Since, furthermore, the resis-
tance shall be high-ohmic (condition: R >> 2R
ent resistance of the measuring input can be neglected. The resistance is then
calculated from the pickup current I
Calculation Example:
For the 5-A CT as above
desired pickup value I
an
For the 1-A CT as above
desired pickup valueI
an
2.7 Single-Phase Time Overcurrent Protection
, i.e. the maximum through-fault current below which
SL
= 75 V and R
S
2
cross-section; results in R
= 350 V and R
S
a2
and half the knee-point voltage:
an
= 0.1 A (corresponding to 16 A primary)
= 0.05 A (corresponding to 40 A primary)
= 0,3 Ω
i
= 5 Ω
i
2
cross-section; results in R
+ R , as above mentioned), the inher-
i2
≈ 0,1 Ω
a
≈ 0,75 Ω
a
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